Carb issue?

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Christopher Vastine

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Location
COLORADO SPRING
I recently built a 408/410 stroker for a 1979 Lil Red Express with bellow:

360 bored. 040
Kieth black pistons
Zero decked
.028 head gasket
63cc edlebrock heads
Harland sharp rockers
Rpm performer intake
650cfm edelbrock avs carb
7177 cam for now, but will eventually change
Duration at 050 inch Lift:234 int./244 exh. Advertised Duration:308 int./318 exh. Valve Lift with Factory Rocker Arm Ratio:0.488 int./0.510 exh.
2500 stall torque converter
3.73 gears posi

Currently i have low unstable vacuum at idle of around 5-8". The carb will not allow me to adjust idle screw low enough for the idle circuit. This is causing a very rich idle issue. If i hold the secondaries open a little rpms jump and vacuum goes to a solid 10"-12" depending on rpm . I have looked for vacuum leaks and evan pulled the intake to insure is was sealing. I have tried timing from 14-20 degrees with little affect. I have triple checked the lifter preload as well. I have replace all ignition components and verified that everything is working. Any suggestions on how to fix the idle circuit issue or what can be wrong? I am at high altitude and the cam is rated at 10" of vacuum at sea level. I was told i will probably need to drill the butterflies on the carb or go to a 750, but seems like it should work without. Also have fuel pressure regulated down to 4.
 
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Here’s my experience.........
Long duration cams that result in low vacuum...... really aren’t a good match for Ede carbs.
Some sort of Holley(or Holley “type”) carb that has 4-corner idle, along with an adjustment for the secondary throttle blade position is what you should be looking at.
 
I agree. Long duration and low vacuum isn't ideal conditions for eddy carbs because vacuum dominates the metering rods. You may get it closer by playing with the springs, but still. I would move to a Holley style carb...
 
I agree. Long duration and low vacuum isn't ideal conditions for eddy carbs because vacuum dominates the metering rods. You may get it closer by playing with the springs, but still. I would move to a Holley style carb...

I put the lowest springs which stayed closed at idle and eventually removed them the springs for troubleshooting purposes. If i create a vacuum leak at the base of the carb it leans out the air fuel mixture and idles great.
 
I put the lowest springs which stayed closed at idle and eventually removed them the springs for troubleshooting purposes. If i create a vacuum leak at the base of the carb it leans out the air fuel mixture and idles great.
I'm familiar with the 6,000 ft elevation at Colo Springs. Long duration cams and poor air quality wants to fatten up the engine mixture, especially at idle. And your poor vacuum at idles becomes worse once you put the car in gear. I don't claim to be a eddy specialist, but I feel I have more control with a Holley style carb. No, I'm not anti-eddy. I have 3 eddies in service right now on a couple of my classics.
 
When it’s idling as good as you can get it........ if you look into the primary boosters....... is there fuel dripping from them?

With the blades adjusted for best idle, if you remove the carb for a visual inspection ........ how much of the transfer slot is exposed below the blade?

Is the ported vacuum hole exposed?
 
I'm familiar with the 6,000 ft elevation at Colo Springs. Long duration cams and poor air quality wants to fatten up the engine mixture, especially at idle. And your poor vacuum at idles becomes worse once you put the car in gear. I don't claim to be a eddy specialist, but I feel I have more control with a Holley style carb. No, I'm not anti-eddy. I have 3 eddies in service right now on a couple of my classics.

I understand and have been thinking about buying a holley, but happened to have the avs new on the self. Also have a 1406 that causes the same issue. I have rebuilt over 40 motors, but none at this performance level.
 
Just drill some idle air bypass holes in the front side of the primary throttle blades. Start around .093
 
When it’s idling as good as you can get it........ if you look into the primary boosters....... is there fuel dripping from them?

With the blades adjusted for best idle, if you remove the carb for a visual inspection ........ how much of the transfer slot is exposed below the blade?

Is the ported vacuum hole exposed?

I have taken carb apart cleaned and checked the float levels. Also put the fuel pressure gauge and regulator on it set to a steady 4. I don't see fuel dripping but pretty sure it is, and yes the port vacuum hole is exposed as well as the whole transfer slot.
I can up the timing but still not enough to get it in the idle circuit. I have the ability to lock ot the mechanical advance once i figure out what i set the timing to.
 
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Just drill some idle air bypass holes in the front side of the primary throttle blades. Start around .093

I am considering it, but wanted to make sure i was on the right track. Also realize i maybe a little small on the carburetor, but at the higher altitude a smaller carb is usually better. Would probably drill the 1406 and see how is does rather than the avs.
 
I have taken carb apart cleaned and checked the float levels. Also put the fuel pressure gauge and regulator on it set to a steady 4. I don't see fuel dripping but pretty sure it is, and yes the port vacuum hole is exposed as well as the whole transfer slot.

If the blades are open enough to expose the ported vacuum hole and the entire transfer slot, the fuel is dripping from the boosters because the main circuit has been activated(from the blades being open too far).

Simply put, the carb lacks the idle circuit capacity to supply adequate fuel flow with the low vacuum signal.

There is nothing “wrong” with the carb...... it’s just not a good match for the application.
If the cam was 10-15* less duration(resulting in more vacuum), the carb would work just fine.
 
If the blades are open enough to expose the ported vacuum hole and the entire transfer slot, the fuel is dripping from the boosters because the main circuit has been activated(from the blades being open too far).

Simply put, the carb lacks the idle circuit capacity to supply adequate fuel flow with the low vacuum signal.

There is nothing “wrong” with the carb...... it’s just not a good match for the application.
If the cam was 10-15* less duration(resulting in more vacuum), the carb would work just fine.

That's what i figured but now to replace cam or drill the 1406 and do a cam swap later. Any recommendations for a cam?
 
Drilling the blades might get you to a satisfactory idle situation..... or not.
It doesn’t really alter the idle circuit capacity.
It just allows more airflow with the blades being in a more correct position.
There’s really only one way to know if it will work in your application.

In the mid-80’s I did several low buck 400 re-ring jobs for guys who wanted a low buck street/strip big block.
These were still low compression when done, and I pretty much always “over cammed” them, since it worked well on the track if you had enough stall and gear.
I used the MP484 cam in a few, which did the job for power, but was a little miserable on the street if the carb wasn’t right.
These motors didn’t make any real power, or flow much air...... so they didn’t need a big carb.
At that time there really weren’t any off the shelf small carbs with 4 corner idle, and the 2 corner idle of the small carbs just didn’t cut it.
My preferred carbs for those was a Holley 4777, 650DP....... but they only had 2 corner idle.
I would modify them to have 4 corner idle, which would really transform how they behaved at idle and low speed operation.

Nowadays, you can buy a 650 with 4 corner idle from a variety of sources.

As for the cam.....”if it were me”, and I was looking to maximize the affect of a cam swap...... I’d go straight to a SFT.
 
I’m quite comfortable messing with Holley style carbs, so even though this appears to be a new model, and I have no previous experience with it...... it looks like it checks off the boxes for your application that I’d be looking for.
-650DP
-4 corner idle
-electric choke
-budget friendly price
And, as a bonus it also has changeable air bleeds.
Brawler BR-67276 650 CFM Brawler Diecast Carburetor Mechanical Secondary
 
The carb will not allow me to adjust idle screw low enough for the idle circuit.

Re-reading the OP.......

Are you saying the engine won’t idle down when you close down the blades?

Or, if you close the blades to a more “correct” position...... the engine won't run(or runs poorly)?

My responses were based on me thinking it was the latter.
 
Re-reading the OP.......

Are you saying the engine won’t idle down when you close down the blades?

Or, if you close the blades to a more “correct” position...... the engine won't run(or runs poorly)?

My responses were based on me thinking it was the latter.

It will idle down, but too low to run. Have to adjust turn the idle screw in too far for the idle circuit to get it to idle. Then it runs way too rich at idle and had the off idle hesitation as expected. If i introduce air via a vacuum leak or manual cracking thw secondaries rpms jump and vacuum becomes higher and steady.
 
Elevation 6000 ft?
IMHO, you got the wrong cam for that elevation.
But lets see if we can get something out of it.

Ok so a lil explanation might be in order;
At idle
Your engine is pulling fuel from the Idle Discharge ports and the transfer slots, and should not be getting fuel from any place else. You control how much fuel by the synchronizing of the mixture discharge with the transfer discharge to ballpark it lean enough to idle but not so lean as to have a tip-in issue.
However, at this setting, the long-period cammed engine is usually begging for more air or more timing. Too much timing will just drive the rpm up, so that is not the answer. So you pick a timing that is known to be in the ballpark, by what others have empirically amassed like 14 to 20 degrees. And after that, you have to decrease the excess fuel, or supply the additional air, to get the AFR back into a decent working range. And that's all there is to it,basically.
So to help you figure it out;
Try this;
Remove the springs from underneath the metering rod pistons.
Defeat the Vacuum advance.
Close the mixture screws to 2.5 turns.
Close the secondaries up tight but not sticking.
Prove the fuel level is correct and stable.
Make sure the PCV SYSTEM is plumbed correctly and working.
Make sure the distributor flyweights are at minimum setting and stay there to at least 1200rpm for this test.
now
Fire it up and
crank the idle speed up to where the engine idles nice, say 1000 rpm, or whatever it takes.
Without regard for the timing numbers, just advance the timing until the rpm peaks.
drop the rpm back to 1000
readjust the timing for max rpm. Write the timing and rpm numbers down. I might want to refer to these later. I just want you to see how much Idle-timing your engine is asking for
Next;
start taking timing out of it, while simultaneously readjusting the idle speed screw to maintain a minimum 800/850rpm.... until you get down to 18/16 degrees advance. Forget about the vacuum reading. Forget about how she runs, just see if she will do it.
Once you got her running there, you will need to figure out if she is rich or lean. You already proved that she is rich by cracking the secondaries and noting the increase in rpm.
So, the question is how to; give the engine the air she is craving, or subtracting the too much fuel that she is getting. Which method you chose will depend on where the throttle blades are at idle, relative to the transfer slots.
Ok then
Remove the carb, drain it, put the primary throttle on the curb-idle screw and keep it there; then flip it upside down and go find the transfer slots. They will be on the very front of the bores, and are the only vertically oriented slots there. You should be seeing them as slots a little taller than wide. NOT more than about 30% taller than wide, and not less than 20% taller than wide. I'm sorta guessing but that will ballpark them. Make them so. Then bolt her back on, and fill her up.
After this, DO NOT adjust the speed screw again! If the rpm is not right (which is a personal preference, somewhere between say 700 and 850), you will need to adjust it with timing.
Ok now you are baselined.
To subtract idle fuel you only have three options;
1) mixture screws, and
2) IABs aka Idle-Air-Bleeds, and
3) fuel level
To add idle air, IMO you only have two options;
4) drill holes in the primary throttles, as mentioned, or
5) cracking the secondaries.

Of these, I am no fan of introducing dry air into the secondaries, and no fan of messing with the factory fuel level.

I also don't like to get too far away from 2.5 turns of the mixture screws.
So really, you only have two options; numbers 2 and 4.
Unless your carb was designed to run at 6000 ft, it has the wrong IABs in it, and so that is where I would start. But I can almost guarantee you that, your engine will still need bypass air thru the blades sooner or later.

So at this point, I am out of my comfort zone with your combo, and someone else with hi-altitude experience can take over.

But I want to reiterate, to leave your transfer slot exposure, underneath the primary blades, very close to the above ballpark of a little taller than wide. Don't care what the idle timing is to get it. If you get too far away from there, you will have driveability issues.

Now after your idle is bugged out, there is a small matter of the missing springs under the step-up pistons to figure out, and at some point in the near future, the Power-Timing will have to be addressed, and later, the missing Vcan.
That's all I got.
Hopefully someone smarter than me will stop in.
 
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Currently i have low unstable vacuum at idle of around 5-8

Why do you gave an unstable idle? I ran a bigger cam with an 800 avs in similar cubes and still managed to get 9 inches. If your idle is fluctuating you need to find out why.

Have you checked your plugs?
have you check your leads?
Have you done a leak down test to make sure all your valves are sealing.

You can try another carb but I don't think that is the real issue here.
 
Elevation 6000 ft?
IMHO, you got the wrong cam for that elevation.
But lets see if we can get something out of it.

Ok so a lil explanation might be in order;
At idle
Your engine is pulling fuel from the Idle Discharge ports and the transfer slots, and should not be getting fuel from any place else. You control how much fuel by the synchronizing of the mixture discharge with the transfer discharge to ballpark it lean enough to idle but not so lean as to have a tip-in issue.
However, at this setting, the long-period cammed engine is usually begging for more air or more timing. Too much timing will just drive the rpm up, so that is not the answer. So you pick a timing that is known to be in the ballpark, by what others have empirically amassed like 14 to 20 degrees. And after that, you have to decrease the excess fuel, or supply the additional air, to get the AFR back into a decent working range. And that's all there is to it,basically.
So to help you figure it out;
Try this;
Remove the springs from underneath the metering rod pistons.
Defeat the Vacuum advance.
Close the mixture screws to 2.5 turns.
Close the secondaries up tight but not sticking.
Prove the fuel level is correct and stable.
Make sure the PCV SYSTEM is plumbed correctly and working.
Make sure the distributor flyweights are at minimum setting and stay there to at least 1200rpm for this test.
now
Fire it up and
crank the idle speed up to where the engine idles nice, say 1000 rpm, or whatever it takes.
Without regard for the timing numbers, just advance the timing until the rpm peaks.
drop the rpm back to 1000
readjust the timing for max rpm. Write the timing and rpm numbers down. I might want to refer to these later. I just want you to see how much Idle-timing your engine is asking for
Next;
start taking timing out of it, while simultaneously readjusting the idle speed screw to maintain a minimum 800/850rpm.... until you get down to 18/16 degrees advance. Forget about the vacuum reading. Forget about how she runs, just see if she will do it.
Once you got her running there, you will need to figure out if she is rich or lean. You already proved that she is rich by cracking the secondaries and noting the increase in rpm.
So, the question is how to; give the engine the air she is craving, or subtracting the too much fuel that she is getting. Which method you chose will depend on where the throttle blades are at idle, relative to the transfer slots.
Ok then
Remove the carb, drain it, put the primary throttle on the curb-idle screw and keep it there; then flip it upside down and go find the transfer slots. They will be on the very front of the bores, and are the only vertically oriented slots there. You should be seeing them as slots a little taller than wide. NOT more than about 30% taller than wide, and not less than 20% taller than wide. I'm sorta guessing but that will ballpark them. Make them so. Then bolt her back on, and fill her up.
After this, DO NOT adjust the speed screw again! If the rpm is not right (which is a personal preference, somewhere between say 700 and 850), you will need to adjust it with timing.
Ok now you are baselined.
To subtract idle fuel you only have three options;
1) mixture screws, and
2) IABs aka Idle-Air-Bleeds, and
3) fuel level
To add idle air, IMO you only have two options;
4) drill holes in the primary throttles, as mentioned, or
5) cracking the secondaries.

Of these, I am no fan of introducing dry air into the secondaries, and no fan of messing with the factory fuel level.

I also don't like to get too far away from 2.5 turns of the mixture screws.
So really, you only have two options; numbers 2 and 4.
Unless your carb was designed to run at 6000 ft, it has the wrong IABs in it, and so that is where I would start. But I can almost guarantee you that, your engine will still need bypass air thru the blades sooner or later.

So at this point, I am out of my comfort zone with your combo, and someone else with hi-altitude experience can take over.

But I want to reiterate, to leave your transfer slot exposure, underneath the primary blades, very close to the above ballpark of a little taller than wide. Don't care what the idle timing is to get it. If you get too far away from there, you will have driveability issues.

Now after your idle is bugged out, there is a small matter of the missing springs under the step-up pistons to figure out, and at some point in the near future, the Power-Timing will have to be addressed, and later, the missing Vcan.
That's all I got.
Hopefully someone smarter than me will stop in.


Thank you that was very detailed. This is basically how ajusted everything this last time, but with the timing up to 20-24 i still cant get into the idle circuit. It is
Why do you gave an unstable idle? I ran a bigger cam with an 800 avs in similar cubes and still managed to get 9 inches. If your idle is fluctuating you need to find out why.

Have you checked your plugs?
have you check your leads?
Have you done a leak down test to make sure all your valves are sealing.

You can try another carb but I don't think that is the real issue here.
Elevation 6000 ft?
IMHO, you got the wrong cam for that elevation.
But lets see if we can get something out of it.

Ok so a lil explanation might be in order;
At idle
Your engine is pulling fuel from the Idle Discharge ports and the transfer slots, and should not be getting fuel from any place else. You control how much fuel by the synchronizing of the mixture discharge with the transfer discharge to ballpark it lean enough to idle but not so lean as to have a tip-in issue.
However, at this setting, the long-period cammed engine is usually begging for more air or more timing. Too much timing will just drive the rpm up, so that is not the answer. So you pick a timing that is known to be in the ballpark, by what others have empirically amassed like 14 to 20 degrees. And after that, you have to decrease the excess fuel, or supply the additional air, to get the AFR back into a decent working range. And that's all there is to it,basically.
So to help you figure it out;
Try this;
Remove the springs from underneath the metering rod pistons.
Defeat the Vacuum advance.
Close the mixture screws to 2.5 turns.
Close the secondaries up tight but not sticking.
Prove the fuel level is correct and stable.
Make sure the PCV SYSTEM is plumbed correctly and working.
Make sure the distributor flyweights are at minimum setting and stay there to at least 1200rpm for this test.
now
Fire it up and
crank the idle speed up to where the engine idles nice, say 1000 rpm, or whatever it takes.
Without regard for the timing numbers, just advance the timing until the rpm peaks.
drop the rpm back to 1000
readjust the timing for max rpm. Write the timing and rpm numbers down. I might want to refer to these later. I just want you to see how much Idle-timing your engine is asking for
Next;
start taking timing out of it, while simultaneously readjusting the idle speed screw to maintain a minimum 800/850rpm.... until you get down to 18/16 degrees advance. Forget about the vacuum reading. Forget about how she runs, just see if she will do it.
Once you got her running there, you will need to figure out if she is rich or lean. You already proved that she is rich by cracking the secondaries and noting the increase in rpm.
So, the question is how to; give the engine the air she is craving, or subtracting the too much fuel that she is getting. Which method you chose will depend on where the throttle blades are at idle, relative to the transfer slots.
Ok then
Remove the carb, drain it, put the primary throttle on the curb-idle screw and keep it there; then flip it upside down and go find the transfer slots. They will be on the very front of the bores, and are the only vertically oriented slots there. You should be seeing them as slots a little taller than wide. NOT more than about 30% taller than wide, and not less than 20% taller than wide. I'm sorta guessing but that will ballpark them. Make them so. Then bolt her back on, and fill her up.
After this, DO NOT adjust the speed screw again! If the rpm is not right (which is a personal preference, somewhere between say 700 and 850), you will need to adjust it with timing.
Ok now you are baselined.
To subtract idle fuel you only have three options;
1) mixture screws, and
2) IABs aka Idle-Air-Bleeds, and
3) fuel level
To add idle air, IMO you only have two options;
4) drill holes in the primary throttles, as mentioned, or
5) cracking the secondaries.

Of these, I am no fan of introducing dry air into the secondaries, and no fan of messing with the factory fuel level.

I also don't like to get too far away from 2.5 turns of the mixture screws.
So really, you only have two options; numbers 2 and 4.
Unless your carb was designed to run at 6000 ft, it has the wrong IABs in it, and so that is where I would start. But I can almost guarantee you that, your engine will still need bypass air thru the blades sooner or later.

So at this point, I am out of my comfort zone with your combo, and someone else with hi-altitude experience can take over.

But I want to reiterate, to leave your transfer slot exposure, underneath the primary blades, very close to the above ballpark of a little taller than wide. Don't care what the idle timing is to get it. If you get too far away from there, you will have driveability issues.

Now after your idle is bugged out, there is a small matter of the missing springs under the step-up pistons to figure out, and at some point in the near future, the Power-Timing will have to be addressed, and later, the missing Vcan.
That's all I got.
Hopefully someone smarter than me will stop in.

Thank you for the very thorough post. It very close to how i went about setting everything last time, but i adjusted the idle screw ar the end. If i barely hold the secondaries open i can get it in the idle circuit and vacuum becomes steady and jumps to 12" . I may try the method again but think i will need a holley or larger CFM carburetor. Thw larger butterflies supposedly allow more air around them and may allow me to get to the idle circuit with Timing around 20 degrees.
 
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Why do you gave an unstable idle? I ran a bigger cam with an 800 avs in similar cubes and still managed to get 9 inches. If your idle is fluctuating you need to find out why.

Have you checked your plugs?
have you check your leads?
Have you done a leak down test to make sure all your valves are sealing.

You can try another carb but I don't think that is the real issue here.

I am not sure what your elevation is but you lose an average of 1" of vacuum per 1000 ft of elevation. The manufacturer rates the cam at 10-12" of vacuum at sea level. Also as previously mentioned if i allow a small amount of air it via a small vacuum leak or barely holding open the secondaries my rpms and vacuum raises to 10-12" and becomes steady.

An 800 cfm has larger butterflies that allows more air to get by and may work. As mentioned in previous post the 650cfm may be a little too small for the build, but i had a new one on the shelf already.


- Compression test was 160-165 cold and dry on all 8 cylinders.
- plugs are becoming black from being way too rich
- machine shop went through the new heads and performed a leakdown test on the bench, but possibly perform one with everything installed.
 
If i barely hold the secondaries open i can get it in the idle circuit and vacuum becomes steady and jumps to 12" . I may try the method again but think i will need a holley or larger CFM carburetor. The larger butterflies supposedly allow more air around them and may allow me to get to the idle circuit with Timing around 20 degrees.

that is not correct thinking, even tho I think the 650 is a bit small.
Idle bypass air is correct thinking; , we all had to figure this out. It begins with about 270* advertised intake duration, depending on a few factors. Your Eddy cam specs are
308/318/112+5 @.006 "lift", and 234/[email protected]
This cam has exceptionally long and slow ramps, I mean the intake is (308 less 234)/2 =37* opening and 37* closing if symmetrical, and more typically a FTH cam would be 46*/2=23*; so 37 is 60* more than typical.
So at idle it acts like a much bigger cam than it is.
And the reason for that, is, in at +5 (107*) the Ica is 81*. At 81*, the piston is almost halfway up on the compression stroke when the valve is advertised to be closed, but is still only at .006 lift. It will be many more degrees before it is actually on the seat and not leaking.
This means at idle, the piston has ample time to push a lot of mixture back up into the intake manifold. And because of your altitude, that air has a lot less oxygen in it than it does at sealevel, so that makes it even worse. This is the principal cause of your low vacuum. The second cause is the lack of EFFECTIVE Compression Ratio; and the third is the lack of timing.
No matter what, I believe, you are gonna have to give it some bypass air.

If you are scared to drill the plates, you can introduce filtered air into the PCV circuit, but I recommend not to pull it out of the crankcase. BTW, do not run the PCV off just one runner; that cylinder will very likely foul the plug. The PCV circuit should enter the base of the carb just below the primary throttle valves. This is a perfect place to bring in bypass-air.
 
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If i barely hold the secondaries open i can get it in the idle circuit and vacuum becomes steady and jumps to 12" . I may try the method again but think i will need a holley or larger CFM carburetor. Thw larger butterflies supposedly allow more air around them and may allow me to get to the idle circuit with Timing around 20 degrees.

I can see logic both ways. I think that the right track is the idle air bleeds. I read a similar post when trying to adjust yesterday.

- I removed metering rod spings
- vacuum advance is disconnected
- secondaries are closing(i have not adjusted or did anything to hold them open
- installed fuel regulator and gauge adjust to steady 4psi. Floats checked no fuel entry and proper float levels
- idle mixture screws are currently adjusted 2 turn out from gently seated
- PVC is plumbed and in good working condition
- have the FBO mechanical lock out and spring kit

At 14 degrees timing motor fires right up and idles great, but rich with idle screw in past idle transfer slot. As i advance the timing and turn down thw idle screw i get to 22-24 degrees.

It will run idle easily at 800 to 850 rpm at as low as 16 degrees timing, but again not in the transfer slots.

I have drained and flipped the carb over and adjusted to a square on the idle transfer slot. With the idle screw that way i cannot add enough timing to get it to idle, but will try again tomorrow.

I also have rhe strip kit with all the different rods, jets, and springs to set up the carb later. I just cant seem to get it to run on the transfer slot.
 
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