Timing help needed

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Ran it twice last week at the track. All seemed well, but still fat and unhappy at idle.

Contemplating trying out that smaller bushing and bumping the initial up before playing with the carb again.

Not to get off track but does everyone agree that if you are under 1.5 turns on the idle mix screws to go with .010 bigger bleeds and if you are over 2 to go .010 smaller?
 
roccodart440 said:
Ran it twice last week at the track. All seemed well, but still fat and unhappy at idle.

Contemplating trying out that smaller bushing and bumping the initial up before playing with the carb again.

Not to get off track but does everyone agree that if you are under 1.5 turns on the idle mix screws to go with .010 bigger bleeds and if you are over 2 to go .010 smaller?
Click to expand...
Don't get to concerned with the number of turns. The idle/transition circuit is a mini carb within the carb. The IAB and IFR work together to set the fuel ratio. The idle mixture screws meter how much of that air/fuel mixture goes to the idle passage. The only way to change that air fuel mixture to the transition slot is to install T-slot restrictors. Fix the timing first, then play with IFR IAB to get it right. I hope you are adjusting the idle mixture with the engine fully warmed up. If the motor idles when you first start it with no need to feather the throttle for a minute you are pig rich when warmed up.
 
I am adjusting when at operating temp.

When I start it up I keep a high idle for 3 seconds to a minute, then put in neutral to get the pump turning then it will idle.
 
roccodart440 said:
Not to get off track but does everyone agree that if you are under 1.5 turns on the idle mix screws to go with .010 bigger bleeds and if you are over 2 to go .010 smaller?
Click to expand...
It's not off track. Someone smarter than me once posted about the time it takes for the fuel to clean up is time lost in establishing good combustion. Point was that getting the idle and off idle cleaned up is definately relavant to the runs.

To add to what Mderoy said.
Important thing is that the idle mix screws noticible change the idle characteristics when turned.
The t-slot under the throttle is restricted but at least as important is the slot above the throttle acts as another bleed.
My experience has been that a test for too large of idle bleeds is very gentle low speed loads. Go up a small hill at 30 mph. If it can't do that, it probably needs smaller IABs.
If you can't do that test (track only car) then prob have to just be very systematic with experimentation.
First do a very slow increase in rpm from idle in neutral. The idea is minimize any pump shot. If there is any hesitation, then it certainly won't like it under load.
Try a slight increase in IFR if you can. If thats not within easy possibility, try smaller primary IABs. Or wires in the IABs to reduce the effective area.
Of course its best to test one change at a time but adjust the idle mix screws if need be for stronger idle.
Always remember idle in neutral will be happy when leaner than it should be for handling even a small load.
 
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roccodart440 said:
Ran it twice last week at the track. All seemed well, but still fat and unhappy at idle.

Contemplating trying out that smaller bushing and bumping the initial up before playing with the carb again.

Not to get off track but does everyone agree that if you are under 1.5 turns on the idle mix screws to go with .010 bigger bleeds and if you are over 2 to go .010 smaller?
Click to expand...


When I do it, I tune the idle circuit so the screws are out 3/4 to 1 turn with 4 corner idle. If you are over 1 turn, IMO you need to either open the IAB up to lean it out, or reduce the IFR to get the idle screws .75 to 1 turn out.

IIRC, the bigger the idle air bleed, the longer you stay on the idle circuit, which may or may not be a good thing.

Again, IMO any carb’d car should be a bit pissed off at cold start. If it’s not, it’s fat at idle.

EDIT: @Mattax I would like to hear what you think.
 
My experience was that I made a lot of tests changing the IFR, then the IAB, then the IFR, then the IAB until the low speed driving seemed the best.
That didn't help me understand the relationships, but some stuff posted by Shrinker and Mark did.

I generally don't worry about how far in or out the idle mix screws are unless it is extreme. At least a half turn out and less than two turns out (on most carbs) is the range I'm confortable with. I think Tuner is too but you can check with him on that. But mark and Shrinker say what you guys say. Try to get it in more in the 1 to 1.5 range. So rereading this I learn more. :) See snips below

Here's one more IFR 'test' but its also done in neutral so like the one I posted before (which comes from Urich's book) its seems only useful identifying if the IFR is way off.
"raise the idle speed two or three hundred rpm with the idle speed screw, turn in the idle mixture screw 1/2 turn. If the engine speeds up, the mixture was rich, if the engine slows, it may be lean.
Now, turn the mixture screw out 1/2 turn, if the engine speeds up, the mixture was lean, if the engine slows, it may be rich." That's from Troy Patterson (TMP carbs) who doesn't always know what he's talking about. But this seems to be a reasonable test. Another one is to increase the throttle opening (off idle position) and block or partially block the IABs to see how the engine reacts. These are quick tests to get a sense of what is going on and what directions to explore for improvements.

Idle Bleed - Idle Jet Relationship
jmarkaudio Fri Oct 17, 2014 10:25 pm"Idle Circuit Function"
Smaller idle jet/bleed will provide fuel farther in the throttle opening than a larger jet/bleed combination that provides roughly the same amount of fuel.
...
The smaller bleed will maintain a signal at a larger throttle opening than the larger bleed, hence the name air bleed. Larger bleed bleeds the signal sooner.


08-13-2007, 06:42 AM
shrinker
The idle mixture screw should normally be around 1 to 1.5 turns out and the t-slot maybe .020" exposed up to .040", depends. If the IFR is too large and the idle air bleed too large then you have to restrict the idle mixture screw to get it correct at idle in neutral. When you put it into gear the revs drop a little and then the vac drops and then the flow restriction of the idle mixture screw has too much influence and it leans out and the engine dies. The idle screw restriction needs to be so that it doesn't restrict it too hard and early as the revs slightly. The t-slot comes on very slowly at first and the idle has to make up the difference so that the mixture is consistent. If the mixture is set correct without using as much idle screw restriction the mixture from neutral to in gear will be more consistent.
...
If the IFR and air bleed are too big then the fuel condition exiting the slot at low openings will be blobs. Thats no good for distribution. If distribution is poor then you have some lean cylinders and the engine has no power. Cylinder power is affected more by being lean than it is by being rich.

That's the shortened version from Shrinker.
wink-gif.gif
 
One more snip from Shrinker.
Some of these things are context dependent, not absolute rules...

" If the vac is less at say 9" to 11" then the t-slot needs to be more exposed so the idle mixture screw will probably be best idle at around 0.5 to 0.75 turn off the seat. The Idle mixture screw is really a trim control for the slots contribution."
 
Mattax said:
My experience was that I made a lot of tests changing the IFR, then the IAB, then the IFR, then the IAB until the low speed driving seemed the best.
That didn't help me understand the relationships, but some stuff posted by Shrinker and Mark did.

I generally don't worry about how far in or out the idle mix screws are unless it is extreme. At least a half turn out and less than two turns out (on most carbs) is the range I'm confortable with. I think Tuner is too but you can check with him on that. But mark and Shrinker say what you guys say. Try to get it in more in the 1 to 1.5 range. So rereading this I learn more. :) See snips below

Here's one more IFR 'test' but its also done in neutral so like the one I posted before (which comes from Urich's book) its seems only useful identifying if the IFR is way off.
"raise the idle speed two or three hundred rpm with the idle speed screw, turn in the idle mixture screw 1/2 turn. If the engine speeds up, the mixture was rich, if the engine slows, it may be lean.
Now, turn the mixture screw out 1/2 turn, if the engine speeds up, the mixture was lean, if the engine slows, it may be rich." That's from Troy Patterson (TMP carbs) who doesn't always know what he's talking about. But this seems to be a reasonable test. Another one is to increase the throttle opening (off idle position) and block or partially block the IABs to see how the engine reacts. These are quick tests to get a sense of what is going on and what directions to explore for improvements.

Idle Bleed - Idle Jet Relationship
jmarkaudio Fri Oct 17, 2014 10:25 pm"Idle Circuit Function"
Smaller idle jet/bleed will provide fuel farther in the throttle opening than a larger jet/bleed combination that provides roughly the same amount of fuel.
...
The smaller bleed will maintain a signal at a larger throttle opening than the larger bleed, hence the name air bleed. Larger bleed bleeds the signal sooner.


08-13-2007, 06:42 AM
shrinker
The idle mixture screw should normally be around 1 to 1.5 turns out and the t-slot maybe .020" exposed up to .040", depends. If the IFR is too large and the idle air bleed too large then you have to restrict the idle mixture screw to get it correct at idle in neutral. When you put it into gear the revs drop a little and then the vac drops and then the flow restriction of the idle mixture screw has too much influence and it leans out and the engine dies. The idle screw restriction needs to be so that it doesn't restrict it too hard and early as the revs slightly. The t-slot comes on very slowly at first and the idle has to make up the difference so that the mixture is consistent. If the mixture is set correct without using as much idle screw restriction the mixture from neutral to in gear will be more consistent.
...
If the IFR and air bleed are too big then the fuel condition exiting the slot at low openings will be blobs. Thats no good for distribution. If distribution is poor then you have some lean cylinders and the engine has no power. Cylinder power is affected more by being lean than it is by being rich.

That's the shortened version from Shrinker.View attachment 1715608618
Click to expand...
Thanks
Interesting idle-mix test; I'm gonna try that one. What I've always called the 'quick n dirty' idle check was to give the pump arm a little bump so it squirts just a little fuel in. If the idle speeds up, it's lean, and vice versa.
.
 
yellow rose said:
Again, IMO any carb’d car should be a bit pissed off at cold start. If it’s not, it’s fat at idle.
Click to expand...
Agreed. I know mine are too rich if they idle on their own on first startup of the day, in much less than a minute, and that's in summertime. And then a few minutes of driving to get the intake good and warm.Of course, I also never use chokes!
 
Edited earlier post where I correctly wrote bleeds then incorrectly wrote IFR while still talking about bleeds. :rolleyes:

What Shrinker suggested for IFR check is similar to Urich-Holley.
shrinker
08-16-2008, 02:41 PM
"Hello; tuning tip; place car in Neutral---start engine---very slowly open throttle by hand or by idle speed screw. Listen very carefully or use a digital tacho for slight increases in RPM for every movement of the throttle blades. If you move the blades the tiniest bit and the engine doesn't increase revs you have a lean spot you need to deal with."
---
The reason for opening the throttle super slowly is to eliminate the pump shot.
 
The Idle air bleeds are .070 and the high speed air bleeds are .032.

This thing will start and idle pretty easily and pretty quickly with no choke. It is a proform 950, milled air horn etc.

I checked where my machinest had them set, which I should have wrote down, but didn't. I believe they were about 3/4 turn out on the rear and half on the front. After a while of messing around and my eyes burning and the place stinking of fuel I set them back to about where he had them sewt and walked away.

One thing I did do however was use a piece of tubing to listen to the barrels and get them to sound the same. This made me open the rears more and close the fronts a tad.

I was thinking of trying .005 - .010 bigger on the IAB's.

Thoughts?

Off idle or blipping the throttle it runs great. It's just very fat at idle and not entirely happy in gear at idle.
 
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roccodart440 said:
The Idle air bleeds are .070 and the high speed air bleeds are .032.

This thing will start and idle pretty easily and pretty quickly with no choke. It is a proform 950, milled air horn etc.

I checked where my machinest had them set, which I should have wrote down, but didn't. I believe they were about 3/4 turn out on the rear and half on the front. After a while of messing around and my eyes burning and the place stinking of fuel I set them back to about where he had them sewt and walked away.

One thing I did do however was use a piece of tubing to listen to the barrels and get them to sound the same. This made me open the rears more and close the fronts a tad.

I was thinking of trying .050 - .010 bigger on the IAB's.

Thoughts?

Off idle or blipping the throttle it runs great. It's just very fat at idle and not entirely happy in gear at idle.
Click to expand...


If you mean .005-.010 on the IAB’s I think that’s a good idea. I wouldn’t be afraid to get into the mid .080’s if you have to.

Or, you can reduce the IFR’s .004-.006 and see what you get.
 
yellow rose said:
If you mean .005-.010 on the IAB’s I think that’s a good idea. I wouldn’t be afraid to get into the mid .080’s if you have to.

Or, you can reduce the IFR’s .004-.006 and see what you get.
Click to expand...

Yes, .005 -.010

Seems easier to try the IAB since it's on the outside. Is changing one more beneficial than the other.
 
roccodart440 said:
Yes, .005 -.010

Seems easier to try the IAB since it's on the outside. Is changing one more beneficial than the other.
Click to expand...


Not really. Changing either basically gets you to the same place. I’ve never gotten the IAB so big it did something goofy.

I agree, the IAB is easier to get to, and that’s what I change first unless the IFR is some huge size for the application.

@Mattax have you ever went big enough of the IAB for things to get weird??
 
can't think of anything really weird. First times I changed IABs I was surprised that they also affected the fuel mixture at dead idle. I had thought they 'only' affected the top of the transition before the mains started up. But now I understand that IAB affects the fuel density in the idle down well - so pretty much everything that feeds from it.
Going too large will result in a low speed dead spot.
First time was the worst that I can remember and we hadn't even made any changes to the IAB.
All we did was drill a couple holes in the throttle plates so they could be closed down a little to get a lower idle rpm.
But that was enough change in the relationships that the car wouldn't go up a 30 mph hill without feathering the throttle. I knew there was no way I'd make it back to my place.
We found some wire and stuck it in the IABs until the low speed power came back up. At the time I didnt really understand what we had done, but my coworker who was helping me did.
So thats the one 'too lean' in the IABs experience that sticks out in my memory.
 
yellow rose said:
Not really. Changing either basically gets you to the same place. I’ve never gotten the IAB so big it did something goofy.

I agree, the IAB is easier to get to, and that’s what I change first unless the IFR is some huge size for the application.

@Mattax have you ever went big enough of the IAB for things to get weird??
Click to expand...

I'm going to make a strong guess that the IFR's are stock sized just like the IAB's.

I need to order some IAB's. I was thiking .073, .076 and .79 to start. SOund reasonable?
 
roccodart440 said:
The Idle air bleeds are .070 and the high speed air bleeds are .032.

This thing will start and idle pretty easily and pretty quickly with no choke. It is a proform 950, milled air horn etc.

I checked where my machinest had them set, which I should have wrote down, but didn't. I believe they were about 3/4 turn out on the rear and half on the front. After a while of messing around and my eyes burning and the place stinking of fuel I set them back to about where he had them sewt and walked away.

One thing I did do however was use a piece of tubing to listen to the barrels and get them to sound the same. This made me open the rears more and close the fronts a tad.

I was thinking of trying .005 - .010 bigger on the IAB's.

Thoughts?

Off idle or blipping the throttle it runs great. It's just very fat at idle and not entirely happy in gear at idle.
Click to expand...
Give the motor more initial timing and see if the idle improves. If it does recurve the ignition.
 
mderoy340 said:
Give the motor more initial timing and see if the idle improves. If it does recurve the ignition.
Click to expand...


Right now i'm at 24/36

I have a 10 degree bushing. I could go 26/36. I guess ever 37 or 38 total might be ok but I know 36 on premium is safe.
 
roccodart440 said:
I'm going to make a strong guess that the IFR's are stock sized just like the IAB's.

I need to order some IAB's. I was thiking .073, .076 and .79 to start. SOund reasonable?
Click to expand...


Yes, sounds reasonable to me.

I’m cheap...I buy blanks and drill my own but it is easier to just order them up.
 
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