New 408 stroker runs rough at idle

-
is the vacuum readings you mention at sea level? I’m getting 7-8 inches at 5000 feet above sea level.
We're at 2150.
You loose approx 1 inch per 1000. But our ratings are conservative. Your reading still feels low.
Will it rev clean once you get it to "catch up" and pass the stumble?
What do your plugs look like?
What is total timing at?
 
We're at 2150.
You loose approx 1 inch per 1000. But our ratings are conservative. Your reading still feels low.
Will it rev clean once you get it to "catch up" and pass the stumble?
What do your plugs look like?
What is total timing at?
So with it not wanting to run it turns out it already ran out of the 5 gallons of gas I gave it. Put another 5 gallons in it and now it runs. It just seems like at idle, it runs rough, but once you raise the rpms to about 1500 to 2000 it doesn’t shake so bad. The plugs are black already. I have yet to check total timing. On the site, it says 10-16 initial, if I can shorten the amount it advances, so it doesn’t go above 34 degrees, could I have the initial timing considerably above 16 degrees, like 20-25 degrees or is that really not recommended? I just have the initial set at 16 as of right now. I haven’t yet checked total timing.
 
So with it not wanting to run it turns out it already ran out of the 5 gallons of gas I gave it. Put another 5 gallons in it and now it runs. It just seems like at idle, it runs rough, but once you raise the rpms to about 1500 to 2000 it doesn’t shake so bad. The plugs are black already. I have yet to check total timing. On the site, it says 10-16 initial, if I can shorten the amount it advances, so it doesn’t go above 34 degrees, could I have the initial timing considerably above 16 degrees, like 20-25 degrees or is that really not recommended? I just have the initial set at 16 as of right now. I haven’t yet checked total timing.
I asked you to check the plugs and the RPM that it cleans up, because you very easily could have fouled plugs from the poorly tuned first carburetor. You could also very well have a vacuum leak on the intake. Also the total timing is actually more important than initial timing.
Unless an Edelbrock operates differently than a Holley on the idle circuit, you should be able to screw in the two mixture screws the complete way, and kill the engine. If the engine continues to run with those screws bottomed out, the carb has a fuel leak somewhere or is running on the secondary being out of adjustment.
I'm not sure how experienced you are in initial carb tuning, checking timing, and sealing an intake. But the Blackness of the plugs and the fact that total mechanical timing has not been checked, is really putting the cart before the horse.
First thing I would do is buy a new set of plugs. Check mechanical total timing. And check for vacuum leaks.
You may want to give Tech a call on Monday or we can have a call on Monday.
A super rich mixture gets gas into the fuel, thins the oil, and can be a big problem.
 
I asked you to check the plugs and the RPM that it cleans up, because you very easily could have fouled plugs from the poorly tuned first carburetor. You could also very well have a vacuum leak on the intake. Also the total timing is actually more important than initial timing.
Unless an Edelbrock operates differently than a Holley on the idle circuit, you should be able to screw in the two mixture screws the complete way, and kill the engine. If the engine continues to run with those screws bottomed out, the carb has a fuel leak somewhere or is running on the secondary being out of adjustment.
I'm not sure how experienced you are in initial carb tuning, checking timing, and sealing an intake. But the Blackness of the plugs and the fact that total mechanical timing has not been checked, is really putting the cart before the horse.
First thing I would do is buy a new set of plugs. Check mechanical total timing. And check for vacuum leaks.
You may want to give Tech a call on Monday or we can have a call on Monday.
A super rich mixture gets gas into the fuel, thins the oil, and can be a big problem.
Ok so I replaced the plugs, checked the total timing and I am getting 32 degrees at 3500 rpm. I did have to bump the distributer down a little because as I was revving it up it was starting to creep past 34 degrees. But now it goes 32 degrees max. At idle it now reads about 12 degrees. It still actually does seem to miss even when you rev it. It just seems to run rougher at idle. I sprayed carb cleaner around the intake gasket area on both sides, sprayed carb cleaner at the vacuum line connections at both the pcv valve port and brake booster hose where it connects to the intake. I also sprayed carb cleaner around the base where the carb mounts to the intake manifold and the idle speed didn’t change a bit. I also clamped both the vacuum line that runs to the pcv valve, and the brake booster with no change in idle or vacuum readings. I also plugged both valve cover holes to see if the crankcase was going to pull a vacuum, and it builds a small amount of pressure within sealing it off for 3 seconds. I honestly can’t find a vacuum leak on it.
 
Ok so I replaced the plugs, checked the total timing and I am getting 32 degrees at 3500 rpm. I did have to bump the distributer down a little because as I was revving it up it was starting to creep past 34 degrees. But now it goes 32 degrees max. At idle it now reads about 12 degrees. It still actually does seem to miss even when you rev it. It just seems to run rougher at idle. I sprayed carb cleaner around the intake gasket area on both sides, sprayed carb cleaner at the vacuum line connections at both the pcv valve port and brake booster hose where it connects to the intake. I also sprayed carb cleaner around the base where the carb mounts to the intake manifold and the idle speed didn’t change a bit. I also clamped both the vacuum line that runs to the pcv valve, and the brake booster with no change in idle or vacuum readings. I also plugged both valve cover holes to see if the crankcase was going to pull a vacuum, and it builds a small amount of pressure within sealing it off for 3 seconds. I honestly can’t find a vacuum leak on it.
What can you adjust the idle down to?
Have you tried turning the idle mix screws in all the way to ensure the engine dies? (Lightly until they bottom out)
Does this have a choke? is it open?
have you put a temp gun on every cyl and make sure all 8 are firing?
Ignition system ran flawlessly on the other engine?
 
What can you adjust the idle down to?
Have you tried turning the idle mix screws in all the way to ensure the engine dies? (Lightly until they bottom out)
Does this have a choke? is it open?
have you put a temp gun on every cyl and make sure all 8 are firing?
Ignition system ran flawlessly on the other engine?
The idle speed goes down to about 350 r
What can you adjust the idle down to?
Have you tried turning the idle mix screws in all the way to ensure the engine dies? (Lightly until they bottom out)
Does this have a choke? is it open?
have you put a temp gun on every cyl and make sure all 8 are firing?
Ignition system ran flawlessly on the other engine?
so I can turn the idle speed screw down to about 350-400 rpm then it dies. But yes I turned the idle mixture screws in until they stopped, and it stays running, doesn’t effect idle speed at all. The choke does open on its own all the way. I disabled the spark at each cylinder, and found cylinders 1 and 3, the engine runs noticeably worse, all other 6 cylinders, the engine runs exactly the same when they’re disabled. There is a pretty hot spark at each cylinder. This is an all new ignition system, this is the only vehicle I ran it on.
 
It should die with the idle screws in. You are bleeding through somewhere.
Vac secondary mis adjusted.
My main guess if no vac leak. Send me your serial #
 
Brake booster with low vacuum? Add that to your problems. You will not have a strong braking system.....

JW
 
It's going to be very, very, VERY difficult to get a metering rod style carburetor to run well with that big of a camshaft.......without a LOT of tuning. The low vacuum signal reeks havoc on metering rods at idle, because it causes them to "bounce" which results in EXACTLY what you are experiencing. I have no doubt that's what's going on. Find a friend with a known good Holley and try it.
 
It should die with the idle screws in. You are bleeding through somewhere.
Vac secondary mis adjusted.
My main guess if no vac leak. Send me your serial #

Naw, not with the metering rods bouncing into oblivion. He needs a Holley on there.
 
It's going to be very, very, VERY difficult to get a metering rod style carburetor to run well with that big of a camshaft.......without a LOT of tuning. The low vacuum signal reeks havoc on metering rods at idle, because it causes them to "bounce" which results in EXACTLY what you are experiencing. I have no doubt that's what's going on. Find a friend with a known good Holley and try it.
Do u have the wires on the right plugs and at the cap? What is ur ignition? Kim
It should die with the idle screws in. You are bleeding through somewhere.
Vac secondary mis adjusted.
My main guess if no vac leak. Send me your serial #
The serial number is 346053. I don’t think you can adjust the secondaries at idle. It looks like they open up by the throttle linkage. But if someone who knows a thing or 2 about edelbrocks please chime in.

I did notice on that carb that at certain idle rpms you can feel the metering rods hitting the covers, but for the most part they’re sucked down when I open the covers. But I could lighter springs on it and see?

The plug wires seem to fit snug over the porcelain on the plugs. I’ve seen worse fitting ones cause no issues. And fit well on the cap
 
Last edited:
The serial number is 346053. I don’t think you can adjust the secondaries at idle. It looks like they open up by the throttle linkage. But if someone who knows a thing or 2 about edelbrocks please chime in.

I did notice on that carb that at certain idle rpms you can feel the metering rods hitting the covers, but for the most part they’re sucked down when I open the covers. But I could lighter springs on it and see?

The plug wires seem to fit snug over the porcelain on the plugs. I’ve seen worse fitting ones cause no issues. And fit well on the cap
We can get you the exact carb that would have come on it as a dressed engine if you are interested. I'm not an edelbrock expert. But sounds like it's your culprit.
 
We can get you the exact carb that would have come on it as a dressed engine if you are interested. I'm not an edelbrock expert. But sounds like it's your culprit.
Yeah, if you can give me everything you ran on the dyno with it, that would be great. But first, I want to see if I can make my edelbrock one work. Is there anyone you know that I could talk to that has experience with edelbrock carburetors that works at blue print engines?
 
Yeah, if you can give me everything you ran on the dyno with it, that would be great. But first, I want to see if I can make my edelbrock one work. Is there anyone you know that I could talk to that has experience with edelbrock carburetors that works at blue print engines?

I had an Edelbrock initially on my blueprint 408. The rods bounced and the carb sucked as a result. Lighter springs helped, but then it had issues transitioning to cruise or going WOT.

In order to get proper idle I had to also drill the butterflies some to add air at idle, otherwise the transfer slot was too exposed. Do some searches on the transfer slot and you'll see its a big deal.

Even once I got my edelbog as tuned as I could, the bowls would evaporate dry as soon as I shut it down. Then I'd have to crank forever to get enough fuel back to fire it up. Finally gave up on that carb.. (I went efi, but none of my holley friends need to crank for 5-10 seconds either..).

I was running an airgap with a thick spacer too.. Didn't make any difference.
 
Yeah, if you can give me everything you ran on the dyno with it, that would be great. But first, I want to see if I can make my edelbrock one work. Is there anyone you know that I could talk to that has experience with edelbrock carburetors that works at blue print engines?
PM me a phone # and i'll get tech to call you. I've owned one edelbrock ever, so I want you to talk to someone better then me. If it was a holley, i can mess with those in my sleep.
We can go from there. we can also sell you the same Holley 750 we would have used.
since you have an MSD and good coil...i'm going to assume thats not your problem (but make sure the rotor is tight...and not wobbling when engine cranks for the heck of it)
 
Does your engine have a PCV valve?
What if that valve was flowing at maximum capacity due to the low idle vacuum........ That's a pretty big "air-leak".
Pinch it off and retune, see what happens.
 
I already pinched off the vacuum line to the pcv valve. No changes. I figured after hearing you have to drill holes in the throttle plates, I just set the idle speed to where it starts to smooth out, and turned the mixture screws out 3.5 turns. However I notice it seems to bog down really bad when you first step on the gas, and sometimes you’ll hear the exhaust pop a little bit, And then it suddenly jolts forward and does just fine until your at another stop. You can see white smoke coming out of the exhaust at idle. I then decided to drill holes in the throttle plates, started at 1/16 and worked my way up to 3/32, and now it’s to where you can have the idle speed screw to where it’s turned all the way down and it idles at 1k rpm, and the mixture screws still do nothing.
 
I already pinched off the vacuum line to the pcv valve. No changes. I figured after hearing you have to drill holes in the throttle plates, I just set the idle speed to where it starts to smooth out, and turned the mixture screws out 3.5 turns. However I notice it seems to bog down really bad when you first step on the gas, and sometimes you’ll hear the exhaust pop a little bit, And then it suddenly jolts forward and does just fine until your at another stop. You can see white smoke coming out of the exhaust at idle. I then decided to drill holes in the throttle plates, started at 1/16 and worked my way up to 3/32, and now it’s to where you can have the idle speed screw to where it’s turned all the way down and it idles at 1k rpm, and the mixture screws still do nothing.


Post a picture of a couple of plugs. You are asking that carb to do something that is very difficult for it to do.

It sounds like it’s pig rich at idle, doesn’t have enough initial timing, total timing and the curve is wrong.

Just because a car worked on the dyno doesn’t mean it will work in the car as the dyno doesn’t replicate idle and low throttle opening situations.

Why are you married to 32 total? That seems low. You need 35-36 total and 18-20 at idle a you can NOT have the curve lose timing when you pull it into gear.
 
My 360 idle better and cleaner with an 800 AVS than a Holley with 9 inches vacuum. Granted the Idle systems are different between the 2 but its not hard.

It's going to be very, very, VERY difficult to get a metering rod style carburetor to run well with that big of a camshaft.......without a LOT of tuning. The low vacuum signal reeks havoc on metering rods at idle, because it causes them to "bounce" which results in EXACTLY what you are experiencing. I have no doubt that's what's going on. Find a friend with a known good Holley and try it.
 
My 360 idle better and cleaner with an 800 AVS than a Holley with 9 inches vacuum. Granted the Idle systems are different between the 2 but its not hard.

9 inches of vacuum is a lot compared to some camshafts. What I am talking about is 5 or less. The Holleys are just flat out better in those situations.
 
9 inches of vacuum is a lot compared to some camshafts. What I am talking about is 5 or less. The Holleys are just flat out better in those situations.

I can't see the logic in building an engine that only pulls 5 inches of vacuum.
 
IDK if this is gonna help but I'll tell what I would try.
Before we start;This is an idle only post.
Mechanical;
> If you have a Solid Lifter cam, Lash the valves to the loose side of the spec.
Ignition;
>At idle you can drive that engine with tons of advance, Same at low roadspeed, as long as you do not feed it so much throttle as to drive it into detonation. So for this test, I'm gonna assume you won't floor it.I will not buy you a new engine
>for this test you must disable the V-can and plug the carb port.
Fuel
>You must ensure that the float level is adjusted right and is stable at all times.
Air
>at 5000 ft your atmospheric oxygen molecules are very far apart. That engine is gonna want a lotta lotta idle-air to get the oxygen it needs; and the fact that it doesn't respond well at idle, proves it.
>the black plugs are also proving the lack of air, and the metering-rod propensity to flood the carb.
Bypass air;
>Bypass air; All air into entering the engine has to be controlled by the combination of;
the primary throttle adjustment,
the Bypass from the PCV, and
the Bypass thru the holes you drilled, and
thru any other Bypass we might have to introduce. That engine wants air and lots of it.
>The reason we have to supply Bypass air from other sources, is so that the transfers don't flood the engine.As they currently seem to be doing.
Coolant;
>Ok now, this is very important, the coolant temperature has to remain stable. You MUST have a stat in it. I like-em hot. I shoot for a minimum coolant temp of 195. Whatever you are running, the important thing is that the temp is stable at idle; you cannot properly tune the idle if the temp is wandering around. So check it every couple of minutes. Do not run less than 185. Do not continue with this test, with an unstable water temperature.
Other:
>You must prove that the PCV circuit is working. I don't care if it works poorly or not, but the circuit has to work. With the engine running, pull the PCV hose off the carb port. The rpm MUST change.On a stock engine, they will usually stall. At 5000 ft, with primary throttle valves too far open, Ima guessing yours should rev up a lil.
>Install a vacuum gauge on the sparkport.
> bring your IR temp-gun
> hook your power brake hose to the correct port on the carb, or better is to the intake plenum, but not to one of the runners. For this test, pinch it off.
> move the car out of the building and bring your fire extinguisher. I also will not buy you a new garage, or a new car, if your stuff catches fire

Ok, with the stage set;I'm only gonna work on the idle in this post.This is an idle only post.
1) Pop the M-rod covers and pull the springs out. re-install the covers. This is gonna make her hard to start shortly, so you may have to splash a few ccs down the primaries later.
2) remove the carb. Put her on the curb-idle screw, and keep her there. Drain it. Flip her over and set the Transfer slot exposures to a lil taller than wide. Close the secondaries up tight but not sticking.
OK, set the mixture screws to 2.25 turns out from lightly seated.
Blow out the IABs with compressed air. I have to assume they are working,and I can't be there so it's up to you to make sure they are gonna work.
>You should have one of those thick paper insulator gaskets on the intake,and I like one paper gasket on each side of it. Make sure it covers all the sealing surfaces of both the intake AND the carb, and does not interfere with the closing of the throttles.
Re-install the carb and fill it with gas.
>After this;
DO NOT TOUCH THE CURB-IDLE SCREW
> While the carb is off and empty;
Blow out the PCV port to prove it's working. Blow out the PCV valve from the engine side. Blow out the hose.
>Other;
Check that your crankcase is NOT overfilled.Check that the rad is properly filled.

In a second we are gonna twist the key, but first; loosen the clamp on the distributor just enough so that you can tug some timing into it, then splash a few ccs of gas into the intake.

Are you ready? I said ARE YOU READY!
Ok hit the key
As soon as the engine runs, tug some timing into her so she will run at 850 OR more;
Do not even put a timing lite on her!
Let her come up to temp. Check it with the IR gun. Find the hottest spot, and let that spot be your test-target.
DO NOT TOUCH THE CURB-IDLE SCREW


With the engine now warmed up, and idling at 850 by the timing adjustment; Hop in, put your foot on the brake pedal, and put into gear, any gear. Measure the rpm drop.If it stalls yur in big trouble,lol. I mean, it's a stroker right, I thought they made tons of low-speed torque,lol.
Yur target is a 50 to 100 rpm drop no more.. This will be aggravated by a low-stall TC, so we gotta tune for that.
If you are already in the zone, badaboom lets move on. if not, see note1
Ok put her back into park.
Now, lets see where we are at. The engine should be idling really sweetly. Grab a shop-rag and stuff it into the secondaries. The idlespeed should not change much, nor the quality. If it changes more than 50 rpm,see note2.
Ok so lets see where the timing is; but first check the sparkport signal. There should not be any vacuum there; if there is see note 3. move the gauges to read manifold vacuum. What are you getting. I need an idea of what we are dealing with, since I have not seen your camspecs. Until then, I'll assume 9 inches minimum.
Ok so now read your idle-timing; and I'll assume you have proved your index mark is on the money. If you haven't see note 4. What have you got?
I'll guess mid to high 20s.
Low 20s is fine for idle, but hi 20s usually causes too much idle drop into gear. with a too-tight convertor.
Ok so with low 20s we'll leave it there for awhile.
Next, lets fix your idle fueling.
Pull the shoprag out of the secondaries.Observe the speed change and direction.
If the speed went down,see note5. If the speed went up, then the engine wants more air.or less fuel. so mess with the mixture screws and see what happens;
if it seems to run better with less fuel, then reduce the transfer fuel by backing out the curb-idle screw 1 turn, and diddle the mixture screws some more. If the idle speed drops below 800, pull in some more timing.
If it seems to run better with with more fuel, then do the opposite..TIMING will be your speed control.
Your idle fueling, and quality is set by the transfer fuel together with the mixture screws, and the mixture screws tell which way to go, by what the engine likes or doesn't. You will try to end up with about 2.5 turns on the mixture screws, with the transfers at a lil taller than wide. So keep track of what you are doing with the transfers.
If if, if the diddling does not produce the expected results, then, the system is out of adjustment range; and either the engine is not getting enough air, or too much. So take your shop rag and begin to cover the primary bores, from back to front. If the speed drops, she wants more bypass air. If the speed goes up, she wants less bypass air.
I'll assume she wants more bypass air.
So, your first go-to is the PCV. The 318PCV is not gonna cut it. You will need something off a radical low-vacuum engine, like IDK, I have never run a stroker, but Ima thinkin maybe a 383Magnum from the late 60s. Whatever you use it should be new, so the spring is fresh.
Here's the deal with the holes you are drilling in the throttles, which is the next go-to;
1) if you go too big, the idle speed will rise, and you will have to take out timing to reduce it; so if you are already in the high 20s for timing, then happily drill away.
2) if you go too big and end up at less than 14*, then you may begin to have problems at throttle tip-in, or stalling when going into gear.I hear other fellas recommending hi teens to low 20s, and IDK for a stroker.
3) I run 3/32 to 5/64 holes in my 750DP on my 230* cammed 367@14* idle-timing. I got up to 1/8 but that just increased my speed too high, and I had to reduce timing which led to other problems

4)if you drill too big, you can close them up, move over, and start again, see note 6..
5) the holes need to be chamferred on the topside only and burs removed on the underside.
6) after every size change; you will need to return all carb settings to the start point, and reset the idlespeed to 800/850 using timing. Then start over to determine if the engine liked the change.
7) if you reach a frustration level you just can't get past; pop the top and reduce your wet-fuel level in the bowl by 3/32...... and yes, start all over. 3/32 is a big jump and should immediately provide relief.

> When you are getting close;
the mixture screws will again work, and
the rpm drop into gear will come into the zone of 50 to 100 rpm,
and the timing will come down to less than 20*, and
when you pop the PCV hose off the carb, the rpm is most likely to want go down or stall, and
The engine will continue to run sweetly down at 750/700/650, maybe even 600, all by retarding the timing. My 367, not quite as radical maybe as yours, happily pulls itself on hard level flat pavement at 550rpm, at 5* idle-timing, with that 276/286/110 (.008 tappet), 549/571 lift Hughes cam. I don't drive it at 5, I just idle it down there when I need to drive 4 mph with a manual trans and 3.55s.
Ok so I'll fill in whatever notes you need, as you may need them
Happy HotRodding.

Edit1;
Just to reiterate, this is for idle-tuning only.
Your Power-timing may have to be revisited when yur done, and after you get that sorted, then the Vcan has to be sorted.
Later on you can learn to trade one timing for another, to better driveability .

Edit2
IDK why that colored part is, but I cannot make it black, so I made it dark brown,lol. Computer is happy.

notes here
 
Last edited:
-
Back
Top