I just put a set of dart iron eagle 180 heads and a voodoo 268 cam in my 327. Previously had 461 stock heads and a 292 magnum cam. Now I just can't seem to get rid of this off idle stumble I have and sometimes it happens up to about 2000 rpm as well. I've tried a holley 600 and a holley 750 (both ran well on my prior set-up with no stumble at all), both are vacuum secondaries. They both stumble now! I've played with the timing, adjusted the accel pump to give more squirt, changed the plugs, new wires, and nothing has worked. After it stumbles, it accelerates very well. The wiring is in the correct firing order-18436572. Idles fine and part throttle is good, but when floored just stumbles for about 1/2 second, then hauls ass. Even when sitting in neutral and reved, the engine hick-ups right off idle, even with no load. I'm really starting to get frustrated. The cam was installed straight up with 4 degrees of advance built in. The sprocket dots were lined up perfectly. No vacuum leaks that I can find. When I open the throttle with the engine off, it seems that an adequate amount of fuel is squirted from the accell pumps. Could it be the heads or the cam? The heads have the 2.02 intake valves. I have 1.6 rockers on the intake and 1.5 on the exhaust (I read in David Vizards book that that can be worth some power). Could that be causing my problem? I just can't think of why BOTH carbs are acting the same???????
How to cure this off idle stumble??????
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Just a couple of things to keeps in mind. If your primaries are too lean, you will get a "tip-in" stumble that no amount of accelerator tuning can fix (my experience), even though at cruise it seems to be OK. Make sure there's no slack in the accel. linkage. Add some more timing, maybe alot. If you dont have enough you will always stumble - I would try this first. I think just reving it in neutral and the hesitation is there tells you that it probably isn't the carb.
Sorry, didn't see the part about part throttle being ok. I saw "off idle" and assumed you meant also part throttle. If it runs good on the primarys, it's probably not the accell pump. If it just seems to happen when the secondarys open, they may be opening too fast, causing the stumble. Try a different spring to slow that down.
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Vacuum Secondary carbs wouldn't cause an off-idle stumble for the secondaries unless they open IMMEDIATELY, which is unlikely.
As said, make certain your timing is up first, then hit the carb.
I think jbird nailed it - put your accel pump linkage back to Holley specs, pull out your squirter nozzle to see what # is stamped on it, and go up about 6 sizes. Assuming you've got a 28, jump up to a 35 or so, then test it. Bump up as needed from there, but if you need to go over a size 40 you will also need the hollow screw.
As said, make certain your timing is up first, then hit the carb.
I think jbird nailed it - put your accel pump linkage back to Holley specs, pull out your squirter nozzle to see what # is stamped on it, and go up about 6 sizes. Assuming you've got a 28, jump up to a 35 or so, then test it. Bump up as needed from there, but if you need to go over a size 40 you will also need the hollow screw.
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Good point.
If floored from a dead stop and stumbles, look to your shooters.
If floored while cruising and stumbles, look to shooters first, but also could be:
slightly lean primaries
secondaries opening too soon, and
check the rating on your power valve, it may be too low and is not opening timely.
Given that it's only "up to 2000 rpm", I'm still betting on timing and the shooters. Vacuum advance is hooked up, correct?
If floored from a dead stop and stumbles, look to your shooters.
If floored while cruising and stumbles, look to shooters first, but also could be:
slightly lean primaries
secondaries opening too soon, and
check the rating on your power valve, it may be too low and is not opening timely.
Given that it's only "up to 2000 rpm", I'm still betting on timing and the shooters. Vacuum advance is hooked up, correct?
Yes the vacuum advance is hooked up. I had the timing "way up" as well when the engine started to idle rough. No matter where the timing is, the stumble remains. Keep in mind again that BOTH OF THESE CARBS WORKED EXTREMELY WELL WITH NO STUMBLE ON MY PRIOR SET-UP. It is just since I changed the cam ( to a smaller one) and changed the heads ( to slightly larger ones 165cc-180cc ) that this happened. Could the 1.6 rockers on the intake valves cause this, or if the valves are out of adjustment, could that cause this? I'm just not sure why all of a sudden I would need bigger nozzles when that was not the problem with EITHER CARB before. Thanks for all the advice so far.
I agree. I'm having the same problem...along with a few others but my stumble startred when I changed initial timing. I would start there first, keep bumping it up to either correct the problem or rule out initial timing as the source.bb489 said:
My stumble thread here
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It's hard to describe WHY you would need them - any change to an engine changes how the carb reacts, particularly during "transition", such as from idle to cruising or WOT. Faster airflow, slower airflow, MORE air at low-rpms with smaller cam, but less at higher rpms, who knows for sure.
Whatever happened, the combination of a smaller cam with bigger runners is decreasing the amount of fuel the engine sees when it first gulps that air. Timing is a big culprit also, but you seem to have tried that already.
So, for about $5 you can up the shooter size and see if it works. Bottom line, if it works, well - IT WORKS!
Whatever happened, the combination of a smaller cam with bigger runners is decreasing the amount of fuel the engine sees when it first gulps that air. Timing is a big culprit also, but you seem to have tried that already.
So, for about $5 you can up the shooter size and see if it works. Bottom line, if it works, well - IT WORKS!
Thanks guys! I think I'll try increasing the nozzle 5 or 6 sizes and see what happens, then maybe check the vac springs too. I drove it again tonight and if I'm going 20-40 mph and nail it, it does stumble a little. Now in everyones experience, does just changing the nozzles make the difference or do I have to adjust the accel pump spring or change the pump cam position?
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sounds to me like you have a valve that it too tight, or maybe more than one. i don't think it's a carb problem when you have the same problem with 2 different carbs both of which worked fine before. nor do i think it your timing if you've moved it all around also. is there any popping through the carb or the exhaust when this hesitation happens?
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Just changing the nozzles will do it.
Note while that may fix the problem, there still could be other issues. Normally, the stumble you speak of is off-idle, not from cruising. As others have said, look at secondary spring opening too soon, valve adjustment (good thought), etc. I've always seen the stumble from cruising to be one of 3 things, in order: timing, secondary springs, nozzle. Either way, good luck! - remember to change one thing at a time or you'll go nuts.
Note while that may fix the problem, there still could be other issues. Normally, the stumble you speak of is off-idle, not from cruising. As others have said, look at secondary spring opening too soon, valve adjustment (good thought), etc. I've always seen the stumble from cruising to be one of 3 things, in order: timing, secondary springs, nozzle. Either way, good luck! - remember to change one thing at a time or you'll go nuts.
I have the same issue. What you are dealing with is strictly accelerator pump / squirter related. First thing to rule out is the accel pump linkage. Unless the motor has a real low idle, most likely the pump shot will be too small due to the position of the cam at idle. Readjusting the accel pump will help, but if the carb is up on the idle circuit at all, you need to first change the accel pump cam. I am in the same boat as you. I was able to play with it some and reduce the stumble but not eliminate it. Only thing that will clear it up is changing out some parts.
Thanks guys! I think I will try adjusting the valves. I always like to do them hot (even though it is kinda messy!) So I guess I will do the valves, then the squirters. I hope it is the valves since these carbs BOTH ran awesome on the same engine with 292 cam and the 461 heads. Can someone explain how an out of adjustment valve or valves can cause this stumbling problem? I definately don't hear any valve chatter, so I would guess they would be too tight if anything?
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Thats what I'm thinking. Nozzles and cam are all important, but I cant understand how the factory setup would be so off that it would cause a bad stumble. I've never had a factory spec carb stumble bad Then again, I needed to go up 9 sizes for the jets on my new 770. If your seeing a hesitation reving in in neutral something is very wrong! Not to beat a dead horse, how far up did you try setting your initial?bb489 said:
I know this was mentioned, but make sure there is no slack/play in the accelerator linkage. Just lower the idle speed will create slack. Check your advance - make sure its advancing. Sometimes the weights stick.
A flat lobe will cause an erractic idle and backfiring b/c fuel/exhaust cant get in or out -and a bunch of other stuff I don't even know about. My 396 lost a couple of lobes. Oddly enough, it ran good till wot where it would pop and backfire. Cruising around it ran good! Had a rough idle anyway (crane 284 .544 lift), so it kinda masked the erractic running at idle.
Well, I adjusted all of my valves hot. Some were about a turn to turn and a half too tight. However, this had no effect on the stumble. I guess I'll try bigger nozzles next. I had the timing up till the engine ran rough, and then backed it down 2 degrees each time until I was at 8 initial. No change. I'll keep everyone posted on the progress.
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richen your idle screws 1/2 turn
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