Idle problem

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bbeep71

bbeep71

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The engine will be idling OK, then starts shaking very badly. Give it some gas and it idles ok again. When it is shaking the vacuum gauge is pretty steady. It's a 66 Cuda with 360. Vacuum advance is not connected. Comp 280H magnum cam, air gap with 3310 750 Holley. Vacuum is 10" in gear. MP electronic ignition with orange ECU.
 
Adjusting the mixture screws doesn't seem to make much of a difference. The timing was adjusted until it sounded right my just moving the distributor. The carb was a new 3310 Holley. It idles in neutral about 1000rpm.
 
i have been having problems allso. One of my problems was motor temp. boiling gas in carb kills idleing circiut.got to have shroud or electric fan.
 
Sounds like a pretty decent cam you've got there.......I'd say it's lean also. When you hit the gas and it idles good again, probably the accelerator pump's shot just made it happy again. Vacuum leak? Is your intake tight? Cup your hand over the air horn and see if it stops shaking.
 
It will shake whether the engine is hot or cold. The car has a vacuum gauge in the dash, so I can see that the vacuum is pretty steady even when the engine shakes. If there is a vacuum leak, what should the gauge display?
 
bbeep71 said:
It will shake whether the engine is hot or cold. The car has a vacuum gauge in the dash, so I can see that the vacuum is pretty steady even when the engine shakes. If there is a vacuum leak, what should the gauge display?
Click to expand...

HA, sounds like where I was with my 3310 last summer. If the idle screws
are not doing anything, sounds like the transfer slots are showing but that
contradicts that it sounds lean. Couple of quick questions for you,
Are you using a PCV valve?
Does it have a standard Power valve in it, and is it stock out if the box?
Did you adjust the secondary throttle plates at all?
What is your initial timing advance at, this is very important!
Does it idle better at say 1200-1300? Curious on this one.


I only run 12InHg on my cam (edelbrock performer RPM) and Eddy says
10-12, so you are not all that far out at 10. Is the gauge in the dash
the stock vacuum gauge or an aftermarket gauge?
 
Holleys are bad about getting dirt in the air bleeds,use carb cleaner and put the little tube right down on them and spray.Make sure that you are running a seal between the carb and aircleaner housing.And then you can have someone sit in the car with it in gear and while you adjust the mixture screws that way
 
bbeep71 said:
Adjusting the mixture screws doesn't seem to make much of a difference. The timing was adjusted until it sounded right my just moving the distributor. The carb was a new 3310 Holley. It idles in neutral about 1000rpm.
Click to expand...


timing sounds right...get a timing light and make sure it is adjusted......

once the timing is set...if still runs after you close the idle mixture screws...the idle speed is set too high...the throttle blades are open too much...

you need to open the secondary throttle shafts a hair to allow move air in the engine..this will allow you to close the front throttle blades..idle speed to allow the mixture screws to work...

on the side with the vacuum cannister on the 3310...look at the throttle shaft you will see a small set screw...the carb will need to be removed to get at this screw....

best thing is to remove screw and rescrew it in from the top..this allows adjustment from the top without removing carb...make small adjustment with secondary throttle blades and make opposite move with front blades...

just did this with a 3310 on w2 head small block with mopar 590 cam....
 
It has PCV valve. The power valve is stock, I believe it's 6.5. I haven't adjusted secondary butterfies. The higher the idle, the better the idle. When I put timing light on it and adjust for around 12 degrees initial, it won't run. That's why I just adjust distrib. until it runs OK. Car has plenty of power and starts well. I did check damper for correct zero against timing mark with TDC tool, it was right on. When I first built engine I had a 750 Edelbrock on it and I had the same problem where it would start shaking. I thought it was defective and replaced it with Holley. My opinion is that the transfer slot has too much showing and like you Guys said, I should adjust secondaries or drill holes in primary butterfies. I guess I'm trying to rule out other options before I try messing with the carb. Gauge is stock gauge in 66 Barracuda. I did try another gauge hooked to vacuum port of carb.
 
12 initial might not be enough for that cam. Try bumping it up to 16-18 before,
that will bring the idle up and allow you to close the primaries some to bring
down the idle speed and then your idle mixture screws should work again.

Make sure to limit your total so you do not go over 36 before total on your
timing. You will probably find your vacuum increasing with more initial, if not,
eventually you may have to change out the power valve. If so PM me I have
a few new ones you can have.
 
idrift said:
12 initial might not be enough for that cam. Try bumping it up to 16-18 before,
that will bring the idle up and allow you to close the primaries some to bring
down the idle speed and then your idle mixture screws should work again.

Make sure to limit your total so you do not go over 36 before total on your
timing. You will probably find your vacuum increasing with more initial, if not,
eventually you may have to change out the power valve. If so PM me I have
a few new ones you can have.
Click to expand...

Exactly. More initial will allow you to need less throttle opening to idle, then your idle mixture screws should be more effective. BTW, when you say "shaking", you don't mean that you're developing a "miss" on certain cylinders, do you?........because since you mentioned that this has happened with 2 different carbs, it sounds a bit strange. Better check that out.......if you've got a lean condition, the spark may be "jumping out" somewhere before it gets to the plug.
 
cudavert said:
Exactly. More initial will allow you to need less throttle opening to idle, then your idle mixture screws should be more effective. BTW, when you say "shaking", you don't mean that you're developing a "miss" on certain cylinders, do you?........because since you mentioned that this has happened with 2 different carbs, it sounds a bit strange. Better check that out.......if you've got a lean condition, the spark may be "jumping out" somewhere before it gets to the plug.
Click to expand...



Good point. Make sure you are firing on all cylinders.
 
It,s not a miss. It shakes or rocks like it's in a paint shaker. The plug wires appear to be good, MSD 8mm wires. Thanks for all the input. I think I'm going to start by cupping the carb to see if there is a vacuum leak and go from there. When I installed the Airgap, I had a bad time trying to start the bolts in the manifold. Almost like the heads were milled, but they were not.
 
A PCV valve will cause to happen what you have. It is leaning out and struggling to stay lit. If it is not the PVC valve, look for a vacuum leak somewhere else! That 3310 is an excellent carb. If you take the curb idle screws out 2-full turns, it should be sufficient idle fuel. If you have to turn the idle screw up to make idle a thousand rpm or more, too much of the idle transfer slot is being uncovered and then the high speed system tries to come into play. You'll see fuel dripping from the boosters or at least weeping fuel. Idle vacuum should not be 10 with the car in gear...a sure indication of a vacuum leak. Disconnect the PCV valve and plug the carb nipple and see what it does. If the power valve were the culprit, it would run overly rich and you would smell it. Those carbs have a 6.5 powervalve that does not open until vacuum drops below 6.5 inches of water. Your engine is smoothing out when you hit the throttle because it is getting the fuel/air ratio it needs to operate as it should. I am glad to see you have a vacuum gauge! I wouldn't be without one on my cars. Low vacuum also shows late timing. I'll bet the engine tries to run on when you shut it off too....right? Find that vacuum leak!
Pat
 
I know this is old. VeRy old! But any updates on what the problem was......I'm having the exact same issue is every way,
 
I found many little problems before it ran pretty good.

1. My set of MSD super conductor wires had a 1 bad wire from the factory. MSD replaced bad wire.
2. Cast valve covers had twist on PCV hole. Put push in grommet and pcv in place. Did not work well, found twist on PCV.
3. MP electronic distributor had advance springs for full advance at around 1000-12000rpm from the factory. Changed springs to 2500rpm and 22deg full advance.(bought Mallory spring kit for this distr.) My MP ignition kit built around 2003 is known for erratic idle. If your dist. was bought between 2003 and now you may try a diff. one. Pertronic' Firecore,etc.
4. Replaced 3310 Holley with 600 Holley. It runs better with 600 but not sure if 3310 would run better now that other problems were found.

5. Do not think because all parts are new that they are OK. It took me a long time to check MSD wires because I felt that they couldn't be bad.

6. Adapt policy that the part is bad until proven good.

7. Good luck. Post solution if found.
 
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