440 idle high and low

[Green Machine]

1970 RR 440, 750 holey carburetor Brawler double pumper. Engine starts fine and it will idle for a for several seconds then the idle drops down. I can adjust the idle up but then it will go up to high. I adjusted the timing, the carburetor but nothing works. anyone with the same issue. I also check for vacuum leaks I don't see anything.

 

[beanhead]

Wandering idle is usually a vacuum leak....the pisser with our big blocks is you can have one under the valley pan that can't be detected with the usual methods. For that you need to pull the carburetor off and look down into the ports, for signs of oil getting in there. Could also be the advance mechanism, vacuum advance set improperly, crap in the air bleeds or fuel orfices, or improper carb adjustments. Fuel pump/pushrod or filter issue also possible. Check the choke (if applicable)for proper operation. Check all the throttle and kickdown linkages. It could be a lot of things, so you want to methodically attack and rule out one thing at a time..
Whatever you find, please report the fix.

 

[Green Machine]

Wandering idle is usually a vacuum leak....the pisser with our big blocks is you can have one under the valley pan that can't be detected with the usual methods. For that you need to pull the carburetor off and look down into the ports, for signs of oil getting in there. Could also be the advance mechanism, vacuum advance set improperly, crap in the air bleeds or fuel orfices, or improper carb adjustments. Fuel pump/pushrod or filter issue also possible. Check the choke (if applicable)for proper operation. Check all the throttle and kickdown linkages. It could be a lot of things, so you want to methodically attack and rule out one thing at a time..
Whatever you find, please report the fix.

I will let you know what works, thank for the info.

 

[davek]

do you have a choke and does it work proper? spray carb cleaner around carb to see if engines idle changes this will tell you have a vac leak.

 

[kramer]

Do you have the vacuum advance hooked up on the distributor? If you do unplug it and see if the problem goes away, what might be happening is the vacuum advance part might be advancing and retarding when it shouldn't be and causing idle fluctuations.

 

[MoparLeo]

For safety sake NEVER spray a flammable chemical on the outside of the carb. You are only checking for a difference in engine speed and water is much safer. Instead of the rpm going up with "carb cleaner spray" it will go down with water. Same diagnosis, different method. For under manifold leaks propane is used by an experienced tune-up technician under controlled conditions. Propane is heavier than air and can seep downward under the manifold ports. What idle rpm are you trying to set your carb for ? If the idle speed drops after warm up, it is doing what it is supposed to do, assuming that the choke is still connected. . You should be able to completely back off the idle speed screw( not the mixture screws) almost completely first and unless you are running a non stock, longer duration camshaft it should not die when initial timing is set correctly. Best to just start from scratch and not make a bunch of changes. If the carb was new in the box, installed properly and the engine was running properly before you installed this carb. It should have run with very little to no adjustments necessary.
https://documents.holley.com/199r11202.pdf

 

[hunt2elk]

Do you happen to have your vac advance hooked to a manifold port?

 

[Wietse]

Using a PCV valve?
Maybe temporary disconnect and block the port to see if that solves the issue.
Could be faulty allowing too much air in, what is your manifold vacuum at idle?

This issue just appeared or you had work done and just put everything back together?

 

[BSB67]

Put you hand over the carb to choke it off while it is running. If the idle rpm increases slightly for a second before it dies, there is a vacuum leak. This is the only method that works for me all the time. I've had identified leaks in this fashion that I could not find by spraying stuff.

 

[Green Machine]

responding to all. now that I read all the comments, I have done most of your suggestions. I don't have a choke, no PVC valve, put hands on carb intake and it dies when I do that, the vac advance is hooked up to carb. Now that I hear all you guys and thinking about it, I should clean the carburetor first because it did sit for few months and even though it was clean maybe something got in it.

 

[BSB67]

......put hands on carb intake and it dies when I do that...

Just to be clear, they always die when you do that. What you're watching for is what the engine rpm does just before it dies.

No PVC can be a bit of a problem.

Is the carb the only thing you changed?

 

[threewood]

Lots of good suggestions here.

If you are hooked to manifold vacuum for advance that could certainly mess with rpm. Set everything with the vacuum unhooked and plugged and see if it still occurs.

Another spot to check would be the mechanical advance plate. I've seen worn out slots / guides that caused the rpm to fluctuate. Pull the cap and see how much play there is in the rotor.

Keep us updated.

 

[Green Machine]

the engine is not stock I don't know much about it, edelbrock intake, brawler carb 750 double pump, electronic ignition, 10-1 flat pistons and a thump Cam and Lifters, Hydraulic Flat Tappet, Advertised Duration 295/312, Lift .507/.494, Chrysler, Big Block, Kit, I'm going to try some of your ideas and let you know what happens. I cleaned the carburetor, will check for vacuum leaks I'll send pictures to show what is hooked up to it.

 

[Green Machine]

 

[Green Machine]

more pictures

 

[Wietse]

No PVC can be a bit of a problem.

Agree with that, better get one and install it because if you are setting the tune now you will have to do it again later on if you decide to install one...which is recommended to do.

 

[MoparLeo]

So how long have you had this car and how long have you driven it ? Any track record wit it or just starting to get things figured out ?

 

[Green Machine]

So how long have you had this car and how long have you driven it ? Any track record wit it or just starting to get things figured out ?

The car I had it since 1978 but the engine was rebuilt and it runs fine at high RPM but at idle it just doesn't want to cooperate but I will try some of your ideas I just haven't had time to work on it until this weekend. I will let you know the progress. I will upload some pictures of the car later. just got it back from the body shop and is 95 percent done just need to get the engine to cooperate.

 

[Wietse]

What vacuum readings do you have at idle with that cam?
If you have a low vacuum i would strongly recommend a Wagner PCV valve so you are able to tune it in on your application.
Standard PCV does not work well with low vacuum at idle...ask me how i know.

 

[biomedtechguy]

strongly recommend a Wagner PCV valve

M/E Wagner PCV valve
They are THE BEST PCV valve on planet earth, and absolutely have a positive impact on ANY modified engine!
So good that after installing one on my 440 6bbl, and seeing how superior the performance is, I got one for the 421 Tripower in the GTO.
GET ONE, tell them biomedtechguy with the GTO and Roadrunner referred you, and prepare for a happy lesson in how truly significant and crucial the role of the PCV valve is in overall engine performance and longevity!
Be sure to call them and refer to my recommendation, you may get a slight discount!!
http://mewagner.com/?p=444