Help with poor idle/performance
Thread Starter
Supporting Lifetime Member
Joined: Oct 2005
Posts: 454
Likes: 4
From: Lovettsville Virginia
Help with poor idle/performance
I'm looking for some advise on a problem that just popped up on my '68 4 speed. It has a <1000 mile new 454, Holley 770 Street Avenger carb. Pretty much everything on this engine is new. When driving this weekend, I first noticed a high rpm miss and "popping" from the exhaust. Thinking it was timing/ignition related, I reset to 4* as it was out. That made some improvement, but did not solve completely. In fact, the idle is getting progressively worse. I've now replaced the plugs, points/condensor, cap/rotor, vac advance, and coil. Did not solve.
Here are the symptoms:
1. high rpm miss and exhaust popping
2. unsteady, rough idle
3. set idle correctly, then drive. It wants to die after engine load is removed (ie: coming to a stop)
4. turning mixture screws all the way in (lean) increases the rpm. I always thought that taking them all the way in kills the motor??? The highest rpm/vacuum reading I can get is with them bottomed out. Enriching the mixtures causes the rpm to drop and idle worsen.
Holley says with this carb that you can't blow the power valve. Is that true?
Could it be a vacuum leak (I did check and haven't found one)?
Any other suggestions???
Here are the symptoms:
1. high rpm miss and exhaust popping
2. unsteady, rough idle
3. set idle correctly, then drive. It wants to die after engine load is removed (ie: coming to a stop)
4. turning mixture screws all the way in (lean) increases the rpm. I always thought that taking them all the way in kills the motor??? The highest rpm/vacuum reading I can get is with them bottomed out. Enriching the mixtures causes the rpm to drop and idle worsen.
Holley says with this carb that you can't blow the power valve. Is that true?
Could it be a vacuum leak (I did check and haven't found one)?
Any other suggestions???
Last edited by jon68l79; Jun 1, 2007 at 09:33 PM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Is it a crate engine, something you had built, or a rebuild of your motor? If a rebuild (you or a shop), I would suspect a gasket problem...possibly on the intake manifold. Do you have a vacuum gauge? Do you know what manifold vacuum was with the engine when you installed it? Check vacuum level with all vacuum systems disconnected and let us know what you get.
Tech Contributor
Joined: Aug 1999
Posts: 14,384
Likes: 6,408
From: At my Bar drinking and wrenching in Lafayette Colorado
You have 2 problems:
First, your timing is way off. You need to set it up for 36 total, which should get your initial timing up into the mid-teen range. Then, hook up your vacuum advance to manifold vacuum so it pulls in another 16 degrees-or-so of timing at idle. This will get your actual timing at idle up into the 25 to 30 degre range.
Then, richen up the primary side of that Avenger by about 4 jet sizes, and increase secondary jetting so that the secondary jets are 8 sizes larger than the new primaries.
Then, set and verify float levels.
Finally, crack your secondary idle speed screw open far enough so that the primary idle speed screw does not open the primary butterflies any more than exposing .020" of the transition slot below the throttle blades.
Once done, set idfle mixtures to about 3/4 turn - that should get you in the ballpark.
With your current setup, you are not idling on the idle circuit due to your extremely retarded timing and the lean condition of the carb. This has you idling on the main discharge circuit, and will indicate a false rich condition.
First, your timing is way off. You need to set it up for 36 total, which should get your initial timing up into the mid-teen range. Then, hook up your vacuum advance to manifold vacuum so it pulls in another 16 degrees-or-so of timing at idle. This will get your actual timing at idle up into the 25 to 30 degre range.
Then, richen up the primary side of that Avenger by about 4 jet sizes, and increase secondary jetting so that the secondary jets are 8 sizes larger than the new primaries.
Then, set and verify float levels.
Finally, crack your secondary idle speed screw open far enough so that the primary idle speed screw does not open the primary butterflies any more than exposing .020" of the transition slot below the throttle blades.
Once done, set idfle mixtures to about 3/4 turn - that should get you in the ballpark.
With your current setup, you are not idling on the idle circuit due to your extremely retarded timing and the lean condition of the carb. This has you idling on the main discharge circuit, and will indicate a false rich condition.
Thread Starter
Supporting Lifetime Member
Joined: Oct 2005
Posts: 454
Likes: 4
From: Lovettsville Virginia
I understand what you are saying and will try that tonight. What I guess I'm not smart enough to know is why the manufacturer specs initial timing to be 4*? Is it solely for emissions or hp ratings? Seems to actually have less to do with engine performance that it does for other reasons.
Thread Starter
Supporting Lifetime Member
Joined: Oct 2005
Posts: 454
Likes: 4
From: Lovettsville Virginia
OK, went thru the proceedure to set total timing at 36 degrees. It comes in right at 2500 rpm. Manifold vacuum is around 15 (unfortunately do not know what it was after rebuild). Have not changed the jet sizes because I'm just not that familiar with carbs at this point to do it. May need to learn.
With total timing at 36*, the engine overall does run better, but not perfect. When cruising with minimal engine load, it sputters and bucks. Also, still have an idle issue. I set it to 700, but when driving, it won't hold that. When I come to a stop, the engine wants to die. When I take the load off, the idle goes up over 1000. Very unsteady idle.
Also, tried mixture screw settings. Same problem as before - the highest vacuum and rpm reading I get is with the screws all the way in.
Where do I go from here? Could a bad manifold gasket or other vacuum leak be causing my symptoms?
With total timing at 36*, the engine overall does run better, but not perfect. When cruising with minimal engine load, it sputters and bucks. Also, still have an idle issue. I set it to 700, but when driving, it won't hold that. When I come to a stop, the engine wants to die. When I take the load off, the idle goes up over 1000. Very unsteady idle.
Also, tried mixture screw settings. Same problem as before - the highest vacuum and rpm reading I get is with the screws all the way in.
Where do I go from here? Could a bad manifold gasket or other vacuum leak be causing my symptoms?
Thread Starter
Supporting Lifetime Member
Joined: Oct 2005
Posts: 454
Likes: 4
From: Lovettsville Virginia
Update - this is a new problem
Just walked back out to the garage 20 minutes after I shut it off and noticed gas leaking out the gasket between the carb base plate and carb body. Fuel had puddled on the intake. There was also a steady drip of fuel from the secondaries. I quickly pulled the secondary float sight plug and fuel poured out. The float is either stuck or set high. Once it drained down, the drip stopped.
I'll admit, yesterday I did not check the seconday float level for some reason. I did check the primary and it was correct.
Could the float level cause these problems?
If fuel was leaking out of the carb between the base and body, could that be the source of a vacuum leak that is also causing problems (a bad gasket)?
I'll admit, yesterday I did not check the seconday float level for some reason. I did check the primary and it was correct.
Could the float level cause these problems?
If fuel was leaking out of the carb between the base and body, could that be the source of a vacuum leak that is also causing problems (a bad gasket)?
Melting Slicks
Joined: Nov 2000
Posts: 2,738
Likes: 53
From: Bogart GA
Yes. Always check floats when carb is new or just removed. I recheck mine periodically. Can cause all sorts of issues. All your adjustments are based on a correct float level. That is the first thing to verify before any additional adjustments are made.
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Thread Starter
Supporting Lifetime Member
Joined: Oct 2005
Posts: 454
Likes: 4
From: Lovettsville Virginia
Problem Solved - Engine runs great!
Just thought I would post my experience and thank everyone for their advice. The solution was actually very simple in the end. I learned a great deal about timing and best of all, it didn't cost $$$.
In the end, there were 2 things that needed correction:
1) The total timing needed to be set correctly,
2) The secondary float in the carb needed to be set correctly.
Those 2 things were causing all of my problems. Now it runs great, smooth, and with a ton of power, which is the reason I put the big block in anyway.
Lars advise that my timing was off was dead on (as were others), and the post at the top of this page from BarryK about the proceedure for setting total timing was very easy to follow.
Thanks again. I'm going for a drive.
In the end, there were 2 things that needed correction:
1) The total timing needed to be set correctly,
2) The secondary float in the carb needed to be set correctly.
Those 2 things were causing all of my problems. Now it runs great, smooth, and with a ton of power, which is the reason I put the big block in anyway.
Lars advise that my timing was off was dead on (as were others), and the post at the top of this page from BarryK about the proceedure for setting total timing was very easy to follow.
Thanks again. I'm going for a drive.
Last edited by jon68l79; Jun 1, 2007 at 09:35 PM.
