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Idle fluctuates 500 to 2500 and back with Holley


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I have a clifford manifold and a holley 4 barrel on my '78 with stock internals. MSA headers carry everything away. After cranking, the idle fluctuates between 500rpm and 2500 rpm every 10 seconds or so. (I'm getting these numbers from my tach) The idle slows until the engine almost dies and then revs quickly up to 2500 and then rpm starts slowly falling. The cycle will repeat three or four times until the engine eventually dies. Could someone please explain what might be happening that causes these fluctuations and how I could potentially make this thing work right?

 

I have adjusted the idle mixture screws to their 1.5 turns. I have checked the fuel levels in the bowls, and I have set the fuel pressure to 4.5 psi. I have the "ported spark vacuum" run the the distributor, and every other vacuum port is plugged. The car has a manual transmission.

Edited by gtg704a
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To be able to answer your question, we'll need more information about your Holley

 

What size is it? 390, 450, 600 etc

Is it a square bore or spreadbore?

Vacuum or mechanical secondaries?

Mechanical or electric choke?

New, reconditioned or bought from a swap meet and used as is?

Does the carb need an overhaul?

 

I think its your carb playing up but what about the rest of the engine?

 

Is the engine in good condition?

Does the engine sound like it runs on all six cylinders?

Do you have a vacuum guage hooked up and what does the needle do when the revs drop/increase?

Does it do this when hot and/or cold?

Have you installed the induction yourself?

Was it running good and suddenly started to play up or have you just done an induction upgrade and it's been doing this ever since?

How much static advance do you run? Points or electronic ignition?

Has it been sitting for a long time?

Have you done any other mechanical work to this engine recently?

Ported head, cam size and compression ratio?

Do you have any experience with L Series engines and/or Holley carbs?

 

Your intake manifold is something that I ran on my L28 for a year or more. That's an awfully big plenum under the carb so getting the engine to be torquey and responsive can be challenging! That setup will give you a good top end (for a 4 barrel setup) compared to others that I've used over the years.

 

I 'think' you have a vacuum leak and it's probably the secondary throttle blades not seating correctly and your idle is rich (if you've got 1.5 turns out on the carb) but I need to know what carb you've actually got to confirm this. Do you use an o2 sensor? How are your plugs looking?

 

Get back to us with some more info and we can go from there.

 

:)

Edited by ozconnection
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The engine seems ok under partial throttle. I can run 3500 rpm with 12 inHg vacuum. To run at all, I need to advance the timing as far as the distributor will allow. Idle vacuum readings go from 8 inHg at the 2500rpm condition all the way to almost 0 inHg. It creeps up from ~0 to 2 inHg right before it races back up to 2500 rpm. This has all been done with a "cold" engine in 90-ish degree weather. It sounds like it is surviving on one cylinder at low rpm, but at 3500 and 2500 rpm conditions, I think I hear all 6 cylinders operating.

 

I got the carb and manifold together over ebay. They were allegedly bolted up to a 280 that somebody was parting out. Best guess is a 390 cfm. It has a dichromate finish, vacuum secondaries, electric choke, and square bowls, so according to holley's website it should be a 390. It has some carbon deposits, but I sprayed carb cleaner all over including into the idle mixture screw holes. It squirted back out the top just under the choke. I don't know what size jets or power valve it has.

 

The car has been sitting for about 2 years, but it has new oil and freshly flushed fuel lines. (flushed means I hotwired the fuel pump and drained all the crap out into a bucket. I saw no rust or particulates, but I did get a good high. I don't know the full history of the engine, but I have no reason to assume that is has anything fancy. It is still using the stock electronic ignition module. Thanks for your help, folks. I am learning as I go, and I have know enough now to realize how ignorant I am.

Edited by gtg704a
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Start by setting the timing. Use a buzz-box, light bulb, or an ohmmeter and dead time it at 7 or 8 degrees BTDC. This should be good enough to get it to idle.

Make sure the idle setting is not holding the carb open a lot. Set it so the throttle plate is not exposing the transfer circuit.

 

It sounds like the mixture is lean. Check for vacuum leaks. Check the simple stuff first. Is the manifold gasket installed correctly? Bolts all tight? Carb base gasket and mounting bolts. Did you miss a vacuum port when installing the plugs?

 

If you do not find anything obvious, use a can of spray can of something that burns well (carb cleaner or starting ether) and spray around the manifold gasket area. If there is a vacuum leak the fluid will seal it and enrich the mixture. The engine will start to run faster and better when you spray the leak.

 

Be careful not to catch the engine on fire with the ether. Do this while the exhaust is cold and be quick about it.

 

EDIT: I realized you probably do not have points so accurate dead timing is not possible.

Edited by beermanpete
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Start by setting the timing. Use a buzz-box, light bulb, or an ohmmeter and dead time it at 7 or 8 degrees BTDC. This should be good enough to get it to idle.

Make sure the idle setting is not holding the carb open a lot. Set it so the throttle plate is not exposing the transfer circuit.

 

It sounds like the mixture is lean. Check for vacuum leaks. Check the simple stuff first. Is the manifold gasket installed correctly? Bolts all tight? Carb base gasket and mounting bolts. Did you miss a vacuum port when installing the plugs?

 

If you do not find anything obvious, use a can of spray can of something that burns well (carb cleaner or starting ether) and spray around the manifold gasket area. If there is a vacuum leak the fluid will seal it and enrich the mixture. The engine will start to run faster and better when you spray the leak.

 

Be careful not to catch the engine on fire with the ether. Do this while the exhaust is cold and be quick about it.

 

EDIT: I realized you probably do not have points so accurate dead timing is not possible.

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I agree it sounds like a vacuum leak. There may also be an issue in the fuel delivery. Do you have a good pressure gauge you can put into the fuel line to check this? If you see pressure issue check the fuel filter (if you have one), and you might want to pull apart the fuel pump to see if there is anything caught inside it.

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The starter fluid test revealed a leak around cylider #2. I have tightened down all of the bolts to FSM torque specs (10-13 ftlbs) (actually, I wen't a little tighter). It idles super great now at 2000 rpm and 17 inHg. However, if I back off on the idle speed screw, everything dies. There is a terrible screaming sound coming from the front half of the engine. It could be a belt thing, but I'd bet y'all are going to tell me this sound is a vacuum leak. The starter fluid test reveals there is still a slight leak up there, but I'm scared to tighten down the manifolds any further.

 

I have a pressure gauge mounted in my fuel pressure regulator, and everything seems ok in that area.

 

I had to modify my intake/exhaust manifold gasket with a rotary tool when I got it because it didn't have holes for the clifford. I suppose I might have damaged it, and I'm cool with replacing it. However, is there a gasket that I could buy that fits my '78 heads and has the correct bolt locations? That'd be way easier.

Edited by gtg704a
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You should get an intake/exhaust gasket meant for the carb'd Zs, not the FI ones. I'd imagine your intake manifold bolt holes use the SU bolt pattern, not the EFI one. When your manifold is off, I'd also check for straightness to make sure that there is no warpage in the flange.

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Leon, do you (or anybody else) have a suggestion for which year I should ask for at the parts store? It is my understanding that the exhaust ports were square(-ish) before 1977, but changed to a rounder shape after that. I'm getting that info from this site as well as from pictures I found online at MSA.

Edited by gtg704a
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According to my bathroom mirror (the flattest surface I could think of in my house) The flange on my manifold isn't as flat as it could be. I'll take it to the shop first thing tomorrow (correction: coffee first - shop second). I ordered a '78 gasket today, and I'll just modify it the same way I did the last one, but it should be easier this time because I have a template. I'll get back to you fine people this weekend when my parts are ready and I get back from visiting relatives out of state. Feel free to discuss in my absence.

 

I'm still a little curious about the huge swings in rpm and vacuum I had earlier. It isn't a symptom anymore, but it sure was neat, and I'd like to know what the magical carburetor fairies were up to. Perhaps the vacuum leak leaned everything out until only one cylinder survived. In that condition, fuel dribbled slowly out into the manifold until a minimum fuel/air ratio was reached when "whoosh!" the engine revved up and consumed it all before the vacuum leak leaned everything out again. Does that theory make any sense? Anyway, thanks for everybody's input so far. Hopefully, this will be settled soon!

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While you have it apart pull the carb off the manifold and change that gasket also. When you put it all back together do not hook up any vacuum connections to the manifold or the carb and make sure all ports are caped with rubber plugs (get a full set from Napa or O'reilly). Get it running with the timing mentioned by beermanpete. If you have the vacuum fixed it should run really well regardless of the carb settings. Now tune the carb, set the idle, set the idle mixture. From there reconnect the vacuum stuff, vacuum advance for the distributor, PCV, vacuum for the brake booster. If you have any dramatic changes undo the last thing and diagnose why, fix the leak then move forward. When you fix everything, pick up a holley specific book by hanes or someone and tune that carb. When you finish I bet that engine will rock!! Good luck!

 

Paul

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