Holley carb problem

[Malcolm]

I have a 4 barrell Holley 650 double pumper on my car. It has sat there happy for over ten years giving good service. Now I have a problem in that the car idle does not drop all the time back to the set idle when you lift off the throttle. Mainly it runs about 500 to 750 rpm higher than it should.

The throttle cable is fine. It has two return springs and operates smoothly by hand when disconnected. The problem is in the carb.

Please can anyone advise where best to look when faced with this problem.

The only mod to the carb is a secondary meter plate has been put in so that I have both set sof jets set as I want them. The actual jet sizes are ok for my engine as they haved worked well for many years.

Just this racing engine speed to sort. Any ideas?

 

[Russ Noble]

What about a manifold air leak Malcolm?

Cheers,

 

[StanJ]

There are several possible causes for this problem, Malcolm. How long has it been since you had this carb rebuilt/serviced? The fiber gaskets between the venturi body and base plate (along with the gasket just beneath the carb itself) will become brittle over time and can begin leaking, also the base plates on the older "non HP" series double pumpers were/are particularly susceptible to warpage from over-tightening of the carb mounting hardware.

One other very common cause of the problem you mentioned -- particularly on carburetors that have seen a lot of miles -- is simply wear to the bearing surfaces of throttle shafts. This allows misalignment of the throttle plates within the throttle bores, causing them to rub/dig in to the sides of the bores themselves. This problem is excalibrated by the fact that Holley carbs have no designed-in provision for controlling axial movement of the throttle shaft (although most of us in the aftermarket industry address this in one fashion or the other as a matter of course). Take your carburetor off the engine and turn it upside down (being careful to drain the fuel from the float bowls first), and examine the sides of the throttle bores in the area immediately surrounding the throttle shafts. You'll be able to see whither or not the throttle plates have cut into the bores in this area and are impeding proper closure.

All of these problems are pretty simple (and fairly inexpensive) to straighten out. PM me if you have other questions.

Stan Justus
Stallion Racing Components

 

[Jac Mac]

Just a Quick check before you go thru Stans check list, when the motor is 'racing' check that the secondary throttle shaft has returned completely to its stop, it may be the link bar needs a tweak or you could have a tiny bit of grit in the spindle.

Jac Mac

 

[Howard Jones]

If you can't find any reason that the carb is causing a high return to idle RPM then look to the advance pieces in the distributor. As they get old they start to stick and hold the advance in. Thus higher that normal revs at idle.

 

[Malcolm]

Thanks guys, this is most helpful.

As Chinese proverb says....

"Go to bed with problem in hand, wake up in morning with solution!"

 

[Rick]

Put some lubricant such as Anti Sieze or White lithium grease on the accelerator pump cams. Then lubricate the pump arm pivots too (Oil for them).

 

[Rick]

??????

 

[Malcolm]

I guess the ???? is asking what was the fault? If so I will post it when I find out, rest assured. So far dealing with other issues but the comments above hae given me a head start to solving my problem, or at least I hope so! I have a big job list to work through at present.

 

[John]

Malcom,

Rather than going through all those time consuming machinations, I suggest that you pick up a rebuilt double pumper and bolt it on. I just picked up a rebuilt 750 CFM for $255 at the Carlisle Spring Show.

John
epper:

 

[Russ Noble]

Then he won't find out what the problem was and he won't have learnt anything!

With 'rebuilt' anything I'm always worried about how good the rebuild is...... Much better to do it yourself and KNOW that it's right. IMHO

Go for it Malcolm!

Cheers

 

[John]

Russ,

If the rebuilt carb works and eliminates the problem, he would have "learned" that he had a bad carb. The question is how much detail he'd need to know about what was wrong with a bad carb!


John
epper:

 

[Malcolm]

Somewhere I have a Holley carb book. I will try and dig it out tonight for some bed time reading. Worked all day on the car today so moving forward. Deadline of 26th May approaching all too quick. The Prosport is prety much ready to go now, will hopefully post some pictures of the garage contents where I sent it for some mods to the gearbox. Dream race cars galore!

Chances are a rebuilt carb is harder to find in the UK than a new one. I will be dismantling mine first before I buy anything else. Wendy has said I can even use the kitchen table. Wonder if the kids will get high on carb cleaner? Hopefully tomorrow I can pick up some gaskets so the damn thing can be reassembled later on.

 

[Phil]

Malcolm
As Stan was referring to I would look at the throttle plates and shafts. I have seen Holleys with leaks in the base plates thru the shaft bearings, which are mostly plastic sleeves. I believe most of this is caused by heavy return springs placing an undue load on the shaft. If the shaft and base plate are not damaged it is easily fixed. Hope this helps.
Cheers
Phil

 

[Malcolm]

I think we have a root cause of the problem. With the carb off the car and flipped over it is easy to see the rubber blanking plug for the vacuum take off from the base plate has simply perished over time. Hence air getting in and racing the idle speed. Haven't tested this theory yet as exhaust in peices at present but it looks a bit obvious from my perspective!

Throttle plates etc in fine condition. I wondered if my return springs might have been too strong but no signs of problems there.

£1.69 got me a pack of replacement blanking plugs so will change all of them on the carb so now good for another ten years of carb neglect! Don't you just love set and forget Holley's? I may give it a quick squirt of carb cleaner though, just to placate the polishers out there.....

 

[Howard Jones]

Don't spray carb cleaner on any carb you don't want to take apart and rebuild. The gaskets will soak up the carb cleaner and start leaking. That stuff also damages anything that isn't metal. Use a small paint brush, spray bottle, and some simple green type cleaner mixed into some hot water to clean up the exterior of your carb.

Glad to see you fixed it.

 

[Russ Noble]

Russ,

If the rebuilt carb works and eliminates the problem, he would have "learned" that he had a bad carb. The question is how much detail he'd need to know about what was wrong with a bad carb!


John
epper:

You're right of course John, but about $3 for a blanking plug set as opposed to $255 for a rebuilt carb, because he's taken the time to find out what's wrong with it, seems to make good sense, and is knowledge gained. epper: Plus he's still got his own carb, which is a known quantity, rather than someones castoff that has been rebuilt.

If you notice, I did point to an airleak in the first reply, although I suggested the manifold gasket as a likely culprit.:dead:

Of course it still has to be verified that this is the problem. I could yet be
eating my words!oke:

Cheers,

 

[Malcolm]

Of course it still has to be verified that this is the problem. I could yet be
eating my words!oke:

Cheers,

Yeah, me too! I'll keep an eye out for leaks due to carb cleaner! At least I have new spare gaskets in the toolkit....

 

[John]

Russ,

You were right on this one - sometimes it pays to dig deeper into a problem - sometimes it pays to do a blunt force repair. The question is how much time and detail you wish to invest.

I'm about to install my rebuilt carb - I hope the rebuilder did a good job - he seemed very professional and even attached a tag showing the size of the jets and power valve!

John
epper:

 

[Malcolm]

Fired the engine up yesterday and it started on the button and idled without input from me vritually immediately! I don't run any form of choke so previoulsy I had to blip the throttle to keep it going. Clearly air leaking in weakening the mixture, now it can't do that so better starting behaviour.

All problems solved, thanks for the help guys. I really should have done this years ago.....