Technical 302 won't idle below 2000 RPM, thoughts on what to look for?

Little background, 302 {5.0} Ford, stock roller cam motor changed from FI to Edelbrock Performer intake with Summit 600 CFM carb new out of the box. HEI electronic ignition also new. Engine starts up fine after you pump the gas a few times, but won't idle below 2000 RPM, it shuts off just like you cut the switch off. Have ran a straight wire off of battery to ignition, no change. Still cuts off if you try to let off gas. Took top off of carb, bowl is clean, I ran a small wire down through the jets, they were open. Pump is putting out [electric], keeps bowl halfway on sight glass in carb as it's supposed to, plenty of volume so it's not running the bowl dry. Have to keep throttle open or put choke on half or 3/4 to keep it running, so I'm thinking it's running lean, and idle circuit isn't doing anything. When it cuts off, even pumping the pedal doesn't keep it going, so the shot may be too small. Will turn up to 5000 RPM easily, just quits below 2000.

So I'm thinking the idle circuit is plugged somehow. Carb is two years old, but never used until recently, so I doubt Summit would make it up as it only had a year warranty. Any ideas on what or where to look for what that might be clogged, and how to open it up?

 

Maybe a vacuum leak at the manifold .also, check for any vacuum ports not capped

 

Intake manifold leak...


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Vacuum. Leak.

Tommy

 

Forgot to mention, checked for vacuum leak, sprayed starting fluid around base of carb and at manifold, no change. All ports plugged, only hoses hooked up are power brake booster, pcv valve, and distributor advance. No sucking sounds like a leak.....

 

check brake booster for leak also

 

Fresh install? Wire for the HEI large enough? I recall I converted my 235 with an HEI from a late model Chevy 6, and had to keep the rpms up high (don't think 2K tho, maybe 1200 or so) to get enough to the unit. Did some research and switched to 14 or 12 ga, been awhile, don't recall. Lastly, you did bypass the ballast resistor, or resistance wire?

 

What carburetor? You take the top off so I think Edelbrock but then you say sight glass do newer Edelbrock have a sight glass? I try to stay with AFB Carters.

 

Vacuum leak. Try checking around the intake.
r


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Did you try spraying starting fluid in the valve cover to see if it's an internal vacuum leak? The gasket might have squished out on the bottom of an intake port.

 

Hadn't thought of that one, will check.

Fresh install, Speedway wiring harness, worked fine with Chevy HEI on previous engine, I even added a additional wire straight from battery, so I'm thinking more fuel/carb related.

Summit 600 cfm, their copy of the old Autolite carb that Ford used back in the 60's.

I did, but will check again.

I didn't, will try that. New FelPro gaskets, don't remember now the part number though.

Thanks all for the suggestions, will do more checking as I get time.
Keep'em coming, I want to get this old girl back on the road.

 

Follow up:

Thanks to the HAMB troubleshooters, I think I may have found the problem. I carefully sprayed starting fluid into the valve cover, around both sides at the intake and heads, blocked all vacuum lines, no effect. Sprayed around carb base on passenger side, no effect. Sprayed around drivers side, engine went dead. Cranked it back up and tried again, same result. Tried for third time, same result, so I took carb off. Not sure where I got this spacer, it's a factory style, but it looks like it wasn't sealed all the way around. Store is closed today, tomorrow I'm going to get a couple of gaskets or a sheet of thick gasket material and make some.

Will report my findings when I get them, stay tuned.....

 

Beer box is thick enough cardboard to make a gasket. Cant get shirt cardboard anymore. If Im buying them, I like to get the extra thick ones with the plastic inserts. Good Luck

 

The give away was when you mentioned the symptoms,,and the cure was by closing the choke down about 3/4.

Brand new carb.
Runs good way above idle,,,even high rpm..
Just won’t idle at a low or reasonable rpm.

Dead giveaway!
I’m glad you posted your results,,,this will help the next guy.

Tommy

 

It's had me bumfuzzled for sure. I had sprayed around the carb first thing, but the way the carb and spacer overhang the manifold, I apparently didn't get the right spot. Hope this does it, then on to the water leak, looks like it could be a freeze plug on the end of the head, under the alternator bracket.

 

Well, that was a waste of time. New carb gaskets, different spacer, torqued intake bolts again to be sure, same shit. Did manage to take the choke off, but still won't idle below 2000 rpm. Retarded timing a bit, no help. The sob may sit there and run 30 minutes at 2000 rpm, then again it might shut off after one minute. Tried backing off the idle speed screw, as soon as it gets below 2000, it quits. Never had one fight me this bad.

 

A competent Mechanic

 

what distributor do you have in it? You say its an HEI but its a 302... so is it one of those aftermarket HEI for a ford motor?

Regardless, If you are unable to find a vacuum leak, but it will run above 2K RPM, try looking at the centrifugal advance in the distributor. I just had an old Mallory distributor that when you first fired it up it was fine, but once RPM went over about 2K RPM, it would fall flat on its face and cut off under 1800 RPM. The culprit was the mechanical advance. Once full advance was achieved at around 2300 RPM, the advane would stay stuck at full and throw the timing off so hard it would pop through the carb and cut off. Sometimes the advance would pop back and release to base, sometimes it would stay there at full advance. Check and make sure the advance is moving freely and not sticking or hanging up.

 

Best advice yet since I seem to have forgotten everything I ever knew.....

 

Yeah, it's one of those small cap china hei's from ebay, probably my first mistake. It will hit and go dead unless you give it about half pedal to crank it, then it will settle down to 2000 rpm, but no lower. Vacuum advance is on ported hookup per Summit's diagram.

May still be a vacuum leak, but I be damned if I can find it.

 

Had one that had a vacuum leak inside power brake booster canister .That caused stalling when stopping .

 

does the spark quit below 2k? pull a plug wire and check. maybe a bad hei module

 

I would throw another carb on it just to rule it out. Oh, and check the firing order


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My 67 z28 did same thing when brand new.changed distributor and it fixed it.

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Firing order is correct now, it runs like a top from 2000 rpm up to 5500 rpm, as high as I've turned it.

I've got a 2GC Rochester and a two to four adapter, I may just rig it up when I get time. It ran good on the 350 that was in it before.

 

I capped all vacuum ports except the vac advance and ruled that out already.

 

When you had the carb off, did you check for a cracked baseplate on the carb or perhaps a loose throttle shaft? If it cut out 3 times in a row after drowning it in starting fluid on the drivers side, It might be worth investigating. I know you said it was new, but I have a hard time trusting some parts house repro parts. At this point what do you have to lose? I feel for 'ya Mav, this shit can get frustrating real fast!

 

Find a friend who has some knowledge to look at it to see if you missed something. Sometimes we can't see the forest for the trees. You've covered all the simple stuff and didn't find the answer. The other pair of eyes might be what you need. Good luck with this. If you do enough, you will always have something that stumps you for a while.

 

Take another look at the intake, the Edelbrocks will leak at the bottom of the intake ports if overtorqued.
Ford 5.0 HO firing order is same as 351W, not the same as non-HO 302.
Summit Holley knock-offs have had QC issues, check the idle circuit...

 

My guess is the idle circuit is not functioning on the carburetor. At 2 k you have passed the idle circuit.
It may be a blockage.
I have read sometimes these Summit carburetors can have a lot of trash in them,