GHOSTOWLGRID

71 olds 455.
C heads.
stock bottom end.
stock cam, seems like it.
Eddelbrock dual plane performer intake.
Holley 750 CFM mechanical primary and mechanical secondary double pumper carb model 4150.


Been running fine until the weather warmed up then it foweled the plugs and wouldn't stay idling unless I reved it.

Changed plugs, checked carb, seems ok?
vacuum was about 17 HG on the high side and bouncing between like 17 and 10 HG during idle.

I did bump up the idle a bit to help compensate.

Changed HEI cap and rotor with known good and helped a little, changed plugs with new factory ones for a 71 cutlass.

Idle and free revs perfectly fine now.
Put it in drive or turn the AC or put any kind of load and it hesitates, stumbles, and will stall without assisting with the gas pedal.

I reset my fuel level and reset the 4 idle screws and still has this trouble. Timing never changed from before should still be ok.

Mechanically it seems like its ok if it idles and revs fine with no load. Possible vacuum leak?
Not sure what to check.

76olds

I would look for a vacuum leak, if its bouncing from 17-10 at idle. Have you tried turning the idle mixture screws in (seat them and turn them out 2 1/2 turns then setting them with your vac gauge?
Hear any leaks around the carb base? Spray some WD40 around the base of the carb and see if it stumbles as well.

Hope this helps,
Eric

76olds

Is it electric choke or vaccum pull off? If vaccum try opening up the butterfly when warm if it appears to not open fully and see if it idles better.

GHOSTOWLGRID

Forgot to say its electric choke.
Choke is turned off, full open.

We set the BTDC timing at 1100 rpm to 12 degrees with vacuum disconnected.
plug vacuum back in and reset carb again, Its now idling with AC on but still seems a bit weak and prone to stalling if RPM drops to low about 650?

I am trying to adjust the holly 4 corner idle for best vacuum. Best I seem to get with AC on is about 15 HG.

when AC off idle goes a bit high. I don't have an idle compensator for AC.

Vacuum gauge is bouncing between about 17 and 5 HG now as it runs. Is that normal?

joe_padavano

Pitch the double pumper. Your heavy Caddy with stock 455 will run MUCH better with a Qjet. The Holley DP is a great wide open throttle carb for track use. It is nowhere near as flexible for part-throttle use in a heavy street-driven car. If you're one of those people who MUST have a Holley, then at least get vacuum secondaries.

classicmuscle442

X2 on pitch the holly, when you get a Q-jet set up right you won't believe it's the same engine. If your not a carb guy, maybe ask some members near you for Q-jet specialist in your area.

shiftbyear

consider replacing/testing the power valves on your holley, if the diaphragm is leaking it will effect idle drastically. good luck

GHOSTOWLGRID

I will look into it.
The initial timing because I have HEI I set to about 14 degrees with total about 35. Then connected vacuum to the port on the carb.

I had a family member help me, The engine now stays running but it looks like everything went bad because of something else.
Sounds like 1 or 2 cylinders are not working.
But the engines got good torque though, once I fix it and retune spark and carb it will be better.

For now it can be driven, I have to take it apart later.
I will look for someone who can rebuild/fix it and tune it proper without charging to much to do.

Might be a burned valve, probably needs heads rebuilt.

I will look into the Q Jet carbs. I bought the Holley because it was recommended to me at summit.

76olds

You could check the backlash on your timing chain, it may need to be replaced. Lift the distributor cap off , put a socket on the crank and turn the engine by hand noting the movement back and forth .
Have a read on your plugs and check spark may help as well.
Joe seems to know you own a caddy, so he would be able to coach you on what to check next, be it compression leakdown etc.
Hope this helps
Eric

oldcutlass

Do not open the 4 adjustment screws on your Holley 2.5 turns. Google tuning your Holley and get familiarized. Here is a link to how to trouble shoot with a vacuum gauge.
http://www.secondchancegarage.com/public/186.cfm

GHOSTOWLGRID

I will try that for checking chain slack.

Yah, Its in a cadillac. I think I mentioned it once before. Its something I wanted to try for daily/ ocassional-racing.

For the Holley I did the 1.5 turn out and adjust after doing the fuel level adjust according to their videos online. Never went past 2 turns out.

I am seeing about 15 HG maybe 17 HG at most around 1k rpm on my vacuum gauge. I couldn't make it any higher.

I will try a compression test later, if it lost compression I will leave it until summer because I don't have time to pull heads at the moment. :/

76olds

Dam, thanks for correcting me on that suggestion Eric , I guess that double pumper Holley works differently than most other carbs.
Would 2.5 turns screw the carb up from a starting point on adjusting?

oldcutlass

Most are 1/2 - 3/4 to start. There are 4 A/F screws on a double pumper.