455 stumbles at low RPM

Hi guys,

My car has decided that it no longer likes to run smooth at low-mid RPM.

build background:
73 455, bored .30 over
70 455 heads
Stock replacement cam, internals
Edelbrock B4B
750 Q-jet at-home rebuild
Everything in the fuel system is new- tank to carb
Everything ignition is new, 72 455 distributor.
Power timed according to Larry’s thread. 34 degrees of timing, 30* dwell. Runs at 180* hot, and 10 pounds of oil pressure hot.

I’ve had issues getting it dialed to 15 on the vacuum gauge. I’ve also had cold start issues even with a properly set up divorced choke. Not sure if these are related. When I first installed the carb, it ran strong. At that time, timing was around 38* and dwell was 40*. My stumbling problem started after I dialed back the timing and properly set the dwell. The points are set at .016.

Whether I plug the vacuum advance or not, it still stumbles and runs rough. It does sometimes surge a little at idle.

I think my next steps are to ensure the fuel filter is clean, and hook up a fuel pressure gauge. The pump is a S1 Deleo unit with no return line. Attached is a video with a vacuum gauge hooked up, and a photo of the number 1 plug. Am I on the right track with diagnosing? Other thoughts?

 

The power pistonn spring is likely too weak and not enriching the fuel mix fast enough.

 

Did you replace the throttle plate bushings on re-build of carb?

 

Does it consume any coolant after driving around under various loads/cruise?

 

Could be, I did not get a new replacement spring in the carb kit

No I did not, the throttle blades moved clean and freely for what it’s worth

Hello wizard. Yes, stock replacement cam based on previous owner’s word of the engine rebuilt. Dwell was reset before adjusting timing. I have another set of plugs, I will change them over. 14* initial timing. Correct on the PN

This was a fear after looking at the plug, but I haven’t added any coolant - maybe 50 miles driven so far. It doesn’t smoke, and the oil is clean.

 

Many experts say that since the use of unleaded gas the plugs are not a reliable tell on the fuel mixture, unless it is way off or oily. An O2 sensor is best. Spray some starting fluid around the base while its running and see if the idle changes. Look for broken or loose vacuum hoses. Change plus like Larry said. Report back!

 

I would change that spring right immediately. You can get a kit with 4 springs. The spring has to overcome vacuum to open up. If its too weak it creates a wicked lean condition that results in a bog/buck. It's a very important part of the rebuild and for some reason the springs dont come in the kit!

 

Seems you have thing backwards!
As bigger then stock Cams and or overlap specs drop idle and low speed vacuum levels you WANT a weaker power piston spring to allow for the needed control of the primary metering Rods again.

 

Like the other guy said, spray some starter fluid around the base of your carb, because it needed a rebuild you should have done the bushings, they become worn and allow air to pass thru one of the biggest mistakes on Qjets and makes them untune-able

 

Good places to start. I already checked the intake, carb base, and every possible Vacuum fitting for a leak. Everything checked out ok but I’ll do it again to be sure. Anyone have a link for the correct spring?

 

No. Read his first post. The engine is essentially stock. That engine should make a ton of vacuum.

 

It could be refreshed with a few careful tugs to stretch it out as a test.

 

" At that time, timing was around 38* and dwell was 40*. My stumbling problem started after I dialed back the timing and properly set the dwell. The points are set at .016."

Have you tried going back to where it ran well?

 

You did set the dwell first, right?

 

I haven’t. I believe the car likes the timing changes. At 38* and 40* dwell, it had a delay in starting hot consistent with too much timing advance. It also broke a starter. I believe it to be in a good place but I’m willing to try again if all else fails.

Yes that is correct

 

I would check the vacuum gauge at different rpm's. Maybe do a compression check.