Carburetor

[Brusco]

Clutch is fixed but now I'm running too rich and drops out at mid to high rpm's., I have a Holley 650 Dbl. Pumper and an running a 7psi electric fuel pump.Should I get a regulator to cut the pressure back to 5 1/2 to 6 psi? I am also running the titianium Autolite plugs.Should I go back to the original Autolite plugs that came with the 273 Commando engine?

[Chryco Psycho]

7 psi is too high so a reg will help

[Brusco]

Holley says it will handle 7 but you are the third person that said to cut back,probably 5 1/2 to 6. Going to get an inline regulator today.

[MEK-Dangerfield]

I'm questioning if the 650 DP is too big for a 273, if that is indeed what you are running.

[Brusco]

 Ok so it ran like a bat out of hell when it was dynoed so I don't believe the 650 D P is too big but here is where I am as of today. took the plugs out and they were black from too much fuel (smelled like gas and they had a hint of liquid residue on them) and gapped at .28. I cleaned them up and gapped them to .35. I also put in a fuel pressure regulator and set it at 5 1/2. Fuel pump puts out 7. I then started it up and ran better than it has since I got it running in the car. However, it backfired thru the exhaust one time when I revved it up to 4,000rpm's and the secondaries kicked in but it sounded smooth and clean and until I hit 4,000rpm it sounded better than ever. I immediately shut it down because I didn't want to have my plugs dirty again. I think I need to rejet the primaries down two sizes and see if that helps but if it ran so good during the dyno , could it be that having it in the car with the drive train and car weight it is running rich because it is not as free running as the drag of waterbrake would put on it and now its not since its pulling actual weight drive train and all? HELP! I've got until Monday to get it roght because I have to take off to N.C. for my grand opening show and GOLDEN COMMANDOS induction to the East Coast Drag Times Hall of Fame. Oh and it is a 273 Commando LA block registering 381 HP AT 6,500 rpm. The cam has .517 lift and 296 duration.

[Chryco Psycho]

I would check everything from float height , power valve , make sure the bleeds are clean , I assume on the dyno they did not use an O2 to measure mixture so the carb could be dialed in , if everything checks out OK I would down jet 4 sizes if you are on the right track the response should be a lot crisper

[jimynick]

Backfiring thru the exhaust shouldn't be necessarily be a carb problem, more likely ignition. That's a pretty big cam and it's possible the overlap may cause it. You can seat-of-the-pants it, but either a O2 sensor or a tailpipe sniffer would make life a lot easier. The black plugs show that it's rich and CP's suggestion would be a good place to start, since the dyno run would be WFO and it could use the fuel versus a part throttle application that may/probably is too rich. You can either guess or take the time to know and if there's a shop with a chassis dyno, they'll have a sniffer. Just my 

[DocMel]

Your carb in a sense is "not too big".   Carbs are demand devices.  I could have a 750 cfm on a 6 banger, and it will only draw the fuel/air level of what the engine actually demands. 

The prob comes into play when the carb is not jetted all the way around properly for the demands that the actual engine in play wants to draw, the accel pump is out of whack, or  other associated probs:   Actual size of the carb really only comes into play when the carb is too SMALL for what the engine demands

ID lower that fuel pressure.  I agree with the other guys  Maybe go to 6 pounds first and see what that does it for you   

[Chryco Psycho]

A backfire through the carb can be a lean condition as the throttle is opened so you may need more accelerator pump shot

[HP_Cuda]

A simple route is to take a stock 600cfm vacuum secondary Holley slap it on there and see what happens.

Probably a combo of ignition timing and over carbing IMHO.

B