i have a 250 chevy with cam,hei, header,heated offenhauser intake, dual carter/webber carbs backed up with a t5 trans. the engine has great power at all speeds,plugs run a nice light brown color. the problem is if i floorboard the gas if falls flat on its face! if i backoff and squeeze back into it it pulls really well. id appreciate any help

Pull the plugs and read them after flooring it. Don't let it idle long. Could be rich, lean (do you have backfires?), or a timing issue.

thanks for responding . ive looked at plugs like you said nothing different as far as color . carbs dont spit and no backfire . as long as i let it wind up it will rev to the ski . im not much of an expert on carbs but i suspect maybe the secondaries are not openning !? unfortunately in this great corrupt and broke state of illinois its 11 degrees out and theres no driving the truck right now. p.s. i balance the carbs using a pair of calibrated vacuum gauges one to each carb and all other ports plugged off carbs pull equal vacuum at all rpms

Do you have enough heat in the intake manifold? Not sure how much is enough but too cold will cause this sort of problem. Water or exhaust heated?

I HAVE AN OFFENHAUSER INTAKE WITH TOM LANGDONS WATER HEAT PLATE . IT SEEMS TO BE WARM BUT LIKE YOU SAID HOW MUCH IS RIGHT?

Wonder if your not getting vacuum advance? If engine is just running on mechanical advance and the vacuum advance isn't working that will cause some hesitation at lower rpm. Does the vacuum gauge fluctuate at all? You could have a vacuum leak somewhere. What are you using for a fuel pump???

THATS POSSIBLE . I HAVE TOM LANGDONS HEI WHICH WORKS WELL BUT MAYBE I HAVE A LEAK . I ONLY HOOK UP THE VAC GAUGES WHILE TUNING BUT IDONT SEE ANY FLUCTUATION THEN. IT HAS A STOCK CHEVY MECHANICAL FUEL PUMP. I HAVE A WHOLE NEW ENGINE BUILT THAT WILL GO IN AS SOON AS ITS BROKE IN I PLAN ON REROUTING THE FUEL LINE AS IT GOES RIGHT BY THE FRONT OF THE HEAD POSSIBLY GETTING TO HOT . ALSO IM PUTTING IN PHENOLIC INSULATOR SPACERS UNDER THE CARBS THEY TEND TO GET HOT AND FLOOD WHEN I STOP (I PUNCHED HOLES IN THE CARB PLATE PER TOM LANGDONS ADVICE WHICH HELPED BUT THEY STILL FLOOD SOME )

I suspect with it 11° outside the problem with the carbs is not them getting too hot, but more likely an improperly set float level. What is your timing set at, and did you verify the timing actually advances when you plug it in and rev the engine.

I have a 235 with the same carbs hei and headers. I was having the same issue. The issue was the carbs were opening too much too quickly. I think its safe to say your secondaries are opening fine if you can work up to wide open slowly and still run fine. I set my secondaries to only open a max of about 75%. Works perfect now.

RE;CNC-DUDE i only have heat problem when its pretty hot out --it wants to flood after its been run then parked . i will check floats . i think timing is around 10-12, it advances when revd but i dont know to what degree as i dont have a read type gun but its up aways.

RE;WOLFFCUB i will check into resetting secondaries . I never thought about that
will have to look into how to do that
THANKS TO ALL THAT HAVE OFFERED HELP

Might be a bad float in the secondary.

Here's what was giving me headaches:



The arm that broke was on the back side so you couldn't see it when you pulled the bowl. I had to take it out to find it. Dumb luck found it in my case.

RE;GBAUER WOW!!! ILL BE LOOKING AT FLOATS ON BOTH CARBS THANKS!


RE; WOLFFCUB WAS LOOKING AT CARBS I CAN SEE THE ARM THAT PICKS UP SECONDARY HOW DO YOU ADJUST IT ?

Sorry for not responding sooner. I never actually adjusted the secondaries separately. That adjustment cant actually be made without manufacturing some new parts. What i did was limit the actual overall travel of the throttle. When i first started when the gas pedal was to the floor the carbs would open 100%. That much opening too quickly is not good. Granted once you ease into it and get at full throttle it runs perfect. I have it set now so when i put the pedal to the floor the carbs don't open fully.

Thing revs to 4g with no issues granted im never really over the 3200 area. It pushes down the highways with no issues and decent gas mileage as far as im concerned.

I honestly think the secondaries could be delayed just a bit in opening in relation to the primaries to get a bit better acceleration. I have found a way to do this by manufacturing a new secondary opening arm plate. Once we get out new laser table at work i might have to try it out.

Peoples thoughts on this?

Here is a video i just did showing the underside of the Carter Weber carbs with my spare carbs.

https://youtu.be/DU83dJzCYQM

thanks wolffcub i appreciate you going to all the effort ,clears things up a bunch. just broke in my fresh 250 merccruiser build ( 3 years in the making ) on saturday yeeha ran awesome!ran it with stock intake/carb etc will put all my other setup on when done i can make adjustments to secondaries then