i have just installed a 450hp 385ci stroker motor in my 91 firebird that was originally a 305 TBI car. Going into this projected i decided i did not want the dash to light up like a x-mas tree, nor did i want to hack all the wiring out of it and put in a carb(seemed like the hillybilly way of doin things to me). So i sent my TBI unit to ExtremEFI in texas. they bored the throttle bores to 50mm each side, made new throttle butterflys, raised the injector tower 3/8",
removed the air horns, and machined the top of the tbi unit smooth. they then supplied me with a set of injectors for a 454ss truck, a custom made pressure reg(adj. from 12-50psi) and a late model corvette fuel pump.

Problem is..... it starts and idles about as well as one can expect with a monster cam, but if i rev it above 1500rpm IT KEEPS REVIN ON ITS OWN!!!!
all the way up to like 4 or 5,000rpm. i first assumed vaccume leak, however all the lines checked ok, i have good vaccume on the gauge(14psi) and even resealed the TBI base gaket and adapter gasket to be sure. THIS MADE NO CHANGES TO THE WAY IT RUNS!!! I even put an extra return spring on to make sure the throtlle is not sticking.

HELP ME PLEASE!!!!!!!!!!!!!!!!!!!!

 

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!PLEASE HELP ME!!!!!!!!!!!!!!!!!!!!!!!

 

Monster cams with TBI are pretty much a taboo, for starters.
It might be a problem with the IAC too.
Suggest posting on the TBI board.
Also suggest getting a 4bbl intake and go carb, if it can't be made to work right.
I can't remember the exact reason TBI won't work with a large cam, I canned my TBI years ago.
210deg @.050" intake duration is supposedly about max for a 350 with a TBI, even with custom chip, IIRC.

 

I would check to make sure your throttle return spring is doing it's job. Even if the IAC was somehow opened all the way by the computer I doubt it could rev it up to 5000 RPMs. I think this is a simple problem with the throttle returning back to idle once it's got some serious air flowing past the throttle plates.

Good luck getting that motor to run with TBI. Definitely check with the TBI guys on this board- if anyone can point you in the right direction they can.

FOR SURE you need a custom burned chip. A simple injector change and some fuel pressure tuning ain't gonna get it.

 

i have a custom chip from tbichips. any other ideas?

 

Can you manually manipulate the throttle blades? are you sure they are bored correctly? if there's a gap in the butterflies, there's an air leak right there.


Also: Are you 110% sure you hooked up your power brake booster line?

 

I guess the throttle position sensor (TPS) would have to be out of adjustment for the computer to allow it to rev high like that with the throttle closed.
Right?

 

PROBLEM SOLVED!!! DAMON U WERE RIGHT!! i checked my throttle operation a dozen times with the motor off. but when i checked itwith it running, sure enough, the airflow was to great for them to shut al the way. I now have the sotck spring on the tbi and a dual throttle return spring for a carb on there, and that barely closes it!!! gonna get heavier springs tomorrow. and believe it or not, it runs and idles pretty damn good with a tbi setup, needs some minore tuning(fuel pressure, idle speed, etc), but i am impressed. THANK YOU FOR ALL THE HELP!!!!