okay, so im borrowing a carb from a friend to ascertain whether it is actually my carb itself thats been giving me problems.......the only difference is its an 850 as opposed to a 750, shouldnt make any difference on the track (unless the car needs the extra flow, it'll go faster)

however, i can't get the damn thing to run right, i tuned the curb idle mixture, and put some big squirters on it.....now it runs mint in the shop, the second you take it out on the road, and put a load on it it hates life.

mashing the pedal at say even 3500rpm creates a nasty nasty stumble that hangs on....it doesn't just stumble and then rev up, it keeps stumbling.......

any ideas?

my thought was the motor is actually ingesting more air through the 850, and therefore needs bigger jets???

im mental junk at this point, any ideas would be helpful

 

I would say lean it out it sounds like its loading up on you... bad. Try leaning it out and see if that helps.

 

its not even remotely possible that its too rich

, besides its extremely difficult to get such a bad rich condition that it causes a stumble....you almost have to try to do it.

 

I'm still thinking fuel pressure. Have you checked fuel pressure at idle? What is it at say 3000 rpm in the garage? Should be close to the same. Bigger carb, more air, more fuel needed. Maybe your bypass regulator is bypassing too much fuel. Do these carbs have power valves? Are they correct for your engine? What jets are in the 850? Is this engine poping back thriough the carb? If it has any pop through the carb when the throttle is mashed it is lean.

 

fuel system is all set, i checke the free flow on it several times now

its got 82 jets in it, which where it made peak power on the dyno at around 13.1 a/f with the 750.

ya it has a powervalve, the carb is setup correctly

 

here is a test that usaully works.look up what jets ,shooters and powervalves were in the carb stock and put them back in.most box stock carbs will work right out of the box.


somethin else may be wrong as i can't see why you need such big shooters.i only have 35's on mine and it makes 950hp.

did you put in a notched float when you installed jet extensions?

 

something to remember is a car is way different than a dyno.it may have shown lean on the dyno but out on the track where the motor is loaded harder(or less) but on the track may be way different.i know several peaple that tuned on a chassis dyno to make peak hp but on the track it was rich and would stumble.

do you pull a plug after every pass while in the tuning stages?get you a spark plug magnifier/light as the plugs will tell you what the motor needs.

 

ya it has a notched float.....the jet extensions aren't in this carb though,

i jetted it up 10 sizes to account for the larger (therefore slower) carb, according to barry grant.....and it runs alot better, but it still has a pretty decent hesitation when you whack the throttle at low rpms.....especially right around idle.....but something tells me that this motor is never gonna have a snappy throttle off idle with the cam in it (244 duration at .050, 308 adv)

i dont know, but there is a HUGE lean spike on the dyno with my 750......and therefore stumble.........we put 50cc pumps on it, and tried all the way up .040 nozzles, and it STILL stumbled off the tree

BG thinks that its something to do with the main metering circuit.......

 

You might have already checked this, but alot of stumble can be caused by timing being off or the advance curve not properly kicking in. Have checked your advance with a good timing light lately? A tuning expert I knew once said it seemed to him, 90% of the "carb problems" he's had to fix was really a timing problem. If you havent already, It might be a good idea to see what your ignition system is doing right now.

 

ya i checked both the initial and the total which is around 34 degrees right now

and its all in by 3000rpm

 

You still haven't said what your fuel pressure is. My 383 likes alot more timmimg than that. I'm running 43 total. That is where it runs the best times. I haven't had it on a dyno, but I have had it on the track for three seasons. I can't get by on less than 5 lbs. fuel pressure either, with a modified holley 800. I'm also running a larger cam than you, and it has very snappy throttle responce. Pulls hard to 6700, where we go thru the traps.

 

its has 7-8psi all the time

i dunno.....this thing is all over the place, everytime i turn around its completely off

all i know is the thing is amazingly large squirters and still ends up with a huge lean spike on the dyno when you mash the throttle....and its obvious at the track as well

 

whats completley off at times?

i agree with cp87gta.mine has over 300duration at .050 and it very responsive.

you should try what i said.put the 850 back all stock(or yours) and only put the jet extensions and float in it.you need a baseline to go off of.

and do it at the track not on a dyno.

good luck ,i hate gremilns.

 

383backinblack,. Did you ever get your carb running right? Still wondering what it was.