stumble

[dustytrails]

i have 3 2 bbl rochester carbs on a mild cam 350 chevy,msd ignition,slow take off no problem,step on it and it almost dies and then takes off,tried timeing from 8 to 15 degrees,no difference,acel pump shoots a big shot in carb no problem ,vacume advance is on a constent vacume not ported and pulls a constent 15 lbs of vacume,there is no ported vacume on these carbs,anyone have any idea what to do next?

 

[rooster57]

Does your distrib have mech advance weights. Mine were sticking and cause a sim problem.

 

[dustytrails]

yes the lightest ones that came with dist.

 

[Ceec]

Do you have progressive linkage? I am converting to that now. Two Holley 450's so the problems are not equal. The guy that helps me look less embarrasing says with out progressive linkage, the carbs flood the system.

Cecil;

 

[Tbucit]

Are the carbs orignals? If so there should be different bases on the two end carbs. Too many times when the carbs went bad people replace them with ones that have the idle circuts in the base. That cause the engine to flood and then hesitate.
Randall

 

[dustytrails]

the front and rear are dum carbs no idle cercits

 

[409T]

I'm chasing the same problem with 6 two barrel carbs. I went from .031" squirters to .035" and it cut the problem in half. Now I am waiting for even larger ones to come. Be sure you don't have any sort of vacuum leak also.
Sometimes it is necessary to take off the secondaries and block off their manifold holes, then get everything running correctly on the primary only. After that you can re-install the secondaries and have only them to monkey with if there is still a problem. I'm slowly learning that there is a very delicate balance between fuel pressure, float levels, air bleeds, power valves, discharge nozzles, syphon breaks and on and on and on.

Jeff

 

[Ted Brown]

on a 3/2 setup with the end carbs no idle jets, you can just disconnect the linkage from them to make sure the center carb is doing it thing properly... If you can stand on it, and take off with no hesitation with just the center carb, that will tell you loads about whats happening and why... if not, that also narrows it down...

 

[railroad]

I think you stated your probem in running manifold vacuum. Stepping on the gas quickly, results in the manifold vacuum decreasing, most times to "0". This lets your vacuum advance reduce the ignition timing. You are actually pulling advance out, as opposed to ported vacuum, which would add timing. Pull and plug the vacuum line, set your timing, and see if it isn't better without the manifold vacuum. Ideally you need ported vacuum. Second best, set the tip in mech advance as light as possible, with the vac adv locked. You can do a lot with the springs and wts. Let me know how you do.
I would love to get my hands on it, for you.

good luck,

 

[railroad]

I thought of another option on the ported vs manifold vacuum for the dist vac adv. In 1978 Ford used ported vacuum on the distributor for their 302 in a Fairmont, others probably also. To keep from having the hesitation they spliced a one way choke in the vacuum hose. It would let the manifold vac pull adv for the dist and when the vac fell off, the choke would hold the vac on the dist side of the choke to a slow bleed off rate.
I owned one of these cars and changed the set up over to ported vacuum. The little choke was a plastic housed diaphram marked for proper oreintation in the vac line. I am sure the logic for its use, was pollution control, but if you are not able to drill and tap a carb for ported vac, it may be an option.

good luck,

 

[Wild Mango]

I think Railroad is onto it. When the manifold vacuum vanishes so the ignition goes big time late. Time it (say 12deg BTDC) with the vacuum line plugged and try it on the road with the line still plugged. Mild cam the idle vacuum will be way too much to use as a signal and ported vacuum (or none) is the way. What ported vacuum will do is give you a good advance at cruise (say 45 - 50deg) so she burns a leanish mix well and doesn't load the engine up with carbon and heavy ends as well as turning in some decent mileage.

 

[dustytrails]

found the problem vacuume leak from front and rear carb,blocked off both at the base and it runs fine no more stumble

 

[Screaming Metal]

on a 3/2 setup with the end carbs no idle jets, you can just disconnect the linkage from them to make sure the center carb is doing it thing properly... If you can stand on it, and take off with no hesitation with just the center carb, that will tell you loads about whats happening and why... if not, that also narrows it down...

Sorry folks....be a racin' and dodging blower belts.....Teds got it by the horns....also RR is right too. If your running vaccuum advance....do it off the center carb....but watch how much your running....you might want to try running it without your vaccuum advanced hooked up and with your light set it about 9 or 10........try it bumping it up a little at a time.......I wouldn't think you'd want to go more than about 14 or 16. With a lot of carb throat area to deal with , you'll want it all coming in as last as you can, with your engine RPM up. Make sure that you don't have ANY vaccuum leaks

If you got all these things dumping at the same time....your gonna have trouble, if your RPM's are too low. Get all your advance in by, lets say 2400.....every engine likes the timing a little different......You wanna run off your center carb. I've experienced the vaccuum setups that Mopar and GM have tried to run on their 3 duece setups. They 're great as long as they're in 'like new' shape.....a little bit of age on these systems....they start going south really fast.

What I do is rather elaborate, I build a 2 stage progressive for these type of engines.......Idle and putt around on the center carb, at 5/8's throttle, the rod the activates the front carb, comes in farly fast....then at 3/4 throttle, the rear comes in with all 3 honkin' at WOT.

I have a piece of small flatbar with a slot connecting the center and front carb. A small bolt serves as a adjustable stop for tuning. From the front to the rear carb, I have a cable that attaches to another piece of flatbar at the rear carb with another adjuster bolt. Its a pain to get WOT set....but you can dial in exactly where you want your carbs coming in.....with this setup.....I have pulled the motormounts loose in more than 4 cars.......I always run torque straps now!...................................