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Author Topic:   Motor problem PLEASE HELP
Dirt Roller

Total posts: 9
posted August 31, 2002 11:44 PM
Engine stumbles at 3000 rpm when throtle is opened very quickly. This problem occures when trying to accelerate through the corners. If I dont romp on it hard it will accelerate ok. Changed the following parts: rebuilt the carburator tried jets, power valve, accelerator pump and changed the little plastic cam, float level (when nothing helped I swaped carbs with a friend with no help), complete distributor(cap, rotor, coil, module, pick-up, and housing), tried using advance weights instead of locked dist, ignition wires, plugs, and tried timing from 36 down to 30. Checked for vacuum leaks, droped exhaust to check for restrictions and all seems ok. Finaly hooked up vacuum guage and found the following: Has 15" at 1000 rpm and at 3000 rpm drops to 5". What should I check next?
It's a Chevy 350, dished pistons, 76cc heads, 2bbl intake with 4412, .500" lift solid cam, uses alternator (gets 13volts to distributor). 1/4 mile banked clay track. The only other things I can think of are: cam timing but engine has power to 6000rpm, internal vacuum leak but idle vacuum is ok and engine is not lean, valve springs but they don't float at high rpm, valve not sealing but compression is 140psi on all cylinders, cam way to radical but that should mostly affect low rpm.

If anyone has any ideas I would greatly appreciate ant input you may have.

Dirt Forum Champ
Total posts: 620
posted September 01, 2002 09:38 AM
sounds like you've covered most of the bases with the engine. you may have a cam going flat. check your oil to see if it is excessively dirty and the valve train. if it's not the the car hooked up really hard? sometimes, if your car's really hooked and doesn't have enough power to spin the tires, it will stumble when you flat-foot it all at once. given all of the information that you've provided, that's my guess. try adding a few lbs. of air to the rear tires.

also, a big solid cam needs compression to work effectively. a dished piston 350 with 76 cc heads and a big solid cam isn't an ideal combination.

[This message has been edited by outlawstock17 (edited September 01, 2002).]

Dirt Maniac

Total posts: 106
posted September 01, 2002 11:06 AM
One thing on the carb maybe try a bigger squirter.Mine had a little stumble in the turns and it ended up being that the float levels had to run a little lower than holley recomends.It must have came through the vent and was loading up.

Dirt Full Roller

Total posts: 26
posted September 01, 2002 11:11 AM
To me is sounds like you squirters are to large. I had the same problem & went to smaller squirters and helped the stumble off the corner. What size are you running?
Just my 2 cents

Dirt Full Roller

Total posts: 73
posted September 01, 2002 01:31 PM
we run a gearte 750alky carb on our modified, the driver said that it wasnt crisp when he put it to the mat, but it would clear out, we also changed the jets, up an down, and we had a spare carb laying around and i swapped the squirters on the secondaries. so the carb went from 52's down to 28's and the bog went away, so i would go that route.

[This message has been edited by DodgeBoy76 (edited September 01, 2002).]

Dirt Freak

Total posts: 467
posted September 01, 2002 10:57 PM
You could also have a fuel supply problem , and your running the float bowl's dry by the end of the straight . Once your in the corner , and pick up the gas , all you have for fuel is the accelerator pump shot . Check over the fuel system , loose fuel line , hole in line , pick-up su*king the bottom of the fuel cell ...etc.

Hendren Racing Engines
Rutherfordton , NC

Dirt Roller

Total posts: 9
posted September 05, 2002 03:08 AM
It's not a fuel suply problem (stumbles a the shop). What could be causing the low vacuum reading at 3000 rpm? Also noticed that choke horn apears to get wet with fuel when engine is reved up. Seems like it is something mechanical in motor like valve springs or bent/burned valves or is the cam just way too big?

Dirt Maniac

Total posts: 135
posted September 05, 2002 08:41 AM
That low vacuum reading at 3000 could be intake reversion just before the engine 'comes on the cam'. The intake mixture is being pushed out of the chamber, even back out of the carb! This would explain the stumble and the choke horn getting wet. I have actually seen a fog of fuel ABOVE the carbs on some 2 stroke engines from this(they call it 'stand off'), and have seen the same problem on drag race engines with automatic transmissions and huge cams(transbrake cured it). You did not mention your cam duration or separation, but you probably don't need more than 245-248 or so intake duration(@.050). Reversion is a problem on all racing engines, but can be minimized with proper intake/cam/exhaust tuning. You can try: cam with less duration and/or overlap(widen lobe separation); different exhaust(smaller header tube size will help); different intake; play with carb spacers. Exhaust restriction(mufflers) will make the problem worse. The cam and intake will have the biggest effect, UNLESS the exhaust is totally ******* up. Keep us posted.

Dirt Roller

Total posts: 9
posted September 05, 2002 12:42 PM
Cam is .498 lift 246 duration @ .050 (264 adv)108 lobe center. This cam worked very well on my last engine with same heads and intake but with flat top pistons. I never checked vacuum readings on that engine. We have to run log exhaust manifolds with single 2" exhaust pipe and muffler.

Dirt Roller

Total posts: 9
posted September 05, 2002 12:50 PM
I checked vacuum readings on my buddies motor and it is 20" @3000 rpm the only difference is he has flat tops.

Dirt Roller

Total posts: 9
posted September 05, 2002 12:51 PM
he has same cam too

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