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Old 10-11-2005, 12:02 AM
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383 Engine Misfire And Popping

I Just Bought A 4-barrel 383 Chevy Engine.
It May Have A Leaky Intake Manifold.

At Fast-idle Speed, It Pops And Wants To Stall.
Cam Is A 300 Degree Race Grind.
Crane Amplifier, Crane Coil, Mallory Distributor, 8mm Plug Wires.

I Have Not Checked Anything Yet - Like The Plugs, Compression, Timing Etc.

I Will Surely Let You Know What I Discover.


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Old 10-11-2005, 12:11 AM
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it could be 1 of a number of things, leaky intake gasket, valve train, carb is out of tune, incorrect timing... we need more info before we can start trouble shootin...
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Old 10-11-2005, 02:59 AM
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Trouble shoot

Doc here,

BASICS FIRST ..Always....

Check your static timing..
#1 TDC, Timing marks lined up, both valves closed (no tension on rockers) Rotor pointing to #1 plug tower..

FO:1 8 4 3 6 5 7 2..Direction of rotation is right..#1 plug tower is about 5 O'clock (In the stock location) VERIFY hand over hand if necessary that 5 and 7 are not reversed..this will give you exactly what you have now...and is a common mistake.

Start and run the vehicle, set static timing, with no advance at 750 RPM M/T..950 RPM A/T..to about 12 degrees...

Get a vacuum gauge on a MANIFOLD source, should read STEADY 17 to 20 in Hg |(unless you have a super lumpy cam) with No needle flux..If you get flux..

Check the vacuum system for leaks..be sure all the hoses are good, and hooked up..then with an air cleaner installed (over-spray will give you a false trigger), spray some carb cleaner around the manifold to head mating surfaces, and the base of the carb, EGR if so equipped, and anything that goes into vacuum..ANYWHERE it speeds up, you have a leak, repair and continue...You may have a warped intake manifold, this will help to detect that.

What do the plugs look like? , They will tell you volumes..
too white=too lean..(Carb, induction,fuel delivery)
oily=mechanical failure, (rings, Valve guides, Valve seals, cracked pistons, Leakey intake manifold)
black and sooty=too rich, (Idle jet adjustment, re-jetting)
brownish = normal..

Make sure you have no little silver flecks on the ceramic element..this is detonation and remnants of melting parts..

If you still have flux and popping do a compression test, anywhere you get slightly lower than normal compression ( more than 10%) note it and move on until all are done, then pull the valve covers, and check the valves on that (those) cylinders..too tight on intake will cause popping in the carb. A stuck or bent Valve can cause this too, it will show up on compression test...Re adjust and reassemble..then Run and check your vacuum again..SHOULD be 17 to 20 in Hg and STEADY..(a little less and a tiny bit of flux is OK if the cam is really radical)

IF the compression is real low all the way around (like 90 or 80 lbs) look at Cam timing..This would also be apparent while setting the Static timing..TDC and timing marks wouldn't line properly and both valves wouldn't close at the same time..

If the ignition and all is good and the condition still exists, get a pressure tester on the fuel lines (before and after the pump) a lean condition can cause poor performance and a lean pop...a failing pump or clogged filter can cause this..

A Dying ECM , (ignition module ) can cause this problem, idle good, under load breaks down, take it to the local auto parts house and get it tested...

Cap Carbon trails or water or unspecified life forms living inside the cap will cause the problem...check when you have the module tested..

Since the car is new to you, I suggest you do the whole list, so you know where you stand and any future surprises maybe headed off..all you need is basic hand tools ,a compression tester, a Vacuum gauge and a Fuel pressure / volume tester and a GOOD timing light...

That Should get you through the basics..

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