green_doctor

Hey,

I just transplanted a 600 holley from a 302 2V to a 351 4V. The carby was dyno-tuned to the 302 and is giving me issues: idles really rough, backfiring alot, revs okay but blows too much smoke with missing when returns to idle. Have checked the timing but its at top dead centre (TDC). Does anyone have any suggestions? I had the carby rebuilt less than a year ago and the motor was running before I put it in.

Any help would be appreciated!

northey

Is the smoke black, white or blue? Timing should be before TDC - approx 6-8 degrees would do for a starting point. Are points, etc clean?

Cheers

fat071

Check that u dont have any shit in the needle n seat from fuel lines ect.

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green_doctor

I have all new plugs and leads, and coil. The coil was running on another motor (just to test it). The smoke is black. I've noticed a small leak around one of the extractors but that should be compensated for by the size of the cylinders.

TruBlu351

Idling rough = low timing.

As you increase the timing as mentioned, just decrease your fast idle screw to keep the rpm about 800+ ish. You'll need to set the idle mixture next.

Turn your car off and measure how many 1/2 turns it takes to wind each mixture screw right in (gently). Wind them back out to roughly the same spot, but make sure they are identical. The best way to set these it to obtain max vacuum at idle, but without a gauge just wind them in 1/8th turn each after you car is idling at normal temp (if you screw one, you screw the other). The rpm should increase very slightly as the mixture leans out. Keep doing this until it stops rising then wind it back out about 1/4 to 1/2 turn. This should get you into the ball park.

Normally black smoke = too rich. But if its jetted for a 302, then there sounds like other issues??

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musclemustang8

on my set up i have an initial timing of 15 degrees because of the rough idle i have. (it then andvances about another 15 degrees at full advance.) from what you describe and the changing from a smaller cid engine with heads with less chanced of detonation to a larger cid and larger port heads, it would make sense that it would be running a lean condition. the smoke makes no sense if you are running lean unless it is burning oil (say by valve seals, etc.) once again you didnt say the color of the smoke. i would pull a plug and check the mixture. ide mixture set up is goin to be different between the two set ups as well. every engine is different, my timing scheme may not work in your situation just as the carb situation surely isnt the same between your two engines i would get an idea on what jet sizes you are running and how your plugs look and i am sure most of us on here could help you more.
also is it backfiring through the exhaust or popping back through the carb.
if back firing through the carb- too lean or too much advance or both
if backfiring through exhaust- not enough advance.
good luck-ryan

Racer

I wil bet my sheep station that it is one of two things:

1) Stuck needle and seat
2) Faulty power valve

Look into the carb when idling, and if you see any fuel coming out of the booster, thats your problem.

xdclevo

I'm with TruBlu351. Low timing will do it everytime. Also, with low timing,
you have to wind up the idle screw, and when the butterflys are exposing
too mush transition slot, it won't be running on the idle circuit anymore,
causing it to run shite and bellow black fuel smoke.
How far open are th carby butterflys??
Get some timing in that sucker!!
(anything under 18 degrees btdc in mine and it runs crap)

Racer

But the carb hasn't been touched. The most likely things to deteriorate (as apposed to be wronly adjusted) are the two things i have listed. I presume the backfiring is through the exhaust?

xdclevo

It a completely different motor he has put it on, so time to start from scratch. I mentioned the timing could be causing it, not the carby. I do assume he tweaked the idle screw on the new motor, so who knows where it is at.