Inliners International Forums General Discussion Engines 250 runs rough HELP

ok i have a 250 chevy block had under 60k (miles) on it

i have a circle trach head (big valves, big ports and its milled as well)
R.V. cam
GM H.e.i ignition
offenhauser intake (4 barrel)
holley 390 cfm
and split exhaust manifold with true dual exhaust
and an automatic transmision(power gulide)

the motor runs like crap, shakes at an idle

i set the idle to 600 rpm i put it in to gear it stalls up the idle to 700 rpm same thing (idle set in park)

also when i try to set the timming theres an "A" and an "0" i assume a= advenced and "0" is 0* ???

there is no wat it will run withthe timming set in between the "A" and the "0". the timming on the "A" it will run its best (still shakes)

i have checked the plugs - all are firing, but look sooty)

help please its driving me up a wall

thanks

have u had it running good before or has this happened from the day u started running it...u may need to advance/retard the cam, u could have a really big vacuum leak somewhere, Dry sooty plugs indicate a rich mixture or weak ignition. This causes misfiring, hard starting and hesitation. The recommendations for this is to make sure the spark plugs have the correct heat range, check the airfilter to see if its clogged or a problem in the fuel system or engine management system. Also check for ignition problems....I got this out of my haynes manual that shows different plug conditions and stuff...but thats a pretty crazy setup u have there and will take a bit to get it running good...but when u do...heheheh.ooohh man

Hi Johnson . . . I was here once before and it turned out not to be much fun.

I had a v-thingy that had thrown a rod. Didn't think that there was any piston/valve contact - I was wrong!

After a bottom-end rebuild and reassembly the engine just would not settle down. The engine just shook and shook at idle. I tried everything until a buddy told me to do a compression test.

This showed one cylinder was a dead zero . . . it was a slightly bent exhaust valve. Visual inspection did not reveal that it was not on the seat.

Replaced that bent valve and everything was fine.

Have you done a compression test to check for balance? Highest and Lowest readings should be within 10% for best results.

Good luck.

stock49

....Have done a compression test on the motor, and they are all between 125 and 130. It has approx 17 inches of vacum at an idle. Can't find a vacum leak. The carb has only one vacum port that goes to the distributor,and the two on the intake I have pluged. I am going to pick up new plugs, wires and cap tonight and try again. It has to be a simple fix, but I can't seem to put my finger on it. Oh well, we will keep trying. If anyone has a idea, let me know.
Thanks

Johnson, I would first verify the ignition timing. This could be done in two ways:
1) with the engine off, bring #1 up on compression to the top and check that the balancer lines up with 0 on the tab.
2) loosen the distributor hold down bolt a bit and with the engine idling, try rotating the distributor each way to see if it makes an obvious improvement.

Also, where is the vacuum line from the vacuum advance connected? Above or below the throttle plate (ported vs. manifold)?

Greg

When I put the 390 on my 250 w/ a turbo 350 I used the V-8 convertor and the engine would die going into drive.
Is the powerglide convertor a stock one or a smaller, higher stall model?
BTW: the idle speed specs are for trans in "Drive"
I ended up tweaking the idle circuit inside the metering blocks to get more fuel so engine would idle.
This has to be done carefully, and as a last resort.
You should address the timing, vacuum leaks, torque convertor, ignition and carb settings, etc, etc, first.
Though if you have access to a 450 metering block you could try that and see if it makes the 390 act better. It was reported that it works and have no doubt it does.
Search for 390 in the Engines and Hi Performance forums and there is some info that might help you.

The vacuum line that goes to the distributor is above the throttle plate on the carb. The convertor is the stock power glide that came from the factory. I would like to up grade to a 200r4 later on, but for now it would just be nice if it ran right. It I set the idle at 700 in park, it drops to about 350-400 when put in gear and stalls. If is set the idle speed in gear then in parks it is far to high to put back in gear.