View Full Version : misfiring 340

10-03-2012, 04:16 PM
I have a 72 340 that seems to be acting up. When it idles, it sounds rough and when i unplug wire number 1 or 4 it don't change the idle, just like the cylinders were dead. Wires are firing and pulled plug out of hole and it is firing. Thought it may be vaccum leak, pulled off intake and put new gaskets, still the same. Now pulled heads off and there was a film of oil and antifreeze between gasket and head and block. Is this normal? Also was a trace of antifreeze in cylinder 4. Anyone ever run into this? Just looking for advice.

10-03-2012, 09:40 PM
a compression test is one of the best ways of finding a bad cylinder

10-04-2012, 07:30 AM
I think your system of diagnosis leaves a little to be desired. As just suggested, a compression test should have been done when your dynamic test gave you a positive. Just making a quick judgment about what the problem is usually doesn't work.
The oil and coolant you noticed after pulling the heads might be normal drip for head removal. You didn't say anything about the condition of the gaskets. Since you had the heads off, did you do a quick check for valve sealing?
1 and 4 are on opposite sides, so a vacuum leak would not be the first choice for diagnosis. Anyway, vacuum leaks are more often at hoses and fittings than at the manifold gaskets.
Did you check the distributor cap for cracks, the wires for cross-firing?

10-04-2012, 01:08 PM
Good time to look for cam or lifter damage?

10-15-2012, 04:33 PM
I had done a compression test before disasembling. everything seemed fine. The head gaskets looked normal, no visible rips or holes. Verified the operation of camshaft before removing heads. I will have to get my heads checked by a reliable shop, as far as I can see valves seem all good. As for wires, plugs and distributor cap; changed them all in the spring and motor worked fine then. Thanks for the comments, in the future i'll have to make better judgement calls.

Rich Kinsley
10-16-2012, 02:55 AM
The water and oil could just be leakage from pulling the heads. If the plugs and cyl's looked really clean in the suspect cylinders then you probably have a crack or head gasket issue. Been there. Done that.

10-16-2012, 07:38 AM
But he said he had 1 and 4 not working. Usually, head gasket problem will get adjacent holes.

10-16-2012, 02:26 PM
If you had a duel plane intake #1and #4 same runner ,was carb mounting bolts tight? also did engine shake while it was idleing? If it did pull wire off #1and #4 did shake go away?If so problem is in that cylinder.A shake is a intake valve not sealing right.cracked dist cap,wiped exhaust lobe in that cylinder .If no shake it is a lean condition.carb adjustment,intake runner gasket,vacuum port running off runner for 1#and#4 also a rare but did happen pcv valve came apart. I hope this helps .

Rich Kinsley
10-17-2012, 01:03 AM
But he said he had 1 and 4 not working. Usually, head gasket problem will get adjacent holes.

You are right. Sorry.

10-17-2012, 03:37 PM
Thanks for all the info guys, and upon futher inspecting the cylinders I found large crack in number 4. None visible in 1 but al least i now know what the problem is. Again thanks for all your info it will be knowledge for the future.