|07-26-2012 11:40 AM|
I see you tried another distributor, but what about the vacuum can? Maybe your can is pulling in too much timing too soon for your set-up.
I would disconnect the vacuum advance (plug the lines), and see if the miss goes away. If so, you know where to look. If not, you've easily eliminated one possibility.
|07-26-2012 06:04 AM|
I agree with what others have said and I would start with a vacuum gauge connected near the manifold to check for leaks at different RPM's
There is one other thing I would also try and that is adding one quart of Marvel Mystery Oil into the crankcase.
I had an intermittent miss on my 350 that drove me crazy!!!
I changed plugs, wires, vacuum lines, distributor and everything else I could think of that would cause a miss.
An 88 year old friend of mind who had worked on engines his entire life told me to try and Marvel Mystery Oil because he said I think you have a lifter that is sticking just some of the time.
Long story short I tried it and never had the problem again.
Hope that helps. Jimbo
|07-26-2012 05:53 AM|
Going down the electrical road,as mentioned above, the coil can break down under load if its faulty. As the revs increase the time taken for the coil to build up a charge decreases so if there is going to be a coil-related problem it will become worse as engine revs increase. I have found a simple High Voltage tester to be useful to see if a 40,000v coil will jump the preset gap on the tool. Plug wires can also be tested, one by one at cranking speed with this tool. Its handy if you can get a spring loaded push buton switch rigged to your starter solenoid so you can watch the spark as you crank it over.
( wire from bat+ to switch pole A. wire from switch pole B to starter solenoid +. )
If you have swapped out the coil for a "good" one maybe you could hook up a timing light and watch your advance curve on the HB to see if you have a smooth rise with the revs. Apart from plugs, coil, dist, leads and earths,and vacuum leaks, then you can only go down the mechanical road as co327 has suggested.
|07-26-2012 01:36 AM|
I would want to rule out a broken valve spring.
I don't know if a new cam/lifters was involved w/the rebuild but I would also want to rule out a bad cam lobe. Cam and lifter break in is more critical now than ever, given the changes to over the counter 'regular' motor oil.
|07-26-2012 01:32 AM|
|07-25-2012 10:18 PM|
|Ranjet||Thanks for the help. Just finished doing that before logging on and am more convinced than ever that I have the wrong HB. This is an aftermarket 6-3/4 balancer listed for SB Chevys and I think it has the timing mark set at the 12 o'clock position. I think an 8 inch would be better for my purposes. It appears to have the timing mark in the correct 2 o'clock position as well.|
|07-25-2012 05:24 PM|
|bigblockragtop||If you know how to check for top dead center with a piston stop that will answer you question.|
|07-24-2012 09:05 PM|
|Ranjet||Have already replaced the coil. Am starting to think that I may have the wrong harmonic balancer. It is an aftermarket one but i think the timing marks may be in a different position than what I need. It seems to me that some Chevy engines had a timing mark on the centre of the timing cover while others had it on the side. I have advanced the timing far past the timing tab located on the side of the cover and the car is running much better and the misfire is almost gone. I can actually drive the car now but it still has some issues. Talking with the guys at a cruise-in tonight leads me to believe my issue is all timing. Will keep you posted.|
|07-24-2012 08:14 PM|
|bigblockragtop||Black or sooted up plugs could either be oil fouling or too rich. Is there any blue color in the exhaust, that would be oil burning. When you say it is popping is it popping out of the intake or exhaust. I had a similar problem once recently when my coil leaked out all the oil it only misfired under load. There are ways to check the coil but going back over what you wrote maybe just see if you can get a coil(even borrow one) and see if that helps.|
|07-24-2012 09:24 AM|
|Ranjet||The miss/popping is more noticeable under load. But the idle is rough. I have replaced intake, carb base and and exhaust gaskets. dosen't seem to be any leaks on the carb. I pulled the plugs and they are sooted up quite bit. When I pull way in first gear the car pulls hard but when it shifts to second the popping starts and the power drops right off. The car will hardly pull itself. I have to take my foot off the gas and let it return to idle before I can nurse it back to the shop. By the way this a four speed auto overdrive transmission.|
|07-23-2012 06:05 PM|
|bigblockragtop||Does it miss at an idle only? Try pulling one plug wire at a time to isolate the miss. Pull the plugs and look to see how they are firing. Also check for vacuum leaks.|
|07-23-2012 03:27 PM|
70's 350 misfire undler light load
I am having a similar problem and am just about ready to give up. My setup is similar to yours only with a Holley 570CFM carb and headers. I have a severe misfire under load. I have swapped out the carb for a 600 CFM HOlley, changed the coil, cap and finally the complete distributor, reset the valves countless times, bought .100 longer pushrods, and nothing seems to help. The car ran fine last fall and then started acting up just before I put it away for the winter. I haven't driven it hard enough to jump a tooth on the timing gear and am using a double roller chain set up so that is unlikely. I have even checked the harmonic balancer (which was new) to make certain that hasn't spun. Only thing I haven't done yet is replace the fuel pump, which is a Holley mechanical HP model. Anyone have any suggestions at all?
|06-01-2012 10:36 PM|
|bigblockragtop||The driveshaft would cause a vibration at certain speed not certain rpm or the faster you went the more vibration. A rich condition can cause strange misfires too. I guess eliminate one problem at a time. Fix the driveshaft then look into other problems. This could be a vacuum leak in the passenger side causing a lean condition too. Too much air getting into one side of the engine. There are many ways to detect vacuum leaks just do a search lots of info.|
|06-01-2012 09:29 PM|
most my friends only have 2 bbl stuff due to racing rules.
Anyways, after messing around a bit tonight i established a couple things.
I think i have internal engine issues, and driveline issues like greg t suggested. a little more history on the vehicle; i did the 700r4 trans swap into my vehicle which required the driveshaft to be shortened. We shorten driveshafts frequently on our racecars (which turn far more rpms than street vehicles) and we decided to attempt shortening one for this one and see what happens.
Well driveshaft is either bent, slightly twisted, or just has horrible runout. i had on jackstand and put in gear and could see the driveshaft go up and down. where this fits into my issue, when i hear the popping and banging going on its hard to diagnose just with sound, but could possibly be the driveshaft causing some sort of noise/issue.
When driving under light acceleration, i rarely hear any sort of problem but under heavy acceleration from a slow speed or stop, i get the nasty noise.
I ALSO decided to look into a possible engine misfire and while the vehicle was on jack stands, and running. (As stated above, i have open headers on vehicle) I watched the end of the headers and the passenger side (suspected side of any engine issues) shot flames out and popped with quick rev up of the throttle, the driver side would never shoot the flames out.
Also did the cheap quick burned valve/paper myth. held piece of paper to the end of the exhaust, and signs of burned valve it was supposedly supposed to suck back in for a moment. Showed no signs of real evidence.
If the lean misfire were to be occuring, wouldnt it affect all cylinders, or at least both sides of engine, due to how the intake is designed?
I appreciate all advise and im not neglecting any suggestions. I realize its difficult to give any direct answer when i cannot give all information needed right off hand. You guys give good information and i try to put it all to use.
|06-01-2012 08:30 PM|
What are you the prince of darkness , dont you got any friends?
Kidding aside there are 4 or 5 hotrodders within my neighboorhood, nice people most of which are always ready to help with this kind of thing. Lend a carb/distributor/etc. The hobby is all across the usa. It is hard to zero in on this kind of problem from the information given. Though these guys are giving you some great addvice.
Soyou suspect internalissues, go with that. You checked cranking compression, can you see if it holds it?
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