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danderduff 05-31-2012 11:22 PM

70's 350 Misfire under light/medium load
I will do my best on giving all information related to the subject to help figure out a solution to this.

To start off, this is an engine i rebuild last summer. It is a 70's chevy 350 bored .040, 305 H.O. heads, slightly aggressive hydraulic cam (specs unknown) small dished pistons, headers, HEI ignition and 1406 edelbrock 4 bbl carb. Timing set to 34 degrees total advance.

As for the symptoms, a nasty misfire in the 2000-3000 rpm range generally while acceleration and generally in second gear. (running 700r4 trans) That is about the only symptom i have to work with. The car idles as it always has before the problem occurred and generally gets better once you step on it and wined it past those rpms. The problem was noticed after was at an intersection in the middle of no where and feel the urge to tromp on the gas and broke the tires loose and the rear end started to "hop" fiercly.

I have messed with all sorts of the "simple" things. I have tried a different distributor, tried setting the timing differently and didnt have much luck with any significant changes. I have adjusted the valves 3 times. First without the engine running (how i usually set valves, and usually no problems) and 2 times with the engine running. After setting the valves with the enigne running the misfire was hardly noticeable for a while, but a little while later it became nasty and fierce again and re-setting the valves again made no change. I checked firing order multiple times, checked for plugs arcing in darkness and everything seems good.

An IMPORTANT note i should add, im currently running open headers being i just swapped into a street car and ran out of money to finish getting the exhaust properly done. With my first trip home from the shop, i heard an exhaust tick and assumed it was a loose header and was minor. I put it off for about 75 miles before it got fixed and there was a noticeable gap between header and heads. Did i possibly burn the exhaust valve? I ran a compression check on a few cylinders, but not all due to headers causing a pain in the ***** to remove the plugs and run the check and i suspect cylinder #8 and the cylinders i tested have generally acceptable compression

Being that i have open headers, i can really hear the misfire and it seems to be on the passenger side of the engine (cylinder #8 is suspect due to the header coming loose on that cylinder) Also, i have not experienced an engine that has had knocking, pinging, or a severe misfire like i am experiencing and cannot determine what is truely going on. My guess is burnt exhaust valve. any thoughts?

bigblockragtop 06-01-2012 08:16 AM

Although there is a lot of info there is still a lot left out. I am not a pro but if there was a burnt valve the miss would be at all rpm ranges. If the miss only happens at certain rpm then it is either fuel or spark and likely not mechanical. I am more familiar with Holley but it sounds like a lean spot like a bad power valve. Can you get an afr meter to test for lean? Hope this helps.

Greg T 06-01-2012 09:41 AM

It's possible that you're not experiencing a miss at all. Could be you twisted the drive shaft, cracked U-joint, or fragged the spline or pinion shaft. Nasty wheel hop can cause some terrific damage. Can you sit with the car in neutral, wind it up slowly and feel the symptoms? My guess is that you can't. The damage is just right to cause a harmonic imbalance in the drive line that has a resonant frequency in the 2k to 3k RPM range. The vibration is still there throughout the rpm range but you just can't feel it until you hit the resonant frequency. Check the drive shaft and joints for play and slop. I'm thinking you'll find your problem.

Custom10 06-01-2012 10:14 AM

Among whats mentioned, Could be a lean miss, if it is a stock 1406 (.098 jet) then buy a set of 1454 rods for the eddy carb, drop them in there, takes 5 minutes no need to change the jet (open up carb). This will get you 2-3 stages richer in power mode. 1406 is lean out of the box.

Check fuel pressure 4-5 psi WOT no more.

danderduff 06-01-2012 10:54 AM

Thanks for the information so far. Im glad to hear reasonable answers. I suppose it could be in the driveline. I have had this cross into my mind as being associated with the problem. i do feel like something is loose (rear-end, driveshaft etc...) Ill have to check it out.

Thing is, it feels and sounds like a popping and harsh banging sound with minimal vibration and makes me feel it is coming from the engine.

With reguards to the edlebrock being lean, why would it all of a sudden just randomly decide to start causing a severe misfire? wouldnt it be a gradual thing?

OH and i can NOT seem to get the problem to occur with trans in park/neutral slowly revving engine, also cannot get anything to occur while power braking the engine (hold brake and rev engine to put load on) although i cant get any decent rpm ranges that way.

Custom10 06-01-2012 11:36 AM

A lean miss can be intermittent. It may not occur through all RPM or load conditions but only under certain conditions. There are lots of varying factors effecting engine operation. Temperature, humidity engine load, fuel conditions etc.

the fact that it is missing only while driving is proof enough there is a problem that shows up only part of the time under certain conditions. Changing carb to run a bit richer is one easy way to rule out a lean miss.

danderduff 06-01-2012 07:01 PM

Ah, i have not dealt with a lean misfire before and would like to do something as simple as slap on a different carb but i do not have another to work with. I will mess around with things tonight and see if i get anywhere and post any updates if i find something.

bigblockragtop 06-01-2012 08:11 PM

custom 10 seems to have given you an answer without changing carbs just parts

spinn 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?

danderduff 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.

bigblockragtop 06-01-2012 10:36 PM

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.

Ranjet 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?

bigblockragtop 07-23-2012 06:05 PM

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.

Ranjet 07-24-2012 09:24 AM

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.

bigblockragtop 07-24-2012 08:14 PM

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.

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