we had a miss on one of lower end circle track cars with the HEI distributor.....couldn't track it down so we changed the module in the distributor and that solved it.
Engine Miss.. rounded lobes?
- Thread starter MrSony
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lean mixtures need more ignition lead to burn completely. is that popping noise an exhaust pulse that is still on fire coming out of the closest cylinder(s) to your down pipes? how is going with the manifold vacuum gauge hooked up? tune your machine for a strong and steady vacuum signal. with more exhaust pipe on that thing, you might find out that it don't like being dumped to atmosphere like that, and reversion is all you're hearing. 8:1 compression doesn't help it out much either...
I think your idle is to low. I idled my stock engine at about 750 in gear and timing was at 34* total.
If it's a misfire that goes away above a certain rpm it's a tuning issue.
I had mechanical, electrical and tuning issues causing misfires on my caddy.
I started with mechanical. At 2000rpm the engine would miss, seen by 1 jerk of the engine at exactly the same time interval. So let's say every 0.8 seconds it would shudder once. I took off my valve covers and pushed on every rocker and two of them bled down. A hydraulic lifter shouldn't bleed down on its own for many hours. I replaced the lifters and that issue was solved.
Next up electrical. I had a weak ground and digital msd ignition. The msd boxes are very sensitive to electrical issues so i upgraded the battery cables and replaced a few bad wires going to distributor coil and it helped alot.
Finally was the vacuum leaks and valve sealing issues. You have to remember that vacuum leaks don't just come from above they can also be from below. You might be sucking up through the valley. A leak down tester is the only sure way to know. Carb cleaner and propane help in a pinch but they cannot fully eliminate possibility of leaks. Check your plugs of they are black and wet (oil) this might be your issue. At high rpm the oil burns off and you don't get a rough running engine.
Last one which is probably not your issue is valve seat corrosion. My car sat in humid storage for a few years and my bros car does the same. When we take them out they don't run right for a while but clean up as the summer goes on. Seafoam or ATF can help lubricate them and speed up the process a bit.
The reason i asked for flexplate is my 350 cracked and i didn't notice it until it physically started to separate. Get under there and try to pry it away from where your converter is mounted and look for cracks. This might be why you're shaking.
A vacuum gauge is an absolute essential tool for diagnosing issues. When i was in highschool the budget was very tight, i get that. Go to home depot or similar and get a small gauge that's meant for a compressor. They read vacuum and pressure and are about $4. Then use some 3/8 hose and plumb it to manifold pressure. Do a Google search on vacuum gauge diagnosis. But essentially if the needle bounces around you have valve issues. You should be able to see the miss in the gauge if cylinder or two is down. At idle they should be fairly steady at 1 value. At cruising they should be rock steady.
Start with the simple and cheap stuff. Don't spend money on parts till you are 100% sure what the issue is. That means you verified the issue in a measurable way and not from a hunch or "some really good advice".
Good luck
If it's a misfire that goes away above a certain rpm it's a tuning issue.
I had mechanical, electrical and tuning issues causing misfires on my caddy.
I started with mechanical. At 2000rpm the engine would miss, seen by 1 jerk of the engine at exactly the same time interval. So let's say every 0.8 seconds it would shudder once. I took off my valve covers and pushed on every rocker and two of them bled down. A hydraulic lifter shouldn't bleed down on its own for many hours. I replaced the lifters and that issue was solved.
Next up electrical. I had a weak ground and digital msd ignition. The msd boxes are very sensitive to electrical issues so i upgraded the battery cables and replaced a few bad wires going to distributor coil and it helped alot.
Finally was the vacuum leaks and valve sealing issues. You have to remember that vacuum leaks don't just come from above they can also be from below. You might be sucking up through the valley. A leak down tester is the only sure way to know. Carb cleaner and propane help in a pinch but they cannot fully eliminate possibility of leaks. Check your plugs of they are black and wet (oil) this might be your issue. At high rpm the oil burns off and you don't get a rough running engine.
Last one which is probably not your issue is valve seat corrosion. My car sat in humid storage for a few years and my bros car does the same. When we take them out they don't run right for a while but clean up as the summer goes on. Seafoam or ATF can help lubricate them and speed up the process a bit.
The reason i asked for flexplate is my 350 cracked and i didn't notice it until it physically started to separate. Get under there and try to pry it away from where your converter is mounted and look for cracks. This might be why you're shaking.
A vacuum gauge is an absolute essential tool for diagnosing issues. When i was in highschool the budget was very tight, i get that. Go to home depot or similar and get a small gauge that's meant for a compressor. They read vacuum and pressure and are about $4. Then use some 3/8 hose and plumb it to manifold pressure. Do a Google search on vacuum gauge diagnosis. But essentially if the needle bounces around you have valve issues. You should be able to see the miss in the gauge if cylinder or two is down. At idle they should be fairly steady at 1 value. At cruising they should be rock steady.
Start with the simple and cheap stuff. Don't spend money on parts till you are 100% sure what the issue is. That means you verified the issue in a measurable way and not from a hunch or "some really good advice".
Good luck
I've got that same manual. Spark plugs all look damn near new, if not a bit greyish from being used. R45TSX gapped at .045, TA HEI Distributor, Accel 8.8mm Super Comp Plug wires, all tested good with getting spark and with a meter. I think the only problem would be the choke being half closed (other thread) and that is dicking with everything.
It does pop out the exhaust. Kind of like that burble that those tuner kids crave. If I leave the car in first and engine brake it it burbles on decel, but I think it's supposed to in that instance. while just idling, it snaps and pops out the exhaust, seems to be coming from the passenger side. I've adjusted the idle mixture screw on that side in and out, nothing really changes other than the engine wanting to die with it all the way in of course. Both screws are 2 1/2 turns out from lightly seated. My dad might be able to snag a vacuum gauge for me to use from his work, and if so I'll post my findings on that later. Summit has a diagram of what the vacuum readings mean. For a street motor, 20 hg of vacuum?
a quick way to find a burnt or stuck valve is to take a dollar bill and hold it close to the tail pipe with the car running. If the bill gets sucked into the tail pipe you have a burnt or stuck valve
20" Hg is great! however, might be more plausible with the original cam. Buddy and i tuned my 383 the other day, now i'm running a different combo than yours, 226/[email protected], .501/.510 w 112deg. the best idle we achieved is avg 15". different heads(alloy), 1.625" full length headers, 3" duals with x-pipe, dynomax vt muffs. he ordered the same roller cam for a 406 build and he is ecstatic with those numbers, as am i. with a 327 running 041 heads, same headers on 2.25" duals, it achieved 17". that was with a edelbrock tow truck cam(204/214, .420??? lift). if the gauge Pops picks up is a "tuner's" gauge, there'll be some green and red banding along the sweep to help along with diagnostics... kinda like traffic lights...
without having been around a buick 350 2bbl for quite some time myself, maybe a dozen or so cars back, i cannot advise on a nice vacuum number.
the best number you can obtain with a steady reading is the best approach. start with static timing in the 8-12 degree adv range with distributor's advance disabled.(disable it from the vac can and plug that rubber with a screw or golf tee, that is unimportant!!!). if cold, jelp it out with SOME fuel, don't overdo it. watch that gauge and "read" it while idle stabilizes. a heavy air sucking noise means it is struggling for something, your gauge will will help you with that. whatever you adjust on idle screws MUST be done equally on both of them, and give that thing some time to react to the changes. that is a must. i would like to add that you must recheck your coolant hoses during this process. a gentle gloved squeeze on thew upper hose will inform you of rising heat build up. check that often. the closer you get to optimum, the less of that sucking noise you will hear. give yourself a safe zone to jump into while performing this operation. if the pressure release is not flexible under that rad cap, you don't want to step on a shovel or rake or your dog... (don't ask!!) treat your tune-up like the cruise control in grampa's leasbare. tuning changes will have an elastic like feel/sound to them and certain fuel circuits need some time to adjust. your motor will thank you by being pleasant sounding, not too noisy like a little brother when he doesn't get what he wants from ya. if your throttle is too far advanced to keep up, "tip-in" will not be graceful when transitioning to cruise, just off idle. jetting will be next approach.if idle screws can't accomodate
there will be more to follow, and would like others to chime in from here
without having been around a buick 350 2bbl for quite some time myself, maybe a dozen or so cars back, i cannot advise on a nice vacuum number.
the best number you can obtain with a steady reading is the best approach. start with static timing in the 8-12 degree adv range with distributor's advance disabled.(disable it from the vac can and plug that rubber with a screw or golf tee, that is unimportant!!!). if cold, jelp it out with SOME fuel, don't overdo it. watch that gauge and "read" it while idle stabilizes. a heavy air sucking noise means it is struggling for something, your gauge will will help you with that. whatever you adjust on idle screws MUST be done equally on both of them, and give that thing some time to react to the changes. that is a must. i would like to add that you must recheck your coolant hoses during this process. a gentle gloved squeeze on thew upper hose will inform you of rising heat build up. check that often. the closer you get to optimum, the less of that sucking noise you will hear. give yourself a safe zone to jump into while performing this operation. if the pressure release is not flexible under that rad cap, you don't want to step on a shovel or rake or your dog... (don't ask!!) treat your tune-up like the cruise control in grampa's leasbare. tuning changes will have an elastic like feel/sound to them and certain fuel circuits need some time to adjust. your motor will thank you by being pleasant sounding, not too noisy like a little brother when he doesn't get what he wants from ya. if your throttle is too far advanced to keep up, "tip-in" will not be graceful when transitioning to cruise, just off idle. jetting will be next approach.if idle screws can't accomodate
there will be more to follow, and would like others to chime in from here
I've done that test. Plus I like to put my hand over the exhaust to feel the pulse whenever I adjust carb stuff. Always a nice pulse, never any suction sounds or feel. I don't think I'd have a burned or stuck valve. Heads were done up by a reputable machine shop that builds all kinds of BOP, Chevy, Ford, and Dodge motors. I also disassembled the heads prior to installation to check their handywork. All valves were in great shape.The shop did replace to galled valves and their respective guides when they did the heads.
The motor, well I guess I should say carb, does make a fairly audible sucking noise. Like that suction thing at the dentist's office. That level of noise. It comes from the carb. I put my hand over the choke...hole(?) and the hissing/suction noise stops and the car dies. I should also mention, this is all with the choke half closed. After reading up on it, factory electric chokes were wired to the oil pressure switch, they would only get 12v if the engine was running and creating oil pressure. My engine doesn't have an extra spot, just the singular hole for the oil line to the gauge inside. I suppose I can get a little T fitting, get the switch, hook the choke wire to that switch along with the wire for the idiot light on the dash, and that might make it all work. Or I may just tap into the wiper motor like everyone does, just to get it to work until the weekend.
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