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Old 09-17-2010, 04:39 PM
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Need more ideas for miss fire in 350 sbc

Hello guys I guess I am starting a new post here cause I have been spending time trying to figure out what the heck is causing a miss fire in my engine and causing me headaches.

Engine is 350 with an hei distributor, 212/218 @ 50 hydraulic roller cam with world sportsman 2 heads with a holley 1850 600 cfm carburetor with edelbrock eps intake. Timing is set at 28 to 30 degrees with vacuum advance hooked up and such timing is all correct and should not be the factor.

Vacuum gauge is nice and steady at 19 inches till the miss fire happens then it starts to drop just a .5 inch to an inch of vacuum then pick back up and stay steady at 19.

The miss fire just happens when it wants to and is the worst at cold startup and mellows out some once warmed up but there is no cycle of when it happens it just does. Some times it will be worse then others. It is most noticeable at idle.

Idle is smooth at 800 rpm and then all of a sudden the idle will stutter then go smooth again and then some times it will stutter a couple of times and some times just one time and it will just happen.

Everything is brand new I mean everything from ignition system, engine and all accessories and all charging system parts etc.

I have used two different distributors, three different control modules, two coils, 4 sets of spark plugs of three different brands. I have also went through several brand new sets of spark plug wires as well. I have checked voltage and it reads fine. There are no vacuum leaks that could be found using carb spray everywhere.

I also plugged off all vacuum accessories hooked up to the carb one at a time including pcv valve and miss is still there. I have added extra ground wire and have also re routed spark plug wires so there should be no cross firing. I have also un hooked my plug wires and lifted them up one at a time while engine was running and they are all firing just fine.

I have also re adjusted the idle mixture screws on my carburetor and checked all setting's on it and they are fine. This problem plagued me before when I had a edelbrock carburetor on it as well. I don't really know what else to check anymore and am getting tired of this engine to the point that I want to get rid of it but I actually wouldn't. I am just lost here anymore. Thanks guys for any help.
Eric

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Old 09-17-2010, 05:18 PM
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Since you`ve narrowed everything down in the ignition system, that pretty much leaves one other area that can cause occasional miss fires and that would be in the valve train, it sounds as if you have a valve that`s sticking open partially. It was some years ago a friend of mine`s engine had a valve that was stuck half way open and that caused a noticeable skip, upon further tear down we found the valve was jammed in the guide, the head guy apparently didn`t give the valve enough clearance. You may just have a valve that`s not fully seating sometimes. Since it does it more at cold start, remove the power wire to the distributor, crank the engine over and listen, you`ll hear the cycle make a steady noise, if it finds a cylinder with little cylinder pressure compaired to the others, you`ll notice the noise of the starter suddendly change. If that sounds okay, pop the valve covers, inspect each rocker and valve springs, have a buddy bump the engine over so you can check the operation of each valve. With some luck, you may find one that`s sticking. good luck.
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Old 09-17-2010, 06:23 PM
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Well when i got the cylinder heads they where brand new world heads and they where already assembled but all the clearances on them where checked and such.

And as far as me having a sticking valve the vacuum gauge does not show a sign of that going by looking at online videos of what it would look like on your vacuum gauge if you had a sticking valve. I will have to look further into things but as far as I can tell it should not be that.

Valve lash has been set several times since this problem has been there. With my previous cam you could not notice it as much cause of the mild rough idle it had. I also have done a dry compression check on my cylinders and they are all the same at 180 now since I have stuck in a smaller cam.

While turning the engine over with the distributor un hooked it sounds normal with no difference in pitch of sound or any difference in how it works? I also am running a 1 inch 4 hole carb spacer but I don't think that is causing my miss fire though.

I recently tried a one inch open spacer and it took away way too much bottom end and made my engine even worse from low to mid but anyways it still miss fired then. Am going to one more time go through all of my vacuum connections and run some propane around my hook ups just to one last time rule that stuff out.

I am going to use a propane torch but just let the gas leak out and see if I find anything but using carb cleaner I did not.
Eric
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Old 09-17-2010, 06:32 PM
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You should be able to pin it down to one bank or the other by listening at the tailpipes.
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Old 09-17-2010, 06:37 PM
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Well cobalt way ahead of you. I learned that trick from my Dad and what sucks it does it on both sides so man it just sucks all the way around. This has plagued me for months now and no matter what I do I just can't find what is causing this issue. I am about ready to just go nuts on this thing. Pretty sad with everything new here. I have plenty of spare parts for future use though. I will do a nice and long listen to both sides and see which is worse.
Eric
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Old 09-17-2010, 06:44 PM
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Quote:
Originally Posted by eric32
Well cobalt way ahead of you. I learned that trick from my Dad and what sucks it does it on both sides so man it just sucks all the way around. This has plagued me for months now and no matter what I do I just can't find what is causing this issue. I am about ready to just go nuts on this thing. Pretty sad with everything new here. I have plenty of spare parts for future use though. I will do a nice and long listen to both sides and see which is worse.
Eric
Try changing the float level to a bit higher than where you now have it.

Reason is, I am thinking this might be happening right when the float opens the needle and seat. If the float is right at the point where the engine wants to go lean due to a too-low fuel level, raising the float to the high side of "normal" may help.

I don't see where you swapped carbs for a known good one. Might try that.
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Old 09-17-2010, 07:01 PM
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Ok cobalt I will try that tomorrow after I get off of work. I will post late saturday evening or Sunday at the latest and let you guys know what I hopefully will find out. Have a nice weekend guys
Take care
Eric
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Old 09-17-2010, 07:09 PM
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Yes I guess I forgot to mention my carb is a brand new one I got last fall so it's not even a year old yet. I am waiting to go that route though want to rule everything else out till I try a different carb. When I had my old edelbrock on there before I sold it last year it did the same thing but I had a lopey idle cam in my engine then and now that I have a smooth ideling camshaft its easier to hear the miss and detect it better now.
Eric
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Old 09-18-2010, 07:55 PM
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Well guys sad to say I spent another 2 hours on this thing today and I have nothing to report. I first off adjusted my floats and still no change. I then listened to my tail pipes and it is more on the driver side to where you can hear the drop in the tone when it does it's spit and sputtering.

I then unhooked all my vacuum accessories the pcv was undone and the hose was plugged and no change. I then went to unhook the brake booster and plugged it with my vacuum gauge. I then checked everything and the idle was still smooth then the motor will spit and sputter for a 1/4 of a second or so here and there and then go smooth again. After all the checked out fine I hooked all my stuff back up and then did them one at a time and nothing.

I would assume this would be caused by a miss fire. Driving down the highway its hard to tell if its there at higher rpms. The engine seems to run smooth accelerating and such but when your at idle in gear that is when it will spit and sputter when it wants too like it wants to quit of a thousand of a second but never does. That is the best way to describe it.

I then took and let some propane all around my intake and all of my hookups and there was no change in the idle so no vacuum leaks. I then undid my plug wires one more time and started out with 1.3.5.7 in that order and pulled them all the way off so that cylinder was not getting fire and I still would have the same exact sputter regardless of which plug wire would be unhooked.

They are all getting good spark cause trust I got shocked a couple of times. All of them came out ok on the passenger side as well. So the last thing to try was my idle mixture screws on my carburetor. They are currently set at 1.5 turns out.

If I put them in at 1.25 of a turn out it will stall in gear after a few seconds so 1.5 is my best setting and highest vacuum. Just for fun I turned them clear out at 2 turns plus to rull the idle mixture screw adjustment as the culprit and it still did the same thing with no change until it started to get to rich. So put them back to 1.5 turns.

Also I tried with the vacuum advance unhooked and just with initial timing and and curb idle readjusted for a 800 rpm idle and there was still no change.

Hooked it back up to full manifold vacuum and turned idle back down to 800 rpm and engine definitely like full manifold way better. So guys dam I am out of ideas here as everything has all went over several times with no change. The motor would seem to drop a cylinder then back up at a 1600 rpm during warm up cause the engine tone would change to low then high again. So I don't know what to do anymore.
Eric
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Old 09-18-2010, 11:54 PM
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My truck is doing the exact same thing except its a 350 TBI. It will run good then it will start doing what yours is doing... sounds like its misfiring! I think ive replaced everything Ive could for it besides the plugs cus there a pain in the *** to do because of my headers so i think that will be next.. but im interested to hear if your problem gets resolved cus i would like to know also!
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Old 09-19-2010, 08:52 AM
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One thing you may want to check is the possibility that your alternator is leaking AC voltage, which will drive an ignition module nuts. Just hook up a DMM set on AC to your battery terminals, if it shows more than about 300mv of AC, that could be your problem.
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Old 09-19-2010, 09:12 AM
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Question for Eric

Eric, I know this sounds out-there, but having worked on chev engines for 30 years or so, are you running any type of egr on this system .. or perhaps more importantly are you absolutely certain the egr remnant passages are sealed off well if you are not? Just a thought of somewhere else to go on this.

Another thought on the ignition, I assume you have checked this, but inspect the interior of the cap and under the rotor especially to make sure there is absolutely no signs of carbon-tracking. Keep in mind that by pulling a plug-wire like you indicated the coil voltage can go as high as 60,000 volts momentarily on that rotor -- if the removed wire was not grounded and rotors just can't handle that kind of voltage very long. I caused a few good rotors to burn a small hole in them and later fail prematurely just because I wasn't aware. That may not be your problem but it is worth considering.

Good luck, and I hope you hit on the problem soon. (I feel for you - intermittent problems can be a bugger-bear).
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Old 09-19-2010, 09:43 AM
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Well guys I know my cap is good as well as my rotor. The rotor is brand new and I just installed a brand distributor and used the coil and cap from my previous one and they are not even a year old and I have had this problem with the last distributor as well.

I also used a older cap and coil along with a different control module and still the same thing. I will hook up my DMM to my battery like suggested and will post my results later. Also my motor does not have any polution stuff just a pcv valve and nothing else. This is in a s10 with headers. Not running any air conditioning as well. Also while having the cap off before I did not notice any carbon tracking just on the brass terminals where the rotor makes contact.
Thanks for the tips guys will post back
Eric

Last edited by eric32; 09-19-2010 at 09:49 AM.
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Old 09-19-2010, 09:55 AM
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One thing I keep coming back to in my mind, is the chance there's a fuel distribution glitch w/the carb and intake you're using.

Do you have a 4-hole or even an open spacer to put under the carb? The EPS has small-ish runners, I'm thinking that overall, you might do better w/a RPM (w/o air gap). But that's not for now, I realize. It should idle smooth (I mean w/o dropping the cylinder momentarily, that is), unless there's a glitch in the design and combination.
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Old 09-19-2010, 10:22 AM
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I've bought a few brand new holleys right out of the box and have problems with floats sticking,and they didn't always stick all the time,only when it felt like it.
When you're having the miss in the engine,try looking directly down the carb while it's running and see if it is dumping fuel out of the overflow into the manifold..

Just another food for thought...
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