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Discussion Starter #1
OK check it out.68' 327" HEI, demon carb.My car started missing a couple weeks back and ive done allot to check things out. It keeps on coming back to the #5 cyl not firing. I just got done replacing its spark plug and it was ok for about a day and then came back when i started it in the morning. Ive swapped the wires around and the #5 cyl is still cold(the header) what could it be? im getting strong spark there. Could it be some sort of vacuum leak only on that cyl? like the intake gasket? what else could it be? the comprssion is good(150psi) so i dont know what else it could be.. any help is appreciated. thanks

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That's odd.

I had the same problem when I ran a points distributor. Cylinder 1 & 8 didn't fire, and when the plug was changed, it fired for about 10 mintures. I then changed to HEI, problem solved. But you already have an HEI, so I'm not sure what the problem could be, except that you may have the #7 and #5 plug wire crossed, which would cause it to misfire. Other than that, I'm not sure.

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1969 Base Camaro
Vortec 350, Perf. RPM, Demon Carb., TH-400
All sheetmetal is NOS GM
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What does the plug look like when you pull it out?
 

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A broken valve spring is what caused mine to do the same thing you are describing. Fire to the plug wire, and fire to the plug, but no fire in the cylinder.



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68 Coupe, 350 4-speed
Jim's Camaro Corner
 

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Discussion Starter #5
Would a broken Valve spring still cause the compression to be ok? cause all of my compressions were good(about 140-160)
And ive checked moving the spark plug wires around to check if there arcing to a differant spot with no luck. Ive done a slight visual inspection on all of my valve springs and they "looked" ok... do broken valve springs pass a visual test usually? or is it easy to tell that their broken? Oh the other thing is ive been gapping my plugs at about .47", is that too much? i didnt think it was too much... but hey im clueless right now, ive check SOOO much stuff on this motor and everything looks ok.... im getting frusterated


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Camaro6666,
Static compression only tells the sealing abilities of the combustion chamber, piston and valves. You need to make sure the valves are opening, cam lobe isn't wiped out ect. A dynamic compression check is what you need to do next.

to save a lot of space here just go to this URL and it will walk you through how to do a dynamic compression test and explain the science of what is going on. Hope it helps, I found a wiped out exhaust lobe on one of my cars recently using this, I mean it was gone, car ran fine, compression was great, no fault codes, but the dynamic compression test showed that that particular cylinder wasn't able to take in it's intake charge because the exhaust wasn't opening. Car ran fine at idle, just backfired badly under load. http://www.misterfixit.com/dyncompr.htm



[This message has been edited by SY1 (edited 02-12-2002).]

[This message has been edited by SY1 (edited 02-12-2002).]
 

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SY1, I was reading your post, and couldn't understand why you didn't have a backfire under load....until I got to the last line!


Sounds to me like camaro666 might just have a wiped out intake lobe if the igniton checks normal and the plug isn't coming out oil fouled???

I find that if I suspect a bad cam lobe, pulling the valve covers and watching the rocker action is much easier than running a dynamic compression test. If a lobe is round enough to cause a severe performance problem, it's usually pretty obvious that the rocker doesn't move like the rest. just my $.02

[This message has been edited by PDQUICK (edited 02-13-2002).]
 

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Discussion Starter #9
SY1 i tried to go to the link but it doesnt work for me..
The thing is guys, i looked at all of my rockers as it was running a couple days ago and they all looked cool. Maybe i overlooked #5 but geez i though they all looked about the same, movementwise. all of my plugs look pretty black and some with oil on them. Now i know what your thinking, its just my 170K motor burning the oil right? but ever since ive had the car the plugs have looked like that.. and it was busting out some high 13 sec. 1/4 times with all the oil burning and all. I just found it odd that right after i cleaned out my carb it started doing it... any suggestions?

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Discussion Starter #10
UPDATES.....UPDATES.......UPDATES
Ok i just got done replacing my #5 spark plug agan and now it runs good at idle,part throttle,3/4 throttle, But still starts to back fire at full throttle. I also took off my valve cover and checked all of my springs and rocker arm movement. They ALL look and act the same with the engine running. Now for my question... Is it normal to be able to compress the valve spring with your own fingers? If i push down pretty hard on my rocker arms i can get the valve to go down about 1/4". Is that normal? Or at least normal for a 170 motor? thanks

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PDQUICK,
Yeah I should have added that part sooner, it was when I was proof reading I realized I'd missed it and added it in. Funny thing is how quickly this came on. Car ran fine and in about 2-3 weeks time went from a very slight miss to very bad miss under load, always idled normally. My son drives the car, so I wasn't aware of the problem until it had gotten real bad. Part of the problem is I just didn't want to admit anything could be wrong with this engine, so I was willing to look at everything but the cam because I knew that it would be a lot of work on a front wheel drive GM car. I'd changed oil every 4,000 miles since car was new and now had 98,000 miles. Many of the other cam lobes had pits on them. That's a bad sign.

Camaro6666 did you try just going into www.Misterfixit.com and see if that works for you? Once in there go to engine mechanical in his auto troubleshooting and scroll around until you see the blue highlighted test "perform a dynamic compression test". If you still can't get in I'll walk you through doing one.
Dave
 

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You still haven't told us what the plug looks like when it comes out?? Have you??

Even weak valve springs should be about 50 lbs to seat. Did you have to put 50 lbs of pressure on the valve springs to open the valve? Are they all the same? A weak valve spring could cause the backfire, but shouldn't foul the plug. Maybe this engine is just due for a long needed freshen up!

SY1,

I had a simillar experience with my Mom's 1980 Monza way back when. The car only had 20K miles on it and I couldn't believe that it had a round cam lobe!! I tried everything before believing that there was a gross mechanical failure. I had to eat the labor, but I did eventually get GM to pay for the parts!!
 
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