#5 & #8 not firing? need help.

[mako_22]

Ok lets see the carb, the carb looks brand new the previus owner had just replaced it. I think it a rebuilt carb. I took carb cleaner a sprayed around the base & nothing. I foged over the top and it deid back, I covered with a rag to snuff it out and it started to die back. Timeing fireing order good, cap rotor & wires new and it was doing this with old ones too. Dont know anything about carbs realy to be able to check jetting, wouldnt know what was right or wrong. OK and EGR valve does not have one, was taken off and coved with plate & I also spayed around it too to check for leaks, nothing.

 

[mako_22]

If the intake were cracked underneath along 5&8 runner would that do this? or would I have been able to tell that when i put the rag the over carb? Because when I replaced the intake gasket I checked the intake for cracks because I thought that might have been the problem sence 5&8 are on the same runner but you cant see the bottom of the intake cause theres like a heat sheild under there.

 

[drags1998]

In a word NO!!!!!! If the intake was cracked on the underside it would suck oil into the intake and foul the plugs on that cyl. Iknow this is giving you fitts. I STILL say Look at the carb. or maybe try a different one. I think you said that It ran OK off idle, If so, I am quite sure you will find it in the carb!!!!! let me know!!!!!!!!! Paul

 

[brusso]

Oh boy. Well not to beat the EGR thing to death but, under the plate that is covering the holes, is there a good gasket? If I'm not mistaken there are two holes under that plate. If there are then there should be gasket material isolating them from each other. I hope someone can verify the two hole theory for me. Man this is a good one!

 

[dman]

one hole exst other is intake ....

 

[Chas]

i had this similar problem ...

I just recently bought an 80 corvette and I’m having a problem, #5 & #8 cylinders are not firing. I changed the plugs, wires, cap, and rotor and still the same. Checked compression and vacuum and both were good. I replaced intake gasket just to rule out, still there. I am getting a good spark on those wires and I made sure timing is set at 8 before TDC. What could this be, what should I check?

it was on my kids car ,not a vette....its fuel injected and i thought it needed an injector...not the case, it was the ignition module or computor.....not sure if it applies here or not. just seems kinda the same symptoms to me.good luck pal, you will find it im sure.

 

[dman]

the 50 post rule applies for this one ... if not fixed by the 50th post... sell it !!!!!!

 

[cntrhub]

Walk up to your (never started for the day) cold car. Pop the hood. Start the car for literally 10 seconds or less and shut it off. Now go and place your fingers right at the edge of the exhaust header, near the head. You should feel the cast iron header warm at that cylinder. Now to make sure #'s 5 & 8 are firing or not, check to see if these header's are cold to the touch or warm? If they are warm like the other cylinders.... in theory, they are firing.
Low compression cylinders will cause a rough idle. Now this is awfully rare but possible. You could have good compression and good leak down with a broken connecting rod(s).

 

[dman]

Walk up to your (never started for the day) cold car. Pop the hood. Start the car for literally 10 seconds or less and shut it off. Now go and place your fingers right at the edge of the exhaust header, near the head. You should feel the cast iron header warm at that cylinder. Now to make sure #'s 5 & 8 are firing or not, check to see if these header's are cold to the touch or warm? If they are warm like the other cylinders.... in theory, they are firing.
Low compression cylinders will cause a rough idle. Now this is awfully rare but possible. You could have good compression and good leak down with a broken connecting rod(s).

????????????????????????????????????????????????????????........not!!!!!!

 

[VRGSVET]

What kind of carb is it? Does it have the right carb to intake gasket? Does the carb fit the intake or does it need an adapter? You could be getting a wierd mixture at idle because the gasket or adapter is interfering with the idle circuit. But when your primaries start to open it runs right. Don't know if this is even close, but thought it was worth a shot.

 

[mako_22]

Ok when I took intake off the two holes to the EGR were filled with something, on top of being capped off. On the ignition module what do you guys think? could the module make it consistantly missfire on 5&8? On the 50 post rule, maybe at 50 I should go with a ZZ4 replacement. with the hot manifold thing it was Kind of hard to tell on 5 being in the center of the man. & heat shield in way but 8 was cold. dont under stand the rod though. The carb is a Q jet and I spreyed around the base with carb cleaner, & nothing.

 

[brusso]

I'm at a loss. At least you are now one post closer to 50!

 

[KOPBET]

I am assuming at this point that the compression test you performed was a "dry" compression test (vs. "wet"). Yes? Not to nitpick but it could make a difference if you did a wet test only. Was the engine warmed up at the time?

 

[dman]

leak down test tells all .. dry test . test good . 155 psi @ up . no need for wet , comp for all cyls , from second comp test is ok. problem not to lack of comp

 

[KOPBET]

leak down test tells all .. dry test . test good . 155 psi @ up . no need for wet , comp for all cyls , from second comp test is ok. problem not to lack of comp

That wasn't my question. The point was whether he did a wet instead of dry. But I understand what you are saying.

 

[dman]

very good question

 

[drags1998]

Hate beat this to death,TRY ANOTHER CARB!!!!!!!!! IF the thing runs OK (Imean on all cyl) everywhere except idle, dosent common sense tell you,1. Its not ignition 2.compession . Now if you have both of these,& timing is anywhere near correct,& it runs wellup top,then It is INSIDE THE CARB!!!!!:eyerole

 

[KOPBET]

Hate beat this to death,TRY ANOTHER CARB!!!!!!!!! IF the thing runs OK (Imean on all cyl) everywhere except idle, dosent common sense tell you,1. Its not ignition 2.compession . Now if you have both of these,& timing is anywhere near correct,& it runs wellup top,then It is INSIDE THE CARB!!!!!:eyerole

Don't blow a gasket. If he has the desire and the access to another carb I'm sure he will. Until then, what's the harm in more discussion?

 

[dman]

Don't blow a gasket. If he has the desire and the access to another carb I'm sure he will. Until then, what's the harm in more discussion?[/Q UOTE].........discussion ....... good for the brain and wallet ?????

 

[mako_22]

Yes I did a dry comp. test. Dont have another carb, Im just gona have to take it to someone and let them play with the carb. Well thanks for all the help, I guess its time to spend some $$:cry$$