#5 & #8 not firing? need help.

[mako_22]

Well I just could not get past the fact that 5&8 are on the same runner and soI thought it had to be something to do with intake. So, I bought an edelbrock performer intake replaced it and still there. Back to the carb. I dont know anything about carbs, but F#@$ it im gona play with it myself.

 

[cntrhub]

"Allready adjusted valves and all seemed to be moving same."

I doubt you need a carb rebuild, or a new carb, or a cleaning. Does the car start right up and idle fine (short of the misfire)? If it does, that means the idle circuits are open and not clogged. Does the car respond to throttle response throughout the rpm range? Sounds like the mains are also free of contamination.
Now back to your quote above. Maybe you went too far on the adjustments.... setting valves on 5 & 8? All you have to do is move the valve adjustment past the point where on a cold compression reading will show good compression, but when the engine warms up, the valve stays off it's seat from heat expansion and will never close. I would re-set 5 & 8 intake and exhaust valves. Try setting the valves more accurately by using the EO-IC method. Just as the Exhaust valve begins to Open = (EO), stop turning the crank and set the intake valve by loosing the rocker nut and having the push rod spin freely. Then tighten the rocker nut until there is the slightest drag by spinning the push rod. Once that happens, make a reference point with your wrench handle and turn the rocker nut one full turn and stop. Do the same adjustment with the exhaust. This time, find when the Intake begins to Close = (IC). Stop at that point and adjust the exhaust valve as described above with the intake valve.

 

[dman]

how did you test spark @ 5 & 8 wire end ??? 2 other test plugs grouned @ idle , did you see spark, crisp blue spark ,,, ???"??

 

[mako_22]

Yes Ive adjusted them twice once taking them in a full turn, and then my neibor told me just to take them in a 1/2 turn so then i tried that, same. I hadnt checked the spark at the end of the wire because the car was idleing rough when I bought it. So I changed the plugs, wires, cap & rotor still was there so I didnt think to check at end sence they were new wires I only checked at distributor. I just checked the end of wires and the spark is orangish blue.

 

[dman]

best way to valve adj is eng running .. pita .. only 4 ways to kill a cyl ,,,, no comp .. no spark . no fuel ,,or any of the 3 @ wrong time .????? this is A BItc &^%$$*^#@$%

 

[Chris Sanchez]

I am having a very similar problem with my 81. Compression good in all cylinders...145 lowest and 155 highest. Has a pronounced miss especially under load and over 2500 RPM (hardly noticable at idle or up to 2400 no load). Changed cap, rotor, wires and plugs. Checked the timing at idle (doesn't have a vacuum advance??) it was set at about twenty degrees btdc. I reset it to 6 deg. and just performs worse. It seems to have a vacuum leak at cold idle but goes away once warmed up. It also seems to be #5 that is giving me the problems. I DO have access to a scope etc. and I will be taking it to the gurus tomorrow. I will certainly post the results here tomorrow night in hopes that they will help us both! BTW This forum is the best thing that could happen to a new vette owner!

CS

 

[dman]

pull egr vac hose . see if miss goes away ..sounds like plug or wire brakeing down under load ... plugs are new..wires new cap new ..???? most pos egr kicking in... that is prop rpm for egr op... is this prob under norm driving....or brakes on @ stop loading eng.. if @ stop loaded..this is norm ......

 

[grapeknutz]

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?

Check the valve springs! I knew of another vette that did the same thing, and it turned out to be the a valve spring was broke!

 

[Chris Sanchez]

Well I pulled the valve covers off and the problem became apparent. The intake valve in #4 barely moves off its seat when the engine is cranked over. Checked all the appropriate issues (valve adj. etc.) and the verdict is a flat cam. several of the cam lobes are not lifting completely with #4 being the worst....soo...new cam and I think I'm gonna just go ahead and pull the engine and do rings and seals while I'm at it. FYI I have learned that you can still get a deceptively good comp. reading if you have to roll the engine too many times. The compression should come up on the guage within a couple of turns. If it "pumps up" more slowly..you may have a valve that is not opening far enough. This is also why the car ran fairly well at idle when not much fuel was needed. Good luck with yours MAKO_22. I've gotta go find an engine hoist.

 

[dman]

cam that flat and no rocker noise ????????????

 

[grapeknutz]

cam that flat and no rocker noise ????????????

I replaced alot of cams in the dealer in the 80s because the cam was bad and yes no rocker noise just popping thru the carb!

 

[dman]

poping up carb if exst valve is not opening ..... not if intake not opening info for this states # 4 intake not opening chris..pos meant exst

 

[Chris Sanchez]

cam that flat and no rocker noise ????????????

Well...I think that what happened is that the last (expletive deleted) person who had the car tried to "fix" the problem by adjusting the valves tighter.:eyerole There WAS a tiny bit of clatter but not nearly enough to send the red flag. I'm just grateful that my mechanic buddies got me going in the right direction. There is nothing worse than chasing a problem the wrong way and I would have been looking for an ignition problem forever. It was the normal seeming comp. test that threw me off. Live and Learn.BTW you're right dman exst valve.

 

[mako_22]

havent been able to get on in a couple of days, just found something though. I took the valve covers off to look at springs, everything looked good... Not sure wether this would have proved anything or not, but back to 8&5 on the same runner on intake. I thought maybe if there were a crack in the head some how, that would leak across from one cylinder to the other that it could cause this. So I took out #8 intake push rod and ran motor taking #5 wire off and on to see if I got change in RPM, nothing. Then the oposit took out #5 rod & checked 8 wire, nothing. But when I walked around the back of the car there was a light spray of water on floor from exaust. By taking out intake rod it made like a pump and sucked water from what I guess must be a crack in the head. Where normally their would not be enough vacuum to pull enough water to make it out of the exaust to tell.... Does this sound right??? I guess I have a cracked head.

 

[grapeknutz]

Check and see if there is bubbling in the surge tank if there is that head will have to come off to see what going on.

 

[mako_22]

Just checked it, let it run for while and revved it up too no bubbling...????

 

[mako_22]

I wouldnt have thought that would have been condensation after driving the car and then running it a while when I was playing with the valves, but maybe with those two cylinders not fireing I guess it just built up condensation.??

 

[drags1998]

HEY MAKO 22 Ireally hope you dont think I am being A D--K, I have worked on an AWFUL LOT OF SMALLBLOCKS!!! and EVEN A FEW BIGBLOCKS, Im not sure what to tell you about the skip or misfire.I have been reading all of the posts on this one and you seem to have checked ALL the normal stuff, I suspect the moisture you saw at the rear of the car was just NORMAL.A lot has been said about the EGR valve, I though I read that it was removed earlier, if this is not so PLEASE email me and let me know. Lets see if we can solve this before the 65 post rule .Paul:confused

 

[bowty]

could be burnt exhaust valves if it is backfiring on acelleration,camshaft also loose timing chain, put a timing light on check to see if it jumps arround

 

[Chris Sanchez]

Hey Folks,

Got the cam out of my 81 and not only was #4 intake Completely wasted but at least three other lobes were all but totally gone! I reiterate there was no noticable rocker noise and the car ran fairly smoothly at idle. I also was getting up to 155 psi. and less than 10% variance in compression. I strongly recommend that you check the actual amount of lift that you are getting on all rockers. Hope you get it figured out soon MAKO.
Chris