hi we've just started up our fully reconditioned 394 and have 6@8 cylinder not firing, they have spark ,we have used 10/30 oil in engine.its been suggested that the hydrolic lifters may be the problem and we may need to chim up the rocker mounting blocks to compensate for the heads being machined? before I start taking things off the engine has anyone had this happen to them,thanks brad

 

How do you know that those two cylinders are not firing?

What do the plugs look like? Wet? Sooty? Dry?

Have you confirmed a good, bright, blue-white spark at those spark plugs?

Have you removed the valve cover to confirm that the valves are opening?

- Eric

 

hi thanks for reply plugs are wet when we pull them out after running the motor ,they through a nice blue spark .the motors running ruff to see if every cylinder was operating we pulled plug leads off each cylinder one at a time while ildling, there is no change in engine speed when either 6 or 8 are removed.i'll get valve covers off and see what weve got or notgot happening thanks brad

 

Triple check the firing order. Are the plugs wet with oil, fuel or coolant?

 

I agree that wet plugs plus a good spark would seem to point to bad timing (misplaced plug wires) before anything else.

If you get no results from rechecking the order, and maybe swapping the plug wires around, then I'd do a compression test next.

- Eric

 

It could also be bad plugs. I would eliminate everything prior to pulling valve covers.

 

hi thanks have tried new plugs still no fire on 5&8 cylinder but on closer inspection the plugs are dry not wet as i first thought.will compression test next thanks for your replys brad

 

hi have done a compression test all cylinders they are arround 170 .we have run the motor with 5&8 plugs in but pulled leads off to check spark while running and definatly have spark. a question i cant answer is if we have compression ,spark and fuel why arn't 5&8 cylinders firing?

 

Are you sure that you have fuel?

You said that those plugs are dry.

With fuel and a poor spark, they should be sooty.

With fuel and no spark they should be wet.

- Eric

 

Was the firing order checked? It is different for the earlier Olds engines such as a 394... it should be 1-8-7-3-6-5-4-2. Can someone please confirm that this is the correct sequence? I'm guessing it is on the intake but I don't know for certain on a 394. Thanks

Good luck!!!

 

hi firing order is correct to manifold as above thanks .so when i pull the plugs they are not sooty or wet ,infact it looks as if they,ve just come out of the box,so it looks like fuel? but why only cylinders 5&8 ,which both look to me feed from the same side of the intake maniflod,we have put a edelbroke(excuse spelling)carberator on and used a 4 hole gasket as origonally on motor could this the problem?
thanks brad

 

At this point, I really don't know.

Was this car sitting for a long time? Any chance of an obstruction in a manifold runner, like a wasp or mouse nest?

If you have a running engine with a cylinder not firing at all, the non-firing plug should look as though there's been fuel on it.

Do the plugs from the other cylinders fed by that side of the carburetor look any different from the others?

- Eric

 

I'm stumped also after reading this thread. Originally you said it was 6 and 8.

 

X2 on possible intake blockage. Please confirm which cylinders. If you have access to a borescope, look in the intake all the way to the valve. Another method would be to put compressed air in the cylinder with the intake valve open to see if air flows out of the intake on those cylinders. Then open the exhaust valves to see if air flows out of the tailpipe on those cylinders; although, if the exhaust was blocked the spark plugs would probably be sooty.

 

Have you tried pulling plug 5 and 8, dump a little fuel down the holes, replace plugs and see if you pick up those two cylinders even for just a few seconds.It would tell you if you truly have a fuel problem.... Tedd

 

hi ,confirm 5&8 cylinders,we pulled intake manifold and there are no blockages,lifters and valves seam to be all working as they should ,the other plugs from that side of carbureter have a slight brown colour, to me they look like there burning nice. will reassemble and try putting a little fuel in cylinders 5&8 and see if they fire, thanks brad

 

do those two cylinders share the same intake runner? if so maybe there is an open manifold vacuum port/fitting on that runner causing an extreme lean condition on those two cylinders. just throwing that out there, not super familiar with 394s

 

It's difficult to confuse cylinders 5 and 6 on that engine. They are on opposite banks of the engine and the cylinder numbers are cast into the intake manifold. I'm going to assume that 5 & 8 are the problem cylinders. While defects are unusual, don't assume that new plugs are "good". I have seen manufacturing errors that make it to sales. Performance test all components to assure that they are "good". You can check them in a plug tester/cleaner or swap them with a known good component to see if the problem moves. Also #5 and #8 are opposite each other on the distributor cap. BE SURE that wires are arranged in the proper firing order, and that there is no error with the cap or the routing of the wires. Your compression test and manifold inspection indicates that the engine is sound. Recheck things CAREFULLY!

 

hi ,yes the 2 cylinders do share the same intake runner i'll check for any leaks thanks brad

 

Did this problem exist before the engine reconditioning? What was done during the reconditioning? Was the camshaft replaced?

 

hi motor never run when we got it.it was in bad shape .we had cam reground not replaced, pistons bearings lifters all new though,havent found any vacume leaks while running ,thanks brad

 

Was the crankshaft reground or was the camshaft reground or both? Trying to find out if the camshaft was custom made. There is a very remote chance the camshaft could be ground incorrectly/out-of-sequence. The opening and closing of the valves must follow the order of the ignition firing order. You could check by pulling the valve covers, putting number one at TDC on the compression stroke, turn the engine and note the order in which the valves open and close.

The problem is much more likely fuel or vacuum related. A camshaft being ground incorrectly is very unlikely but...

Good luck!!!

 

hi yes both where reground ,camshaft was ground for street ,a custom grind not the standard 394,could this be a problem? .having good compression i presumed cam is ok but i will lift valve covers to see if it follows ignition firing order. thanks brad

 

Yes it could be a camshaft related problem if the sequence in which the valves open and close is not the same as the ignition firing order. Determine the correct rotation of the engine, then starting with cylinder one on the compression stroke, rotate the engine and record the order in which the valves open and close. Next compare the valve sequence to the firing order stamped on the intake.

If you have the old camshaft it may help to make a comparison.

If the valve sequence and firing order differ, depending on how they differ, you MAY be able to use the valve sequence for the ignition order. This is done intentionally to some engines to alter performance.

 

hi just a thought, if i've installed camshaft and put timing chain on 1 tooth advanced would engine run but create enough back pressure in intake manifold to cause 5&8 not to fire?just a thought i had while reading our manual .camshaft we have in the car is the original that has been reground,thanks brad

 

About running one tooth cam advance causing the problem, probably not as it would likely affect other cylinders in a similar way. However, I do not have experience with changing the cam advance/retard while others members probably do have experience with that technique.

If the camshaft is the one that was in the motor then it is unlikely that the valve sequence changed; unless, it was modified already. Although quite unlikely that it is modified, the valve opening/closing sequence is not too difficult to check.

Good luck and keep us posted!!!

 

gs72 mentioned this...are there any vacuum ports near that intake runner for example a brake booster connection? Try plugging every vacuum connection.

Were the heads milled? If yes, this can cause the intake not to seat well. Do you have the old intake gaskets from when you removed the intake to check for blockage? Check to see if the gaskets made a good crush/seal particularly at that intake runner.

With the engine running can you feel a vacuum over the breather port?

 

hi, we think we've found the problem?have taken out rockers to check #8 and found the exhaust valve stem is lower by aprox 1/16-1/8" have yet to comfirm measurement.reconditioners may have replaced valve with another of there shelf that is shorter than the origonal ,work done 4&1/2 years ago, have yet to talk with them.thinking #5 will be the same and that valves are not opening correctly , due to the shorter valve stems,instead of taking heads of to replace valves we replace push rods in #5&#8 with 1/16" longer ones to compensate for the difference in height or would this create another problem I'm unawear of,
thanks to everone that has repied to this post, brad

 

Doubt if a 1/16 to 1/8" difference in valve length would cause those cylinders not to fire as those valves would still open, just not quite as far.

 

So, are we correct in assuming that the engine has never been fired in 4 1/2 years? If it has run during that time frame it would seem the valve in question did not stop it from firing?

 

hi yes engine not fired until 2 weeks ago, reconditioned 4&1/2 years ago .been working on body and paint.i'm thinking of trying a 1/6" longer pushrod to see if #8 will fire before I take drivers side rocker off thanks brad

 

As I said in post #29 the push rod being 1/16 to 1/8" too short won't cause the cylinder to not fire, it will just less efficient. Any chance the 2 lifters aren't pumping up?

 

hi thanks will recheck lifters more carefully ,thanks brad

 

hi, rechecked lifter and is definitaly opening valve ,this has got me beat ,am going to get another person to go through motor to see if he can find what I've missed ,thanks for all your help will keep you posted on what he finds ,brad

 

I don't recall whether you did a compression test.

- Eric

 

hi yes compression tested all cylinders ok @160-170 ilbs, brad

 

Have you checked to see if the rotor is pointing to #5 and #8 terminal respectively when those cylinders are at tdc on the compression stroke?

 

I wish I had a good answer for you on this one.

This is one of those situations where having the engine right in front of you, instead of having it on the other side of the interwebs, can make all the difference.

- Eric

 

hi ,yes being on the other side does make it harder. have pulled distributer out to check it again and will start the process from beginning again setting timing etc thanks for all suggestions, have a great Xmas everyone brad

 

As I keep reading this thread I keep asking myself, "Could it be that the the crank and cam shafts are not in sync?"