Technical Chevy 6 with misfire at low speeds

I'm working on a stock Chevy 250 in a 64 truck. The engine runs rough at idle but smooths out at higher speeds. Someone else spent time chasing the problem and replaced the intake/ exhaust gaskets and put in new plugs, points ,wires and what looks like a factory rebuilt carb....There are no vacuum leaks...Only vacuum items are PCV and distributor vacuum, both checked out no leaks
I checked the compression , it's 115-125 psi on a warm engine, good enough to fire all cylinders. The dead cylinders at idle are 2 and 5 confirmed by shorting out plug wires .And all plugs do fire....
With the engine warmed up at a slow idle I adjusted the hydraulic lifters....Something unusual was going on..As I back off the rocker nut and the lifter started to clack, the engine seemed to run better.. Then slowly tightening the rocker just to the point when the clack stops and no further,the engine started to misfire again. it did not clear up in a few minutes..Note that the head was very clean and all rockers were moving up and down ,push rods spinning and lots of oil flowing....The heavy layer of crud on the outside of the engine says this motor has not been disassembled in a long time if at all.
It appears that a few lifters might be pumping up at idle and holding the valves off the seat???..Could be bad lifters or weak springs ???
Has anyone else has a problem like this with hydraulic lifters?

 

All the cylinders have good compression?

Have you tried adjusting all the valves, with the engine not running? get each cylinder in firing position, back off the rockers so there's play, then tighten just enough to remove all play, then tighten an additional half turn to preload the lifter.

 

Yes, I adjusted them that way the first time trying to avoid the oily mess. Another guy said he came on a problem like this on these engines used in fleet service because a valve spring would go soft causing the lift to pump up at idle...
maybe I need to check the springs....

 

If a spring is broken, then it probably would have little compression in that cylinder. And from what I understand about valve springs, when they are weak, it results in loss of power at higher rpm, not rough running at idle.

 

Throw a vacuum gauge on it and check the readings at idle. If the valve springs were bad, the needle will bounce back and forth. If the reading is steady, it's not the valve springs.

 

You could have worn valve guides.

 

Bob, nice addition. This test (although scorned by service reps that have married themselves to the scanners) is most conclusive when trouble shooting.
My late friend Rudy (had the shop across the tracks from my shop) gave me a 9" diameter vacuum gauge that I hang from the hood or windshield frame and can SEE!
Old eyes...
I draw it like a gun.

 

Collapsed or stuck lifters
Dave

 

I just did a vacuum test....Engine at operating temperature, timing 4 degrees at 500 rpm idle. At idle the gauge reads a bit low at 15 inches and pretty steady......Rev the engine and the vacuum drops ,then as the engine slows back to idle the needle jumps to 22 and then gauge needle fluctuates between 15-17 inches and remains there for a few seconds at normal idle before settling back to a steady 15 inches.......I can hear the steady cylinder misfire at idle that disappears when the engine is reved up...

 

Pull the side cover off and remove the lifters that are causing the problem. See if they are worn down.

 

I also went through shorting the plugs again,one at a time. at idle #5 makes no difference. I checked the compression again on #5 ,105 psi on the third stroke of the gauge needle. Low but should be enough to fire the cylinder on a low performance engine.
I checked the vacuum again and when the engine is reved rapidly, the vacuum drops to about 5 inches, then 22 inches as the engine returns to idle...According to the vacuum info it means the rings are ok...
I can can and feel the cylinder misfire at idle and low speeds...But rev the engine and it's got the smooth sound and feel of all 6 cylinders.... Maybe ay higher RPM's there's less time for the compression to get lost...

 

Did you watch the rocker arms to make sure they're all moving all the way? A worn cam, as suggested, will cause problems like this. Usually a stuck or collapsed lifter will make some clattering sounds.

 

I'm going to throw something out here that may be completely irrelevant. All the high mileage chevy sixes I have owned in the past have had distributor wear that made the static points setting pretty useless. Worn shaft.

 

Good thinking! 2 and 5 being across from each other on the cap tells me it's possibly a worn drive gear or bushing. Maybe even a worn breaker plate. Throw a dwell meter on it, see if it's jumping around at idle, off idle and just passed when the advance is all in. Let it idle down and see if it settles back to the same dwell. Something the look into. If those items looked ok, I'd pull the lifters on the offending cylinders. See if they are cupped. I wouldn't bother replacing just one.
Good luck, keep is posted.
Tim

 

What he said!!! At some point, to save a few pennies, GM decided that instead of putting a bushing at both the top and bottom of the distributor housing, they'd just use a single, slightly longer bushing in the middle. Didn't take long for the single bushing to start wearing in an hourglass shape. Timing and dwell would go all wonky and it would want to die at idle. Might contact GMCBubba and see if he offers a re-bushed housing. Doubt you can get a new one from GM any longer and it might still just have a single bushing.

 

If distributor is suspect, the later model HEI self contained work really good and will eliminate a worn shaft problem

 

Yes, I watched while the engine was running with the valve cover off and adjusting the valves... Rockers were all cycling the same and all the push rods were spinning...I was talking more like a pumped up lifter holding the valve off it's seat....
But that or any valve issue should have showed up as a bouncing needle on the vacuum gauge..

The last few comments on distributor shaft wear got my attention....The shaft does have some play...perhaps the points cam doesn't open the points properly for the #5 hole.....During testing I slowly pulled the #5 wire off the cap and it has a snappy blue spark.. I don't have a dwell meter but there's other ways to check and I will do that...

 

The symptoms are the same as I ran into (caused) many years ago on a 216.

It sounds like 2 & 5 are trying to fire dead (no fuel) holes. 2&5 are 180° apart on the cam as they are on the distributor. Have you checked the plug wire position on the cap or are you using what the other guy put in???

Just a possibility...

 

I'd try new plugs and wires. Not Champions! Or just swap the plugs and wires around to different holes, see if the miss moves with them.

 

This morning I pulled the valve cover and went through the valves without the engine running... All the rockers do open and close the same distance...My previous valve adjustment was ok...
I checked the distributor closely. The shaft play isn't too bad ...
I started the engine ,warmed it up and drizzled water into the carb at a fast idle...
I substituted a copper core wire on the number 5 cylinder......No difference.
Although shorting the # 5 plug seems to make no difference at idle it does change the exhaust sound slightly ...
It's just tired engine and 105 PSI compression is just too low at idle to fire the cylinder I suppose.. The truck is used to haul whatever. It's also one of them godawlful things with a front drive axle that we ain't suppose to talk about here. And it's not a daily driver...Just going to drive it and during the winter the engine will be pulled for repairs...

 

105 is lots to fire at idle. Some engines never make that much on any cylinder!

 

My 283 had 55 lbs compression. Despite the nay-Sayers it ran suprisingly well not great, but well. Went through a quart every 30 miles or so though. 105 lbs is a darn healthy engine.

 

I understand 105 psi should fire at idle no problem...It just doesn't with this engine... And this may be another clue....When the engine is fired up on a cold start is runs like shit no matter how the hand choke is used.The driver thinks the engine is lean so pulls out the choke gradually and nothing happens until it's out all the way and the motor starts to choke literally..Then after a few minutes it settles down and actually runs pretty decent for 5.5 cylinders. Yeah maybe the carb is off......But when does a carb issue cause one cylinder to drop out? The idle mixture screw does seems to work properly.. Could the carb be very lean on the idle jet and the #5 cylinder is too lean fire? Number 5 and 6 share an intake runner, how come 6 is ok????

 

Do you have the intake heated?

 

Yes, the heat riser works and it gets hot.
Makes it more of a mystery because all the stuff needed to make it run better is in place....

 

Just out of curiosity what do you have for exhaust. I have seen engines that run like crap on open exhaust. When a good muffler with a little back pressure they run great. If you have a 6' head pipe through an open glasspack it may not be creating enough back pressure.

 

The engines I have seen were both straight 4's and straight 6's. Higher RPM's smoothed them out.

 

Stock exhaust with a shorter tail pipe.

 

A friend of mine had a similar problem but at high rpm. Low rpm was perfect. It acted like it was starving for gas. He found the condenser bracket screw was loose and it wasn't grounding properly. But , he has a perfect fuel system now !