KRC 65VETTE

Had respectable machine shop (65 years racing build history) rebuild my 60K numbers matching C2 L76 327 4 speed for the purpose of changing to Hydraulic roller and lowing compression to allow the low octane fuel of today. Shop complimented tear down stating engine was super clear and very little wear and I commented it only had 60K. The rebuild consisted of the normal process with the following:
.030 bored out
New 0303 TRW pistons and tops shaved to 10:1 down from stock 11:1
New rods for pistons
Comp Cam Thumper 12-601-8 the original 3030 solid cam removed
Cam Button 200
Timing chain C-3023 Cloyes
853-16 lifters
New rocker arms 181-1049
Pushrods 7635
Heads worked with seats, guides and valves int 10650 Exh 10649
oil pump m-55
Holley 2818 carb rebuilt (ran fine prior rebuild)
Distributor stock with points and tach drive (bearings replaced, tach gear, points, condenser, rotor, caps and drive gear and dimple aligned correctly)
Dyno completed and engine only run a short time to get readings 352.8 HP and 321.5 torque at 6300 rpm

Engine installed and noticed a small miss in #1 which RPM did not drop as much when wired pulled. Thermal gun showed #1 was running 120 degrees cooler than other seven, Vacuum also appears lower but steady at 10-11 at idle. Wires changes twice plugs changed three times using new AC and NGK equivalents twice, cap replaced twice, rotor and points twice (dwell set a 30 and timing 12 degree at idle and 34 degrees at 2500 -2700 rpm Distributor also changes twice. Carburetor changed three times with new Holley 4160 on it now with new base gaskets using carb base manifold vacuum for advance as 2818 does. Stock high rise Edelbrock tested for gasket leak at heads and tested negative. Machine shop finally pulled manifold and said appeared to have small leak at number 1 and milled to provide even surface (no issues prior to rebuild and ran great). Compression on all cylinders is consistent 160-165 psi. Coil and firewall ballast changed twice.

After all this engine still has miss at all speeds and more noticeable at lower gears and in forth with 3.70 rear end still there but not as bad with lower gears. Engine temp stays at 180 F. Engine does not hold in idle under 1000 RPM and appears to run rich when all settings are set. Valve lash per factory settings with rocker arm no play and additional half a turn which has been tried plus and minus quarter turns. No water in oil and no metal in filter. Oil and water readings normal.

Sorry for long post and open to all thoughts and suggestions since I and machine shop techs are coming to the end of our rope. Last suggestion was possible valve seat leak from machine work on #1, possible small crack in #1 cylinder from bore and last one is defective can grind for #1 on new Comp Cam that has happened to person with suggestion.

I am baffled and running low of ideas what is causing this miss and lower cylinder temperatures in #1 cylinder only.

Any help or input greatly appreciated.

ukester

do a leak down test you have a problem-could be a vacuum leak on that cylinder.

larebob

Cranking speed (your compression test) isn't running speed. Since the heads were just done I'd look to a guide problem, which may seal the valves at ~200 rpm, but has them hanging up at running speed. You're doing so much, maybe you could pull the valve springs from #1 and wiggle test them. I understand the Chevy dealers were told to do this to LS motors which had guide issues.

road pilot

I always start with the simplie things first than go deeper into a problem.
You might need to run a hotter plug. I agree with all the guys on other ways to find the problem.
Good luck

thoyer

If you know of anyone that has one of the older Sun or Snap On engine analyzers, you can easily see if you have an ignition problem on #1. I have one of the Snap On ones and while it sees little use anymore it is invaluable in situations like this.

Leak down test will help.

Valve adjustment?
Screw in rocker studs - if so has #1 pulled one out?

Interesting issue...........

Tom

Tampa Jerry

I know it's a roller cam, do you have a dial indicator? Put it on #1 and check the lift. It may be a bad cam lobe or lifter. Good luck. Jerry

427Hotrod

What RPM do you notice the miss? That cam is pretty healthy for a mild 327 and has a tight LSA of 107*. By design that cam has some long slow lobes with a lot of advertised duration. It's going to be rougher and have more random misfires at low speed light throttle. Is it a dead miss...or one that seems to "skip"?

Cams like that typically like a LOT more initial timing...like up in the 20's with the total limited vs only 12* and bringing it all in fast. How much does vacuum advance add and when?

JIM

larrywalk

The thermal reading on #1 of 120* cooler than the others is cause for concern. I'd do a leak-down check. If that's good, do an intake valve lift check at the tip of the pushrod with the rocker disconnected. If that lobe lift is in spec, do a valve lift check at the valve retainer.

Boyan

So the reputable shop took your perfectly running engine with very little wear and generally good condition inside and COMPLETELY removed everything inside that made it an L76 and now doesn’t run correctly……??!!

The L76 will run fine with today’s gas, but the damage is already done so I will try to help with your current issue.

Pull the plug from the dead cylinder. Does it look like it may have coolant on it compared to the others? Does it smell sweet as compared to the others? Buy a cheap borescope….or borrow one…does the cylinder look washed with coolant? If the reputable shop replaced the valve seats there is a high probability they hit a water jacket and you are getting coolant into the cylinder.

This condition will still give good compression but leak when valve is open. Set the cylinder so that the exhaust valve is off its seat, pressurize the cooling system and see if the cooling system leaks down. If it does, you will see the leak in the cylinder..probably coolant dripping down valve.

larrywalk

So, what did you find?

KRC 65VETTE

Found two bad roller lifters that machine shop is finding a lot in their new lifter installs

Factoid

Both in cylinder 1?

Glad to hear you you got it fixed. How does it run now?