Hey everyone, hopefully someone can steer me in the right direction. I have a 72 comet with a 250/auto. It has been sitting for quite a few years and after some new fuel, plugs, wires ,cap, rotor, carb rebuild, etc. got her running. Not running well, but running. Upon further inspection, cylinders 3 and 4 had no compression. The rest of the cylinders 1,2,5 and 6 were between 145-155. Anyway, pulled the valve cover and had a few bent rods. I replaced the lifters and rods, and tore down the valve train, carefully inspecting and cleaning each piece. The head was reassembled in the exact orientation as original, each piece in its respective original place. Now I cleaned a ton of varnish out of the intake ports and a ton of barnacles out of the exhaust. Night and day difference. So when I got everything back together, now she wont fire. Has spark, at least in the first cyl I checked. Now it has 65psi of compression across the board. All 6 cylinders. Any ideas? Any help would be greatly appreciated. Thanks!!! Ron
Did you use any solvent/cleaning agents in the cyl bores while the heads were off. IE: cleaned the carbon off the tops of the pistons?? Did you at least lap the valves when you reinstalled em. Cleaning all the carbon off the valves/seats will kill whatever minor seal the carbon was creating in situations where the valves/seats are really bad. The margins (sealing surface on the valves) looked pretty beat up/pitted on the two valves you took pics of. My guess is the seats are not much better. Are the valves closing fully...You said you replaced the pushrods/lifters. Are the pushrods the same length as the originals??? New head gasket right???Gotta ask... Have you tried a little oil in the cyls and re-test?? Cleaning in the bores will wash all the oil off the cyl bores and cause low compression on a tired engine till the oil gets back up there. If you have spark/fuel...Lack of compression will be the only thing keeping the engine from running.Pull the coil wire off the cap, shoot some oil in the bores and crank the engine over a few times. Then put the coil wire back on,try to start it and see what you get...
... love the patina on that work bench... can you see all the rocker arms moving when cranking it over?
I take it you changed the points. I would concentrate on the points to make sure the dwell is correct. In years gone by I often had trouble getting the points correct on the first attempt. MD
Well today I worked on it a little more. Yes the valves were lapped. Yes it is a new head gasket. Oil in the cylinder did not boost the compression. The push rods appear to be the same length. The valves look like they make a decent seal. I didn't pour any gas or anything down the intake to see if they leak. I wouln't imagine the timing would have jumped. As far as replacing/cleaning/gapping points and condenser, I have not. I did the rest of the ignition system, but like I said it was running on 4 cylinders. And I drove it up and down the road a little bit but it was missing. When I got to the problem, cleaned it up and got it back together I have low compression and no fire.
Am I not tightening the rocker rail correctly? I am starting at 1 TDC and tightening until I cant turn the pushrods with my fingers. I know it says to go one full turn after that, but it seems like a lot of force. So I have been tightening them with reasonable force. Then I go in the firing order @ tdc and repeat.
Well if you can find the fire it should at least start, won't likely have much Hp... The old 3½Hp B&S lawnmower ran on approx that compression... BTW just tighten the rockers till slop in gone, worry about tightening then farther later...
Those rockers are on a shaft...Tighten the pedestal bolts down from front of engine to rear of engine. Torque bolts to 30 to 35 ft/lbs and your done. There is no valve train adjustment other than longer/shorter pushrods as needed. Unless you have a solid lifter cam then you have adjusting nuts/ball end studs on pushrod end of each rocker...You don't have these...Not tightening down the rocker all the way can cause you to have low compression as well. IE: Intake valves not opening fully...
Ok, just got home from work and was reading about the straight six. Apparently the if you use a new gasket, you need to shave .025" off of the head. Apparently I am in need of an old style steel shim head gasket. Any ideas of where to find one?
The gasket won't make the difference you have, loose maybe half point of compression at most and the valve train should be able to make up the difference... Yes I've used composition gaskets for shim several times...
