Heatingman 971 #1 Posted October 10, 2022 This motor is on a cub original. Trying to get this thing to run. finally got it to crank, had spark, fuel was gelled up. Put in a spare carb, and new fuel line. Interestingly there is no fuel pump, but tank is above the motor. Motor would sputter but only with starter fluid. decided to check compression. There is none. And the crank turns very easily. When I open it up, what should I be looking for? Share this post Link to post Share on other sites
Snoopy11 5,718 #2 Posted October 10, 2022 (edited) When you turn the engine over by hand, do you see/feel valve movement? Check to ensure valves are sealing/seating... stuck valve most common culprit of loss of compression. Leegit worn cylinder wall/rings could be another culprit... and you would know if that is an issue as soon as you peer down inside... Don Edited October 10, 2022 by Snoopy11 Share this post Link to post Share on other sites
Heatingman 971 #3 Posted October 10, 2022 I could see the intake valve moving through the carb. Exhaust still connected. So cant see that. I can feel suction through the carb. So the piston has to be moving. Share this post Link to post Share on other sites
Snoopy11 5,718 #4 Posted October 10, 2022 Just now, Heatingman said: I can feel suction through the carb. So the piston has to be moving. Just as you can feel suction through the carb, you should be able to feel the exhaust pushing that air out... Don 1 Share this post Link to post Share on other sites
Snoopy11 5,718 #5 Posted October 10, 2022 Head gasket would be another part to check/replace. If the engine ran at all, I could address diagnostics for cylinder head leaks other than visual. Since it doesn't run, you are just going to have to go over the cylinder head with a fine tooth comb... check the gasket, check for cracking... etc... Don 1 Share this post Link to post Share on other sites
squonk 43,770 #6 Posted October 10, 2022 Exhaust valve would stick sooner than the intake. Valve face could also be shot. Pull the head. 3 Share this post Link to post Share on other sites
John2189 541 #7 Posted October 10, 2022 I have had valves not closing. I had to remove the valve and grind some off the stem 1 Share this post Link to post Share on other sites
Snoopy11 5,718 #8 Posted October 10, 2022 59 minutes ago, squonk said: Exhaust valve would stick sooner than the intake Precisely... And that would be why it would pop with starter fluid but not run... Ya' haffa' pool' da' hayad awff... AND replace the head gasket when you do!!! Don Share this post Link to post Share on other sites
Snoopy11 5,718 #9 Posted October 10, 2022 10 minutes ago, John2189 said: I have had valves not closing. I had to remove the valve and grind some off the stem I saw one guy a long time ago do a valve job with wd40 and a hammer... seriously! I wish I could find the video... Sprayed some wd40 around the stem... and hit the valve closed with a hammer... I couldn't believe it when I saw him do it... but the engine did run after he did that... I'm not sure if it was the intake or exhaust valve that he did it on... I can't remember... Don Share this post Link to post Share on other sites
rmaynard 16,292 #10 Posted October 10, 2022 (edited) You would have to have some compression to get a sputter. And remember that most K181's have ACR. So you won't be able to use a compression gauge to test. Edited October 10, 2022 by rmaynard 1 Share this post Link to post Share on other sites
squonk 43,770 #11 Posted October 10, 2022 If the engine is original to the Original it will most likely NOT have ACR. Early small blocks had spark advance cams. 1 Share this post Link to post Share on other sites
Heatingman 971 #12 Posted October 10, 2022 Does this look like blow by around the exhaust port? Share this post Link to post Share on other sites
moe1965 758 #13 Posted October 10, 2022 You can remove the breather cover to check if your valves are sticking. Share this post Link to post Share on other sites
Heatingman 971 #14 Posted October 10, 2022 The valves both move up and down. And when in the down position, they will hold a paper towel in place on all sides. Again may some wear but not enough to keep from running at all Id think. Cylinder walls look OK, but I’ll have to measure for out of round. It looks well enough it should at least run. I’ll order a gasket kit and see if she’ll go. Share this post Link to post Share on other sites
squonk 43,770 #15 Posted October 10, 2022 Look at the piston edge closest to the exhaust valve. Top edge is eroded away. That engine was rode hard and put away wet. Example; Share this post Link to post Share on other sites
Heatingman 971 #16 Posted October 11, 2022 Cylinder bore is beyond wear limit at some points. Per the manual : original bore was : 2.938 max wear limit : 2.941 max out of round : .003 max taper : .003 Here is what I have, measurements taken near the top, the center, and near the bottom, in 3 indexes around the circle (exhaust, intake, middl) is what I am calling the index locations. Int Exh Mid top. 2.942 2.941 2.943 Center. 2.940 2.942 2.940 bottom 2.940 2.943 2.941 Is the motor just beyond useful life. And I need to scrap it, or machine the bore? Think it will run with a new gasket? I just want it to run. It has no useful implements, so its not like its gonna be used for work anymore. Share this post Link to post Share on other sites
Heatingman 971 #17 Posted October 11, 2022 Could I just get a .003 over bore piston, hone it and call it good? 1 Share this post Link to post Share on other sites
Snoopy11 5,718 #18 Posted October 11, 2022 5 minutes ago, Heatingman said: Could I just get a .003 over bore piston, hone it and call it good? Only if it is completely round... which you really can't get without a machine job... If you really wanted to ghetto-rig it... you could throw some new rings in it... but nobody can make promises on how far your mileage will be... Does the piston have slop (i.e. side to side movement)? Don Share this post Link to post Share on other sites
Heatingman 971 #19 Posted October 11, 2022 (edited) Piston does have side to side movement. The rings do not. At least not when I move the piston. But only side to side when pushing to and from the port side. Edited October 11, 2022 by Heatingman Share this post Link to post Share on other sites
Heatingman 971 #21 Posted October 11, 2022 Intake valve also not closing tight. I can spin it easily in the down position. Exhaust port I cant. Found there is no tappet clearance. Should be .006. Would that all be cause by wear of the valve seat? Share this post Link to post Share on other sites
953 nut 59,885 #22 Posted October 11, 2022 10 hours ago, Heatingman said: Could I just get a .003 over bore piston, hone it and call it good? Share this post Link to post Share on other sites
squonk 43,770 #23 Posted October 11, 2022 (edited) Up to you what you do with it. But that engine is worn out. Honing and valve adjustment will probably get it running but it's going to smoke, maybe rattle and not last very long. Bore it and new valves and guides is the best course if you really want to save it. And what's the reason for no valve lash? Those valves don't grow longer by eating their Wheaties. The valve is probably knackered. Edited October 11, 2022 by squonk 2 Share this post Link to post Share on other sites
John2189 541 #24 Posted October 11, 2022 And what's the reason for no valve lash? Those valves don't grow longer by eating their Wheaties. The valve is probably knackered the valves do not grow longer, the valve lip and seat wears which will lessen the valve lash. If they are not burnt, you could remove the valve and grind some off of the stem to correct the lash 1 Share this post Link to post Share on other sites
Heatingman 971 #25 Posted October 11, 2022 Just found a machine shop nearby that will bore the block. Guy said about 70 bucks. Think I will give that a go. Gonna tear it down the rest of the way this afternoon and make sure nothing else is totally jacked with it. 2 Share this post Link to post Share on other sites
