Compression Leak

Dave31

Dave31

Active Member
Have this motor that blows a little compression between the head and cylinder when you first go to fire it up. Once it starts it stops leaking.

I've replaced the gasket several times, tried different heads on it; same problem. Tried two gaskets, no go.

The only thing left is the jug? I've checked it as best I could, is it possible I have a warped jug? I cannot find any evidence of a warped jug or head with my straight edge?

What am I missing? Anybody run into this problem?

I am about ready to head over to the machine shop and see about resurfacing this head and jug. i don't know what else to do?
 
Dave,
Have you lapped the head and cylinder? You know, the old wax on, wax off method on a sheet of glass and sandpaper?

Take a close look at the gasket mating surface on the cylinder. I saw on, a new one, that had a tiny hairline crack. Probably a bad casting but it never saw service; went in the scrap metal can. What probably is happening is engine heat is expanding things so that they seal up when hot but leak when cold.

Let us know what you find.

Tom
 
Yeah Tom, I waxed on and off until I was waxed out. I've never had a head and cylinder I could not get to seal up?

Was spending too much time pulling it on and off I gave up and stabbed a spare jug I had in the junk pile.
 
Dave31 said:
Yeah Tom, I waxed on and off until I was waxed out. I've never had a head and cylinder I could not get to seal up?

Was spending too much time pulling it on and off I gave up and stabbed a spare jug I had in the junk pile.
Click to expand...

try some red anaerobic sealer...
 
First of all, how the **** do post a new thread???

Second, my new engine has low compression. I got from 90 up to 125 psi when i switched the cylinder gasket for RTV sealant. It still won't start. Checked the engine, ignition, etc. What is the minimum pressure to run?
 
Yeah, sure have. I have some good experience with chainsaws/weedwhackers, so this one is a little confusing. I even checked for crankcase leakage. Actually, I had to replace a leaky crank seal already. Again, this thing has never run. Timing is right, it has spark and I think it should be strong enough for ignition, and I squirt just enough fuel in there to start. I even tried with the carb on there. I have sealant on the top and bottom of the cylinder. Compression should be 150 right? Is that the minimum?
 
We are assuming you have a Chinese 2 stroke bicycle engine. It always helps if you give us at least basic information to work with.

"I even tried with the carb on there." What do you mean by this?

How are you trying to start this engine? Is it mounted on a bicycle? Is the chain installed? Is the clutch cable installed and adjusted?
Why did you remove the cylinder and how do you know that a crankshaft seal was bad if the engine has never run? It is doubtful that you'll get it to run without the carburetor properly installed and fuel supply connected to it. It isn't a chainsaw or a weedwhacker. It is a gravity flow fuel system. No reeds, no fuel pump. The carb feeds fuel through a main jet via a siphon system. The carb meters the correct fuel/air mix to the engine for combustion. Simply dumping some fuel into the intake won't do it.

Lots of questions, I know, but we need some help here in order to offer any advice.
Also you'll need a head gasket. Silicone sealer won't be enough. It will work as a seal on the cylinder to case area but the head is a different story. You need a gasket and the head bolts properly torqued. 120 to 140 inch pounds is sufficient.

Tom
 
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You are correct, it is a Chinese 2 stroke from Bikeberry. I do a test all the time on engines, 2 stroke and 4, squirt a few drops or so of fuel in the intake, crank it at WOT and see if it pops off. If it does, you know you have compression and ignition. I tried this many times, then I tried with the carburetor mounted. The bowl fills up and seems to be working. Even if I flooded it by squirting it, it will eventually purge itself. I tried my pull starter and using a drill, which is much easier. Cranked is for a good ten seconds sometimes.

I also do have the chain installed. The clutch cable is on there too and works. Head bolts are torqued to 12.5 ft. lbs, I did a pressure test on the crankcase to find any leakage, which I found on a lip seal. So I replaced that, now it's sealed up. I put silicone sealant on the top and bottom of the head gasket, I did not remove the gasket. That actually gave me 5 more psi. I did remove the paper gasket from the cylinder to case and used silicone there instead to lower the cylinder, which it did, and gave me 30 more psi. I hope that's everything you needed to know. I am certainly stumped here. Help!
 
You didn't say which crankshaft seal you replaced but if it was the one behind the magneto rotor (magnet) there is a possibility that when you reinstalled the rotor you put it on backwards. Check this old thread and look at the photos of the rotor installed correctly. http://motorbicycling.com/showthread.php?t=35334 Post #4
Let us know what you find.

Spinning the crank for ten seconds isn't a lot. Unless the fuel to air ratio is close or right on it could take a lot more than ten seconds of spinning to get the engine to fire. Again let me stress that the proper fuel/air ratio is critical in these little engines. Too rich or too lean and it might not fire at all.
Based on your small engine experience we are assuming you know the basics of testing for ignition. Also make sure the choke is open and there is nothing restricting the fuel flow inside the carburetor. There have been a number of reports of finding the main jet not screwed in and laying in the float bowl. That will prevent starting too.

What fuel to oil ratio are you using? If you're trying the 16:1 ratio recommended in many kit instructions you'll be better off to ignore that and go with no more than a 24:1 mix. 16:1 has been proven to be too much oil.

Tom
 
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