I have been working on my sons Aldy 90cc 2-stroke ATV for what I believe is a low compression problem? Last year ATV ran fair but hard to start, stalled when cold, and lacked power. Compression tested at 70psi and although I do not have specs on this unit, I felt it should be at least 90psi (??). Started by replaced rings, put a new cross-thatch on cylinder, new base and head gaskets only to get 75psi. Then replaced crank seals and the entire top end new – cylinder, piston, rings, and new gaskets. Now getting 83psi. Before replacing the entire top end, I verified ring gaps (top ring 0.008", bottom ring 0.012"). Also, pressure tested the engine which did not lose any pressure in 10 minutes. I put engine together with and without Hondabond HT and no difference. I feel all clearances are within a decent window and should be getting more than 90psi. When trying to run the engine at 83psi, it seems to start better but wants to stall once warm. Poor throttle response at low rpm, good mid and high rpm power as long as you do not let the rpm drop too close to idle. Installed new carburetor, but old one performed better after cleaned and soaked in Chemdip. New air filter and reed valves were perfect. Am I worrying about the wrong thing here?

 

I would be thinking of an idle jet blockage or idle mixture problem. Some engines don't have a high compression, and seeing as you have a good piston seal, it looks like your engine must be a low compression one. Don't forget, on a two stroke you loose a large portion of the "compression stroke" as there is no compression increase until the piston is past the exhaust ports.

 

Thank you merryman! I was thinking the same and already re-soaked the carb in Chemdip (including the jets) last night. Have not had the time to put back together but since it is still open, i will double check needle clip position as well. Unfortunately I cannot remember if it should be in the 3-up or 3-down position. I live in Minnesota so altitude is not a concern. Back in the day I used to set the mixture screw to 1 1/2 turns out to start, is that a good starting point in your mind?

 

My ideas with idle mixture screws is never touch them unless someone else has fiddled in the past, or if a bike simply won't idle at all. I guess 1.5 to 2 turns is a good starting point if you do have to alter it.

 

Compression. Cheapie china kit is biting you in the ****.. imo..

 

I am sure leaning toward your opinion LT80 but 2-strokes are very simple engines. For compression problems it is either worn cylinder (should see that in ring gap or bore to piston clearance), worn/broken rings, piston, ring gap, cracked cylinder or head, or head gasket. Am i missing anything? I checked all those variables and makes no sense i have such low compression, unless Merryman is correct. I have thoroughly ruled out carb or either jet. I am having a very difficult time convincing myself to purchase another new top end. Adjusting the air/fuel screw does nothing. Since playing with it so much, there is no idle unless I finger restrict the intake (serious choke) and poor response throughout mid and high ... unless I finger restrict the intake. This leads me back to not enough compression (LT80 opinion) and subsequent lack of ventury to pull enough fuel into the cylinder. Is it possible the CDI or coil is weak and not getting enough spark power? I am all thumbs with electrical.

 

Thought i would circle around and give an update on the above thread. After 3 rebuilds including entire top end from OEM (Taiwan), the compression never reached above 83psi. I basically gave up and decided to just put the plastic back on to sell/give it to someone more knowledgeable. On a whim i decided to try starting it one last time before taking pics to sell and to my surprise, it started right up, idled (albeit slightly high), had power, and ran rather well considering!!! The compression is still just 80psi. The old battery i thought was junk (because it didn't have enough power to turn it over) was cranking the engine over like it was new all of a sudden! My son has been riding it since end of April and loves the fact his scrawny legs can kick start it because it starts so easy now. The only think i did was put the plastic back on, i am certain of that. Guessing i must have jiggled a bad wire connection when installing the plastic? I guess the points to take away are there are 2-strokes out there that do run on low compression - thank you merryman, and the problem was most likely an electrical one from the start.

Q: What electrical component would you folks look at first when this problem resurfaces ??

 

If it is AC CDI the only thing the battery system and ignition system have in common are earths.