Low compression on 4 cycle Robbins engine 350 cc

[gmcman317]

So I tested the compression on my engine and it’s 90 in one cylinder and 150 on the other. It still seems to run good but doesn’t have nearly as much power as it used to. I have 295 cc that was rebuilt to replace it. Should I pull the one that’s currently in and replace the rings or just swap engines and rebuild the other as I have time? Just need a second opinion before I pull the trigger. Planning on new gas lines, gaskets, belt, and cleaning the carb and new plugs while I have it out.

[Etu]

Have you adjusted the valves?

[gmcman317]

Yeah I adjusted the valves and same result

[JPonLKN]

I guess it depends on how much you use the cart, how often, and if you are willing to hold out using it till its done. Me personally, I would prefer the 350, but there isn't really a ton of difference between the two if both are properly running. As easy as it is to pull and swap motors, you could do a swap in a few hours and have a running cart with the 295, and then do whatever work you want on the 350 till it gets back together. Before buying anything, I would pull the head and look at the condition of the valves. If it isn't using oil, smoking like a camp fire, then it is likely the top end. If the valves are sunk into the head, easy to replace them, lap them in, and will likely solve your compression issue. Once you have the head off, you can roll the pistons down, look at the condition of the cylinder walls and decide from there with known info from observations rather than guessing.

[big mule]

Originally Posted by JPonLKN

I guess it depends on how much you use the cart, how often, and if you are willing to hold out using it till its done. Me personally, I would prefer the 350, but there isn't really a ton of difference between the two if both are properly running. As easy as it is to pull and swap motors, you could do a swap in a few hours and have a running cart with the 295, and then do whatever work you want on the 350 till it gets back together. Before buying anything, I would pull the head and look at the condition of the valves. If it isn't using oil, smoking like a camp fire, then it is likely the top end. If the valves are sunk into the head, easy to replace them, lap them in, and will likely solve your compression issue. Once you have the head off, you can roll the pistons down, look at the condition of the cylinder walls and decide from there with known info from observations rather than guessing.

I second this !!

[CP241]

I third

[Cartmaster]

From what I read, I would go ahead and buy new valves and gaskets. Top end can be done in a couple of hours, or a day if inexperienced. Why swap an engine only to do it again after rebuild.

[gmcman317]

Well I ended up taking out the engine with low compression and I’m gonna clean up everything in the engine compartment and clean up all the grease and dirt and see why my rear ended is leaking gear oil while it’s out. Probably just fix up the 350 when I have time and then I’ll have another fresh motor for it on the shelf. We only ride it on the weekends when I have time so not worried about how long it takes to put new engine in. Thinking about putting in a new clutch spring while I have it off the engine. Thanks for the advice guys.