Got a question about checking compression I have a gauge I checked compression while bothe plugs were out bike not running one side at a time roughly 90lbs each cylinder is this standard? If not where do I go from here
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Low compression
- Thread starter 51sandra
- Start date
Did you have the throttle open during the test or carbs off? If not try that. If it's still low, let a little oil into the cylinder. If the compression goes up, you have ring problems. If it doesn't go up at all, you have valves that are not seating. Did you do a ring job at all? If you did, you won't get proper numbers until you break in the motor.
No I haven't done the rings yet.
Carbs were off and tried it with carbs on throttle open
I did add oil no change still around 90lbs
Looking for a manual my first Yamaha kinda new to this never done a valve job just looking for direction I guess thanks again for the help
Carbs were off and tried it with carbs on throttle open
I did add oil no change still around 90lbs
Looking for a manual my first Yamaha kinda new to this never done a valve job just looking for direction I guess thanks again for the help
Well before you worry about the seats being bad, you need to check and make sure that your valve lash isn't on the tight side. If you are too tight on the clearance, it could cause problems as well. Do a search on here for the proper procedure and specs, it's also in the manual. Start there, as getting that right is better than pulling a head and sending it off to a machine shop.
XSLeo
XS400 Junkie
Gas in the oil comes from a bad petcock. With a bad pet cock it can't stop the flow of gas to the carbs.
The float and float valve are not fuel shut offs, they are metering devices. They control the fuel level in the float bowls when the engine is running. As the gas over flows the float bowls it runs into the engine, collecting in the crank case, thus the gassy smell of the oil.
Fix your petcocks.
Clean the carbs.
This excess gas in the oil, dilutes the oil. This cause excessive engine wear if run that way. One of these excessive wear points is the rings. This will cause low compression.
But if adding a oil to cylinders doesn't bring up compression, this indicates poor sealing of the valves.
I might suggest removing the exhaust headers and carbs. roll the engine over to open each valve. Watch each valve to see that it opens and closes fully. Sticky valves could be part of the problem. Use a cleaner, carb, brake, whatever and a wire tooth brush and clean off the valves and seats. Reach in through the intake and exhaust ports.
Once clean, set the valves to proper clearance or a bit looser. Set a bit loose you know the valves can close fully. Once the compression test is done set the valves to spec. Spray a bunch of oil in around the valve springs and valve stems, this should help fix any sticky valves.
If you have a good penetrating fluid, spray that around the valve springs and into the cylinders. If you don't have any, get some ATF and acetone at the parts store, acetone is in with painting supplies. Mix the acetone and ATF 50-50. Makes the best penetrating fluid ever.
This helps loosen sticky valves and rings.
Leo
The float and float valve are not fuel shut offs, they are metering devices. They control the fuel level in the float bowls when the engine is running. As the gas over flows the float bowls it runs into the engine, collecting in the crank case, thus the gassy smell of the oil.
Fix your petcocks.
Clean the carbs.
This excess gas in the oil, dilutes the oil. This cause excessive engine wear if run that way. One of these excessive wear points is the rings. This will cause low compression.
But if adding a oil to cylinders doesn't bring up compression, this indicates poor sealing of the valves.
I might suggest removing the exhaust headers and carbs. roll the engine over to open each valve. Watch each valve to see that it opens and closes fully. Sticky valves could be part of the problem. Use a cleaner, carb, brake, whatever and a wire tooth brush and clean off the valves and seats. Reach in through the intake and exhaust ports.
Once clean, set the valves to proper clearance or a bit looser. Set a bit loose you know the valves can close fully. Once the compression test is done set the valves to spec. Spray a bunch of oil in around the valve springs and valve stems, this should help fix any sticky valves.
If you have a good penetrating fluid, spray that around the valve springs and into the cylinders. If you don't have any, get some ATF and acetone at the parts store, acetone is in with painting supplies. Mix the acetone and ATF 50-50. Makes the best penetrating fluid ever.
This helps loosen sticky valves and rings.
Leo
Thanks I'll try that I did adjust valves went from 90 to 120 but still a little low I'll clean valves and re adjust them see what happens rebuilding the carbs as well.hope I don't have to do a ring job not sure I'll be able to swing it but we will see thanks again
jthiessen
XS400 Addict
Finally had a chance to work on my engine. (it is currently stored 1.5hr from my house 
) The bike is suffering from low compression, and my wallet is suffering from low funds... So I've take the valves to a shop where I will ask them to take a look and do some work on them. Hopefully that will solve the problem as a wet compression test didn't improve the results. Here are the cylinders, how do they look to you? I left that part in tact in hopes that there is a glaring issue with the valves etc
View attachment 26145 View attachment 26146
) The bike is suffering from low compression, and my wallet is suffering from low funds... So I've take the valves to a shop where I will ask them to take a look and do some work on them. Hopefully that will solve the problem as a wet compression test didn't improve the results. Here are the cylinders, how do they look to you? I left that part in tact in hopes that there is a glaring issue with the valves etcView attachment 26145 View attachment 26146
