TOPIC:

Ag3NT wrote:

I took my bike to the garage today. The mech told me that the problem is lack of compression. He said that it is barely 20lbs and should be at about 80lbs. not sure if it is a holed piston or worn rings. If it is rings/pistons does anyone know of a good source for parts????


Steve

Insufficient valve clearance can also cause lack of compression (usually gets worse as engine warms up). Have clearances been checked (and adjusted if needed)?

After assuring correct valve clearances, suggest repeating compression test, and thereafter putting a teaspoon of fresh motor oil in each cylinder (through the spark hole) and again repeating the compression test. Take note of the compression readings after each test. Results of these re-tests?

A leakdown test may better diagnose cause of insufficient compression.

After "running fine all weekend" would be reluctant to limit diagnosis to "holed piston or worn rings."

How much crud was found in the tank and petcock?

The tank and petcock were practically clean. The bike only has 40000 miles on it. I agree it is really premature for the engine to be worn this badly. As it warms up the bike runs great. It's only when cold that it doesn't run. Could there maybe be a hole/leak in the head gasket that is swollen closed when the bike is warm? The bike has always run really lean could this have any bearing?The mech is telling me that the only way to tell of any damage/wear to the pistons is to take the head off. Is there any other way that would be less time intensive?

Steve

Ag3NT wrote:

...As it warms up the bike runs great. It's only when cold that it doesn't run. Could there maybe be a hole/leak in the head gasket that is swollen closed when the bike is warm? The bike has always run really lean could this have any bearing?The mech is telling me that the only way to tell of any damage/wear to the pistons is to take the head off. Is there any other way that would be less time intensive?

Could be wrong, but "a hole/leak in the head gasket that is swollen closed when the bike is warm" would not be my prime suspect.

Compression test should be performed with throttle held in wide open position while spinning engine over at cranking speed.

What are psi readings from compression test without oil added through spark plug holes?

What are psi readings from compression test with oil added through spark plug holes?


Glad to hear fuel supply to carbs has been assured. If not already done, might also check to assure vent in gas cap is clear.

Post edited by: Patton, at: 2007/02/03 10:09

Pattons comments are great, and how you should proceed.

I don't know about your mechanic, he says the compression should be 80 lbs? It should be much higher than that. When new my bike should be around 150-160 lbs, maybe more. When I measured it recently at 18,ooo miles I get from 130-140 lbs. The throttle has to be held open to get any meaningful reading. It doesn't sound like that happened, and if the mechanic doesn't know this then he's not much of a mechanic. If the compression is really 20 lbs (and I doubt it) how could the bike run?? It does not seem possible with compression that low.

Don't let him pull the head. Ask him to check the plugs, points if you have them, timing, cam chain adjustment, and valve adjustment. Then see what you get for compression.

By the way, that stuff is all something you can do, as a beginner, with some simple tools and a manual.

I asked the Mech about the pressure he said that it had to be atleast 80 to burn gas. Right now it won't even light starter fluid. He said that to check the valves I would have to remove the head anyway. He also said that if a valve was stuck or not working the engine wouldn't build any compression at all. Makes sense, but if there was a hole in the piston or the rings were bad it seems that it wouldn't build compression either. I don't know. I'm bringing it home tomorrow, is there anyone around San Clemente CA willing to help?

Steve

What year is your bike? Isn't it early 80's? Then it probably has electronic ignition. Not much to adjust there.

Do you have a manual?

If the bike was running fine and now it won't start, it sounds like it is simple, but frustrating for you I'm sure. Your plugs may be fouled from flooding if you smell gas, so spend ten bucks and pop a fresh set in ther. Some plugs don't take to being flooded and just crap out after it happens.

And if the compression really is low, it's possible that the cam chain was pretty loose and jumped a tooth, causing the valves to remain open.

Before trying to turn it over anymore, you might want to remove the cam cover and check the camshaft timing.

How would I check the cam timing? Do I need any tools to do that? The plugs are new less than 6 months old. I'll try new ones. My bike is an 82 and has electronic ignition. I feel that it is most likely something really simple, but I have no idea how to fix it. If it was a stuck valve would it even build any pressure at all?

yes some simple tools and a shop manual. Do you have a manual? It's all in there. You can also search the forum for cam timing and find out what other riders have done. I think the 750 is a bit different from the 650 in regards to the chain and tensioner that is in the head area, but otherwise the whole process is straight forward.
Basically you need to remove the gas tank, coils, and cam cover. Then remove the ignition cover. Rotate the engine with a wrench on the bigger ignition bolt to the suggested mark that your manual tells you to set it on. Then you look at the cam marks and see how they line up with the head. I'm hoping to hear they line up.

I don't have a manual, it is out of print and not so easy to find. I have bid on a few on ebay, but I haven't gotten one yet.

Steve

I'm sorry to just realize you have a twin, not a four! Yeah I see where manuals are scarce. I don't see any on ebay at the moment. Why not post a wanted ad in the forum classifieds, I know there are a couple guys here who own the big twins, maybe you get lucky?