I just looked through my manual for my 76 250 MC 5, and could not find the compression for it. Does anyone know the correct amount is?
Thanks, James
Most of the ones I've checked are in the 150-165lbs range, but I dont know what the factory specs are. I've ridden a few that when rebuilt were intentionally lowered to the 130 range and I think they run better at that lower number. For instance, I've ridden two recently, one with 165 and one with 130, the one with 130 hits darn near as hard on the bottom, but revved a lot better on top.
Brian
'72 Berkshire
Thanks Brian! Interesting information. James
Of note also is that Carl Cranke lowered the compression of the KTM engines when he hopped them up.
Brian
'72 Berkshire
Okay Brian, I will show my ignorance..... How do you lower the compression; increase the base gasket thickness? James
Yeah, the KTMs have multiple thickness base gaskets and you us combinations of those to get the deck height you want. If you want to lower the compression just add a bit more base gasket.
Brian
'72 Berkshire
Adding base gaskets is certainly a quick and easy way to lower compression. However, if you are a stickler for accuracy, you need to remember that adding gaskets will also increase deck height (squish area), and slightly reduce the mean effective stroke (distance from TDC to top of exhaust port). A better, but more difficult, method to decrease compression is to increase the volume of the combustion chamber by grinding or machining the dome of the cylinder head after first setting the deck or squish dimension.
Nelson Lingle
73 Jackpiner
74 Jackpiner
71 DKW 125
Thanks, that is what I thought. Would it be fair to say, that if the compression in in the 110 - 118 range, that the piston is too loose, and I should bore the cylinder to the next size overbore? James
Ok I'm an absolute villager when it comes to tuning but I have seen factory spec "low-compression" heads for the MC-5 400 and perhaps they exist for the 250s too.
Skiracer,
I wouldn't use compression alone to determine if it is time to bore the cylinder. You really need to carefully inspect the cylinder and rings, and measure the piston-to-cylinder clearance at the top and bottom of the stroke, as well as the "roundness" of the cylinder.
Nelson Lingle
73 Jackpiner
74 Jackpiner
71 DKW 125
There is much knowledge passed along in the previous messages. One needs to isolate exactly what one is attempting to do to the engine, or what one is attempting to determine or measure. Yes, one should periodically tear down the top end and do an inspection of it. Yes, one should periodically perform a compression check on the engine. Do it when new or newly rebuilt, after break-in, and then periodically after so many races, hours, or events. Keep a record of the readings. It is not reasonable to compare an old, worn-out engine with lower compression with a specifically built lower compression engine. As a previous POG member (Nelson) said, adding base gaskets changes port timing, also. It effectively raises the top of the exhaust port. As we know, the true compression reading starts when the piston reaches that point. All the other ports (heights) are changed also (intake, and tranfer ports). This affects the performance of the engine, also. So therefore, adding base gaskets not only changes compression, but also changes port timing. Shouldn't the ignition timing be adjusted also?
Thanks..... Some very good points; isn't this site great?????? My intent is to find out why the bike is very very hard to start when it is cold. I redid the lower end last spring, and had the motor sitting on a shelf for nine months. I put the top end on last fall, and there is where my problems started, or, not started. The top end came off a running motor, but maybe it was close to being too loose. My thought is the low compression is preventing the bike from starting as it should. James
James, what nelson has said is true.. there could be several reason for low compression. It the cylinder is glazed this could cause a low compression or it could be that the rings are broken or way beyond servicable. Any time you put a top end together, even if it had come from a running bike the cylinder should have had a hone run through it to clean it up. If the piston is worn out then certainly it needs to be replaced but I have to tell you, that my MC 5 still has the stock piston in it but I have replaced the rings. The stock pistons are very good and they should last along time if they have been taken care of....
Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)