175 Bogging Down - Why

[KJDonovan]

Guys,

Looks like my problem may be a compression issue.  I just checked it and it's only 80 PSI.  I need to know what the compression should be at for this bike?  I can't seem to find this number anywhere.

Thanks,

KD

Kevin J. Donovan
Foster, Rhode Island
72 Jack Piner (My Ride)
72 Six Day (Wifes Ride)

[tomale]

It sounds like it is time for a new piston, rings and a bore job, I am not sure exactly what it should be but your Penton has a high compression ratio.... I believe it is 11:1 what that means is that it should have 170 Psi or so. That is really not the point though, 80 psi is really low, I am amazed that it even runs. I hate to even say this but, are you sure that your reading is accurate? Not that I doubt you or your tools but 80 psi is really low, I have had bikes with cracked rings that had more compression than that! It is time to pull the top end and find out what is going on.. I think you found the problem, It now makes perfect sense.

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
75' GS400 (project bike)
72'sixday (project bike)

[slvrbrdfxr]

KD,
Was just going back and reading all the posts again and something I noticed about the deck height. Your reply says you set it at 7mm. Did you mean you set it at .7 mm ?? If you set it at 7mm then that would explain the low compression.
Dave McCullough

[454MRW]

It would take an awful lot a gaskets to achieve a 7mm deck height! I read that the engine was freshly rebuilt. Did you specify to the machinist what to set the piston to cylinder clearance? Gary Ellis pointed out to me when the 175 I bought in May with a fresh top end turned out to have a badly scuffed piston, that most machine shops are used to water cooled bike engines with much tighter cleannces than for an air cooled bike, and it  ruined a brand new piston, but fortunately the cylinder is still OK to hone out to fit another NOS piston without having to rebore it. This time it should be done right! I would look through the intake and exhaust port and see if you can see whether the piston is scuffed like mine was, or smooth. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 250'S

[KJDonovan]

Dave,

I meant .7 (left off the decimal point).  Looks like it is back to square one for this bike.  After coming up so low on the compression, I have already pulled the engine from the bike and tore it down.  Needless to say I am a whole lot less than pleased with the guy who bored it for me.  He came recommended by the local KTM shop too.  Not really sure what the next step from here is.  The piston I just pulled was 65.5MM and I am not sure how much larger I can go before re-sleeving.  Thanks again for everyones suggestions which helped my track down this problem.

KD

Kevin J. Donovan
Foster, Rhode Island
72 Jack Piner (My Ride)
72 Six Day (Wifes Ride)

[john durrill]

KJ,
 can you measure the piston to cylinder wall clearance. You can use a feeler gauge to slide between the piston and the cylinder walls at 90 Degs from the wrist pin. Let us know what the clearance is. 65.5 is an .080 over piston . Thats a lot off the cylinder walls.
John D.

[slvrbrdfxr]

KD,
I thought it might be a typo on the deck height but just wanted to be sure. Sorry to hear about the poor machine shop work you had done. I had a similar experience with my Jackpiner earlier this year only mine was too tight and I ended up scuffing a brand new piston just like Michael described. Once you determine what the piston to wall clearance is let us know. If it's not too far out of limit then maybe you can send the piston to Swain Technology to have it moly coated. This will add about .0015" to the piston diameter and might get you back in limits again.
Dave McCullough

[john durrill]

KJ,
 can you measure the piston to cylinder wall clearance. You can use a feeler gauge to slide between the piston and the cylinder walls at 90 Degs from the wrist pin. Let us know what the clearance is. 65.5 is an .080 over piston . Thats a lot off the cylinder walls.
John D.

[slvrbrdfxr]

KD,
I thought it might be a typo on the deck height but just wanted to be sure. Sorry to hear about the poor machine shop work you had done. I had a similar experience with my Jackpiner earlier this year only mine was too tight and I ended up scuffing a brand new piston just like Michael described. Once you determine what the piston to wall clearance is let us know. If it's not too far out of limit then maybe you can send the piston to Swain Technology to have it moly coated. This will add about .0015" to the piston diameter and might get you back in limits again.
Dave McCullough