Competive MC-5 125?

[Mike Lenz]

I am building a 76 125 MC-5 for MX. It will primarily be a "fun" racer as I am WAY heavy for a 125. However 125's are so much fun I want to run one...for fun.  I also want it to be a Penton and a PV bike.  So that leaves me to try and make that heavy motor go.  I have the lightened clutch basket and alum inner hub and plates that came on that engine, an internal roter Ign and I found some gears that were lightened. Im also going to run a smaller chain and 1980 125 wheels. What Im wondering about is the crank.  Nobody wants a slow reving 125 for MX and from what I remember this engine revved pretty slow? Im wondering if I should try and have the crank lightened.  Anybody out there had any experiance doing that type of thing, or have any other ideas?

[Dwight Rudder]

Mike,  The 76 125 motor was very hard to keep running. Lots of issues.
I know of a couple of the enduro bikes that were thrown into the dump with hardly more than 30 miles on them. Broken cases, Siezures, etc. Best to keep this motor as a collectors item.
Dwight

[Mike Lenz]

Do you know why it was hard to keep running? Same basic engine as all the others??

[Mike Lenz]

I did a bit of searching on the site and found a post that thought the stock compression was too high.  That would maybe explain the top end problems.  I have a set of 125 cases that are cracked. The cases are the same as all the others so mine may have been from improper assembly, however early 250 and 400 cases did crack on the mc-5 at the left engine strut before it was redesigned.  Maybe the case story was do to that?

[TGTech]

Mike,

I never tried to make the 125 KTM engine light, but I did ride the bike for some time, and never had any problems. No seizures, broken cases, etc. I don't know where Dwight found all that stuff. The biggest difficulty at that time, was making sure that the Lectron carburetor, was "jetted" right.

My brother still has my 76 ISDT bike that is a 125 now. (was a 175 for the Six Days)

Dane

[Dwight Rudder]

I worked for Butler Motors in Jackson MS and know Don Burgess who owned Competition Cycle in Kenner, La.  In fact down south I never saw one run more than a few miles. I have heard more stories of the 1976 models unreliability than of those who finished races.  I heard of no problems with the 1977 models.
In fact didn't I see you with a KTM 125 top end on a Sachs bottom end ?
Dwight

[thom coles]

I have to agree with Dwight on the 1976 125 (KTM engine)we had numerous problems with the 1976 model---- seized, blubbered, hard to start etc. It ran poorly compared to the earlier Penton/sachs 125 engines and was not reliable. We ended up getting rid of it after a short time (3 months)and moved on with the mc5 250 and never had problems with the KTM 250. Never paid much attention to the KTM 125 model after that to find out if the bugs got worked out, luckily I kept the 75/76 Penton 125 with the lay down suspension and the sachs D series motor, which still runs great.

thom coles,  paw paw, michigan

[Chakka]

I have seen a few 76' 125 KTM engines with Sachs top ends on them. In an old Keeping Track there is an artical about a Carl Cranke built Penton 125MC with a 100cc Sachs top end. I am interested in this thread because I currently have a 76 125 enduro. I finished restoring the bike but carb issues have kept me from running it.

Chris

[Mick Milakovic]

Hi Mike, I say FINISH THE PROJECT.  I respect the opinions of both Dwight having problems and Dane not having problems.  The bottom line is your satisfaction with what you're doing, AND it would be a cool bike to have [8D] When you're finished if you don't like it, sell it and do another project!  

This group is all about saving the brand of Penton motorcycles.  Last weekend I picked up two 1976 YZ250 bikes and a 390 Husky.  I look at the Husky brand like the Penton:  Those two YZ's will get sold long before I part out any of my 5 Pentons or 5 Huskys.  Now if only I could find the time to restore my own projects......

[thom coles]

Chris,
  Cannot be much help, the bike performed poorly and it was later sold. It had been taken back to dealer/shop several times for repairs but never had much luck with it in competition, just did not run right compared to the Penton 125 Sachs model. I did hear later the bike was just used for trail riding and ran okay? Good luck with your project...

thom coles   paw paw, michigan

[Mike Lenz]

I have put a Sachs 100 and 125 top end on a 73 Jackpiner with success.  Do any of you know the difference in the 125 KTM 76 top ends compared to the 78 top ends? They have different part numbers.  Different pistons too.  I am just going to rubber mount the carb.  34mm I guess, although they were using the big 36mm Bing on the mx model by 78!!  Timing and deck hight were alot different between the two also.  Thanks to all for all the input!

[Mike Lenz]

Oh, By the way anyone got any 54.5 pistons for this bugger?
Thanks
Mike

[Mike Lenz]

Oh, By By the way anyone got a spare 78 style cylinder for this bugger?
Thanks
Mike

[john durrill]

Mike,
 How does a Sachs base gasket fit the 125 KTM bottom end? the 76-77 175 base gasket fit the 125 KTM? should let you know what options are their to work with.
I don't know what class your looking at running but Dane said his had a Lectron.  
 Lectrons would be a good carb for a wild port, pipe set up. Using base gaskets to keep the cranking pressure down to 160 LBS max should be safe #. 150 to 155 might work better I would think.  Thats just rule of thumb to keep a track or trail with long high speed straights from causing a heat soak  issue. We can weigh a Sachs 125 B crank if you would like and post the reading if it would help.
 Its a neat project. Let us know how you are doing.
John D.

[Chakka]

Mike,

I had no idea you could put a Sachs top end on a Jackpiner bottom end. That is very interesting. What type of modification to the parts needed to be done? I have a Sachs GS cylinder and head off a Monarch...now the gears in my head are spinning.

Chris