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Old 09-07-2011, 06:17 PM   #1
Abdelhub OP
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revving a vintage 125

I have been into vintage mx bikes for a bit. Mostly big bore and 250 two strokes. I have a couple brit four strokes as well. Over the winter i rebuilt a DKW 125 Motocross. It is my first foray into the 125 realm. I am well aware you have to really rev a 125 to make it work but it seems this SACHS 6b will scream to the moon. unfortunately im not to comfortable (or used to) slipping the clutch at 7500 rpm in second gear. I guess its just a riding adjustment but i feel im killing the motor. as an aside when it hits 12000rpm (a guess) it feels like im in the scene in the first star wars when the millenium falcon goes into lightspeed

For those of you that raced small bores back in the day or those who still do....was there a feeling, vibration, intuition that made you grab the clutch and upshift? Is it just a practice, practice, practice thing? this motor only seems to come alive above 7k rpm (a guess). seems like if i miss the powerband on an upshift the motor just bogs and dies...

i would especially like comments from those of you who have had penton, dkw's, monarks or the like with a 125 sachs motor. The numerous neutrals are also a pain....

heres the bike. ignore the bastard honda tank on it...its just my tuning tank. I have one nice race tank and a beautiful show tank.

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Old 09-08-2011, 08:32 AM   #2
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right when it hits lightspeed, shift.

careful with that trans. they are fragile...stabbing the shifter and no clutch can definitely cause problems.
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Old 09-08-2011, 08:41 AM   #3
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Never fall on the shifter side....that shoves the shot shaft in just enough to grenade the tranny and makes life horrible theses days since no one has affordable prices and parts on a Sachs motor. On the technique for shifting a Sachs 125. rev to moon, then continue to Saturn and shift......There was a reason they never posted torque ratings in specs for the Sachs 125, they don't have any.
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Old 09-08-2011, 08:43 AM   #4
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I guess no one told you holding a video camera sideways is not the same as holding a 35mm camera sideways?
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Old 09-08-2011, 09:01 AM   #5
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You have to develop a feel for where you are in the power band. There is a sweet spot in the rpm range where the most HP & torque is developed and you need to adjust your riding style to keep it in that range when ever possible. Yes, it is easy to to both lug it and over rev it. Lugging it will most likely cause plug fouling, wringing it's neck won't really hurt it, but it isn't nearly as good as shifting just before the power starts to flatten off on top.


The pipe and silencer you have has the bike choked down compared to what we used to race back in the day. Although it is holding back potential HP, it prolly spreads what HP it does make over a wider RPM range than a race, bike making it that much easier to ride and keep in the power band.

Proper jetting will help you feel where you are in the power band. If you are too rich it may never "clean out" allowing you to feel where you are. Too lean will run like a raped ape allowing for a good feel, but you can easily damage the engine.
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Old 09-08-2011, 11:46 AM   #6
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Back in the day, you would polish and hard chrome that shifter shaft and undercut the dogs on the gears to get them to stay engaged. Look up John Uhlrich for info on road racing your Deek.

Keep the throttle wide open and shift up when you aren't accelerating hard anymore, if you are still accelerating hard and you are in 6th, you need to get a smaller rear sprocket. If you never get wound out in 6th gear you should go to an industrial hardware store and buy a pair of big, brass nuts. I hear that a pair of those holding the handlebars makes any motorcycle faster.
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Old 09-08-2011, 12:19 PM   #7
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I raced a 125 Penton, and loved it! The transmission is easy to miss gears, you need to make sure the gear change has been completed each shift, that's the best thing I can tell ya. You can over rev a little, cause they are so slow to shift, you will come out better in the end. These motors have lots of torque compared to a 125 CZ!
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Old 09-08-2011, 12:54 PM   #8
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Quote:
Originally Posted by lemieuxmc View Post
Back in the day, you would polish and hard chrome that shifter shaft and undercut the dogs on the gears to get them to stay engaged. Look up John Uhlrich for info on road racing your Deek.

Keep the throttle wide open and shift up when you aren't accelerating hard anymore, if you are still accelerating hard and you are in 6th, you need to get a smaller rear sprocket. If you never get wound out in 6th gear you should go to an industrial hardware store and buy a pair of big, brass nuts. I hear that a pair of those holding the handlebars makes any motorcycle faster.
We used to re-shape the shift shaft so it more like little footballs rather than squares, then after re-shaping hard chrome the shaft. We copied a works Zundapp ISDT motor we got our hands on. There was a guy in So Cal who used to make the KOBA shift kit for the Sachs. It changed the shift shaft to a spring loaded paul, kind of like a Hodaka style shifter. Discussed here:http://www.pentonusa.org/forum/topic.asp?TOPIC_ID=5701
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Old 09-08-2011, 02:48 PM   #9
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I used to pull second gear starts on mine, by revving and dumping, at about 9 million rpm! Then, if it bogged, just clutch it, and away you go! For me that saved the time shifting first to second. Get weight off the bike, and just get that back tire spinning like crazy, I got good hole shots that way. Mine was a green fiberglass tank six day, I worked at the shop were they sold them and uncrated it myself. We cut off the big muffler, and put on a smaller race muffler. I love the way that bike handled, felt like a Macio, low to the ground, and very forgiving. The motor ran good stock, big cooling fins=good power.
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Old 09-08-2011, 05:21 PM   #10
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+1 on most comments so far. I raced a DKW a few times and my reccomendation is to find a nice open area where you can let it fly a bit and then in 2nd and 3rd gear rev it out at full throttle while listening closely to the motor sound, feel it as it revs and when the front end starts to drop at all, that is shift time, well just a tic before that preferably, but after some practice you will get it.

And YES it does sound ALOT screamier {is that a word?} at full tilt that a big bore wing ding and even quite a bit revier {?} than a 250. It will sound like it is coming unglued at times, but it should NOT sound overly clattery {?}, that could mean it needs a top end. It should sound like it is screaming but not banging and clattering, if that makes sense.

Pin it and see, what could a new top end and crank cost?
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Old 09-08-2011, 08:40 PM   #11
Abdelhub OP
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deek

I am still carb testing. The carb that came on the bike was a bing 26 or 27mm. The jets in it looked positively tiny compared to the Keihin I have on it now.

Cleaned up the bing really screamed and revved out nicely. Only problem is the old zinc slide that would stick open. also old bing parts seem to be tougher to find. Ive gotten a keihin off of a honda cr80. It had 50 pilot and 125 main in it. on install it bogged when quickly wacking open the throttle and would never clean out. i went smaller on jets to a 38 pilot and 120 main but it still bogs when wacking open the throttle. It almost cleans out on the 120 main. i think ill try a 115. I just hate the jetting guessing game when switching brands of carbs. I really like the simplicity of the mikuni and keihin roundslides so ive got them on most of my bikes.

weird thing is to my thinking how would a cr80 be jetted fatter than a larger 125. maybe reed vs piston porting? is the mid 90's cr breathing that much more than the old sachs?

Ive noticed on this bike with the large sunburst style head the rattling is really loud. about as loud as my Rickman vr250.its got new top end bearing and 20 over wiseco so hopefully the top end is good for a while.

as far as the silencer goes i absolutely had to put in on. I would be deaf after a few minutes of wide open runs with just the pipe. Its a noisy sucker.

enough of my rambling.

Mike
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Old 09-09-2011, 07:59 AM   #12
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No comparison between the CR 80 and the Dee Kay. The CR prolly makes more HP than a built DKW back in it's hay day. What size is the carb off of the CR80? 28mm? For piston ported DKWs we would go as large as 30mm Bing, super built Reed Valved DKWs sometimes got 32mm Bings. I always liked the older Bings as they were easy to jet and made good HP.
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Old 09-09-2011, 08:00 AM   #13
Sniper X
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Quote:
Originally Posted by Abdelhub View Post
I am still carb testing. The carb that came on the bike was a bing 26 or 27mm. The jets in it looked positively tiny compared to the Keihin I have on it now.

Cleaned up the bing really screamed and revved out nicely. Only problem is the old zinc slide that would stick open. also old bing parts seem to be tougher to find. Ive gotten a keihin off of a honda cr80. It had 50 pilot and 125 main in it. on install it bogged when quickly wacking open the throttle and would never clean out. i went smaller on jets to a 38 pilot and 120 main but it still bogs when wacking open the throttle. It almost cleans out on the 120 main. i think ill try a 115. I just hate the jetting guessing game when switching brands of carbs. I really like the simplicity of the mikuni and keihin roundslides so ive got them on most of my bikes.

weird thing is to my thinking how would a cr80 be jetted fatter than a larger 125. maybe reed vs piston porting? is the mid 90's cr breathing that much more than the old sachs?

Ive noticed on this bike with the large sunburst style head the rattling is really loud. about as loud as my Rickman vr250.its got new top end bearing and 20 over wiseco so hopefully the top end is good for a while.

as far as the silencer goes i absolutely had to put in on. I would be deaf after a few minutes of wide open runs with just the pipe. Its a noisy sucker.

enough of my rambling.

Mike
I suggest finding someone in AHRMA racing one of these and picking their brain about a carby swap. you are going to right way in mo opinion though, probably smaller jets till it runs well is theticket on the Kehin.
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