RM 100, no low end power

[bake]

So I rebuilt an old RM 100, new piston, cylinder rings plug. Cleaned the carb (twice) and timed it. Spark looks good.

It starts OK but no low end power(will barely pull itself forward) if I get the revs up it's all there. Any ideas? I just tried dropping the main needle 2 notches with no real difference.

 

[Britt Boyette]

If it's not ignition, it maybe be a clogged pilot jet or, if it has reeds, it could be a chipped reed. Whats the plug look like? Is it black, wet, brown? You might want to post this over in the 2 stroke section too. I think you will get a lot more help there.

 

[wirefryer#85]

Did you pull out the Pilot (slow) jet and clean it really good? Those little bitty holes can clog up quick.

If it runs like a raped ape on top it's getting enough fuel from the tank, and can breath thru the air filter enough to run at top RPMs.

Other than the choke system loading the engine up and barring a compression test problem I'd look at the reeds like Britt mentioned or put a new plug in, fire it up, run a few minutes at JUST above idle speed, stop, pull the plug and see what ya' got per the manual.

Did you have the cylinder bored and honed to fit the piston? No offense, but that is a part of the ritual when doing a top end pre-82 scoot depending on make and model.

Tim

 

[bake]

I will tear into the carb and recheck the pilot jet before pulling the head to recheck the reeds. Yes cyinder was honed to match piston, compression is good. I am a lot closer than when it came to me in boxes but I'd sure like to sort it out and get on with the next project.

 

[dirt bike dave]

It is a 100cc bike without a power valve.

It is not ported for low end power.

The bike probably makes 20 hp at 10,000 rpm and about 8 hp at 7,000 rpm. The hp below 7,000 rpm is negligible. Think mo-ped.

By your description, it sounds to me like it is running like it is supposed to.

In my experience, Old school 100cc two stroke MX bikes are on the pipe and pulling, or they are falling on their face. Lots of throttle and an active toe on the shifter will keep you going forward. On level ground.

These motors are not known for usable power over a wide rpm range. On or off is more like it.

 

[2stroke]

so when you pull off, is it actually bogging out or sputtering, or is there just no "ooomph"? If the motor is firing fine, then I would tend to agree with the poster above. Even 125s of that era were that way....squat for power except when pushing them. Lots of clutch work to get the RPMs up when leaving a corner and stuff like that.

Having said that, I would think ignition timing or carb. The ignition timing may be such that it is off at low RPMs, but with the natural advance of a faster spinning crank, it comes into phase so to speak. I would almost sneak it forward a bit (the stator plate) and see what you get. How did you set the ignition timing? I do my older points units with a meter, but Ive never come up with a way to do it on a CDI machine except trial and error.

 

[bake]

I had the timing done by a competent mechanic, I had played with timing a bit myself previous.

It's bogging only but really bad, this is not the way it ran new and I know something is wrong (it won't move forward in 1st gear without lots of clutch slipping) Lately it's been way to hot here to bother with it, some day real soon.

 

[2strokerfun]

These little bikes had plenty of low-end power for what they were, small bikes. When running correctly, it should easily pull wheely with 140 pound rider. They aren't tractors, but should pull itself along nicely when running right. I'm leaning towards clogged pilot or problem with choke lever.

 

[kingbrian]

i have no doubt in the yz 100, i had a 1975 yz80 that i completely restored and completely rebuilt it I MEAN EVERY SEAL AND EVERY BEARING. had it bored and ported with a weiseco and that was the meanest little bike you ever did see.. we clocked it on a dry lake at 89 mph before i was too far away and it would outrun my dads toyota on top end.... ahh the good ol days.. the cylinder was paper thin... literally

 

[deathpunk]

oh god.
I am well aware that older bikes could make good horsepower. I have ridden CR480s et al. I have seen 40+ HP Maico 501s as well.

I also raced MX for years. I raced YZ80s when they were so frigging outdated that magazines did not even include them in shootouts. I had to run an 89 cylinder + intake tract + porting on my 92 and 93 YZ125s just to hang with stock CR125s on the straights back then.

I don't really ride a Team Peak CR125..which were the 'it' bikes of the day...ya know, the Team Peak of Buehl, Lamson, Swink, and uh this kid named Jeremy Mcgrath...they were unstoppable in 91 and 92. Mitch Payton was on the verge of tears (his words!) when he found out that pro circuit would be switching to KX125's for 93.

The 82-83 YZ100 is a really cool bike and I came VERY close to buying one this past summer to restore and display in my house. I actually entertained thoughts of racing one in the Ultima 125 class in 07. I'm just saying that there is no way short of running nitrous oxide and a YZ125 top end (as you are probably aware, the 82-83 YZ100 has pretty much a 1980 YZ125 bottom end and will accept an a/c 125 barrel without much work) plus crazy porting to get anywhere near 30 hp.