Vintage Dirt Bike Q & A

Guys

I have just rebuilt a CR125 1976 and took it for a blast at the local track.

The spec is a stock standard engine. It has very good compression and a good spark.
I have put a 32mm Mikuni on it because the old carb was shot. It is jetted with a 6F5 needle and a 270 jet. The sprockets are 14 front and 53 rear. The timing has been set as per the manual. I am running B9EV plugs as per the manual. It has an aftermarket pipe on it.

Other than that...bog stock standard.

It starts third or fourth kick and seems to rev freely in neutral.
When I put it in gear and you really have to slip the clutch to get it going and on the pipe. Change into second gear and you have to start slipping the clutch again to get it revving and motoring.
I know you have to keep these motors on song but there is no low end power at all it would seem?

Its almost as if it is gassing up?

I have checked all the specs for timing, plug gaps and everything seems to be in order.

I weigh 210lbs (95KG) so it has been suggested to me that I am just to heavy (fat) to ride it?

Other than me having a drastic weight loss what would you guys recommend I do in a sequence to check through what could be wrong?

All help much appreciated and thanks for the space Nathan

Hi mate, i feel exactly the same as you. The reality is that its normal! I weigh 90 kgs myself,i just got it going for a title meet and thought something was wrong. Mine is standard except for pipe, pod filter and jetting. I felt cruel the way i had to ride it (like i stole it!), but it got good starts and i placed well at the meeting. So all i can say is... hold it on! Cheers,Brad

Have you experimented with main jet sizing?
My 75 CR125 with stock carb is really fussy about main jet size. With a rise in ambient air temperature or increase in elevation of riding area it tends to bog out. A smaller size can make a big difference.
Be sure to check spark plug colour to be sure you're not overheating!!

I own a 76 and 77 125's I weight 170 lbs. both bikes run like raped ape when on the pipe. The 76 has stock carb and actually has better low end and mid range but fall off the pipe and they both will load up. These were raced by riders weighing 125 to 150 lbs in the day so you may consider a 250. I run a 34 mikuni on my 77 but have port work and DG pipe Top end is were it likes to excell. but I am an older expert rider age 49. some guys including myself are 125 riders others are 250 and open riders. Not many are good on both. I been riding and racing since 1972. currently racing FLVMX series. FLVMX.com
Kevin

_________________
Kevin Politte

Bet a 13 tooth front sprocket would do wonders. Maybe even a 12. 210#'s is a heavy load for a 125 ...Van

_________________
1980 Honda CR125R

Yep I run a 13 tooth front sprocket, the same carb and jetting you have on mine and it hauls my fat 110kg ass around the track without much trouble.(i some times get to pass other riders ) Darcy

Well, I guess that takes care of my idea about getting a 125.

_________________
Garry Mainer
#33m (retired)
Honda Cr250r 1980
Honda Cr250r 1980
Yamaha TY250C 1976
Arctic Cat 650 H1 2007

I have the stock Keihin carb on my 76 and 77 CR-125. The natural state of the bike is pipey even with the jetting spot on, but besides that you may want to check your clutch adjustment. The bottom adjustment is through the threaded port on the right crankcase. Loosen the 10 mM nut and then adjust by turning the inner bolt in with a small straight end screwdriver, then retighten the 10 mM nut. If the clutch still slips after adjustment you may need new clutch plates.

Carl, his clutch ain't slippin'. He's slippin' it puposely to propel the weight and not bog down ...Van

_________________
1980 Honda CR125R