Vintage Dirt Bike Q & A

New member to this fourm. I've been buying and selling parts for awhile on the swapmeet. Lot of good people on this site.

I have a problem with my 79 cr125r.
Have a major hesitation/bog on the low end. The bike idles fine, runs good on the main.
When I take off it just bogs, tried working the clutch that does not help....the bike is almost unridable. I can completly unscrew the air screw and the engine will still run.

Install new Wiseco piston standard bore. This new double ring piston has a higher cut on the intake side of the piston than the orignal single ring piston. Could that cause a problem.

I used the Keihin carb after the rebuilding the top end. So I bought a VM 32mm Mikuni and still have the same problem....but the bike does run much better with the Mikuini.
The Mikuini has 300 main/40 pilot...but the cut away is a 3.0

I have not replaced the crank seals....no fouled plugs or oil out of the exhaust manifold.
Don't no what the problems is? to much air, not enough fuel? I'm 1,000' above sea level or less.

I did squeez the Mikuni to fit in, little long. Do I cut it down to fit like the Keihin?
I've read a couple of previous posts.

Any advise could really help. I'm trying to get ready for the upcoming D-36 cross country seires.

Chris

I'd try checking the reeds for condition and proper seating, adjusting the air screw, maybe raising the needle one notch, a bigger pilot jet if needed, etc. If different needles are available for the Mikuni, that might also be an option down the road. Can't hurt to check for air leaks either.

I have your same Keihin on my 175cc CR hybrid. It was also lean-bogging a bit off idle. I adjusted the air screw in to 1 turn (which really helped) and will probably have to go up one size on the pilot (currently 42). I also raised the needle, and will test it today.

I don't think the piston skirt height should affect things too much on a reed-valve motor, if the reeds are in good shape and seating well. I'm also more inclined to modify the air boot/manifold than the carb, although I have done it once or twice.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks for the reply Ray,

On the Mikuni carb the intake side of the carb is just longer than the stock Keihin, fits in the boot diameter just fine. I don't know how to check for a leaks, can you give some advice on that.


I'm going to but a larger pilot jet and raise the needle try that first. Then also use 40to1 mix is what I use on our KTM's as a second take.

See what happens. Need to get the air screw to work properly and I will figure it out.
The reeds seem good....not sure of the crank seals they are the originals.

Just seems more than just jetting problems

Thx Chris

Chris,

I've seen posts on the Web where people have used WD-40 and squirt bottles of gasoline to check for leaks, spraying around gaskets, seals, and other suspected leakage areas. I've never tried this myself. My approach is to assume that everything may be leaking except the right crank seal (if this is leaking on your motor, you won't need to troubleshoot it much) and then inspect/replace/resurface/regasket as needed. I've had one air leak (left crank seal) and I never want another one, even if it means a little more initial work and expense.

I'd adjust the air screw before doing anything else. It will give you a better idea of what pilot jet (or range of jets) to get.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Screwing the air screw all the way out should kill the bike I am pretty sure if it can go all the way out its too big a pilot and if it can go all the way in its to small a pilot. As I understand it the air screw leans it out as you turn it out and richens as you turn it in so if your leaning it out that much and its not killing the bike its too rich. The thing is I may have it backwards I do that sometimes. This is for a mikuni I dont know a whole lot about kehins.

You are right.
I tried installing a smaller pilot jets and the hesitation got worst.... with the air screw all the way out then engine would still keep running, just like an idle.

I must have a air leak somewhere. I do have another intake boot. Also try running the engine with out the air filter just for trouble shooting. I have the orignal style filter ( not foam )

I don't want to, but to trouble shoot I think I'll order some crank seals.
But I will keep trying some more jetting..ect.

Thanks for the input guys.....I'll let you know what comes up.

Chris

A couple quick questions - you said it idles fine just bogs when you try to ride it correct?

Is it idling fine with just the choke on or will it idle fine without the choke on?

This might sound strange, but have you checked your compression? A bike with low compression will be really boggy but once you get it going and up on the pipe (and mainjet) it'll feel like it's running pretty good... You can get inexpensive compression guages at just about any auto parts store ie: Auto Zone etc...

The diff carb you put on - was it new or a used carb you were putting on? If the pilot air circuits are dirty (the small hole on the back of the carb) and open just enough to idle, it can bog when you try to give it more gas - guess what, you can even take out the pilot air screw if that happens - but if that 2nd carb is a new one, then that eliminates this potential problem.

If you had bad main seals on the left it'd be sucking air and prob revving up high or if it's only "pushing" you'd see fuel under the stator (or wetness). If it were the right hand side (or wet side) it'd be blowing blue/white smoke everywhere like a fog machine...

Mike

_________________
The Race Isn't Over Until YOU Cross The Finish Line!

A stock 1979 CR125R Kehein carb can be replaced with a Mikuni VM 32mm round slide carb. I am at sea level and use a 300 main jet, 40 pilot jet and 2.5 cutaway slide. I did shave off a few mm or more from the bell to make it the same length between the carb and air boots. Just measure your stock carb to get the correct length. I had to drop my needle down to one notch below center to lean out the first 1/4 throttle.

good luck