Vintage Dirt Bike Q & A

Hey all,
I have posted before on the subject of getting a Mikuni 34mm VM round slide carb jetted properly for a stock 1979 CR125R. Since my last post, I had the cylinder replated to standard bore and bought a new wiseco piston, rings, bearings etc.

I am still not there on the jetting though. Here is the history:
Bought the Mikuni VM 34mm with 159-Q6 needle jet, 6FJ6 needle, 260 main and 40 pilot. (too lean)
Tried 159-Q2 needle jet, 6DH4 needle, 310 main and 35 pilot (had to drop the needle all the way down and it was still rich)
Got the cylinder plated and kept the above settings - except raised the needle position 2 clips to the middle position - better but still a little rich.
Tried 159-P0 needle jet, 6DH4 needle, 310 main and 35 pilot (needle in middle position)

As you see, I keep going leaner on the needle jet. I hope I didn't get too lean with the P0. With the latest settings, I'm getting a bog in the 1/4 throttle range. I'm pretty sure the pilot is lean and I will try a 37.5 and a 40 that I have in the shop. I'm also getting a little rough riding at near full throttle. Since the bike is not broken in yet, I'm a little hesitant to rip it up yet. So I'm not sure if its a too rich or too lean condition. I am also thinking about going back to the richer needle (6FJ6). I may also need to try a larger main. I have a 320 and 330 in the shop as well. It's just such a pain to keep switching out these jets!!!!! I do try to limit what I change at one time to keep it as simple as possible, but since the replating, it looks like I'm starting over again.

Does anyone out there have any sudgestions for me? I know some of you guys out there have VM34's on your 79 CR125's, do you guys know what needle jet size etc you are using?

In the mean time, I will keep plodding along.

thanks guys,
nola_fiddy

At this point, it sounds to me like a richer (smaller cutaway) throttle valve may solve the lean bogging problem. On paper the TV cutaway supposedly acts about the same as the straight needle diameter, but in practice it has a (vacuum-related?) effect on throttle response the needle D does not, at least in my experience.

If the main jet roughness is due to a lean condition, it might be best to go one up on the main and then wait until break-in is finished to sort out the main jet.

This is what has worked for me:

1) get the main jet reasonably close to 'correct'
2) get the needle jet/diameter/clip position close
3) find a pilot jet that will allow the air screw to achieve max idle revs in less than 2.5 turns out
4) if throttle response needs work, try different throttle valves if available (may require pilot change)
5) fine-tune main jet
6) fine-tune needle position and taper if needed

This took me 6 months with one carb, but the results were worth it. I had the advantage of using the stock carb and the OEM baseline jetting, some of which I ended up using (after trying every other possible combination ;>).

You're obviously making all these changes one at a time, not several at once? And taking notes? It's a pain, but I would have been lost without my notes, especially a few months down the road when I (almost) started repeating things I had already tried.

Ray

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

Last night I took out my carb and put in a 40 pilot jet, 320 main jet and went back to the 159-Q2 needle jet. I left the needle itself alone with the 6DH4. I started it up and it idled fine. It was 11pm and I live in the suburbs - so I couldn't let it run more than a few secs. Sure way to get the neighbors angry! But anyway, before I changed to this current setting, I wasn't getting any smoke outa the pipe at idle (too lean). Now it was puffing away like it should.

When I get home from work today, I will take it for a ride around the block a few times and let you know how it goes. I'm hoping to be able to let everyone know of a good base point to start with when jetting the 1979 cr125R with a Mikuni VM34. It just seems like this info should be out there already.

There is a website with jet sizes listed for bikes with Mikuni conversions but I cant remember what it is.

Will have a look for you.

Well the bike ran pretty good with the following: 40 pilot jet, 320 main jet, 159-Q2 needle jet and a 6DH4 needle. I had to drop needle down by putting the needle clip on the highest position. It screams on the top end but the low end is a little weak.

I am thinking that the bike, all stock, would run better if I switch to the Mikuni 32MM VM carb. I should get more out of the lower end. I plan on buying one and using the jets and stuff from the 34mm. I will update this thread once I get the new carb and give it a try.

thanks,
nola_fiddy

Hey all,
I removed the 34mm Mikuni VM round slide and replaced with a 32mm Mikuni VM round slide. I put in a 37.5 pilot jet and a 310 main jet. I left the needle and needle jet alone but did drop the needle to the leanest setting. I will have to take a look and see what size they both are and report later. I did shave down the air intake bell by 7mm to make it 93mm in total length as well.

The 1979 CR125R is now running a lot better with much better low end power. It still screams when it hits the high end power band though. It still seems a little rich when I hit wide open. I have a bunch of jets and a few needle jets and needles. I will play around with the main jets first to see if I can dial it in any better.

later,
nola_fiddy

Hey guys, I've been ridding my cr125r around the levees surrounding my neighborhood (lake pontchatrain) every now and then. I really can't ride too much because it is a residential area. But I've ridden enough to be able to discribe the bikes issues.

It runs fine all the way to about 3/4 throttle. Some times when I get on the throttle (3/4 and above), it will stumble and keep stumbling until I let off the throttle. Most of the time I can bring it up to full throttle with no stumbling, but it has to be a slow even acceleration, not a quick jolt. Every now and then I can jam on the throttle and it just takes off like it should. At full throttle the bike screams and runs fine.

Again, the bike has exellent compression (new cylinder and piston), pipe is clean and not plugged, air filter is new and not over oiled and I believe the electronics are fine.

Is this a rich symptom? It sounds like a too rich needle jet/jet needle to me. I did have to lower the needle to its leanest setting to get the bike to run well at all.

thanks!
nola_fiddy

I agree - it definitely sounds like a rich condition to me too. I think it's the main jet that's too rich, though, not the needle circuit. I've tried to reduce top-end richness by dropping the needle, and ended up with the same symptoms you've described.

If it were my bike, I would drop the main jet one size at a time and see how it runs. At some point the top end should clear up - then you'll probably notice the mid-throttle positions are too lean, so you can bring the needle back up where it should be. This should give you a bunch of your low end back.

The '79 CR125R Keihin carb came stock with a 155 main jet, which translates to a 320 Mikuni jet - so you're definitely in the ballpark. If it doesn't clear up by 290 or 300, though, I wouldn't go any lower - the problem may be somewhere else.

Just for what it's worth - I don't 'sign off' on any jetting change as being good or bad until I've tested it for at least an hour of hard riding, in the dirt. I've been wrong too many times with jetting that worked completely differently during a short ride than it did in the woods. Maybe you can designate a 'jetting day' where you take the bike to a good riding area and spend a few hours testing/jetting/testing again, taking notes for future reference. Yeah, it's a pain, but the payoff can be huge.

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 input rayivers. I already have a 300 and 290 main jet ready to try. I will see if I can get to it this weekend. I agree with not going any lower than the 290 as well.

I should be getting a couple chain rollers in the mail today. They were not made to fit this bike but I will try to fabricate something. I have a MIG welder and all the machine tools to do the job. Once this is done, I will feel comfortable getting the bike out on the trails for an extended ride. Then I can put the jetting changes to the test.

thanks again,
nola_fiddy

I made a few changes over the past week. I added two chain rollers. They were intened for a 1983 CR125 but I was able to make them work. I also replaced the steel reeds with reeds from Boysen. I dropped the main jet from a 310 to a 300 and lowered the needle clip one position. The bike is running pretty good now. Maybe a tad too rich still. I will take it out to the trails for a good long ride before I drop the main to a 290. I'll keep posting till I'm done.