Mikuni 38mm for 1979 250

Hi guys, looking for a steer if any help is available!

I was racing an Enduro last weekend and there was a lot of flat out fast sections. The top end felt far too rich with noticeable loss of power. The main jet is a 430 and needle is 6DH3 (looks like that anyway!), the handbook spec is a 430 main but needle says Q8?

I’m in UK, so climate is cool / temperate and no riding above 1000ft.

Any suggestions? I guess the main jet could be worn, so maybe just replace with a new 430 and see what it’s like? Or are these generally regarded as too rich? How about needle and position?

Appreciate there are a lot of factors, but a general steer would be great!

In what rpm range was it running rich? 1/4 throttle, half, 3/4 or full? The Q8 is the needle jet size no the needle size. You said top end felt rich so probably safe to going to leaner main jet to begin. I have never had a 250 needing a 430 jet.

Marty

thanks for coming back to me Marty. it's at full throttle that the bike is rich, the rest of the range feels pretty crisp to be honest. So I'll try dropping down a size or two on the main jet. Do you have any knowledge of what the needle size its self should be? Many thanks, Ed

All my 250's never had to run a 430 Main either.
78 250 CR Jetting
Needle 6DH3
Slide 2.0
Main. 390
Pilot. 35
this is my current jetting I'm at 800-1400 feet above sea level.
Bill

Awesome, thanks Bill

A handy carb chart showing which circuit (jet) comes into play at corresponding throttle opening:

Your # 6DH3 needle is correct. What fuel mix ratio are you using?

Marty

Excellent, thanks for the help guys. I’ll replace the needle anyway as I guess it could be worn.

I’m running 45:1 with 99 octane and Putoline fully synthetic - thoughts?!!

Remember the original mix ratio back in the 70's was 20:1 so when you go 45:1 then the factory jetting specs will be rich. Replacing the needle is a good idea if you suspect it to be worn. The needle jet may also be worn.

Marty

thanks Marty, yep that makes sense

Thanks for your advice guys - was racing today and the bike was going like a train!!! Top end transformed, some real pulling power.

That was my last event of the season, so time to get some more jobs done on the old girl. Think I’ll look in to the 40mm fork upgrade (I’m on 35’s) and have to get the Works Performance shocks sent off for a rebuild.

Roll on next season

What did you do to change the outcome? Goes like a train as opposed to running rich at full throttle
I didn't think you could run the mix that lean(45to1) and not have to buy a piston!

I’m running the jetting same as Bill, I’m at the same altitude or lower.
In regard to the mix ratio, I know there’s been a lot of conflicting recommendations on here. My take, as with all the guys I’ve spoken to who race in the vintage enduro series, is that the old manual recommendations of 30:1 or whatever was relevant to the oil quality available. With modern oil (I use fully synthetic Putoline) most of our guys are running 50:1. I’m just going on the slightly cautious end of that.
Most of the events im doing are pretty open terrain, so rarely am I thrashing the bike in slow going, it certainly hasn’t seemed to have got too hot so far…
Of course happy to listen to others opinions on the matter

Heres an excellent read on two stroke oil for the vintage motorcycle.

http://www.klemmvintage.com/oils.htm

thanks for that. Interesting article and certainly educational to me.

I'm not doing much sustained high RPM, just the occasional section of a track across open fields when i can let it rip properly - the suspension slows me down from going hard the whole time!!

Regardless of this, would the suggestion of other members on air cooled / older bikes be to use Maxima 927? I'm only doing 10 riding days a year so the additional cost wont really factor in to it.

Opening a can of worms asking about oil!
Personally I wouldn't be comfortable running my bike on 50:1 though I know many people do with no problems. I ran fully synthetic racing oil from Westway Oils, a company in the UK for years at a ratio of 35:1. After three years of hard use I stripped the top end and found virtually no wear, I really don't see what advantage you get from running less oil.

For the last couple of years just really to try it, I switched to Maxima 927. I find it burns clean, smells great and when I lifted the cylinder a few weeks ago everything still looks like new. The only disadvantage is that it's harder to get it to mix with petrol, especially when it's cold. That said I'm not sure if I'll stick with it, I'm not convinced it's that much better that a good synthetic oil and it does cost a lot more.

True Statement. Ask 10 motorcyclists what the best 2 stroke oil is and you'll most likely get 10 different answers. The performance shop that wrote the article I posted above has been building their own race motors since the 70's. IMO they know vintage motor architecture well. The article is long and full of stuff not related to this thread so here's what I got out of it.

"A two-stroke engine actually has two very different kinds of lubrication needs. The lower end crank and rod bearings prefer a slow drip of a very heavy viscosity oil, while the top end parts wear best with a deluge of a very lightweight oil. Since these are two very opposite lubrication needs, you have to choose whom you want to please. For most two-stroke owners, it’s a no brainer. You can buy 100 cranks and crank bearings that will all offer the same performance, but every owner wants to do whatever can be done to preserve a good running piston / cylinder set. Given all this, heavier premixes of lighter viscosity oils are more desirable to most owners."

"The lesson here is that your premix ratio should be a function of the average operating rpm that your vintage two stroke runs at. If you are at peak rpm all the time, 20:1 is a good idea. However for recreational level riders that don’t “scream” their engines constantly, leaner premixes will yield excellent long term wear."

"For vintage machines that spend very little time held at peak rpm (in the higher gears), API-TC oils are the better choice. Among these oils, our favorites are Yamalube R, Kawasaki K2, and Maxima Super M."

I seem to be getting a server error on posts

Right - the above was a tester, I seem to be able to post again! Does anyone else get a server error?

Anyway, thanks for the input guys. On balance, seems folk run a richer oil mix than what I’m on. Shall richen the mix to 35:1 and see how the bike feels - if the difference isn’t noticeable shall just go with that for a while