Jetting Advice 28MM FS Mikuni, 200X race motor - DUMB AGAIN!!

[Dirtcrasher]

Dirtcrasher, I would think that If the choke makes no difference, then you are still lean. If the choke being on doesn't change anything, and the pipe Is glowing your not getting enough fuel. If the timing was retarded, like Mosh suggested that could cause excess heat In the pipe. What kind of carb Is on It? I know It's a Mikuni, but is the air/fuel screw In front of the slide? or behind It?

Thanks Jason, the airscrew is in the back half of the carb at 1 turn out. Thats the only place we had any 0-1/4 throttle response and it made it idle.

Correct me if I'm off..... But if I can start a freezing cold motor doesn't that mean my pilot is too rich?? It's cold as hell outside, I kick it with the choke on - it bogs right away - so I take the choke off and it runs freezing cold....

Doesn't that mean the Pilot is too rich?? but the headpipe says lean??

The choke and plunger are brand new also!!

I always thought that if the choke did nothing, you were rich in the pilot/airscrew circuit??

Thanks guys for the thoughts and quick responses!!!!!!!!!!!!!!!!!!!!!!!!!!

[edog]

This is it-

Maybe you should try a bigger carb?

[Jason Hall]

I edited my last post, but you might have missed it. If the A/F screw Is behind the slide you are adjusting AIR, not fuel like a normal 4 stroke carb.

[Dirtcrasher]

Does the carb have an Air/Fuel SCREW?

This is it-





I believe it is an AIRSCREW not a fuel screw.......

Anyone know what the "AIR JET" does?????????? It's in the back half of the carb, just where the air box boot mounts.....

[Dirtcrasher]

I edited my post because you answered my question. If the carb has the A/F screw behind the slide, you are adjusting AIR, like a 2 stroke. A 350x or normal 4 stroke carb has a FUEL screw In the front of the slide. Remember now you are adjusting air, and not fuel when you adjust the A/F screw on your carb. What happens when you tunr the A/F screw all the way In?

Thanks Jason......


So that means that if I turn it in I am taking air away from the starter circuit correct??

I'm at 1 turn out that idles but if I turn it in more, I imagine I am letting more fuel into the starter/idle circuit??

But, if I let more fuel in, how can the choke be doing nothing at this setting I'm at now? More fuel with closing off the air would continue to allow the choke to be useless - no??

[Jason Hall]

Ok, I know your not a Dummy LOL, but Is their a chance that the thumb throttle Is holding the slide up? It also looks like thats a 2 stroke mikuni. I guess from what you have said, the choke being turned off does make a slight difference. So I would for sure say you are correct to say It's rich. If the slide Is held up by the cable, then the air gap could be Inconsistent & could make the Idle Inconsistent.

[Dirtcrasher]

Sorry........ I made the dummy comment because some people are beyond the very first critical steps and thoughts

Nah, there is plenty of cable slop...

Everything I've ever owned needed the choke for 30 seconds to a minute.

This does not, which (I thought) told me that the starter circuit was too rich.

Mind you that I have been dickin with this for awhile now, it wasn't a perfect running motor at any point. It ran at TF with 98 octane and 50% of the power it has now....

To me, I should drop down to a 40 pilot, but that would be even leaner and the headpipe would get even hotter... Is a glowing headpipe normal?? I don't think so myself with ANY mods.....
]
Maybe I'm wrong??

Of course I'm wrong, this just doesn't seem right to me.

Is it at all possible that I have to go leaner on my pilot and richer on my airscrew?? Just throwing it out there guys......

I do seem to be somewhat flat on the bottom.

[Billy Golightly]

THis already sounds similar to the gremlin I've been chasing on my 86 ATC250R thats driven my to extreme ends of insanity.


Sparkplug gap, and heat range? Good closeup Picture of the plug would beneficial to me.

I'm leaning towards timing, myself at this point. I don't agree with the red glowing 450 pipes being normal. I've seen it happen but its not a real common thing here and any of the pros or pro am riders I've seen don't ride with glowing pipes.

Are you SURE, you don't have any airleaks? How do you know?

On the Keihin's and my limited experience with the bigger Mikunis, the airscrew controls air and fuel mixture at idle and just the very beginning of the throttle range where the pilot jet picks it up.

And I'm not positive, but I'm pretty sure the bigger the #, the bigger the jet with the mikuni's just like the Keihin's but just a different range. Like a 320 mainjet is still richer then a 280 main for Mikunis. I have a few pilots for a bored 38mm TMX series and bigger=bigger and smaller=smaller.

[Jason Hall]

Ok, I would drop the pilot back down. I feel that you are tuning a 4 stroke with a 2 stroke style carb. I'm not sure It will come Into tune!!! Most all 4 strokes have a fuel screw, not an air screw. Remember this. Air goes through the pilot hole In the rear bell of the carb. With an air screw you are adjusting the air before It contacts the fuel to be pushed through the carb. With a fuel screw, the air has allready pushed fuel against the screw. I think that the Idle circuit of that carb Is to big for a small 4 stroke. I'm not that familiar with the flat slide Mikuni's. I would put a 32mm FCR on that engine.

[Mosh]

I thought all 4 stroke carbs had the air screw after the carb towards the cylinder?

[Jason Hall]

Thats what I'm saying Mosh, the 4 stroke has a Fuel screw. If the screw Is In front of the slide your adjusting fuel. If the screw Is behind the slide your adjusting air.

[oscarmayer]

i always thought the pilot was for idle and low range. now assuming this is true, why a 50? that means you really need a 36-38 pilot most likley.
now from what I understand the MAIN jet is what handles everything from about 60% and up on the throttle if i rememer correctly. assuming i'm correct(my memory has been having issues latley), then that means your main jet is too lean.

drop back to a 38 pilot, and then up the main by like 3 sizes to start. set the needle in the midle clip (if the flat slide has one i forget if it does or not) and start from there. if your at 2/4 your out of the pilot jet range and running only on the main jet if i rememebr correctly.

[Daddio]

It does look like a 2stroke carb. That could be whats causing the problem on the bottom. Check this link.
http://www.thumpertalk.com/forum/arc.../t-595079.html

[Jason Hall]

It sounds like he has the needle and the main set pretty close. The pilot Is where his trouble Is. Idle to 1/4 throttle. Is there a chance the slide Is In backwards? If the cutout Is facing forward, It will pull fuel through the main jet.

[Dirtcrasher]

This might be a 2stroke carb modded to work with an 86/87 200X......

People say 2 stroke or 4 stroke carb, but what is the difference anyhow??

I just went for another blast.... It is fast as can be on top and loves life!! Down low it is erratic and stumbles.

This is the plug which says "RICH" to me and I'll bet I am a bit rich on the main but it seems to love to be WOT....

If it is rich, why is a 50 pilot glowing the headpipe!!



This is also RACE GAS which definitely seems to affect the plug color reading...

I think I can take that main down a few thousandths...... but I certainly have a 0-1/4 throttle problem....

Dam, I even downloaded the Mikuni tuning manual and not one frigin page says " the air jet is primarily responsible for.........."

Thanks so much for any help guys and I WILL get this and I WILL tell you all what I find....................

Again "Jetting is no easy task!!"