Author Topic: Carb Tuning (Read 5348 times)

Knock10090 · « **on:** August 12, 2016, 06:54:52 am »

Ok.. almost have it completed.. .need to know if Im going in the right direction or am I setting myself up for failure.

I just rebuild the top and bottom ends of my 250. My carb (TM34) was set back to stock config.. 37.5 pilot, 6fp60 needle on middle position, 240 main, air screw at 2 turns out, airbox lid on, frame boot off, 110 at 32/1 R50, FMF pipe and silencer (new packing), boyesen carbon fiber reeds on stock reed cage. Im in NE PA at @ 1000 ft

I can light it up and did 3 heat cycles for break-in, re-torqued, and started to cruise around yard at 1/4 throttle. It seems to load up with heavy smoke. Now...

I cant seem to keep it idling. I reset cable with slack, adjusted idle screw to adjust slide height to @ 2mm. and dropped pilot incrementally. To keep it idling I have to keep the idle screw all the way in, the air screw at like 3-4 turns out. I ended today at 27.5 on the pilot, 3 turns out on air screw, lid on and idle screw all the way in... it will idle decently for @ 1 min then cut out. I restarted it and removed airbox lid.. not I could back the air screw down to 2 turns, it would idle consistent and would chug on for several minutes. Smoke was still present but not as heavy as before.

Im starting to grasp the concepts here but seem to be confused why my pilot is way below the stock settings.

Need some help with this. Thanks!

MotorGeek - Jerry Hall · « **Reply #1 on:** August 12, 2016, 07:29:56 am »

Quote from: Knock10090 on August 12, 2016, 06:54:52 am

Ok.. almost have it completed.. .need to know if Im going in the right direction or am I setting myself up for failure.

I just rebuild the top and bottom ends of my 250. My carb (TM34) was set back to stock config.. 37.5 pilot, 6fp60 needle on middle position, 240 main, air screw at 2 turns out, airbox lid on, frame boot off, 110 at 32/1 R50, FMF pipe and silencer (new packing), boyesen carbon fiber reeds on stock reed cage. Im in NE PA at @ 1000 ft

I can light it up and did 3 heat cycles for break-in, re-torqued, and started to cruise around yard at 1/4 throttle. It seems to load up with heavy smoke. Now...

I cant seem to keep it idling. I reset cable with slack, adjusted idle screw to adjust slide height to @ 2mm. and dropped pilot incrementally. To keep it idling I have to keep the idle screw all the way in, the air screw at like 3-4 turns out. I ended today at 27.5 on the pilot, 3 turns out on air screw, lid on and idle screw all the way in... it will idle decently for @ 1 min then cut out. I restarted it and removed airbox lid.. not I could back the air screw down to 2 turns, it would idle consistent and would chug on for several minutes. Smoke was still present but not as heavy as before.

Im starting to grasp the concepts here but seem to be confused why my pilot is way below the stock settings.

Need some help with this. Thanks!

Your carb sounds rich. Here are some other possibilities:

1. It may be getting fuel through the choke circuit becausse the choke plunger is not sealing.

2. The float valve is leaking a little because the o-ring on the outside of the seat is bad or the needle and seat is bad.

3. The float level is too high.

4. You are using pilot jets that are not sized correctly. A common problem with the cheap not genuine Mikuni jets.

5. The needle jet and or metering needle is worn. The needle jets give the most problem with ware. Replace it just to be sure.

6. A piece of reed petal is missing.

The slide being set at 2mm is not set in stone, it is just a setting to give you a starting place for idle speed

Mikuni and Keihin tuning manuals give the theory for the throttle positions vs the circuit that is responsible for supplying fuel at those throttle positions. If one circuit is too rich or lean it will affect the circuits on either side of that throttle position.

Knock10090 · « **Reply #2 on:** August 12, 2016, 08:31:23 am »

Damn.. sounds like I may be chasing my tail now... Is there another carb solution if I decide to go new? To try and eliminate the wear (slide, etc) I may go with a new carb. What would you recommend? Ive read on the Lectron and got mixed reviews... I have a good understanding of the Mikuni TMs now. I put a Keihin on my wife's blaster and have great results with jetting.

MotorGeek - Jerry Hall · « **Reply #3 on:** August 12, 2016, 10:57:04 am »

Quote from: Knock10090 on August 12, 2016, 08:31:23 am

Damn.. sounds like I may be chasing my tail now... Is there another carb solution if I decide to go new? To try and eliminate the wear (slide, etc) I may go with a new carb. What would you recommend? Ive read on the Lectron and got mixed reviews... I have a good understanding of the Mikuni TMs now. I put a Keihin on my wife's blaster and have great results with jetting.

The things I mentioned above are where I find about 90% of the problems with carburetors. Do not omit any of the above steps.

A worn slide of slide channel will not cause the carb to have the running symptoms that you described. A new carburetor that is not jetted correctly will or may have the same problem with your old carb!

Nekrofilliak · « **Reply #4 on:** August 12, 2016, 12:26:55 pm »

X2 1,needle jet, 2, needle 3, lower the pilot jet

Knock10090 · « **Reply #5 on:** August 12, 2016, 08:14:57 pm »

Ok. I will keep trying. A few items I can check off. I checked reeds. They are good. Float parts are new. I need to check the float level.

Thanks for the help. I'll check back tomorrow.

Knock10090 · « **Reply #6 on:** August 13, 2016, 06:28:50 am »

Ok. Located a needle jet locally. I'm thinking that's gunna be my problem. I'm figuring it's probably 29 years old. Next question is what needle to get. The stock 87 is an 6FP60. I cannot source it and most locally are totally different numbers. Does anyone know of a comparable profile? I do have a Y95 which is a 6FP55, but it is a richer needle.

Do the needles overlap? In that the lowest clip position of one needle is like the second from top position on the next richest needle. In other words. Can I run the next richest needle on a leaner clip position and get similar results?

Knock10090 · « **Reply #7 on:** August 14, 2016, 12:46:04 pm »

Ok.. Re-cleaned entire carb, new float needle and seat, new needle jet. Now.. I put the next richest needle in and put it on the leanest clip position. Based on the comparison of the both needles they do overlap and setting the rich needle on the leanest position it equals the stock needle on the middle position. So... It idles great... seems just a bit rich on the needle now so I have to get a new stock needle so I can lean it out.

Thanks for the help... Ill report back after I get the rest of the quad dialed in.

Nekrofilliak · « **Reply #8 on:** August 15, 2016, 07:11:54 am »

needle is used to tune the mid throttle, if the idle only is too rich , lean the pilot jet !

MotorGeek - Jerry Hall · « **Reply #9 on:** August 15, 2016, 08:34:10 am »

Quote from: Knock10090 on August 14, 2016, 12:46:04 pm

...................It idles great... seems just a bit rich on the needle now so I have to get a new stock needle so I can lean it out.

What throttle position does it seem a bit rich. If it seem a bit rich from about 1/8 to 3/8 throttle opening install the next leaner needle jet and or.......Measure the old needle at the straight portion of the needle a few mm above where the taper begins and compare the diameter of the new needle at same distance from equal clip positions. A difference in needle diameter of .0004" will make a noticeable difference in the 1/8 to 3/8 throttle position.

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Author Topic: Carb Tuning (Read 5348 times)

Knock10090

Carb Tuning

MotorGeek - Jerry Hall

Re: Carb Tuning

Knock10090

Re: Carb Tuning

MotorGeek - Jerry Hall

Re: Carb Tuning

Nekrofilliak

Re: Carb Tuning

Knock10090

Re: Carb Tuning

Knock10090

Re: Carb Tuning

Knock10090

Re: Carb Tuning

Nekrofilliak

Re: Carb Tuning

MotorGeek - Jerry Hall

Re: Carb Tuning

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