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Life is better on a R1
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Discussion Starter #1
I have tried every dirt bike forum, no one has any opinions to help me :nopity:

I have a stock TTR-50 that never idled from day one. I tried everything, new pilots, all air screw settings, cleaned it, read tuning guide, blah blah... shot story, the carb is gone and in place is a new Keihin PC18.

The PC18 is possible a little 'too big' for my stock 50. Can someone explain what 'too big' means in terms or mechanics/physics?

Also, with this new carb, it idles perfectly. If I slowly increase the throttle to wide open, everything is fine.

If I whack it open from idle, it will sputter and die. Perhaps this is where 'too big' comes into play.

Any ideas? I have tried a 75, 85, and 95 main jet, and tried the needle up and down the clip range. No discernible difference with the stumble.

So does anyone know why this occurs? On a google search, I read so many contrasting opinions, I think this ranks up there with oil questions.

THANKS!

Derek
 

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Not Again
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1,314 Posts
had the same prob on my ttr50 put a thicker filter on the bike and now she frickin rocks , but i have an azz load of work done to mine :fork

it was all related to the air/fuel jet size was never a real issue
 

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Life is better on a R1
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1,087 Posts
Discussion Starter #3
had the same prob on my ttr50 put a thicker filter on the bike and now she frickin rocks , but i have an azz load of work done to mine :fork

it was all related to the air/fuel jet size was never a real issue
I tried a 15 pilot, up from the 12.5 stock one. Both did not work, it would only idle with the plunger on. Seems like many people have this issue, some were able to fix it with a 15 pilot, some were not...

What pilot do you have?

Derek
 

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Yamahammer
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288 Posts
At a minimum it sounds like your new carb's main jet is too big, which makes the fuel mixture too rich. A smaller main jet will lean out the mixture. What was the original main jet size? Did you go up in sizes from there or down? There may be additional problems with the jetting other than the main, but getting the main jet close first will enable you to diagnose other issues.

If the bike is still stock, I would put the stock carb back on (with original jets), turn up the idle some, put fuel/air screw back to stock position (or about 2 turns out), check for air leaks, and tune from there. As you know, small changes can be made with the fuel/air screw. I would adjust it first, then try pilot changes.

There is a great description on Thumpertalk.com, in the jetting forum, on how to adjust the fuel/air screw. Here is the linkhttp://www.thumpertalk.com/forum/showthread.php?t=327405

By the way, I also responded to your post on TT as well. Hopefully, it will generate responses with others that have experienced the same problem.
 

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Life is better on a R1
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1,087 Posts
Discussion Starter #5
At a minimum it sounds like your new carb's main jet is too big, which makes the fuel mixture too rich. A smaller main jet will lean out the mixture. What was the original main jet size? Did you go up in sizes from there or down? There may be additional problems with the jetting other than the main, but getting the main jet close first will enable you to diagnose other issues.

If the bike is still stock, I would put the stock carb back on (with original jets), turn up the idle some, put fuel/air screw back to stock position (or about 2 turns out), check for air leaks, and tune from there. As you know, small changes can be made with the fuel/air screw. I would adjust it first, then try pilot changes.

There is a great description on Thumpertalk.com, in the jetting forum, on how to adjust the fuel/air screw. Here is the linkhttp://www.thumpertalk.com/forum/showthread.php?t=327405

By the way, I also responded to your post on TT as well. Hopefully, it will generate responses with others that have experienced the same problem.
Thanks for the response. Believe I tried all last fall and this summer. There is only so much you can do with the air screw and pilot. I even downloaded the Mikuni VM tuning manual and followed the procedure. I have tuned a lot of carbs - I couldn't even get this one to idle, much less tuned. On TT, there are many people with the same issue, and some with no issue at all. I can't understand it - maybe different batches of carbs? I did all the air screw adjustments - without that starter plunger on, it will die, even after 20 minutes warming up.

The new carb idled instantly after I adjusted the air screw on that one.

The new carb - It came with a 85 and a 92. It is meant for the 88cc big bore kit, so trying it on a 50cc was a little questionable. I put in a 75 main and will see how it works today after a warm up, but in my garage, it didn't help.

Yes, the TTR should just work from the factory, but I am definately not alone. I will check your post on TT - thanks!

Derek
 

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Ghostrider Squadron
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9,251 Posts
okay try this.. seen it few times... its not carb.. if u have done all that.. its not carb.. my bet.... would be.... check valves.. bet exhaust valves have zero clearance at all. intake will be fine, but my guess check vavles....
its not big deal.. should have little cap on valve cover.. just take it off..roll motor around till u see rocker arm go all way down... then grab rocker arm and pick it up, move it a little see if there is any slack at all.. they dont run much clearance, so if u can fell it come off valve any.. it should do..but my guess is that there is no clearance.
 

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Life is better on a R1
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Discussion Starter #7
okay try this.. seen it few times... its not carb.. if u have done all that.. its not carb.. my bet.... would be.... check valves.. bet exhaust valves have zero clearance at all. intake will be fine, but my guess check vavles....
its not big deal.. should have little cap on valve cover.. just take it off..roll motor around till u see rocker arm go all way down... then grab rocker arm and pick it up, move it a little see if there is any slack at all.. they dont run much clearance, so if u can fell it come off valve any.. it should do..but my guess is that there is no clearance.
But my new carb I just put on, the PC18, idles fine, so my assumption when I did that was that the engine was fine - true?

Derek
 

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Ghostrider Squadron
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9,251 Posts
could be.. but for 5 minutes it takes to check it..i would at least look. even if ur not vavle guy and not gonna do it urself.. seen too many of little bikes do that. they run such small clearance and heat stretches vavles and gets rid of .002 clearance they have. makes them not idle and not take throttle. like i said u dont need feeler guages to check them or set them.. just so they lift up a hair off valves.. they are good. worth checking.
 

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Life is better on a R1
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1,087 Posts
Discussion Starter #9
could be.. but for 5 minutes it takes to check it..i would at least look. even if ur not vavle guy and not gonna do it urself.. seen too many of little bikes do that. they run such small clearance and heat stretches vavles and gets rid of .002 clearance they have. makes them not idle and not take throttle. like i said u dont need feeler guages to check them or set them.. just so they lift up a hair off valves.. they are good. worth checking.
Ok - I'll check it out, it is all out in the open on this bike, fairly easy...
 

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Yamahammer
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288 Posts
I'm not sure about the TTR, but usually you get turn the crank by removing a plug on the left side of the engine. Under the plug there will be a nut and you can see the marks for top dead center (TDC). If you can't find that, put the bike in gear and turn the rear wheel. You can check for TDC through the spark plug hole (with a wooden dowel or something plastic).

I have doubts about the valve theory because you said the bike won't idle even when new, and that other TTR owners have the same problem.

I'm not questioning you that Yamaha didn't get it right from the factory, but it at least should be close. Minor changes to the jetting can make a big difference, especially on the low speed circuit that affect idling.

If the new carb is idling fine and runs good, except when wide open, try smaller main jets. Get several sizes and keep trying. Otherwise, you may want to try the factory carb again and work with the pilot jets and fuel screw.

Also, check out the TTR forum on Thumpertalk, it is under the Yamaha category. Guys had a few different solutions there:

Shim under the factory needle (which isn't adjustable)
http://www.thumpertalk.com/forum/showthread.php?t=643517&highlight=ttr+50+idle

Cleaning filter and new plug b/c of oil getting to the carb:
http://www.thumpertalk.com/forum/showthread.php?t=541250&highlight=ttr+50+idle

Other guys went to the next size up on the pilot.
 

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Life is better on a R1
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1,087 Posts
Discussion Starter #13
I'm not sure about the TTR, but usually you get turn the crank by removing a plug on the left side of the engine. Under the plug there will be a nut and you can see the marks for top dead center (TDC). If you can't find that, put the bike in gear and turn the rear wheel. You can check for TDC through the spark plug hole (with a wooden dowel or something plastic).

I have doubts about the valve theory because you said the bike won't idle even when new, and that other TTR owners have the same problem.

I'm not questioning you that Yamaha didn't get it right from the factory, but it at least should be close. Minor changes to the jetting can make a big difference, especially on the low speed circuit that affect idling.

If the new carb is idling fine and runs good, except when wide open, try smaller main jets. Get several sizes and keep trying. Otherwise, you may want to try the factory carb again and work with the pilot jets and fuel screw.

Also, check out the TTR forum on Thumpertalk, it is under the Yamaha category. Guys had a few different solutions there:

Shim under the factory needle (which isn't adjustable)
http://www.thumpertalk.com/forum/showthread.php?t=643517&highlight=ttr+50+idle

Cleaning filter and new plug b/c of oil getting to the carb:
http://www.thumpertalk.com/forum/showthread.php?t=541250&highlight=ttr+50+idle

Other guys went to the next size up on the pilot.
This whole carb issue is a mystery for sure. The problem I have with the new carb is not WOT itself, that works. All the throttle ranges work fine, as long as I move the throttle in a smooth fashion. If I whack it from idle to WOT very quickly, it stumbles. My theory is that the engine speed is too low and is not generating enough low pressure to pull enough fuel into the air stream (the air volume is high due to WOT position). Because if I crack the throttle and get the revs up, then whack it open, its fine. It is only from low rpm to WOT.

As for the oil in the filter - I had that problem, but I rerouted the crankcase vent to not go to the airbox, so it never fills with oil anymore.

The other issue with the shim - the needle is for 1/4 to 3/4 throttle, not idle. That stock carb did not idle but ran fine in all other areas. The only things used in idle is the pilot jet and the air screw, both of which I messed with endlessly. I have never seen such a idling issue in anything, ever. Thus I concluded the carb was a piece of crap after a year of messing with it...

There are many on TT that never resolved the issue. There is even a guy who has TWO TTR50s, one has always worked great, and the other has never worked. Very strange.

Its all very depressing...

Since I have the throttle limiter on for my kid, the bike is running good now. At least he can ride it without stalling. Before he would cut the throttle and it would stall, then he just falls over. If I ride it without the limiter, I just have to be careful to get the idle to WOT transition right, then it will run fine in higher RPMs.




Derek
 
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