Need advice TT500

Hello, I have been fighting with a lingering problem with my 79 TT500 for about 6 months now and as of today I am officially out of bullets.

 

Background:  Last year I rode all the local vintage mx races with a basically stock bike. The only upgrades were a 36MM Mikuni , K&N pod filter and a Supertrapp silencer with a stock headpipe. Last winter I modified the bike to a HL replica. I installed a GMC left side pipe, YZ400 forks with emulators, 78.5 RM250 swingarm, new shocks, 76 YZ125 seat , tank and plastics. After the upgrades the bike started and ran fine after I got the suspension dialed in and the carb jetted for the bigger diameter pipe. (I run straight 110 octane race fuel and did last year as well) Jets in bike are 240 main, 37.5 pilot and clip in 3rd notch. Last year with the old pipe the main was 230 and pilot was 32.5.

 

At the first race of the year the bike started missing and popping on the last lap. Since then the problem has gotten worse at each race until the last race where the bike ran great for the warm-up lap but ran like crap for the entire race. It was a very hot day. The problem is worse as the ambient temp. rises.

 

The bike starts easily and runs great when cool. As soon as it heats up (after 3 or 4 minutes of hard riding) it starts to backfire, pop, and has no power. It will idle fine and if you roll the throttle on very slowly it is not as bad but if you nail the throttle it will just sputter and pop.

 

What I have done so far:

changed sparkplug (BP8ES) Color of plug looks good

Checked valves (.005 on intake and .007 on exhaust when cold)

Installed new sparkplug cap 

Checked points gap and condition

replaced condenser

checked timing

checked governor plate to make sure it moves freely

Installed new exhaust gasket where pipe bolts onto the head

changed fuel filter

cleaned carb.

upped main jet to 250 and then 260...this did not help so I went back to 240

None of this helped at all so yesterday I started swapping parts from another TT500 that I have that runs good:

 

swapped carbs

swapped coils

swapped wiring harness

 

 

None of these things helped either, today I guess that I am going to put the old pipe back on and see if that helps. The problem is as soon as the bike gets hot it runs like crap, hot weather outside makes it worse. I have a race this weekend so if I dont make any headway I guess I will swap motors out but I hate to do that as I will need to get this resolved eventually! The thing that I dont understand is that the bike ran great last year with a very similar mechanical set-up. All I did to the motor over the winter was change the pipe.

 

Any help will be appreciated

Thanks

Mark

 

 

Hi Mark:

I've heard of a similar situation that was caused by the magneto coil.   The coil resistance measured fine at low temperature but caused rough running 

in high heat.    It was assumed the coil had an open winding that made contact and worked okay at cool temepratures, but as the coil expanded with heat 

the winding separated.  In his case too the problem was more severe as time wnet on.

 

I'm not sure if  you;ve swapped magneto coils or just the spark coil..

 

patuca

Hi Mark:

I've heard of a similar situation that was caused by the magneto coil.   The coil resistance measured fine at low temperature but caused rough running 

in high heat.    It was assumed the coil had an open winding that made contact and worked okay at cool temepratures, but as the coil expanded with heat 

the winding separated.  In his case too the problem was more severe as time wnet on.

 

I'm not sure if  you;ve swapped magneto coils or just the spark coil..

 

patuca

I did not swap out the stator plate. I am not much of a motor guy (evidenced by this thread:)

 

I assume I will need a flywheel puller and then just unbolt the plate from my other TT500 and swap out?

 

Will I need to re-time the bike or just bolt into existing holes? 

 

Mark

Hi Mark,

I have had a similar issue as Patuca mentions above, though it was on a much different motorcycle (70s Ossa 2 stroke). Symptoms were slightly different on the 2 stroke though, it would stutter and pop once it was warmed up, then foul the plug. I still think that the magnito could be the culprit for you!

 

Curious if the the re-built bike was ever running good when warmed-up during your jetting and suspension tuning trials? Like pre-race 1 did you do any longer rides without issue?

 

Also one more thing to check - does the seal between the header pipe and the exhaust pipe leak at all once warmed-up? This would cause a loss in back pressure and potential throw things off

Hi Mark,

I have had a similar issue as Patuca mentions above, though it was on a much different motorcycle (70s Ossa 2 stroke). Symptoms were slightly different on the 2 stroke though, it would stutter and pop once it was warmed up, then foul the plug. I still think that the magnito could be the culprit for you!

 

Curious if the the re-built bike was ever running good when warmed-up during your jetting and suspension tuning trials? Like pre-race 1 did you do any longer rides without issue?

 

Also one more thing to check - does the seal between the header pipe and the exhaust pipe leak at all once warmed-up? This would cause a loss in back pressure and potential throw things off

I'm going to say that I think the problem has been there all of this year. In the early races this year the temperature was cooler and I saw the problem at the end of the moto....maybe the last half lap or so. As the summer progressed and hotter temps were the norm the problem arose much sooner in the moto. 

 

This Geoff Morris pipe that I put on may not have much backpressure, maybe I will just throw the old supertrapp system back on and give that a try before I dig into the stator coil.

 

Mark

Hi Mark:

 

Changing the stator plate requires the standard Yamaha flywheel puller tool.  Timing on your bike is done by the points and there is no critical positioning of the stator just replace it and hook up the plug.  It's an easy job and your existing timing would remain as is.

 

Trying the S-trap wouldn't take long and is worth looking at.

 

patuca

Hi Mark:

 

Changing the stator plate requires the standard Yamaha flywheel puller tool.  Timing on your bike is done by the points and there is no critical positioning of the stator just replace it and hook up the plug.  It's an easy job and your existing timing would remain as is.

 

Trying the S-trap wouldn't take long and is worth looking at.

 

patuca

Well....here is an update.

 

I cut some of the old brackets off the frame so I was unable to put the old pipe back on without fabbing some temp. brackets. We had a race Sunday in New york so Saturday I decided to swap motors. When I swapped motors I also swapped wiring harness and intake manifold to further eliminate possible causes. (I had previously swapped coils and verified that they were good. 

 

So I put the new motor in with the Geoff Morris pipe.......go to the race and the bike runs great until the first lap.....whammo, same problem!!!!! So I guess that by process of elimination I have narrowed the culprit to the pipe. The thing that I dont understand is why this pipe wont work. The motor(s) are stock with no top end work and as I said the bike ran great last year with the old (smaller dia. headpipe) supertrapp. When I put the new pipe on I replaced the exhaust gasket where the pipe bolts on and when it is running I have no leaks. I am sure this pipe flows much more than the old one.

 

I have one more race this year so I am going to have to fab up some brackets to mount the old pipe....and the old motor goes back in today

 

Mark

Well....here is an update.

 

I cut some of the old brackets off the frame so I was unable to put the old pipe back on without fabbing some temp. brackets. We had a race Sunday in New york so Saturday I decided to swap motors. When I swapped motors I also swapped wiring harness and intake manifold to further eliminate possible causes. (I had previously swapped coils and verified that they were good. 

 

So I put the new motor in with the Geoff Morris pipe.......go to the race and the bike runs great until the first lap.....whammo, same problem!!!!! So I guess that by process of elimination I have narrowed the culprit to the pipe. The thing that I dont understand is why this pipe wont work. The motor(s) are stock with no top end work and as I said the bike ran great last year with the old (smaller dia. headpipe) supertrapp. When I put the new pipe on I replaced the exhaust gasket where the pipe bolts on and when it is running I have no leaks. I am sure this pipe flows much more than the old one.

 

I have one more race this year so I am going to have to fab up some brackets to mount the old pipe....and the old motor goes back in today

 

Mark

Is it possible that the pipe is not fitting correctly to the head allowing air to be sucked back in? I have a hard time believing that the design of the pipe is wrong, Geoff sells lots of them. As I said, I replaced the gasket at the head but maybe I dont have it sealed up well enough.....does anyone know if a leak where the exhaust bolts to the head would cause this type of problem?

 

Mark

Hi Mark:

 

I think your bike might just be starving for fuel at wide open throttle.....The bike runs okay while testing and only has a problem under race conditions....both setups act the same and you have a bigger carb, jets, pipe, and the bike would require a huge gas flow under racing conditions.    I don't believe the pipe would cause this problem and you've eliminated almost all the other possiblities.

 

A faster flowing petcock and larger  tubing to deliver more gas might be the key.  The symtoms sound like fuel starvation, is that possible?

 

patuca

Hi Mark:

 

I think your bike might just be starving for fuel at wide open throttle.....The bike runs okay while testing and only has a problem under race conditions....both setups act the same and you have a bigger carb, jets, pipe, and the bike would require a huge gas flow under racing conditions.    I don't believe the pipe would cause this problem and you've eliminated almost all the other possiblities.

 

A faster flowing petcock and larger  tubing to deliver more gas might be the key.  The symtoms sound like fuel starvation, is that possible?

 

patuca

Well.....I guess that is possible. The tank and petcock and fuel line is another constant between the 2 motors I had in. And I did swap tanks over the winter at the same time I installed the new pipe. It runs like a bear wide open until it gets hot though. Wouldn't it be starving for fuel all the time cold or hot?

 

BTW...I really appreciate your responses, I am totally stuck

 

Mark

Now you got me thinking, Actually if that was the problem it would explain why I see the problem later in the moto's. The little YZ tank is about 1/2 the size of my old one and it has a different petcock with a smaller diameter hole. I just went out to the shop and did some flow tests with the old tank vs. the new one.

 

When both tanks are full up they flow a lot of gas but when the little YZ125 tank gets like 1/2 empty the flow slows significantly. I also have a filter on it that appears to be restricting the flow some. 

 

I had the problem this weekend but I did not top up my tank before the race and then I ran it for 5 or 10 minutes before the start because I wanted to get it hot to see if the problem still occurred with the different motor.

 

I just cant ever remember during my testing if I ran the bike until the problem happened and then let it cool and did not have the problem until it got hot again without adding more gas to the tank.  I guess that it is possible that I am right on the razors edge of fuel starvation all the time and when it gets real hot it put it over the edge, I am not sure if a hot motor uses more fuel than a cool one....anybody know?

 

Mark

Maybe you could try the regular XT tank, make sure the tank-breather is clear, remove any in-line filter and check the tube.  Also the flow is dependant on the "head-pressure" of the tank (small volume of gas, close to the carb = low flow and low pressure).  Most race bikes get around this with big tubing and no filters and even hi-flow petcocks.

 

I'm not sure about the heat increasing fuel consumption...it seems to me that a warm engine would be the same as a hot engine, but for sure the gas flow would be higher during race conditions than any other time with prolonged WOT....maybe all the factors come together :  reduced fuel pressure, lower flow, longer periods of WOT eventually reducing the flow to the point the float bowl can't keep up. 

 

I wonder how the bike runs immediatley after the moto at reduced throttle?  If it's back to normal it might be worth experimentally changing tanks and removing the filters....

 

patuca

Maybe you could try the regular XT tank, make sure the tank-breather is clear, remove any in-line filter and check the tube.  Also the flow is dependant on the "head-pressure" of the tank (small volume of gas, close to the carb = low flow and low pressure).  Most race bikes get around this with big tubing and no filters and even hi-flow petcocks.

 

I'm not sure about the heat increasing fuel consumption...it seems to me that a warm engine would be the same as a hot engine, but for sure the gas flow would be higher during race conditions than any other time with prolonged WOT....maybe all the factors come together :  reduced fuel pressure, lower flow, longer periods of WOT eventually reducing the flow to the point the float bowl can't keep up. 

 

I wonder how the bike runs immediatley after the moto at reduced throttle?  If it's back to normal it might be worth experimentally changing tanks and removing the filters....

 

patuca

On the way back to the pits on a much reduced throttle it runs fine. I am going to put on a higher flowing petcock and remove the fuel filter. The tank is real clean inside so I am safe to try that at the next race.

 

Mark

I wonder how the bike runs immediatley after the moto at reduced throttle?  If it's back to normal it might be worth experimentally changing tanks and removing the filters....

 

patuca

 

Well well well.... we had our last race of the year this Sunday. It was a hot sunny day and the bike ran like a freight train. I removed the fuel filter and kept the tank topped up after every practice session and moto (something I was not accustomed to doing with the old stock TT tank). So I think I can say with certainty that the issue was indeed fuel flow. Reading back through my own post I can't believe that I could not see the forest for the trees!

 

I cannot that you enough for the advice.

 

Mark

001_zps713d0979.jpg

Wow....nice race-bike Mark...good job.

 

patuca

Edited by patuca

Does that pipe sound as AWSOME as it looks ? 

Does that pipe sound as AWSOME as it looks ? 

Its not super loud. I had a stock headpipe and a supertrapp on the bike at one point and it is a bit louder than that. It is repackable and truly a quality piece of work. I wish that I could have found someone nearer to home to build one. Ended buying this one from Geoff Morris concepts in Australia.

 

Mark

GMCpipe1.jpg

GMCpipe2.jpg

I am gathering parts to build a sister bike this winter. I scored a (visually challenged) but very strong TT from New-York that has a bunch of go fast parts in it. I have everything that I need in spares so its just getting time to work on it. I love building Frankenbikes :)

 

Here is a pic. of my current vintage racer, a 73 MX250 Yammie

Newarm.jpg

Actually that is an older picture, since then I put on a set of TT500 inline forks with racetech gold valves and a set of shoulderless TT500 wheels. Those shouldered rims were mud magnets...and heavy to boot!

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