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The TTR250 FAQ web pages -> TTR250 -> Repairs and maintenance -> Back fires
Post Info TOPIC: Back fires
barra8


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Back fires
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kjd1559 wrote:

Just rebuilt engine. reson bent exhast valves. fitted new piston rings, cam chain and sprockets, clean the carb Bought the bike cheap because it did not run.

its running now but backfires and wont idel. is it up or down on the air screw to make it richer?

 This sounds like you got the timing wrong hmm

You may have to move it one or two teeth on the cam shaft sprockets so check this.

 To ''richen'' the air/fuel screw  is in.

.....................

Jarrah.

 



-- Edited by barra8 on Wednesday 19th of December 2012 09:29:36 AM

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barra8


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brindabella wrote:

huh? it is a four stroke, the screw is a fuel mixture screw, not an air screw. it is on the "downstream" (engine) side of the carb so that it can control not air but a rich mixture of fuel and air from the pilot jet circuit - if you have more of this being "injected" into the air stream you have a richer mixture. if you screw it in (clockwise looking from from below) you decrease the proportion of fuel-air mixture being added, so weakening the mixture. if you screw it all the way in it is lean to the point of NO fuel and the engine stops.

so I believe the correct answer is: screw in the mixture screw to make it leaner at idle, screw out the mixture screw to make it richer at idle.

of course randomly twiddling it is a fine way to get it right, but you wouldn't think adjustment would fix a major backfiring problem.

My bad just checking wink

The more you screw it out the more air gets let in....The more you scew it in the more fuel gets in.

Martyns post kinda confused me & i did say in at the start lol but edited it (not sure why ''maybe i'm half drunk now :)

Cheers brindella for clearing that up wink

Although i'm afraid in this case no amount of ''richening up'' will work with the ''backfiring''' problem.

The mixture screw will not change the way the engine runs much except make it more responsive (usually by turning it 2 1/2 turns out instead of the usual 1 1/2).

 If the mixture screw is all the way in (or out) then maybe this is the cause (but not likely)

 

I suggest that the timing is checked properly & carby for flooding.

Also the air filter cleaned (if not done already).

 

...........................

Jarrah.

 


 



-- Edited by barra8 on Wednesday 19th of December 2012 04:47:47 PM

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kjd1559


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Just rebuilt engine. reson bent exhast valves. fitted new piston rings, cam chain and sprockets, clean the carb Bought the bike cheap because it did not run.

its running now but backfires and wont idel. is it up or down on the air screw to make it richer?



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Cubber


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Welcome to the forum, Kevin.

Screw in to richen, out to weaken.

Maybe the wet Welsh air is affecting your carb. smile

Martyn



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brindabella


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huh? it is a four stroke, the screw is a fuel mixture screw, not an air screw. it is on the "downstream" (engine) side of the carb so that it can control not air but a rich mixture of fuel and air from the pilot jet circuit - if you have more of this being "injected" into the air stream you have a richer mixture. if you screw it in (clockwise looking from from below) you decrease the proportion of fuel-air mixture being added, so weakening the mixture. if you screw it all the way in it is lean to the point of NO fuel and the engine stops.

so I believe the correct answer is: screw in the mixture screw to make it leaner at idle, screw out the mixture screw to make it richer at idle.

of course randomly twiddling it is a fine way to get it right, but you wouldn't think adjustment would fix a major backfiring problem.



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barra8


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kjd1559 wrote:

Have just checked the timming and Valve gap everything spot on.

cant get the dam thing to Idle right and its still backfires some one has drill three 3/8 hole in the end of the exhast pipe

would that effect it???

 Thank god wink 

The hole would not matter so much as to do that.

Here are some simple things to try first..

Adjust air/fuel mixture screw. ASSUMING ALREADY DONE ...:) (Would'nt be the problem but worth checking)

Clean carby/fuel lines/tank

Check spark plug has bright spark by grounding it against the motor

Replace plug (if needed)

Bad earth

Check cam chain tensioner.

...........................................................

I will try to think of other possible causes but that should give you something to do for now wink

..................

Jarrah.



 



-- Edited by barra8 on Friday 21st of December 2012 02:35:42 PM

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barra8


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So we can be more clear on your problem can you describe it again adding more detail this time.

Does it idle better when warm....Hard to start ect.ect.ect.

........................

Jarrah.



-- Edited by barra8 on Friday 21st of December 2012 02:30:57 PM

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barra8


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This does not sound good...the only things that will make it red at the header pipe 7 backfire are....

Bent valve

Valves not seating

Timing incorrect (firing on the intake side)

...........................................................

I hope for your sake it's not this & just a blocked pilot jet cry (not likely though)

..........................

Jarrah.



-- Edited by barra8 on Friday 21st of December 2012 02:59:48 PM

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barra8


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How is the compression? If you have'nt got a tester just hold your finger over the spark plug hole to make sure it ''blasts'' your finger off.

Also if you have a kickstarter you should get a rough idea.

 



-- Edited by barra8 on Friday 21st of December 2012 03:08:34 PM

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kjd1559


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Thanks for the info will be trying in the new year



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kjd1559


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can't do anything about the Welsh Air but you could be right its tooo Rich lol and damp not so funny

 



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kjd1559


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Thanks will check everything



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barra8


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If you need a manual i can send you a full repair pdf.

Also see either this thread ...I was trying to find Brians thread also but???

......................

Jarrah.



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kjd1559


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Have just checked the timming and Valve gap everything spot on.

cant get the dam thing to Idle right and its still backfires some one has drill three 3/8 hole in the end of the exhast pipe

would that effect it???



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kjd1559


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Running a lot better now still not ticking over right but will work on that. have now noticed that the front pipe glows red when in the workshop is that what they all do???

 



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kjd1559


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Just done engine rebuild bought bike dirt cheap as I am only using it off road

change cam chain and sprockets new valves as old ones bent, new piston rings just incase.

bike starts from cold with choke 4-6 kick is ticking over now not backfiring as much after adjusting the mixture screw

not leeking air in through manifold checked that New Air filter fitted

Have cleaned the carb, timming spot on checked and checked and checked again

Just cant get it to tick over right and like i said front pipe glows in the dark?



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barra8


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I suggest you get it compression tested.

The other way to see whether it's the valves is to remove the head....put solvent in the ports & see if it leaks through after a while.

............................

I must stress that glowing red at the header pipe is almost a certainty that valves are'nt sealing or the timing is wrong!

Even if you have some small main jet fitted it would be hard for it to affect it so much as to make it red at the header pipe.

They run on alot of different jet sizes with no difference so it doesnt change it all that much. It would only really make a noticable difference if the jets are too big as an engine can run lean without making much difference to anything else.

They has been cases where the pilot jet & main jet were too small & this may cause it to overheat & very well may be  your problem.

Never seen it get so hot as to make it glow red on a trailbike though unless it's valves or timing.

.........................

THINGS THAT WILL HELP US DIAGNOSIS THE PROBLEM

.........................

What main jet is fitted?

What pilot jet is fitted?

How many turns out is the mixture screw?

 

What size is the cylinder?

What is the compression?

.........................................

It is hard to diagnosis the problem unless we know all of the above. The better you can describe your problem the easier it will be. Good work so far

...............

Things to remember...

No amount of adjusting your idle screw will fix a backfiring problem- This merely leans or richens the mixture & will not cause backfiring.

If it ran fine before with the standard jets then it should be okay now (atleast not backfire).

.......................

 

Jarrah.

 



-- Edited by barra8 on Saturday 22nd of December 2012 03:20:28 AM

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domski1


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I would take it for a run and see if things settle down, then check the plug to see if it is running rich or lean, I have experienced similar problems, after finding that it had a 130 main jet in instead of a 137 when I replaced it and readjusted the idle mixture the bike seems to be running a lot better.

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