1987 TT600 - Backfiring -

[sarsipious]

Hey,
My first post on this board. Lots of good info available. Just hoping you knowledgeable types might be able to help me get this thing sorted.

1987 model TT600. Starts easily (now) and idles ok once it is warmed up.

However it backfires under any throttle - mostly when backed off. Right from idle and onwards when ridden.

from what I have read on this forum it could be one of the following, air leak in either the carb, carb mounting, or headers/pipe joins. Or slide upside down.

I will try to work through these items but I believe the bike has (just bought it) had a wiseco piston put in it. Seeing as this may have raised the compression from 8.5:1 to a possible 11.5:1 could these symptoms (backfiring) be caused by the standard plug now being too hot/cool ?

Any idea's would be appreciated. Like I said I have done searches on the forum and will try the above air leak checks.

 

[Ol'89r]

1987 model TT600. Starts easily (now) and idles ok once it is warmed up.
However it backfires under any throttle - mostly when backed off.
QUOTE]

sarsipious.

Welcome to DRN.

First check your valve clearance.

If you have no air leak problem, I would remove the carb and check all the jets. Especially the main jet and holder. Make sure they are properly installed and tight. Also check for a bent slide needle. Take the needle out of the slide and roll it on a flat surface. A bent needle will cause this condition.

Also, check your wiring for a loose connection or bad ground. Remove the sparkplug cap and check for oxidation on the end of the plug wire. Cut about 3/8 of an inch off of the plug wire to expose fresh wire and remove the little resister inside the plug cap and replace it with a piece of copper rod.

Hope some of this helps. :cool:

 

[sarsipious]

Checked the valve clearences - seemed fine - no difference still backfiring. Seems to falter on every 4th stroke a little.

Trying the air leaks next. I have seen generous amounts of selastic applied to carbie and exhaust joints before. Thinking I might try this approach.

Then I will try the needle and seat.

Thanks for your help.

 

[Ol'89r]

Seems to falter on every 4th stroke a little.
Trying the air leaks next. I have seen generous amounts of selastic applied to carbie and exhaust joints before. Thinking I might try this approach.
QUOTE]


sarsipious.

If the backfire is mostly on deceleration, it may be an air leak in the pipe. Instead of silastic, make sure the copper o-ring in the head is in good condition and especially the seal between the head pipe and the muffler. If they are bad, replace them with OEM parts. Silastic will burn out.

If the engine falters on every few strokes, you may have a bad (leaky) valve. Might need a valve job.

 

[sarsipious]

Found the culprit - busted o ring gasket on the secondary carb inlet boot between the boot and the head.

Replaced this and the backfiring is pretty much fixed.

Now I just need to figure out what is the benchmark setting for the fuel screw.

I realised also that I had gotten the fuel screw setting backwards and had screwed it all the way in (lean) instead of all the way out (rich) It seems to screw out a total of about 7 turns. I have seated it at about 3.5 turns in which is around the previous setting that it had when I bought the bike.

I have purchased the manual and it seems to make no mention of this basic setting.

PS. Merry Christmas

 

[sarsipious]

Hmm problem returned after a short ride.

Looks like it seems to be the slip fit connector in the middle of the pipe. I put some selastic around it as a test and that seems to have fixed it when I burbled it up the street.

Once I rode it a bit it cracked pretty quickly and the backfiring returned.

Need to add a boot or some exhaust binding over the joint.

Exhaust is non standard pipe.

 

[76GMC1500]

Check for air leaks where the pipe bolts to the head. Leaks that far down the pipe don't usually cause much backfiring. My 86 TT225 has a pretty leaky joint between the muffler and header, it doesn't backfire much at all.

The carbs on some of those Yamahas have a rubber diaphram that enriches the mixture during deceleration to prevent backfiring. That may not be functioning properly.

 

[snowbugca]

MY 350 XT had a leaking intake manifold. Found it as soon as i took the carb off. My idle adjustment from the factory is 2 and 1/4 turns from lightly seated. I know cause I drilled the plug out. This should be close . Hope it helps Jim