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My ltr 450 backfires and is hard to start after fresh topend rebuild

Just put new rings and piston in and seems to not be getting enough fuel thru the carb

Posted by Anonymous on


6 Suggested Answers

  • 2 Answers

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Posted on Jan 02, 2017

  • 3567 Answers

SOURCE: timing Adjustment issues!

If the bike runs then Top Dead Center (TDC) is not an issue. But to check piston stroke position, Remove the valve cap cover on the intake of the front cylinder. Remove both the spark plugs. Rotate the crank shaft untill the rocker arm starts to move. The piston is now starting down on the intake stroke. Rotate the crank further and the rocker arm moves in the opposite direction. The piston is now going upward on the compression stroke. Look at the marks on the flywheel. Rotate the crank untill the TDC mark lines up with the timing index mark. The front piston is now at TDC.

If your model has carbs then the backfire fix requires you to syncronize and balance the carbs.
Remove the air filter and rubber fittings to allow you to see into the throat of both carbs. You may need a mirror to see inside. You need to be able to see the slides go up and down when you twist the throttle. Now turn the fitting where the cable goes into each carb such that there is just a bit of slack in the cable, 1/16" is plenty. Lock ONE of the cable adjusters down tight. All further adjustments will be made on the other carb.

With the motor turned off twist the throttle very slowly while looking at the slides. Both slides need to begin lifting at the exact same moment. If the slides don't raise at the exact same time then slowly twist the throttle until the locked adjuster slide just barely starts to move. Hold the throttle still and turn the adjuster on the other carb so that the slide on that carb just barely starts to move also. Now recheck the slide movement timing. Do this process until the slides on both carbs begin to raise at the EXACT same moment. Lock down the loose adjuster and re-check the slides.

Turn the throttle screw on the LEFT carb one half turn inward. Now start the engine and get it warmed up. With the engine running, remove the RIGHT spark plug wire. Adjust the idle speed on the left carb to the point where the engine just can't quite stay running and dies. Now put the loose spark plug wire back into place.

Now turn the throttle screw on the RIGHT carb one half turn inward. Restart and rev the engine. Next, remove the LEFT spark plug wire. Adjust the idle speed on the right carb to the point where the engine just can't quite stay running and dies. Now put the loose spark plug wire back into place.

Turn the idle screw on each carb EXACTLY ONE QUARTER turn outward and restart the engine. The engine will be probably be idling very fast. Adjust both the idle screws equally from this point to get to the desired idle speed. Congratulations, you have just balanced and synchronized your carbs.

Please rate this solution. Thanks sgtswampdonk!

Posted on Apr 02, 2009

  • 688 Answers

SOURCE: 1981 Wide Glide Bike is missing and backfiring

open up the ignition cover plate and check the backing plate. it may have come loose and threw the bike out of time.
make sure your plug wires are plugged on tight and the coil wires are secure.
It is most definitely a timing problem.

Posted on Jun 06, 2009

  • 25 Answers

SOURCE: fresh rebuild now no spark

If it ran before, I would double check your electrical connections. Its more likely you didn't get a connector pushed all the way together or left off a ground wire. The ignition coil specs are generally 1 ohm or less on the primary (small Wire) side, and 5K-10K ohms on the secondary (spark plug) side. Each side should be tested to ground. Pick-up coils are typically open or really large ohm reading in Megaohms normally and around 100 ohms when the magnet on the flywheel passes by. there is no way to test the CDI box other than putting it on a know running bike or getting a known good CDI and putting it on your bike. Double check that the spark plug cap is secured to the high tension lead as well. Testing the stator is a little more complicated, but a quick way is to close the gap on an old spark plug to about 0.010" and see if there is a week spark. If so, the stator will need to be replaced.

Posted on Jun 16, 2009

  • 4 Answers

SOURCE: I,m getting fire and gas its in time hits but wont start

I had a simular problem with my 88 xr. It started out just running bad at higher speeds which had me thinking it was a carb or plug problem. I started changing plugs and had varing results which still had me guessing. So I started adjusting and jetting the carb. Still the same varing results. I suspected the CDI box so I tried to get one. (I live in Thailand, not the easiest thing.) The parts places produced several boxes that plugger in but gave me the results you just described. Good spark but no start, I also got one box that started the bike real good but would not accelerate. There is a built in timing in those CDI boxes. Just thought you might check this, By the way the original box of mine eventually failed completely and I was correct in my suspicions. I found a used one and it runs great.

Posted on Jul 09, 2009

ollie reece
  • 1167 Answers

SOURCE: cr125r 2001 cant start after changing piston and cylinder

hi mate you need to adjust the float if theres to much fuel going through you need to take the float out and adjust it to the manual specifications but remember slot the pin out and take the float out but watch the little valve which stops the fuel where the little valve is hanging from that is the part you need to adjust

Posted on Jul 23, 2009

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As you have done most things regarding fuel system i would have to assume it either has not enough compression, or the ignition timing is out, so remove the muffler and check the condition of the piston face through the open port, any signs of scuffing or marking on the piston then the saw has seized at some time and the piston ring is stuck in, if all appears ok then remove the flywheel and check the key has not sheared, the key is alloy and is part of the flywheel..

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watch this video. this should help you understand
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That does seem low, but it's a 2cycle.
It hasn't been run hot or w/o proper gas/oil mix has it ?
Will it fire with a squirt of starter fluid ?
If you squirt some oil into the spark plug hole, does the compression go up very much ?
If it has spark and won't fire w/starter fluid sprayed in thru the carb, it's most likely a ring/piston/cylinder problem.
If it fires with starter fluid, it's fuel not getting into the cylinder.

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I have a 1980 rm125 it does not start it just back fires

usually a 2 stroke will chuff/backfire out of the exhaust if it low on compression, did you put the rings in the right grooves and up the right way and is the piston in the right way around as there is a front and back ,the ring pegs are supposed to face the inlet side of the barrel.

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My 2007 450 sportsman won.t start but she backfires

Remove the seat, and take the cover off the air filter container. Place your hand over the end to aid in fuel flow to the carburator. You might not be getting enough fuel which is causing it to backfire and not start completely.
Attempt to start the ATV once you get fuel flowing you should be all set. Clean that filter as well.

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First of all i had problems b4 with loosing power then i had a new piston put in and its never been the same again cud that have anything to do with it cos its never been the same sins i have had a ne

You put little oil in the fuel is that the problem? I would say that's the problem if a bike is not supposed to have oil thru the fuel lines on a four stroke. You said the answer. It never ran right since a new piston got put in. The break in period didn't get broke in or piston wasn't correctly matched for you cylinder and your cylinder needed to be re crosshatched for a new piston getting put in.

Dec 23, 2013 | 2006 Gilera Runner SP 50

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Does not start when hot. difficult to start when cold but runs well at high rev, but if you shut it off it does not restart until about twenty minutes later

Most likely cause of these symptoms on ANY two stroke engine is scoring of the piston / cylinder. It's hard to start when cold, because you have insufficient compression to get enough fuel into the combustion chamber. After numerous pulls (and lots of swearing), there is enough fuel floating around to allow it to start. But as soon as it does, heat causes the piston and rings to start expanding and then you get metal to metal contact with the cylinder wall. This causes much more heat, which causes greater thermal expansion and suddenly the piston seizes in the cylinder and the engine stops. When this happens, the score(s) in the piston / cylinder get a bit worse. Then, when the engine has cooled sufficiently and the piston has shrunk back to it's normal size, the engine might start again.

Specifically to your question, more info is needed.
1. Will it always keep running WELL once started, or does it sometimes stop of its own accord?
2. You say it runs well at high RPM, but do you have another machine to compare it to, or do you just mean that it seems to be working as well as you remember it working before the problems started?

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Non starter

take plug out put a very small amount of fuel down cylinder, plug back. does it fire up??

Mar 12, 2013 | Motorcycles

2 Answers

I have a 1987 kx 125 just put topend gasket set in with piston and rings and coolent is getting in the case bottom end anyone know why plz help me

It sounds as if you have rebuilt the engine without torqueing the head bolts. Although, it could be that the cylinder base gasket has been torn during the rebuild. Try to re-tighten the head bolts to the correct settings first. Good luck.

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