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View Full Version : Rs 125 Backfiring



phil16016
06-22-2009, 01:56 PM
Hi, im having some problems with my 1999 rs125. Bought it and it rand good for a day, then it seized. Changed the top end (mitaka piston and barrel) and powervalve cable (snapped) and blade (was chipped at the rod where the circular bit it is) and i heard a large knocking noise which was the crank bearings. Took the engine out and replaced the bearings/seals/gaskets etc. Then when i started it, initially there was no spark but this was a losse connection in the HT lead. Then It started. Ticked over and then backfired. After looking for ages for problems like the inlet, bad fuel , spark plug and all hte basics i gave it to a shop who then diagnosed it to be an air leak within the engine. However they wanted too much for the work so i took it back, hwere i saw an ebay advert, for a bottom end rebuild from road and dirt performance, sent it off and he replaced the bearings/seals/gaskets/con rod/big end etc. He also sand blasted clean the crank case. Put the engine back in the bike and the pwoervalve case broke ( i noticed a small crack when i replaced the blade and cable) so i bought a new one. Put it in the bike and it still backfires. It runs slightly better with the choke on and ticks over, but the engine backfires generally after 3k rpm if u push too much it gives a big backfire and cuts out. Any help please? I've spent wayyyy to much money on this i really need it running.

Micah / AF1 Racing
06-22-2009, 10:13 PM
Back firing through the intake side (not very likely, broken reeds are the result in many cases) or the exhaust. If it is just out of the pipe then look at ignition timing, I know this seems impossible to f-up with a keyed rotor/stator but I have seen it enough to know it possible at least. If this is the case, it free revs in neutral very easily but under load the backfiring stops power very early.

Has the carb been off for a while? Any possibility it is filthy or at least has clogged jets? Super lean or rich mixtures can cause this too, choke stuck on for instance of a rich late firing event.

Little more information por favor. Not trying be an ass, just assess the situation.

phil16016
06-23-2009, 05:51 AM
yep no worries mate. It backfires through the exhuast as far as i am aware, and it does it in neutrtal or in gear (with clutch pulled in)., i havent really tried running it cause it barely gets past 3k revs. Without choke ,gives a little pop pop, but if you push it too far it goes bang and cuts out. As i sed it revs slightly higher with choke on.

jokerio
06-23-2009, 10:18 AM
Sounds like the carb is plugged-up.......

phil16016
06-23-2009, 05:36 PM
cleaned out the carb today, no improvement..., when i first took out the seal it made a scratch in the side where the crank sela goes. Could this cause an air leak or would the seal seal it up, and on the clutch side abit of rubber sheared off, but i was told this just meant a tight fit. Just remembered that, but you think it would have been replaced if my engine was rebuilt after that but im scared i mighta been ripped off.

jokerio
06-24-2009, 11:12 AM
cleaned out the carb today, no improvement..., when i first took out the seal it made a scratch in the side where the crank sela goes. Could this cause an air leak or would the seal seal it up, and on the clutch side abit of rubber sheared off, but i was told this just meant a tight fit. Just remembered that, but you think it would have been replaced if my engine was rebuilt after that but im scared i mighta been ripped off.

When you say you "cleaned out the carb" do you mean that you removed the jets and made sure that the tiny passages (pilot jet) are clear ? Just removing the float bowl and flushing that out is not enough....

phil16016
06-24-2009, 04:36 PM
ypu, got some carb cleaner and took out all jets sprayed it thorughly, checked the passages, and cleaned the fuel inlet and checked the fuel tap.