1991 Yamaha 250 Surging - 3 mechanics - no solution

[bigshrimpin]

My good friend has been through 3 Yamaha service Dept's trying to get a fix an intermittent surging issue on his 1991 Yamaha 250. The motor can run for up to 30 minutes @ cruise 4000/4200 before the surging begins. Surging will drop RPM's up and down from 4000 / 3300 <br /><br />Here's what's been replaced so far: <br /><br />The poor guy has blown several thousand dollars for nothing. If any of you guys have an idea where to go from here . . . I know he'd appreciate it. <br /><br />3 Fuel pumps<br />Carbs rebuilt<br />Fuel Filters<br />Computer<br />Coils<br />Stator<br />TPS<br />Plug<br />Plugwires<br />primer bulbs<br />Check Valve on Fuel Tank.<br /><br />Compression is all around 105/110psi<br />The motor behaves the same way when run off a spare tank.<br />

 

[rodbolt]

Re: 1991 Yamaha 250 Surging - 3 mechanics - no solution

nice looking motor, a quick run with the diagnostic test lamp would have saved him a lot of coinage.<br /> most the time I trace that back to #6 carb running lean due to a bad T-stat or one that is sticking.<br /> the diagnostic test lamp will flash a code for bad pulsers,bad TPS or overcooling,"code 33".<br /> while that motor is complex its not rocket science. if you can recreate the problem squirt premix down each carb throat to see if any are lean.<br />at least he is fairly sure of whats not wrong now.<br /> the test lamp is about 70 dollars from yamaha.

 

[bigshrimpin]

Re: 1991 Yamaha 250 Surging - 3 mechanics - no solution

Rodbolt - Thank you!!! I follow everything here except for understanding how the carb could run lean b/c of a "bad T-stat or one that is sticking." Could you explain this to a dummy like me. I can understand how some crud/loose gasket material in one of the carbs might restrict one of the high speed jets . . . but I'm failing to make the connection how a bad/sticking t-stat would effect fuel flow?? <br /><br />I really appreciate the help . . . I bought a squeeze bottle today and will go to the yamaha dealer for a test lamp.

 

[Propnuts]

Re: 1991 Yamaha 250 Surging - 3 mechanics - no solution

Rodbolt.......thanks for the heads up on the Diagnostic indicator working on the carbed older BB models..I thought it was only for the EFI 2's and 4's.

 

[rodbolt]

Re: 1991 Yamaha 250 Surging - 3 mechanics - no solution

diagnostic lamp works on the V-76 stuff, not all the carbed stuff.<br />bigshrimpin<br /> if ya study the subsystems and how and why the carb has enrichment valves all will be clear.<br />those valves,controled by the ECU have several functions, acceleration,konk,decelleration and COLD START.<br />the cold start functions main control is the temp sensor. not to be confused with the two temp switches.<br />at high speed if the ECU "see" the temp and starts enrichment, all the enrichment solinoids draw from #6 fuel bowl. at high speed #6 goes dry as more is being used than the needle can allow in.<br />you should also check fuel system vacum at the lower lift pump inlet, the check valve between the pumps and the engine filter can be removed if it has not already.<br /> its a fairly complex setup but it works well. most techs are terrified of it and start tossing parts and hoping it will go away, it rarely does.<br />with the correct test eqipment and knowledge of why the stuff is there usually that motor will trouble shoot itself.