yamaha dt175 won't rev past 7k

[Dr Rockso]

i'm trying to get my 81 dt175 going 100% , but i'm having some issues . you can give it as much gas as you want and it does fine untill you hit 6k , then it just flutters there . i checked the float height and it was off so i set it to factory spec . the slide needle isn't adjustable , the autolube pump seems to be adjusted right (peg lines up with the dot) , all jets clean . once i'd done all that i could get it to go to 6.5k rpm and sometimes a little higher but it would start gurgling . then i thought hey , maybe the air cleaner isn't as clean as it looked , so i took it out and found the back side of it sopping with oil and pretty much falling apart . i removed it and i took it for a spin up the street and now i could get it up to 7k maybe a little higher but not up to the 8k red line so i'm guessing the clogged up old air filter had something to do with it (i have a new one on order) . but shouldn't i be able to get her up into the red (not that i plan on riding in the red , but theroetically shouldn't it be able to get there ? atleast to the 8 ? ) and if so what could be holding it up ? there's nothing else adjustable on here . compression is good and she starts first kick , idles great , the plug looks a little wet but the tip is somewhat tan .


i just noticed that in the diagram for my air box there's a small snorkel that you set into the box before screwing the cover down and mine doesn't seem to have that ...that couldn't cause this could it ? i wouldn't think that an inch long plastic snorkel would make that much of a difference tho . anyone else have any ideas ?

 

[Dr Rockso]

ok , well i just tried running it with the fuel off to see if it was still float height , it ran as usual untill it didn't have enough gas to keep going , so i don't think it's float height . then i came down the street as fast as i could , hit the kill switch and pulled the clutch and rolled back to my house , pulled the plug . it was a little wet but the tip was slightly tan . i haven't checked the reeds yet , i'm going to let the engine cool down some first . i think i'll also pull the exhaust off and make sure it isn't all blocked up near the head .

 

[Dr Rockso]

i just pulled the muffler off and besides being loud as hell i can now get it up to 7.5k rpm . i think i'm going to pull the whole exhaust and make sure it's clean ....i really hope this is it.

 

[Dr Rockso]

just for the hell of it i fired it up with the entire exhaust off , same deal , it would get to 7.5 and that was that , the exhaust port is fairly clean and the pipe doesn't seem to be blocked up ...i don't think the exhaust is the problem . ehhh , i'm really starting to get tired of this , every single bike i've ever had has always worked 90% because of some undiagnosable mystery problem that bugs the hell out of me untill i give up and sell it .

 

[Dr Rockso]

i just put everything back on and took it for a spin , it still only wants to pull 7k , tho when i pulled up back at the house for the hell of it i turned the gas off but left it running again . i kept revving it wide open , the first 2 or 3 times it got to 7 , the third and fourth time it got to 7.5 , and on the last rev before it ran out of gas i got it to 8 , maybe i should try lowering the float a little more and see if that does it . when i have the carb off i'll check the reeds , but i dunno why it would run prefectly up to 7k if there was a problem with the reeds but i've seen weirder stuff before . if anyone else has any other ideas feel free to chime in .

 

[Dr Rockso]

i pulled the carb and pulled the reed block out , they look brand new and they're the metal kind , no missing hunks and they all flap back just fine so i rebuilt the carb again ,i'm running the stock 22.5 pilot and the stock 150 main , i reset the float height again just to see . went out and fired it up again .....still tops out at 7 . i don't think there's anything else i can check . the bike is 100% yet it's not 100% and thus is my off road bike curse , a problem that can't exist because everything is correct , yet it still exists . not one out of the 50 or more dirt bikes i've owned has ever worked right , it's always something . i'm starting to think i should swear off anything that's not meant for the road and just be done with it .

 

[jackflack44]

how are you measuring the RPM's?

 

[Dr Rockso]

with the tach . here's a video you'll see what i mean

http://www.youtube.com/watch?v=dPOSvUlYRMg

 

[2strokerfun]

When are you ever going to get past 7k riding this bike?? They run differently under a load.

 

[Dr Rockso]

well it doesn't hit the powerband till around 5k , so when you're on the street it's nothing nothing nothing , powerband , 6k , nothing . and off road it's a pain , if you're going up a hill and you need all it's got then you're screwed .

 

[Someone]

I wouldn't ever run my 2 stroke without exhaust OR running the carb dry.

 

[Dr Rockso]

neither would i if it was running right , but i'm trying to find out what the problem is .

 

[flyingfuzzball]

ignition maybe?

 

[Dr Rockso]

yeah , the carb is clean , and i've been in it about a million times now . i haven't gotten a new plug but i'll throw one on the list . i wasn't riding it with the filter and exhaust off , i just ran it long enough to rev it up and see if it would go to 8 , while it sat on my driveway . i dunno about the ignition , the spark looks nice and it's cdi .

 

[_JOE_]

Maybe rich on the main jet? If you get the exhaust cleaned out and a fresh plug and it still doesn't rev try experimenting with jetting.

 

[rmc_olderthandirt]

You should be able to operate without the air filter for testing purposes, just stay out of the dust.

On a two stroke the exhaust, specifically the expansion pipe, is a vital part of the engine operation. I would fully expect the engine to run very poorly if you attempted to operate without the expansion pipe.

If you suspect the exhaust then take the pipe and silencer off and inspect them. Make sure that some tiny critter hasn't made a nest inside the expansion pipe. I doubt it, but it is possible.

Next take the silencer apart and replace the packing. It is possible that the old packing has reduced to a pile of goop that is clogging the output.

The next most likely problem that would be RPM dependant would be the ignition. If the magneto is weak, or the CDI is bad it may not be able to produce spark at the higher RPMs.

Have you inspected the reeds? My experience has been that reed problems affected the low RPM more than high but it is certainly worth checking.

Rod

 

[Dr Rockso]

the reeds look good , i'm working on the exhaust right now , but after that i'm going to check the timing , i know it's not adjustable , but it takes 3 seconds to check so why not .

 

[Dr Rockso]

ok , well today i got that restricter out of the exhaust , without a rotary tool . there was alot of chiseling and the plasma cutter took care of the bulk of the cutting . even tho it's badly out of tune i can already feel more oomph . i also checked the timing and it was dead on . it still doesn't rev up tho .