Hello all,

Sorry, I know that this example is not for an ATV but a dirtbike, but I figured someone would help out. =)

My son's '98 YZ80 seems to bog on accleration - it has great power on top (enough to lift the front wheel in the 3 lower gears just on acceleration without popping the clutch). When you crack the throttle from a start, it bogs and makes a "boowaa" sound momentarily and seems to have very little power - until the revs are up, and then it hits hard. The powerband is in the upper rev range, and I kinda expect that due to the small displacement. I noticed that when we rode it recently during some 40 degree weather, it was less noticable than before, which leads me to believe that it is jetted too rich maybe. Does this sound logical? Is the best way to jet the bike just by trial and error and "feel"?

On an unrelated note, are Banshee's known for restrictive stock fuel petcocks? I have to keep mine on "RES" or I seem to run out of fuel if I'm wide-open for any length of time with the petcock on "ON". I noticed a similar problem on a friends '05 Raptor 350 too..........???? Thanks!

 

It sounds like any two stroke dirt bike in the 125cc and less classes to me. My youngest son's KTM 65 SX is the same way, you hit the throttle at a low RPM, ... and ... nothing, ... until ... WHAM!, almost over backwards. I have a hard time riding the 65 because I can't keep the front tire on the ground with that little bugger.

I would suggest you check your spark plug though. If it is black and oily try going a step leaner on the needle. If it is white try going a step higher on the needle. Too rich usually causes a blubber when you gas it, not the "boowaa" type of problem, at least that is what I have seen. I also wonder if you could have a partially clogged pilot jet. My oldest son's KTM 300 XC started running badly just last Sunday, and it was a clogged pilot jet. It would start and idle fine, but as soon as you cracked the throttle it would die. If you pulled the choke out it would actually run good at lower RPM, but load up at higher RPM. I pulled the carb apart and blew out the jets with compressed air, and it runs like a raped ape again. So I would suggest doing a plug reading, and making sure the carb is good and clean. Good luck.

 

Are you sure its a jetting issue and not compression? i had a yz 80 that did the same thing till i rebuilt the topend.

 

Well, I doubt it is a compression issue - the bike is very fast. It just doesn't have ANYTHING on the low end. I have never had a small 2-stroke, so I have nothing to compare to. I ride a Banshee, so no comparo there, and all the dirtbikes I've had were either 4-stoke or over 125cc. It makes the bike difficult to ride, except all-out. I suppose that is fine for the track though, which is mostly where it will be ridden (my son will start motocross this spring).

I'll examine the plug, and the carb, and see what turns up. My understanding about 2-stroke top-end rebuilding is that it needs to be done rather frequently on hard-ridden bikes (like motocross bikes) - like every 10-20 hours - is this true? Thanks.

 

Every 20 hours of racing would be resonable, with practice time thrown in for free. That makes for 10 thirty minute motos, or two rebuilds per year if you race every race in a series.

Ever watch the 85cc pro class? They really do ride those things wide open all the time, and the YZ 85 has been the top choice for pro class mini riders for the last few years.

 

How are the reeds?

 

I know there is a lot of controversy on this but here are my thoughts.
If running the proper mix and keep in proper tune, with a good air cleaner a two stroke should last for years on a rebuild, even an engine held at WOT for hours at a time.
At 10 hours X lets say 20MPH average that is only 200 miles on a brand new engine!!!!!! That is NOTHING.
The equivalent on a 4 stroke would be around 100 miles!!!!!! Again NOTHING. Would anybody rebuild their auto engine with 100 miles on it?
I have seen this happen many times at the race track. Some winning pro has his engine always tore down and is replacing pistons and rings and bearing and everthing else. There is a crowd of racers around seeing what's going on and half go back to their pit and tell their crew chief that the reason they don't win is because their engine is tired. So they through a piston and the rest of the junk in their engine and now they are set to take on the world.
It's the pro's job to put on a show for his sponsors!!! The reason is they sell parts. Winning pro's get everything for free because they sell parts.
I asked a well respected racer in my area why he was rebuilding his engine. His answer..........because it was time.
Then I asked him how did he know it was time? He told me after a certain number of races he rebuilds his engine. I then asked him what determined the number of races? He told me they have always have done it that way.
I suspect a lot of racers do this. No leak down, no compression test, no nothing; just, let rebuild it because "it's time."
The only real way to determine if an engine needs rebuild is a leak down or compression test. Get the rings seated in and do a leak down. Write that figure down and every few months take another. If you are over 5% or so take the jug off and give it a fast hone job, throw in some new rings and have at it again.
A lot of good parts ar being thrown away if you ask me.
That's my 2 cents on this subject.