Help please with tuning

Ok here goes....My 02 426 is acting up. It is topping out way too soon, seems like it's full throttle at 1/3 throttle. Classic signs of an air leak and/ or lean jetting right? Well I've tried to find an air leak and can not find one anywhere. Jetting is as follows:

Main 170

Pilot 42 (was using 45 for a long time but it had same issues lately plus hard starting)

Needle JD Blue clip in #5 position from top down

All else is stock in carb.

Adjusted the AP squirt and did oring mod today. Didn't help my problem.

Bike has Hotcam auto decomp exhaust cam,FMF powerbomb and FMF Q4 (newer version custom fit from a 450).

Test rode the bike today, it is FAST but almost too fast and out of control.

I've tried all I can think of triple checking everything like timing and such. What am I missing here?!

we thought my buddies 450 had an intake leak. To test we just sprayed carb cleaner around intake boot that goes from the carb to the engine...while its idling.... sure enough it idled it right up.... he just needed a gasket and a new intake boot

you might try this, if you have a leak there the carb cleaner will cause it to idle up real high... note to self, try to keep the spray off the hot exhaust!!! whoooooosh:thumbsup:

Yeah I have done the same test while checking for air leaks. Sprayed carb cleaner, brake cleaner WD40 at different times during this whole process. Sprayed all around the intake and exhaust flanges and also where the muffler slips on the head pipe. Never changed the idle....

I guess the best way to describe it is like if you were to pull the hot start and rev it. It's not a smooth powerband, it just takes off and revs out right away. I guess it could be something electrical also?

I know you can't really go by spark plugs with fuels nowadays, but I just checked mine out and it is a nice tan color on the porcelain with black soot on the very end of the threads. Better than white I guess...

Ok test rode it again today and still having the same issues. I replaced the intake boot as the stock one had a suspect crack that goes half way through, but still no change in idle with the carb cleaner test with either boot. I also replaced the seal that is on the slide in the carb just because, the stock one seemed ok but new one seems better. Put in a new 45 pilot as the older one was a bit plugged. None of these changes made any difference. I have played with the jetting from pilots to needles to clip positions to mains richer and leaner. Gets worse if I go leaner but even going way rich doesn't help.

I am convinced there is no air leak and that it also isn't a jetting issue. I'm thinking electrical? Like timing or the TPS possibly? I bench tested the TPS and could not get a reading on it no matter the throttle position. That tells me it's bad I would guess. I ran the bike with it unplugged and it ran the same way.

I've also read that others have unplugged it and their bikes still run as good as plugged in.


There is no more cruising on my bike, anything over 1/8th throttle is hauling A**!

OK i just know borrowed my friends multimeter (better than mine) and got a good reading on the TPS. It reads 4.97k Ohms both closed and fully open throttle. That falls in the spec range for full throttle but not closed. So that tells me it's bad as it reads fully open all the time. Ok that's a problem sure.

But why do some people have no issues when they unplug or go bad?

Be sure you test the right two leads. If you go from blue to black, it will read the same regardless of throttle position. If you read from yellow to black and don't see a change as the throttle moves, it's defective. Disconnect it and try the bike.

The function of the TPS on a carb equipped engine is to provide the CDI with information it can use to set the ignition advance curve at part throttle load situations. The system is set up so that low TPS output voltage at the CDI means full throttle, which means that if the TPS fails open, or is disconnected, the timing is mapped as if the engine is at full throttle all the time. In most cases, there is a slight loss of fuel efficiency and overall cleanness in the way the engine runs at a cruise, but most people don't notice it, especially when compared with an engine with a faulty TPS.

Ooops that's what I get for not being able to find my actual manual and using a PDF version. Should have wrote down the correct pins instead of going by memory hours later.

Turns out my TPS measures within spec afterall.

If it passes the resistance checks, the thing to do then is to run the voltage check.

My advise would be to get rid of the jet kit and use stock known jetting. This is of course if there is not another mechanical issue going on.

Yeah I have messed around with the jetting, lots of trial and error with no luck to a fix. It used to run great with the JD needle that is in there now so it's hard to believe that would be the problem. I'm going to do a leak down test tomorrow just for the heck of it.

The bike just revs out too quickly and vibrates more than it used to yet I find nothing loose like motor mounts, or timing chain, etc. Imagine the noise your bike makes when reved out at full throttle, well mine starts to get to that at 1/4 throttle and only gets worse as you give it more. By the time I get over half throttle I'm affraid the thing will blow it's so loud and feels like it's revving so high. It's definitely not normal and there is no way a hotcam exhaust cam would make this happen as some have mentioned in other threads. Nobody would ever run this cam if that was the case and hundreds of people have run it to get auto decomp.

hhhmmmmm...the latest adds some clarity in my feeble mind...check your flywheel key. It's possible that the key sheared off and the flywheel has rotated a bit...that would 'splain the increased vibration, and would also impact the ignition timing...

Thanks Birdy, yeah I have been thinking about checking it just haven't broke down and done that yet. I did check today if it had any play but was only doing it through the timing hole so couln't see if spun or not, no play though up and down or side to side. When I did the top end I replaced the woodruff key with a new one, but I guess it can shear also. I will probably check it out tomorrow.

Hmm probably check the timing with a timing light first and that should tell me if the flywheel is ok right?

Hmm probably check the timing with a timing light first and that should tell me if the flywheel is ok right?

I would think so...

Ok I pulled the flywheel today and everything is fine, woodruff key is still good, flywheel all lined up. I went to my friends work today to do a leakdown test but it turns out we didn't have the right hose to screw into the small spark plug hole, auto shop not a moto shop. We checked the timing but the H was all over the place, not steady as I would think it should be. Hmmm....what's up with that?

Also lots of air comes out of the timing hole while it's running. We weren't sure if that was normal so we figured rings or something may be bad. But my friend's bike does the exact same thing, also an 02 WR426 he just bought the other day, and his bike runs fine. Is there supposed to be lots of pressurized air coming out? I could see a little but this seemed excessive. Like I said his bike runs fine so I guess it's normal

Good grief.

Have you never noticed the blast of pulsing air that comes from the breather? The breather vents the crankcase, and if you run the engine with the timing plug out, you'll get the same thing from there. It's normal. The piston displaces 450cc of air on both sides, you see.

For future reference checking the timing with a strobe will NOT verify the key in place. The ignition signal comes from a lug on the flywheel passing the signal coil on the stator, and it makes no difference where the crank is when it happens, it's between the flywheel and the stator, and the way the CDI reacts to the signal, and the key doesn't have a damn thing to do with where the mark on the flywheel is when the spark fires.

Of course, the whole idea IS to be sure the timing is right relative to the piston. You just can't check the key with a strobe. Do that by finding TDC with a screwdriver in the plug hole, and see if the mark on the flywheel agrees.

Now, then, is the Hot Cam made for a 450 or a 426?

Yes of course I have noticed the air coming out of the breather tube, I just thought this amount of air was excessive like I said. I had never had the bike running with the timing plug out so it was a bit of a surprise that's all.

The Hotcam is made for the 426, all cam timing is spot on. New timing chain, chain is tight and tensioner tests fine and retracts smooth.

The flywheel does line up with TDC.

As far as the timing test goes, I was simply following the instructions in the factory service manual that said to use a timing light and to make sure the H was in the range of the pointer mark on the ignition cover.

I'm sure I'll figure out what the problem is someday.....soon I hope.

Any chance you have the vacuum release plate on the engine side of the slide installed upside down? The "square" end with the hole near the edge goes down.

It's in right side up you can read the M on the plate as well. I wish it was upside down that'd be an easy fix.

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