Modified RM85 Bogging

My son's bike is having a problem with bogging. Here's a video of the problem; you can hear it from the beginning through about 45 seconds of the video. It seems to be from 1/2 throttle to 3/4 throttle. It fouls a plug in about 15 minutes, not so bad that it won't run, but it is wet and kinda black.

I put the carb on before the porting and switch to race gas, and it ran all right, but had a darker plug than I would like, kind of a dark chocolate color.

After the video below was shot, I moved the clip up on the needle from #2 to #1, which (maybe, kinda) helped a little bit, but hardly at all.

Bike specs:

2004 RM85

PWK28 carb (somewhat new addition)

-48 pilot

-122 main

Ported cylinder

VForce Reeds

New Wiseco topend

Runs VP U4.4

Klotz Castor 32:1

Thanks in advance for your help.

Matt

Try a #45 pilot jet it seems to really clean the bike up. What size sprokets Are you running?

First try this;

#45 pilot

Keep the #122 main for right now.

Air screw at 1-1/2 to 1-3/4. If you got to go in or out more than that, then the pilot jet is a little off.

Lets mix up some 24:1 premix. More oil, leaner fuel.

And move that c-clip back to 2nd groove. Your son is backing off alot coming up to a corner and this makes it hard to jet for. When he gets to charging at corners and staying on the gas longer in the straights, then your black plug will clean up.

First try this;

#45 pilot

Keep the #122 main for right now.

Air screw at 1-1/2 to 1-3/4. If you got to go in or out more than that, then the pilot jet is a little off.

Lets mix up some 24:1 premix. More oil, leaner fuel.

And move that c-clip back to 2nd groove. Your son is backing off alot coming up to a corner and this makes it hard to jet for. When he gets to charging at corners and staying on the gas longer in the straights, then your black plug will clean up.

+1 on everything plus sprockets, except for the 24:1 premix. The Race fuel is definitely going to make it run richer so you are going to have to lean some. But I would stick with the ratio you are going to use, and adjust/jet for that mixture. That way you will not be adding another variable to the issue...JMO though.

Good Luck.

My son's bike is having a problem with bogging. Here's a video of the problem; you can hear it from the beginning through about 45 seconds of the video. It seems to be from 1/2 throttle to 3/4 throttle. It fouls a plug in about 15 minutes, not so bad that it won't run, but it is wet and kinda black.

I put the carb on before the porting and switch to race gas, and it ran all right, but had a darker plug than I would like, kind of a dark chocolate color.

After the video below was shot, I moved the clip up on the needle from #2 to #1, which (maybe, kinda) helped a little bit, but hardly at all.

Bike specs:

2004 RM85

PWK28 carb (somewhat new addition)

-48 pilot

-122 main

Ported cylinder

VForce Reeds

New Wiseco topend

Runs VP U4.4

Klotz Castor 32:1

Thanks in advance for your help.

Matt

I had a lot of the same work done on my son's KX 85 to include cutting the head down. A better choice of fuel is VP/ C12. The U4.4 is designed for 4 stroke race bikes. I dont believe the U4 is high enough octane. You should be running atleast 110 octane rating so you dont burn a hole in your piston. We mix 32:1 maxima 927 and I forget where it is jetted right now. He has a pro-circuit pipe also. The bike runs awesome and the plug is that nice clean brown color.

thanks, everyone. I already ordered a bunch of new jets, which is what i would've tried the other day at the track, but the jets from the stock PE28 were all bigger.

To answer other questions people had:

gearing: 14/48 (1 down, 1 up from stock)

exhaust: FMF SST, shorty

thanks, everyone. I already ordered a bunch of new jets, which is what i would've tried the other day at the track, but the jets from the stock PE28 were all bigger.

To answer other questions people had:

gearing: 14/48 (1 down, 1 up from stock)

exhaust: FMF SST, shorty

We run 13/50 with the gearing, and I would run C-12 instead of U4. I think the C12 is cheaper, and much better fuel for 2 stroke.

14 front sprocket is stock try the 13 with the 48 rear.:thumbsup:

UPDATE:

OK, so I took that carb apart and cleaned it out. It wasn't really dirty or anything.

It actually had a 42 pilot in it, not the 48 that I thought previously. I swapped the 122 main for a 120, checked the float height (it was correct at 19mm), and put the needle clip back down to #2.

Same problem.

I tried messing with the air screw. It raised the idle when going out on the screw up to about 4 1/2 turns. After about 2 1/2 turns the idle was VERY fast, which would lead me to think that it needs a smaller pilot, right? The idle was so fast that it could no longer be controlled by the idle screw at this point. I put the air screw back to 2 turns out to bring the idle down.

I also tried draining the gas and going back to straight 91 unleaded at 32:1.

Same problem, maybe a little better.

So, what should I do, keep going leaner?

Thanks,

Matt

did u jet for race gas? if you didnt you need to re jet

Well, it's not that it's jetted for race gas or not; it's just that it's a new carb to the bike and not jetted correctly at all, and I'm trying to dial it in.

I ordered new 35, 38, and 40 pilots for it. Hopefully that will do it.

Matt

Look up the stock specs for a KX85 or a Honda CR85, both of those bikes came stock with the PWK28 carb. That should give you a good starting point.

So a KX85 is 140/45, which is a lot bigger than the 120/42 than what we're running now. Hopefully that 35, 38, or 40 will fix it.

Also got some new reeds coming. One of them has a corner missing, nothing too bad though.

Your 120 is kinda scaring me. In our RM85L Supermini we ran the PWK28 and our numbers were #135 main and #45 pilot, thats the same jetting as the RM100. Our motor just had port work, light milled head and we ran the stock pipe and silencer (my son liked it the best). My son rode the bike pinned everywhere, he never let off after the gate dropped.

Get the new reeds in and start over. I also posted above about 24:1? We ran 28:1 with the above mods. It realy did help bringing the jetting around. With a mod motor and a rider ringing the hell out of it, you need the residual oil in the crank. If you just got to run the 32:1 mix, then jet for that, but watch it close when your rider hits a speed streak. You got to jet to the rider, not to the guy in the next pit. Good luck.

Padgett:

I think you're right about the jets. I've been reading, and maybe the reeds could be causing the rich condition, which would explain why changing the jets has had basically no effect.

I've got some new ones coming here in a day or two. I'll try them and the 135/45 combo and see how it goes.

Once we get that straightened out, I'll switch back to the race gas, and then to the 28:1.

Thanks for your time and everyone's thoughtful and helpful advice.

Matt

NO go rich time to look at the yz85 it comes with 138 main jet i have used 160 when my 138 went bad until i got my new replacement just get yourself a 138 main going by yamahas book it will work trust me im a doctor

given that you have a ported bike i would recommend going 145 main it will start first kick and go

After the new reeds and switch back to 45 pilot and 135 main, everything is running great!

I'll probably bump it to the 138 main until I have a little more time to get it sorted out. It runs fine with the U4.4, too.

Thanks everyone!

Now to install this Vortex programmable ignition and mess with it some more.

Matt

That's the problem with bikes. you work like hell to get them running great, and then you want more!

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