Am I fighting a losing battle??

[Midhigh]

I have posted a few threads on this subject. My son rides a 1989 Kx 80. He is a beginner, and this bike runs poor/fair at lower rpm's and jerks your arms out of their sockets when it hits the pipe.
has new dual stage reeds, Fatty pipe. After reading Eric Gorr's site on power band selection I have two questions:
1. I am thinking of putting a new Carb. on it. Is the Keihin PWK 28mm the way to go? Or should I try to mod. the intake (engine side) to a smaller size(26mm) to get better low end response?
2. Is the V-Force Delta 2 reeds any good for the money? And would they help get a better low rpm ride?

Am I fighting a losing battle?
Just trying to make it an easier bike to ride without going to a 4-stroke

 

[Jasle]

Well your really fighting a loosing battle. You ahve a 2 stroke without a power valve. Although going to a big bore kit usually tames down the hit a bit I think the answer to your question still lies within Eric Gorr. You need porting for more bottom end. You will see some help with the 26mm carb, some help from reeds. But the big help will be from porting. The bike was designed to have top end from the factory. Their ports were designed for this. Everything on the bike was designed for this.

Have you experimented with sprokets? You would be very surprised at what going down one up front or up 2 in rear would do for the bottom end. When he come to the powerband tell him to shift earlier. I went up one tooth today. The difference on bottom end was HUGE. This was a 50cc machine so it took away topend due to no gears but I don't think the kid would have won without the 1 tooth change in gearing.

 

[SuzookKING]

I think going higher numerically (less teeth on front/more teeth on rear) on the gearing would exaggerate the "yank your arms outta socket" feeling, You might check into a flywheel weight if you don't want to spend the $'s on porting/head mods.

 

[Midhigh]

Originally posted by Jasle
Well your really fighting a loosing battle. You ahve a 2 stroke without a power valve. Although going to a big bore kit usually tames down the hit a bit I think the answer to your question still lies within Eric Gorr. You need porting for more bottom end. You will see some help with the 26mm carb, some help from reeds. But the big help will be from porting. The bike was designed to have top end from the factory. Their ports were designed for this. Everything on the bike was designed for this.

Have you experimented with sprokets? You would be very surprised at what going down one up front or up 2 in rear would do for the bottom end. When he come to the powerband tell him to shift earlier. I went up one tooth today. The difference on bottom end was HUGE. This was a 50cc machine so it took away topend due to no gears but I don't think the kid would have won without the 1 tooth change in gearing.

 

[yzJoKeR]

I had the same problem with my daughters 1996 yz 80. I put a set of power reeds and changed the sprockets to a 12-52 which is 4.33 to one and it helped her alot.
Hope this helps
JoKeR

 

[Midhigh]

I did do sprockets too: 13/51 from 14/47

 

[lwsmithjr]

I think the port job (low to mid) is the way to go. Of course after my suggestion on the stator, you may not want my advice! The port job only costs $100.00. That's cheaper than the Delta Force or a new carb. And Eric will reverse it after your son gets more experience. I did not do this with my son and it took him almost a year to learn to ride an 80 the way it was meant to be ridden -- revved to the moon. And woods riding would still be difficult because there is no power for tight situations.

I played with different gear ratios but what happens is the power band seems even narrower -- he's going slower but the power has not been moved. It's still in the upper reachs of the rev range and it makes the throttle MORE responsive to rider input. In other words, he can twist less and the bike will react quicker and the "hit" is still there. Porting is the best bet. It will move the power from the top to the bottom and no more unexpected wheelies.

 

[Midhigh]

I think I am going to do the Port with Eric Gorr! But Just for kicks I pulled the carb one last time and measured the intake (engine side) of the carb. It is suppose to have 26mm Keihin PE carb. It is a Keihin PE, but it has been bored out to 29mm.
Will 3mm make that much difference in the low need power band? I know on the Eric Gorr site it says that an 80cc shouldn't exceed 28mm for the mid to High power band & not exceed 26mm for the low to mid power band.

 

[gwcrim]

It is suppose to have 26mm Keihin PE carb. It is a Keihin PE, but it has been bored out to 29mm. Will 3mm make that much difference in the low need power band?

3mm over 26mm is an 11.5% increase in size! On an engine that's already tweaked for top end, it would definitely make a difference. Though, it would only be 'real' noticable at WOT.

Though the smaller carb would help, nothing beats a good port job.

 

[lwsmithjr]

You may want to call Eric and talk to him before you ship it off just to make sure. And I think I read on here somewhere that the actual carb bore is measured at the slide and not at the opening. (If I can find it again, I'll post the link to that thread.) That could explain why it measures so big. Then again, I guess 3mm over is possible.

 

[bbilliot]

Try the gearing change first. Cheapest trick.

 

[yzeater]

Originally posted by bbilliot
Try the gearing change first. Cheapest trick.

Gorr = $100
Gearing = $50 chain + $20 front sprocket + $40 rear sprocket = $110

one of them changes the horsepower. Take your pick

 

[bbilliot]

I didn't know he wanted more hp. Just thought he wanted it to be in the low end. I think he can change just the front by one less tooth and not have to change the chain and rear sprocket. May have to shorten the chain.

 

[Midhigh]

I called Eric Gorr! He said I really needed to get my Carburetor right first. He also said that boring a carb. on these 80's are one of the worst case situations. That little engine needs the right carb and jetting. Then if it still runs to high of a power band consider porting. Stock carb. on this engine was a 28mm. Eric seem to think going to a newer year carb. and a 26mm would do wonders. New carb. should be here by Tuesday. He also said that this bike would much better and have better response at low rpm's if I started using 110 oct. racing gas. $30 for 5 gallons!!!!!! I'm going to give it a try at least once.

 

[SuzookKING]

Get some Phillips B32 from Red Rock Distributing here in OKC. :flame:

 

[Midhigh]

I got the new carburetor (Keihin PE 26mm). I took the carb. from a bored out 29-30mm(old one) to a 26mm. Should have done that long ago. This bikes runs clean and smooth. And the high RPM power band does hit near as hard. The RAD valve is still at 28mm. I'm guessing if I change it to a 26mm also this thing will be just the way I want it. It is already a ton easyier to ride.