OzVMX Forum

Clubroom => Tech Talk => Topic started by: Curtis on October 11, 2015, 03:27:09 pm

Title: 2 stroke idle with a Keihin carb?
Post by: Curtis on October 11, 2015, 03:27:09 pm

Just fitted a Keihin PJ36 flatslide to my 125, just wondering can you get these things to idle? They dont have a throttle slide stop screw like the Mikuni. I dont expect a 2 stroke to sit there and idle like a 4 banger, but if it could hold idle for even 5-10 seconds would be nice.
Is it just a case of having the correct pilot and mixture screw setting?

Title: Re: 2 stroke idle with a Keihin carb?
Post by: Hoony on October 11, 2015, 05:16:42 pm

the idle adjuster is the choke lever knob.

with the knob depressed (choke off) 

simply turn it one way to increase and the other way to decrease.

now go and try it in the shed :)

Title: Re: 2 stroke idle with a Keihin carb?
Post by: Nathan S on October 11, 2015, 05:26:50 pm

Quote from: Curtis on October 11, 2015, 03:27:09 pm

Just fitted a Keihin PJ36 flatslide to my 125...

Why? Did you also fit a BSA Bantam motor? Maybe some nice Thermalflow shocks too? Some lovely trials universals?

PJs are shit. They trade away a decent idle and decent throttle response for maybe 2% more top end power. They're a road-race carb that some manufacturers were stupid enough to put them on dirt bikes.
Fit a PWK, be happy.

Title: Re: 2 stroke idle with a Keihin carb?
Post by: Hoony on October 11, 2015, 06:47:35 pm

they get a bad wrap, sure they are old tech' (30+ years old by design)  and although i cannot compare it with a PWK i am happy with the PJ on my CR500RF, it seems to suit the short rod engine fine.

Title: Re: 2 stroke idle with a Keihin carb?
Post by: Nathan S on October 11, 2015, 09:03:01 pm

It's not about age or technology, it's about design compromises.
The PJ was released in 1985, while PWK came out in 1988... We're not talking about decades of development between them - the issue is that the PJ was designed for upper-RPM power, and nothing else.
Meanwhile, the PWK was designed to be a decent carby...
There is no question about which is the better dirt bike carb.

Title: Re: 2 stroke idle with a Keihin carb?
Post by: John Orchard on October 11, 2015, 09:50:28 pm

I've had good results from Both PE's (round slide) and PJ's.

Title: Re: 2 stroke idle with a Keihin carb?
Post by: Curtis on October 11, 2015, 11:31:00 pm

Yeah thought that choke was the idle adjust,
Haha...Just experimenting Nathan, it definetly revs out better, but lost a bit of response down low, hoping to fix that with correct jetting. Just trying to bring this RM to life. Got a set of thermal flow shocks to sell me?.... Or a cheap PWK? ;)

Title: Re: 2 stroke idle with a Keihin carb?
Post by: Lozza on October 12, 2015, 09:44:35 am

Try a Dellorto ;)

Title: Re: 2 stroke idle with a Keihin carb?
Post by: Nathan S on October 12, 2015, 10:28:05 am

You'll never get the low-end jetting right. The vacuum signal at low RPM is too weak, and the idle adjustment too coarse and too variable.

But good luck trying...

35mm PWKs are cheap enough. Look for an 88-94(?) KX125 or 89-94 KDX200E.
36mm ones have a larger body and may or may-not fit the RM boots.

Title: Re: 2 stroke idle with a Keihin carb?
Post by: Curtis on October 12, 2015, 11:14:48 am

How come Honda CR go so well with them? They ran all through the 90s.

Title: Re: 2 stroke idle with a Keihin carb?
Post by: Nathan S on October 12, 2015, 08:24:55 pm

And they all idled shit, and had reputations for lousy bottom end power...
Same with the RMs and the rego-able RMX250s