Lectron carbs

iagsxr · Post by **iagsxr** » Fri Dec 25, 2009 2:12 pm

I see Tom Garcia is using Lectrons on his project and some other engine pictures with them installed, but don't want to jack anyone's thread so..

Been trying to get some info on putting a Lectron on an 81 YZ250 I'm restoring to race vintage mx with. Have gotten some very conflicting info, anyone have any input?

thanks

tz375 · Post by **tz375** » Fri Dec 25, 2009 11:36 pm

What would you like to know? For road race they are great because they improve pick up. I don't recall people using them for motocross and I'm not sure why that is.

Maybe because I didn't follow it so closely or maybe because for mx, a roundslide is quite sharp enough.

iagsxr · Post by **iagsxr** » Sat Dec 26, 2009 8:43 am

Mostly interested in throttle response.

The stock carb on my bike is a 38mm Mikuni roundslide. I've been told that they're lazy, looking at it I believe that's true(never ridden the bike).

pjmcburney · Post by **pjmcburney** » Sat Dec 26, 2009 5:56 pm

tz375 wrote:What would you like to know? For road race they are great because they improve pick up. I don't recall people using them for motocross and I'm not sure why that is.

Maybe because I didn't follow it so closely or maybe because for mx, a roundslide is quite sharp enough.

I have an '81 KTM 495 in the process of being restored for (maybe) some Classic Motocross .
Hopefully it will end up looking like this one:

In the process of researching all things early KTM, I frequented a few Classic MX meetings locally and bumped into a fellow KTM'er.

He had a Lectron on his (I think) 250. I asked about it and his response was, 'it works well, certainly better than the Bing it came with.'
It looked the part, what with the transparent float-bowl and all...

BTW, I have a 39mm Keihin PWK to go on my bike when it's the motor is finally installed in the frame.
The original Bing is a traditional round-slide design, the PWK is a semi-flat/oval slide and is said to be much more responsive and easier to tune.

The PWK's are a bit exxy, but are a damn fine carby (certainly a more modern design than the Lectron's).
I know I'd be tempted to grab one of those if my budget allowed for it.

Cheers
Paul

tz375 · Post by **tz375** » Sun Dec 27, 2009 10:01 pm

The advantages of the Lectron are the flat slide which creates a shorter path and stronger signal to the main jet and the flat on the needle which amplifies that signal.

The combination results in a faster throttle response ie it's crisper and feels more responsive. Mikuni introduced their flat slide carbs specifically to improve throttle response on MX bikes, though I don't recall anyone claiming that their YZ250 was lazy before.

If you can find a second hand 38mm Lectron it would be worth trying it.

iagsxr · Post by **iagsxr** » Fri Jan 01, 2010 11:54 am

Looking at the stock Mikuni, the intake bell is almost the same size as the slide and wraps around the sides of it. I know they make the UFO thingies that go on the bottom of the slide because they have reversion.

The guy that's helping me with the engine said right away that the lack of velocity in that particular carb made it a little hard to tune and a bit lazy.

I should mention that the this bike had sat for a very long time and the PO buggered up the original carb getting it apart. I'm buying another carb one way or another.

If this were a four-stroke I'd figure airflow requirements and work backwards from that. Any idea what I should use for a VE number or max RPM on this thing? Are cfm ratings available for anywhere for aftermarket carbs?

When did the PWKs come out? I've had a couple of those that worked well. By the rules I could run one for sure, but I'd like to to look like it belonged on the bike not an old bike with a modern carb.

tz375 · Post by **tz375** » Fri Jan 01, 2010 1:50 pm

UFOs seem to be loved by some and hated by others. there are very few professional tuners that use them and that's enough for me.

I don't understand your tuner when he claims it's lazy and the flow velocity is too low. Compared to what? Were they considered to be slugs back in their day? If so, he could be right.

Flow velocity is a function of a number of things including engine efficiency, size, intake configuration, rpms and of course RPMs and throttle opening.

Sure, WFO at low revs with no through flow, the velocity will be low, but velocity and flow are very similar on the same sized flat slide and round slide. We are not concerned about flow at full open unless that's how you ride.

This is about part throttle application and at the same engine speed, the motor will create the same pressure drop into the carb. The difference is that with a flatslide, that translates into a stronger signal to the jet ie a larger pressure differential at the jet as the throttle opens.

A change in pipe will probably make more difference than a carb change, so it's important to define your objectives. For example if what you are after is more punch out of corners, consider more compression and a good race gas. For better (more subtle) feel, try a flat slide.

BTW, 4 stroke formulae don't work well for two strokes, but 38mm is about right for a 250 MX bike of the time.