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Grand Prix Classics
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La Jolla CA
Joined: 06/02/2016
Posts: 4

Okay i've been searching for an answer for this question for some time, so I'll finally just go ahead and ask.
After working on, and servicing an R42 for the last few months I am still somewhat ignorant as to how the carb actually functions.
I have managed to make it run, driven it for long distances at high speeds and i'm still not sure if i'm doing it correct.
The fact that it is NOT LIKE ANY OTHER CARBURETOR I HAVE EVER SEEN certainly doesn't help things.
Having two separate slides, I am unsure what one is doing?
Currently, the top paddle which operates the rearmost of the slides is just left wide open from start to finish.
Any attempt to start it with this slide closed is in vain. Any closing after running causes stalling.
The lower paddle controls the forward slide and is used exclusively to start and operate the machine.
This method seems to work, but I have no idea if it is correct. Also I am finding the plugs extremely rich after running.
Anybody have some experience with this carburetor set up? A lesson would be greatly appreciated>

Kenny

Grandprixclassics.com

Darryl.Richman
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VBMWMO #6285
Surf City, USA (Santa Cruz, CA)
Joined: 10/27/2006
Posts: 2185
R42 carb setup

These older 2 slide, non-needle jet carbs don't really function the same way as "modern" carbs do (eg, carbs with slide needles and needle jets). Needle jet carbs continuously vary the amount of gas by the taper of the needle. These older carbs can't do that, and the amount of gas that goes into the mixture is basically controlled by how much gas is pulled out of the main and idle jets by the vacuum that results from the air passing by.

To control how rich the mixture is, you change jets. Probably you have an 85 main jet, which is the "sport" jet for the sidevalve bikes and stock for the OHV bikes; the stock jet on a sidevalve is an 80. But don't worry too much about richness yet; too rich is much better than too lean, and changing the jet size is a pretty gross adjustment. If the bike runs well and doesn't foul the plugs, then you're probably ok.

The rear slide - the air or choke slide - controls how much air gets into the carb. Usually you choke it down a bit to start cold, and then generally run the bike with it wide open, possibly richening the mixture just a bit when pulling a hill, if the motor needs it. Choking it not only reduces the amount of air entering the carb, but it also increases the air speed, which creates a bigger vacuum and pulls more fuel out of the jets.

On the R42, the idle jet sits in the floor of the carb throat between the two slides, so it always contributes gas to the mixture. The main jet is under the front slide - the throttle slide - and doesn't contribute when that slide is close to closed. As the throttle slide is raised, the tip of the main jet is exposed, and the air rushing over it pulls gas into the air stream.

(If you have an R52/R57/R62/R63 or an early R11, even though the parts inside the carbs look very similar, the operation is somewhat different.)

You should try to start with the carb set up so that, with the levers closed, the slides are just seated at the bottom of the carb throat. To do this, you need to remove the cover on the rear of the carb to be able to see in, or remove the entire carb from the transmission so you can see in from the front. Use the choke lever to lift the air slide all the way, and then adjust the throttle cable adjuster to get the throttle slide to just hit bottom when the lever closes. Then you can do the same for the air slide.

This is the starting point for tuning the carb. Start the bike - the starting ritual may have changed, dramatically - and let it warm up. (Don't just let it run with no air, put a fan in front of the bike.) With the choke lever open fully, try to get the throttle as low as you can. (This is in conjunction with retarding the spark, you don't really want to idle the bike with the spark fully advanced.) Adjust the throttle cable to pull up on the throttle slide if the bike won't idle with the lever closed.

Having changed the adjustments, you'll now have to relearn how to set the choke and throttle for cold starts, but from this point you can reproduce the carb settings at any time and so the new operation should become SOP.

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Grand Prix Classics
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La Jolla CA
Joined: 06/02/2016
Posts: 4
Well that all falls pretty

Well that all falls pretty much in line with what I had assumed to be the case.
The rear air slide is unusable in any respect though as I mentioned.
The bike will not start with it closed and dies immediately once running if you shut it whatsoever.
Also the plugs are extremely sooty and hot starting is difficult.
I will check jet sizes in the morning and see what it has in it.
Meanwhile i am running Bosch T-1 w7a is this a good plug for this bike?

Darryl.Richman
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VBMWMO #6285
Surf City, USA (Santa Cruz, CA)
Joined: 10/27/2006
Posts: 2185
Plugs, slides

Oh, your heads have 14mm adapters in them! I run Bosch M8A plugs (M = 18mm, W = 14mm) in my R52, I think that's a step cooler. They are sooty but I have no problems with hot starting. You should be aware that hot starting problems are also a classic sign of a weak/dying magneto coil.

I would pull off the carb and look closely to see how the slides look as you move the levers, and make sure you're getting the best range of movement for the slides.

Good luck to you!

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