VM Carb Push/Pull Throttle Cables Questions

[bgk]

Are push and pull throttle cables absolutely necessary on VM carbs?

Is the push cable just a safety feature in case the throttle return spring breaks?

I bought new push and pull cables this past week, and went down to the basement to install them over weekend.

A while ago, I read somewhere on the GS Resources forum that the push cable is required to close the carb slides, but when I pulled the old cables off my bike, and looked at the throttle-actuating cam/pulley/lever mechanism, I couldn?t see how this is so.

It clearly looked to me that both cables actuate the same cam/pulley/lever system. It?s just that one cable pushes it and the other cable pulls it. (Actually, the ?push? cable is a return cable that pulls it back the other way.)

Also, I know that all the Suzuki GS bikes with CV carbs and most all brand and model newer bikes just use one throttle cable.

So, I just installed the pull cable, and kept the push cable as a spare (both cables are the exact same and have the same part number).

I also installed a lighter throttle return spring that I bought at the local hardware store.

Right now, after the new pull cable install and lighter spring install, my throttle operation is just about perfect, with nice light spring return action. It?s now much, much better on my hand and wrist than the stiff action with the many-years-old push/pull cables and stock throttle return spring I had before.

However, now, I?m second-guessing myself and wondering if I?m missing something in my thinking and limited knowledge on this subject, and I?m asking GS Resources members if I should go back and install the second cable.

I called the Suzuki dealer, and the service department rep (who is probably younger than the bike) simply said ?If it came from the factory with two cables, it should have two cables?.

 

[daveo]

there is an issue with carb icing on VM type carbs, the pull/return cable is needed for when the carb freezes open at WOT...could be a very bad thing without it...you may want to install it

 

[bgk]

Carb icing? Yowza, I never would have thought of that.

Is this a real-world problem for a guy who very rarely sees the far side of 6000 RPM on the tach on a GS1000?

In fact, I don?t think I?ve ever had my bike at wide open throttle (with the engine running, either while stationary during maintenance or while riding).

Where can I read-up on this carb icing problem. You?ve got me curious!

 

[daveo]

http://www.google.com/search?hl=en&q=carburetor+icing&btnG=Google+Search


it's a common thing... the slide barrel carbs are more prone to it than CV type carbs with a butterfly, the slides hang open easier..it's more from relative humidity in the air, coupled with temperature drop inside carburetor, due to the venturi effect upon the fuel mixture,it expands and cools as it does so, then the water content, in the fuel mixture,condenses and freezes. It can happen at any rpm, usually when relative humidity is above 50% and ambient temp is below 70F, very similar to the dew point effect, where temperature and relative humidity combine to make water vapor condense to water.

 

[lhanscom]

With my RS carbs (they have a manual slide too, just flat instead of round) if you don't have a push pull throttle the slides will hang under certain conditions. The air flow will actually get high enough to over come the springs, so it requires your hand to pull them shut.

 

[first timer]

these are good discussion points, i have a set of 29mm vm's and will be keeping the talk in the back on my mind to get use to using my hand to close the throttel and not relying on the spring like i do on my 550 w/ cv's. i thanks for the good tip to practice.

-ryan

 

[earlfor]

Re: VM Carb Push/Pull Throttle Cables Questions

I converted my 79 750 (VM carbs) to pull only throttle shortly after I got it.
In the years I ran it like that, I never had a problem. I also went to the hardware store and replaced the throttle retun spring to suit my wrist and preference for throttle feel. If the throttle return spring breaks, the throttle will remain at whatever position it was in when it broke if there is not a throttle return cable. Just plant firmly in your mind, that if the return spring ever breaks, you will need to flip the kill switch to reduce throttle. (of course, you can still increase throttle if the return spring breaks.)

Earl

Are push and pull throttle cables absolutely necessary on VM carbs?

 

[KEITH KRAUSE]

I would go with just the lighter spring. It's a safety issue. Use both cables.

 

[Anonymous]

I supose a good test would be to run the bike up to about 9 thousand pm in one of the lower gears and with holding the throttle wide open just hit the kill switch.
I wont try it but that should tell if the kill switch idea is sensable or just an opinion.

 

[KW]

When I bought my 750 it had no "push" cable, and was a pain in the ass! When snapping the throttle shut to change gear the carbs didn't close down fast enough on spring alone. The result was that every gear change above about 4000 rpm resulted in a screaming engine until the clutch was let out, then a thunk and jolt forward. Stuck the "push" cable on and no more problems. I think the spring is the safety device in case your "push" cable breaks. You shouldn't rely on it for normal operation.

 

[earlfor]

All you would be doing is turning the ignition off. The only difference between turning the ignition off in your car and turning it off on your bike is the steering doesnt lock.



Earl

I supose a good test would be to run the bike up to about 9 thousand pm in one of the lower gears and with holding the throttle wide open just hit the kill switch.
I wont try it but that should tell if the kill switch idea is sensable or just an opinion.

 

[earlfor]

Fuddy dudd, where is your sense of adventure, your quest for the gusto, your cavalier alter ego? Perhaps if you would ask the wifey to let you out more often, you could..........................LOLOLOLOLOL

Earl

I would go with just the lighter spring. It's a safety issue. Use both cables.

 

[Anonymous]

All you would be doing is turning the ignition off. The only difference between turning the ignition off in your car and turning it off on your bike is the steering doesnt lock.



Earl

I supose a good test would be to run the bike up to about 9 thousand pm in one of the lower gears and with holding the throttle wide open just hit the kill switch.
I wont try it but that should tell if the kill switch idea is sensable or just an opinion.

Mzybe you are overlooking something. EH EH

 

[earlfor]

If youre referring to locking up the rear wheel, I've done that at pretty decent speeds. I dont consider it a big deal. I'll be happy to demonstrate that anytime.

Earl

[quote="Fool

Mzybe you are overlooking something. EH EH[/quote]

 

[daveo]

The problem with killing the ignition, with the slides stuck open, is that the engine can continue to fire from the cylinder temperature alone, like a diesel. Without a way to get the slides closed to stop fuel mixture induction, and a screaming engine, it could get ugly. I'm not saying this will happen, but, under certain conditions it can and has happened. It may never be an issue for you Earl, due to your location or whatever, but I can't say it won't for everyone...

 

[bgk]

Wow. Looks like I inadvertently started a lively discussion!

As for safety, if the slides ever freeze and stick open, AND if the engine then runs on or "diesels" after I hit the kill switch, I could just pull in the clutch lever to save my a$$.

But, at this point in the conversation, it's all academic: I'm simply going to install the push cable!

Thanks for the education, guys.

The info provided on this forum is amazing.

I love the GS Resources!

Thank you all.

 

[earlfor]

I was not considering carb icing as temps here are pretty much in the 90-100 deg range all the time. A cold snap usually means 75-80. LOL

I have never seen a gas engine get hot enough to diesel, but logically thinking, I can see it as a possibility. How much of one, I have no idea.
I suspect the odds are less than they are for being run over by a train while taking a shower. Everyone has the right to choose the train or the shower though.

Earl

The problem with killing the ignition, with the slides stuck open, is that the engine can continue to fire from the cylinder temperature alone, like a diesel. Without a way to get the slides closed to stop fuel mixture induction, and a screaming engine, it could get ugly. I'm not saying this will happen, but, under certain conditions it can and has happened. It may never be an issue for you Earl, due to your location or whatever, but I can't say it won't for everyone...

 

[mcycle-nut]

I have never seen a gas engine get hot enough to diesel, but logically thinking, I can see it as a possibility. How much of one, I have no idea.
I suspect the odds are less than they are for being run over by a train while taking a shower. Everyone has the right to choose the train or the shower though.

Earl

The problem with killing the ignition, with the slides stuck open, is that the engine can continue to fire from the cylinder temperature alone, like a diesel. Without a way to get the slides closed to stop fuel mixture induction, and a screaming engine, it could get ugly. I'm not saying this will happen, but, under certain conditions it can and has happened. It may never be an issue for you Earl, due to your location or whatever, but I can't say it won't for everyone...

I've seen it happen numerous times, mostly on two-strokes. Somehow the throttle sticks due to a crash or something, you go to hit the kill button and it doesn't do ANYTHING! Sort of freaky! 8O I've also seen cars do that, but not for long. You turn them off, and you get a "ca-chug-ca chug" type thing.

As far as the push cable, gas it. It's just extra weight. I've been running my 750 without one for 14 years. If I ever have any problems with a sticking throttle, it's because the throttle cable needs lubing, is routed wrong, or is starting to break. I'm running the stock spring (I think!) and the throttle snaps back with authority! 8)

 

[saaz]

I have a 78 GS1000, have owned it since 1980. It still has the original throttle cables on it, as they do not seem to wear.

The return cable probably is not necessary, but it does no harm to leave it on if the return spring does all the work anyway. I figure it is an easy way to carry around a spare throttle cable if I need it one day!

 

[KEITH KRAUSE]

Like saaz says, it's the spring that causes the throttle stiffness. Brion replaced it with a lighter spring, so that should help a lot with a sore wrist.
It makes the decision about keeping both cables easy.