Motorcycle wants to die at anything below 2000 RPMS please help!

[Zechariah]

Hi all,

I've got a 1975 Suzuki GT250.

I can get it started with about 6-10 kicks and playing with the throttle, but, then I have to work the throttle to keep it going, if I keep the revs at about 3000 then it runs fine, but if I let it go down where it should be at idle (1200 RPMs) then it just tries to die, if I try to rev it back up to 3 then the engine lags and dies. (i.e. 1200 rpms quick full trottle = engine dies)


Could this be a problem with the timing? sparking to soon? (I installed new points and tried to gap them by reading about it in a haynes manual, I think I understand how to do it but, maybe not) Does anyone know of a how to video or article on setting the timing on an older bike?

Thanks,
Zechariah

[richtazz]

It sounds like a carb issue to me, but if the points aren't properly gapped that could cause your symptoms too.

[MagicRat]

I agree..... a dirty, gummed-up float bowl and idle circuit will do this.

[Zechariah]

Well, I already cleaned the carbs, boiled the parts in citric acid used compressed air and made sure all passage ways are clear, put them all back together again using new gaskets and needles but and it's still doing that....

Any other ideas? could it be that the floats need to be adjusted?

Thanks,
Zechariah

[MagicRat]

Originally Posted by Zechariah

Any other ideas? could it be that the floats need to be adjusted?

I think the idle speed is set by an idle screw on the side of the carb. It actually controls how far down the slide goes.

There should also be a mixture screw on the side of the carb as well. It kind of sounds like the carb is running too lean.

Can you post a pic of the carb(s), just as they appear on the bike?

[Zechariah]

Okay, here's the pics.

They're a little big... might take a second to load.








Thanks for helping me.
~Zechariah

[MagicRat]

Okay, looking at your last picture..... the idle speed is set by the larger screw on the side (the one with the spring). Turning it in slightly will raise the idle speed. It does this by opening the throttle slightly.

Note, its best that you open the throttle a bit, (by using the throttle twist grip) when turning this screw in, so the screw head does not bind on the throttle slide.

Also, both idle screws must be turned in or out the same number of turns, so both cylinders are drawing the same amount of air at idle. With the engine off, open the throttle, and turn both screws in all the way. Then turn both screws out two and a half turns. Then start the bike, let it warm up and set the idle speed, by turning the screws in or out. Turn both exactly the same amount.

The smaller brass screw just below the hose clamp in the last picture is the idle mixture screw. With the engine off, turn both screws in all the way. Then turn both out one and a half turns. Then start the engine, warm it up and turn the screws a bit in or out until it idles smoothly. For this, it's trial and error. An experienced 2-stroke guy could set the mixture screws in seconds, based on the sound and throttle response.
On the other hand, you may have to do some riding and adjusting before you find the right spot. Just be sure to turn both screws in or out exactly the same amount.

BTW that's a super-nice clean bike! I wish my 1975 Suzuki (TM75) looked that good.

[Zechariah]

Alright thanks a lot I'll give that a try.

Yeah my father repainted it the original color and now its in near mint condition.

~Zechariah

[richtazz]

Sweet bike, thanks for the pics. I agree with Magicrat on his tuning suggestions. You could also buy/rent/borrow a carb sync tool to be sure both carbs are set the same.

[FNA]

"boiled the parts in citric acid"

Apparently a practice not worthy of comment.

[MagicRat]

Originally Posted by FNA

"boiled the parts in citric acid"

Apparently a practice not worthy of comment.

Citric acid is used as a cleaning solution.

From Wikipedia: Citric acid is the active ingredient in some bathroom and kitchen cleaning solutions. A solution with a 6% concentration of citric acid will remove hard water stains from glass without scrubbing. In industry it is used to dissolve rust from steel.

Now, I am not quite sure how well citric acid dissolves gummy fuel residue. If such residue is alkaline (and it probably is) citiric acid would be okay. I would say its unconventional, but if it did a good job, perhaps we can learn from it.

[Zechariah]

It did a great job, as far as removing the old gas residue. They did come out kinda tarnished but a scotch-brite pad fixed that.

I read that online somewhere about using citric acid... and gave it a try, it worked wonders.

Maybe I'll post some more photos of the bike after I get it all done and put back together.

~Zechariah

[Zechariah]

Okay, so I played around with the Idle and air mix screws and still it's dying around 2200 rpms. Here's a video of it. http://www.youtube.com/watch?v=EERdizNyRhw

What do you think?

Thanks,
Zechariah

[MagicRat]

A video must be worth about a million words

Okay, is that you opening the choke as soon as its started? Sometimes, you can only open the choke just a bit when cold, or else it dies just as the video shows.

I would suggest experiment with choke settings for the first few minutes of running, until it warms up fully. Often, this is perfectly normal for 2-stroke machines, most require lots of choke for the first few minutes.... and even with the choke, they do not run well until warmed-up.

[Zechariah]

Well, yes I did open the choke as soon as it was started but, I already had it warmed up. (ran it at 3000RPMs for 5 mins) And if I didn't open the choke then it would die, but if I tied to start it with the choke off it just won't start at all. The choke is a on/off choke no in between.

So I was messing around with it again today and found that if I have the choke on the throttle is very responsive and the rpms will drop quickly after I rev it to where every I set the idle, but with the choke off after I rev it the rpms slowly go down to the idle... any idea what causes that?

Thanks again,
Zechariah