Question about carb settings

[Sebulba]

As you may or may not know, I own a '93 JDM GS400E. No that's not a typo, it's a GS500 with a smaller engine.

Is there any reson why the recomended jetting and needle settings wouldn't work on a smaller capacity engine?

Thanks

[fred]

I'd imagine the carb settings for a 500 would be too rich for a 400cc engine... You could try them, but I'm guessing they wouldn't be perfect. You could always try the stock jetting for the GS 500, people always complain that it is too lean, perhaps it would be perfect for the GS 400....

[Sebulba]

My thinking is that the carburettor settings affect the quality of the fuel-air mixture not the quantity. So the fact that less mixture is used on every stroke shouldn't be a factor in how the carbs are set up.

[ohgood]

As you may or may not know, I own a '93 JDM GS400E. No that's not a typo, it's a GS500 with a smaller engine.

Is there any reson why the recomended jetting and needle settings wouldn't work on a smaller capacity engine?

Thanks

jdm - jap domestic market  ?

so it's a gs500 frame/stuff with a 400 cc engine ? cool, that proves i know nothing about motorcycles again


got pics ?

[Sebulba]

http://gstwins.com/gsboard/index.php?topic=44493.msg499095#msg499095

The pics are about 1 1/2 month old, I washed it since then

Externaly I can't see any difference, bu I guess it's been debored and/or destroked.

[dgyver]

My thinking is that the carburettor settings affect the quality of the fuel-air mixture not the quantity. So the fact that less mixture is used on every stroke shouldn't be a factor in how the carbs are set up.

Jet size and needle height affect the amount of gas available to the motor at a specific throttle position. The GS500 settings will be rich for a GS400.

[Sebulba]

It seems that I didn't understand how carbs work.
The Suzuki GS500 service manual says that the BRT33SS carburettors are constant velocity, which I understand means that the speed of the air, and thus the ammount of vacuum, are roughly equal across the range of motion of the throttle valve.
The way I understand it is that the needle won't raise as high if there is less vacuum because the engine is smaller.

I'm not trying to be contrary, I just want to understand

[ned37]

generally, a carb jet works as a function of differential pressure from one side to another and is not a function of engine size.  you should be fine using 500 spec jets on a 400 as long as they are the same make and type of carb.   the jets in my gs1000 are smaller than the jets in my gs500. 

[Sebulba]

That's it? Nobody else has any information?

(Buddha, I'm loking at you , since you're the resident carb guru)

[scratch]

Fuel demand is going to be less on a 400.

See what size your current jets are, and go one size up; that is essentially the same changes that we do to our bikes.  So, maybe not the same sizes, but the same changes.

The needle changes are going to be different, though, and you may have to experiment in that area.  I suggest doing only the jet changes first, ride it for a week, then maybe one .5mm shim under the clips of the needles if you need to (I took the shims out of my '93 500 after I found that I didn't need them).

[Sebulba]

The reason I'm asking is it's a second hand bike, I don't know how many owners it's had and if they did any modifications to the carbs.
They might have already changed the jets, so I don't waht to make it run richer if I go one size up.

[fred]

The reason I'm asking is it's a second hand bike, I don't know how many owners it's had and if they did any modifications to the carbs.
They might have already changed the jets, so I don't waht to make it run richer if I go one size up.

Hmm, how is it running now? If it isn't showing signs of running lean, why change the jets at all? Why even bother to figure out what jet size is in there, if it is working, ride it... I guess it would be interesting to see how the jetting on a 400cc version of the engine differs, but if it aint broke...

[Sebulba]

Well, I'm not sure how it should run. It's my first bike so I don't have anything to compare it to.
It seems to run OK, once you get it warmed up, it's just that I'd like it to warm up faster. I haven't ridden it yet in anything greater than 20 Celsius (dunno how much that is in Fahrenheit) so I have no idea on the time it would take for it to warm up in summer. I expect it would be shorter

[fred]

If it is anything like the GS500, it will take a while to warm up, they are fairly cold blooded bikes. If you want to check for signs of mixture problems, pull the spark plugs and have a look at them. There are plenty of posts here that will help you figure out what they are telling you, and if you don't feel confident in your assessment, post pictures...

[Sebulba]

Yeah, I thought about pulling the plugs, but that is going to happen in the spring, I'm putting it to sleep this weekend.