Hello All,
I'm new to the forum, and I haven't had a GS very long. The one that I was given is a bit of a project and has a lot of carb issues. In the meantime, I have a brand new Mikuni VM36 sitting around from my SR500 that I thought might be fun to rig up on the GS. I made a Y pipe and got the bike running on the carb, but of course it's not set up for the bike.
Does anyone have any suggestions on where I could start with jets that might get me in the right area for this bike? It is a '93 if it makes any difference.
Thank you
if it's running, it's running, right? I think you'll have to discover for yourself what the correct jetting is, the old fashioned way. Read the plugs, ride, tune, repeat.
If it's set up for an SR500 then it's probably already pretty close. Clean the plugs, go out and run it at WOT and high revs (like 2nd gear :) then pull the plugs and check. Start there.
I seriously considered switching to a pair of VM-series carbs when I was having all of my trouble with the original carbs that came on the bike. I'm a little envious :)
All Naturally Aspirated engines require the same F/A mixture to run correctly....you are prolly closer than you think :whisper:
EDIT: Other than flow differences and altitude
Quote from: Suzuki Stevo on June 06, 2018, 08:21:56 AM
All Naturally Aspirated engines require the same F/A mixture to run correctly....you are prolly closer than you think :whisper:
Yes, and figuring the displacement of the donor and target bikes are roughly the same, then they will draw in roughly the same amount of air and therefore need roughly the same amount of fuel to go with it. Probably already almost there.
Only thing is the redline of the GS is about 3K rpm higher than the SR so the carb may run out of airflow (and fuel) early. But you can ride a GS at under 7K rpm and it gets the job done just fine for daily riding. I almost never rev mine over 7K. :thumb:
I am guessing it probably works as is.
Excellent, thanks for the info. I don't anticipate riding this bike at high RPM so I'll just give it a few test rides and see how the mix comes out.
VM36 is a round slide isn't it ? If so it would supply the GS, except it may not work as you think it would.
You're not feeding 500 cc with a 36mm, the intake strokes are split far enough apart that you're actually only feeding 250, and a round slide isn't a CV, so - well I dunno.
But 250 cc @ 10k demands about the same volume as a 500 @ 5k and since the sr500 runs fine to well over 6 you should be OK on peak flow for sure.
Cool.
Buddha.
This isn't as simple as it looks.
Gs500 wants to flow 1250l/m per cyl at maxrpm.
SR500 wants to flow 1750 @ max rpm.
So it looks like it should be fine EXCEPT the GS500 is a 180deg twin. There is 96degrees of intake overlap bewteen the 2 cyl. That is a lot. Both cyl will be trying to suck through the carb for half a stroke and that is enough for both intake air speeds to be at max.
Add to that manifold shape is going to be problematic it is going to be a bit messy overall.
The carb size (calculated) you beed is about 48mm ( I already had this from when I was considering doing a single body fuel injuction and discarded it, because of the complication of designing a suitable manifold).
I can be done and it will run, but will probably be a bit flat towards the top of the rpm. from 7000rpm onwards.
But wait there's more The GS carbs are flat slide and the SR carb is round slide. Round slides don't flow as well as flat slides so may lower the useable rpm even more.
Interesting point though. If you open up the intake cam/valve clearance, or better still get a lower angle set of cams made, you could lower the torque point and max rpm, and give it a bit more lower grunt ( sort of like an English bike). That would also not require as much overlap airflow. Bit drastic but interesting experiment, especially with a 2 valve motor that doesn't flow so well up top anyway.
That sounds like a fun experiment. I say go for it and see how it performs. :cheers:
FWIW my 1989 RMX 250 came with a VM38. It's a damn good carb. Great performance and easy to tune. I swapped in a Keihin PWK 36mm Quad Vent and she flat rips.
Just thought of this.
Actually if you add a plenum chamber of about 500cc between the intake runners and the carb you will be able to feed the carbs but your throttle response will be slower and the pulses from the other inlet may affect the vacuum slides of the other carb( maybe).
That and internal size of about 8x8x8 cm so it's manageable. Or 16cm wide and 4 and 8cm ( roughly min). those are guide figures . Simple multiples like that would create harmonic problems so you need to vary them to maintain the volume but not allow reinforceing standing waves. You may have to play with the shape of the internal walls to prevent harmonic problems also.
That would also allow you to tune the intake runners as well pretty easily.
This could be an interesting experiment .
Virago 1100's have had those 2X1 intakes made and use 42mm HSR carbs. Those work very well. So I'm thinking it should be doable.
Cool.
Buddha.
The Virago is passing 3300l/m at max rpm of 6000 but max torque is at 3000 where it's only passing 1650l/m. From the only dyno chart I could find it does keep producing power up to the max rpm so that 42hsr is capable of flowing 3300.
Sorry one thing I forgot to include is the single carb has less internal surface area than two carbs of the same cross-sectional area, so there is less restriction as the gas speed increases. So a single carb will restrict less from boundary effects. This is a square effect, so a simple compare won't show the true picture. The equivalent single throat size will be smaller than the simple calc 48mm calc. How much depends on the comparative actual gas speeds and internal shape of the carbs. I do NOT have the required formulea to calc a more correct size.
Even more interested in how this goes now.
Many V-twins and old parallel twins use a single carb. It's not the most effective for performance, but gets the job done.
I have read that the 270 degree crankshaft is a real problem with a single carb. However, I do not believe everything I read. Also, with carbs, quite often "the bigger the better" is not always true. I used a 34mm on my big-bore stroker sr500 (585cc) and it pulled like a freight train, well, almost. The SR 500 intake port does not flow well--one wonders about the gs500. BTW, I sold the SR, which I had ridden for 38 years and bought a low mileage gs500f. The original reason I switched is that I have developed COPD and therefore wanted an electric starter. I miss the sound, torque, and coolness of the SR, but I really enjoy my GS. It is smooth and quiet, has a huge power band, and handles really well. The SR was a lot like a dirt tracker in the turns; the GS is more like a road racer. Keep us posted on your progress.
Hasn't Yamaha brought back that SR500 or whatever it was - a 1 lunger. With EFI ?
Cool.
Buddha.
Yes--they have made them for years for the Japanese market where there is a cut-off in driver's licenses for bikes of different displacements. (I lived in Japan and have gone back many times.) Therefore, the one imported now is the SR400. The only trouble is that they are slow, slow, slow. They have the same bore as the 500, but a shorter stroke.
SR500 the second worst vibtraing motorcycle ever produced.
The first? Nothing comes close to the Ducati Desmo 450...nothing. Rev it on the centra stand and it actually moves backwards at a walking pace.