GS650 carbs and fuel

[Guest]

I've been chasing this fueling issue for a while.

Not much info out there for the 650s.

My thoughts;

First with CV carbs there isn't as much correlation with the slide and throttle as with the slide and the amount of flow through the carb, which at WOT is going to be mainly RPM dependent.

Why? The kidney shaped opening at the back of the carb provides force/pressure to the bottom of the slide diaphragm. Any pressure drop across the filter or decreased pressure at this port and this changes the pressure differential acting across the diaphragm and either slows and possibly limits the movement of the diaphragm, such as with the decreased pressure of of an air box and restrictive filter, or speeds up and possibly allows it to open further with a less restrictive system.

I think on this bike, the point where the air box and filter would begin to limit the movement of the slide, without the restriction, like in the case of a modded air box, it might cause a rapid opening creating a rich misfire like I've been getting.

It could be that in the normal operation of this bike, the slide isn't intended to fully open and most of the "WOT" fuel metering is actually done by the jet needle.


I think this correlates with the sudden rich misfire I was getting once a certain RPM was reached. This was based on jetting. Larger jetting, sooner, leaner jetting later.

It seems that the general consensus is, modify the air box, go bigger on the main jets. But, I think this applies with the larger bikes because at any given point, with a larger motor running the same diameter carb, the slide would be open wider. So earlier on, compared to a smaller displacement bike, the main jet will be the absolute restriction in fuel flow.

My thoughts are that this bike is jetted fairly rich from the factory on the main circuit and this isn't an issue on a non modded bike because possibly the slides don't fully open with the factory air box.

It's curious because of all the variants of the 650, it looks like only 2 run different jetting. One is entirely different, air jets, mains, pilots, needle, and needle jet. One only has changes to the main jet. That's the 650e, it's running a 97.5 main jet, which is 5 down from the 110 main the other 650s are running. That air box looks similar to the gs550 air box.

To get a rich misfire based on tuning cars, you have to be in a 8:1 AFR range. That's based on most wideband rigs pegging around 9:1. So, with the factory jetting going that rich with a modified air box, IDK.

I think that's why I've been chasing this so much is what I was doing was intuitive, but incorrect.

Time to get some smaller jets I guess.

[Old guy]

Runs great on 102.5 mains. But it's being choked down by the current filters in the upper rpm ranges. No sputtering or breaking, just weak.

Air box will being going back on and I'll try some other stuff. I still think this bike needs to go leaner with the air box modded.

[tom203]

Mine runs great with 110 mains and stock airbox/filter setup! more power than I need plus 50mpg- life is good!

[Old guy]

Took the bike out after some changes. It was feeling good, so I thrashed it

Finally running like what a 70hp bike is supposed to.

It was great. Breaks up about 8.5-9 k, but it is nice and strong. Peak hp is at 9400 rpms, so I got to get it right up there. It's got 102.5 mains in it now.

I'll do some thrashing of a more diagnostic nature once it cools Back down to see if it's better or worse once warm. It seemed to do better cooler, which would point to rich. I've got some 97.5 mains to try.

These bikes might be pretty rich from the factory

And they might need leaner jetting with air box mods, WTF.

I found a shop that is letting me borrow jets, bring back what I don't need and just buy the ones that work. Cool guys.

[Steve]

These bikes might be pretty rich from the factory

And they might need leaner jetting with air box mods, WTF.

I do believe that you would be the FIRST to think that.

I found a shop that is letting me borrow jets, bring back what I don't need and just buy the ones that work. Cool guys.

You have a definite keeper there.


Stock jetting was with 97.5 mains. My concern would be that the jets you were first trying might have been drilled out, making you think that they were a different size.

While you have access to all those different jets, might try some jets that you know have NOT been drilled.

.

[Old guy]

GL 650s were a 110 main. The E model was a 97.5 main.

With the 110 it would break up starting around 7.5k but rev out. The larger I went with the jetting the earlier it would break up, but it was like hitting a rev limiter. It would just stop at a certain rpm.

Going leaner pushes the "rev limiter" further out. I know, it makes no sense.

With a 112.5 and 2 (47.5) up on the pilot it would cut out around 5k.

112.5 and stock pilot (45) 7k.

I'm on a 102.5 and it's going out to 9k before checking out. The plugs look great. The mid range, throttle wise everywhere, and RPM wise is great.

The stumble at steady throttle is virtually gone, I won't even mess with it at this point. I'd feel comfortable riding the bike any kind of distance.

It seemed like the bike got a little worse as it warmed up, which would indicate it's still rich.

Makes no sense.

The air box lid is supposed to clamp on. I have a threaded rod run through the box and a piece of 1/4" ply held on with a butterfly nut. I have 2 4" diameter paper filters stacked together, the threaded rod runs through the middle, to the lid, the filters are slightly taller than the box, but they pull down when the lid is tightened. Lid edges are sealed with foam weather stripping.

What I'm running has way more surface area than the stock filter. Less volume in the box, and I'm guessing less restriction than the factory filter.

[tkent02]

Have fun.

[Old guy]

I have some 112.5, 115, 102.5, and 97.5 that aren't drilled. The 110s weren't either because when I started drilling them the size was correct. I even gravity flowed them vs known good jets.

[Griffin]

GL 650s were a 110 main. The E model was a 97.5 main.

With the 110 it would break up starting around 7.5k but rev out. The larger I went with the jetting the earlier it would break up, but it was like hitting a rev limiter. It would just stop at a certain rpm.

Going leaner pushes the "rev limiter" further out. I know, it makes no sense.

With a 112.5 and 2 (47.5) up on the pilot it would cut out around 5k.

112.5 and stock pilot (45) 7k.

I'm on a 102.5 and it's going out to 9k before checking out. The plugs look great. The mid range, throttle wise everywhere, and RPM wise is great.

The stumble at steady throttle is virtually gone, I won't even mess with it at this point. I'd feel comfortable riding the bike any kind of distance.

It seemed like the bike got a little worse as it warmed up, which would indicate it's still rich.

Makes no sense.

The air box lid is supposed to clamp on. I have a threaded rod run through the box and a piece of 1/4" ply held on with a butterfly nut. I have 2 4" diameter paper filters stacked together, the threaded rod runs through the middle, to the lid, the filters are slightly taller than the box, but they pull down when the lid is tightened. Lid edges are sealed with foam weather stripping.

What I'm running has way more surface area than the stock filter. Less volume in the box, and I'm guessing less restriction than the factory filter.

What really "makes no sense" is dicking around like this when the stock jetting with the stock airbox runs perfectly fine. Put a well-sealed airbox back on, put an oem air filter in the box, put the lid on the box, and all is well.

The only things to drill out are the mixture screw plugs, so you can set the screws to maximize idle and help the bike run smoothly.

All that takes an hour or two, and once you sync the carbs, your bike will run better than you can believe.


Or.........you can screw around endlessly with jets, drilling out jets, messing with different foam materials,messing around with different monkey-rigged airbox designs, and never enjoy actually riding the thing.

[Sci85]

Well for me, 550 with 650 heads, I'm looking for maximum power. Tinkering to me is fun and a learning experience. Hell,if all I ever wanted to do was ride, then I'd by something fuel injected. Why the hell,would I want to even fool with a carb?

Now, on to something that might actually be useful for you. I run the K&N twin filters. Based on my jetting, I'm not convinced these flow more than the stock air box but they are are a heck of a lot easier to remove the carb rack with. Anyway, I ended up experiencing the exact same issue you had. I started rich naturally assuming I would need to richen up due to the pipe, head swap, and filters. I used the dynojet needles and mains. I would hit the same upper RPM limit around 6-7k. Not until I went back to the stock 650 needle and mains would the bike run good. I still have a slight light throttle stumble that I need to shim out, but overall, the bike pulls good and hard now.

As far as slide movement, that can and is manipulates by spring pressure and bleed hole in the slide top (the dynojet kit enlarges the hole). Ultimately though, I really want to switch from CV to a set of Keihin FCR's with accelerator pumps. Now that would be a nice carb based setup. I wonder if there is an aftermarket Adjustable FI option with O2, open loop setup? Nirvana?

[Steve]

GL 650s were a 110 main. The E model was a 97.5 main.

Very true, and the non-L G was also 110.

Sorry about any assumptions, but the ONLY mention of models was in the first post, and you have NEVER mentioned in this thread which model you have.

THIS is exactly why we keep begging you NEW guys to create a signature that mentions what bike(s) you have, because you tend to forget to mention it in your posts.

.

[Old guy]

Well for me, 550 with 650 heads, I'm looking for maximum power. Tinkering to me is fun and a learning experience. Hell,if all I ever wanted to do was ride, then I'd by something fuel injected. Why the hell,would I want to even fool with a carb?

Now, on to something that might actually be useful for you. I run the K&N twin filters. Based on my jetting, I'm not convinced these flow more than the stock air box but they are are a heck of a lot easier to remove the carb rack with. Anyway, I ended up experiencing the exact same issue you had. I started rich naturally assuming I would need to richen up due to the pipe, head swap, and filters. I used the dynojet needles and mains. I would hit the same upper RPM limit around 6-7k. Not until I went back to the stock 650 needle and mains would the bike run good. I still have a slight light throttle stumble that I need to shim out, but overall, the bike pulls good and hard now.

As far as slide movement, that can and is manipulates by spring pressure and bleed hole in the slide top (the dynojet kit enlarges the hole). Ultimately though, I really want to switch from CV to a set of Keihin FCR's with accelerator pumps. Now that would be a nice carb based setup. I wonder if there is an aftermarket Adjustable FI option with O2, open loop setup? Nirvana?

Awesome, another tinkerer! is your stumble at steady throttle or rolling on to the throttle? You might just need to change the idle air jet

I think, it might be the case with the 650s, that rejetting richer, simply isn't needed, even though that's what's being repeated over and over based on other bikes.

That's the whole point of tinkering in the garage and then putting it out there.

A second 650, similar results. Go richer everyone said, two guys that played around with it said stay stock or go leaner.

This is my basic conclusion from playing around:

I still think the 650 stock is overly rich from the factory. In my research I've run across a few posting where the bike was running like crap and it was caused by a dirty filter. That filter would have to be REALLY dirty, or, the bike has to be pretty rich to start with, and the extra restriction throws it so rich it starts having problems. I bet on a totally stock bike that going down on jetting would pick up some power. If it's negligible, then suzuki was doing it for the cooling. I think it's so rich that going to pods or whatever, just takes care of the overly rich situation, hence why rejetting isn't needed. Just going to a 112.5, 1 up on the mains, caused a rich misfire, going up more just caused it to rich misfire at a lower RPM.

I also think the inner and outer cylinders might need different jetting. The 650 is rated @ 73 hp, it's in a pretty high state of tune from the get go.

[Old guy]

Hopefully this can save a few 650s from being abandoned because of jetting issues, or let people pick them up and save them.

[Steve]

Thanks for the new signature.

.

[Old guy]

Steve, Did you have to rejet your 650 when the pipe went on?