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VM setup and fuel economy

J

jknappsax

Forum Mentor
Past Site Supporter
I'm finished with all the preliminaries, valve adjustment, bench carb sync, ignition timing, getting ready to vacuum sync the carbs and fool with needle height (Dynojet Stage 3 kit, headers and K&N pods), but I'm curious about the fuel screw setting. After lightly seating the fuel screw, I set them all at 2.5 turns out, and started it to time it. Seems a little rich, but starts and idles fine. I'm a little confused about this setting. Obviously the mixture is controlled by both the fuel screws and the air screws, but the initial 2.5 turns seems a little arbitrary; if that was a fixed amount of fuel, why even have fuel screws, just a set diameter passage that lets in the proper amount of fuel, then adjust the mixture with the air screws, like a lot of auto carbs do. As far as the fuel screws, can they be adjusted in somewhat, like a quarter or half turn, to improve fuel economy? A method I've heard car guys use for fuel screws, if the carb has them, is to tweak them in slightly until the rpms just start to dip a LITTLE after the air screws are set to max rpm, basically setting them close to optimally, but just to the lean edge. Any opinions/ enlightenment, anybody?
 
They did that because you can change the needle height. This gives you the option to fine tune after changes. I just did mine and found that if you turn the fuel screw to get max RPM's then you need to check the air screw for max RPM's also. At max RPM on the air screw, turn in 1/8th turn so that you won't be too lean. After you have done that, then do a sync check. This will give you the best fuel economy.
 
If you installed a Stage 3 kit I'm sure that streching the mpgs on your bike wasn't a big priority.:)
 
Actually, I installed the Dynojet Stage 3 kit because I got it with the bike, which already had headers and K&N pods. I wanted to go back to the stock airbox with a K&N replacement filter, but without a stock of different sized main and pilot jets, this was the quickest path to a bike on the road this summer. I use my bike as a sport-tourer; ride somewhere and then enjoy the twisties, so fuel economy is somewhat important to me. I want to be able to get decent mpg while touring by being a LITTLE easy on the throttle, but be able to roll it on and have some fun! I know others on this site have managed to get upper 40s AND have bikes that are scary fast. I just want to be able to do the same. Even on a bike, when gas creeps toward $5.00 a gallon, it can get expensive just to get somewhere. For around town, I rebuilt a Honda XL 185S that I'm getting ready to carb tune now to ride to and from work, so running a literbike is getting to be a luxury I can only afford as recreation. BTW Jay, where in MD are you? I have a boat project I keep in Rock Hall, so I'm down that way a lot. If you're in the area, and ever want to ride (if I get the carbs straightened out!), PM me and we can ride.
 
Last edited:
jknappsax said:
Actually, I installed the Dynojet Stage 3 kit because I got it with the bike, which already had headers and K&N pods. I wanted to go back to the stock airbox with a K&N replacement filter, but without a stock of different sized main and pilot jets, this was the quickest path to a bike on the road this summer. I use my bike as a sport-tourer; ride somewhere and then enjoy the twisties, so fuel economy is somewhat important to me. I want to be able to get decent mpg while touring by being a LITTLE easy on the throttle, but be able to roll it on and have some fun! I know others on this site have managed to get upper 40s AND have bikes that are scary fast. I just want to be able to do the same. Even on a bike, when gas creeps toward $5.00 a gallon, it can get expensive just to get somewhere. For around town, I rebuilt a Honda XL 185S that I'm getting ready to carb tune now to ride to and from work, so running a literbike is getting to be a luxury I can only afford as recreation. BTW Jay, where in MD are you? I have a boat project I keep in Rock Hall, so I'm down that way a lot. If you're in the area, and ever want to ride (if I get the carbs straightened out!), PM me and we can ride.
Click to expand...


Go leaner with all four screws, say a half turn at a time, until it starts to run a little poorly, then back out a little until it runs better again. You are looking for smooth throttle response just off idle, missing or surging at a steady low throttle setting, etc. Adjust the airscrews for highest idle after each change. The bigger bikes spend most of the time while cruising at lower speeds on the pilot circuit, if it is too rich it wastes a LOT of gas. 2 1/2 turns out sounds like too much, but each bike is different.

Just bought a Suzuki TS 185 for my daughter to learn on, I'm thinking I'll save a bunch of gas with it getting around town too.
 
Did I miss something here, or does nobody else have a clue what bike we are talking about, either? :-k

My only clue is that you stated 'VM carbs". That would indicate '79 and older. Anyway, with VM carbs, the stock setting for the fuel screw is about ONE turn out, not 2.5, so that could be a lot of your "Seems a little rich". It could be that with the Stage 3 kit there are other instructions, and I have never played with kits on VM carbs, but it is just another point to consider. 8-[

.
 
TK, thanks for the advice; this kind of stuff I was hoping someone might have insight to. Steve, the stock tuning advice for the VMs, including the stock VM 26s on my 1000, that I've read is to start at 2.5 turns out from lightly bottomed on the fuel screws. The air mixture screws get ballparked at 1 turn out, then tuned for max RPM one at a time, unless I completely misunderstand everything I've read here on the GSResources about VM tuning.
 
OK, I don't deal al lot with VMs, so I might be reremembering incorrectly, but I thought that the recommended starting points were 3/4 to 1 turn out on the fuel screw and 1 1/4 to 1 1/2 on the air screw. The idle mixture screws on the BS carbs start at 2 (some people say 3), so it might be easy to confuse the two. :-s

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