77 GS750 dies with lots of throttle off idle

[stubbs]

Hey all,
I've had this 750 for 6 weeks or so. I rebuilt the carbs when I got it, and it was running, but not well. It's got K&N pods and a Vance&Hines 4-1 exhaust on it. The main jets are 115s and the carbs are all nice and clean. Fuel screws are about 3 1/4 turns out, and air screws maybe 2.5-3 turns out (adjusted for highest idle). I'm at around 4500ft altitude.
A couple of weeks ago I tried moving the needles up to the highest position, thinking it may have been lean with those pods, and off-idle and part throttle response was great, and pulled all the way to the redline, but if I gave it anything more than around 1/3-1/2 throttle quickly, it would bog. It seems to bog much more at low engine speeds as well, but I'm assuming the effect is just amplified there.
Today I tried moving the needles down to the middle position like stock, and it now feels better in the midrange and takes full throttle better once it's going, but I have to be very gentle off-idle or it dies. To my semi-trained ear, it sounds like it's getting too much air and leaning out when given throttle quickly. Any ideas? Do I need to enrich the pilot circuit? Raise needles and lean out the pilot circuit? Lower needles more?

I know I need to get out and do plug chops sometime, but I'm hoping for suggestions from this video as well. Have a look/listen! I took a video of it so you can see and hear it.
http://www.youtube.com/watch?v=ZLnhKd8c1Kg
Thanks!

PS, it was also getting terrible gas mileage on the highest needle setting (25-30mpg), so I was pretty sure it was running rich. The exhaust also looks fairly sooty and the plugs are dark and wet looking, but no soot or buildup. (That's just pulling them out and looking, not actual plug chops).

[Dave8338]

Sounds like you are just on the edge, for correct jetting. Raising the needles will richen the mixture, not lean it out. Also it sounds lean on the mains and as far as "exactly" what you need for jetting...the only true way is to run it through all the RPM ranges and see what the plugs look like. You'll KNOW when it's right. You will be able to feel it.

UP on the mains... and the pilots (sound) like they will work, but the fine tuning is in the final jets that you select and where the needles need to be shimmed at.

[stubbs]

Yeah, I figured I was getting there. I think it sounds lean now too. Maybe I'll try one step richer again when I get a couple of hours to play with it. I almost wonder if I need 117.5 mains and needles in the middle position. I just need to take the time to do chops, don't want to order jets and waste money until I know.

[Dave8338]

Yeah, I figured I was getting there. I think it sounds lean now too. Maybe I'll try one step richer again when I get a couple of hours to play with it. I almost wonder if I need 117.5 mains and needles in the middle position. I just need to take the time to do chops, don't want to order jets and waste money until I know.

You'll need more than the 117 mains. That much drop-off on WOT, is at least two sizes under and if it is me, I'll take one size over on the mains (as a poke and hope) as you usually don't hit them until you are at 3/4 plus throttle. Better rich and lighten the oil on the pods, than lean and melt things down.

[stubbs]

Think so? At this altitude, I would have thought that 117.5s or 115s might have been enough. If the chops say I'm lean, then, I'll try 120s. Speaking of which, how does anybody find a safe spot to go WOT for more than 4-5 seconds and not hit crazy speeds (and get ticketed)? I'm thinking it'll take at least a few solid seconds to get a plug color change at WOT, and I'll be doing 70-80mph by the time I hit that kill switch. Sound about right?

[Dave8338]

Think so? At this altitude, I would have thought that 117.5s or 115s might have been enough. If the chops say I'm lean, then, I'll try 120s. Speaking of which, how does anybody find a safe spot to go WOT for more than 4-5 seconds and not hit crazy speeds (and get ticketed)? I'm thinking it'll take at least a few solid seconds to get a plug color change at WOT, and I'll be doing 70-80mph by the time I hit that kill switch. Sound about right?

What altitude? Where are you?

I like to run them WO (less than red line) in 5th gear +- 70-85mph for at least a 1/3 mile to make sure you get a good solid burn color on the clean plugs. In my opinion, you want to start out, just above a "lug" and pull hard for as far as you can, without returning with a ticket.

[stubbs]

I'm in Provo, UT, so around 4500ft altitude.

[stubbs]

Does anybody know what jet part # the vm26ss uses? I'm looking on sudco, but I don't know which ones to get. Small round? Large round? Here's the list http://www.sudco.com/mikunijets.html

Thanks! It's feeling worse than my initial impressions, so I think I'm going to try a set of #120s.

[tkent02]

Get the jets from Z1.
You did vacuum synch the carbs, right?
Fuel levels correct, valve clearances OK? Ignition working right?

Stock main jets are 95 on mine, I ran 115 mains at 6,000ft. with VM 26 carbs, K&N pods and a V&H 4-1 pipe. So it is 20 higher on the main than stock. Still this was slightly lean, as it ran better at 7,000 to 8,000 feet than it did in town. Mostly I ride this one up in the hills so this setup worked well for me, I just try to avoid full throttle until I get up higher in the mountains.

Your 750 came stock with 102.5 or possibly 105 mains, so probably 122.5 or 125 would be about right. Your elevation is lower, so go up another size, 125 or 127.5 maybe? Always it's only a starting point, your results will vary.

You really want to get the low power jetting right before messing with main jets on the VM carbs. Probably the original 15 pilot will work, once you get the screws dialed in. The screws should end up around 5/8 or 3/4 turn out on the fuel screws, 1 1/4 to 1 1/2 out or so out on the air screws. Tweak them for the best throttle response at very tiny throttle openings, put them as lean as it can be and still run correctly, with nice response. The jet needle will likely need to be a notch higher or so than it was stock. Plug chops will tell the story here.

[Spyderman]

Does anybody know what jet part # the vm26ss uses? I'm looking on sudco, but I don't know which ones to get. Small round? Large round? Here's the list http://www.sudco.com/mikunijets.html

Thanks! It's feeling worse than my initial impressions, so I think I'm going to try a set of #120s.

VM28/486 pilot jets and mikuni large round main jets for the VM26 carbs

[stubbs]

Get the jets from Z1.
You did vacuum synch the carbs, right?
Fuel levels correct, valve clearances OK? Ignition working right?

Stock main jets are 95 on mine, I ran 115 mains at 6,000ft. with VM 26 carbs, K&N pods and a V&H 4-1 pipe. So it is 20 higher on the main than stock. Still this was slightly lean, as it ran better at 7,000 to 8,000 feet than it did in town. Mostly I ride this one up in the hills so this setup worked well for me, I just try to avoid full throttle until I get up higher in the mountains.

Your 750 came stock with 102.5 or possibly 105 mains, so probably 122.5 or 125 would be about right. Your elevation is lower, so go up another size, 125 or 127.5 maybe? Always it's only a starting point, your results will vary.

You really want to get the low power jetting right before messing with main jets on the VM carbs. Probably the original 15 pilot will work, once you get the screws dialed in. The screws should end up around 5/8 or 3/4 turn out on the fuel screws, 1 1/4 to 1 1/2 out or so out on the air screws. Tweak them for the best throttle response at very tiny throttle openings, put them as lean as it can be and still run correctly, with nice response. The jet needle will likely need to be a notch higher or so than it was stock. Plug chops will tell the story here.

Thanks for the help, guys. Carbs are only bench synced, don't have a tool to vacuum sync them. Float levels are correct, but I haven't done the valves yet. Need to do that sometime soon.

TKent, are you sure about having the fuel screws 5/8 to 3/4 turns out? Mine ran well with them around 3 turns out! You are talking about the screw in the bottom of the bowl, right?
Mine's got the same setup as your bike right now with the 115s, and is definitely a bit lean around here. I was thinking of trying 120s, do you think I would need 122.5s or 125s?

Also, for you guys who have these 750s, what kind of mileage are you getting? I know it'll be the best when it's running right, but I'm trying to wrap my head around how it'll possibly get better gas mileage (which is already poor at high 20s to low 30s) with bigger jets.

[Steve]

My source does not include a '77 750, but it says the stock main jet for a '78 was 100. Following the suggestion of 20 up, you should be looking for 120s.

The part that has me scratching my head, though, is this:

It's got K&N pods and a Vance&Hines 4-1 exhaust on it. The main jets are 115s and the carbs are all nice and clean. Fuel screws are about 3 1/4 turns out, and air screws maybe 2.5-3 turns out (adjusted for highest idle). I'm at around 4500ft altitude. ...

Down here, closer to sea level, the fuel screws are usually set between 1 and 1 1/4 for your mods, and the air screws start out about double that. To me, it sounds like the pilot circuit is WAY too rich.

.

[stubbs]

Yeah, no kidding! I thought that's what I read in the VM carb rebuild how-to. I'll pull them off and readjust and see how it responds. Did you listen to it in the video link in my first post? Could that be a rich condition? I thought it was lean when it sounded like that.

Anyway, I'll let you know how the tweaks go before I order jets. A super rich idle circuit would certainly explain poor gas mileage.

[Steve]

Yeah, I saw the video. Can't tell too much about the jetting by whacking the throttle, but a quick twist like that will kill just about any engine that does not have accelerator pumps. That is one reason there was a move to the CV carbs in later years.

One way to judge the jetting from engine response is to set the idle up just a little to about 1200. Blip the throttle and watch the response. If the engine hangs a bit at the higher speed, then finally settles down, it's a bit lean. If it drops quickly, goes a bit below the set speed, then comes back up, it's a bit rich.

.

[tkent02]

TKent, are you sure about having the fuel screws 5/8 to 3/4 turns out? Mine ran well with them around 3 turns out!

Can't remember exactly, and I have since sold the carbs, but it was somewhere less than one turn out. If yours runs best at three, something is very wrong somewhere.

Pods don't add much airflow at low power, the airboxes work fine going slow, so the stock pilot jet will be close to correct. Half throttle, half RPM, needs somewhat more fuel so the jet needle will probably need to be moved up. Probably just a small change required. It's at wide open and high RPM where the pods and pipe flow a lot more, so the main jet needs the biggest change...

You need to vacuum synch the carbs if you want to get the pilot circuit working correctly.