WOT stumble

[drhach]

Hello gentleman (and ladies). I worked a fair amount on my bike last Summer and I got the jetting almost perfect. I started with a 1978 GS1000. It had a no name 4-1 and pods. I got it pretty well spot-on, but I hated the pods. so, I took them off and installed the stock airbox. After a lot of trials, fiddling and note taking I got it all pretty good. In fact it's great expcept for one spot that I just can't get right. Around 5-6k at WOT throttle, it really stumbles, after that it takes off and goes great.

I know, someone will chime in and say that it is throttle position not RPM range that matters. I assure you that it is at WOT and only in this range. I chopped the plugs and they are lean. I have what I believe are the stock needles. I don't have my notes in front of me but if I recall correctly, mains are one over stock, pilot is stock and needles are raised to the fourth position (richest). Any thoughts? I'm kind of leaning towards going back and checking the floats. The bike is very tractable in most normal operating conditions and I have taken a few trips and not even noticed it. Still, it bothers me that it is not right. I wish I had stock exhaust to put back on but I don't and I probably never will.

Usually when I conduct my test, it is in third gear, at about 2,500-3,000 RPM. I pin the throttle. It pulls hard, hits that spot, stumbles but still pulls, passes that spot and BLAMMO, it's hauling arse again.

I find it wierd that it only does it under this condition. It's contrary to what I understand to be the case for jetting. Anyone?

[tkent02]

With wide open throttle it is the main jet doing the metering. The pilot circuit has almost no air or fuel going through it, the jet needle is all the way up out of the needle jet. The main jet is the most restrictive and is completely in control of fuel flow. However the amount of airflow will increase with RPM, and the bike can go lean at certain RPM bands... I'd go one size bigger on the main jet, maybe two, see which works better, do more wide open plug chops to make sure it isn't too lean at any RPM.

[drhach]

Thanks. Like I said, I don't have my notes in front of me. But I am recalling that I actually came down a size on the mains (to where it is now) because the plugs were too rich, but stock made it worse again (leaner); hence one size over stock. I also recall that I did a little experimenting with removing the airbox lid to see if I could get the spot to move or change (it didn't).

Honestly, with intake and exhaust setup that I have, I didn't expect to run in to this issue at all. I have gone through the process of changing only one thing at a time and taking good notes. I understand that the mains are running the show at WOT and that fuel requirements can change, thus causing a dead spot. The thing that gets me is that the spot is effectiely in the middle of the RPM band. So, up until that point, on the mains, there is plenty of fuel. After that point, with higher RPM's and seemingly a higher fuel requirement, it is fine too. So, why does it starve right in the middle? Obvioulsy there is a need for fuel that the mains can't provide, I just don't get why not when they can provide it after that point. How is the load requirement being placed on the engine changing such that it can't provide enough fuel?

[tkent02]

So maybe the mains were right before, and the needle was up too high, causing rich plugs?
The main jet in a VM has zero effect on low power running, you can actually leave the main jets out and it will run fine up to about 3/4 throttle.

Did you get rich plugs from doing plug chops or just after riding around for a while?
If it was just from riding around it is likely the lower power circuits that were too rich, then when you went with smaller mains you caused the full throttle lean problem without even effecting the low power richness at all.

[drhach]

I recall that with the first jets, I moved the needles around to no avail. I went stock mains, experimented with the nedles, still wrong. Went to the next jet up from stock, moved the needles to where it is now. In each case, I installed the jets only after going through needle positions. The reason that I went up two sizes from stock was because it was so lean, even with the needles all the way up on a stock main. At that point I way over-shot it and with the needles all the way up and two sizes over on the main, I worked the needles back down. Eventually I concluded that it was still too rich. I went one more size down on the main and worked the needles back to where they are now. All the while making no adjsutments to the stock pilot jet or any other settings. Basically, I have been on bothe sides of it (too rich, too lean). It's been a long winter. I think I need to get out my noted and see if I missed something. I'm trying to aviud what i think I may actually need to do, which is get a $100+ dynojet kit with a better needle taper.

[tkent02]

I'm trying to aviud what i think I may actually need to do, which is get a $100+ dynojet kit with a better needle taper.

Talk to Ray, he had sets of kickass needles for sale for the 1100E a while ago, without paying for all the misnumbered DJ jets you don't need.

[drejal]

drhach-

I'm having a somewhat similar problem on my ride. (79 550l) I have the VM carbs like yours. I was having a problem where it would hit that stumble around 4000 - 5000 rpm, and once past it ran up to about 7500. I found I was getting poor voltage to the coils, and did the coil mod. It cleared up the problem I was having in the midrange, now I just need to tackle the upper range.

Odd thing is, the night I did the coil mod, it was nice and cool out (well, cool for Florida!) around the mid 60's. The bike ran like hell that night, all the way to the redline. The next day, temps in the 80's, it won't go past that 7500-8000 stumble, and even pops loudly if you hold wot. (which I believe is an indication of lean condition?)

fwiw....the setup i'm running (thanks partially to my p.o.) The awful emgo pods, new intake o-rings, clean and new oring carbs, with 102.5 mains and larger pilot jet (not sure what size, but pilot is fine according to plug chops) into an ugly gutted out (stock) exhaust. I know it's a poor setup, BUT, I also know it will run up to the redline, because it ran like hell in the cool weather!

My questions (to anyone, not just drhach ) ....I know that its near the range where it's all main jet, BUT, will raising the needle have an effect in the 7k+ range?....ALSO....I saw you have been adjusting your jet needles....is there an easy way on the VM's to adjust the needle, or do you need to fully disassemble to get to it?

[drhach]

Thanks for that. I will get in touch with him. As far as the coil mod goes. I was thinking of doing that anyway. You know, it's an interesting thing to consider. Maybe there's some natural frequency where the coils break up a little. i don't know. I'm probably grasping. Either way, it can't hurt to give it a try.

[Big T]

drhach-
My questions (to anyone, not just drhach ) ....I know that its near the range where it's all main jet, BUT, will raising the needle have an effect in the 7k+ range?....ALSO....I saw you have been adjusting your jet needles....is there an easy way on the VM's to adjust the needle, or do you need to fully disassemble to get to it?

1. Needle - yes
2. Disassemble - partially, you have to take the tops off to get at the needle

And, to both of you - you can always order needles with a different taper from SUDCO. That's the blessing and the curse of VMs - lots of parts available - different needles, slides with different cutaways, jets, pilot jets

[drhach]

I understood the needles were NLA. Maybe thats just OEM.

[chef1366]

A bog and then the motor "catching up with the fuel" says rich to me. A lean condition won't let the motor accel or burn up.
Either the needle is rich or the main. Get the bike to go past red line with the main and then do the needle.
Is your intake boots sealed? O-rings good?

[drejal]

1. Needle - yes
2. Disassemble - partially, you have to take the tops off to get at the needle

And, to both of you - you can always order needles with a different taper from SUDCO. That's the blessing and the curse of VMs - lots of parts available - different needles, slides with different cutaways, jets, pilot jets

thanks!

(my apologies for kinda hijacking the thread!)

[drhach]

A bog and then the motor "catching up with the fuel" says rich to me. A lean condition won't let the motor accel or burn up.
Either the needle is rich or the main. Get the bike to go past red line with the main and then do the needle.
Is your intake boots sealed? O-rings good?

Boots checked and double checked, o-rings brand new. I'm headed out for a ride right now. I'm bringing the plug wrench and I'll do some thorough checking. "Film at eleven"

[DanTheMan]

Stock airbox, it sounds to me like it's doing what mine was doing WOT, same symptoms. I've got a K & N filter in mine, it was clogged with oil from blowby (I haven't cleaned the filter in two years, it looked clean). The excess oil in the filter was choking the mix and it was too rich. Cleaned the filter today and she runs like a scalded dog.

[drhach]

Ok, the verdict is that it is too lean. My notes are on an old computer, so it will take a while to dig them out and see where i went wrong. But clearly, I missed something. It is quite obviously lean at WOT. I made sure to put a good load on her and to go past the stumble. It is lean in all spots. So, up go the jets. It just goes to show, you can never blame the bike. "Operator head space and timing" as they used to say in the Army.