Choke/petcock/what?

[mike1414]

So I had my wiring harness completely redone to rule out any electrical issues, the bike started well and I rode it for about 7 miles with only a slight lag in the throttle. I just moved from FL to TN, and figured that, although it still wasn't running in FL, maybe a change in jets would help. So I currently have 17.5 pilots in and 115 mains, which is a step up in both categories from what I had in FL. When I had in 20 pilots and 122.5 mains, it seemed to run way too rich. Now, with this set up and my pilot screw about 1.5-1.75 turns out, it runs, but it runs like this:

https://drive.google.com/open?id=0Byq0lwWAWrLccGZScjJZSHVyUXM --- Hope you guys can see this.

That is with it on prime/switch to reserve and full choke. It has great throttle with choke. But as soon as I turn the choke off, I lose all throttle response, and it eventually dies after 15-30 seconds. If I'm at 1/4 choke there is sucking from the carbs, but it still runs well at about half choke. I've changed the valves in it before, it's got new coils, new plugs, new plug wires, so much new new new. Is it just the jetting that I'm worried about still? Do I just have to get this part of it right for it to run decently? Sorry for the rambling, but any help is appreciated.

Mike

 

[Agemax]

have you connected up the vac pipe from the carbs to the petcock correctly?

 

[Steve]

Sounds like it's time for a carb clean/rebuild and putting the proper jets back in.

Which screw are you calling your "pilot screw"? You have a pilot FUEL screw on the bottom of the carb and a pilot AIR screw on the side.

How long have you had the bike before you put on your 7 miles? How much of its history do you know? Might be time to catch up on about 30 years worth of maintenance so you KNOW that things are set properly. Critical things to check include valve clearance, jetting in the carbs, brake condition, tire age/condition.

Until you have all of that checked off as "DONE", you won't really know what you have.

.

 

[BigD_83]

It sounds like the fuel line is collapsing as the revs come up...worth a quick check

 

[mike1414]

I have the vac pipe connected at the port between carbs 3/4. Steve I've only put about 50 miles on the bike since I did a valve job and all valves were in spec once I completed the job. Tires are brand new. When I say pilot jet, I mean when you pop off the bowl of the carbs, the cylindrical jet with a narrower cylindrical bottom with 8 holes. Those are 20; the bike has 4-1 Kerker pipes and K&N pods (how I bought it) so it's been a battle ever since to get it better correctly. And I've taken the carbs off and cleaned each time I've tinkered with the jets, but did a deep clean back (took them apart, submerged them in the carb goo) when I did my valve job.

 

[goflossurself]

I have the vac pipe connected at the port between carbs 3/4. Steve I've only put about 50 miles on the bike since I did a valve job and all valves were in spec once I completed the job. Tires are brand new. When I say pilot jet, I mean when you pop off the bowl of the carbs, the cylindrical jet with a narrower cylindrical bottom with 8 holes. Those are 20; the bike has 4-1 Kerker pipes and K&N pods (how I bought it) so it's been a battle ever since to get it better correctly. And I've taken the carbs off and cleaned each time I've tinkered with the jets, but did a deep clean back (took them apart, submerged them in the carb goo) when I did my valve job.

the port between carbs 3/4? I just had a similar issue and it was the petcock, try it on prime. My issues was the "vac" line to the petcock was hooked up to the nipple on carb #3. turns out that not a vac line lol....

 

[Big T]

the port between carbs 3/4? I just had a similar issue and it was the petcock, try it on prime. My issues was the "vac" line to the petcock was hooked up to the nipple on carb #3. turns out that not a vac line lol....

Sorry, but the port on the side of #3 IS the vacuum port. The line to the petcock attaches to that port. The ones between 1-2 and 3-4 are vents

If You don't have vacuum on #3, your carbs are assembled incorrectly on the rack

If your bike won't run with out choke, your carbs are clogged ad need a cleaning

 

[mike1414]

Gotcha, Big T. I'm sure another cleaning couldn't hurt them.

Another little bit of info with background: I posted long ago, over a year and a half now, about the carbs that came originally on the bike. They looked exactly like the standard VM26's that came on these old GS's, but they weren't the slide type. Also peculiar was that they did not have the pilot screw, what I believe to be the pilot screw, on the bottom where you can adjust with the carbs on the bike. All it has is the fuel/air screw on the side of each carb. They never worked great, and no matter how I adjusted the floats on carb #4, it wouldn't stop leaking.

Fast forward to now; I purchased KZ1000 Mikuni VM26 carbs and have had them on there for the past year. It has run well in spells, but for the most part has been non-running. Now, I just put these old GS Mikuni's on with the same jetting I had in the KZ Mikunis (17.5 pilots, 117.5 mains) and it started, and initially, with choke, had throttle. After about 10-15 seconds, I left it to run on choke as it was dying down a bit, turned my fuel/air screws out to 1.5 turns, and had throttle still, but the RPM's were hanging, so I turned them out to 2 turns and, with choke and throttle, the RPM's surged to around 8-9k with exhaust backfire, and cut off. Backed the screw back down to 1/2 turns, wouldn't start with prime/choke. At 7/8 turn, it started with prime and choke, and turned it to reserve/choke, no throttle. As I turned the screw out more, again with throttle it surged to 8-9k RPM's. There was a minute amount of smoke even at 1 turn out, but more as I turned it out more. Again, still would die without choke.

 

[Steve]

It sounds like you need to learn what the various adjustments do on the carbs. Well, on the GS carbs, anyway, I don't know if the KZ carbs are different or not.

The screw on the bottom is the pilot fuel screw. Stock setting is about 3/4 turn out from lightly seated, but with your pods and pipe, something between 7/8 and 1 full turn would be better. This is assuming that you have the stock size pilot JET in the bowl. I don't have my chart available at the moment to see what the stock size is.

The screw on the side is the pilot air screw. It contols the amount of air that will be mixed with the fuel from the pilot jet and fuel screw to create the idle mixture. The setting on that screw is generally a but less than double the amount the fuel screw is turned out, so 1 3/4 to 2 turns might be close to ideal.

However, since an engine will run better on a rich mixture than it will on a lean mixture, turn the air screws in so they are about 1 1/4 turns out from lightly seated as a starting point. That slightly-richer mixture will enhance the "choke" and the cold engine will warm up quicker and come off "choke" for proper tuning.

When the engine is warmed up and running well, slowly turn the air screw OUT, listening for changes in engine speed. As you cross the point where the mixture is happiest, the engine speed will increase. As you continue to add air to lean the mixture, the speed will drop a bit. Stop there, back up about 1/8 turn, move to the next carb.

All of your adjustments should be in small increments, giving time for the engine to respond, so this is not a quick process. Might take 5-10 minutes, so have a fan blowing on the engine as a precaution.

When you are done, you should have a very crisp response to the throttle, without any hanging when you back off.

Since I don't have my jetting charts with me, I don't know what your main jets should be, but there is no real need to have anything larger than stock pilot jets. Might need to raise the needle a notch or two to cover the midrange.

.

 

[mike1414]

Steve, thanks for the detailed input. I've always read that, when putting 4-1 exhaust on and pods, re-jetting is a necessity. When I got the bike with these additions, it seemed frightfully lean, and my pipes were quite blued at the block. Thus began my re-jetting journey. I have the chart that you are talking about, and the 77-79 750's stock jetting is 15 for Pilot, Main jet 100. I figured that, since I had minimal response in my first 1/4 throttle initially, then I needed to increase my pilot jet sizing (according to this http://www.iwt.com.au/mikunicarb.htm). I remember adjusting the height of the needle once before, but didn't keep a detailed record of what it did exactly. This link kind of details what I've gone through before (http://www.thegsresources.com/_forum/showthread.php?239625-VM26SS-Carbs). Anywho, before I do anything, I will take my main jet sizes down to the smallest I own close to the factory, and take the pilots back to 15, and then raise my needle, and start from there.

 

[mike1414]

Alright, Steve (and crew), I when to change out my carbs today and was reminded of a couple facts about them:

1) My GS Mikuni VM26 carbs do not have this pilot fuel SCREW which you can adjust from the outside of the carbs, only the fuel/air mixture screw on the side of the carbs (see the link to another thread started by myself). It was concluded that, both that is very weird, and that they may be from New Zealand. There are not even threads to which I can screw a jet in




2) My needle cannot be raised in the clip; it is a needle with a clip to hold it in place, but rather with a small and smaller orange piece about either side of the clip. There are no etchings on the needle to move the clip up or down, thus moving the needle up or down.





So that being said, I returned my pilot jets to 15, main jets are 117.5, fuel/air screw turned 3/4 turn out to start. The Kawasaki Mikuni VM's have an adjustable needle, but the throttle slide is shorter than the GS Mikuni's while the needle is longer than the GS Mikuni's; I don't know if these could be interchangeable...For these type GS Mikuni's, do you just buy longer or shorter needles to accommodate the richness/leanness?

Couldn't get it out of the shed to get it started because of a storm, but will pull it out tom to try it out. Any input anyone?

 

[allojohn]

I remember you Mike, sorry you haven't got this sorted yet. You're destined to go round and round with these carbs, a better option may be to find a regular set of VM26's made for your machine and be done with the problems....

 

[mike1414]

I'm surprised you remember, John! But yes that's been my thought the whole time. Honestly, from all my research, the KZ VM's seemed like a suitable replacement and they were very cheap, so I figured I may as well give them a try. They've worked periodically, but haven't been the solution that I was hoping for. I'll scour the internet/forum for someone willing to part with a set for a reasonable price