1982 GS750E Manic Issues (please help!)

[luke]

Good day everyone!

Just want to start off by saying this is my first post here and I'm looking forward to becoming a part of this community! If I've posted this in the wrong forum please let me know, and I swear I've tried searching for this information throughout the forum first.

I bought a 1982 GS750E about a year ago, and the previous owner put 4-into-1 pipes on it and lost the original airbox, so I had to get pods. I've been tinkering with it a bit (my knowledge is mostly with turbos and fuel injection) and this is my first time trying to get a 4 carb setup running properly, especially with modifications.

So, I had it idling nicely with 112.5 main jets (that's what they had in there, not sure if it was stock) stock pilot jets and stock needles. I took it for a quick 5 minute run and noticed bogging/lagging throughout 3000-6000 RPM range and couldn't get it any higher. I pulled the plugs and they were white, which made sense due to the amount of flow going through the pods and the exhaust.

To enrich the mixture I installed 120 mains. Brought the air screws out to 2 turns each, and it ran a lot better. After another 5 minute burn (after letting it reach operating temp of course) there was still a dead spot between 3000-6000 RPM, but there seemed to be good power over the rest of the RPM range. I assumed a bit more tuning was required, and I left the bike sit for the afternoon to run errands. Came back to it and the bike wouldn't even hold an idle, sounded like it was misfiring (It sounded completely different). It would start to idle around 1000 then quickly jump up to 3000-3300 and bounce.

I thought maybe I fouled one of the plugs from running lean, so I replaced them all since I had another set. I also guessed that I needed even larger jets since it was still running lean (plugs were still very white). The local shop only had 132.5 mains as the next size up so I decided to give those a shot. I was also going to adjust the needles one or two notches richer to try to get some mid-range power, but the clip tool I had didn't fit so I decided to just see if I could get the bike back to a clean idle.

I reinstalled the carbs and started the bike. I still had the jumping idle and now I have fuel coming out the front of the carbs above the bowl. I've never had float issues since I bought the bike.

My questions are:
1) Can larger main jets without changing the needles cause fuel to leak into the carb inlet? My guess is no, but again I'm completely inexperienced with carb modifications.
2) Does the new bouncing idle have anything to do with the carbs or could I have damaged something else in the engine by running so lean?

I'm very stubborn when it comes to getting things working so I refuse to bring it to a shop, I'd rather take the time to learn but I don't know anybody with enough experience to help. If anyone has any suggestions for me that would be greatly appreciated.

Thank you all in advance!

 

[tom203]

Welcome..we like stubborn folks! You've seemed to have immersed yourself in pod tuning fun. Other guys have found that opting for the dynojet carb kit makes pod living a lot easier,especially if you need a decent midrange. The dynojet needles have different shape than stock ones. Read this..

https://www.z1enterprises.com/dynojet-carb-kit-gs750-1980-83.html

 

[Nessism]

Float valve is sticking leading to overflow. And agree with tom on the importance of a Dynojet kit.

Also, you might want to read through the carb rebuild tutorial and Newbie Mistakes thread to get ahead of the curve.

 

[Rob S.]

Can I, a misinformed non-mechanic, make one observation?

You made two adjustments (mains & air screws), then reported the difference in performance.

Later, you changed the plugs and upped the mains, then noticed a difference.

If you adjust more than one thing at a time, isn't it difficult to know exactly which adjustment resulted in the performance difference?

Dynojet in my bike also, (it runs perfect).

 

[luke]

Thanks Tom, I?ll look into those.

Nessism, thanks for the input. I was just baffled as I?ve had the carbs off multiple times without float issues. I have to stop trying to rush things!

Rob, very true. I adjusted the air screws (maybe I?m calling them the wrong thing, the screws on the top of the carbs nearest the engine) as it was running with the new jets trying to get those right. Is that the wrong procedure when trying to get the air/fuel mixture right after installing different jets?

My plan now is to try the kit and investigate the sticky floats. Does that seem like the best way forward?

Thanks in advance!

 

[Nessism]

My plan now is to try the kit and investigate the sticky floats. Does that seem like the best way forward?

Thanks in advance!

You would be wise to go through the carb tutorial and do a full rebuild. Replace all the O-rings with a kit from cycleorings.com. And be sure to ungang the carbs to replace the interconnect tube O-rings since they are the ones most likely to leak.

 

[Steve]

I adjusted the air screws (maybe I’m calling them the wrong thing, the screws on the top of the carbs nearest the engine) as it was running with the new jets trying to get those right. Is that the wrong procedure when trying to get the air/fuel mixture right after installing different jets?

Well, yes, no, sorta, not really.

The screws that you are referring to adjust a MIXTURE, not just air. Some call them "mixture screws", but Suzuki calls them "pilot screws".

You say your experience is with turbos and fuel injection, so at least you have some idea of what works. The Mikuni BS-series carbs that you have on your bike have four independant fuel circuits. One is completely separate, the other three overlap a bit.

First, is the "enrichment circuit". It performs the duties of a choke by richening the mixture for cold starting. It is imperative that you know how this works to make life easier. Set the amount of "choke" desired with your thumb lever. (About 1/2 usually works well.) Turn the key ON, pull the clutch lever (unless that 'safety' switch has been disabled) push the starter button. DO NOT touch the throttle. The enrichment circuit works by opening a plunger (right behind the pilot screws) that allows a rich mixture of fuel and air to be sucked up by the high vacuum of a CLOSED throttle. If you open the throttle, you defeat that high vacuum and the rich mixture. When the engine is running, adjust your cold idle speed by moving the thumb lever. Under 2000 RPM works well. When the engine is warm enough to respond to throttle blips, start riding. By the time you are a couple hundred yards down the road and into third gear, you can turn the "choke" off.

Second, the pilot circuit provides fuel for idle and low-throttle openings. Fuel is sucked through the main jet, then goes through a passage to the pilot fuel jet, where it is mixed with air that has come through the pilot air jet to provide a pilot mixture. That mixture is then sent through a passage along the top of the throat to some small holes at the butterfly. One hole is downstream of the butterfly and open all the time, but it can't provide enough mixture, so there is another hole with an adjustment screw that allows you to add a bit more. Back at the butterfly, there are two more holes that are progressively uncovered as you open the throttle. These are transition ports, and allow a bit more fuel until the throttle is open far enough to get into the ...

Needle circuit. The jet needle is housed in the slide, which is the moving part of a variable venturi. The needle slides in and out of the needle jet, which is also called an emulsion tube. Fuel for this circuit also comes through the main jet, but the minimal clearance between the needle and the top of the jet is the limiting factor. The needle is tapered, so as it rises, the clearance gets larger and more fuel is allowed to flow with the increase in air flow. This circuit controls fuel flow from about 1/4 to 3/4 throttle opening. Evidently you are blessed by having a Canadian bike, where you have several notches in the needle for adjustability. We lost the extra notches in 1980.

The main circuit. When the throttle is fully open and there is enough air flow to lift the slides, the main fuel jet and its accompanying air jet provide the mixture for running. The air jet is pressed into the body in the intake throat and is basically not changeable, only the main jet is changed to richen the mixture.

Adjusting the pilot screws will only affect low-throttle operation. You do not use them to tune anything for different main jets.

By the way, the specs for a US version of a 750 have a main jet of 112.5, pilot fuel is 42.5 and pilot air is 170. With your pods and a pipe, I would expect probably 125-130 for the mains. You should not have to change any of the others, but it might be good to raise the needles a notch. Whatever you do, change only ONE thing at a time, record your change and its results. Systematic changes will keep some sort of sanity in the process, and springing for a Dynojet kit is definitely worth it.

.

 

[storm 64]

Excellent explanation Steve. This should be a sticky!

 

[Steve]

Just doin' what needs to be done sometimes.

If it helps somebody, it's worth it. :encouragement:

.

 

[luke]

WOW. I never expected to get so much help so fast. Steve, thanks so much for the summary. Understanding the functionality of the different carb circuits is definitely a huge help. I actually ended up springing for the dynojet kit and I will do a full carb rebuild so I can see how everything works and make sure I don't have any nasty bits in the wrong places. Now if there's any suggestions for a cheaper rebuild kit (what I've found is a K&L kit for ~$40 CAD per carb body) that would be appreciated, unless all I need is the o-ring kit from cycleorings.com (Nessism, can you confirm?). Any suggestions are totally appreciated.

 

[steve murdoch]

Welcome to the site, luke.
If you are determined to use pods i would have a look at the APE ones first.
They have a good reputation with this site.
http://gszone.biz/carbs.html

 

[roeme]

Addendum re. overflow; rubber bowl gaskets may shrink, rubbing the floats, ergo sticking valve.

 

[1948man]

WOW. I never expected to get so much help so fast. Steve, thanks so much for the summary. Understanding the functionality of the different carb circuits is definitely a huge help. I actually ended up springing for the dynojet kit and I will do a full carb rebuild so I can see how everything works and make sure I don't have any nasty bits in the wrong places. Now if there's any suggestions for a cheaper rebuild kit (what I've found is a K&L kit for ~$40 CAD per carb body) that would be appreciated, unless all I need is the o-ring kit from cycleorings.com (Nessism, can you confirm?). Any suggestions are totally appreciated.

Most of the rebuild kits are not so good. You should be able to reuse pretty much everything but the o-rings. The cycle-orings assortment will be what you need, If you need a few other bits, you can buy spare parts from members here or from Z-1 Enterprises ( I think they're still in business.)

 

[luke]

Quick update:

Thanks everyone, finally on my journey to forever learning. I've ordered the dynojet kit, o-rings and float bowl gaskets. I should have them all by next week so I can start reassembly by next weekend. I'm looking into the APE filters, can't wait to eat that cost. Is it customary on the forum to reply to the thread once I've completed the work? I mean, I'd like to thank everyone who chimed in once I have some progress!

Thanks

 

[Rich82GS750TZ]

Quick update:

... Is it customary on the forum to reply to the thread once I've completed the work? I mean, I'd like to thank everyone who chimed in once I have some progress!

Thanks

I believe more people should do this. Folks here are genuinely interested in progress, otherwise they wouldn't contribute to helping you get the answers. Some newcomers just come and take the free advice, never to be heard from again, leaving everyone wondering how it all worked out. Sounds like you got a lot going on at one time with this one. I image you'll be back with more questions. If not, please pop back in, post some pictures. We like pictures. Stick around, join the pic of the week game over in the campfire. Good luck.

 

[steve murdoch]

Progress or not, follow-up is good.
As Rich mentioned take some pics. We all like to see members bikes and they may help you when it is time for reassembly.

 

[luke]

Hello again!

I just completed the full carb rebuild as well as dynojet stage 3 kit install. Just fired the bike up and the intermittent issue is still there. It will idle and run beautifully cold and then all of a sudden the engine speed will spike and sounds horrible, like it?s misfiring. Seems to me that it could be an ignition issue. Any thoughts? When it did idle it sounds beautiful so I don?t suspect a lean condition.

Thanks again!

 

[luke]

My plan now is to clean up/test the ignition system, I feel as if I'm getting an inconsistent spark for some reason. Other than that, is there a common issue(s) to look for when doing so? Otherwise I'll just test everything. Here's hoping for one of those "simple" fixes.

 

[Carter Turk]

I've had a few cracked intake boots over the years, hard to see underneath 2-3 and sometimes they crack underneath the clamp on the intake manifold side.
Might be worth a check as the carbs have no support from the airbox, all the weight is supported by the boots and the rpm increase may indicate a vacuum leak.
A support can be fabricated to hold the carbs up on the inside of 2-3 and attached to 2 6mm threads on the frame where the airbox used to attach. Or just use a beefy ziptie.