'81 GS750E Lurches Going Down the Highway

[Guest]

First I want to say this is my first motorcycle, and while I've had experience working with older vehicles and snowmobiles in the past, the world of cv carbs, wet clutches, sequential transmissions, and chain-drives is very new to me, so don't be afraid to dumb it down.

Anyway, some quick background information. I bought the bike last fall, it's a 1981 Suzuki GS750E, it ran and drove fine but wouldn't idle off choke. I got it back and did a valve adjustment, using the info off of BikeCliff's website, and took the carbs apart to clean them. Unfortunately, some stuff came up and I was forced to stop working on it and put it away for the winter. This spring I took the carbs completely apart and scrubbed the holy hell out of them. The insides were fairly clean, but one of the pilot jets and one of the idle air jets was broken off. I didn't use chem dip, but I did use carb cleaner, compressed air, and little pipe-cleaner sized brushes to scrub out the passages. Before re-assembly, I replaced the broken jets and the rubber stoppers and checked for any pinhole leaks in the diaphragms. After assembling the carbs, I put them back on the bike with new rubber boots. I then, using a Clymer manual as a reference, replaced the plugs and air filter, and adjusted the chain and clutch tension. After everything was done, I fired up the engine and synced the carbs with a vacuum gauge kit. After the first test ride, I pulled the plugs and they all appeared to be the right color, not dark and wet, but also not white and burned.

It's fine around town at slower speeds, but after about 55 mph the bike will start lurching, almost like the throttle is being momentarily cut, but the rpm's stay the same. The problem persists in every gear, both under acceleration and cruising, but cruising in 5th seems to be the worst. It makes the back-end feel almost unsteady whenever it happens. It isn't very violent, and it doesn't feel like it's going to stall, but it definitely gets your attention. It's not rhythmic, but happens every 10 or 20 seconds.

I apologize if this is a fuel or carb issue, and therefore under the wrong category, but it feels like a drive train problem to me. Does this sound like a chain, clutch, or transmission issue? Any ideas are appreciated, thanks!

[posplayr]

I would check charging (quick test) and voltage to the coils. If you have not already done so you should clean all electrical contacts. I used to have the same bike and it was very reliable pretty close to bone stock. Only thing I could imagine is intermittent low voltage.

[Steve]

Welcome to the forum.

I must say, though, that your method of "carb cleaning" is scary, at best.

There is no way the passages can be cleaned with pipe cleaners. They are not even cleanable by bare wires. Carb spray is good for some things, but to CLEAN the carbs, they really need to soak in carb cleaner at least overnight, if not a full 24 hours.

Posplayr has the right idea about checking the voltage to the coils, but he needs to change some settings to allow his keyboard to work correctly.

One other thing I would suggest is to verify the routing of the carb vent hoses. They should go over the airbox and end either behind the plenum chamber or on top of the filter assembly. They don't get connected to anything, they just need to end in relatively calm air.

.

[AlexReitenND]

Appreciate the help...the pipe cleaner like things are skinny wire brushes from a carb cleaning tool kit that I bought off of amazon, they fit in all but a few of the tiniest passages, but I will look into some chem-dip. I'll also check the electrics and the vent hoses. I've heard some mention of cutting the tips of vent hoses a certain way to avoid problems with turbulent air; is that something to consider or more of a myth?

[LAB3]

If it seems to run OK with just an intermittent problem like you described I'd be inclined to think it's something going on with the electrics. Something that's easy and cheap to do is checking and cleaning your ground wires where they attach to the frame. Pull them off one by one and use a wire brush, some rough sandpaper or whatever else works to get those lugs, rings and spades nice and shiny making sure there's no paint getting in way of making a good connection. Next would be to look over connectors in the wiring harness. Same principal here, make sure everything is clean and shiny. Start the bike up in complete darkness and rev the engine, looking for any arcing from the spark plugs wires, if you can can rig up a temporary fuel supply you can look at the coils as well with the tank off. There's no guarantees with any of this but something that's good to do anyhow on an old bike, even if it doesn't fix the problem it may help another one from popping up in the future.

[posplayr]

If it seems to run OK with just an intermittent problem like you described I'd be inclined to think it's something going on with the electrics. Something that's easy and cheap to do is checking and cleaning your ground wires where they attach to the frame. Pull them off one by one and use a wire brush, some rough sandpaper or whatever else works to get those lugs, rings and spades nice and shiny making sure there's no paint getting in way of making a good connection. Next would be to look over connectors in the wiring harness. Same principal here, make sure everything is clean and shiny. Start the bike up in complete darkness and rev the engine, looking for any arcing from the spark plugs wires, if you can can rig up a temporary fuel supply you can look at the coils as well with the tank off. There's no guarantees with any of this but something that's good to do anyhow on an old bike, even if it doesn't fix the problem it may help another one from popping up in the future.

This is a list I prepared. Contacts especially related to charging need to be chemically cleaned because it is the crimps that get corroded and can't be cleaned with a brush.

https://www.thegsresources.com/_forum/showthread.php?237763-Recommended-Products-for-Preparing-Electrical-Connenctions&highlight=electrical+cleaner

[posplayr]

Appreciate the help...the pipe cleaner like things are skinny wire brushes from a carb cleaning tool kit that I bought off of amazon, they fit in all but a few of the tiniest passages, but I will look into some chem-dip. I'll also check the electrics and the vent hoses. I've heard some mention of cutting the tips of vent hoses a certain way to avoid problems with turbulent air; is that something to consider or more of a myth?

It sounds like your bike runs well enough till you put it under more load then it quits running. That is electrical and have seen it before. You need to perform a general (see the link I posted) cleaning and measure the voltages in the Quick Test and the voltage drops in the Stator Pages Phase A. In order to pass that your fuse box will probably need to be taken apart and have the crimps soldered. When I did this to mine I remideied teh chargings problems, resolred the charging voltages, and had a solid spark from stock coils and ignitor.

[posplayr]

Posplayr has the right idea about checking the voltage to the coils, but he needs to change some settings to allow his keyboard to work correctly.

.

That was so cryptic I coudl barely figure out what i typed. IPhone, I usually reread what I post and see what gets mangled. There were now 3 lbs words in that post so it is pretty amazing how they got swapped around.

[fixmybrokecomputer]

Your problem sounds like the problem I had on my GS1100GK. I was all set to replace the carb to intake boots and o rings, carb to airbox boots, clamps and pull the carbs to clean and replace o rings. I pulled the left side cover and disconnected and reconnected all the electrical connectors multiple times to scrub the connections a bit. My carb problem was electrical. It has performed well ever since. The previous post had it right. You would be doing yourself a big favor if you meticulously cleaned every electrical connector on the bike and gave each a dose of dielectric grease after cleaning.

[Gorminrider]

there's lots of complicated things but simply, you're losing spark or fuel. spark has been discussed.

Fuel: Bike needs more fuel running faster, and petcocks can be a problem on these bikes.
I've cured very similar symptoms on the highway by turning petcock to PRIME, which bypasses the vacuum petcock operation. so, I suggest that before you go next time, Test tap for full flow of fuel (take it off the tank) at prime setting and then,next time it happens, or before you speed up, turn tap to prime . It might just fix it.