My GS...not running well

[james750tz]

I have a 1982 GS750T that is been my baby for many years. I brought it back from the dead recently after a long park. I rebuilt the carbs, and it seemed to run okay for several months. I noticed it backfiring a bit (exhaust) a few weeks ago and then it began to run progressively worse as if the carbs were getting dirty again. I haven't used a filter on the bike because when I tried to run one the first time it ran poorly with the filter on it, so I figured the carbs were full of trash, so I decided to take them off and build them again. After getting them off and dumping the bowls into a tray I noticed a LOT of water in the carb bowls. I checked the fuel in the tank, and it was uncontaminated, so what gives? Anyway, I built them again, but they were basically clean. Another problem sprung up after getting back together is that it was running pretty poorly. I tried the filter thing again with no success again, so I ran it straight but it still ran crappy but a lot better. I noticed awhile back that the bike tends to run like crap unless the tank is basically full and it's on about a half tank right now, so I am going to fill it up and see if it straightens out.

Question: Would a bad fuel cock cause the full tank phenomenon? Are the rebuild kits worth doing or should I buy a new one?

Question 2: How the heck did the water get in the carbs but not the tank? The only thing I can think of is that one of the vent tubes from the carbs were missing.

Thanks in advance for your help!!!!

[almarconi]

If your petcock is original, its probably time for a new one. Water will condense in your tank and carbs from sitting, changes in temperature..etc. You could try adding some gas drier (methanol) to your gas or better yet dump it and add some fresh gas. There probably is water in your tank..you just can't see it. It's kind of hard to see into all the nooks and crannies of the tank. I never ran a filter on my bike either until I realized that fine sediment kept finding its way into my carbs. Get a filter designed for motorcycles or clean and seal your tank. The filter on the petcock may also be shot. Check the vent on your fill cap to make sure its clean.

[Colin Green]

Have you drained the carb bowls and checked for more water?

Does the engine run just as badly across the whole rev range?

Have you checked the plugs for sign of one cylinder missing? or a temperature difference between the headers.

Have you tried running it with the petcock set at reserve?

If it runs better when set on reserve, I'd recommend replacing the vacuum hose from the carbs to the petcock along with the petcock.

If the bike runs better without a filter it is suggestive that all is not well with the carbs. When you had the carbs apart did you check the jet sizes against spec? How many turns out are the air pilot jets? Has the carb been tuned and/or balanced?

[mcloud]

+1 on above
Could be vaccum related as the problem seems progressive.
The water could be sucked in from a cracked line and slowly accumulate in the float bowls.
Also check the air box. Water could get in from there also.
Check the air filter. Check your boots between air box and carbs.

Two Aspirins every 4 hours. Call us tomorrow

[Nessism]

Water is heavier than gas so if there is water in the tank it will settle to the bottom and you might not know it unless you fully drained the tank. Water could also enter the carbs if the bike sat out in the rain with the vent tubes off the nipples.

I suggest installing the vent hoses and draining the tank into a gas can to make sure everything is clean. While you are doing this, check the petcock function.

Regarding the carbs, what did you do to clean them? Did you fully dissemble them and dip in carb cleaner followed by replacing all the O-rings? There is no need for "carb kits" as many newbies feel is necessary; just get a $15 O-ring kit from cycleorings.com and maybe some new bowl gaskets if the old ones are torn.

Good luck,

[james750tz]

If your petcock is original, its probably time for a new one. Water will condense in your tank and carbs from sitting, changes in temperature..etc. You could try adding some gas drier (methanol) to your gas or better yet dump it and add some fresh gas. There probably is water in your tank..you just can't see it. It's kind of hard to see into all the nooks and crannies of the tank. I never ran a filter on my bike either until I realized that fine sediment kept finding its way into my carbs. Get a filter designed for motorcycles or clean and seal your tank. The filter on the petcock may also be shot. Check the vent on your fill cap to make sure its clean.

What type of filter do you recommend? The bike doesn't seem to like them, and it is very difficult to find a way to make it fit under the tank.

Also to answer a few other questions:

The bike runs like it is starving for fuel. You can crank it up with the choke on wide open and it will run at around 3500 rpm. Pushing the choke off will kill it immediately. It acts as if it is running lean because if I hold a shop rag in front of the airbox, it runs significantly better and I can push the choke off. I have the air screws turned all the way flat (bottomed out) I'm going to go get a couple of gallons of fuel and see where that takes me and I'll check back in. I still think it's that b%$tard fuel cock that's most of my problem...I had a very similar situation when I built the carbs the first time and the bike only had 1/4 tank of fuel in it. I took the bike for a ride one day and it had about 1/2 tank in it and it performed poorly. Filling it up made it run fine. HMMMM.

Thanks!
-james

[almarconi]

What type of filter do you recommend? The bike doesn't seem to like them, and it is very difficult to find a way to make it fit under the tank.

I'm using a 5/16" inlet/outlet emgo motorcycle filter. It is cramped under the tank. I used gasoline compatible tygon tubing which is flexible and thin walled. It is translucent blue, I think I got it at Cycle Recycle II. Since the bends are tight I ended up bending a piece of 5/16 id thin wall tubing in the shape of an L so that the fuel tubing will not kink. A 90 degree barbed fitting would have been preferred but I couln't find one. Don't think it would work with the thick wall automotive type fuel line. The alternative would be a small filter. Alot of people don't use a filter. I don't have one on my KZ and have not had any problems. If your tank is clean, your petcock screen works..you may not need one. I usually buy gas at the BP station near my house and noticed that their gas is not always clean. I'm careful to clean my gas can out before filling it with gas and somehow always seem to get crud in the gas.

Pushing the choke off will kill it immediately

More than likely your carbs are still dirty. I would get the petcock replaced before messing with the carbs. Clean your tank and dry it out with some acetone to get rid of any water.

[james750tz]

I'm using a 5/16" inlet/outlet emgo motorcycle filter. It is cramped under the tank. I used gasoline compatible tygon tubing which is flexible and thin walled. It is translucent blue, I think I got it at Cycle Recycle II. Since the bends are tight I ended up bending a piece of 5/16 id thin wall tubing in the shape of an L so that the fuel tubing will not kink. A 90 degree barbed fitting would have been preferred but I couln't find one. Don't think it would work with the thick wall automotive type fuel line. The alternative would be a small filter. Alot of people don't use a filter. I don't have one on my KZ and have not had any problems. If your tank is clean, your petcock screen works..you may not need one. I usually buy gas at the BP station near my house and noticed that their gas is not always clean. I'm careful to clean my gas can out before filling it with gas and somehow always seem to get crud in the gas.



More than likely your carbs are still dirty. I would get the petcock replaced before messing with the carbs. Clean your tank and dry it out with some acetone to get rid of any water.

Okay, here's what's going on now. I just filled it up with gasoline, and what do you know, it ran better immediately. Smooth idle with no choke. Went for a test ride, and it ran real crummy over about 2500 rpm. Realized I hadn't adjusted the air screws. Went back to the house and adjusted them. It ran better, but still cuts up at around 4500 rpm and won't go over that range...it just blahs and refuses to accelerate. What are your suggestions for setting the air screws? I guessed at it last time and did okay, but apparently not this time.

After this fiasco, I am definitely going to put a new fuel cock on it since I am almost 100% sure that it is most of the problem.

-james

[almarconi]

I guessed at it last time and did okay, but apparently not this time.

Adjust each carb for the highest idle. With stock pilots you should be between 2-3 turns out. You also need to vacuum sync the carbs. If this has not been done, the bike will run like crap.

[james750tz]

Okay...more stuff.

I attempted to set the idle screws. I propped the tank up and then noticed gasoline pouring out of the exhaust on #3 as if the needle valve was stuck. This only occurred on #3. I put the tank back down, and it stopped and didn't give me any more hassle. This an indication that something is going on with #3? Does it prove that the fuel cock has issues? I can put a rag over the air filter and it runs quite a bit better which suggests to me that it is either starving for fuel or getting too much air. The bike idles pretty good so I'm confounded.

On the subject of #3 when I initially built the carbs the first time, I had a very hard time getting the pilot jet out and the threads were a little messed up. I managed to get it back in there and it seemed everything was cool and the bike ran great. The second time I built them I had problems getting it to seat again, but finally managed to get it back in there. It seemed the correct number of turns so I don't think it was cross threaded. However, that shouldn't have anything to do with gas pouring in there like that right? That is controlled by the needle valve am I not correct? After setting the tank back, no more issues. WTF.....!??!?

[almarconi]

I attempted to set the idle screws. I propped the tank up and then noticed gasoline pouring out of the exhaust on #3 as if the needle valve was stuck. This only occurred on #3. I put the tank back down, and it stopped and didn't give me any more hassle. This an indication that something is going on with #3? Does it prove that the fuel cock has issues? I can put a rag over the air filter and it runs quite a bit better which suggests to me that it is either starving for fuel or getting too much air. The bike idles pretty good so I'm confounded.

I would suggest you open up the carbs and check the float needle valves and the float height on all the carbs. Float height is critical so make sure you take your time and get it right. Are you running the airbox without the lid? Why do you need to put a rag over the airfilter?
It's my guess your problems lie within your carbs and your petcock needs to be replaced. Simply replace it and finally rule out this possibility.

[james750tz]

I just ordered a rebuild kit for the fuel cock. I will say this: I rode the bike later this evening and it only runs crappy now when you accelerate more than about 50%. and it is regardless of rpm..only affected by throttle position. It idles like a champ and starts on the first push of the button without choking it most of the time. I don't think I have a problem with the needle valve because I checked them thoroughly before I reassembled the carbs because that was what was going on when I had to build them the first time. Needle valve issues usually come with crappy idles, smoke, seizing, etc, which of I have none. I think the fuel was running too fast out of the petcock because it seems to be fine now. The only running well situation when full is telling me it has to have problems with the fuel cock.

How do you guys set the float height? I did it the first time with a mm ruler against the carb body and it seemed to work okay and that's how I did it this time.

Anyway, thanks for the help, and I'm going to rebuild that damn fuel cock and see where it goes from there. I think it is at least half of the problem. It's never been rebuilt as far as I know and it is a 27 year old bike.

[almarconi]

I just ordered a rebuild kit for the fuel cock.

Good luck with the rebuild, I've never had much success with the kits. I just usually buy a new one.

[james750tz]

One more question. Are the carbs from a GS850 or a 650GL the same except for the jets? I'm looking at some parts on ebay. They look identical to me but I want to make sure.



Update...I just rode it and got several good pulls out of it along with some stumbling afterwards. I am even more sure than ever it is the fuel cock now and not the carbs. When I started it today cold, no choke and idled immediately. Still curious about the carbs though because I sure could use some spare parts.

[james750tz]

Does anybody have the answer for me on the carb crossover? From what I have read the 650-850s are all 32mm. Can anybody confirm this for me??

Thanks.