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My carbs are driving me nuts - GS850

  • Thread starter Thread starter 86turbodsl
  • Start date Start date
8

86turbodsl

Guest
Just pulled the bike out of the barn first time this year.

I've been starting it up and running it every couple of weeks this winter to keep the bike fresh.
It has been running fine at idle and warming up enough to rev nice and high in the barn.

The last ride of the season last year, I was almost home and it started to feel like it was starving for fuel. Wouldn't rev, kept slowing down etc. I parked it when I got home and didn't worry about it. Since it was running ok in the barn, I didn't know if there was still a problem. Well there is.

I started it up tonight, got it out on the road and hit it. It ran fine for about 20-30 seconds. Then started bogging again. Stopped on the road, wouldn't start up for a while. Then I got it to start and it didn't want to run well, only runs ok on choke, not at all without. If I can get the rpms up to about 5000, it runs like normal if I am not moving. It revs right up. Moving down the road, it just bogs. Once in a while, it will act like it just got a shot of nitrous and surge forward, but it's always short lived. I limped it home and parked it. It starts better if I wait a while, but it's still not very responsive.

Last year, I did an EXHAUSTIVE rebuild on the motor. New rings, set timing+valve clearances, went through the carbs TWICE, new dyna ignition, etc. Sync'd the carbs, new K+N filter, rebuilt the stock airbox, etc. It was running really sweet until that last ride last year.

I *think* there might be some more crap in the pilot circuit, but how do I explain the surging ?

Anyone got any ideas before I tear this stupid thing apart AGAIN?

Thanks!
 
Not revving and not making much power is not the pilot jets, they are all about idling and very low throttle openings... How about the petcock, how much fuel will it flow if you just put a hose into a bucket and suck on the vacuum line? inline fuel filter, gas cap vent, needle valve screen, one of these maybe clogged, maybe something else in the fuel supply... How clean is the gas tank inside?

Once I found a small piece of wood blocking the main fuel line... WTF?
 
No inline filters, clean tank, gas tank vent checked, didn't do a fuel flow check though.

What explains being able to rev to the moon if I can get it over 5000?
 
86turbodsl said:
No inline filters, clean tank, gas tank vent checked, didn't do a fuel flow check though.

What explains being able to rev to the moon if I can get it over 5000?
Click to expand...
Dude having identical problem with my 81, except I can't get it to 5000 if it is in gear. Its driving me nuts as well. I think I am just gonna take it to the shop and tell em to fix it.
 
How much fuel do ya have in the tank?
Run it on prime see if it changes.
New plugs might help.
 
It has happened to me. I used automotive type fuel line and cut it too long. It was kinking and collapsing the line. It was really hard to see what was happening. It would rev all day long, but under load the line would collapse at the kink and stall from starvation. I went to the next smaller fuel line, cut it shorter and have never had it happen again. It could still be a petcock, but the symptoms sure sound like what I caused by trying to get enough line on it to make it easier to get the clamps on the fuel line. One of these days I will get the proper line that doesn't need a clamp.
 
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I used tygon line from mcmaster. It's very flexible and it's a tiny bit long. Could there be an airlock forming if the line has a high spot in it? The line curves up over the carbs then down. Seems like the fuel would keep flowing, but what do I know? I deal with EFI all the time. Carbs baffle me.
 
86turbodsl said:
I used tygon line from mcmaster. It's very flexible and it's a tiny bit long. Could there be an airlock forming if the line has a high spot in it? The line curves up over the carbs then down. Seems like the fuel would keep flowing, but what do I know? I deal with EFI all the time. Carbs baffle me.
Click to expand...

Gravity, it's all gravity

And Boyle's Law

and the Venturi prinicple
 
Do all the suggestions, especially check for good petcock flow to carbs. Since this problem surfaced on last fall's ride, maybe you picked up some bad gas, Try Seafoam dose, it sometimes works and you might get lucky. Also make sure no critters have taken up housekeeping in your air filter box.
 
In contrast to the other opinions, I will bet one or more of your pilot jets are clogged or partially clogged. That part of the system is active up through about 7000 RPM. Our old carbs don't care much for modern alcohol in the gasoline and are quite prone to gumming up after sitting.

Although I don't think it's the problem, what was the condition of the slide diaphragms? Were any of them marginal when you rebuilt the carbs?
 
cross fuel delivery off the list first

cross fuel delivery off the list first

86turbodsl said:
No inline filters, clean tank, gas tank vent checked, didn't do a fuel flow check though.

What explains being able to rev to the moon if I can get it over 5000?
Click to expand...

The petcock is supposed to replenish the bowls at a certain rate, if there's a fuel delivery problem, the bowls could run dry before they get more fuel. If it runs for a bit (even poorly) and then very different after 10 minutes or so, that sounds like starvation.

I would do what the others are saying.

Test1 - I would take the vacuum variable off the petcock troubleshooting list and do a test ride with the petcock on PRI.

Test2 - Check your fuel flow. Disconnect the fuel line from the petcock, stick one hose end onto the petcock and the other end into a gas can, put the petcock on PRI, gas should flow freely into the can once the gravity feed takes over - let it drain for a while and see if it continually flows.

I would think the steps after this are all carb blockage related (assuming all your spark and ignition are good) so, crossing fuel delivery off your trouble list is easier and faster to do first.
 
I am down to a gallon or so in the tank. I don't know how well the petcock is flowing, so first thing I will do is pull the tank and verify good flow on all settings. It didn't get any better on prime either. I did notice that I had air in my fuel line at the top of the hump. I put a block of wood under the back of the tank and tried again and it didn't get any better. I'll start with the tank.
 
Ajay said:
In contrast to the other opinions, I will bet one or more of your pilot jets are clogged or partially clogged. That part of the system is active up through about 7000 RPM. Our old carbs don't care much for modern alcohol in the gasoline and are quite prone to gumming up after sitting.

Although I don't think it's the problem, what was the condition of the slide diaphragms? Were any of them marginal when you rebuilt the carbs?
Click to expand...

One of my diaphragms was a little marginal. It was very wrinkly and I had to fuss with it to get it seated. I did pull the cover later and found it still seated. I don't think the slides are an issue at this time. I'm betting on fuel.
 
Pull your gas cap apart again, if it is not venting your bike will starve, but my money is on water in the gas.
fwiw
 
gustovh said:
Pull your gas cap apart again, if it is not venting your bike will starve, but my money is on water in the gas.
fwiw
Click to expand...
Y'know, that's not a bad thought. As you pulled more gas, you'd start to unport the water sitting on the bottom. At a minimum, open your drain screws and drain the bowls. This will also let you see if your petcock flows on the prime setting.
 
86turbodsl said:
One of my diaphragms was a little marginal. It was very wrinkly and I had to fuss with it to get it seated. I did pull the cover later and found it still seated. I don't think the slides are an issue at this time. I'm betting on fuel.
Click to expand...

I guess you've got a later model engine or head on your '79 850. The original carbs were manual slides (without diaphragms).

Give your tank a through checking over for a clogged petcock filter. If you have an inline filter, ditch it and try again.

Failing that, my money is on clogged pilot jet circuits from leaving the bike sitting too long between runs. You can isolate this by checking if all the headers are hot. If one is cold at idle, that is the carb that needs additional cleaning of the pilot circuit .
 
Update: I fiddled with the fuel line and got it shorter and so there wasn't an air bubble in it. I verified that the petcock has full flow on all settings. Put it all back together and took for a spin. It was better, but still had the same issue. Acted like it was running out of fuel at low rpms. If I could get it up in the rpms, it ran more normal. My last act for the evening was to run and get a few gallons of gas. I dumped the gas into the bike and took it for a spin. To my surprise, it acted almost normal. Just a little bit of surging, but nothing major. So apparently the problem is sensitive to fuel pressure. My reasoning is that more gas means more fuel pressure due to increased fuel level / delta Z. I took it out for a couple of miles, far longer than I was able to before the new gas. It ran the same when I got back as when I started. So what makes the pilot circuit so sensitive to fuel level in the bowls? And why would more fuel pressure fill the bowls better? Restrictive fuel inlet valves? Crud build up or something on the screens? Wouldn't that prevent ALL circuits from working correctly? I'm stumped. Anyone have insight?
 
Enough thinking- just ride it some more to refresh it's memory! When I bought my bike, it had sat out two riding seasons- it ran OK, but improved noticeably after a couple of tankfuls.
 
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