1980 GS850GL high speed power problem, take 2

[randyshipp]

Hi folks,

Well, today was a disappointing day. My petcock and brake pads came in from Z1 Enterprises (amazingly fast shipping, accurate...they’re my first option for such things), and I went over to work on the bike.

To recap:

I have a 1980 GS850GL with about 18,000 miles on it. The bike starts instantly upon hitting the starter button, idles pretty well, and rides smoothly on side streets. It’s not going to give me whiplash with its acceleration, but it’s fine for my riding style and experience. However, once at highway speeds, a problem appears. At speeds ranging from 60-70 mph, it feels like a cylinder drops out or something. The RPMs drop, power noticeably drops off, and speed will consequently fall off until 50-55 or so, when the power comes back a bit.

An earlier problem was that the bike was only running on three cylinders (#2 was cold, wet plug, etc.), and that went away with a new set of plugs. Earlier suggestions from fellow GSers here included rebuilding or replacing the petcock. The reasoning was that if there was a hole in the diaphragm, for instance, then fuel could leak down the vacuum line into the #2 (inboard left) carb at low speeds, leading to a super rich condition, fouling, and eventually failure to fire. Additionally, the argument went, a leaky diaphragm would keep the petcock from opening fully at highway speeds, leading to the kind of fuel starvation symptoms I’ve described. All of this made a lot of sense, so I ordered a new petcock.

Today we pulled the tank and did a little test with the old petcock. With a new piece of tubing installed on the vacuum port of the petcock, I sucked and got the fuel to flow, though it seemed to take a pretty mighty suction to get a decent flow. Furthermore, both my dad and I detected a taste of gasoline, though no liquid gas came into the clear line we were using. We decided to go ahead with the replacement. We put the new petcock on and reinstalled. After a quick rear brake pad replacement, I hit the road for a test.

At first, things were going well. As always, it fired right up and behaved perfectly on the back roads. Once on the highway, I wound it up to about 65-70 mph behind a truck and things seemed OK. I started smiling, figuring my daily driver was now squared away. A little further down the road, however (say 3 miles or so), I detected a faint hesitation. At first I thought I’d just been buffeted by a gust of wind, so I shrugged it off, but half a minute later, there came the undeniable sag in power and I realized the problem wasn’t gone. Deflated, I pulled off the highway and went back to the shop.

So, what’s a good thing for me to try next?

Randy...

[DimitriT]

Randy, I think your symptoms sound like inadequate fuel supply. The flow rate into the carbs can't keep up with the rate of you are drawing it away to the engine. Eventually the bowl(s) empty and you hit a lean condition until you can refill the carbs.

The reasons this can happen: petcock, kink or air bubble in the fuel line or external fuel filter. Clogged tank filters. There is a fine screen filter in the tank (on the petcock) and in the each of the carbs; maybe you have an external one too.

There's also a possibility that the main jet or needle jet in the carb(s) is clogged. This will result in a flat spot at open throttle. But from the way you describe I'm leaning towards my fuel supply theory.

Have you gone through and cleaned the carbs?

[randyshipp]

Well, the petcock and the screen that fits on it are new as of today. I certainly was going with the fuel starvation theory, too, but it suffered a blow when the problem didn't respond to the new petcock. There is no external fuel filter, and the fuel line seems fine. I can certainly try putting a new length of fuel line in there, just to be sure. Frankly, some blockage of the fuel line makes more sense to me than something farther down in the carbs, though I'm willing to be shown why that might be the case. Wouldn't a dirty carb problem happen at other times than just 60-70 mph? If the clogged main jet would give trouble at wide open throttle, then I'd get the bogging down when I pull hard away from a stop, which I haven't seen (though I probably have never opened the throttle all the way).

The fact is, I have a newly cleaned and rebuilt set of carbs in freezer bags. If nothing else is suggested that might help, I'll probably swap them in, though they were intended for another bike. But I haven't been convinced why generic carb dirtiness would cause this set of symptoms.

Thanks very much for your advice...I'm learning a lot from everyone!

Randy...

[dpep]

So it drops power under load? Is it always at the same rpm? Does it happen at a lower speed/rpm if you are going uphill? if you are two-up? Have you tried riding with the petcock in Prime position? Does it drop off at the same rpm in 4th gear (at lower speed).

[randyshipp]

So it drops power under load? Is it always at the same rpm? Does it happen at a lower speed/rpm if you are going uphill? if you are two-up? Have you tried riding with the petcock in Prime position? Does it drop off at the same rpm in 4th gear (at lower speed).

Good questions. I'll try testing those things out the next time I'm over there...hopefully tomorrow. Just out of curiosity, what might it mean if hills affected it, or if it does it at the same RPM in 4th? Thanks for the help.

Randy...

[DimitriT]

Does the bogging go away or improve if you add choke to it? Then I would suspect the carbs, if not were back to fuel delivery. Take a look at the fuel hose does it point straight down from the tank to the carbs to does it loop upwards and then down (where it can trap an air bubble). I had this happen to me - it would happen more often when I was low or on reserve.

I guess another possible cause would be a sticky float or float valve. If your float sticks, your may not get it to release the valve until the bowl is almost dry - but usually they stick in the open position (too much fuel). I'm just throwing out ideas here. Are the vent tubes plumbed properly? Does the tank vent ok from the tank cap? Get it to 1/4 tank and try running it without a cap - see if the problem goes away.

[GS850_Coug]

Typically if it happens at the same RPM in different gears, then there is a fuel flow issue in one of three or four circuits in the carbs.

Because it didn't seem to happen right away, and got worse as the bike warmed up, I would think it is running rich on one (or more) cylinders. This could be a float sticking, the needle valve sticking, a bad spark plug wire, etc. OR, a fuel flow issue prior to the fuel bowls. This could include plugged up filters on the float valves, pinched fuel line, or too small of fuel line.

Also, could be the vent in the gas cap no working properly. You can check this by loosening the gas cap then taking your highway drive again. If problem is gone, replace your cap. Check your plugs for fouling by shutting the bike down in the middle of it acting up, coasting to the side, and pulling the plugs on the side of the highway. Ideally, take photos of each plug to reference them in further diagnostics.

[randyshipp]

All great ideas, folks. Thank you so much for taking the time. My dad already mentioned some suspicion of the gas cap, so tomorrow we'd already planned to try a ride without the cap. Glad to see we might have been on the right track. I'll post a reply to this thread when we've tried some of these ideas out. Thanks!

Randy...

[Bananaman]

I am watching with interest as mine does the same thing.
If I sit on 4000 rpm on the highway it breaks down after 15 mins.
If I sit on 3500 rpm I can feel it hesitating a little, but she holds in there.\\/

Mine has an inline filter so I plan to replace it all, just have to work out how Im gonna get the hose off at the carby end, without removing the airbox:shock:

Having extensively researched GSR over this problem, another thing you might want to check is the coils. I have seen that mentioned as a possible cause as well.

[beby99]

randyshipp....When I first got my bike it started, idled and ran very well. But when in 5th gear at around 65-70 it would bog and sputter until I backed off a bit. Took carbs off to clean and found 1 main jet laying in the bottom of the bowl.

I think it's time to clean your carbies.

[randyshipp]

Randy, I think your symptoms sound like inadequate fuel supply. The flow rate into the carbs can't keep up with the rate of you are drawing it away to the engine. Eventually the bowl(s) empty and you hit a lean condition until you can refill the carbs.

This was the theory I was leaning towards, too. Replacing the petcock didn't prove useful in solving the problem, and my experimental ride yesterday has me looking beyond the bowls for the problem now. In an effort to test the faulty gas cap vent, I found my second key and took off with the key in the gas cap. The idea was that I'd find the spot where it started to bog, maintain that throttle position for a few seconds, then reach down with the left hand and pop the filler cap; if there was some kind of vapor lock, that should relieve it and I'd get results after just a few seconds.

I noted two things as I took off down the road. First, I was able to smoothly accelerate -- somewhat hard acceleration, but not wide open at all -- all the way up to 65 or 70 with no problem. I cruised along at 70 mph for some time, and I didn't detect any problem. Quite a bit later, throttled back to about 60 mph, and about seven or eight miles down the road, it started stumbling. After letting it go on long enough that I could tell it wasn't just a momentary blip, I popped the gas cap, to no effect. Scratch that easy fix. :?

To answer another question someone asked, I took back roads on the way back home and did some testing where, in 2nd or 3rd gear I got it up to the same RPM range (4400-4600) where the problem occurs on the highway. In both cases, there didn't seem to be any problem, and that included a bit of uphill riding. I have no idea what it means that the problem only seems to be in 5th gear at highway speeds.

So, I feel like I learned two related things. Obviously, the gas cap wasn't the problem. But I also feel like it can't be the kind of fuel starvation problem I originally thought it was. If, at highway speeds, the engine was emptying the bowls faster than the fuel supply could fill them, then it would starve at that speed or throttle setting and everything faster, right? It's not like going 70 mph draws less fuel than going 60.

So...

Have you gone through and cleaned the carbs?

...I have a set of carbs that were cleaned not too long ago and were sitting in Zip-Loc bags waiting to be put on another bike. Guess I'll get them back together, get them adjusted per the Clymer's manual, and try them on this bike, unless anyone has any other ideas.

Randy...