Yet another carb issue. I think...

[newbie1993]

My gs is doing some weird stuff. So it all started during a ride to work. Started fine, idled fine, and took off fine. Toward the end of the ride ( about 12 miles later) I noticed that all torque was gone, horrible acceleration during takeoff, dies at idle unless constantly revved and quite a bit of backfiring. I haven't had any issues like this in a while. Any help is much appreciated.

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[ajensen]

The most common advice is to put the petcock on prime. I am fairly sure that you have a fuel-flow problem.

[mr72]

That was exactly the behavior my bike had when it had bent valves.

BTW the bent valves were not caused by the normal things people associate with bent valves, which is over-revving, valves coming in contact with pistons. But in my case it was because carbon build-up on the back side of the valve caused the valve to not be "level" with the seat when it closed so over thousands of miles riding like this it over time bent the valve ever so slightly so that it would not seat fully. As a result, poor compression, and exactly the behavior you mentioned. I initially thought it was a carb problem (so did the shop that eventually did the top end rebuild).

Check for this condition by doing a compression test.

Hopefully you have a fuel flow problem.

BTW other things that you could look into might be failing signal generator, you could have one of the pickup coils open-circuiting when hot and that would cause this. Also a sticking float needle, if sticking closed, could cause this, the theory being it sticks when it's hot, but in that case it'd be just as likely to stick open as closed so you might have some days it does what you describe and some days it runs fine and floods when you park it. Could also be a vacuum leak that only manifests when hot, which depletes the vacuum to the petcock enough that the petcock closes when the engine is hot (unlikely IMHO but possible). Or of course it could be poorly seating valves and low compression like I mentioned due to carbon build-up or just worn out valves.

[newbie1993]

I don't know if telling you the gs ran like nothing happened a few hours later and then the problem came back.

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[mr72]

the gs ran like nothing happened a few hours later and then the problem came back.

Yeah this is exactly what mine did when the valves were bent. I'd do a compression test before tearing into the carbs again.

[newbie1993]

Right side was great. Left side not so much. Where do I go from here?

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[newbie1993]

Here is the left side reading

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[mr72]

Right side is OK, but left is pretty bad. Mine was 120psi one cylinder, 80psi the other, and all four of the valves were bent to some extent.

At this point you should check the valve clearances just in case the problem is zero valve clearance, but I wouldn't get my hopes up. If you have >0 clearance then I'm afraid the next step is to pull the head and investigate further.

When I got to the point you are at, I hired a shop to do a top-end rebuild.

[newbie1993]

The needle started at 90 and then started to drop. If that makes a difference at all.

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[newbie1993]

Also is there a video or in-depth thread that explains how I would go about checking valve clearances.

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[ajensen]

I am an old-time motorcycle mechanic. I started when I was 16 and worked in various shops on various motorcycles until 1986, when I was a couple of months shy of 40. I worked on bikes from Aermacchi to Zundapp. Back in those days we were simply mechanics, so I have a little different view from modern motorcycle technicians. All this being said, I have never seen a valve that was bent from carbon. I've seen valves bent from hitting pistons, ingesting foreign objects, and seizing in valve guides. However, never from carbon build-up. I have seen valves lose their seat (sealing) because of carbon. I have seen exhaust valves burn from overheating—usually lean mixture, tight valves, or bad ignition timing.

Therefore, I am skeptical about bent valves on your GS500. I am sure that the previous poster was told by the repair shop that he had four bent valves, but I think the technician was not telling the truth. It was a good idea to replace all four valves on a high-mileage engine, but I do not think they were bent.

If I were you, I'd check the valve clearances. If a valve is too tight, you may have found your problem and just need to put a thinner shim in. If you find excessive clearance, you may have a bent valve.

If anyone wants to dispute me, I am happy to be corrected. It may be that the GS500 is different from the things I worked on so long ago. No matter what, I hope you get your bike running correctly soon—GS 500s are so much fun to ride. I rode a 1978 Yamaha SR500 from early 1979 until last February, but I have gotten too old to deal with kick starters and ancient parts that can fail without notice. For me, my 2006 GS500f is a "modern" motorcycle that I enjoy a great deal.

[mr72]

Whether you've seen it or not, that's the best explanation for how my valves got bent. IDK why the op has low compression, just sharing what caused it for me. But it doesn't matter why at the end of the day. Failing compression and these symptoms points to poorly seating valves. I think I said, check valve clearance and if it's not zero then you have to pull the head to know the cause of the low compression. Whatever caused it originally the solution is most likely a valve job, so arguing about whether my valves were bent due to carbon build up is not really productive.

[J_Walker]

failing compression could be a failed piston rings, or scored walls too, to check, add a tablespoon of engine oil down into the sparkplug hole and test it again, if it fails it's the valves [or headgasket, but the GS500 doesn't have this issue really] but if the PSI rises, its the piston rings, or badly scored walls.

[gsJack]

I'm still surprised every time I hear of GS500s with bad valves causing low compression usually worse on one cylinder, I found GS valves to be nearly bulletproof at least for the first 100k miles.    Shows a sad case of neglect usually by previous owners of valve checks on these bikes.

I rode my 97 GS bought new for 80k miles with no compression loss.  When it was totaled I bought my 02 GS with about 4k on it and rode it to over 100k miles and checked compression with the finger over the hole, crank, and pop test every time the plugs were out on both bikes.

On the 02 curious about the actual compression at 100k miles I bought a compression guage and checked it and found it to be 135 psi cold and 140 psi hot on both cylinders and 190-200 psi after squirting a bit of oil into the cylinders showing ring and cylinder wall wear over 100k miles and valves that were still good.

http://gstwins.com/gsboard/index.php?topic=67148.0

[J_Walker]

GSjack do you happen to know where both your GS's where made?.. I have a running theory on "particular" GS's assembled in a certain part of the world... I don't wanna say what part, because I don't wanna start a rumor as I don't have enough conclusive proof of it being true, just yet...

[mr72]

failing compression could be a failed piston rings, or scored walls too, to check, add a tablespoon of engine oil down into the sparkplug hole and test it again, if it fails it's the valves [or headgasket, but the GS500 doesn't have this issue really] but if the PSI rises, its the piston rings, or badly scored walls.

Yeah but with the running symptoms it doesn't sound like rings or cylinders. But still bad valves warrants a top end rebuild.

[newbie1993]

Should compression be tested with both plugs out while using a compression tester? And also would appreciate a walk through of what I need to do and also a possible list of tools necessary to do everything I need to do.


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[J_Walker]

Should compression be tested with both plugs out while using a compression tester? And also would appreciate a walk through of what I need to do and also a possible list of tools necessary to do everything I need to do.


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I was taught to pull a single sparkplug at a time.. so I'd say, pull a single sparkplug at time, unless someone can provide proof that it has no effect otherwise.

[mr72]

And also would appreciate a walk through of what I need to do and also a possible list of tools necessary to do everything I need to do.

If you are seriously considering attempting a top-end engine rebuild YOURSELF then you need a factory manual, which can be downloaded online if you search, or a Haynes manual that you can purchase on Amazon etc. This will have the details of tools and procedures needed to accomplish this.

While I am no expert, and I hired someone to do this but I would have likely been capable of doing it myself if I had the time and inclination (done cars, etc...), here's the rough outline.

1. you have to get access to the cylinder head just like if you were doing a valve adjustment/check. Actually I recommend you do a valve adjustment/check to be certain you need to pull the head. The instructions to do that are on the wiki. Follow the links.

2. once you determine you need to pull the head (pending outcome of valve clearance check), then you already have the tank and valve cover off, so now you need to pull the head. Use your shop manual for instructions on this. If you get the head off of the bike then you will at the very least need a new head gasket before putting it back. You're going to have to have an array of ordinary mechanic's tools to do this. Once the head is off you can inspect the valves directly. You will have to disassemble the head, pull the valves, need a valve spring compressor, etc. IMHO this is the time to take the head to a machine shop and order a valve job. Worth it to pay to replace the valves since they are only $30/ea and there are four of them, so this is a rather small cost once you have committed this far.

3. Again while the bike is this far apart, might as well pull the cylinders ("jug") and deliver it to the same machine shop to get the bores honed. Again, shop manual will tell you how to do this. Then pull the pistons and clean them thoroughly, preferably in an ultrasonic parts cleaner, with the rings removed. Get a new set of rings. Put it all back together and wait for the head and cylinders to come back from the machine shop.

4. When the parts come back from the machine shop, measure and verify everything is done right, verify valve clearances etc., then put it back together according to the shop manual procedure. Break in procedures apply.

All in all this is probably 6 hours of total work for an experienced home mechanic, but split over two days I'd figure a Saturday on each end of the machine shop's schedule. If you are asking what tools you need, and I am not trying to be a jerk, but the reality is you might be in over your head so I highly recommend getting some hands-on help or letting a shop handle it. But in the grand scheme of home mechanics, this is not a hard job by any stretch. As an example, my daughter's boyfriend just bought a car that needs a clutch, and I am considering helping him replace it (it's a FWD car, which adds greatly to complications of clutch replacement), and that is probably 3-4x as hard as the assembly/disassembly for a top-end rebuild of a GS500 plus probably 50% more time consuming, but plenty of first-timers will tackle this job themselves armed with little more than a Haynes manual, a couple of jack stands and access to their dad's tools.

OTOH, the reality is in the condition your bike is in, which is it will run enough for a test ride, it'll be cheaper and easier to flip this bike and buy another one that doesn't need a valve job. I mean, even if you are dead honest about what you KNOW of the condition (low compression and running problems when it gets hot), you should be able to get $400-500 for an otherwise good/working GS500 and put another $500 with that, which you'd spend on gasket kit, rings, valves and machine work to do your own top end rebuild, and you can probably get an equivalent GS500 that doesn't need this work. Unless there is sentimental value in keeping this particular bike, I'd flip it.

[gsJack]

I've always pulled all the plugs when checking compression on 8, 6, 4, and 2 cylinder car, truck, and mc engines starting back in 48 when I went to work in a garage the summer I turned 16.  Have I been doing it wrong all these these years?

Should compression be tested with both plugs out while using a compression tester? And also would appreciate a walk through of what I need to do and also a possible list of tools necessary to do everything I need to do.


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I was taught to pull a single sparkplug at a time.. so I'd say, pull a single sparkplug at time, unless someone can provide proof that it has no effect otherwise.