This relates to my project bike, and now my only bike, that I hope to have ont he road soon.

For starters, it runs, and it runs pretty strongly on both cylinders, but at idle, before it's fully warmed up it feels like it might be either missing on the right or only running on the left. Last time I started it I was definitely getting some pops and sputters from the right cylinder. I've recently had the carbs apart and I thought I cleaned them thoroughly, I also installed a Buddha stock jet kit which seemed to improve things.

Yesterday I bought a compression test kit. When I pulled the spark plugs, the right-side one looked very white (lean?) while the left looked (to my knowledge) healthy, if a bit rich.

Both cylinders measured 120psi cold, but I didn't have the opportunity to test the bike hot. Should I be concerned about a somewhat low compression? I've been reading that they should be around 150+, with a bare minimum of 120-ish, but people seem to disagree on whether it's the cold or hot compression that you should care about.

Thanks for your help, guys! Any ideas would be appreciated.

[attachment deleted by admin]

120 cold is ok. As long as they are about the same left and right.

Probably just need your carbs cleaned.

No mention of a float height check, so I'd suggest doing that and ensuring the proper amount of fuel is getting to each carb first.  See if that tells you anything, then go on.  You may have an air leak on one of the carb boots if float height checks out okay... or something else... if float heigh is off, well you can troubleshoot those reasons in that direction. You just have to look at this in a systematic approach, otherwise you'll be chasing a moving target or wasting time at the very least.

By its symptoms I'm inclined to think toward mixture issues, and fuel delivery is the easiest to troubleshoot.  Then move onto air... if its not a mixture issue, then look to electrical(spark, etc..), then to compression.



- Bboy

Quote from: Big Rich on May 14, 2013, 05:49:51 AM
120 cold is ok. As long as they are about the same left and right.

Probably just need your carbs cleaned.

Doye. I told you I just bought the compression tester, never having used one before.

I just realized that I totally forgot to hold the throttle open while running the starter. I'll bet that probably has an effect on my cold readings, no? I'll warm the bike up after work and run it the RIGHT way, throttle open, engine warm.

Quote from: BockinBboy on May 14, 2013, 06:51:36 AM
No mention of a float height check, so I'd suggest doing that and ensuring the proper amount of fuel is getting to each carb first.  See if that tells you anything, then go on.  You may have an air leak on one of the carb boots if float height checks out okay... or something else... if float heigh is off, well you can troubleshoot those reasons in that direction. You just have to look at this in a systematic approach, otherwise you'll be chasing a moving target or wasting time at the very least.

I guess I'll test the easiest stuff first. I haven't yet tried a float-height check, which I've read you just do with a clear hose on the drain nipple at the bottom. If I'm not mistaken, you just hold the hose up next to the carb to compare how high the fuel level sits?

Quote from: BeerIsSoAwesome on May 14, 2013, 06:59:36 AM
I haven't yet tried a float-height check, which I've read you just do with a clear hose on the drain nipple at the bottom. If I'm not mistaken, you just hold the hose up next to the carb to compare how high the fuel level sits?

Yep, the fuel height should be equal to the carb bowl gasket height when you open the drain.  Don't forget to postion the frame petcock to prime.

- Bboy

Quote from: BockinBboy on May 14, 2013, 07:12:20 AM

Yep, the fuel height should be equal to the carb bowl gasket height when you open the drain.  Don't forget to postion the frame petcock to prime.

Thanks for the tip. I assume once I have the carbs out of the bike to adjust the float, I'm going to want to get it right before putting it all back together again. Have you ever tried this technique for measuring float height?

http://cgi.stanford.edu/~sanjayd/gs500/Maintenance/FloatHeight

I have digital calipers which are pretty accurate.

Quote from: BockinBboy on May 14, 2013, 06:51:36 AM
You may have an air leak on one of the carb boots if float height checks out okay... or something else... if float heigh is off, well you can troubleshoot those reasons in that direction. You just have to look at this in a systematic approach, otherwise you'll be chasing a moving target or wasting time at the very least.

I picked up a can of starting fluid on the way home from work today. It turns out I don't have any tubing small enough to fit on the drain valves, so I had to skip the float-level check for today. I decided to start the bike and spray some starter fluid around the boots. Found the leak almost instantly. Right-side cylinder (the lean one, of course) right where the carb met the boot that goes into the manifold. I took it all apart and removed both boots, gave them a thorough cleaning with WD-40 and a rag as well as any and all corrosion on the mating faces of the cylinder head and the carb inlets and outlets. I did the same to the airbox boots. 90 minutes later I had it all back together, it's VASTLY improved. No more coughing from the right-side cylinder! It could probably still use a throttle sync, but any stumbles in the idle are very minor.

I can't thank all of you guys enough for giving me suggestions. It was the simplest thing.

Oh, also I re-tested the compression with just a 5-minute warm-up and with the throttle open, and got just about bang-on 150psi from both cylinders. I plan on testing it once more with a full 20-minute warm-up once the bike is street-legal, however now I feel like it's "real". The idle is good, the power is good. It still pops more than I'd like on deceleration, even with the Buddha jet kit, but it doesn't feel like it's running wrong anymore. I just ordered brand-new tires today and hope to have it on the road by next week!

Awesome! Glad you got it sorted. 



- Bboy

hey beer, look into replacing the faulty boot. cause you DID clean it up, however still may have a bit of a leak there. ( or clamps if possible etc.

Quote from: yamahonkawazuki on May 15, 2013, 07:21:38 AM
hey beer, look into replacing the faulty boot. cause you DID clean it up, however still may have a bit of a leak there. ( or clamps if possible etc.

Thanks for the tip. I don't think the boot is faulty. The carbs have been on and off the bike a few times, and everything was quite dirty, plus I think I had failed to align/tighten that one properly when I put it back together last time. I don't see any cracks in the rubber.

If it starts acting up again, I'll consider replacing them. I shot a quick video of the idle, now, after a 5-minute ride down the street to the gas station and back.

http://www.youtube.com/watch?v=zNByxd8eEFM

Sounds alright to me, no? I sprayed starter fluid all around the boots after reassembly with no response, so I think I got a good seal this time.

Granted, I think the carbs could be synced.

Judging from the sound your carbs are out of sync. Also, is your exhaust modified in any way? It sounds like you've got an exhaust leak.

Quote from: Kijona on May 15, 2013, 06:14:52 PM
Judging from the sound your carbs are out of sync. Also, is your exhaust modified in any way? It sounds like you've got an exhaust leak.

Thanks for the input. Yes, I'm certain the carbs are out of sync. Unfortunately I left my homemade glass-bottle sync tool at my buddy's workshop an hour away. I'm debating buying a "real" one, but I don't really need the expense.

And who knows about the exhaust. This bike was acquired for $500. It has a title, crash damage, and most of all *mystery*.

I agree it doesn't sound like a stock exhaust. I don't know if anything funny was done to it, except I can tell you there's a little round hole on the bottom of the can (where the pipe meets the muffler) that I assumed was a drain of some sort.

I have a fresh set of header gaskets coming in the mail, because I assumed that a leak at the header might be causing the popping on deceleration. What do you recommend I do next?

Quote from: BeerIsSoAwesome on May 16, 2013, 06:47:31 AM

Quote from: Kijona on May 15, 2013, 06:14:52 PM
Judging from the sound your carbs are out of sync. Also, is your exhaust modified in any way? It sounds like you've got an exhaust leak.

Thanks for the input. Yes, I'm certain the carbs are out of sync. Unfortunately I left my homemade glass-bottle sync tool at my buddy's workshop an hour away. I'm debating buying a "real" one, but I don't really need the expense.

And who knows about the exhaust. This bike was acquired for $500. It has a title, crash damage, and most of all *mystery*.

I agree it doesn't sound like a stock exhaust. I don't know if anything funny was done to it, except I can tell you there's a little round hole on the bottom of the can (where the pipe meets the muffler) that I assumed was a drain of some sort.

I have a fresh set of header gaskets coming in the mail, because I assumed that a leak at the header might be causing the popping on deceleration. What do you recommend I do next?

You are correct on the drain plug hole on the bottom of the pipe. Also, popping on deceleration is an indication of a lean condition, as is the color of your plug - white ash is definitely a lean indication. My guess is you're going to find either one missing gasket or all 4 header bolts loose. I wouldn't even so much as touch the carbs until you figure out if your exhaust is leaking or not.

Don't worry, the GS500 engine is quite a tank. She can take a lot of abuse.

Since you only have two carbs you sync you can make a VERY inexpensive tool out of vinyl aquarium tubing and water & food coloring or oil.



Click for bigger.

Quote from: Kijona on May 17, 2013, 12:24:18 AM


You are correct on the drain plug hole on the bottom of the pipe. Also, popping on deceleration is an indication of a lean condition, as is the color of your plug - white ash is definitely a lean indication. My guess is you're going to find either one missing gasket or all 4 header bolts loose. I wouldn't even so much as touch the carbs until you figure out if your exhaust is leaking or not.

Don't worry, the GS500 engine is quite a tank. She can take a lot of abuse.

Since you only have two carbs you sync you can make a VERY inexpensive tool out of vinyl aquarium tubing and water & food coloring or oil.

Dammit, I wish I had read this comment before digging into the carbs. I actually went ahead and built a version of that with aquarium tubing, except I did the one with the bypass valve up top that allows for sensitivity adjustment. It seems like a very good design, and made it a $9 tool instead of a $4 tool. I started rushing because I was losing daylight, got frustrated trying to sync them, kept having the engine race uncontrollably, lost track of my original idle setting and original sync position, and left it in far worse running shape than it was before. Hopefully the gaskets arrive today so I can install them, bench-sync the butterfly valves and start fresh.

EDIT: Also, I used motor oil in U-bend because I figured it would be less likely to harm the engine if it got sucked in. Problem is, it's too viscous. When the left cylinder pulled much harder than the right, it sucked it HARD through the lead tube and ended up coating the walls of the tube with oil that refused to collect back in the bottom of the U-joint. I'm guessing that water doesn't quite have that same problem, so now I have to figure out how to flush out the oil and fill it with water, or simply replace the tubing for a few bucks. My tool looks like this:

Another option is to use ATF that's red or something like that.

You can use a set of feeler gauges to set the idle and make sure they're close to the same position. I'm sure someone will come along with exact specifications, I don't remember off the top of my head. Seems like 1mm of opening for idle but I can't be certain.

If you're worried about fluid getting sucked in just make the tubes really long.