'81 GS650G Engine Knocking

[Guest]

Good Morning Fella's!

So, I bought this bike from my buddy who wasn't riding it anymore, it was sitting in his shed for 2 years (got it for $400, has 25,000 miles on it). I noticed it was burning a lot of oil on start up so I figured it was bad valve seals. I did a compression test and was getting pressures of around 125. I also had a stuck float as I was spilling fuel out the left side overflow line, and sometimes out the airbox drainage line. I took the carbs off, took them all apart, and dunked each one in carb dip, reassembled with new O-rings, set float height and then synced them with a carb sync. I then and went and tore the top half off and put new valve seals in, lapped the valves into place and cleared the tappets to the .03 - .08. I also got new plugs and properly gapped them, new gasket, put it all back together, fired it up and was running fine from the beginning (although checking plugs seems to be running a little on the lean side.) That was earlier this week. So now after around 50 miles, I went to start it up in this morning while it was on the side stand. When I stood it up to sit on it and leaned it level, it started making this knocking sound (first video link below). This goes away when RPMS around 1800-2000. I rode it home from my girls house (where I spent the night) and when I got home, the sound had diminished a little (second video link) but it was still there. After letting it sit for a bit, the sound was back. I decided to take it to work anyways because my girl is using my car while hers is getting a new transmission (but that's a whole other story). Bike didn't seem to be running or sounding bad, and when got to work the sound was gone.

Now, after reading some other posts, people were saying that this could be an exhaust problem, which could make sense, because the exhaust bolt busted on the #3 cylinder so I had to drill it out and tap it for one size larger, which created another problem in that couldn't get the proper alignment to get exhaust bolts to fit directly in the hole and had to keep opening up the bracket holes to get more clearance to get both bolts to thread. But, if it were exhaust related, how would this be any different after bike has warmed up? Maybe thermal expansion closed off the leak when engine is warmed up? Let me know your thoughts.

First video link: https://www.youtube.com/watch?v=fCGg-3ITLXw
Second video link: https://www.youtube.com/watch?v=0jJRg7kH494

This is my first motorcycle and don't have that much experience with engines but I love tinkering (am an engineer) with them and they make sense to me in their operation. I'm basically looking for is the bike safe to run for the rest of the year (have plans this winter to tear it all down and clean it up, replace all gaskets, etc.) or is this more of a serious issue that needs to be taken care of right now? Thanks in advance guys!

[tom203]

What's with all these 650 knocking sounds lately- you guys are making me nervous- good thing that I have bad hearing or I'd be real paranoid.
anyways,MrFirstgs, if you going to keep working on this thing to bring it back, please get a new petcock.. There's a good chance yours is and has been leaking gas into crankcase oil diluting it- this might explain the random knocking as gas is a lousy lubricant and this is a plain bearing motor unlike most bikes. Once you know petcock is good, then you can blame carbs. Try Z1 or this florida guy for a good as OEM petcock- don't waste your time fiddling with an 30 yearold petcock.

http://www.georgefixs.com/brand-new-suzuki-gs300-gs450-gs550-gs650-gs750-gs850-gs1000-gs1100-petcock/

[FirstGS]

Thanks for the reply Tom, and I will get around to replacing the petcock soon (which sounds like just a good wear replacement idea anyways.) My question regarding this is (and not trying to fight you on it! Honestly!) if cleaning the carbs solved my bowls from overflowing, how would this then cause gas to get into the crankcase? Also, I had heard earlier that carb overflow can cause this problem, how would this work? Would the gas eventually get past the piston rings? Genuinely curious because was wondering how that would be possible since I heard about that problem.

[seanarthurmachado]

My half-a-clue understanding (and that's generous) is that the old petcock vacuum fails, the tank drains down into the carbs, and these fancy new fangled CV carbs without an overflow tube to the ground, the gas goes to the valves down into the cylinders and to the oil washing all lubricants away in short order.

(Anyone with more ed-u-kay-shun feel free to correct me)

Sean

[tom203]

If petcock fails, when engine is off fuel can dribble down constantly overflowing fuel bowls (. the needle seats won't hold it back long if at all) . Fuel can rise up into carb throats where most will run down towards intake valves, into cylinders, past rings, etc.
The petcock can also fail by dribbling gas down its vacuum line- connected to #2 cylinder, washing out that cylinder with gas which will then sink to crankcase.

these carbs are real fussy about cleaning- cutting corners will give you plenty of exercise at doing it over and over. Go here and do some reading... Scroll down and you'll find the proper carb cleaning method on rightside.

http://members.dslextreme.com/users/bikecliff/

[FirstGS]

That makes sense Tom. I didn't even think about the vacuum line being a possible culprit. Also, the Bike Cliff page you referred me to is where I got all the manuals as well as the rebuild/cleaning page for the Mikuni BS carbs. As far as redoing the carb cleaning, the first time I cleaned them with just the can cleaner. Decided to do it again so that is when I got the carb dunk gallon. So my plan is to replace the petcock and do an oil change on the bike. Any other ideas you (or anyone else) can think of to look at?

[tom203]

You got the dip and the carb o-rings from cycleorings.com, so off you go! After the dip, blow/poke out all passages that you can. Pay attention to "choke "circuit passages in each carb. Extra fuel is picked up by small hole in float bowl side and sucked up into carb throats when "choke" is activated.
catch up on all the maintenance to enjoy happy motoring

Edit: here's a pic of how one guy did his carb fun- note that he keeps all carb parts seperate to avoid mixups

[FirstGS]

Yup Tom, have already done all of that. I just need to rejet my main because my buddy that I bought if from just told me that he put a K&N filter in it, so that is why it was running lean. It took me around 3 hours to get everything all done with the carb cleaning and "rebuild" and not looking to do that again, that's for sure.

[eil]

If it's a K&N filter in a stock airbox, you should not have to rejet. In most cases you only have to mess with the jets if you remove the airbox and install pod filters, or switch away from the stock exhaust.

I just watched your videos and (bearing in mind that I'm far from a GS engine guru) one likely candidate for that knocking sound is a loose (or worn) cam chain. Our own bwringer has a site showing how to rebuild a tensioner and check for proper operation: http://www.bwringer.com/gs/