1981 Suzuki GS650G - dies under load when throttled past ~1/3

[seinwave]

That price is doable, but I'm a little concerned about buying NOS rubber vs repops; rubber degrades over time whether or not it's installed, though it being out of the sun slows down the process.

The bigger thing is: I don't exactly know what's wrong with the #2 boot! It's not hard or anything, it just... doesn't seem to match the shape it needs to be to fit in the carb. Tried rotating it and it only made the problem better or worse, not fixed. I'll post a picture in a new thread at some point and see if anyone can identify it. It *does* seal, though, so perhaps that's a problem for when next summer's high heat keeps me off the road.

[dgates1]

Next I'll probably sync the carbs. I also think the #2 airbox->carb boot is misshappen or mis-selected or something, but I can't for the life of me find a replacement, and it's sealing, so... who knows.

There’s a set of all 4 on eBay. Kinda spendy, but they are new.

https://www.ebay.com/itm/275979066905?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=ewfGyMTrQnq&sssrc=4429486&ssuid=5ueBAimr Qo m&var=&widget_ver=artemis&media=COPY

[salty_monk]

Congrats!

[seinwave]

I didn't want to update too fast -- I've already pulled a Bush Jr. and declared mission accomplished way too soon once. But it's been over a week and a few dozen miles since I've done anything to the bike so I feel comfortable it's fixed. I owe it to future forum trawlers following in my footsteps to bring the story full circle.

Long story short: I didn't *quite* have it fixed after my last post. It started and ran alright, but I quickly realized it was running on only two cylinders - the exhaust pipes for 2 and 3 were ice cold, and I could barely get it to move without the clutch. I confirmed a lack of spark on those cylinders.

The biggest thing I did was bite the bullet and do the relay coil swap; the voltage drops had jumped from one-point-something to two-point something and I was tired of relying on 43-year-old wiring and bullet connectors. $20 worth of parts later I was getting real strong voltage to the coils, and (a visible) spark on 2 and 3 - but still running on two.

Just because I'd already bought them and wanted to leave nothing to chance, I made the third (and hopefully final) swap of coils and hooked them into the (now entirely costume-ized) ignition system. I headed down to the Advance also and picked up four shiny fresh sparkplugs to replace the fouled ones.

I also retarded the timing a little bit - I think I may have had it slightly advanced on initial setup. This was precautionary.

Now it runs and pulls! Before it's properly warmed up I have a tiny momentary stumble the *first* time I pass 4 or 5k revs under load, but it barely lasts long enough for me to realize it's happening before the plant powers through.

Next I'll probably sync the carbs. I also think the #2 airbox->carb boot is misshappen or mis-selected or something, but I can't for the life of me find a replacement, and it's sealing, so... who knows.

[tom203]

Sorry, wrong 650 guy! In the carb forum, there was guy looking for air filter element cage who mentioned using 115 mains. I merged you two together to save brain space

[seinwave]

What do you mean in reference to the mains? I don't think I posted my specs here and I'd have to hunt down the notebook I wrote them down in, but IIRC the mains were the default (110).

[tom203]

Progress is always good! Nice work….but I still think the 115 mains are too big

[seinwave]

Well, looks like this chapter of my journey can be entered in the comedic annals of "foolish novice mechanic catastrophizes, pessimistically declares sky is falling" rather than the much sadder book of "foolish novice mechanic trusts buyer too much, optimistically buys lemon he then can't unload". The bike runs again!

Went down truly at the end of my rope to try a few things you guys suggested on this page. Decided to take the plug off the vacuum tube and reattach both hoses to the [still-original] petcock (I'd been trying to run it off just the fuel line, topping off from the tank every so often). Caused the engine to upgrade to a big cough, but only one, when I tried starting it. I think? My memory of this is sort of erased by the next thing. I might have actually done them simultaneously, and never tried to start it just after I plugged in the vac tube?

I removed the air filter and jammed a big rag into the D-shaped orifice. Hit the starter a couple times. Cough, cough, cough, and then... the glorious sound of starting combined with a "schloop" noise as the rag disappeared from sight.

Shut the engine down (it had pulled to 6k - idle was set way high). Retrieved the rag (it was just around the corner). In short order had it settled down to a nice idle and easy starts.

In my defense, the last time I had the tank on and off and tried to run it (after the first coil swap) the bike had no problem starting, running, and restarting just with fuel in the feed line and nothing in the vac line. So I hadn't even considered an open vac line could cause a problem (and I'd plugged it with a properly sized smooth stud, so I thought I'd eliminated it).

The circumstances of it starting again aren't ideal for diagnosis - not sure if it was just the vac line, if it just needed a little manual choking (the rag) to prime itself again, or if I accidentally did an "Italian carb clean" and made enough vacuum to wrench some obstruction out of the orfice it was blocking (leaving it free to settle at some unknown future date in another orfice, or an oil gallery, or or or).

I've already pulled a dubya once so I don't want to say mission accomplished yet. Haven't tried to ride it. Haven't taken it to redline (pulled smooth to 5 or 6k with about as much throttle as I expected). It started off idling way lopier than before, but after running for a while it settled down to "maybe only a little lopier than before". The left pipe spat white or gray smoke when I revved it for the first few exploratory twists of the throttle, then stopped. The right pipe oozed a little white (or gray) smoke after I killed it (not the first time it's happened). Having the air filter on or off didn't seem to do nearly as much to the idle smoothness as it should (I do think it helped a little to have it on), which means I should fast track the long-delayed Operation Weatherstrip The Airbox. But I'm really glad I'm (for the moment) back in the realm of fettling and tuning rather than lifesaving.

Thanks again to everyone for not just putting up with my longform diary posting but trying to help out, too.

[dgates1]

Does it start with starting fluid sprayed into the carbs? Would tell you if it's a fuel delivery issue.

I recently cleaned my carbs. Found that the hole that goes to the bottom of the bowl for the choke circuit was pretty clogged, even after ultrasonic cleaning. Had to spray quite a bit of carb cleaner through each one before it finally blew a chunk out of there and they were "actually" clean and clear. Also, my number 2 cylinder would not fire the first time I started it. No fuel in that one bowl. Found a needle was sticking in the closed position just barely, like it had a little varnish still in the seat. Recleaned the seat and needle. Polished the seat with some rolled up cardboard. Put back together and had fuel to all bowls and fires on all 4. Not having that filter on the end of the needle valve seat might let a little chunk of stuff block that up.

Did you take out the fuel mixture screws and replace the O-rings there? Clean those passages all good? Did you get the old O-rings out (don't have 2 of them down there)?

I was getting the same issue where it would start fine but fall flat at 3-4k rpm, then want to die. My fuel mixture screws were about 2 to 2 1/8 turns out at the time (thought this was the factory setting on my year carbs). I reset them all to 1 1\2 turns and put the airbox in place and now will rev great all the way through. Haven't vacuum synched them yet, Morgan Carbtune tool is in the mail.

I think I read that you replaced your petcock. Make sure your new petcock is working. Make sure the vacuum line to number 2 is attached and not cracked and leaking, or hook up a temp tank. If using a temp tank, don't forget to block the vacuum port on number 2 carb.

If you adjusted your floats too far, they might not be opening up to needle.

Just sharing my thoughts and own experience... Really does sound like a fuel issue to me, but who knows now with all the ignition work you did.

[seinwave]

I tried a spritz of WD40 (I know, I know, but I had no starting fluid on hand, and also I'll take a little solvent action over starting fluid blowing a valve if I get heavy handed) in one intake (carb->engine). Nothing. I tried a spritz of WD40 in one cylinder (not the same one I tried the intake on). I *think* nothing? Maybe a tiny cough.

I haven't tried on the air filter.

If I saw any evidence of fuel getting to the cylinders I'd assume a weak spark, but I didn't. But I also don't have enough confidence that I've provided enough fuel for it to try and start, so I can't bring myself to focus on the fuel either.

I don't expect anyone to try and remember the two months of stuff I've tried. Pretty close to the point where I start going in circles, and really considering selling it to cut losses. I hate to do it to something I know *can* and *has* ran really well but this is getting moronic and I'm getting to close to literally pulling out my hair over it rather than just figuratively.

[rphillips]

Too much to remember back on what's been tried and done. Thinking out loud, will it start or try to start if spraying small doses of starting fluid on air filter.

[seinwave]

I've tried every combination of choke, throttle, and idle speed I can think of. I'll try blocking the airbox today.

I can't really find any good information on how wet a wet plug should be. Should I be looking for 'soaked' or 'slightly damp'? How long do I have before it evaporates off?

If I don't have fuel after all is said and done I'm going to be completely out of ideas. What could possibly cut off all four (or at least more than one) choke/idle tube in the carbs in the span of three days? I guess diagnosis doesn't matter much; next step will be spraying the *@%^$*@%^$*@%^$*@%^$ out of the choke pickups from below.

[tom203]

Ok, you got fuel in bowls, spark at plugs. Activate “choke” ….make sure plungers fully extend. Use no throttle and crank….any life yet? No ? Then remove air filter element, place hand over opening to airbox to block…crank again..still nothing? remove a couple of spark plugs and look for fuel. The idea here is to ensure fuel is getting up the choke pickup tubes into carb throats.

[seinwave]

Hmmm, I don't think that's possible. When I timed the ignition that would have shown up in big blazing letters. The 1-4 coil (white) went high when it was supposed to (aka when I turned the advance mechanism with the advancer pointed at the full-advance mark).

I did swap wires around in the past. Might as well do it again.

My current goal is to clean up the connections to the coils and see if that does anything. Despite not seeing anything on the plugs I swear I smell gas after trying to crank it (and I really don't have a very good sense of smell). Maybe the spark really is just weak enough? The voltage drops have crawled up to 2 whole volts after I swapped the ignition (probably due to the *@%^$*@%^$*@%^$*@%^$ty hardware-store connectors I used).

[Nessism]

Could the spark timing somehow be 180 degrees out of phase? This has been a long thread, and I'm too lazy to read back through, but did you try switching coil wires from side to side? If not, it's worth a try...