GS 750 Carb popping

[guygs750]

I just got a 1979 Suzuki GS 750. It ran a little rough and was dying when I was coming to a stop if I pulled the clutch. It also was popping a little out around the carb boots.
So I sprayed down the boots with brake cleaner while it was running and the idle smoothed out, picked up, and then came back down to it's sputtery idle.

I got a set of intake boot o-rings and screws from cycleorings.com and put them in and now when it's cold it idles really smooth. But now when I try to accelerate after letting the choke out its really boggy. As soon as it stops being really boggy when accelerating and starts to warm up the idle goes up really high (2000-2200 rpms). Also after this point, if I get the rpms over about 4k it starts to pop in the exhaust occasionally, more when I'm accelerating on the highway or passing.
Also, today when I was on the freeway I was going about 70mph and around 5k rpms with slight acceleration and it started to get a little boggy and popping in the exhaust, then boggier, and boggier, and boggier, until it was so hesitant that it wouldn't accelerate in any gear. I pulled over to the shoulder and it died. I whirled the thing over again, and again, and again but it barely even popped off. Then I pulled the choke and kicked it over and it fired up, but was idling at about 3k with the choke pulled, 2.5k with the choke off.

I rode it another 5-10 miles and it seemed to be okay, other that an occasional pop in the exhaust. Then I goosed it a little and it was boggy/hesitant again. I exited the freeway and when I pulled the clutch in it died and was a pain to start again. Again I pulled the choke, again 3k with choke on, 2.5k with choke off.

If I let the clutch out a little it brings the idle down for a second, but it comes right back up.

I also replaced the points and condensers.

Then I figured out why it was bogging out completely. It was that the PO had put the wrong petcock on the tank, took the diaphragm and spring out of it, replaced the cover with a piece of 1/8" aluminum, and siliconed the "cover" in place. That worked fine until the silicone broke down and clogged the outlet.

I've since replaced the petcock with the CORRECT one, and hooked the vacuum line up to it.

When I test-rode it the bike ran just like it did before..... Popping and sputtering in the airbox.

I had the covers off and I did look at the snorkels going into the airbox and the outsides were a little melty, but they were still 95% intact.

I'm not suspect of the carb intake flanges because I've tested them with brake-cleaner/ether several times after replacing the o-rings. The idle has never changed hitting ALL angles of the flanges since I've replaced the o-rings so I'm not concerned about them at the moment.

It's like the thing actually ran better when all of the intake boots were leaking... Which kind of points me in the direction of it just being WAY too rich.

I was going through my manual and looked up the air screw adjustment, but it's not very specific. I took a crack at it and got the thing idling at 1200, bottomed out the screws, and then adjusted them out 1 1/4 turns. Then I turned them to the factory setting (which is stamped into the carb body) and then it barely ran. I adjusted them so it idled well and didn't die when you accelerate, but then it would die as soon as I let the clutch out.

Then I adjusted them individually. I turned them in to the bottom (with the idle at 1200), and then adjusted them out until the engine was screaming, gave them about another 1/8-1/4 turn out, and then adjusted the idle down. Then the bike was taking off from stop signs and picking right up off of idle.

BUT, I still have the same problem at around 3k+ rpms. It still "runs" pretty well, and it still goes and moves, but it has an occasional pop in the airbox. I have the low-range worked out pretty well, but the mid-high range is still way off.

SO, I pulled the covers off of the top of the carburetors and noticed that the right two are popping and the left two are not. It starts popping at about 1/4 throttle, gets the worst at around 1/2, then tapers off as the throttle increases.

The thing that's weird is that it's only the right two carburetors that are popping. What would cause this? The only thing that those two carburetors share in common is the muffler. What are my options here?

[Wrider]

It could be that the previous owner started a carb job and never finished it. What are the stock jet sizes and what are the jets that are in there? Are you running the stock airbox? Stock exhaust? And what altitude are you at? (If you're not sure just tell us a location.)

[guygs750]

I couldn't tell you what the jet sizes are. It has the ridiculous cam pull-arm 4 carb setup so it's a bear to check the needle setting. I had the whole carb rack off once to change the o-rings on the boots and never thought to crack the bowls off to even look inside. I was thinking the massive air leaks from the destroyed o-rings were the problem, but I guess hindsight is 20/20.

I did get a used muffler/pipe from the bone-yard and swapped it out. It was off of a GSX750, but it was almost identical. I bolted it up, got everything all sealed up, and took it for a test drive. It still popped a little bit around 4k rpm. I filled up the tank with 91 octane non-oxy and hit the freeway at 70 mph. I put about 15 miles on at 65-70 mph with only a slight pop maybe twice. Then I turned around and went back the other way. No popping (except on decel) all the way back. Even on the side roads at 30-35 mph and 3.5-4.5k rpm I got no popping out of it.

I'd hate to say anything too soon, but I think it's pretty close to "fixed". Maybe.