1978 GS400 dies under 2k rpm

[hoburger]

Howdy y'all, here's some background info. I recently bought a 1978 gs400 for my girlfriend (it's her first gs and my 4th), and it came running like junk because it sat for about a year or two. It also leaked gas through the overflow so I figured I'd do some carb work. I cleaned them pretty thoroughly, tightened a loose mainjet housing (which was the source of the leaking fuel) and put some 115 main jets (over the 112.5 stock) in them because it already had a tendency to run lean/pop and I had also done a small holesaw mod to the exhaust which probably leaned it out even more. After the cleaning and before the new main jets, it had some trouble starting and staying one. Idling was a nightmare, the acceleration hung, it popped like a mother, and it died easily after deceleration which all pointed to it being way too lean. After the new jets, it can now hold an idle and has normal throttle response again. However, the bike always dies under 2k rpm without fail. After it dies, it usually takes a good 3 or 4 kicks (and maybe some starting fluid) to get it going again. The carbs haven't really been synced, but they sound decent. I don't think a poor sync can be fully responsible for this issue. I checked my plugs, and they're still incredibly white; I also gave them a good sniff and they were still incredibly lean after the new rejetting. I'm thinking of going up a size for pilot jets to 17.5 (from the 15 stock I believe) because it's still popping a great deal on idle. In addition, I think I might go up another main jet size to 120 because the plugs are still reading lean overall. Might this fix my idling issue? I read that a super lean fuel/air mix can lead to poor idling. Anyone have any advice? Thank you for your time and attention, I look forward to hearing back from everyone.

 

[Guest]

You may have a vacuum leak. Have you tried spraying some carb cleaner at the intakes. If you do have leaks the idle will surge from the spray. The intake o-rings may be flat and leaking and the intake boots themselves may have cracks.

 

[whiterabbitt]

+1^
This is likely the source of your problem.

 

[hoburger]

The intake boots are in pretty good shape (surprisingly), and I've already checked for vacuum leaks by spraying starter fluid.

 

[Guest]

Have you tried the A/F mixture screws. Done the fast idle adjustment?

 

[hoburger]

Yes, I turned both a/f mixture screws to the point where the RPMs would stop rising, but they still wouldn't prevent the bike from dying under 2k rpm. To be honest, the a/f mixture screws didn't seem to do any good.

 

[hoburger]

UPDATE**

The stock pilot jet size is 22.5. I inspected the ones in my carbs which turned out to be only 17.5 which was way too lean. I changed it to a stock 22.5 and tweaked the fuel/air screws. In the process of reassembly, I also finely cleaned the carbs this time. She purrs like a champ now and doesn't even backfire. Good stuff.