Revs off starting but then just idles.

[Keithatron]

I have a 78 gs750, the bike has a 4-1 exhaust and standard air filter, 15 pilot and 107.5 main. I recently got this bike back on the road and have travelled about 300 miles since. The bike is now behaving weirdly and i'm hoping someone can point me in the right direction to solve this. The battery hasn't had enough juice in it to use the electric starter however starting with the kicker has been fine. At the moment the bike will start from a kick and then i can rev the bike into the thousands no problem. The second i let off the throttle the idle drops to 1000 (lower than normal) and sounds off, if i then apply the throttle nothing happens. A slight change in sound, if i open the throttle fully the bike dies. This is so weird because if i catch the throttle after starting i can rev and rev the bike, but as soon as i let it idle the engine speed will not change.

I have checked that fuel is getting through from the tank, the plugs look to say the bike is running a little rich, but they have been that way for a while and this problem is new.
Any help would be appreciated. Thanks.

 

[Guest]

Don't you think solving your electrical problems might be a good first step in diagnosing your problems? The split rectifier, regulator system abandoned after '79 was terrible, worse than what came after (also pretty bad). So right off the bat you know there might be something you could fix that will effect the way the bike runs. I have a '78 GS1000E and '79 GS1000E (parts bike). Both electrical systems were overcharging, damaged the stators, burnt electrical connections and boiled out the battery before I discovered the problems. If you are committed to keeping the bike it would be a good time to replace the regulator, rectifier with an integrated Series R/R, and go through the rest of the electrical system doing required maintenance. It's likely the stator may require replacement. The problem might be in the carburetors, but clearing one problem at a time that you know of will eliminate a lot of tail chasing in the future.

 

[Keithatron]

I think the battery only needs a charge and i will do that soon. The bike has a modern R/R as the old one was toast when i bought the bike a few year ago. The charging system was working fine, the battery was flat because the bike had sat for a while whilst i was sorting out paper work for it. Although it crossed my mind that an undercharged battery may affect the bike, the behaviour doesn't seem to reflect this as the problem. Of course charging it will rule it out, but i was wondering if it sounded more like a fuel problem to someone or perhaps a faulty ignition system.

 

[Peteortiz11]

this may sound a bit strange and I'm sure i'll get blasted by the more experienced guys but my GS850 had a 4-1 exhaust, 15 pilot and 115 main jet when I purchased it long ago and it did the exact same thing. through experimentation, I learned the bike ran the absolute best with 20 pilot, 130 main jet and the stock breather. when I install K&N pods, 140 jet was the best. give it a shot keep us posted

 

[Keithatron]

thanks, i have had a 17.5 pilot and 115 main both of which didn't improve the engine response. The mid range needs improving which probably means moving the needles. Did you do anything else as this issue is new and doesn't seem related to the jetting as the bike had been working fine before the sudden break down.

 

[Peteortiz11]

What I noticed on my carbs was that I could not move (or notch up) the needles. simply through experimenting different jet sizes was I able to get the bike going. and boy! does it go!!

 

[Keithatron]

I appreciate the replies, but i think both are probably better suited to fixing the overall performance/reliability of the bike, as the bike has been running quite well up until now. I'm still trying to identify what might be the specific cause of this fault. if anyone has another angle for me to come at it from that would be great.

 

[dorkburger]

Personally, I would verify the charging is ok first, especially since it ran fine for 300 miles.
When my stator / R/R was dying, the bike would not idle, but once started would stay running if I held it at around 2k RPM. If I let off, it stalled. (This was 200 miles from home leading to an eventual tow truck ride).

If the coils aren't receiving proper voltage it won't run right. I've also had the boiled dry battery syndrome as well.

 

[Keithatron]

ok, thank you. i'll get on the battery and then report back.