cold bike starts, hot bike doesn't

[bimmerboy750]

After searching anonymously and fixing some issues, I'm now stuck and need help on two things.

My bike 1981 GS550TX, no pods

Starts up fine when cold, cranks but doesn't fire up when hot. starter cranks the bike fine when hot or cold.

Idles fine at 2000 RPM, won't idle below 2000 RPM; quickly dies out, even if I twist the throttle. in fact twisting, cuts off the motor even quicker. above a 2000 rpm idle, a twist will have it rev quickly and smoothly.

carbs were dipped per this site's instructions
stator, R/R, and battery checked per this site's flowchart
plugs were cleaned and gapped
coils were swapped with another set
petcock checked
checked for air leaks too


Thanks, this forum is great!

Rick
81 Gs 550 TX

[nabrams]

All your symptoms point to one thing: Too rich idle circuits.
The evidence is as follows:
1. The bike won't idle below 2000 RPM.
2. When bike is fully warmed, shut off and then try to restart - no luck. This is consistent because when the bike is cold it can handle the super rich mixture at zero throttle. A warmed up bike will naturally run richer (which is why you need choke to start a cold bike, but not a warm bike), so the mixture is getting WAY too rich (at zero throttle) once the bike is hot.

So what do your plugs look like if you keep the bike at 2K RPM (AFTER fully warmed up) for 10 minutes. If they're black, then you've found the problem - you need to lean the mixture for low RPM settings. The mixture screws will be useful here. Also (and perhaps more importantly), make sure you have the right size idle jet!!

[bimmerboy750]

I'm not at the bike right now. I'll warm up the bike and then check the plugs.

Which way to turn the mix screws if it's too rich? Out or in?

Thanks,

Rick
80 GS 550TX

[DimitriT]

If it runs better cold than hot then you are too rich on idle. Check the plugs for color to and verify that your air filter is clean and the airbox isn't obstructed. What's the history on the carbs? Any rejetting work done in the past? Your carbs should have #92.5 idle jets. I'd verify that. Other than that I would check the idle fine tuning mixture screws and make sure they are not too many turns out.

[bimmerboy750]

DimitriT, vbmenu_register("postmenu_655888", true);
I'm the 2nd owner. bought the bike in 89. used it on and off all these years. CV carbs haven't been rejetted. airbox is clear and filter is clean.
From your last sentence mix screws in equals leaner, mix screws out equals richer.

thanks,

Rick
81 GS550TX

[chef1366]

Old igniters can be heat sensitive. Also bad connections. Are you getting spark when hot? If you are I'd agree with the rich pilot circuit. Did you replace the o-rings with the carb dip?
CV carbs idle mixture screw Tighten=lean Loosen=rich.
I use the highest rpm method on these screws. Tighten one down until the bike almost dies and take it out until you hear the smoothest and highest rpm. Repeat on all.

[bimmerboy750]

I spent some time with it last night, but didn't use chef's procedure. i'll give it another try this morning chef's way. i had some success last night, and even got it to settle down to 1100, but it wasn't smooth at all.

[chef1366]

Synchronize those carbs.

[bimmerboy750]

got it sorted out

Well, I got it sorted out. It seems that my fuel mix screws needed to be at 3.5 turns out. Also my idle screw was way off. The plates were opened up too much.

Now onto a potential R/R issue.

Thanks folks!

Rick
81 550 TX

[Dave8338]

Now onto a potential R/R issue.

Rick
81 550 TX

Clean all the connections first...then retest and replace if needed.