Misfire When Warm

[Andrewb.95]

Okay, so i’ve Been trying to get my 1977 GS750 running well before I park it for winter so I can start fresh in the spring. I’ve had an issue where the bike runs well for 10-15 min, then starts to run very poorly, misfiring and backfiring, can’t give it more than 1/4 throttle. This is Especially noticeable after riding for 15 min, turning the bike off, and starting it up again 20-40 min later.

So what I’ve done to try and fix this: Got rid of pods, and got new air filter, resealed airbox, new boots for airbox and carbs, had shop clean/rebuild carbs, had shop adjust valves, I replaced points (not condensers), retimed (static) ignition, replaced r&r with SH775 (because I had a charging issue and thought it could be related to this, getting 14.8v at 3k rpm now, and 13.5ish at idle. ), ignition coils were replaced by previous owner (generic eBay ones, shop said they were good but I haven’t tested yet), and tested stator (good). When I first got it I checked for spark and it was good, but I am thinking I might have some sort of heat soak issue? What is a valid check for pet cock? And should my fuel line have a hose clamp on the side that connects to the t shaped thing on the carb?

I have searched the forums extensively and that’s how I’ve found out about everything that I have checked since I didnt know anything about vintage bikes before I got this. i am starting to run out of things to check. Just looking to see if anyone has had similar issues and if they could point me in a more clear direction because I feel like I expanded my search so wide that I can’t really tell what I’m looking for anymore.

 

[Big T]

New condensors, or better yet, a Dyna S

 

[Rob S.]

Get a shop manual.

Put your year and model in your signature.

Some resident experts will chime in within 24 hours with instructions/suggestions. Good luck.

 

[LAB3]

Sounds like an electrical issue to me and condensers are meant to be replaced from time to time.

 

[Grimly]

Condensers would be the prime suspect, if they're old and original. The symptoms are typical of one going bad, leading to an odd misfire that gradually gets worse, eventually causing a hard start conditon.
Sometimes they go straight to failure, which can be preferable, because it forces you to look at them and toss them in the bin instead of hoping they're ok and trying to avoid spending coin on new ones.
There are thousands of bikes laid up over the years because of mysterious running faults - some of them caused by duff condensers, I've no doubt. When the GS range passed into the cheap-as-chips old bike bracket, many owners didn't want to spend the sometimes ridiculous cost of OEM replacements, and the bike got put up the back of the garage or garden and accumulated round-tuits that passed into years of neglect.

 

[tom203]

Probably condensers...but measure the primary ohms on one of your replacement ignition coils. Originals were likely 5 ohm, while many aftermarket ones are 4 ohm or less - the lower ohm units stress the point/condenser more.