1979 GS750 running problems

[tomo]

I know this is not the section for tech questions, but not sure whether problem is fuel, electrical or something else. So, bike had been running fine but one day began to fluff, lose power and cut out. Would not start up again, until about 20 minutes later. Set off again only for the same thing to happen a few miles further on.

I suspected fuel issues, as bike did not cut out straight away, but after reading a few posts on here and talking to a couple of mechanics, started to suspect a dodgy coil or maybe condenser. Decided to change the coils, plugs, points and condensers. First test ride bike ran great with no issues, but next morning garage stank of petrol and sure enough puddle of it under the bike. Took tank off and petrol poured out the petcock when I disconnected the fuel pipe. Think I mentioned all this in previous posts. Anyway, bought a re-build kit which seemed to do the job, but after a couple of rides out (bike ran ok) started to leak again. So, fitted an aftermarket fuel tap, unfortunately as I have read on here before, only taps out there have the fuel outlet coming out the back which makes for tricky fuel pipe fitting.

So, with new coils, plugs, points, condensers and petcock, went out for a test ride. Bike seemed fine for about 10 minutes when started to fluff and lose power again, picked up if I twisted the throttle hard. Seemed fine at higher speeds, but fluffed at lower speeds. Eventually cut out again. Would`,t start up again, until I switched the petcock to prime position, then it fired up and I was able to ride the short distance back home.

Needless to say, feeling pretty pissed off just now and puzzled. Wondered whether the fuel flow might be restricted as pipe had to bend quite tightly to connect with that petcock, but looks ok and does`nt appear kinked. Any thoughts/suggestions appreciated.

 

[dgates1]

With that many times of a leaking petcock, did you check to see if you ended up with fuel in the oil??

 

[tomo]

Yeah, that did occur to me and think another guy mentioned that in a previous post about the petcock. Think I got away with it, no change in level and no smell of fuel when the oil filler cap removed. But will change oil anyway as it is due an oil change. Thanks for the input though, much appreciated.

 

[allojohn]

Can't happen if you're on level ground. VM carbs flood out the overflow tube first and if the tubes are blocked they will flood out the back into the airbox......

 

[chuck hahn]

When you took the line off the petcock what position was the lever in? You only use PRIme to refill carbs if it has sat a few weeks, Maybe a minute then put it back to ON.

 

[crazyramen]

Did you go with a cheap aftermarket petcock? The $17 one I got had some gunk in it when I bought it. Maybe your "on" position got clogged? It'd be work taking it apart to check IMO. I'd also be nervous that stuff could've gotten into the carbs and while it sounds like your carbs aren't your issue it couldn't hurt to clean them out.

I've got a '79 also, and I found that my fuel line doesn't have to bend very sharply with the one I got and I'm running the standard parts store 5/16th hose. Is this what yours looks like? https://ebay.us/m/SGoWBT

 

[lizzywhite45]

Hey man, sounds like a classic fuel flow issue. If it runs on PRI but not ON, the petcock might not be flowing enough in ON. Try a longer fuel line with less bend just to test it.

Also check if your gas cap is venting. If not, it can create vacuum and stop fuel flow. Next time it dies, open the cap and see if it starts.

Good call on the oil change. You're close. Keep going.

 

[tomo]

Thanks for all the advice/suggestions, much appreciated. Chuck, the petcock was defo in the run position, not on prime. I have taken the tank off many times over the years, so that is why it caught me by surprise when fuel flowed out the petcock. Crazyramen, I did get a cheapish petcock, not quite the same as yours but similar. On mine, the vacuum port is on the left and the fuel outlet on the right, but it points up at a 45 deg angle. I did check the cap vent and seems clear. Wondering if the bend in the fuel pipe may be restricting flow, so need to check that out. I do have a couple of old tanks with petcocks in my shed, did wonder about taking a punt and fitting one of those. Guess the rubber parts in those may be goosed too, but maybe worth a try.

 

[crazyramen]

Thanks for all the advice/suggestions, much appreciated. Chuck, the petcock was defo in the run position, not on prime. I have taken the tank off many times over the years, so that is why it caught me by surprise when fuel flowed out the petcock. Crazyramen, I did get a cheapish petcock, not quite the same as yours but similar. On mine, the vacuum port is on the left and the fuel outlet on the right, but it points up at a 45 deg angle. I did check the cap vent and seems clear. Wondering if the bend in the fuel pipe may be restricting flow, so need to check that out. I do have a couple of old tanks with petcocks in my shed, did wonder about taking a punt and fitting one of those. Guess the rubber parts in those may be goosed too, but maybe worth a try.

For a bench test you can hook up a longer fuel hose to drain into a clear cup and attach a syringe to the vacuum port. When you pull vacuum with the syringe it should flow just as quickly as on prime and stop when you disconnect the syringe (personally I don't ever push the stopped back in because I'm worried about busting the diaphragm in my cheap petcock.) You can also hook it up to your bike from the bench (as long as the tank is above the carbs) and test run it for a while. That was the setup I used to sync my carbs and it worked just fine.

If my description isn't great, lmk and I can post some pics.