Only runs when fuel is on Prime

[Izzard Drake]

Hello there, I been on the site a while but first time posting.

I have a duh GS, 750E 1978.
I noticed a problem whilst riding where I would loose power.
Got it home and thought carbs might be an issue so took them apart and cleaned them up along with air filter.
No joy once done.
Then noticed leak on Oil sump plug. Fixed all that, cleaned all filters and gauzes (new oil filter and O ring) New oil too.
No joy.
Noticed a leak on the fuel tap, took it apart and cleaned out some crap from the o ring (I had done this 3 months ago aswell). Checked the Diaphram and that seemed to be doing its job. Put the thing back on and there the leak has stopped, but still had problem with performance.
I now have noticed the engine runs fine when the fuel is on prime, but when turned to 'On' it back fires / mis fires / stalls when I accelerate.
The only thing I can think of that makes sense is the carbs are the fault, that they can't open the diaphram.
Maybe points / condenser?
The spark plugs are sooty black apart from one that is wet aswell. (they are new plugs).

Sorry for the lengthy post but im at a loss and thought you might need all relevant info.
Hope someone here may be able to help,

Cheers, Izzy

[GS750]

if it runs OK on the PRIME position then it HAS to be to do with your fuel tap, or the vacume source.....

Go back to what you said in your post and think about it

It cant be anything else because you said:

I now have noticed the engine runs fine when the fuel is on prime, but when turned to 'On' it back fires / mis fires / stalls when I accelerate.

[Izzard Drake]

To my knowledge everything on the tap works fine.

I thought there's still something wrong as even on PRIME, despite sparking on all 4 in only seems to work on 2

[GS750]

Yes, I was going off what you said that it ran ok when on PRI....

Check the points and condensor but the best thing you can do is to ditch them and get an electronic ignition setup!
You will rarely have to touch the timing again!

the plugs willl stay wet if there is no spark, carbon builds up (as dry sooty look) when the plug fails to fire correctly and burn properly, usually from Air/fuel mixture being too rich or your choke stuck on. Sometimes it can also be from an ignition problem.

[Octain]

Ya need a new fuel valve.

[chuck hahn]

Youn said you had to clean crap off the oring a few months ago.. that tells me that something up stream of the actual lever is polluted. I think i would drain the tank, pull the petcock. There is supposed to be a periscope looking filter installed in the petcock. Is it there? Is it clean? does it have any tears or holes in it? Does the inside of the tank look rusty? any flakes of rust will idicate that the tank will most likely need a very very thorough cleaning and then sealed with the POR 15. If stuff is getting in the petcock you can bet a fat paycheck its also getting to the carbs. Back up from the tank down and clean the enitre fuel system again.. and yup that means the carbs again to be sure little rust particles havent made it down there and clogged the tiny holes again..Sorry to tell you all this, but thats just b how i see it. You either go from gas cap to carbs all the way or keep having performance and tuning problems..

[chuck hahn]

Todd..me and you had to learn the hard way about how the full fuel system has to be super clean too.. I hope he takes the advise and does it all..

[Izzard Drake]

Ive cleaned all the petrol tap and filter today.
Carbs were done last week.
I spose it would be best to go from start to finish. Its a massive ball ache
And ive only just got the airbox back on after 2 days of trying, there not on all the way either. Scrappin the airbox would be the 1st but i want to keep it origonal.
I was worried about the long task i would have to do. but im the kind guy that would do all these things and it would have turned out to be a simple 2 min thing that i missed

[chuck hahn]

Sooty black plugs and one is wet..I also suggest that once you have cleaned the ENTIRE fuel system again, that you go to my recent thread entitled..Carb Mecanic Wanted.. I just last week went thru a lot of the same iisues We found out the carbs had the wrong main jets, the wrong pilot jets, the guy that rebuilt them had the pilot screws and air screw all the wrond settings, and i can go on.. Read the thread and i think you will have plenty of a refrence point to look at, The only thing that may be different is the jet sizes you will need. That will depend on if you have stock exhaust or are running a 4 into 1 pipe. If they are stock pipes then the mentioned 110 mains and the #15 pilots woule be the correct sizes. If you are running a 4 1nto 1 then im sure someone will be able to properly direct you on jetting sizes...hope i have been of some help.

[Izzard Drake]

Got the Harris 4-1

[chuck hahn]

Heres what im findn the best as far as wrestling the airbox.. Put the carbs in first and tighten all the clamps and hookm up the throttle cables. The put in the airbox.. Put it in from the right side of the bike.Youll jjust have to wiggle it a bit, but its intended to go in from the right side..trust me here. Then get the boots lined up and wiggle them over the carbs throats. Some may dig in and snag up on the throats. Just use a flat tip screwdriver go around from top to bottomm and get the catchy spots undone and get the boots all the way onto the carbs and tighten the clamps..Then put in the 2 airbox mounting bolts that got in the frame at the top of the box. Thing is to do this in the steps the did it at the factory. And if you look at the engine mount youll see the ones on the left of the frame are welded on and the ones on the right bolt on. They put everything into the frame from the right at assembly..Hope thats aof help also.

[chuck hahn]

harris 4 into 1s. Are they VM26 carbs?

[Izzard Drake]

Getting the airbox in isnt the problem is getting the 'pipes' over the carb throats.
I don't know what kind of carbs they are. They have the same appearance as origonals but I have no clue about the insides.

[chuck hahn]

Well as i said..you take a flat tip and youll have to circle the bike and peek up underneath to see where the boots are snagging and pick the puckers out so they will slip over the carbs. Look all around each carb and pick the hanging up places loose so the boots will slip over. Also to ID the carbs will help the guys get you up to speed as well. Not many answers are availbale if the carbs arent properly IDed. I can tell you what i know from my learning on my bikes as far as the fact is that my bikes have stock VM 26s.. I can ASSUME that yours are stock 26s as well, but that may or may not be the case. I took a set of VM 29s off a parts bike that the guy said was all stock..Seems i have given you many things to check and verify for now.. I will stop and not overload you. Best to do the fuel system cleaning things and ID the carbs positvely. You know where to measure to be sure of the diameter.right?

[DanTheMan]

Get rid of the inline fuel filter. It alone can cause the running problems, fuel won't flow as well with its restriction there. The fuel is filtered twice without it.