Vacuum Petcock - running lean

[ghwrenchit]

Ever since taking delivery of my 1980 GS750E, the bike has allways ran way lean, except number 2, with a wet plug. After reading ton's of posts (thanks to you contributors) regarding carb's, petcocks, fuel systems, etc. I rebuild the carbs, proper float settings, new airbox boots. Upon further investigation I did find that the vacuum petcock diaphragm was leaking fuel into the number 2 intake, causing the wet plug. Until I had time to rip in, I thought I would just vacuum cap the intake and petcock ports and run in the prime position. No go!!! I tore this thing apart and based on the design of the petcock, I can't see a feasable way that gas can flow in the "prime" position without vacuum. There is a piston/seal and spring stopping the flow of fuel without any sort of vacuum, regardless of the position of the "Prime" screw. So I did a little cobble until the diaphragm comes in. I wiped the entire diaphragm clean with rubbing alcohol, coated the back side (vacuum side) with a fine coat of "SealAll" using a modeler's paint brush and let it dry. Put it back together and WaLa \\/ , holy cr@p :-D , the bike has never ran better!!!! Put a few miles on, pulled the plugs, and they are now the choice brown color.

Greg

[ddaum]

Innovation at it's best! Good job! Now get that new diaphragm!

[ghwrenchit]

The inquiring mind just can't let this one go....:? I'm baffled by the design. The only thing I can come up with is that a tool was used, AKA a syringe plugged into the screw hole where the prime/run screw is. Perhaps the screw when turned to prime blocks the port to the intake, allowing a vacuum to be drawn by a device plugged into the screw hole, such as pulling a syring out, attached with a hose. Any former 80's Suzuki Mechanics out their familiar with a factory procedure???? I've got it cobbed and working, I'm just curious about the mechanics......

Greg

[Footy.]

The inquiring mind just can't let this one go....:? I'm baffled by the design. The only thing I can come up with is that a tool was used, AKA a syringe plugged into the screw hole where the prime/run screw is. Perhaps the screw when turned to prime blocks the port to the intake, allowing a vacuum to be drawn by a device plugged into the screw hole, such as pulling a syring out, attached with a hose. Any former 80's Suzuki Mechanics out their familiar with a factory procedure???? I've got it cobbed and working, I'm just curious about the mechanics......

Greg

Hi Greg.

I have just modified the vacuum petcock on my 78 GS750 to manual operation, in a simple way that can easily be reversed back. I am doing this to see if the vacuum device was responsible for blueing the outer exhausts last season.

I think the basic design is the same on these Suzuki petcocks, so here's what i found. Sorry no pics!

The vacuum operated plunger seals one side of a hole that is bored "through" to the other side where the tap "disc" rotates. The outlet pipe that goes to the carbs, is taken from the centre of this drilling at 90 degrees.

Both on and reserve tap positions feed fuel into the vacuum side, so the plunger controls both these.

When in the prime position, a quarter circle groove in the tap disc connects fuel taken from the reserve drilling in the petcock, to the other side of this "through" drilling. So on and reserve, feed via the vacuum on one side, and the "direct" prime feed comes in from the other side.

My simple "manual" mod was this.

The through drilling is 6mm, so i made an aluminium plug that was a tight push fit in this drilling, by 5mm long. This was fitted in the prime (tap) side of the drilling with weak loctite. It has to fit just below the surface so as to not score the tap disc, and not be so long as to interfere with the 90 degree drilling that feed the carbs.

The alternative is to fill the quarter circle grove in the tap disc face with suitable filler and sand flat. But i went with the "plug" way because i can knock this out to revert to vacuum if i want later.

All you have to do then is to remove the back cover and the vacuum mechanism, and make a plain plate and gasket to replace this. I made one out of 6mm plate aluminium so i could use the original four screws.

Now the on and reserve positions remain the same, with the prime position now being "off".

All you need to do to return to standard, is replace the vacuum bits, and knock out the plug!

Footy.

[ghwrenchit]

Great post. I think this may be alittle different as the bike has a fuel gauge and no "reserve position" on the petcock. It only has "prime" and "Fuel". The only other thing I can think of is that when in "Prime" the amount of vacuum is greater causing the bowls to fill up quicker, as opposed to "fuel", which would reduce the vacuum and supply less flow during riding. Iv'e looked this thing over and over, and with large spring pushing up the diaphram and sealing the tank opening, I can't see any way that without vacuum this thing will flow fuel.

Greg

[Footy.]

Great post. I think this may be alittle different as the bike has a fuel gauge and no "reserve position" on the petcock. It only has "prime" and "Fuel". The only other thing I can think of is that when in "Prime" the amount of vacuum is greater causing the bowls to fill up quicker, as opposed to "fuel", which would reduce the vacuum and supply less flow during riding. Iv'e looked this thing over and over, and with large spring pushing up the diaphram and sealing the tank opening, I can't see any way that without vacuum this thing will flow fuel.

Greg

I dont think there would be any basic difference in the design of the "porting". In the petcocks with both "on" and reserve, the extra position just lines up another slot in the tap disc. But this is independent of the prime position porting.

The prime position feeds from the tap side, and has nothing at all to do with the vacuum mechanism. It is designed to bypass the vacuum side, and is a direct feed.

Regards.

Footy.