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Author Topic: 1981 GS750E (GSX750E) Rat Cafe Racer  (Read 5545 times)

Offline LKS

  • Posts: 61
Re: 1981 GS750E (GSX750E) Rat Cafe Racer
« Reply #60 on: Jun 09, 2019, 16:45:43 »
Short lived happiness..

The two carburettors on the left side of the bike are leaking a lot and there was also a leak between the two on the right. The small leaks between the outermost carburettors was fixed by replacing the O-rings on the connecting tubes. I changed theese on both sides. Se pictures below:

IMG_20190609_125606 by Lars Krogh-Stea, on Flickr

IMG_20190609_125534 by Lars Krogh-Stea, on Flickr

I connected a one liter gas tank to the carbs to see what was happening. The gas is pouring out of the two holes on the next picture:

IMG_20190609_125512 by Lars Krogh-Stea, on Flickr

Nothing comes out of the overflow tubes. Removed the floatbowl on the leftmost carb and the float is moving unrestricted, the needle seems fine too. The float hight might be off but I checked this when I cleaned them. I might have done the measurements wrong. I sodablasted the carbs so there is also the possibility that some passage is clogged. Does anyone know what might cause gas flowing out from the holes on the intake side? Any tips on how to go about fixing this? Is it better to measure float hight with a ruler or check with gas and clear tube and measure on the outside? These are BS34s instead of the original BS32, but they are rejetted and worked well last time I ran the bike three years ago.
 





Offline LKS

  • Posts: 61
Re: 1981 GS750E (GSX750E) Rat Cafe Racer
« Reply #61 on: Jun 09, 2019, 18:45:02 »
Found a good rebuild tutorial online:

https://thexscafedotcom.files.wordpress.com/2014/06/mikuni_bs-cv_carburetor_rebuild_tutorial.pdf

2019-06-09_11-33-11 by Lars Krogh-Stea, on Flickr

2019-06-09_11-33-00 by Lars Krogh-Stea, on Flickr

Stupid as I am, I didn't bring the carbs home with me. I'll make some tube adaptors for the drainholes while I wait for my next wrenchday..
« Last Edit: Jun 09, 2019, 19:38:25 by LKS »

Offline LKS

  • Posts: 61
Re: 1981 GS750E (GSX750E) Rat Cafe Racer
« Reply #62 on: Jun 09, 2019, 19:58:04 »
I'm learning a few things about carburettors today..

The hoses I thought were overflow-circuits are really just for venting. The reason for connecting hoses to them is so gas will flow away from the engine in case of a crash where the bike is left lying on the side. Makes more sense since they're located so high up.

The leftmost hole on the intake side is the real vent and is where gas will overflow. I guess that points to the float/needle.

2019-06-10_12-50-50 by Lars Krogh-Stea, on Flickr


Offline LKS

  • Posts: 61
Re: 1981 GS750E (GSX750E) Rat Cafe Racer
« Reply #63 on: Jun 09, 2019, 20:22:05 »
I've also read page 4-5 and onwards in the workshop manual for an explanation on why gas flows from both holes, so I'm now confident I have it narrowed down to wrong float setting or some obstruction or similar problem connected to the needle not making a good enough seal to stop the gas from flowing.

Offline teazer

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Re: 1981 GS750E (GSX750E) Rat Cafe Racer
« Reply #64 on: Jun 10, 2019, 00:36:01 »
The two holes in the intake on a BS carb are the air jets for the pilot circuit and the main circuit.  if the floats don't close the fuel valve (float valve/needle and seat) fuel will rise until it pours out of those orifices.

In this case it is most likely that some of your valves are sticking and not closing.

Offline LKS

  • Posts: 61
Re: 1981 GS750E (GSX750E) Rat Cafe Racer
« Reply #65 on: Jun 10, 2019, 06:46:37 »
The two holes in the intake on a BS carb are the air jets for the pilot circuit and the main circuit.  if the floats don't close the fuel valve (float valve/needle and seat) fuel will rise until it pours out of those orifices.

In this case it is most likely that some of your valves are sticking and not closing.

Thanks, I'll check it out. I didn't remove the floats when cleaning the carbs as I knew they worked before. Probably got some soda powder in there when cleaning.

Offline LKS

  • Posts: 61
Re: 1981 GS750E (GSX750E) Rat Cafe Racer
« Reply #66 on: Jun 10, 2019, 11:23:42 »

This is a picture of how my bike was assembled. Some kind of thick radiator hose sits loosely over the intake boot and carb opening.   

48036867868_135740661c_o by Lars Krogh-Stea, on Flickr

Next picture shows my brothers bike (same model and year). This is an airtight assembly:

riktig_muffe by Lars Krogh-Stea, on Flickr

My bike is sucking in lots of unfiltered air and at the same time the intake plenum is not alowed to do its job as it's supposed to. Next picture shows one of the intake boots. It's inside diameter is about 45mm and on the carb the outside diameter is about 53mm. Not an optimal solution..

feil_muffe by Lars Krogh-Stea, on Flickr

 They also don't make a particularly tight connection with the holes in the plenumbox. This is also why gas poured over my engine.. If everything was done correctly, excess fluid would flow out the boots and down the draintube in the bottom of the plenum. One less firehazard to worry about when this is corrected :) Guess the bike will run smoother as well.