Fuel flow restriction?

bavarian · Post by **bavarian** » Sun Aug 20, 2006 9:09 pm

I have completed the carb coversion to Mikunis on my '88 750 Paso, but always the same problem plagues me. The floats won't shut off the fuel flowing into the carbs once they are full, that is with the fuel tank return line connected to the tank. Everything works fine if I route the return line to a gas can instead of the tank. I used an air compressor and blew out the line that is inside the tank and I even ran wire through it. I can hear gas returning to the tank but once again the floats apparently can't overcome the pressure. I even replaced one of the float assemblies at a rediculous cost of $50 to no avail. I also have a fuel pressure regulator installed (and turned all the way down to .5PSI)on the line coming from the Y to the carbs. What am I doing wrong/missing here?

Gwenadu · Post by **Gwenadu** » Mon Aug 21, 2006 11:08 am

Nick,
your return rubber pipe is it connected to the good pipe of the tank? One of these pipes is blind...

Harigeharry · Post by **Harigeharry** » Mon Aug 21, 2006 8:16 pm

I use the rear pipeconnection. No problem with overflow yet!

bavarian · Post by **bavarian** » Tue Aug 22, 2006 3:50 am

I have mine connected to the rear pipe as well. I just don't understand why nobody else has had this problem. I am seriously debating just putting the old weber back on and forget this stupid mikuni conversion all together. I have not ridden the bike this year!

Gwenadu · Post by **Gwenadu** » Tue Aug 22, 2006 9:09 am

how is connected your Y? the pump is it connected on its "foot"?

bavarian · Post by **bavarian** » Tue Aug 22, 2006 3:00 pm

The way I have it plumbed now is: the fuel line comes out of the fuel tap, goes to a filter, then the pump, then about four or five inches of hose and then it y's off, one leg goes about a half inch then is connected to a fuel pressure regulator, then another half inch then it goes to the fuel inlet of the carbs, the other leg of the y goes to the rear spigot of the tank.

Gwenadu · Post by **Gwenadu** » Tue Aug 22, 2006 3:59 pm

when you test your bike with the return hose to another tank, does it work fine with this 2nd tank at the level of the bike tank? (remove your regulator... simplest the best (?)).

Harigeharry · Post by **Harigeharry** » Tue Aug 22, 2006 4:06 pm

bavarian wrote:The way I have it plumbed now is: the fuel line comes out of the fuel tap, goes to a filter, then the pump, then about four or five inches of hose and then it y's off, one leg goes about a half inch then is connected to a fuel pressure regulator, then another half inch then it goes to the fuel inlet of the carbs, the other leg of the y goes to the rear spigot of the tank.

I have the same configuration, exept of the feul pressure regulator. I don't have this.

In my thread (finally started with the rebuild, you see in one of the last pictures we made a test with a fuelcontainer. We could clearly see the fuel going back into the container instead of the carburettor. Why not try to make a similar test situation to check out what happens with your fuel lines???

bavarian · Post by **bavarian** » Tue Aug 22, 2006 4:37 pm

The test I did with the return line I did with the gas can sitting on the ground. The problem seems to be the pressure required to send the excess pressure back into the tank is more than the floats can handle. That is why I'm trying to figure out the specs for float height and also how to measure the height.

Gwenadu · Post by **Gwenadu** » Tue Aug 22, 2006 6:46 pm

the level of the gas in the carb must be 7.9mm (+/-0.5mm). i don't remember were are the mark on the carbs. you have to connect a clear hose on the draining pipe of the float place and you will see the level in this hose...

bavarian · Post by **bavarian** » Tue Aug 22, 2006 8:37 pm

I don't really understand how exactly to connect this clear rubber hose you speak of?

ducapaso · Post by **ducapaso** » Wed Aug 23, 2006 8:19 am

Maybe I'm not so good in english translation.
Did you put the fuel return connected to the second pipe in the tank? (I mean the first one is the fuel outlet)
This will not work. You need a second Y connector and put it before the fuel pump (better before the filter).
The second pipe in the tank is an iron pipe starting from the top end inside the tank and ending about 2" out and it only have to let gas and fuel vapour in and out the tank. If you use it to make fuel return in the tank, you'll face two different problems: 1)you need too mutch perssure to push the fuel so high. 2) gas and vapours will make a vacuum or an excessive pressure inside the tank.

Harigeharry · Post by **Harigeharry** » Wed Aug 23, 2006 8:56 am

ducapaso wrote:Maybe I'm not so good in english translation.
Did you put the fuel return connected to the second pipe in the tank? (I mean the first one is the fuel outlet)
This will not work. You need a second Y connector and put it before the fuel pump (better before the filter).
The second pipe in the tank is an iron pipe starting from the top end inside the tank and ending about 2" out and it only have to let gas and fuel vapour in and out the tank. If you use it to make fuel return in the tank, you'll face two different problems: 1)you need too mutch perssure to push the fuel so high. 2) gas and vapours will make a vacuum or an excessive pressure inside the tank.

You are right!
Fool that I am

The backline is the one that runs down to the reartyre.
The firstline is the returnline.
But mine Y-connection is behind the pump in front of the craburettor. So excessive fuel retuns to the tank.

Gwenadu · Post by **Gwenadu** » Wed Aug 23, 2006 8:41 pm

ducapaso wrote:Maybe I'm not so good in english translation.
Did you put the fuel return connected to the second pipe in the tank? (I mean the first one is the fuel outlet)
This will not work. You need a second Y connector and put it before the fuel pump (better before the filter).
The second pipe in the tank is an iron pipe starting from the top end inside the tank and ending about 2" out and it only have to let gas and fuel vapour in and out the tank. If you use it to make fuel return in the tank, you'll face two different problems: 1)you need too mutch perssure to push the fuel so high. 2) gas and vapours will make a vacuum or an excessive pressure inside the tank.

Nicola,
when I bought my paso, the return hose was connected to the tank at this rear pipe. I think it is the original place . A friend of mine have also a P750: same connection.I think that, like this, the pressure at the carb is more regular -does not depend of the level of fuel in the tank...
Nick,
To control the level of the carb, open the plug under the carb and you will see where the gas drops... connect tour clear hose there ... the plug screw is on your left-hand side if you look at the carb from the front of the bike.
Alain.

ducapaso · Post by **ducapaso** » Sat Aug 26, 2006 8:11 am

I don't think you're right, Alan. See tav 26 and 27 in spare parts catalogue for further information. There you will see the pipeline from the fuel lock (#21), both the Y union (#24) the original filter (#22) and the original fuel pump (#27). It doesn't matter the kind of carb you have, this plant is almost good.
Actually I put a pressure regulator between the Y union instead of the jet(#26) that opens at 0.3 bars. This it to make vapour lock flow by troght the carb after a long period of parking