Fuel vacumm diaphram 79 special

[rob carroll]

both petcocks and the vacumm diaphram were rebuilt with a kit. Is there a simple test to determine if they are operating as intended.

tired and dingy, Rob

[DiverRay]

put a pan under the fuel lines, turn the petcocks to "on" with fuel in the tank, and check that NO fuel will flow. Then turn to "reserve" and check for no flow once again. Turn to "OFF" and verify no fuel flow.
Now turn to "PRIME" and the secondary fuel lines SHOULD flow fuel. Turn to "ON" and the fuel should stop. Now put a vacuum pump on the line to the diaphram, and when you apply vacuum, fuel should flow. When you release the vacuum, the flow should stop.

[rob carroll]

diver dan

I performed the test you indicated. I detached the two fuel lines directly from the carb banks. However fuel flowed out from either side in the on, prime, and reserve positions. The petcock kit used by the mechanic contained parts for both petcocks and a new vacumm diaphram. Could this condition cause the engine to keep fouling plugs with too rich a mixture or is this just an issue of turning the petcocks off when not riding. Also I expected to see a hose full of gas streaming from the fuel lines in the prime position, but instead it ran out rather feebly in all three positions.

thanks ,Rob

[tarzan]

Even if the octy dont work right the float needle and seats should stop flow when not running.It sounds to me like you are leaking through the needles in the seats or if there is an oring under the seats it could leak there.Or if your floats are set too high.Either way you probably need to pull the carbs to inspect things.Good luck and keep at it.You will prevail.

[rob carroll]

I called the mechanic that charged me an arm and half a leg to build the petcocks, octy, change tires and clean carb. He agreed to inspect the carb if I bring it in. I pulled the carb and the octy for his inspection. How can I tell if my carbs are the ones listed as stock in clymers? Are there any helpful hints I can give this mechanic concerning inspection and adjustments for the 79 special carb? Should he follow the clymer instructions for laying the carbs on their backs for the float adjustment? He last set the float height by holding them in such a way to compensate for some movement. Does anyone know how many cc's of fuel should exist in the bowl with a correctly set float on this 79? He seems to know the amount for many bikes but not the 79 special.

Why is there an exsive under my name for the threads I generate? Does that mean I'm a pain in the ass?

I'll be leaving in half an hour for the shop. Would appreciate some advice.

Thanks ,Rob

[Ken Talbot]

DiverRay - he's got Special with an octopus, so the fuel flow at various petcock handle positions is all different.

[bikerphil]

79 spec. carbs

The carb type is a BS34-11, it is identified by the visible idle mixture screws and having 4 lines (2 fuel, 2 vent) running in between carbs 1+2 and 3+4. The later carbs have only 2 lines (fuel) running to the carbs, and the idle mixtures are concealed. Stock float height (brass floats) is 25.7 mm. +or- 1mm. Float adjustment should be done with the carbs upside down, not on an angle. HTHU

[boyat68]

rob carroll:

One of the problems with having a house mechanic do the carb job is that they won't take the extra time to get the floats adjusted correctly as it is time consuming and the cost for doing the job is up in the 300-400 range. Since the clearance of the floats forward is critical, i.e the arc swing causes the forward edge of the floats to bind against the forward side edge that is built out from the corners of the bowl, you have to bend the floats so they do not hit that triangler forward edge.
/ <-----> \
when they drop down. In most cases when the float bowls fill up the mechanics just tap on the bowl and this lets the binding float loose. As long as you are riding the bike the floats do not drop very far down because as soon as the fuel is used more flows in. When you park the bike and cut off the fuel, the remaining fuel in the bowl boils off and the floats drop further down and bind. Judicious bending of the floats and the back post tang that allows drop will cure this but it takes a lot of trial and error unless you have done it a thousand times.

[tarzan]

They shouldnt bind unless someone bent them I would think,but maybe the mechanic did I suppose.But it sounds like the mechanic didnt adjust them right.Also I thought I read that when he got the bike back that he had to reroute the hoses and also put filters on.And that he found rust or debrit from the tank.Maybe some junk got in the bowls before he added the filters and are keeping the needle from seating.JAT