I've noticed alot of people having problems with the frame mounted petcock and being told to run in PRI and not worry about it. Here is the problem with that. If the diaphragm in the petcock is cracked you have a vacuum leak that will cause the left cylinder to lean out. if you are running in PRI because of a diaphragm problem you also need to plug the vacuum line from the left hand carb. switching frum ON/RES to PRI does not shut off the vacuum; it only redirects the fuel through the petcock. So if you are running in PRI and the bike has lost some power/throttle response plug the vacuum line or replace the diaphragm/petcock.
is there any way to check this damn diaphragm??
if you have a vacuum tester you can hook it to the petcock and apply vacuum. the reading should be steady and not drop. also if you disconnect the vacuum hose from the left side carb and suck on it (lol suck the hose). you should not be able to suck air threw it. should feel like you are sucking threw a straw with 1 end plugged. you should get a little air then it should stop.
perhaps i can clarify for you Kt, there are 2 issues here dealing with the frame petcock.
1. In the higher PRM's the vacuum pulse is short and weak compared to the lower RPMs, the diaphragm valve doesnt stay open very well, letting less fuel pass into the carbs and in turn getting sucked dry. Running in prime prevents that.
2. When the diaphragms get old, they may suck air a little, but ive actually found what happens is fuel starts to bypass the diaphragm through the little fissures and holes and get sucked right into the left-hand carb, making the left side super rich at idle. And cuz the diaphragm is leaking it again doesnt get opened enough to allow enough fuel to flow into the bowls for the higher RPM ranges.
I now have the vacuum plugged and just use the frame petcock as a on/off valve for the fuel. Pri works well enough.
I understand what the problems are, but also any un-metered air or fuel entering the engine will cause poor performance. switching the petcock to pri does not stop the un-metered air/fuel from entering. if someone is going to run in pri due to a faulty petcock, the vacuum hose needs to be plugged
or you could just run the two hoses from the tank down to a T fitting and run a short line to the carbs. just put a inline valve on the res line so when you run out of gas you just open it up. works great. and you don't have vacuum problems.
I suspect the petcock as my problems recently. It was running fine after rejetting but after 2000 miles it started starving for fuel when I tried to maintain a speed of 55mph for any extended period. A quick turn of the petcock to the prime position and let up off the throttle for just a moment and it seems as if the fuel refills the carbs and its ok after that. But now it seems to have a really weak feeling while cruising highway speeds. I'm gonna plug the vaccum line and see if it helps. Mine is a 96 that sat a lot of years and think the diaphragm is either cracked or lost its "flex". :dunno:
if the diaphram is cracked you may be sucking air (or fuel) ad fouling up the whole works. let me know if plugging the line makes a difference
Well now, interesting. Looked at the clear vaccum line I installed at time of rejet and found it full of fuel!!!! Diaphragm is definitely blown- cracked.
I just plugged the line & it seems to run pretty good so far.
When I did the rejet I had done the "suck" test on the line checking the diaphragm at the time and it did run for 2000 miles ok but due to its age its obviously toast. I do not want to run it without the reserve option. I've not heard of anyone finding a kit for it so I think before I replace it I'll try making a diaphragm. I have some kits for rebuilding propane vaporizors on lift trucks. Plenty of thin diaphragm material there.
Last Sunday I rode it to Cedar Point up in Sandusky. 380 miles on the bike that day. Its a wonder It didn't fill the crankcase with fuel.
I'm having a lot of trouble with little brothers fuel chicken..
Who (if anyone) makes a really good rebuild kit?