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83 650 Fuel delivery - next logical step

  • Thread starter Thread starter Doug650L
  • Start date Start date
D

Doug650L

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Hi everyone. I haven't posted in a while because it has taken me a bit to get around to cleaning the carbs and replacing o-rings on the bike, including o-rings from carb to engine. My first time doing these things and thanks to everyone for previous help. Bike fires up when petcock is set to prime but will not stay running when petcock is in the "on" position. I did replace the vacuum line with clear flex plastic tube because original black tube was cracked at ends. I suspect the petcock needs replacing. Is there something else I may be missing?
 
Is the clear tube sucking itself flat and blocking off the vacuum?
Is it kinked? Did you hook the vacuum line up to the right port on the #2 carburetor?

Did the petcock work before?
 
tkent02 said:
Is the clear tube sucking itself flat and blocking off the vacuum?
Is it kinked? Did you hook the vacuum line up to the right port on the #2 carburetor?

Did the petcock work before?
Click to expand...

Petcock was temperamental before but do not know for sure it was working ok. I did observe the vacuum line while it ran and there were no kinks nor did it compress on itself (didn't think of that one and I can see where that would be an issue with certain tubes - I was not able to find a rubber replacement).
 
If it has been a LONG time since the petcock worked, it's possible that it's stuck.

There is an o-ring on the tip of the plunger in the center of the diaphragm that moves in and out of an orifice to control fuel flow.
If it is stuck in the closed position, no fuel will flow when vacuum is applied.

Some petcocks will use a spring to push on that plunger when the lever is turned to PRIme, others will simply use another set of holes for fuel flow.

You can take the petcock apart to verify proper operation.

.
 
Since it was recommended anyway I purchased a replacement fuel petcock and it fits nicely. Hooked everything back up and still have the same problem ? it runs fine in the prime position but not in the on position making me suspect that it still a vacuum issue. Do I need to re-examine the carb end of the vacuum hose to see if somehow there is a clog somewhere? Carb was disassembled and dipped already.
 
Remove fuel outlet hose at petcock; remove your clear vac hose at carb nipple and suck on it. In "ON" position, fuel should flow (catch it in cup)- stop sucking, fuel won't flow. Remember that you need to have enough fuel in tank for "ON" spot to find it- "PR" (and "RES" ) draws from very bottom
 
As Steve suggested, take apart the petcock for inspection to confirm proper working order. In all honesty, if it is not working perfectly I would buy a new one. Seeing as how it has, most likely, never been replaced or rebuilt. I have heard, and read, many times where rebuilt petcocks can fail at terrible times.

tom203 said:
Remove fuel outlet hose at petcock; remove your clear vac hose at carb nipple and suck on it. In "ON" position, fuel should flow (catch it in cup)- stop sucking, fuel won't flow. Remember that you need to have enough fuel in tank for "ON" spot to find it- "PR" (and "RES" ) draws from very bottom
Click to expand...

Hm will have to do that when I finally get my damn tank on the bike.
PS also going to put part of this in my sig if you don't mind tom... I have a dirty mind ;)
 
I confirmed the petcock works with suction when set to "on" position by sucking on the hose - fuel flowed only when there was suction. With the engine running, I confirmed there is vacuum/suction from the engine with my finger tip over the opening (hose is connected to port on engine side of 2nd carb from left - looking down while sitting on bike). Problem persists: engine stays running only when petcock is in prime position. I believe someone noted that fuel flow in prime postion is lower rate than when vacuum is working properly. Are there adjustments to the carb that could be causing maybe not enough suction? How do I measure the vacuum?
 
Strange, engine vacuum at idle should be better than your lungs ;get some small diameter wire and push it thru that vac fitting on #2 carb (second from left as you sit) to make sure it ain't blocked.
 
I'll do this and I'll also see if I get fuel flow in On position by priming the carbs then disconnecting the fuel line and have it empty into containers while vacuum tube is connected.
 
Doug650L said:
Are there adjustments to the carb that could be causing maybe not enough suction? How do I measure the vacuum?
Click to expand...

There is no adjustment on the vacuum port itself, it is just a hole bored straight through to the venturi after the slide.

Just stick a vacuum hose on the vacuum port and run the engine, you can feel the suction. There should be a lot of it.
 
Have you done a compression test , and or adjusted the valves, low compression , because of a bad valve adjustment can cause low vacuum
 
I have not yet done these things because they're probably beyond my skill level and may need to get an experienced mechanic help me out. Question: Is the principle of the vacuum actuated petcock such that the amount of fuel changes depending on how many RPMs are generated Or is the rate of fuel delivered constant no matter how many RPMs? Can I simply set the petcock to the prime setting when running the bike and turn it to the on setting when not running the bike until I figure this out or will this not deliver enough fuel under road conditions?
 
It's basically an open/shut valve. The petcock just serves to fill the float bowls, and the required fuel for combustion is then metered by the jets. Yes, you can run it on PRIME, but do NOT forget to turn it off, especially if you have concerns that your float needles are not sealing properly.

One other note, IIRC, the PRIME orifice on the barrel is a little smaller than the ON...this theoretically could be a problem at extended high speed/high rpm operation, but in practice you're probably not going to run into that.
 
Once the engine is running, vacuum will open the petcock, assuming it is working properly. You should not have to turn it to PRIme to get the engine to run. The rate of fuel flow is regulated by the floats in the carbs, not the petcock.

If you have vacuum from the port on #2 carb and the petcock flows if you apply vacuum, there is no reason why it should not provide enough fuel for the engine to run.

Where are you? One of us might be close enough to drop by and lend you a hand.

.
 
So .... Funny thing. It seems that if you intentionally kept your fuel tank low for this sort of trial and error testing and taking the tank off often, there may in fact not be enough fuel in the tank for the "prime" setting to function properly. Such is the case with my problem. I turned the petcock to "reserve" after reading other posts and it remains running. I had examined and disassembled my old petcock valve carefully and could not see how the reserve function worked, but someone had pointed out that on reserve, the fuel is drawn from the bottom of the tank. it simply never ocurred to me that there wasnt enough gas for the prime position. reading the manual, i believe there is maybe a gallon in reserve. So chalk that one up to inexperience. Thank you to those that helped educate me on the vacuum system. I'm moving on to other aspects of the performance with a new thread.
 
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