vf500f fuel petcock issue

I just got a clean 85 vf500f but it doesn't run but I've narrowed it down to a fuel supply problem. While searching this topic I ran into this site and a discussion about tossing the fuel pump and going grav feed instead and it sounds like a good idea mainly because it simplifies things. I saw the site that shows what mods need to be done to the petcock vacuum diaphragms but my question is do these mods have to be done in order to make it grav feed? The reason I ask is because if the diaphragms are left intact and the vacuum line still connected then once the engine starts it will supply the vacuum the petcock needs to open anyhow with the only diff being the fuel line itself is going straight to the carbs rather than the pump. Does modifying the petcock diaphragms help improve the flow of the petcock?

 

From my understanding, the petcock mod makes the fuel flow more reliable. At this point, do you know if it's the pump or the petcock causing the problem? Personally, I say elimate the vacuum diaphram in the petcock to make sure that's not the problem. From there, make sure it gravity feeds to the pump. If that's ok, try cranking it over with the pump outlet disconnected and see if it pumps fuel. If not, go straight to the carbs (through a filter) from the petcock. My bike had a filter restriction, and it would run fine for a while, then it would starve for fuel. I did the petcock diaphram modification, then ended up taking the filter out of the line, and putting the pump back in, just because it's there.

 

THAT petcdock mod is NOT ADVISABLE because certain carb problems could cause one or more cylinders and the crankcase to fill with fuel while the bike is sitting(on gravity feed without fuel pump)......UNLESS you remember to always set selector knob to OFF after riding.

Gravity feed + petcock mod= bad idea

 

As it stands now I don't know what the problem is other than the fact that it has something to do with fuel delivery and that could be a bad petcock, pump, filter and or the carbs need to come off for a thorough inspection/cleaning. The reason I know it's a fuel related problem rather than electrical as in ignition related is because I removed the air filter box and shot gas directly into the carbs, hit the starter and it fired up albeit only for a few seconds. If it was electrical it wouldn't have fired at all. The fact that it did fire up was a huge relief to me because I had no clue what was wrong with it when I purchased it and was crossing my fingers that it wasn't a major internal problem. I do have a factory service manual so I'm in the process of making sure everything is doing what it's supposed to be doing.

As far as the petcock mods being a bad idea due to the fact that carb float can stick and flood the cyls due to a petcock left on while the engine is off is no surprise to me. I have a 71 cb350 and cb175 as well as a 75 cb550-4 all of which have plain jane grav fed petcocks so shutting off the petcock when the bike is shut off is already a standard routine. Thanks to all for the input.

 

1 thing for sure is that this baby has sat too long because gas in carbs were starting to varnish. I've seen a whole lot worse but in this case it was just enough to freeze the slides. I bought the bike at an auction and the auctioneer said only that the bike needed some love and he was right. It'll take some time for me to thoroughly clean all the carbs and make sure they are getting adequate fuel flow from the tank and pump but I'm confident this bike will be running again.

 

Mine was in similar condition (it sounds like) when I got it. I had to go through the carbs twice to get them clean enough.

 

YEAH BABY IT RUNS!!!!!! but still needs a lot of TLC before it's streetworthy. I still don't know if the petcock or fuel pump is a problem but my carbs are clean and in my process of elimination I rinsed out an empty bottle of gear lube, filled it with fresh gas, stuck a hose on the end and used it as my temporary gravity fed fuel tank. The bike started up AND stayed running which was a huge relief to me though the carbs still need fine tuning. With that said I now know that if I have a problem once the tank is installed that the problem will either be the petcock or pump. I do however need new bowl gaskets and possibly new o-rings all around because the carbs were leaking fuel a bit while it was running. The only reason I didn't replace them in the first place is because I'm finding out the hard way how expensive parts are for this bike and I wasn't about to spend any more money on it than I had to until I knew it ran and therefore worth spending the time and money to repair it.