1990 VX800 fueling issues

Hi, I have just bought this bike which has been off the road since 2009. Mileage is a reasonable 18k and it is in pretty good condition after the initial clean up. The main problem is that it is running mainly on the rear cylinder. The front cuts in and out momentarily. 

I've checked compression, spark, and fuelled with fresh petrol - all are good. Fuelling is the issue; I removed the tank and set up a temporary fuel supply via the input line to the fuel pump. The bike has a clear fuel line that runs from the rear carb to the front carb (up and over the engine) and the fuel supply here is erratic - you can see the fuel falling back from the front carb and starving the cylinder, then randomly the pressure rises and the carb is resupply accompanied by a significant increase in revs from c.1100 to 2-3k....only to full back again as the fuel supply cuts.

Fuel supply to the rear carb is constant so I an suspecting a bad fuel pump - (I've checked that it holds vacuum with a Mityvac vacuum tester via the vacuum line from the front carb intake manifold) - The pump isn't leaking or perhaps the poorly synchronised carbs are not allow sufficient vacuum to be generated in the intake of the front carb?  but either way, it looks like this is reason the bike was removed from the road. There is evidence that the carbs have been apart (new screws holding the float chambers on suggest rebuild kits?) and the settings for the carbs and synchronisation seem way out resulting in a really rich mixture and soot covered plugs in both cylinders.

First question is, can the pump be rebuilt - is there a kit available. Also what is the standard way to check the viability of the pump? Secondly, as the fuel feed is via the rear carb and then on to the front carb, is there anything in the rear carb that could be interrupting supply to the front carb?

Finally, where can I get a replacement pump from....EBAY has some very expensive looking USA supply units...which is fine if it is the pump...but expensive if not.

Thanks for reading and thanks in advance for any responses/advice

Regards

Jon

The only reason for the pump is the cabs sit higher than the bottom of the tank, to prove that it is/isn't the pump, just connect the temp fuel supply up (making sure its higher than the carbs) and bypass the pump, I have never heard of a pump failing, but its possible, it could be the float sticking in the rear carb, my money would be on the carbs and not the pump, especially if it has been unused for so long, every thing will be gummed up.

v-twin wrote:

The only reason for the pump is the cabs sit higher than the bottom of the tank, to prove that it is/isn't the pump, just connect the temp fuel supply up (making sure its higher than the carbs) and bypass the pump, I have never heard of a pump failing, but its possible, it could be the float sticking in the rear carb, my money would be on the carbs and not the pump, especially if it has been unused for so long, every thing will be gummed up.

Thanks for your reply  Andrew,

I rigged up a supply to both carbs attached to the handlebars (higher than the tank!) with separate feeds to each carb. The bike ran for about an hour, mainly at idle speeds but with a number of trial revs up to a maximum of 4k revs. It is certainly running rich as ther new plugs were very black when I shut the bike down.....I wanted to get it to running temperature to exclude any other issues - that at least went to plan with the fan cutting in an out as expected.

I'll pull the carbs off and strip them to see what is presented and hopefully be able to fix the issues reasonably quickly.

best regards

Jon

As you have probably found out, the carbs are a bit of a pig to balance. Good luck, keep us posted

Thanks again.

I've downloaded a full set of workshop manuals and bought a tanker load of carb cleaner. Tomorrow I plan to pull the fuel system carbs and pump apart for a proper clean and set up. I'm used to balancing and setting up the Delorto's on my old Ducati F1 and the monstrosities that were on my Yamaha XV550...so I thought it was worth a go.....

Wish me luck

While the carbs are in bits check the jet sizes. As someone could have changed them if they fitted a different air filter and exhaust but not changed the jets back if they put the stock filter & exhaust back on. From memory the rear one runs a one size larger main jet but I can not remember the sizes. Also check the 'O' ring on the float valve etc as it could be perished and could cause it to stick/let fuel past.

v-twin wrote:

While the carbs are in bits check the jet sizes. As someone could have changed them if they fitted a different air filter and exhaust but not changed the jets back if they put the stock filter & exhaust back on. From memory the rear one runs a one size larger main jet but I can not remember the sizes. Also check the 'O' ring on the float valve etc as it could be perished and could cause it to stick/let fuel past.

Yes I will check everything....also just off to buy fork oil and change the seals as the front legs are a bit weepy...the bike that is, not mine

atvb

Jon