Checking Fuel Level in carbs

[rd84]

Hi,

Just about to check the fuel level in the carbs as per the service manual. The manual says to check the level with the engine running - however is there any reason I can't check it with the fuel tap turned to PRIme instead ?

Thanks.

Cheers
Paul

[Speed Freak]

I think they want to have a certain fuel flow while measuring.
There should not be a big difference between "PRI" and running engine.

[rd84]

I think they want to have a certain fuel flow while measuring.
There should not be a big difference between "PRI" and running engine.

Thanks - I’ll try with the tap at PRI to see if there are any gross errors then I’ll try with the engine running in case it’s important.

Cheers
Paul

[silverstrom]

There will be no difference. The level will only rise as high as the float will let it, running or not. Engine off petcock in PRI will work just fine. That will tell you the current float level is correct, or not.

If you test it with the engine running in the ON position the only thing you are doing differently is proving the petcock diaphragm is working.

[gpaddict]

The prime feature WILL slowly overrun your float needles. Therefore it can cause errors in float readings.

[silverstrom]

If the needles are very old and very hard possibly, and the fuel tank is completely full. If you are really concerned you can do this test on the bench with a portable fuel bottle. All of my carbs are done that way.

[Speed Freak]

They should close 100%.
If not take an ear tip and polish the valve seats on the inside and the needle sliding surfaces.

There might be a small difference as there might be a hysteresis between opening and closing.
But this difference might be smaller than the measuring tolerance.

What you can find out with running engine is if the needle works how it should. If not the needle will not close again after opening and the fuel level will go up.
Happens if the needle and the seat is covered by fuel residuals.
Again... polish them

[rd84]

Thanks everyone - I’m going to both checks to see if there is a difference or if there is any problem with the carbs - hopefully the carbs will be in good order and both readings will be the same - so fingers crossed

Cheers
Paul

[rd84]

Ok - I’ve done the static check and all 4 carbs have fuel levels that are exactly 1mm high
When I switched the fuel to PRI one of the carbs immediately started to overflow - my heart sank, but a quick tap of the float bowl with a screwdriver sorted it out - however it now left this carb with a fuel level that was about 5mm too high - I was thinking about removing the carb to set the float height - then it occurred to me that perhaps the overflow had left it with a high fuel level - so I drained the float bowl using the drain screw - tried again and this left the carb with a 1mm too high fuel level.

I’ll do a engine running check tomorrow - but I’m wondering if the float valves still need adjusting to get the correct fuel level ? Is 1mm too high on the fuel acceptable ?

I set the carbs level with a spirit level to make sure this wasn’t having any effect.

There is about 4 gallons of fuel in the tank.

The float valves held the fuel levels for about 30 minutes without letting any fuel through with the tap on prime - I decided this was probably ok?

Cheers
Paul

[phildu31]

Fuel level has been corrected in the manuals.

Original value was 21mm for the float level with disassembled float chamber.
Corrected value is 24mm ( fuel level is lowered ).

For the fuel level with a gauge connected to the drain pipe, original value in the "Service Information" manual was 1mm under carb level.
Corrected value is 3,5mm +/- 1mm because of the lowered level.

Hope this helps.

[Speed Freak]

And polish your needle seats - it will avoid that your needles get stuck before they close as it happened.

[rd84]

Thanks for the advice everyone

Looks like my fuel levels are all 3.5mm too high instead of 1mm - so do I go in and correct this or see how it works out ? - it was set up on a dyno in this condition so hopefully the only consequence will be overflow problems ? If I change the float heights I will polish the float valves as suggested.

Cheers
Paul

[silverstrom]

If you have to go back into the bowls you might want to consider new float valves (needle and seat). New means you'll have years of use without any issues. Available from Yamaha and also as a Japan made reproduction part. It is a small cost for long term benefit. Yambits sells the Viton tipped reproduction.

[rd84]

If you have to go back into the bowls you might want to consider new float valves (needle and seat). New means you'll have years of use without any issues. Available from Yamaha and also as a Japan made reproduction part. It is a small cost for long term benefit. Yambits sells the Viton tipped reproduction.

Sadly the float valves are almost new OEM Yamaha - new replacements for Keyster ones just a few months ago. The ethanol had eaten the diaphragm in the new OEM fuel tap in less than a year. I've replaced the OEM fuel tap diaphragm and seals with Viton replacements from NRP - I've bought some Viton tipped float valves from the same place - so I'm going to use those if I have to go back into the carbs. Right now in the UK fuel is E5 - ie 5% ethanol - its shortly due to go to E10 which I'm dreading - this might make some old bikes almost impossible to use long term - I've started using super unleaded which I'm told has less ethanol in it in the UK, but I'm not sure. I'm going to start using a fuel stabiliser to help the internals to survive. I'm wondering if I should replace all the fuel lines with viton ?

Cheers
Paul

[silverstrom]

We have had E10 in Canada for many years. It isn't a huge concern. Use a product to prevent phase separation if you have fuel for more than a few weeks. Phase separation happens after 30 days. I treat all fuel here and my tanks are always full to the top. No problems.

Make sure any stabilizer you buy specifically says it prevents phase separation. Star Tron is what I use. Aside from stopping phase separation it keeps fuel fresh for 24 months. I change all fuel out in the fall and in the spring. Empty all tanks and dump it in the SUVs where it lasts just a few days before being used, and then refill the bikes and Jerry cans with fresh treated fuel. The bikes here don't get out much and as mentioned above the tanks are always kept 100% full.