Petcock question

[Bob]

The left side petcock on my 79 xsf started leaking. Have not been able to get a rebuild kit yet (partsnmore are out). And I'd rather walk than get anything from our local Yamaha dealer (besides, he never has anything for an xs11). So, for the time being, I eliminated the left petcock and am feeding all four carbs off the right one via a "T" connection. My gas mileage has dropped dramatically---like from 32 or so to 20-22 mpg. Is my set-up to blame or is it just coincidental and I need to look elsewhere? BTW, I plan on replacing both petcocks with new ones as soon as I can let go of an extra couple hundred bucks.

[sixtysix]

K&L sells replacement kits for the petcocks....available at any dealer (yamaha or other). They work great, I used one on my 80G.

[James L]

Bob,
This may sound stupid (forgive me), but what is your method of determining mpg?
Here's what I do:
1. Fill up the tank and zero the trip odo
2. Ride until low fuel light comes on
3. Refill the tank, taking note of how many gallons is required, and how many miles I travelled
4. Divide miles travelled by gallons needed to refill. The result is mpg.

When you're only drawing fuel out of the right side of your tank, I think you're not totally emptying it. It occured to me this could account for your discrepancy in mpg readings (if you are using another method for calculating mpg than I do).
It's hard for me to believe that you'd experience such a dramatic loss in mpg from your 1 petcock setup (unless it's leaking profusely!)...
Is the bike performing well? If anything, I'd think you could potentially see fuel starvation symptoms under heavy loads.

Again, sorry if I've stated the obvious about the mpg, it's just the only thing that came to mind!

[Texas_Steve]

Yo Bob,

Steve from San Antonio here. I may be able to help with your petcock issue. Have some extra parts left over from my recent(last week) overhaul.
You can contact me off-list at smcnealy@satx.rr.com or call my place after 5 pm. My # is (210)408-6880.

[Bob]

mpg

James....I calculate mpg exactly like you do. I figure that I have some other problem, that the timing is just "coincidental". The bike is running fairly good, but it has never been up to optimum since I've owned it (3 years now). When I first got the bike, I was getting only 23 mpg. Found several air leaks in the carb boots, repaired them, got between 32 and 37 mpg, but now, it's back down in the lower 20's. Think now that I have a Co. truck to drive to & from work, I can take the time to check out the carbs, boots, vacuum lines, etc., thoroughly. It really did not make sense that one petcock feeding four carbs could drop mileage so dramatically, but I'm new to motorcycle mechanicing and wasn't sure if there were some "odd" quirks involving the carbs. Hopefully I can find and fix the problem without resorting to a "professional" mechanic. Guys, thanks for the input. This site has a bounty of information & help for us do-it-yerselfers. BTW, James....not stupid at all....no offense intended, none taken. I've known guys that get ticked because they didn't get as many miles as usual out of a "tank of gas".

[TopCatGr58]

Hey there Bob,
I'm no guru!! I vaguely remember reading about a guy who had somehow gotten one of the fuel lines connected to one of the vacuum ports on the carbs, was sucking gas straight into the intake boot instead of thru the carbs!?!?

As for your leaking petcocks, I found some neoprene rubber washers (large) at a local hardware store for like 50 cents, used scissors to cut out a circle similar to size of petcock washer, then used a single hole punch to make the 5 holes needed in the washer, also found the "O" ring that fit between the petcocks and gas tank there as well real cheap. Just another suggestion for home do-it-yerself technique!

You say your bike is a 79SF, do you still have the Octopus, or have you bypassed it? I threw mine out 2 years ago. Like you said, while you were doing this redirection of the hoses, could have developed some more vacuum leaks of boots, other hoses, and you'll have more time to recheck everything, like James said, I would think an actual starvation situation could occur at sustained high rpms, running a little lean, which is actually supposed to increase mileage!?!? Good Luck.

[Ken Talbot]

Hmm, maybe TC's on to something. I wonder if somewhere along the way the volume of gas that was being delivered by that one small nipple just wasn't enough to keep all 4 bowls filled. Maybe one carb was getting nothing much more than 'fumes'? Having one dead cylinder would certainly bring the mileage down..

[Bob]

TC: my bad...typo...I have a 79 standard, not a special, but that doesn't mean I can't have vacuum leak(s) somewhere. Many thanks for the input.
Ken....I hadn't thought about unequal distribution of gas to the carbs....could definitely be a problem. Thanks.
Think I'll pull the ole tank off, remove the bad petcock & see about manufacturing my own repair gaskets. Many thanks TC.
If worse comes to worse, an independent bike shop here in Amarillo has a couple of Pingle petcocks for about 60 bucks each. That's probably what I'll wind up doing.
Thanks again everyone for your input and help.

[sixtysix]

I had a vacuum leak on my standard petcock that caused fuel to be drawn directly in to the boot, you could tell because the vac hose was wet with fuel when you pulled it off.