70 XS F with white plugs

[Science Guy]

I have a 79 F which I recent tore down and cleaned the carbs and petcocks on (about 2 months ago), adjusted the floats, cleaned the screens for the bowls, etc. In the last few days I have been having a real problem. It will start off fine but then after a few miles will start missing and losing power, no back-firing though. If I stop for a few minutes it will restart but still without much power. It will continue to run for a few miles with limited power but then die again with complete loss of power. Sometimes it bucks and surges a lot before it completely dies. I pulled the spark plugs tonight and all 4 have white electrodes which to me indicates lean, but how could all 4 of the carbs suddenly start running lean. I thought possibly vacuum advance (will test that with a vacuum pump tomorrow). Since each pair of carbs run on seperate petcocks I didn't think they could be the cause. Any ideas?

[TopCatGr58]

Hey Science Guy,

Let's use that premise..... a scientific approach! Just because you worked on the carbs doesn't necessarily mean that they are the cause of the NEW PROBLEM!? Lots of things can cause poor running. How about some history of the bike, have you owned it a long time, been running well before the carb work, or is the bike NEW TO YOU, and you jumped in and cleaned the carbs and petcocks because you found that they are the # 1 problem with used XS11's???

The PU coil wires comes to mind. When it's running poorly, have a bottle of water with you, spritz some on the headers to see which ones are not sizzling hot!? IF paired 1-4 and 2-3 usually indicates ignition component problems. 1-2 and 3-4 is usually fuel delivery related. Single cylinder could be anything, fuel, electrical, mechanical, etc.!

ALSO, how's the charging system doing? Do you have the old OEM glass fuseblock and fuses, have you inspected them by removing them from the clips, check by pulling on the ends of the fuses, etc.?? IF the power to the TCI drops below ~10.5, it won't work right, no sparky, no go!!

More info on your diagnostics, and then we can provide more info on where and what to look for and work on!
T.C.

[CatatonicBug]

Sounds like your fuel flow is partially blocked. The carbs fill with fuel, and after a bit, the fuel is used up. You stop the engine and the fuel slowly fills the carbs back up. Try running with the fuel cap open (in your driveway, of course) and keep the RPMs up. Perhaps the fuel cap vent is blocked.

[TopCatGr58]

Also can try PETCOCKS in PRIME to ensure fuel flow vs. the vacuum controlled petcocks. Are the vacuum lines going to the petcocks intact, no cracks, fitted snugly to intake boot ports??

Did you tear down 1 petcock at a time, or both together, and the possibility of mixing up the right and left hand petcock handle/valve with the other petcock??
T.C.

[Bigge_al]

Just wondering did you replace the fuel lines when you did the carbs and petcocks. If so you may have a kink in the line. I found a kink today and I replace the lines about a two weeks ago. The octy and lines are a PITA.

[Ivan]

Missing and loss of power as the engine warms up, when all else has been looked into (Your carbs are probably working since it runs fine to start with...etc) is a classic sign that your valves may be out of adjustment.

As the valves wear the seats, the clearance actually lessens. The clearance, or lash, is calculated to give near zero lash when the engine is up to operating temperature. You'll notice that the spec on the exhaust is always wider than the intake, since exhaust valves run hotter and expand more with heat. When the seats wear, the lash will be fine on a cold engine and the valves will seal. When the engine warms up, the valves expand enough that they don't fully seal, since there is not enough lash to compensate for the heat. This shows as a miss that gets gradually worse until the engine won't run, and gets completely better when the engine cools off again.

The good news is, it only takes pulling the valve cover to check. The clearances have been posted here a few times, so a search should pull them up easy enough.

[LoHo]

Geez, as I read this, I was really wondering...

The guys says he has fuel starvation symptoms and white plugs. That has to be fuel, yet we get ignition and valve suggestions.

It starts and runs, then breaks down, so carb problems are not likely.

Make sure you have gas flowing to the carbs and I think your problems will be solved. Look at kinked lines, broken vac lines to the petcocks, clogged petcock filters, etc.

I hunted for a lean condition for quite a while once, adjusting carbs and such, until I took the tank off...the empty tank...then I switched to clear fuel lines.

[Larrym]

Horsing around

I'm going with "running outa gas". Plugs will look white after running a bike completely out of fuel. The bucking and surging is when the fuel level in the float bowls is at that "magic" level where it "gurbles" up the jets in spurts from one carb to another before they all run dry.

Something is not letting the fuel in fast enough to meet the demands of the engine. Everything upstream from the carbs is a suspect: fuel line kinks, petcocks, petcock tower screens, tank cap.

If you take a gas line off the petcocks and turn the gas on through "prime" or by applying vacuum to the petcock diaphragm then the flow should look like a draft horse.

[Cobia]

Starving for fuel

One other thing to consider is the vent on your fuel tank cap. Make sure that the vent hole in the middle of the underside of the cap is unobstructed. Otherwise it creates a vacuum lock that prevents the flow of fuel to your carbs.