High altitude issues.

[BA80]

Just got back from our trip out west. I really had no issues other than above about 8000 feet altitude my bike would sputter and misfire if I gave it more than about 1/2 throttle. The lower I got the better it ran.

trbig's and JWHughes' bikes seemed ok up there but mine SUCKED.

Any help. ( real help, not guesses).

I'm not sure if it was carbs or spark. Changed the plugs and that didn't seem to make a difference.

[motoman]

Just got back from our trip out west. I really had no issues other than above about 8000 feet altitude my bike would sputter and misfire if I gave it more than about 1/2 throttle. The lower I got the better it ran.

trbig's and JWHughes' bikes seemed ok up there but mine SUCKED.

Any help. ( real help, not guesses).

I'm not sure if it was carbs or spark. Changed the plugs and that didn't seem to make a difference.

A definite-possibility may................just gonna sit this one out and watch.

[3Phase]

... ................just gonna sit this one out and watch.

Isn't it a little late in the season for the Cadbury Bunny screen tests? <G,D&R>

.

[TopCatGr58]

Okay, I'll bite!

Greg, it sounds like you may be a bit rich on your jetting, which works okay at lower altitudes with more dense air, but when you're running at that altitude, the air is way thinner, and even though the carbs are C-V type and are supposed to adjust to the less dense air, it seems like you're over jetted. That, or your floats are a bit too high, or both.

That's my theory/take on it, don't know how Tod and John's bikes are jetted/tuned? Stock jetting was always a bit leaner to begin with due to the EPA regs, at least in the latter years, but custom tuning adjustments for strongest sea level performance may not be good for the very high thin altitude air!?

T.C.

[trbig]

When Greg changed the plugs in Grand Junction, they all looked good except #3 which was black and sooty. I thought his #3 slide diaphragm has a pinhole, letting that slide drop, but he thinks it's more electrical.

[motoman]

Isn't it a little late in the season for the Cadbury Bunny screen tests? <G,D&R>

.

Likely.........but this is way better, and I hear the popcorn almost ready.

[motoman]

When Greg changed the plugs in Grand Junction, they all looked good except #3 which was black and sooty. I thought his #3 slide diaphragm has a pinhole, letting that slide drop, but he thinks it's more electrical.

pinhole lets slide/metering rod rise.......good call, worth checking #3 diaphram for issue elimination out of equation.

[BA80]

pinhole lets slide/metering rod rise.......good call, worth checking #3 diaphram for issue elimination out of equation.

If it was leaking it wouldn't rise, it would lag behind the others and LEAN out that carb.

[3Phase]

If it was leaking it wouldn't rise, it would lag behind the others and LEAN out that carb.

Mmm, the slide would lift late, close early and make that carburetor really rich. Opening early and closing late would make it really lean or one of the original 7-11s.

The smaller venturi creates a greater pressure differential (vacuum) across the top of the needle jet/emulsion tube as the intake air velocity goes up along with the engine rpm.

That would lift a bunch of extra fuel through the main jet and if the diaphragm is really bad it'd probably keep digging for whatever's in the bottom of the float bowl too, not just the fuel.

.

[BA80]

OK, I sorta get what your saying Scott but as the slide lifts the needle gets thinner ie making the main jet orifice larger. More flow.

Otherwise, why should it lift at all other than to let more air in?

[Ken Talbot]

I had similar issues with Tsunami the year SWMBO and I attended the rally at Colorado Springs. I started having issues on the way up Independence Pass, and by the time we were on the day ride around Pike's Peak, it got so bad I had to throw in a full fresh set of plugs. Got home from that trip and dropped the floats across all four carbs by a couple of mms if memory serves, and that seemed to solve the problem. I haven't been over anything quite that high since, but mileage increased substantially, and plugs stay much cleaner.

[trbig]

I know when mine had holes, the slides were dropping, letting less air come through, but with the same draw as normal. If it can't draw the air, it'll draw fuel. At higher altitudes, the air is less dense, so the slide would have lifted even less, drawing even more fuel and running even worse.

Not saying it is the problem, but makes sense and worth checking out. If so, a little dab of silicone will work for a while.

[3Phase]

OK, I sorta get what your saying Scott but as the slide lifts the needle gets thinner ie making the main jet orifice larger. More flow.

Otherwise, why should it lift at all other than to let more air in?

The slide needle never does completely block off the emulsion tube and the main jet even when it's all the way down. Also, the pressure differential across the top of the tube is not quite linear or one-o-one.

I hate to go there but imagine blowing air across the top of a beer bottle or a jug of whiskey. The faster the air moves across the top, the more the 'sound' changes. If you drilled a hole in the bottom of the jug or bottle and moved the air fast enough across the top you'd get beer or whiskey to come out along with the sound.

You can control the flow out of the top of your beer bottle or whiskey jug with your thumb (the needle) but if the air's moving fast enough you'd have to completely cover the top to close the hole and that will never happen with a BS-34 carburetor.

The main jet, needle jet/emulsion tube and the jet needle/slide needle are all matched to work with a variable venturi that varies at the same rate and proportion as the other carburetors. They can compensate a little more or less but that's what tuning is all about.


I'll pull some numbers out from under my keyboard that don't have anything to do with reality but you'll get the idea. Imagine a pipe, any pipe, that is one meter long and has one cubic foot of air moving through it in one second.

Volume cf/s = 1 cubic foot/second
Velocity m/s = 1 meter/second

1cf/s at the Inlet     1cf/s in the middle   1cf/s at the Outlet
1m/s at the Inlet      1m/s in the middle   1m/s at the Outlet

Put a crimp in the middle of the pipe and the air will have to speed up as it goes through the crimp to maintain a volume of 1cf/s.

1cf/s at the Inlet     1cf/s in the middle   1cf/s at the Outlet
1m/s at the Inlet      3m/s in the middle   1m/s at the Outlet

For this totally oversimplified example, the air has to travel three times as fast through the restriction in the middle of the pipe to allow the same volume of air to flow and 1cf of air In to equal 1cf of air Out.

Next, drill a hole in the middle of the pipe where the crimp is and stick the neck of that drilled beer bottle or whiskey jug into the hole.

Cover the hole in the bottom of the bottle or the jug with your thumb.

Will you get better sound and more beer or whiskey if the restriction (slide) is more closed or more open?

If the air speed through the pipe is increased to 100m/s and the volume to 100cf/s you will get even more beer or whiskey and even more sound when your thumb gets sucked into the hole in the bottom of the bottle.

.

[CaptonZap]

And now, from the opposing camp;
To paraphrase Motoman, half of carburetor problems are electrical.
The old vacuum tubes worked because the electrons could flow through the space between electrodes because there weren't any pesky air molecules getting in the way. And there are less air molcules at 8,000 feet than there are at 2,000 feet.
If your plug lead insulation is marginal, the electrons can slip their surly bonds and roam free, looking for a ground. And with fewer air molocules plugging the cracks in the insulation, the more electrons can slip out. You can see where this is going.
One problem with trying to find ONE cause for a condition is that in most cases, it is a multitude of causes that contribute.
Like moist air, with built in water to help surface conduction.
So, it could be ignition AND carburetion.
CZ

ps. We don't need no steeenking oil threads to have a good argument.

[BA80]

We don't need no steeenking oil threads to have a good argument.

Ha ha, don't want to argue, want to correct the problem. Not having full throttle on a 10% grade at 10000 feet with the temps dipping into the 30's and 20's is not a comfortable feeling.