high altitude/stock carb

[bazuka]

Last fall my wife and I drove the back trail from Bear River Res. to Silver Lake in the Sierra's. We were at 8,000 to 9,000 feet when the engine started to run bad and would not idle. We were at the "ROCK GARDEN" when this occured and I consequently dented my front pumpkin. Back at camp at Bear River Lake I changed the fuel filter and looked for loose hoses or wires but nothing helped. This conditon lasted a couple of days and then then engine ran ok. We did then entire trail without a sputter. Is there an altitude adjustment for the carb? Is the ECM going bad? Any help would be appreciated.
By the way, this was the week before the opening of deer hunting season and we saw very few people besides hunters on the trail. We also enjoyed the first snow of the season. It was t-shirts and shorts one day and snow the next.
Anyway, if there is an altitude problem, and hopefully a solution, I could use the help.
Thanks

[Bobzooki]

Originally Posted by bazuka

Last fall my wife and I drove the back trail from Bear River Res. to Silver Lake in the Sierra's. We were at 8,000 to 9,000 feet when the engine started to run bad and would not idle.

Huh. OK, so this is a Sammi with a carb? No computer at all?

I live at 9,000 feet. Now back in the day, when there was no such thing as an ECM, or fuel injection, everybody KNEW that you had to tweak the carb for that kind of elevation change. What elevation do you normally run at?

So here's the deal. Keep it running. Cup your hand over the exhaust pipe for a second or two (so the exhaust blows into the palm of your hand) - don't burn yourself. SMELL your hand. Does it smell like gasoline? You're probably running rich. You need to turn the mixture screw in, until it runs better.

Now mind you, you'll have to turn it back OUT the same amount, when you go back down to lower elevations, or it'll run lean, which is way worse than running rich.

I wish I could begin to tell you how I adjust a carb, but it's by ear. I did 's Jeep carb. It used to be that lots of people had this knowledge.

[GeoB]

Originally Posted by bazuka

We were at 8,000 to 9,000 feet when the engine started to run bad and would not idle. We were at the "ROCK GARDEN"

I have found that sometimes on really rough and bumpy terrain my engine starts to miss and sputter. It gets over it really fast when I get to more smooth terrain. This doesn't happen often, but when it has it was like on jumps and stuff where i was really bumping it around.

Bobzuki asked if this had an ECM. I will assume it does since you are in California and not Canada, and since pert near all zukes here in the USA do have ECMs. Are you aware that this is an electronic carb and that it adjusts for altitude? Automatically the ECM adjusts the mixture as it reads the O2 sensor. As the engine gets richer at altitude the mixture is adjusted so it remaions crisp and smooth. Yes, cuz of less oxygen you will have less power, but at least you won't be throwing gas away.

But not all Zukes are running properly, with the ECM controlling the mixture. You may be in 'open-loop mode' which could also be called here the 'limp-home mode'. If say, the O2 sensor was kaput, the ECM would just let the mixture go full-rich and would not adjust for altitude.

There is a separate altutude sensor that switches on at 4000'. It does some stuff like kicking in fast idle. And i think some other stuff... maybe turns off the EGR? So, I think you could get the fast idle and the ECM still might be in open-loop mode. Search the archives, there is a lot on this. You can test the O2 sensor. You can determine if you are in closed-loop too. Note that when cold the Zuke starts in open-loop, and at WOT (wide open throttle) (IIRC).

when this occured and I consequently dented my front pumpkin.

Now wait a minute! This strains my credulity! Only time i bend a pumpkin is when I dodn't see a pesky big rock.. yer telling me a rough running engine....

We did then entire trail without a sputter.

Weird huh? Sounds like mine.

Is there an altitude adjustment for the carb?

One of these posts tell how to adjust the mixture. Please note that the ONLY mixture adjustment screw is for IDLE only! It doesn't adjust the mains. It effects the mixture above idle some, just not much.

When I re-jet a motorcycle for altitude I start with the mains and work down. But it is a lot more than turning a screw. Some carbs exist that are fully adjustable but not on a Zuke.

Is the ECM going bad?

I think anybody who is conversant with the entropy and chaos theorys would tell you 'The ECM ain't gettin' no better!'. Pardon me, it is late, everything seems funny to me. I personally doubt that this problem is associated with the ECM.

Anyway, if there is an altitude problem

No, a properly functioning Zuke needs NO special adjustment to run very well at up to 11,700 feet. After that? Probably still runs good but I haven't been higher.