Anybody riding above 5,000 ft (1,524 meters) using an AMAL GP1 carb? I'd be intersted in knowing what slide, needle, main jet you are using. My setup is running too rich and I need to acquire different parts. TIA Ralph
As long as your Carb is adjusted correctly a main jet reduction of 5% between 3,000 to 6,000 feet should be ok. No need to change anything else.
If you think about what you wrote a bit you'll realize it can't be correct. For example, at partial throttle mostly the needle and needle jet are controlling things (plus the idle mixture screw to some extent), with the main jet largely irrelevant in that range. Hence, if the mixture is rich at partial throttle changing the main jet alone won't do much to the mixture.
This has been addressed before. See the following post and some of the posts that follow it for advice on the changes to make for altitude.
If the GP settings are at original factory specifications and it runs ok, then due to the lower proportion of oxygen in the atmosphere, a smaller main jet will be required to compensate for richness at a higher level. I admit, could be wrong, but in the technical data in the Gold Star Book by Bruce Main Smith, only the main jet size has to be decreased for higher altitude.Main Jet, Pilot Jet Adjustment, Throttle Valve Cutaway Jet Needle, all these settings were done by B.S.A
Just thinking, just a 5% decrease up to another extra 1 to 3,000ft indicates that their is not a significant change in atmospheric pressure to have much effect on the factory base settings, but hopefully Ralph, you won;t need to do much fine tuning, all the best, Goldie John.
in the technical data in the Gold Star Book by Bruce Main Smith, only the main jet size has to be decreased for higher altitude
Yes, the BSA recommendation only addresses the main jet. However, for the reasons I gave in my previous post, those other settings do need to be adjusted to get the mixture correct across the entire range of operation at altitude for precisely the same reasons they needed to be adjusted to get the mixture correct at sea level in the first place. However, as the rest of this post shows, this isn't easy to do which no doubt is why BSA literature fails to address it.
BSA's recommendation to reduce the main jet by 5% for altitudes 3000-6000 ft. is consistent with the overall observation that if the mixture is off by ~5% there is a noticeable deterioration in behavior. This happens when a, say, standard .106 needle jet in a Monobloc or Concentric has worn by only ~0.0005". By the time this jet has worn by that small amount the midrange performance seriously suffers, which is why John Healy emphasizes the needle jet needs to be treated as a consumable.
Unfortunately, this "5% rule" also means it will take some effort to get the GP's settings correct for the "above 5000 ft." Ralph433 asked about. BSA goes on to suggest a further 4% reduction for every 3000 ft. over 6000 ft., which would mean a total of 7% for riding at 7500 ft. (a Mikuni slide rule shows 7.3%, all other things being equal -- however, if the temperature were 30 oF lower at altitude you'd gain back 2% of the air density, requiring only a 5% change). The reason this is a problem is that dropping the needle jet by only one size, from .109 to .108, will lean the midrange mixture by ~10%. Such a big change would take the midrange mixture straight from too rich at 7500 ft., to too lean. But, I said it wouldn't be easy, not that it would be hopeless.
Things get specialized at this point. I have two bore micrometers that that measure IDs of needle jets to 0.0001". Thanks to production variations (and wear), if this were my problem I would gather a batch of new and used .108 and .109 needle jets for my GP. If I knew that, say, .1090" was perfect at low altitudes, I would go through the batch and select one that was .1083"-.1084" to use at 7500 ft.
Just getting the main and needle jets correct might well make the bike run "nearly perfect" enough at 7500 ft., and luckily it is very easy to swap these two jets to change between riding at low altitude and high altitude. However, if the behavior coming off idle were unacceptable the cutaway would have to be dealt with as well. Again, if it were me, I'd get another slide whose cutaway provided "perfect" mixture at low altitudes. I'd then increase the cutaway of that slide a quarter-size (1/64") at a time until I hit on the perfect mixture for 7500 ft.
As the above shows it is possible to get the jetting correct for high altitudes. It also shows why BSA only addressed the main jet.
Sounds like a need for a "worn" 108 needle jet. Something else that comes to mind: when I was trying to solve a problem on my '66 Spitfire that ultimately turned out to have nothing to do with the carburation, I was mucking about with different cut aways on the GP carbs. I had a limited number of slides to play with. Making a slide leaner was easy, I would use a file to increase the cut away. To decrease the cut away, I would rub the bottom flat sides down on a plate with emory paper. Because I hadn't thought the process through, it didn't occur to me at the time that I was also lowering the needle by the amount removed from the bottom. Perhaps what Ralph can do with an extra slide is both remove material from the bottom of the slide, thus lowering the needle, and also, if necessary increase the cut away by removing material there also.