The standard Mikuni carb on the Savage/40 is an altitude compensating carburator. (just like the SU's and Stromberg's). It's slide is pulled up by vacuum created in the venturi. At sealevel we got about 30 inches of mannifold pressure. As we go up in altitude we lose about 1 inch per 1000 ft so in Denver (5000 ft) we only have 25 inches. This causes the slide not to be pulled up as high. This gives us about 20 percent (4 percent per 1000ft) less power. The tapered needle attached to the slide also allows less fuel to be drawn in. So the mixture stays reletively the same (14 to 1). There is NO need to change the MAIN jet. However the idle circuit is fixed. Therefore the bike idles less good. So if you're gonna live there change only the idle jet and adjust the RPM.

vtail wrote on 12/31/69 at 16:00:11:


vtail, I completely agree with the theory, but I'll add some emperical data to the mix.  Before I did the white spacer mod, my stock Savage was running lean with all the usual symptoms: cool weather surging, blue pipe, outrageously high mpg  , etc.  But I trailered my bike with me to Colorado for a three-week business trip (I live in the DC area, sea-level) and all those symptoms completely disappeared.  One variable to note, is standard octane fuel there is 85, here it's 87 so I'm sure that played a role as well.  Now that I've removed the white spacer, it runs in DC like it did in Colorado, except with WAY MORE POWER.  Thoughts?

Barry, when you went to Colorado you placed yourself in a higher altitude where the air is thinner, = less O2, thereby more fuel per volume of O2 = richer mix = less/no backfire.

If there is still a bit of backfiring, then after making sure the exhaust is leak free, adjust the pilot air adjusting screw for a slightly richer low range mix ...WHICH IS THE PRIMARY CONTROLLER for the backfiring issue ... not the needle.  The slight effect of the needle on this issue will certainly be eliminated if the white spacer is shaved even a little.  Personally, I think it should be shaved by 1/2 right off the bat.  This engine is soooo freaking lean from the factory that it needs a richer mix from the get-go.
Where jetting is concerned 95%+ of backfiring is absolutely controlled by the pilot circuit.

A stock Savage engine with just a rejetted carb and higher flowing exhaust will rarely ever need more than a #55 pilot jet.  With the pilot circuit tuned properly with the adjusting screw you should be able to eliminate backfirng completely... keep in mind that some popping & crackling is normal behavior and will not hurt the engine.  In fact, it is evidence that you have done the tuning job well.  Be proud of your skills!  

Procedurally, start with the pilot circuit first and get that jetted/tuned properly ( low range ), then do the needle/spacer adjustments ( mid range ), and wrap it up by working out the best main jet.
Remember that there is overlap by the various needles/throttle ranges, and the lower ranges do affect the higher ranges.  The low range, or pilot circuit, is the foundation, so start there and build up from there.

barry68v10 wrote on 12/31/69 at 16:00:11:


Yes. because the Idle mixture is richer than at sea-level(see above) This same carburator, because it is self leaning ( altitude compensating) is used on Rotax 582/4 engines in some airplanes and works great at all altitudes. They are Strombergs , but work exactly the same. They do have a dampner on top of the slide to keep it from coming up to fast (Like SU's)