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H6 needles

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NutmegCT

NutmegCT

Great Pumpkin
Bronze
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Good morning all.

Yesterday I noticed the front carb piston seemed to "stick" before it hit bottom. Took the piston out and found the needle was slightly bent.

I got two new SM needles; the previous needles were an R and a PT (?). Figured a matched set of standard needles would make more sense ....

So now that the new needles are secure and mounted with the shoulder flat to the piston bottom, I've started to adjust mixture.

Following the "screw mixture nuts fully up to lean, then back off two full turns as initial setting" technique, I find that lifting the pistons 1/16" results in faster rpm. So I lean them more to get a "brief rise in rpm - then return to first rpm". But getting to that stage means I end up screwing the mixture nuts at least one full turn *back* up (six flats). Also, at that stage, the idle is quite lumpy - ca. 800rpm but engine rocks like crazy.

Does that sound right?

Seems the mixture is too lean per plug appearance. After balancing the carbs, what procedure do most folks here use to set mixture?

Thanks.
Tom
 
I use the "lift the piston" test, just as you've described. Sounds to me as though either your balance is badly off, or you have one carb rich and one carb lean. Unleaded fuel doesn't leave the white/brown deposits like the old leaded stuff did, so it takes a lot longer for a plug reading to develop than it used to. (The deposits were mostly lead oxide, same pigment formerly used in white paint.)

I've also had trouble in the past with a rubbing needle wearing the jet inside; which alters the mixture profile. With the idle mixture set 'normal'; the cruise mixture was so lean that the engine would always overheat. A friend's MGA had the problem so bad that the mixture was still too rich even with the mixture nut all the way to the top (and his cruise mixture was too lean). The cure is to replace the jet as well as the needle.
 
Older H6 carbs may be in less than perfect condition. Randall has a good point on jet wear. A check for GROSS wear would be that a .099" drill (#39) would just slip into the jet & a .101" drill (#38) would not pass through. Best would be to start with new jets.

First, synchronize the throttles. (Make sure that the fast idle cam is not preventing the throttles from closing.) You can use the sound test, other gadgets, or simply back off the throttle stops, then advance them until they just lightly clamp a piece of paper, & then turn them in equal amounts to get the desired idle.

With the jet adjusting nuts screwed to the fully up position the actual jet head heights may not be at the flush level to start with. Start by adjusting the nuts so that the jet heads are flush, or at least equal, no matter the turns, & use this as a starting point. Reference all adjustments to these starting points.

If the piston lift test doesn't seem to be correct, adjust the jets for best idle. I realize that there are more "scientific" ways of doing it, but it should get things close.
D
 
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