View single post by Mark Rosenbaum
 Posted: 04-22-2007 03:50 am
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Mark Rosenbaum

Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
Once warmed up, a JH engine with un-worn and properly set-up Strombergs should idle quite smoothly at 1000 rpm, and with increasing roughness down to 400 rpm or so, provided there are no air leaks at the throttle shafts or at the big o-rings between carbs and adapter plates.

"Blue" refers to the paint or dye used to color one end of the spring.  Usually this disappears after a few years of use.  However, the correct spring for the JH should have wire that's 0.036" thick, should have a free length of 5.5 to 5.9 inches (which is not at all critical), and should compress to a length of 2.812" under a force of 117 grams.  Please note that only the "blue" springs should be used in a JH.

I suggest verifying that you have 2.0 to 3.5 psi fuel pressure at the tee that feeds the two carbs, that your ignition timing is correct, and that your cam timing is correct.

Assuming the carb is properly assembled, unscrewing and lifting up on its damper rod shouldn't have any long-term effect on engine speed.  OTOH, if you were talking about lifting the vacuum valve, meaning the cylindrical piston-like object visible from the carb's opening to the atmosphere, then a slight (1/16" or thereabouts) lift should cause engine speed to increase if the mixture is too rich, or decrease if it's too lean.  So if you were lifting a vacuum valve and the engine bogged down, that says the fuel-air mix was too lean, and adding some "choke" would make the mix a bit richer.  I'll note here that if at any time you can see liquid fuel flowing into the engine, there's something wrong with the carb.

Given your description, and assuming that your carbs are in good condition, I'd expect them to be out of tune for air flow and/or mixture.  This can be dealt with fairly quickly using the procedure in the shop manual, but bear in mind you may have to go through the adjustments several times before everything falls into place.

If tuning the carbs doesn't help, it would be necessary to remove, disassemble and inspect them, rebuild them as indicated, then reinstall and re-tune them.