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Thread: 1931 4 running VERY HOT

  1. #11
    Join Date
    Mar 2010


    Full advance is correct.

    Steve Slaminko

  2. #12


    Howdy sir,

    First off a Four is rather a fragile machine compared to nearly any v twin and I would caution anyone in operating one overly hot through poor carb or ignition settings or both, especially when fitted with Babbitt bearings and those in the rod big ends. A motor attempting to seize will stress and pound out the rods resulting in reduced oil pressure.

    Any idle settings should be achieved with ignition fully retarded, advance is there for just that, to chase ideal combustion which fully engages (max cylinder pressure/torque) at about 15-20 ATDC on most engines across the reve range. How much lead you require to achieve that efficiency is a learned skill, as a heavy rider in an extremely hot climate I have never ridden my Fours solely on full advance while underway over all operating conditions. One works the advance under load to where the motor feels free, not labored, and no more, running fully advanced at lower revs cokes your combustion chambers and fills your oil with abrasive carbon .....much faster than it would have done already. With the low speed needle adjusted correctly on a low rom level idle you should be able to roll of in 1st fully retarded feeding in advance with no motor hesitation. Youíve probably covered the carb problems with Cotten, make sure your ignition is timed correctly. Unlike later Fours you have the luxury of a timing plug in the top of your flywheel compartment case.

    Itís been a long time since I pulled the top end on a pre-38 but these later cylinder head pairs are not indexed/doweled in the cylinders so to alleviate some possibility of intake leaks one must lightly clamp up the intake manifold to the heads loose before torquing the latter. If I recall your era motor is not doweled so provides the opportunity to do the same. You might want to lightly spray WD40 around each intake port gasket at idle to see if the revs go up or down which would indicate a leak before undoing anything.

  3. #13
    Join Date
    Dec 2007
    The teeming metropolis


    The early fours had what I call a 'bridge' intake. The cylinders are cast individually, and the manifold is a single casting that bridges all four cylinders. The 4 cylinders are each bolted to the case and if they move the connection to the manifold may open an bit making an air leak. After the motor is warm spray some wd40 at the connection between the cylinder and the manifold, and listen for a change in idle. If it is leaking there, the manifold has to come off, new seals, and possibly cylinders shimmed. Be careful tightening the cylinder base nuts, the ears will break if they are over tightened.
    A. Bernhardt
    AMCA# 9726

  4. #14
    Join Date
    Jun 2001
    Central Illinois, USA


    What would happen, Folks,...

    If you yank the carb on a Four, and pressurize the manifold to a regulated 15psi, even though all the valves aren't shut?

    I never had the pleasure. Thanks in advance,

    AMCA #776
    Dumpster Diver's Motto: Seek,... and Ye Shall Find!

  5. #15


    Howdy chaps,

    For anyone reading this thread having a 38 later Four and at the point where they have the cylinders secrured to the top case and are ready to do the top end be aware the cast alloy intake manifold on these motors is very fragile. If running heat riser tubes assembling the top end can be fiddly while at the same time trying to get the best manifold seal to the heads as it cannot in any way be torqued to down to achieve a seal that is not there to begin with. Any desired gasket crushing, though not encouraged, should be limited to the robust exhaust side and if much required in new not old annealed exhaust studs.

    Loosely fit the heads onto the cylinders with only those head bolts accessible with the intake manifold fitted. Lightly tighten the intake manifold down, and torque exposed head bolts to 20 ft lbs or so. Remove intake manifold, fit remaining head bolts and if running copper gaskets on known flat surfaces torque all to 30 ft lbs (manual recommended torques are for prior inferior fiber based gaskets and destructive to these now 80 year old alloy castings causing crush at the washer lands). Refit intake manifold without fastenings at the same time as risers and exhaust manifold. Do not be tempted to hone riser inserts in either manifold to achieve an easier fit. Heat at the exhaust ports along with a heavy poor heat dissipating iron exhaust header will require utmost caution when fitting nuts to original studs. These machines experience very little exhaust back pressure in such large ports and mufflers with no baffling therefore nuts need only be lightly cinched on machines with known flat aligned mating surfaces.

  6. #16


    Thanks everyone. All your comments are very helpful. Just reset everything and took it out for a quick test ride. It seems...... to be running cooler. It's still feels a bit weak but I'm sure at some point I'll figure it out. Keep the comments coming and again, thank you.

  7. #17
    Join Date
    Aug 2006
    So Cal


    Just my guess - on my Henderson, I would check for intake leaks and timing. If it is retarded a bit, they well get hot quick ! One tooth off can make a big difference in timing and heat.

  8. #18


    Howdy sir,

    May want to confirm the health of your magneto. Weak output mimics your description of leaning out mixture to eliminate black smoke only to have the machine run hot at speed. Unlike battery/distributor set ups having a constant voltage output at the coil, a magneto generates its own voltage to make a spark and its output increases with rpm. A weak spark at idle or low rpm with a carb leaned down to accommodate then can go to lean as revs and magneto output increases closer to its desired output. If closing your spark plug gaps progressively down from the recommended .025 - .030 gap makes for a better or more predictably running motor you have a mag output or condenser issue.

    To clarify on recommending ignition fully retarded when make idle screw adjustment this applies to both the mechanical stop at the bellcrank as well as your low speed needle. You want a steady even idle, not the loping 8 stroking idle that results from trying to refine this adjustment with the ignition advanced. When making that high speed adjustment where momentarily cracking the throttle is required to eliminate backfiring of leanness then, yes, have ignition advanced. There are lots of YouTube videos of idling Fours to give you and idea of what you’re aiming for.

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