Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties Spark plug heat range.

Spark plug heat range.

Post Wed Jul 23, 2008 2:17 pm

Posts: 11
Location: Bergen County New Jersey
I could use some input as to what spark plugs and heat range is being used out there on big twin flatties using the smaller 14mm plugs.I have a 1940 UL with aluminum high compression heads,black widow mid range cams,auto advance 45 servi car timer,and an adjustable main jet Bendix 38mm carburetor.The motor has approximately 350miles since built and the bore is 10 over.The plugs i am currently using are Champion RN12YC.Is this heat range too hot? I cant get any color on the plugs.I have checked for air leaks,found none.A local carburetor shop familiar with these carbs recently has drilled out the main jet twice and now I am just starting to see some "pink" coloring on both plugs .Any info would be appreciated.Thanks
Joe E

Post Wed Jul 23, 2008 4:14 pm
panic

"drilled out the main jet"

What size is the slow jet?

Post Wed Jul 23, 2008 4:50 pm

Posts: 11
Location: Bergen County New Jersey
Hi Panic, I dident know that a bendix carb had a slow jet.Where in the carb is it located? I thought that as soon as you come off idle you are running on the main jet.If you are talking about the main jet adjustable needle hole sizes ,I have already experinented with enlarging the upper hole which only made it basically impossible to lean out the idle mixture and had on other effect.
Joe E

Post Thu Jul 24, 2008 9:07 am
panic

Sorry, senior moment!
No, not all main jet immediately, but the low speed cruise is controlled by things they don't want you to change (idle tube) or difficult to change (transition holes). I suspect the hole in the idle tube is a restriction, but not sure and it's not a tunable part. There are 3 more idle holes after the one regulated by your low speed mixture screw, and it looks like you can reach them through the bowl by removing the plug. Enlarging the last hole (away from the engine) will enrich the cruise mixture; an increase of only a few thousands will make a diff. However: very difficult to reverse.
1. are you sure all these holes are clear (no gummy buildup, etc.)?
2. float level is OK?
3. are you sure the idle tube is correctly seated?
4. carb ever been apart?
Until you sort this out, I'd go down to a 10 or 11 plug just for safety.
What exhaust?

Post Thu Jul 24, 2008 12:11 pm

Posts: 11
Location: Bergen County New Jersey
Panic, Thanks for the info.The carburetor is new,but my carb guy took it apart to check everything and did find that the needle for the float assembly was hardened and replaced it which cured a stumble in mid range.Other than that everything was checked.I am going to an 11 plug.With Champion plugs it goes from 12 to 11 and then jumps three heat ranges to 8.I have contacted Zenith as in their paperwork they stated that if your plugs are still white afted backing out the main jet needle,you should switch to an optional main jet,Part# C66-184-2 ,which is supposed to cure the problem.I have ordered one this morning and will give it a shot. I am running shotgun straight pipes.
Joe E

Post Thu Jul 24, 2008 4:49 pm
panic

That's really weird. Open exhaust frequently causes very rich mixture at low speeds due to reversion, and needs leaner idle mix blah to clean it up.
I see 2 holes in the idle tube, I suspect the lower is just a feed (it's submerged in the bowl), but the top one supplies the idle & transition holes, and the 1st 3 holes have no other restriction (the adjuster only affects the 4th one). Increasing the idle tube top hole may do something, but definitely want to have a spare available since it's thin wall tube. If the top hole is 1/16" (not even a guess, just example), going up to .0635" (#52 drill) is +3.2%, .067" (#51) is +14.9%.
I would prefer to re-size the individual transition holes since they cover a different range, but it's more work and much harder to reverse.


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