ok - here goes - not because I'm very knowledgeable, but because I would like to be more knowledgeable and believe we (at least us neophytes) can learn a lot from this forum. How about if we break it into stock block (ie bolt ons only) and modified, and see what we can document as good starting points for those of us who want to have some fun with their motors. The following is what I've gleaned, from other posts and personal email exchanges with people actually building these engines, and am planning for my 40:
Stock block/bolt ons:
* remove rev limiter - snip "gray wire" on some other rqts on other vintages.
* 50 tuner, with one inch cut off
* reeds - Boyesen or mentioned - I'll dig up recommendations sent to me/read here.
* mill head to increase cranking pressure to ~ 160 psi (input welcome) for use with pump gas (91 octane??)
* early heads w/ offset chamber seem to make more power
* 70 horse carbs - I've received an email saying the old 70 OMC carbs may bolt right on. Rejetting will be needed - anyone want to chime in with jetting recommendations? That would be a help to us dummies as starting points.
* lighten flywheel
my understanding (although never specifically noted by any of the guys who've done a lot of these mods so, guys, feel free to chime in here) is that this will allow the engine to make decent power to the 6500-7k range, and unless a bunch of jumping is done which loads and unloads the engine rapidly, will allow it run a long time without welding the crank. Guessing 70 horse output?
Based on my experience (modifying West Bends for karts in the 70's) and a lot of reading, I would guess the next steps would be:
stock block: combustion chamber mods to hemi and set up proper squish band. Pictures of this are on this site or S & F from a guy overseas who did one.
I am thinking this is about as far as bolts ons to stock unassembled block will go. Inputs from y'all w/ experience?
Modified:
* create port map to measure time areas of porting
* define desired full speed operating range - seems 8500 is pretty sporty for all but pure race guys
* define port shapes (reshape originals or add fingers) Any port maps out there?
* find a friend with a mill to cut the ports
* be ready to spend cubic $ testing and torching a few pistons.
* build custom exhaust tuner, shorter and better flowing than the 50 tuner. Any pics? I know there have been some made.
Full disclosure: mods I've done to my 40 to date: rebuild carbs after sitting with wonderful fuel for 3 years. Inherited recently from my brother w/ ~ 30 hrs on it - he didn't die, just upgraded.
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