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421 cub having trouble launching


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Here is the original post by Matt after the build/dyno (graph is on last page). He ran the carbs as set up by k&t and noted it was rich then at their elevation (we are higher yet).

 

http://www.atvdragracers.com/forum/dyno-results-33%3B/5913-gas-4mm-cub-set-up.html#/forumsite/21322/topics/5913?page=1

 

 

I agree. If it only has 145 pounds of compression at 2600 I'd be looking at new domes.

umm it already has special secret domes :)

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After reading that post, 19cc domes should be plenty more compression IMO even at that height. Typically a 421 cub is looking at race gas around 21cc's. Those domes should be burning that fuel plenty fine up there. I know the exhaust is big, but it's still 196* and there is elevation, but it seems just a touch on the fucky side. Looking at the graph, it should have plenty enough grunt to move out of the hole.

 

I'm curious, has anyone pulled the head off to get a good ol' fashioned lookey at what's going on inside?

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After reading that post, 19cc domes should be plenty more compression IMO even at that height. Typically a 421 cub is looking at race gas around 21cc's. Those domes should be burning that fuel plenty fine up there. I know the exhaust is big, but it's still 196* and there is elevation, but it seems just a touch on the fucky side. Looking at the graph, it should have plenty enough grunt to move out of the hole.

 

I'm curious, has anyone pulled the head off to get a good ol' fashioned lookey at what's going on inside?

 

That graph is from sea level, where it most likely had 160+ pounds of compression.

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I wouldn't pay too much attention to that. It's likely a custom dome, but for closer to sea level.

 

Most engine builders don't like trying to cut custom domes for a motor they didn't build.

 

You may have to send the specs to Noss and let him cut you a set.

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