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CMB Vac 21 in JAE?

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#1 PaulHail


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Posted 13 February 2018 - 12:29 PM

I've read through old posts and it seems that several folks were having problems with the VAC 21 when it came out.  Seems the factory carb was causing grief.  Also sounded like the engine preferred smaller props spun fast.


I haven't measured the stock ports but my plan is to set them 130 for transfers, 190 for exhaust, set the head to 0.006" and run 60% fuel. For pipes I have what appear to be an OPS, Irwin and mystery pipe that seems to be in between the Irwin and OPS for diameter and looks like a Macs pipe but has a weld in the mid section.  I plan to start off with the pipe at 8".  For props I was planning to start with ABC 1815-19-38, 1815-17-45 and Octura 1445.


Just wanted to know if anyone else is still running this engine and if I'm in the ballpark with my plan or I need a different carb or setup to get started.

#2 shoboat


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Posted 14 February 2018 - 07:28 AM



I would stay with 19/38% line up in the ABC family of Game Changers.

I am not sure I would go up to 190 degrees on the Exhaust timing?

The 1817-19-38% works very well also. I would remove some cup out

of the trailing edges. The 1445 will need to be pitched up more or you

could just run the ABC H-5.


Have Fun Testing,


Mark Sholund

#3 Stuart Barr

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Posted 14 February 2018 - 09:21 AM


The problem with the carb on the VAC can be corrected by lapping the face of the drum housing.  Do this on a piece of glass or granite using 400 grit with oil or water.


#4 PaulHail


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Posted 14 February 2018 - 10:39 AM

Thanks for the tips.


Stu is the issue that the carb doesn't seal to the drum housing?