how many shims

chevyiron420 Mon Feb 16, 2015 5:05 pm

On Babe Bee engines, running 20% fuel, how many head shims should I need under a Merlin drop in plug? I have been test running with three(all I have) and they are hard to start sometimes. They want to get the jitters, prop just bouncing back and forth.

pkrankow Mon Feb 16, 2015 6:37 pm

Prop bouncing back and forth is a sign of over compressed. I would expect 3 to be about right, but don't be afraid of going up to 5. I am not sure the maximum that fit well with the threads (I want to say 7, but I have never used more than 5)

Phil

**RknRusty** Mon Feb 16, 2015 7:21 pm

That's a really high compression head. I usually start an untested engine with 5 shims and remove some as necessary. My Tach Race Black Widow had 4 in it(with 35% nitro) and ran well over 21k RPMs.
Rusty

TDbandit Mon Feb 16, 2015 11:52 pm

The Merlin head has a wider squish band as well as a shallower combustion chamber compared to the TD trumpet head. I too experienced the bouncies while I was adjusting the head height. I just added shims until i could finger start it easily and went from there. My tach race engine ended up with around 3 shims for 25% and 4 for 35% (Bandit)

getback Tue Feb 17, 2015 7:43 am

My TD tach race engine was 3 , 35% pressure , while cleaning the BB notice the head was only finger tight / and from what I have seen so far all are very clean from the runs . Telling me the fuel is superb, O coarse wee are talking short runs but does look good . Eric Babe Bee .049

forgot to put what head I was using http://coxengines.ca/insert-for-conversion-a-and-texaco-head-hot.html I have no idea what I was thinking when I ordered the Texaco and extra cooling fins .

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