DiY 1200ccm 1/2VW full case

[flyover1974]

Hello community,
Im proud to announce You my selfmade 1/2 VW first run right now...two hours ago
specs:
1200ccm,
dual Koso (Mikuni copy) 32mm carburettor,
94mm AA performance cylinders
86mm AA Performance crank
F1 prop hub,
Completely changed cylinder heads with 70ccm instead 49ccm head volume,
Compression ratio 8,15:1

Here the YouTube links:



The the carbs are not synchronised and not checked if the right mixture is correct...but it runs very good. 

Ceers
Hermann

[Dan_]

Friggen awesome dude..!

[flyover1974]

Thank you😊
Now its a lot to refine.
1. The left carb spits a lot of gas (right one a little) and checked with a infrared thermometer  its run 20 degrees Celsius hotter. Around 120 degrees measured between head and cylinder. Oil was handwarm and i did not want to run it longer then some seconds at full power because all exept case and modified heads is brand new and not broken in.
2. Than it was not possible to open the throttle more then 2/3 because max rpm slows down over there. I think it runs too rich.
What is the jet size you use on yoir carbs ?

Hermann

[Dan_]

2. Than it was not possible to open the throttle more then 2/3 because max rpm slows down over there. I think it runs too rich.
What is the jet size you use on yoir carbs ?

Hermann

Herman,
Here is a comment on it from the old Yahoo group.  It explains it rather well and his engine is very close to yours in specs...

joe spencer
Message 2 of 5 ,  Nov 26, 2013
the aluminum carbs weigh +- one# each so believe thats what you have. the zinc "a lot more?" what size are they? 28s? 26s? the 28s have been shown to be correct for a 45. the chart indicates that 26s should be too.
experience that i'm aware of has shown that above 28s isn't optimum; the motor can't use the entire slide range and flow is affected. i'm running a 160 on one side and a 150 on the other...that's correct on my particular 45 for 1350+- EGT at 65-80F. the power comes on above 1300 egt. 1400 is max according to casler and that's the target for mine but havent quite gotten there except transients.  on a new motor i would start at about 170-180 the idea to keep it  little rich for a while til it loosens up a little; assuming your motor performs closely to mine. the dual egt is absolutely the way to go IMO. much more accurate than plug color etc and the results of a jet change are immediately apparent. i couldnt tune in 50F increments or target 1400 EGT w/o egts. maybe some of the motor experts can. let us know what you learn. do you have the mikuni tuning manual? i can forward a link. Kiblinger has much hands on mikuni experience
believe i saw you sign an earlier post as John Harmon? the Harmon Rocket man?


Spencer

[flyover1974]

Dan,
Thank you. This post helps me for both questions because Joe speak about 45 cubic inch engine and maximum 28 carbs, so i think the 73  of mine and 32 carbs (not Mikuni copies how i said but Keihin flatslide copies) are the right choice...hopefully.
My heads uses standard 35,5mm inlet valve and 32mm outlet valve but with higher rocker arm ratio of 1,4:1 instead of 1,15:1. This gives a ~28% bigger open for better cylinder filling.

So...if Joe need 140 to 150 jets in the 45er i dont know what to use in the 73er...
I think the best thing to do, is to mount asap the egt sensors to calibrate the carbs to be below 1400 egt.
Some pics of building in attachment. 
Ceers

[Dan_]

The 45 Joe is referring to is the horsepower... 


The displacement is 1200 CC. 


EGT is the way to go on tuning.  But if you had Mikuni 28s you could just stick the jets in and be real close.

Spec page for Joe's engine:  https://www.hummelengines.com/engines-specifications

[flyover1974]

Thats not good news for me...
It means my carbs are definitely too big
So i will check the jet size and mount the egt and than i report.
Edit:
I own a lambda sensor and maybe additionally to egt is good for precise setting. I read for best power lambda 0,85 is the way to go and probably good for cool egt temps because it is on richer side

[ArcticDave]

Very nice! 
With those bigger carbs you may not get enough velocity through the plenum to properly atomize the fuel. The 28's seem to be the sweet spot. 
I wish I was that far along with an engine! Keep it up ☺

[flyover1974]

Thank you but there are 2 ways to go for an engine. Using stock or slip in  big bore cylinders, cranks and so on and finisch it in short time with minor changes and, thats important, with less power or doing mayor changes with needs good skills and the use of probaly external work that costs extra...
I needed 5 monts and probably 300+ hours but i made 99% by myself.

For the carbs,
Its the second time i bought to big ones for a project
Last year i rebuild my '64 Vespa and put too big carb on it. The result was a very hard to setting engine. Changing to a smaller one the problems was gone. Btw the 28s are ordered

Ceers

[flyover1974]

Some more pics of building.
First is the finished heads after weelding.
2 ans 3 are the caps for the oil channels. If you are not using a new case i recommend to put out all oil caps and clean that cannels very good.i had in the main oil gallery obstructions that close it more than 50%. Needs a drill to put that shit out.
My case where sanded with glass pearls.
If you decide to do this, do it only outside and with all holes closed properly. My mistake was to do that work inside too. The result is very nice in and outside but you are not able to clean it and this small glass pearls are very hard and the result you see in pic number 4. This was alfter only 500ml gas running and a few minutes. I had a big brandnew full flow oil filter mounted

[flyover1974]

Hi Guys,

today i found the time to weigh my engine in ready to fly condition.
Dry without exhaust tubes and prop is 88,4 pounds.
The picture says more than tousand words