Which engine?

[Jerry Carter]

So, I need to order my engine. I'm leaning toward the 37 hp Cassler engine. I weigh 170 lbs. I'm just trying to decide between the single zenith vs dual Mikunis. And advantages or disadvantages of full case?

[Sam Buchanan]

So, I need to order my engine. I'm leaning toward the 37 hp Cassler engine. I weigh 170 lbs. I'm just trying to decide between the single zenith vs dual Mikunis. And advantages or disadvantages of full case?

These are not necessarily advantages or disadvantages, just differences.

Full case:
     Less machining required
     Can be used for four cylinder engine at later date
     No discrete engine mount required
     Magneto will protrude into foot well
     Oil cooler not necessary
     Single Zenith carb installation is complicated
     Best with dual carbs

Cut case
      Requires fabrication of engine mount
      Requires a lot of case machining and fabrication
      Can be mounted with magneto outside of foot well
      May need oil cooler
      Works well with dual carbs
      Single Zenith carb installation is simple and incorporates oil cooler and manifold heat

Single Zenith carb
      Simple installation
      Simple throttle cable installation
      May be possible to fabricate cockpit mixture control with some carbs
      Main jet is adjustable
      Seasonal (ambient temp) jetting changes in seconds with no disassembly required
      Built-in choke
      Needs provision for carb heat (incorporated on intake manifold)
      Develops good high-end power for engines up through 37hp
      Does not flow well enough for high-end power on 45hp

Dual Mikuni
      Requires throttle control that can overcome slide spring tension
      Requires synchronization of carbs
      Jetting may require considerable effort and expense of having many jets on hand
      Jetting changes require disassembly of carbs and changing jets
      Seasonal (ambient temp) jetting changes require disassembly of carbs
      Larger engines may require bracing to prevent carb vibration
      No carb heat needed
      Develops good high-end power with all engines

[Steve]

-Well, this is a Sticky topic-;)

One other observation about the single carb 1/2 full-case:

A single intake pipe across the top of the engine with a (plain pipe "T" carb attach in the center) works well with almost any carb...It does need the hot oil pipe wrap...
If you use the Mikuni, the size chart in the Mikuni manual does not apply - a VM30 is about right but larger carbs & brands have worked well too...

Regarding Mikuni main jet change, most of the carbs just require a plug removed from the bottom of the bowl to reach the jet (it's less than one minutes' work with the plug and jet socket sizes)... The running range when found will be within 3 jet sizes all seasons... I found my range with a large jet that I soldered up and drilled out until I got in the ballpark - replaced with the real Mikuni thing - they are two step machined - the solder/drilled version isn't quite right... The drills are so small you will need to hold with a pin-vice and chuck the vice into a drill press - hand held doesn't work out that well...

Mikuni's are a lot cheaper to buy and repair than the Zenith's which show throttle valve wear in a hundred hours - a problem that is daunting to identify...

[Jerry Carter]

Thanks, Guys. My priorities for engine selection are, in order of importance,  as follows: 1) Reliability  2) Maintenance issues 3) Complexity of installation 4) Power/performance 5) Cost.  These may be different from others making the selection. Given the engine configurations and carb choices, I think the 37 hp, full case, dual Mikuni with magneto is the way to go. Cassler says he is 7-8 months to ship from order date, so I plan to order the engine tomorrow.

[grdev]

I'm building a 35 HP Ultra Vair for my XL with a redrive swing a 6' prop with a single carb. Be interesting how well it works

[Aerodude45]

Steve, how did you determine what size drill bit equated to what size jet? I'm thinking of ordering up a set of #60-80 drill bits and a pin vice to try the same procedure...

[Steve]

Steve, how did you determine what size drill bit equated to what size jet? I'm thinking of ordering up a set of #60-80 drill bits and a pin vice to try the same procedure...

Another approach & function clarification attached:

[Aerodude45]

So... modify my jet as necessary for tuning using solder and drill bits for engine performance, then stick a 50cm tube over the end and use a funnel to pour gas carefully into it for... 60 seconds...? (The flow rate is described as being in "cubic centimeters" put without any unit of measuring time). Then purchase a jet with equivalent operational flow volume?

[Dan_]

So... modify my jet as necessary for tuning using solder and drill bits for engine performance, then stick a 50cm tube over the end and use a funnel to pour gas carefully into it for... 60 seconds...? (The flow rate is described as being in "cubic centimeters" put without any unit of measuring time). Then purchase a jet with equivalent operational flow volume?

From what I read spending about 20 minutes on the internet, you are correct about the time unit...  However I read that the factory jets are burnished to final size and flow much more precisely over a drilled hole. 

It is still better to have an assortment of factory jets to pick from,  but drilling can narrow down the number you need on hand.

There is a lot of info out there.


https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=mikuni+jet+numbers+correspond+to+how+many+cc+per+minute


https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=mikuni+jet+calibration+

[Tom H]

I have found that there is a lot of info on the Zeniths, but never did find a source for jets.  I've asked about a source for jets previously, and got no recommendations.  Has anyone found a source recently?

While working out the bugs on DE Stubby, we found that the Zenith carb could not be adjusted to the rich side at higher rpms due to a too-small main jet.  Not having a source for a replacement, I was reluctant to drill/solder or otherwise modify the original jet.  So, I made a copy on the lathe and drilled it out, which is what is on the airplane now, working well.

[Steve]

I have found that there is a lot of info on the Zeniths, but never did find a source for jets.  I've asked about a source for jets previously, and got no recommendations.  Has anyone found a source recently?

[[[[While working out the bugs on DE Stubby, we found that the Zenith carb could not be adjusted to the rich side at higher rpms due to a too-small main jet.  Not having a source for a replacement, I was reluctant to drill/solder or otherwise modify the original jet.  So, I made a copy on the lathe and drilled it out, which is what is on the airplane now, working well.]]]]

[My thinking is the hole can be square and have barnacles in it - if it flows at the right rate you fly nice - smell good - love that clean burning av gas smell]

[Theodore]

Full case:
     Less machining required
     Can be used for four cylinder engine at later date.
i on my third engine, Just for testing on stand(no props yet), ALL are full case and full crank! two cylinders
lightest is 1200cc from Karmann ghia(no cam bearings, bummer)not usable))

[Suomelainen]

Anyone else using full crank on their full case 1/2 engine? Sure it’s a bit heavier but would simplify building significantly.