26mm vs 28mm Mikuni carb

[Bob Wood]

Thinking about swapping out my 2 28's for 26's on my 37hp. Mosler.  Looking at the chart page 7 in the manual the 26 is really a better fit on their chart than the 28.

My question is: will the smaller carb get rid of some of the dead throttle travel between 3/4 and full throttle.  On my 28s full power is about 3/4 throttle travel.

Has anybody made this swap?

Bob

[flyover1974]

Hi Bob, i had the same prob on my 45hp selfmade but with bigger carbs:  32 vs. 28 Keihin flatslide copy (Koso).
But my engine uses different rocker arms. There open 30% more than the standard one because the lever ratio is 1: 1.4 instead 1: 1.15
See here:
https://www.eaglersnest.com/forum/index.php?topic=1558.0
There a lot stuff about main jets and carburettor sizes.
Im sure if you use the 26 instead of the 28, your air flow at wot is faster, and the carb work well.
Too big carbs means less power at wot, complicated setup, impossible to find a clean smooth setup.

[Bob Wood]

Thanks!! very interesting, confirmed some things for me.  My 37 mosler seems to run lean. I am running 180 mains and #5 slot in the needle and my plugs are clean and tan.  My issue is I get random cut outs or RPM drops until i went rich with the needle and mains.  I had 5 flights issue free this month and on #6 I went to 2800-3000 to cruise and got the rpm drop. Never quits and seems to work it self out with throttle rpm changes.

I am tempted to try the VM-26's.  I get no RPM increase from 3/4 to full throttle.

I also show EGT at or below 900 (where my Belite gauge begins.

Engine runs great, pulls strong 3200 static on the ground  3450 in the air.

Thanks for your input!!  Maybe see you in a couple weeks at EAA 

Bob

[stevejahr]

Note that carbs not only have to pass the right amount of fuel but they must convert the liquid fuel to atomized fuel. Atomization is helped of course by heat but also by pressure changes. Larger carbs often have slower air velocity through the throat and thus reduced pressure drop. That can lead to poor atomization compared to a smaller carb. And all kinds of weird stumbles and challenges getting the mixture just right.

On carbs it is always better to err on the small side than on the large side.

[flyover1974]

Steve, that is exactly what i mean. Too big carbs mean slow airflow at wot and this mean bad atomisation of fuel and this results in bad setup at the whole rpm range. Not only the engine work bad but in some rpm range is too rich, in the higher probably too lean, the transition from idle with fast open throttle probably result in sputtering engine....and so on

[Leonard]

I will but your 28 carbs.   Leonard

[flyover1974]

Leonard, i don't understand what you mean?

[Bob Wood]

Leonard,
Great seeing you at Oshkosh. I will keep you informed on my carb swap and send you my 28's if I make the switch.  I am going to try changing my valve lash to .010 as we discussed. I will let you know how it goes. My 26's came in the mail last week and are ready to test.

Happy flying
Bob

[Bob Wood]

Leonard,
The VM-26's seem to have solved my issue.  I have the 2 VM 28's here for you when you want them. I also went with the .010 valve lash setting and it seens to run better. Best of all the weird miss or hesitation has not been seen in the last 2 hours of flight time.

Let me know if you have any 1/2 vw parts to swap I am always collecting to build up another engine. 

Bob Wood

[Leonard]

Just put two new carburetors on the original Legal Eagle 26mm it runs great took off 28 old ones (21years and used) Runs much better.  New ones can be bought at  Summit racing in Arlington TX.682 375 2600 bout $100.00 each

[racerroo]

Bob, what size jets did you end up with in your VM26's?
-Kevin