Carb Spitting update

[Bob Wood]

Saw something interesting today. My engine was dynamically balanced at 3100 RPM and runs very smooth except for a narrow band around 1/4 throttle or so 1800-2000 rpm approx. Just a little more vibration and some carb spitting on right  jug only when I remain in that band for a length of time when landing , not a big problem but puzzling, and aggravating..
My A&P told me some engines have this and are even placarded to avoid running continuously in these bands.

So today I fire up to fly and forget to turn on the gas, all is well for a minute or 2 until the left side starts sucking air which I see in the fuel line. running poorly until I turn on the gas and all is well in 3 seconds.  So I get to thinking, my spitting carb on the right has a higher level as it is not sucking bubbles.  Maybe that is why it is spitting....I set both carbs to the manual(17mm) and rechecked them a couple weeks ago.  I am thinking of raising the float to decrease the level in that right carb a mm or 2 above book to see if the spitting stops.

I am running Viton needles in both VM 28 Mikunis and assumed both were adjusted to the same level.  I remember reading in the forum somewhere lots of us lower the levels to decrease spitting.

Anyone been through this?  How much are you guys running above manual ( 17mm)   on float height? Thinking of trying 18-19mm

Thanks,
Bob

[flyover1974]

I dont know exactly why some engine spitting fuel but i know why my engine did that:

When i build my engine with new valves, remade the seats as well and then you need to broke it in with that special abrasive cream (sorry, no idea how is the english name).
My ones token by a car garage was a very rough one. After finished work i checked the head blowing air with closed valves and was not airtight but i was thinking is not a big problem.

And here is exactly the problem: carbs spitting because on compression the gases inside the cylinder blow out of the inlet valve and this results in spitting gasoline on carbs, because the way was vice versa now.
I broke in the inlet valve for the second time and now perfect airtight...and no more spitting.

My personal opinion: im sure spitting is only possible when gases comes back out of the inlet.
If you use the standard cam or similar (they work very well for a low rpm torque monster) and spitting occours, than you have to check your inlet valve if its airtight.

ceers

[Bob Wood]

Very interesting thank you! It makes perfect sense to me because it seems to have worked itself out. I flew my first 2 hour long cross country last night and it seems to have worked it self out...No Spitting at all.. Maybe crud on the seat that works itself out..

Thanks!
Bob

[flyover1974]

Some important thing to say about i written above, is that this is mostly true only if used a cam like the original.
Speed cams have a larger window where both the inlet and outlet valve is contemporary open. This is needing for high rpm like 5000 and up.
Dont want to explain now why because it needs more than a few lines to write but if interesting, google knows a lot