carb adjustments for Mosler CB-40 on a Mini Max

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Kim Brown

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I have a recently completed Mini Max and acquired a Mosler CB-40 engine for it from a Canadian seller. I am now at the point of determining the condition of the used engine. With a very knowledgeable helper/guide/a&p, we got the engine fired up today after it had not run in two years. Took a little effort but got it going. The engine came to me with an Aero slide carb which I opted to change to a Zenith carburetor having checked with Scott Cassler on what to buy and I got it from him, the model number escapes me but it is what he said to get. We followed what were internet guideline on carburetor settings for main and idle and fired it up. Took a lot of tinkering with the two adjustable features and we finally got all it had with what we knew, and that was about 2800 rpm. I believe ideally we should get 3000-3200 rpm. Prop is a 54 x 22 so it was not the culprit. The other oddity is all of the rpm we could get came in the first half of the throttle travel. In the last half, we got no further rpms. Our sense of it was that we were lean and not getting enough fuel to get the rpm we should. Thoughts that come to mind are air leaks somewhere and fuel blockages somewhere. We had no luck in pursuing these possibilities. Looking for other peoples’ experiences with this engine and Zenith carb to see if we can get throttle response in the second half of travel and close to the desired rpms. Thank you very much for your time and consideration

Kim Brown

Hawk12979@gmail.com
 

Dana

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It's been a few years, but an old not about mine said I was getting 3000 rpm static with a prop marked 56x26 but measured more like 57x27.5. 3200 rpm @ 60mph. This was with a Zenith carb and Slick magneto. Have you checked the ignition timing and valve clearance?

Also how's the compression? I had a lot of trouble starting mine at first, turned out the valves needed grinding.

A fuel flow test is also easy, just measure how fast it comes out the float bowl drain.
 
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Daleandee

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A friend with a Legal Eagle removed his Zenith carb and installed dual Mikuni carbs on his Moser engine (don't recall the model of the carbs) and it really came alive. I believe he got that tidbit of info from Mr. Leonard Milholland ...
 

Kim Brown

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I've been trying to avoid the obvious error in timing, the prior owner had it way at something like 19 degrees btdc, and it needs to be at or close to 28, will cautiously adjust it to the correct point and retry, thanks for all comment
 

Kim Brown

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Today we managed to get the timing up to the correct 28 degrees btdc, started it up and ran it but were still unable to get past 2800rpm. We tried all manner of carb adjustments, none affected rpm, and all the rpms occur in the first half of the throttle travel. We do believe we are running lean more on one side than the other, plugs indicated lean and egt on one side much too high, 1400, other side 800, sensors are notorious for error but one exhaust pipe much bluer than the other and handheld laser thermometer supported the egt sensor readings. We did quick check on lifters, seemed right. Next we are going to do a valve job and lapping on the side that is running hot. We could find no explanation for running lean condition as far as any leaky intake issues. Will let you know how we do with valve work. One side note on propping when you have the timing where it is supposed to be with a CB-4 Mosler, you better put a lot of oooomph into your spin or it does indeed want to kick back and if you are not careful, the results could be ugly.
 

Dana

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Yes, you have to snap the prop through pretty quickly when starting it. What mag do you have, the original Fairbanks-Morse tractor mag? Mine had a Slick 2316 (4316 with two poles shorted out) which has an impulse coupling making it a lot easier to start.

Did you do a compression test?
 

Pops

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I also used the Slick 4316 that has a 25 deg impulse lag and never had any problem with the timing at 28 degs.
 

poormansairforce

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Today we managed to get the timing up to the correct 28 degrees btdc, started it up and ran it but were still unable to get past 2800rpm. We tried all manner of carb adjustments, none affected rpm, and all the rpms occur in the first half of the throttle travel. We do believe we are running lean more on one side than the other, plugs indicated lean and egt on one side much too high, 1400, other side 800, sensors are notorious for error but one exhaust pipe much bluer than the other and handheld laser thermometer supported the egt sensor readings. We did quick check on lifters, seemed right. Next we are going to do a valve job and lapping on the side that is running hot. We could find no explanation for running lean condition as far as any leaky intake issues. Will let you know how we do with valve work. One side note on propping when you have the timing where it is supposed to be with a CB-4 Mosler, you better put a lot of oooomph into your spin or it does indeed want to kick back and if you are not careful, the results could be ugly.
You have a fuel delivery problem. The cylinders not being fed evenly.
 

Pops

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Intake leaks are very common. Spray some WD-40 around the joints of the intakes and watch for a rpm change to find the leak.
 

Kim Brown

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TFF
I have the manual that you referenced, been through it a couple of times and saw no mention of a transitional plenum. Could you elaborate a bit on what that is please? Thanks
 

TFF

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It’s something that’s not there. It wasn’t designed in. Essentially it’s a reservoir. The T area should be bigger in volume. More complicated to make.

First thing is you have to make sure the cylinders are sealing and that the cam is opening valves the same amount. that is important. You also might see it the timing is the same advance for each cylinder. Something internal in the mag offsetting the timing on one cylinder. Not probable but possible.

Sometimes these simple manifolds can favor a flow direction which cuts off the other essentially. Just a potential option.
What is the history on the engine?
 

BBerson

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The 2800 sounds about normal to me for static. What is the book number for static rpm?
Dana said the 3200 is at 60 mph.
The throttle position is not abnormal either for static, I think.
 

Kim Brown

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Hi
I will have the valve job, haven't done it yet. My guy said spraying anything behind a running prop just makes a mess, maybe the wd-40 idea is good for engine checking with no prop, but I think we can't do it with prop. Thanks for all thoughts
 

Pops

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Hi
I will have the valve job, haven't done it yet. My guy said spraying anything behind a running prop just makes a mess, maybe the wd-40 idea is good for engine checking with no prop, but I think we can't do it with prop. Thanks for all thoughts
Engine at idle, no problem. Just takes a little, you are not washing the engine down. Just a little coming out of the plastic tube of the WD can to wet down the seams of the intakes. Idle rpm changes, that is your leak.

My 1835 cc, 60 hp VW engine has a WOT at static of 2950 rpm. WOT on climb out of 3200 rpm with a Culver 60"x 26" pitch prop.
 
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