Thanks Hobbsy,
i think that’s how the factory would have done it. I’ve actually used piston TDC for #2 and #3.
Maybe I’ve overthought it, but wouldn’t it be like doing a single cylinder engine and timing with 1 tooth off? I’ve run the engine and it has good idle and transition. My curiosity tells me to set it up as you say and compare the running between them.
i estimate about 6 degrees of rotation difference. Maybe, it just doesn’t matter with our toy engines!

Hi Jesse,
i have no trouble in finding top dead centre.
My question is TDC with piston fitted to the cylinder or TDC of the crank pin without piston fitted? ( there is a difference )
As Hobbsy said and other information I have seen is that the crank pin is used as a reference in determining TDC. But this does not give piston TDC on #2 and #3 when fitting the camshaft gear.

Ok , for anyone that is following or interested, this is what I have discovered today.
Static rpms on glow are 8300 rpm

Today, I fitted a Morris mini motors timing ring and sensor to the engine. I already had one of Adrian’s 3 cylinder CDI units. I ran same prop and fuel and was only able to achieve 6500rpm! Hang on, all reports I’ve heard say even more rpms!
The timing is set at 32 BTDC. The engine ran smooth with good idle and transition. Cylinders #1 and #2 were cool compared to #3. There was no misses in the engine. Something is not quite right with this system so I took some timing measurements.

Obviously, the three cylinder engine has it’s cylinders spaced at 120 degrees to each other. The timing ring has the magnets spaced at 120 degrees......... all good so far.

I think if I go back to my original question I asked here it explains a lot.
I have found that TDC on cylinder #1 is say at 0 degrees, rotate the hub in direction of rotation then the next TDC should be at 120 degrees on #2 cylinder right! Wrong, it is at 126 degrees ( this has to do with the geometry of the slave rods in how they are connected to the master rod.)

I measured all cylinders and this is what the respective angles are to TDC

#1 to #2 is 126 degrees
#2 to #3 is 108 degrees
#3 to #1 is 126 degrees

None are at 120 degrees as per the timing magnets. I don’t know if the gas engines are the same but I assume they are.

What this means is that Cylinder #1 will fire at the time set correctly, say 30 degrees BTDC.
But cylinder #3 will fire 6 degrees late at 24 degrees BTDC.
And cylinder #2 will fire 6 degrees early at 36 degrees BTDC.

The only fix so that each cylinder gets to fire at the “ set time “ is if the magnets are spaced at 126-108-126 degrees to match the respective cylinder.

This obviously doesn’t affect a single cylinder glow to CDI, but certainly does with a multi Cylinder!
Has anyone had similar results to me in running glow on CDI?
Also, I would have thought both Adrian and Morris would have noticed this mistiming, sure it works but not very well!

look forward to some replies.

cheers Steve







#2 to #3 is 108 degrees

Thanks Gary,
yes your right, but much better to have the timing of the ignition set correct and the same for all cylinders. I wonder if the gas radial engines are setup the same ?
Morris, from Morris mini engines has apparently come to the same realisation in England and is making alterations to his timing rings. He is sending me one with 126-108-126 magnet spacing.

Retarded timing increases cylinder temperatures.
I used to have a box of VW Bug pistons that had swallowed valve heads. All came from #3 cylinders with retarded individual timing on the #3 cylinder.

I will look at the aviation engine handbook later for their take. I personally would lean towards piston position.

Also may be worthwhile to audit the cam timing from cylinder to cylinder.

Some not so pretty pictures there! Once the timing ring is the correct spacing I will not have any retardation issue. I did note above that the #3 cylinder was hot compared to #1 and #2 though. #3 was retarded compared to those.