Ballpark setting for small diesel engines

[Fuzzy-Felt Bloke]

Good evening all

 

I'm looking for a ballpark setting for the needle valve and contra head setting for smal DC diesel engines. Im aware that each engine model would be different, therefore I'm after a good starting point.

 

Kind regards

 

Tom

 

[Jon H]

Cant tune engines by counting. 

 

Close the needle all the way and back off the compression. Squirt a small amount of fuel into the carb, and a bit into the exhaust port. 

 

Flick, flick, add 1/8 turn compression, flick flick, another 1/8....repeat until it fires. Eventually it should fire up and run on the prime. It will then stop as its out of fuel. 

 

Prime again, open the needle, if it again runs and stops you didnt open the needle enough so open it more and try again. 

 

 

This might sound really unhelpful, and it is in many respects as its a gross over simplification, but you simply cannot tune engines by counting turns on things. Model diesel engines in particular are a law unto themselves and you just have to learn how to coax them into life. I run up my model diesels from time to time in order to keep them in good condition and to remind myself how the damn things work. There is no substitute for just giving it a go. 

 

 

[PatMc]

I've attached a couple of guides for beginners in pdf format that you may find helpful. There's more in each than you really need. If you have the "Quickstart" spring & finger mangler I'd advise not to fit them as they're more of a liability than help.

 

Operating a First Diesel.pdf 

 

Quickstart_instructions.pdf

[Peter Jenkins]

One way of finding an approximate position for the sub-piston, is to have the engine at TDC and then while working the crankshaft to and fro using an attached prop, screw down the compression screw until you feel the sub-piston as you jiggle the prop.  Now back off one full turn, flick the prop over and the sub-piston should then pop back up to the approx correct position to run.  Worked for me on a Mills 75 and PAW 249 and 149.

[Simon Chaddock]

Hmmm.

None of my diesels. including my original Mills 75, will move their contra piston by comprssion alone but only when the engine fires.

In this case moving the contra piston so it actually touches the piston at TDF would be unwise. 

As I understand it the contra piston is intended to be a much tighter fit than the piston.

[Peter Jenkins]

Well mine did.  The Mills contra piston moved when you flicked over the prop.  Obviously, I turned over the prop slowly first to make sure there was clearance.  It used to be a standard method of finding an approx correct contra piston position in the '60s when I used to run just dirsels with the Wenmac Hotshot the only glow I owned.  All rather tidly these days but it was all I could afford from my paper round!

[Jon H]

I agree with Simon. Its a much safer idea to start with low compression and work up as you risk bending something if you do it the other way around. 

 

I have never known an undamaged contra piston to be as free moving as you describe and they only move when the engine fires, resulting in the classic 'click' as the contra piston shoots up the bore and whacks the comp screw. 

[Peter Jenkins]

If you guys would read what I 'd written which is that I wiggle the piston either side of TDC till I fell the contra piston, then undo the comp screw by 1 turn and make sure the engine is free to rotate through TDC before flick the engine over.  OK, I accept that the contra piston didn't always go up till the engine fired.  

I never damaged any of my engines by following this route which, as I say, was a standard method of finding an initial comp setting.  I would have read that in my monthly mag, the much lamented Model Aircraft.

[2.4g Shaun]

Tom,

For most DC Diesel's 2 to 2  1/2 turns out is a good starting point. I seem to remember that the glow motors had a finer thread on them and a different spray bar so they were different.  

As long as the motor can be flicked over when  primed keep  tweaking it down until it fires and runs . Then it's simply a matter of small adjustments until it runs sweet but ensure you don't overcompress ( you risk bending the con rod ) or running too lean.