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Needle valve settings, generally...

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Foxfan23/08/2019 12:07:16
883 forum posts
7 photos

Needing to raise some pennies for a small 4 stroke, I am intending selling some older engines and want to test them so I can truthfully say that they run (or not!)

But what, if any, is a good rule of thumb setting to start with on a needle valve?

I'm thinking both RC carb equipped and basic needle valve (DC glow Dart and PAW 149, not that I have any diesel fuel, but I know that runs well).



Jon - Laser Engines23/08/2019 12:21:42
5068 forum posts
217 photos

i always attempt a start and go from there.

When it comes to starting, dont just slap a leccy starter on it and go ham.

Sling some fuel down it, flick it over a few times then try a hand start. The backwards flick trick gets most things going but even if not, it should at least fire and that is a good start. You also get a feel for the compression (or lack of same) and the general feel of the engine. if it fires up but stops that suggests lean on slow run so adjust and repeat, if it starts but wont go up on the revs again its likely lean, probably top end this time.

As a general rule the engine will always tell you what is wrong, you just have to listen to it

Nigel R23/08/2019 12:30:11
3394 forum posts
524 photos

try and blow through the carb with the throttle open

if you can't blow through it, open the needle

if you can blow through it really easily, close the needle a bit

Denis Watkins23/08/2019 13:05:51
4055 forum posts
75 photos
Posted by Nigel R on 23/08/2019 12:30:11:

try and blow through the carb with the throttle open

if you can't blow through it, open the needle

if you can blow through it really easily, close the needle a bit

Tag on to Nigels post

Put a spare piece of fuel pipe on the carb fuel nipple

Then do the above

Foxfan23/08/2019 16:00:12
883 forum posts
7 photos

Haha! I think that might be a good idea!¬



Frank Skilbeck24/08/2019 22:14:23
4569 forum posts
101 photos

Having just watched our club diesel expert flick starting a PAW 149, then too much or too little fuel and you'll flick away to no avail, best to see if you can find somebody with the diesel technique.

Foxfan25/08/2019 00:29:10
883 forum posts
7 photos

Yes, ours has the same trouble, but he makes his own fuel apparently and it still costs him £20 a litre! No wonder when it does finally start he gets it as high as he can see, it cuts and glides to earth as slowly as he can get away with.


Peter Miller25/08/2019 08:25:46
10489 forum posts
1246 photos
10 articles

With a diesel,Start under compressed and slowly increase the compression until it fires. You may need to keep priming it but don't flood the crankcase.

If there is too much fuel you can get it oscillating. In other words it goes back and forth between compression forwards and backwards. Blow through the exhaust ports to clear excess fuel.

Diesels need a really good hard flick.

paul d25/08/2019 08:30:27
101 forum posts
8 photos

That paw unless it's gummed up will not not need much of a prime, a few chocked turns is normally enough.

Mike T25/08/2019 12:46:17
435 forum posts
29 photos
Posted by Foxfan on 23/08/2019 12:07:16:

But what, if any, is a good rule of thumb setting to start with on a needle valve?

Open 1.5 turns from closed.

Foxfan25/08/2019 13:12:10
883 forum posts
7 photos

Thanks, Mike. That was the kind of info I was after.



Martin Harris25/08/2019 14:17:54
9084 forum posts
224 photos

It's not too clear from the title/OP whether you're only looking at diesels which have the additional complication of the compression (effectively ignition timing) adjustment but most of the fuel related tuning advice holds true for any carburettored IC engine. My recollection of diesels is that there's little difference although my experience doesn't extend to RC carbs on them but I doubt if there's much difference although if I'm wrong, there should be enough expertise here to correct that assumption!

Certainly for glow engines, while 1 1/2 turns is a good guide figure for the expected running position on most engines, on an unknown engine I usually start with 2 1/2 turns on the premise that the low end needle is doing most of the regulation and the engine should start as long as there's sufficient flow through the needle valve to feed the idle jet - leaning out of the top end then can be performed and the bottom end fine tuned once the engine has reached running temperature.

Having the main needle open further than the expected running position may reduce problems from partial congealed oil blockages due to storage, allowing easier initial starting and almost certainly allowing the engine to run on full throttle rather than cutting out when its opened due to being slightly too lean. Unless your bottom end was spot on, you'll need to repeat the top end check/adjustment and re-check the bottom end continuing until you are happy with idling, transitioning and top end.

Many people seem scared to touch the low end but it's pretty simple in essence. Assuming the engine will run at low and high throttle (the blow check described in posts above should allow it to start) then lean the bottom end for highest rpm. Check the transition - an immediate cut needs the low end richening a bit and a burbling hesitant transition needs it leaning. Recheck as above...

One extra tip for glow engines is that although they may throttle OK in service, any large drop in rpm when you remove the glow clip indicates that the bottom end is too rich - often this condition can lead to unexpected cuts while idling due to over cooling of the glow element. Alternatively it can be due to a glow plug on its last legs.

Edited By Martin Harris on 25/08/2019 14:24:04

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