JAP 4/2 (Engine no. P15904UVX)

Woodsman, would love a copy!
Got me knickers in a twist as I think I have sent you 2 PM's!

Alan

No worries. Email sent.

Managed some playtime today!
Carb all cleaned out (again, nothing found) air filter left off and belt removed from countershaft/generator. So no load.
Started first pull, let it warm a bit and it's running is exactly the same - good tickover but on opening throttle it goes very rich and won't accelerate.
I found if I open the throttle manually very slowly to let the engine revs build it gets to possibly 2000 rpm and it's as though the spark has been switched off and won't start firing again until the revs drop.
I'm thinking magneto problem!
Believe it's suffering from a weak spark - good enough with impulse to start well but not powerful enough to ignite the enriched mixture encountered once throttle is opened wide.
So I'm looking at a coil/confenser fault and will be having a resistance measurement (do we know primary/secondary resistance of a Wico series A?) or hopefully a dicky condenser, it's the original.

Alan

sounds like its being flooded with to much fuel dousing the spark.

Maryalice

Removed magneto today and extracted the condenser, believed to be the original. Measured it's capacitance at 0.25 uf so appeared to be functional.
The actual condenser was potted in a brown wax presumably to give it some mechanical support and resistance from the elements which was removed with a hot air gun.
Capacitor replaced with a good quality item which was potted in a silicone rubber, once again for mechanical support. HT coil measured 5.6 k ohms which I think is about right.
Got a bit late to try it out so will have to save that for another day!

Alan

Nice job !

Thanks! However it didn't allow it to run any better!
I've come to the conclusion that it's definitely a mixture problem, I managed to get it better than it has been but still a long way from running properly.
Very small adjustments of the mixture screw has allowed it to cater for one bank of load lamps (275 w) but switching on a second causes the throttle to open wide and run very rich, rpm is recovered (about 1800) which causes a slow hunting effect with throttle going from tickover to wide open with plenty of black smoke - very rich.
I am considering removing the carb from my Villiers 25HS which is the same type although I don't know what jets/choke it has fitted but whatever is fitted I think I will give it a try.

Alan

you keep describing an overloaded genny or engine, maybe your wiring has cause a severe load more than you thought. Run the engine without the genny, open the throttle and see what occurs,
an overloaded genny will act like a severe brake causing the engine to over fuel and because it cant rev it floods and creates black smoke.

Maryalice

Something along those lines did cross my mind although the engine is rated at 2.5hp so my load is well within it's capability.
With that in mind I have run it without the drive belt - no load, just the engine and it shows similar results, 'blipping' the throttle just produces a lot of black smoke with the engine sounding gagged. It will not rev to 2000, hence the condenser change.

Alan

have you tried strangling the fuel supply to the carburetor to see if it will rev.

No I haven't although not sure how I would do that? Perhaps just 'crack' the petrol tap. Float chamber level appears ok and it doesn't flood or leak anywhere.
I understand the magneto is designed to advance timing with revs (20 something degrees rings a bell) Although the mag mechanism all looked good, if it didn't could that cause the problems do you think?

Alan

I dont know of a magneto with advance and retard unless its a bike magneto, unless you have fleible fuel hose then cracking the tap to restrict flow will work.

Maryalice

maryalice wrote:I dont know of a magneto with advance and retard unless its a bike magneto, unless you have fleible fuel hose then cracking the tap to restrict flow will work.

Maryalice

Wico A impulse magneto fitted to this engine is retarded to start the engine and auto advances when the speed picks up. However, if the mechanism is not working properly then it may still be at the retarded position. I don't know if this is the issue with your engine, but you may like to re-inspect the engine governor mechanisn also.

Oops missed that one

Must admit I did wonder where I had seen reference to timing so dug back through my info and found it.
I'm sure I set it up correctly but to confirm I have removed cowling to recheck. Job for tomorrow. If the advance was inoperative I guess the fuel charge would not have time to fully combust?
Upon removing plug it was very sooty as is the inside of combustion chamber hopefully if I ever get it running properly some of this will burn off!

Alan

Checked and verified ignition was ok with magneto impulse working well so went for a carburettor swap from my Villiers 25HS. This is a similar cc engine although rated at 3.5hp compared to the JAP's 2.5 although using the same Zenith carb.
Unfortunately it wouldn't fit! Lugs underneath the 25HS carb were in the way of the governor housing on the JAP.
Then decided to swap needles and jets for a trial run and noticed the mixture control needle from the 25 was far more 'needle like' than the JAP's so I decided that this would give finer control of fuel supply.
Fired it up (always starts first pull), warmed up and tried to set mixture - basically no difference!!!
Can't even get it to run at a steady 1500/1800 rpm, produces a slow hunt, up and down in revs with the governor varying throttle from Tickover to wide open including sparks from the exhaust although it still had a lovely Tickover!
Played with plenty of engines but never experienced one like this, it's not going to win though (hopefully!)

Alan

Could it be an issue with the governor mechanism itself? It seems as if it's too sensitive if that makes sense.

Thanks Mattblack, possibly! Looking at the carb linkage and governor arm (which I bought new as the original parts were broken and missing) I would say the throttle spindle operating arm is of a very short radius so a small movement of the governor arm would give a large angle of butterfly movement, possibly not helping?
Anyway today I have had the carb apart and replaced the original jets and needle and once again is all super clean.
For my next trial run I will be disconnecting the linkage and fitting a temporary throttle return spring so running can be assessed without governor influence.

Alan

I've just been going through all this again, and noticed that in an earlier post you said that the carb. is fitted with a 16 choke tube and a 1.5 air jet. If the main jet is an 83; then all this suggests that it is a carburettor for a JAP 6 which is rated at twice as much power as the 4/2.
It may be that you need to fit the correct carb. or, fit a 13 choke tube, a 62 main jet and a 1.75 air jet to the carb that you already have.

Oldgit, thanks for your comments - which in hindsight I think are spot on!
I think because it had said parts fitted when I got it I assumed it had been running ok, especially when it had such a good tickover. Didn't look in anyway suspiciously like it had been apart or tampered with, it all looked original. I guess if bits were changed 40 years ago they could still look original!
I will try and source the correct parts as suggested in the JAP manual - any recommendations of supplier?
Upon dismantling the carbs (I have another on a Villiers) I have been unable to remove the choke tube, even heating the body hasn't helped. If I am successful in obtaining the correct choke it needs to be removed, do you know if it's just a push fit?
The number cast into the chokes appear to be it's minimum inside diameter in mm, if I am unable to source the correct item I would consider sleeving the existing one.

Alan

Alan, I've not had anything to do with this model engine, or this type of carburettor so cannot be much help.
I have been trying to find a good diagram or picture that might show how the choke tube is held in; but no luck. I assume it would be a push fit; just held down by the carburettor top.
During my searches I found, mentioned a firm called Meetens engine parts; they may be able to help with parts.
Maybe your engine was fitted with the whole carb. from a JAP 6 at some time in its' earlier life; if so, it may have other parts that are unsuitable (I note that if you order a slow running tube; you need to state the size).
Maybe some one has a complete spare carb. for a JAP 4/2.
Good luck.

Thanks for your input, I have been researching the topic myself and arrived at a few possible sources of parts or info, one of which was meetens.
It would seem quite a popular carburettor and with suitable components able to cater for a variety of engine requirements. I believe the body castings are common to most with jets/chokes as required.
I shall make a few phone calls in the hope that my carb is able to take the required parts. I believe the choke is freely fitted and as you say nipped down under the gasket - I shall also ask if there's a recommended method of removal!
Being a diecast part it wouldn't take much to wreck it.
All part of the experience!

Alan

Contacted a few suppliers and have placed order for the correct choke tube, the correct main jet and an air jet which it seems is as close as I am going to get (1.8 instead of 1.75, 1.5 fitted at present)
It was confirmed that the carb fitted is not the original but changing these components should make it perform as per original fitment.
So fingers crossed this will help the running! Parts coming from Carburettor Hospital - guy was very helpful and would certainly recommend them.

Alan

deja vue, over rich engine I believe

Maryalice

Okay, okay!

Alan