Spitting Hunter

[R.Herman]

Hi folks,
I have a 1948 NH 350 that spits an oily mist out of the carb bellmouth.
I have honed the cylinder and replaced rings, replaced valve guides and recut seats, lapped in the valves.
Checked cam timing.
Checked valve lash (and added a bit to be sure the valves are seating fully)
Tried an alternative carb.
No joy; it runs okay but leaves an oily film on the outside of the oil tank - and my right leg.

Any perspectives and advice will be most welcome.
Cheers,
Bob Herman

[Bernie Blundell]

Might be running lean, what colour is the spark plug ?

[Simon.Gardiner]

For me the the first question is, are you absolutely sure that the oil on the oil tank (and your leg!) is coming back from the carb?
I've had trouble with oil bleeding out of the oil tank filler cap, both from the lip of the cap and through the bleed hole.
I think all my singles have been guilty of spitting back through the carb to some extent - enough to leave the oil tank needing a wipe-over after a long run but not enough to trouble my trouser leg.

Edit: The other things to check is that you've not got a slight leak from the cylinder head joint or push-rod tubes that's bleeding back across the top barrel fin. (You've obviously had the head off so you've probably checked this already - but sometimes lightning does strike twice...)

SG

[ian williams]

i had am issue with my NH and it would backfire and spit back and not really run right just as you came off of idle when riding it around and it would also backfire on the overun .
i thought of all the things that it could normally be said to be but turned out the opposite .
i found that the needle jet was too big and it was too rich rather than lean , i changed the needle jet from 106 to 105 and it improved greatly .
i had another problem last year where it wouldnt run right and would spit and cough and that was down to blocked pilot gallery and once i had throughly cleaned that all out it ran lovely .
theres a tiny hole in front of the slide in bottom of carb opening that is part of idle circuit but if it gets blocked the engine wont run right at all , you do need a tiny bit of wire to clean it out though , something from a wire brush for instance .
i cleaned whole carb out using gunwash thinner and blew it all out with airline , including cleaning the jets out .
just to mention i am using a monobloc carb 376/8 but ive changed needle jet to 105 .

another problem i had was the plastic "stayup" float swelled a bit and would stick half way up in the float chamber , it rubbed against the rib of the casting for the lower left screw , amal now mill off a bit of that rib to give more clearance for the plastic stayup float but older carbs made before say 2022 i dont think were milled .
i put a brass float into my 376 carb and there is much more gap in the area mentioned and the float doesnt touch anything .

hopefully youll get your NH sorted without much issue .
ian

[R.Herman]

Thank you all for your responses. To address your suggestions:

First, I should mention that I live at 8000' elevation, so all my vintage bikes need to be jetted accordingly. Oddly, the Ariel will not run with jetting leaner than it had at or near sea level (where I bought it). It only runs with the richest of the three main jets I have for it, and it wants the 106 needle jet and protests when I try to use a 105.
This results in a black, sooty spark plug. But any attempt to lean the mixture results in hard or impossible starting and a refusal to run off idle.
With the overly rich jetting it starts first kick (I have an electronic ignition fitted).
I have removed the mixing chamber from the 276 carb and cleaned it using an ultrasonic cleaner, have run a wire through narrow passages, blew it all out etc. My carb has a brass float, not plastic.

The oil definitely is coming back through the carb, not from a leak. There is a visible fog when I rev the engine above idle; I can watch the oil accumulate on the outside of the oil tank. If I hold a piece of paper an inch from the bell mouth and rev the engine, the blowback will push the paper away from the carb.

And I pulled the aftermarket oil pump, confirmed that it works correctly.

As you can imagine, this is very frustrating. A simple engine, and I've scratched my head until it bleeds. I must be overlooking something.

Oh - one more thing: I've pretty much decided to pull the top end and check rings etc. After I removed the rocker assemblies I pulled out the pushrods and found that they are different lengths. The old Clymer manual I have does not mention different lengths. One is +/- 10-3/8" and the other is about 1/4" shorter. Both are steel, as apparently is correct for the iron barrel and head. I don't see how this would cause my problem, so long as the valve lash is correct, but it's just another mystery...

[Roger Gwynn]

Check the float height

[Simon.Gardiner]

It seems to me that a greater than 'normal' (for the type of engine) blowback through the carb must be caused by the inlet valve opening earlier than specified? (OK - or by too tight tappet clearance but that would mean no compression and probably no starting.)
I know these things have been checked, I'm trying to all causes.

It's not unknown for the marks on the camwheels (or the alignment) to be slightly 'of'f' (there are posts from David Anderson about this). Bob - how was the cam timing checked? Also, what cam are you running (single lobe, small base circle twin lobe, big base circle twin lobe)?

You mention also running an aftermarket oil pump, would that be a Morgo? If you're using an earlier-type 3-compression ring piston (not the later 2-compression+1oil ring type) there may be too much oil going into the engine - is the exhaust smokey, and is the breather (in the timing cover) functioning properly?

You've obviously put a lot of time into trying to fix this but it's clearly still being quite a puzzle.

SG

[nevhunter]

The Push rod length depends on the Base dia of the cam used, Spitting back is usually lean (air leak coupled with a retarded spark sometimes Might even be worth checking Leak at the exhaust pipe to Head join. That can cause it on overrun. Low float height ( Roger just mentioned) which will make idle erratic and doesn't help starting either.

[R.Herman]

Again, thanks for all your thoughtful suggestions.
Re: float height - is that adjustable in a 276 carb? Also, I tried a different (Asian) carb with no change in symptoms.
Re: Cam timing - Simon, your questions are beyond my knowledge. This is my first Ariel and I do not have deep knowledge about the variations in their engine specifics. All I know is that when I checked, the timing marks on the cam and crank line up, and the cam has two lobes opposite each other. Don't know about the diameter of the base circle.

The oil pump is not a Morgo but looks like one. Don't recall the brand offhand but I think it's the one Draganfly sells.

And yes, my engine has the 3-compression ring piston, no oil scraper. When I peer into the spark plug hole I see oil atop the piston crown, and there is a lot of oil in the rocker area. On cold startup, there is no smoke in the exhaust, but after it runs for a few minutes it begins to smoke. This is why I wonder if the rings have not seated properly. I did not use break-in oil when I replaced the rings and am tempted to drain the oil tank of its Castrol GTX Classic (lots of zinc) and replace it with 30W lawnmower oil and run it for a while that way. Thoughts on that?

The breather in the timing cover: a previous owner must have removed the check ball assembly. All I have for a breather there is an L-shaped fitting with a copper pipe venting to atmosphere. Could this possibly factor in to the problem?

Nevhunter: Definitely not a lean condition. I'm running very rich jetting (it won't run otherwise) and the plug is black. Spitting back is not on the overrun, but on acceleration. Starts easily.

[j.coleman]

So you are getting too much oil in the combustion chamber. Either
Your oil pump is creating too much pressure and this is being forced around the rings, or the oil feed to the rockers is getting through the valve guide to the inlet valve and putting oil in the fuel mix?? Maybe you need an oil scraper ring? When you have run the engine, is there much oil in the sump? Is the oil quantity returning to the oil tank??