Needle jet behaviour has changed over time

[Scott and Jay]

Hello there,

The symptom I have been grappling with over the last 3 months - is, at a steady 60 m.p.h the engine suddenly dies and splutters. I go down to 50 and it's ok. If I get up to 65-70, first - its seems fine, also, at that speed. During the time this has been happening I noticed a blown head gasket, and rocker box gasket too. Our engineer, on examining the head - said "you better get to the bottom of why it's running so hot, because it was cream colored. I ended up putting a new head gasket on (iron head), new head bolts from SRM. This all seems in tact, but the fading at 60 had got more prevalent. I have now ruled out, after testing: blocked tank filler cap hole, sticky float (needle), float level, blocked fuel lines, water in petrol. I put new plugs in, I have good copper ht leads and good pickups. Mag was rebuilt when I restored my bike 11k miles ago. I cleaned the whole carby. The needle jet seems good. The needle itself looked like it had a build-up of light corrosion, I cleaned and poished that - but to no avail. This morning I had raised the needle from position 3 to position 2 (next to lowest groove). I thought I had nailed it, but it was a fine morning so I tried a longer ride and at 60 for a spell it happened again.
I think I am on the right track, but if I raise it to the highest - I question if I really need a new needle and jet. It was 4.5k miles ago I got this new Amal Premier carb. There must be something wrong when it needs to go so much higher than the original setting, now - when it seemed fine (wasn't running hot at first). I rode all around the outback of Victoria on the International Rally. It is such a sharp symptom, when it happens. Do these new Premier needle-and-jet combinations go bad in this way, in this amount of time??

REgards

[Black Sheep]

If it's running so weak, the first thing I would do is up the main jet a couple of sizes (at least). Next I would consider a larger needle jet. I don't have experience of premier carbs, but with any other Amal carb needle jets gradually wear and the mixture gets richer, not the other way around.

[Scott and Jay]

Thanks for responding, Black Sheep. Ok, first - It is a 389 with 1 5/32 bore. The inlet manifold has been enlarged to match. I do run a KNN air cleaner but Emgo universal not-too-baffled silencers. So, I had to at first go up to a 310 main jet. I have now gone up to a 320 - with no ill effect, in the "main-jet" range. Yes, what puzzles me, too, is the change to leaner - over time..

[muskrat]

G'day S&J
Definitely need a richer needle jet try a 107. With your better breathing going bigger on the main was only half the answer.
Clip position numbers from the top to bottom. Top groove is #1 = lean, bottom groove is #5 = rich.
Cheers

[Triton Thrasher]

With such an oversized carburettor, you may be doing 60 mph with the throttle nearly closed, so the mixture is being controlled by the slide cutaway height.

[JulianS]

Have you tried it with both petrol taps on?

[Scott and Jay]

Thanks for these replies. I did try both taps, thanks, Julian. It made a difference but only for a second. So, for the 107, Muskrat - what is the overlap (if any), between the lower needle positions on this (say, #1 and #2); and the higher needle positions (say, #4 and #5) on the 106?

I have just ordered a 107..

[Triton Thrasher]

Have you tried it with both petrol taps on?

And loosening the tank cap.

How much petrol runs out of the bottom of the carburettor in a minute?

[muskrat]

G'day S&J.
The needle jet is measured in thou's, so a 107 is just 1 thou bigger than a 106 allowing a 2% increase in fuel. It doesn't take much in diameter to make a reasonable difference to performance.
The needle works in two ways. The straight part works with (to an extent) the slide and after. The taper works before the main jet. Lowering the clip allows the taper to come in sooner (lifting the needle) or raising the clip (lowering the needle) the taper comes in later. Increasing the jet richens both parts of the needle.
Cheers
ps: The needles in my Kehin PWK's are a pain with combinations of thickness, length of straight, length of taper, degrees of taper and 5 clip positions. Amals are a walk in the park. 

[Scott and Jay]

Ok, thanks, greatly. The 107 has arrived, already. I have put it on. Based on your explanation I will start with the same clip position, #4. I don't have to dismount the carby that way. I also noticed another (side-) effect of this position, the little backfires I got when slowing down disappeared. Plugs didn't go much (any) darker. I will do the same ride, but tomorrow morning - after rush hour, and its a much nicer, calmer morning. I have skipped the petrol flow test, at this stage, thanks TT - because I can't easily get a funnel or catcher under there, with it mounted, but I'm certainly not dismissing. I hope I can prove the 107 fixes it, and when I earlier turned reserve on as well (as above) - it didn't help.
If the 107 does fix it (and I would like to report back, either way) - I will still be puzzled how the 106 (clip position #3) worked so well for 4k miles. This symptom, sudden dying and spluttering at 60 - started after the National Rally, end of February, though. On the way up the K&N air cleaner would have got soaked, from heavy downpours, with bikes sitting uncovered on a trailer. I know that  because Jay's Bantam wouldn't start until we took his same K&N off - then dried it. After mine would have similarly dried out, naturally - it may have had more flow. I cleaned it and oiled it just before my last test ride, when the symptom recurred - so I can't do much more about stabilizing that factor, in the puzzle, now..

[Triton Thrasher]

Little backfires when slowing down can mean a weak idle mixture.

[Scott and Jay]

Yes, I've done the standard pilot mixture procedure - as per Amal tuning guide. I repeated it on Sunday, after the needle-raising and carb cleaning..Thanks. I had also done the Amal-guide needle-jet tuning procedure - in the past. I had tried a bit of choke, when it was in that range - and it didn't make it run better. So, I concluded it wasn't too lean, then. It seems to have subsequently become too lean - over the last 500 miles. I tried choke, first, when the 60 mph symptom I've been explaining first occurred - and it did improve it, the first time. When I tried it a subsequent time, it didn't help. I think it's tricky getting the right amount of choke, in this test..

[Swarfcut]

Backfiring in the silencer on the overun was the classic sign of a weak mixture, so  if the old carb was set up OK, an air leak on the induction side was a possibility. I understand it ran fine for a while, then acted up when hot....hairline crack somewhere in the inlet tract or the usual bowed carb flange would be  a possible cause, moving as things heated up. If the new carb fixes it, have a critical look at the old carburettor body for anything that could cause an air leak, if not, the head casting requires scrutiny. The joint between manifold, drip shield and carb is a well known source of problems with the gasket order being easily misplaced.

 Retarded ignition, as well as a weak mixture, will also make it run hot, so a quick check here would be worthwhile.

 Swarfy.

[muskrat]

G'day Scott (as you mentioned Jay's bike I assume we're talking to Scott!).
Just to confuse matters. An air leak in the exhaust pipe, either the head joint or the muffler joint can cause popping/backfire in the exhaust. 
Cheers

[Scott and Jay]

Yes, it's Scott. Jay, nominally owns the Bantam (and I the A10) but I ride his a lot and we have always been "partners" in the A10, too. So, upshot is the 107 needle jet wasn't the answer, sorry. I think I put too much of a spin on the diagnosis by naming the topic "needle jet..", my bad.  I should just stick to the symptoms: After about 1 or 2 km at 60 m.p.h it suddenly loses power and splutters and "ticks" - go down to 50 and it's fine again. If you get up to 70 before this, it seems fine, too - but I haven't been far enough at that speed to test it, fully.

The plugs changed to too dark, after this last test ride. Acceleration wasn't as good, either. I think I'm just about ready to rule out carburation/induction air leaks (I can't see any cracks either and joints seem all tight - with master gasket 518), based on this. I put the 106 back (at needle pos #4, still) and it was just great - in a nice ride the 2 of us then went on, round the back country roads behind our suburb, where I never got over 4 m.p.h. Acceleration and all-round performance is great, up to this speed. I could try the petrol flow test next, when I organize a suitable catcher (but earlier it made no difference when I put reserve on as well). However, just to jump to another possible diagnosis -  can the auto-advance suddenly flip out? I've really only got electrics. The exhaust joints seem all tight with exhaust cement in the headers (which aren't loose), and in the header-to-muffler joints. I mentioned before: new plugs; good copper leads; good pickups. I have cleaned the slip ring and gapped the points. I could buy a good points file and do all this and timing again...

Thanks for your tolerance of this, everyone...