Author Topic: mk1 concentric screw in pilot jet - adjustment ?  (Read 150 times)

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hoogerbooger

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mk1 concentric screw in pilot jet - adjustment ?
« on: 13 July, 2019, 17:44:12 »
I have a screw in pilot jet on my Mk1 concentric. I notice in the Amal advice on rebuilding that it says:

"(d)    Unscrew and remove the Pilot Air Screw. If you wish to replicate the setting on re-assembly, screw the screw fully home and make a note of the number of turns required, before removing it."

But I can't find anything in the tuning guides I've scanned so far on what difference screwing in or screwing out the pilot jet makes ?

Obviously it allows replacement and a change of jet size.......but what difference will it make if it's not screwed all the way in ??

Can any one advise ?

(mine is screwed all the way in and that I believe is how it is when I first took it apart, so I've returned it to that position both times I've stripped it)


chaz

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Re: mk1 concentric screw in pilot jet - adjustment ?
« Reply #1 on: 13 July, 2019, 18:10:16 »
normally applies to the mixture screw, in or out varies the air flow. the reason for duplication of what was there.
its not the pilot jet, its the pilot air screw, note... not all carbs have the pilot jet shown
« Last Edit: 13 July, 2019, 18:12:06 by chaz »

JulianS

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Re: mk1 concentric screw in pilot jet - adjustment ?
« Reply #2 on: 13 July, 2019, 19:14:40 »
A detachable pilot jet needs to be fully screwed in. There is no adjustment other than change to a different size.

Are you sure your carb has a matched set of needle/needle jet/jet holder?Not unusual to find mixed parts giving carb problems.

hoogerbooger

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Re: mk1 concentric screw in pilot jet - adjustment ?
« Reply #3 on: 13 July, 2019, 23:33:24 »
Thanks Julian. I shall leave it screwed in then.

It should have a matched set of needle/needle jet/jet holder as I changed them following previous suggestions on this forum.

The Carb came with the bike when I bought it, but seems to be a recycled spanish concentric for a  2 stroke bultaco. I ended up fitting a 4 stroke spray tube, jets,holder and needle to set it up as the Amal website shows for a 1967 Australian WD B40 except that I have a 140 Main jet instead of the WD 130 Main jet as I have the standard B40 star gauze air filter, whereas the WD had a paper filter. It's also a 627 rather than a 626.

Subject to any response on my float height post, the float may be 1mm high which might make it a little bit rich & uneven in running ? but I'm not sure how much 1mm high would be a problem.

I noticed the float bowl flanges were a little bent and so an air leak might be a problem, so I've sanded it flat again and put the carb back on the bike. It's seems a  less uneven idling now but not brilliant. Will give it a run next week and see if it's still lumpy & hesitant on the road on higher revs as well as idle.

 




(No idea then why Amal provide the advice I pasted...which clearly refers to the jet and not the pilot air screw)

JulianS

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Re: mk1 concentric screw in pilot jet - adjustment ?
« Reply #4 on: 14 July, 2019, 09:36:43 »
I would suggest that you try a larger main jet - the standard monobloc main jet is 190 for the B40- so you may be too weak at higher revs.

hoogerbooger

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Re: mk1 concentric screw in pilot jet - adjustment ?
« Reply #5 on: 14 July, 2019, 12:35:20 »
noted. Will get some alternative main jet options and try them.

could a paper filter make that much difference then ? minded that the Australia WD B40 used a 130. It seems a big difference. (is the monobloc a 26mm bore or as near as ?)

JulianS

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Re: mk1 concentric screw in pilot jet - adjustment ?
« Reply #6 on: 14 July, 2019, 13:49:36 »
The WD B40s were detuned when compared to the B40 Star . Had a 26mm carb. Your B40 takes a 1 1/16 inch carb. So there will be probably be differences in the jetting needs of your bike and the Australian Army version.


hoogerbooger

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Re: mk1 concentric screw in pilot jet - adjustment ?
« Reply #7 on: 14 July, 2019, 16:21:40 »
Julian
Many thanks indeed. I did not know about the detuning.Glad I asked the question.

hoogerbooger

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Re: mk1 concentric screw in pilot jet - adjustment ?
« Reply #8 on: 15 July, 2019, 18:47:52 »
Just re-read the Amal site on the effect of a the screw pilot Jet on 4-strokes, hence why they introduced a press in bush instead:

Pilot Jets
When first introduced, all carburetters were fitted with a detachable pilot jet screwed into a threaded hole in the bottom of the carburetter body. This arrangement is retained for 2 strokes, but from 1968, was replaced for 4 strokes by a bush pressed into the gallery behind the pilot air screw.
The detachable pilot jet caused problems with some 4 Stroke engines. The remoteness of the jet from the two pilot circuit outlet holes, caused starting and idling problems on bikes with lower manifold vacuums. This could cause stalling during deceleration, due to a weak mixture. The introduction of the pilot bush moved the pilot jet nearer to the outlet holes, overcoming  this problem. The bush has a flow rate, equivalent to 20cc/min. Carburetters fitted with a bush, retain the threads originally used to fit the removable pilot jet.

I presume the B40 has a low manifold vacuum ? as the engine has a low compression ratio ???

I have a screw-in pilot jet as its a converted 2 -stroke carb & these symptoms fit ... or sort of fit ?? The symptoms I have are:

- bike starts easily
- once warm it may idle evenly for say 20 secs or more , but will dip/flutter every now and then.
- the flutter sometimes is enough to stop the engine.
- the flutter does not have consistent pattern & seems relatively random in occurance

- Up to 1/8 turn ( opening throttle slowly then held steady) a similar pattern of occasional flutter happens, but perhaps at less frequency

- there's a bit of a popping/ringing sound from the exaust ( post combustion of fuel in the exaust ?)

- there is hesitancy and lumpiness if the throttle is twisted quicker.

- if the throttle is closed swiftly the engine can die....or sometimes the revs dip and the carb spits, then recovers or dips and recovers without spitting.

- on the road it seems more even when accelerating, but  lumpy and jerky at constant revs or deceleration.

Any views on if the screw in pilot jet could be a substantial part of my problem ?

(I have other reasons that may justify buying a new Concentric Premier carb. e.g. I have no O ring on mine at the manifold flange, or Stay-up float)











Mark B

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Re: mk1 concentric screw in pilot jet - adjustment ?
« Reply #9 on: 16 July, 2019, 14:59:51 »
Hitchcock's website has some very useful tips for tuning Amals.