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ET: Engine (Twin) Pilot jet


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vibrac

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I have just seen a concentric installation where after cleaning in detail and the pilot screw set as rich as possible all air leaks in tract checked and slides sycronised (or not ) The rear cylinder only runs at tickover with a hand covering the bell mouth otherwise the front cylinder lugs the rear round. The way the rear picks up even after a prolonged lugging when the hand covers the carb suggests it's not ignition. When the throttle is opened it still is reluctant to chime in. They are difficult carbs.
 

greg brillus

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VOC Member
Yep, Just had the same on a Shadow where the floats would not let any fuel in........At low engine speed it would only run on one cylinder.........These were the Mk 1 1/2 concentric's with the choke lever on the sides........I had to do some odd alterations to get the levels right, when the floats were hanging down they were not lifting the needles off their seats and the fuel could not lift the needle of it's own weight........These were the large diameter alloy needles.
 

Chris Launders

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VOC Member
I had one where the float was fouling the chamber body before lifting the needle, it was a brass needle as well so would not lift with fuel pressure, after speaking to AMAL fitted a new stay up float and alloy needle and reset float level, they now recommend the float be level with the top of the float chamber, previously it was recommended .080 below.
 

Bill Thomas

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VOC Member
I have found It's tricky bending the new stay up float, You can do it and you don't get enough drop on the float To allow enough fuel in.
 

Nigel Spaxman

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VOC Member
I have just seen a concentric installation where after cleaning in detail and the pilot screw set as rich as possible all air leaks in tract checked and slides sycronised (or not ) The rear cylinder only runs at tickover with a hand covering the bell mouth otherwise the front cylinder lugs the rear round. The way the rear picks up even after a prolonged lugging when the hand covers the carb suggests it's not ignition. When the throttle is opened it still is reluctant to chime in. They are difficult carbs.
My Ducati did that until I figured out that the cam timing on the rear cylinder was out by 8 degrees. The rear cylinder wouldn't suck well until about 2000 RPMs. The cams are very long duration race cams (Imola) so if it is wrong it really doesn't work. I think the duration is about 340 degrees on those cams.
 

oexing

Well Known and Active Website User
VOC Member
A few months ago I had a very interesting design carb from Aliexpress, extremely low price and a kind of flat slide with round sector around the needle jet. We have not tried it yet , but you cannot be very wrong at € 30.- brand new. A set of jets is a tenner. Well, being an old fart I would not fit it onto a classic but my friend is into SR 500 racing so we got a 38 mm plus twist grip set at € 50.- . . . . .

Vic

flat slide carb

flat side carb Ali

flat slide internals
 

Bill Thomas

Well Known and Active Website User
VOC Member
It will be interesting to hear how you get on, I have seen the advert and always wondered, For that money,
How could they make it.
We made these in 1970, Didn't have enough money for Carb's !.
Just a bit of alloy tube, A rotary jet and a butterfly.
They got me to the IOM and back !.
Cheers Bill.
 

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Vincent Brake

Well Known and Active Website User
VOC Member
Now this is all better than just a mere pilotjet.....

Wonderfull thoughts all

And waiting for a mockup, mockup like them new Triumphs
.....
 

erik

Well Known and Active Website User
VOC Member
To alter the Level in the float chamber of a concentric mk1 is best done by altering the height of the valve seat in the bowl.You can press it out and Fitting different rings und it or milling the small flange more narrow.I hope you know what I mean.Erik
 

erik

Well Known and Active Website User
VOC Member
I want to let you know that brand new concentric mk1 are availiable .They have an altered Pilot Jet System.I ordered a pair today at surrey cycles. when they arrive i will post Fotos.Erik
 

vibrac

Well Known and Active Website User
VOC Member
I hope to see Surrey Cycles next week about some real carbs while I am there I will speak about the pilot mods you never know he may change my opinion on the jokes concentric carb :)
 

MartynG

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VOC Member
To alter the Level in the float chamber of a concentric mk1 is best done by altering the height of the valve seat in the bowl.You can press it out and Fitting different rings und it or milling the small flange more narrow.I hope you know what I mean.Erik
With the new AMAL concentric Mk1's they come with the stay-up float that has a stainless steel tang that can be bent to adjust the float level. OVR #67 (in the OVR archives) has a detailed article on Mk1 float levels and OVR #69 talks at length about setting the needle height as well.

Martyn
 

oexing

Well Known and Active Website User
VOC Member
Well, my choice would be the pre-monobloc - for great looks. But then , prices are a bit high - well, not so much when looking at Vincent spares and values of bikes.
When looks are not a factor but reliable operation, have a look into Aliexpress, amazingly low price offers, even Mikuni styles. Or try those power jet types, semi flat slide construction. I had one of these on the bench, not too bad, really. But we have not tried one on the road, sets of jets a tenner.

Vic
28 mm "Mikuni"

28 mm power jet

38 mm power jet carb:

P1070438.JPG

P1070417.JPG

P1070442.JPG
 

erik

Well Known and Active Website User
VOC Member
With the new concentric mk1 I hope to have the best of two worlds: outlook and function.I dont like the plastic parts and plastic tubing on the Mikunis and similar carbs.The price is a little bit secundary because the carbs ,and especially the premier ones,last for years.Best regards Erik
 

oexing

Well Known and Active Website User
VOC Member
Actually even Bing carbs - and Amal Mk 2s - had these plastic hoses from the seventies onwards and I hated this even then. But really the only function is to vent the float chamber. So I had a close look at the China carb above in my photos and yes, you can cut all connections and bin the hoses. Seems this is an environmental thing since for all these vents.
I have not tried one of these , not for a Vincent of course. But I guess these will work - unlike some dated Amals, as you will know better from own experience. After all, Chinese or Japanese companies have a LOT more knowledge about engines compared to all other worldwide companies as they have produced many , many more millions of bikes in three decades than all other brands added up from day one of Benz or Daimler ´s times.

Vic
 
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