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Concentric carb on a Mark 1 square four

Posted: Sat Jan 25, 2020 6:01 pm
by Paul_Linden
What is the best slide and main jet to use on 626 concentric carb that’s fitted to a mark 1?
Currently I have a number 3 slide and a 150 main jet, and it seems all right-ish, although i only gave it a quick test. I heard that a 3 1/2 slide and 160 might be better?
I have the Solex carb, but it’s been impossible to get it running well and starting well.

Re: Concentric carb on a Mark 1 square four

Posted: Sat Jan 25, 2020 7:24 pm
by GuyWalton
Personally I would persevere with the solex carb . My mk 1 has a solex and runs perfect starts first of second kick and ticks over good . I stripped and cleaned my carb and didnt buy any new parts just meticulously clean .

Re: Concentric carb on a Mark 1 square four

Posted: Sat Jan 25, 2020 8:50 pm
by will_curry
I'm with Guy. My Mk 1 was transformed from a pleasant old friend into a
twitching neurotic simply by swapping the Solex for a Concentric when the
petrol union threads stripped on the Solex.

The slide hangs up from time to time too so make sure you can reach the
ignition switch.

If I'd been on the ball I'd have got David Jones to recondition it while
he was still working on them.

As to what slide and jets I've got in the Concentric I cannot remember and
I cannot find my notes at the moment.

Re: Concentric carb on a Mark 1 square four

Posted: Sat Jan 25, 2020 8:53 pm
by JohnnyBeckett
HI i get my concentric carbs off SURREY-CYCLES he is on the ball with carbs you just tell him what bike you have and he sets it up for you or he will tel you what you need to know i get my new carbs all ready to fit i just have to give it a little tune up and go i have had no running problems they run sweet with them i get all i need off them wen i go to kempton park motorcycle jumble :)

Re: Concentric carb on a Mark 1 square four

Posted: Sat Jan 25, 2020 11:29 pm
by will_curry
There are a number of issues fitting a concentric to a Mk 1. The stud centres are 1 7/8"
not 2" and there is precious little room either. Ariel did fit an Amal to some US export models -
a model 275/154R which was laid on its side. The more-or-less continuous suction from a 4
causes problems with Amal slide operation - hence the Solex and later the SU which while it
has a slide was designed for just that type of application. The Mk 2 with the SU has a horrible
shape to the frame to make room for the top of the SU which isn't there on a 4G or a Mk 1.

Incidentally my Mk 1 is a 1952 'Korean War' model with very little chrome - SV frame prefix and
TM engine prefix - first registered on 8th November 1951. The Ariel model year started in
October (I think) of the previous year. Hence your 350 being a 1954 model - KS frame prefix -
being registered in late 1953.

Re: Concentric carb on a Mark 1 square four

Posted: Mon Jan 27, 2020 4:10 am
by Paul_Linden
So the Solex was overhauled before going on the bike, including ultrasonic cleaning, passages blown through, and all the jets are the correct ones and in usable condition. It’s not working properly, as it’s difficult to start, and once started and warmed up is erratic, and spits/coughs. So the Solex is now on the bench and while I work on it I put the Amal 626 on the bike just so I can keep riding, with the intention of going back to the Solex when sorted. The Amal is much better, as the bike is now rideable, but I want to make sure I’ve got it jetted correctly.

Re: Concentric carb on a Mark 1 square four

Posted: Mon Jan 27, 2020 10:39 am
by keith wainwright
Concentric Carb?
Not an ideal carb to use on a any square four, to much suction going on and also the problem with the slide sticking. I ditched my Concentric, I kept running out of spare underpants. Keith

Re: Concentric carb on a Mark 1 square four

Posted: Mon Jan 27, 2020 7:32 pm
by paul.jameson
See:
viewtopic.php?f=21&t=9663&start=50

about half way down the page. Have you got the slot in the gasket? It takes the undoing of 4 screws to check.

Re: Concentric carb on a Mark 1 square four

Posted: Mon Jan 27, 2020 9:52 pm
by nevhunter
The square is the only butterfly throttle carb you come across generally on Ariels but all butterfly carbs must have the idle passage holes uncovered at the right time to operate in the range near idle. IF there's any wear there that may be out of whack. Bushes and the dimensions of the butterfly itself which must also be centred with the idle screw wound back so it's fully closed . It's critical that this aspect be right, or it will run like a dog.
A slide carby on a sq4 is not what I would like to be contending with. Nev

Re: Concentric carb on a Mark 1 square four

Posted: Mon Jan 27, 2020 11:17 pm
by Paul_Linden
Thanks Paul, that is interesting as my gasket doesn’t have the slot. Looking at the picture I took when dis-assembling there are a few subtle differences (an extra hole in the gasket, and the ridge around the large central orifice) on my Solex.
28E329C2-C937-41BA-867F-0D7BF923D5F2.jpeg
Solex top and gasket.