AJS model 18 rigid monobloc conversion

[z1100r]

From what the OP said ie "Eight stroking, lumpy and sooty black and very rich" I wouldn't bother filing a few thou of the back of the slide - it wont make that much difference.

Bin the 260 main jet, its way too big, buy a new 200.
Bin your existing primary jet and buy a new 25 - maybe some home bodger has done something similar to the jets as you're about to do to that slide.

Buy a new 'D' needle and put clip in 2nd down notch.

You will still find bike slightly rich at slow speed but 1/2 throttle to flat out it will be perfect.

Buy a new 3.5 slide if you want total perfection. Dont go to a 4 - it will stutter on quick throttle openings.Edited by - z1100r on 13 Feb 2012 1:59:11 PM

[Trefor]

I think the valve timing changed in 1955 when they went to monoblocs. I've often wondered if that has any implications when fitting monoblocs to the pre'55 engines- anyone know?

[Samuel]

Well I ran a Mk1 concentric from a '70s Triumph 750 / 3 on my '49 G80 for a while with no particular problems so you can get most combinations to work if you try

[z1100r]

I seem to spend my life jetting carbs. I'm a 2-stroke nut really and am mad on RD/RG500's. I have alot of RD's. Jeting those is critical if you haven't got buckets of cash for pistons.

These bloody Brit singles me and a mate own have all been a pain in the arse jetting wise. Mainly because some wally has stuck a different carb on and its invariably come from a 2-stroke.

Our bikes:-

Pre-65 Tiger cub - had Bantam monobloc carb (killed plugs and bitch to start)

Pre-65 C15 - The worst of the lot, another Bantam carb but upjetted to 'go faster ' by some idiot.

B44 Victor- Monster huge jet - but should of been running on Methanol.

G80s - Replacement carb of a different type, and smaller choke, and not only that but also from a 2-stroke..!! On top of that fitted with jets from the original earlier larger choke carb. Absolutely Hopeless

500 Bullet -ridiculously worn out slide...been filed that much to fit distorted bore it was practically alum foil.

The thing that annoys me is the cost of Amal slides. Every bike had had to have a new slide. They all have different carbs 389 276 926.

I must have £500 worth of amal slides....!!!


I have a brand new 28mm concentric. Was going to fit to the Matchless to help with changing tick over as the bike is leant over. Do concentrics help with this.Edited by - z1100r on 14 Feb 2012 10:09:42 AM

[GRAHAM47]

I ran a Mk1 concentric I was leant on my '47 16M last year and it ran really well. I had to return this carb but have now been given another one. Now then, settings. The one that ran well - 26mm bore, 180 main, 4 slide. The replacement 26mm bore 210 main, 3½ slide. Do I convert this to suit the previous one or would any one recommend alternative jetting / slide? Advice appreciated.

[john]

Yep the concentric will allow the bike to run on the side stand but the return for the oil will be higher and the crank case will slowly fill with oil. After a few mins of tickings over my bike smoked realy badly!

[zwarts]

I am ,once again, confused. Monoblocs are identical in their main bodies regardless of if they are for a 2 stroke or a four stroke. They just require the correct main jet, pilot jet, needle and slide. Start with the original factory settings and proceed from there following the sequence prescribed in the Amal literature. They are very simple carbs to tune but be aware that the original settings can only be a starting point. Modern fuels are quite different to those of the 1950's and some some fine tuning of the carb. and ignition timing is usually needed to get the best results.
Concentrics, initially, did have some minor differences between 2 and 4 stroke applications, but again, all this info is easily accessed via the internet.
Bob.

[z1100r]

They are identical, yes...and it is easy to alter and tune them, but normally you have to replace everything inside and that gets expensive.

Its so simple in fact, just pick a choke diam and look for a same capacity bike in the amal model/carb pdf. Take all the settings from that - being careful of the radically different from the norm ones, (which usually involve an airbox or a twin).
My point was that I found it amazing that folk would bolt on a Bantam carb and not notice anything was wrong. Maybe Bantam carbs were lying about all over the place at one time and these older singles had worn out their std carbs, so people were just replacing them with carbs from the miriad of 2-strokes that appeared all of a sudden.
Edited by - z1100r on 14 Feb 2012 1:49:53 PM

[SPRIDDLER]

quote:
They are identical, yes...and it is easy to alter and tune them, but normally you have to replace everything inside and that gets expensive.

In the back of my grey matter I seem to remember a long and tedious thread on here about 'jetting problems'.
It eventually transpired that the carb had a 2-smoke slide needle which IIRC is different (shorter?)from that for a 4-stroke engine.
Might be having senior moment though.

(Sorry you seem to be having a bad day Z1100r - maybe she only put a second class stamp on it )

[crawsue]

Way back in the day I was booked in to sit my 'bike test on my Cub,weekend before I decided to "tune" itmanaged to strip the inlet banjo thread of the Zenith carb, "OH dear" I said "that is unfortunate" (or words to that effect)....anyway,took the bus into Glasgow's Great Western Road,where all the 'bike shops were and in one of them (Victor Devine?)the storeman dragged out a wooden tea chest full of old carbs.....I bagged the best looking monbloc for £1-12-6p...it didn't work as some eejit had drilled out the main-jet! I fixed the Zenith by hammering a football brass air valve in,smothered in Hermatite...lasted for ages....well until I ran the big-end, again. (Show me again,mister,where you put the oil in ) passed the test though,and I'm still bodging