Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board Mikuni back to Amal for my A65?

Ok, as some of you may know, I've been running Mikuni VM32s on one and then another of my A65's since 1984. But now I'm thinking about converting back to AMAL Concentrics. I have a pair of 930's that I had rebuilt at Lund's with sleeved slides. The Concentrics came off a 1976 Triumph T140. I'm pretty sure the carbs were original equipment on that bike, and it ran well with that jetting, which is as follows:
- Main jet: 180
- Slide: 3.0
- Needle jet:.106
- The needles have no markings on them.

A while ago someone posted a link to a listing of factory jetting for various years and models of BSAs. Does anyone know where to find that document? My '66 and '67 Spares manuals only have jetting specs for Monoblocs.

I'm stuck with these slides, so I'm hoping 3.0 is appropriate.

Ok, I found a 1968 Spares manual online. Unfortunately, the Lightning calls for 2.5 slides! Is this a show-stopper?

All the later 69 -71 A65L s used no. 3 slides, 68 was the first year of the conc, later settings are probably better?

Hi Mark,

I have had pair of Lund-sleeved Concentrics in the past. Great work as far as I can tell The bike they are on runs nicely.
3.0 slides are IME fine on any Lightning, as also stated by Gavin.
The standard main jet for the 69 Lightning is #190, needle jet .106" is fine. My own bike runs fine with that jetting with Pancake filters mesh-type, no paper.

So from my perspective you are good to go.

I'd recommend to install new needle jets and needles anyhow, as they do see significant wear and tear due to the vibration.
The correct needle is the standard-4 stroke needle with two rings above the needle clip grooves.

Good luck!

Cheers!

Phil

these were lightnings? ive never managed to fit a Mikuni in between the head and the frame on a thunderbolt.

Dual carbs, yes. Require use of homemade air cleaners (oiled UNI foam over wire mesh) in order to fit between head and frame. This is one of the reasons I want to convert back to Amals.

this is an option i hsve been considering for BSAs. it would require some thought on filters but lots are available

it is possible to resize the cut away ( but you can only make it bigger) by careful filing ...

grab slide in a vice at 45 degrees so cut away is horizontal and take FLAT cuts with a file ... put something flat across the non cut away part and measure the height of the cut away ,,,, use a known 3 for the gap ...it works ,,, hope this makes sense

I tried something like that once, but it put the air cleaners right into my thighs when I put my feet down, and it looked like hell. Also, you need something very rigid for the coupler, or it will "breathe".

There's another approach to this, one which I don't have the inclination to try, and that's welding a flange onto the carb so it can be bolted directly to the head.

Another reason I want to convert is that the bike had more low-end grunt and a crisper idle with the Amals. The Mikunis are stronger and faster "at speed", but they're soft right off idle, presumably because they're 32mm. (Maybe that's why my clutch has lasted so long!)

i run 34mm VMs on my 1972 T120, but its got a morgo on it and has some mild head work.

bottom end has a bit too little vacuum, but im running 8 degrees retarded too

As Gavin stated, use the specs for 69/70. The 68 would have had the 2.5 slide, 2 stroke spray tube (very important) and the needle clip in the middle position and I beleive a 2 stroke needle jet and 190 main jet which would not have been cross drilled. Whilst it would have ran ok it would run better with the later adopted settings if it had the 2t needle
Jet it would also have had the 2t needle (3 ID rings?)

Later settings are (pre oif). Oif used 200 mains, I beleive the rest are the same.
#3 slide, 106 4stroke needle jet and 2 ID ringed needle. 4t spray tube (flat top) and 180 main jets. Needle clip in top position aka pos1.

The leaner settings would allow the bike to run much cleaner and more responsive.

I’m finding a richer main jet is preferred as ethanol is becoming more prevalent, but then again my bike is no longer bone stock either.

More likely they were not jetted correctly at low end.

Kevin, Mikuni can be a bit soft off idle..Do the pilot jet sizing by the usual tuning, then go one size larger....Or go flatslide....

lol

im resisting the flatslides as long as i can. i already run 30 pilot jets in my 34 VMs, 35s actually smoke. i ran anywhere from 17.5 to 25s in the 32s i used to run.

i have two sets of resistors to run across the pickup leads in my boyers that will reduce the total spark advance by 4 or 8 degrees. when i get around to actually putting one or the other in, ill be able to run the 30 BTDC that i use for twin plugs with something closer to the original advance at idle.

i know the flat slides are better carburetters than the VMs, but im a stick in the mud. i suppose i should test them-- theyre not as expen sive as amals.

you need to post more pictures of your A10.

I feel like I'm in a different world with my essentially stock A65. I don't think it has the "lung capacity" to support the larger bore carbs; I feel I'm compensating for that by running small slide cutaways (2.0). It ran worse with larger cutaways. The change was accompanied by needle and needle jet changes. In fact I've gone through three significantly different sets of needles and needle jets, and two different "air corrector" jets. As near as I can tell (from spark plug readings), mixture is good throughout what I can measure. I still don't think I have the right needles though; if I drop them a notch, it runs like an absolute monster over half throttle, at speed, but the low-rpm performance suffers. Raised a notch, low-end performance is satisfactory, but the bike loses its monster quality at speed. That's my preferred setting, since my go-fast days are pretty much over. I could continue my search for the perfect jet needle, but I've already spent too many days of my life jetting these carbs, and I'm still faced with the necessity to run homemade air cleaners, which do nothing to quell air turbulence. The Mikunis are fine carburetors and they've served me well for almost forty years, but this is the first time I've had some rebuilt Amals to play with....

ANYWAY... Thanks for all the helpful AMAL info; it sounds like I'm on the right track. The needle jets are square on the end, and the needles have two rings above the top groove.

BTW, I'm using K&N "cone-type" clamp-on air cleaners. I tried the Amals once before with stock "pancake-type" air cleaners, and discovered that, without side panels, they looked ridiculous.

amals are perfectly fine. i run a 930 on a 66 A65 and have another on a 69 Tbolt. and other stuff too. i personally like mikunis because theyre so much easier for me to get right. your problem with needles appears to be that you need two tapers on the same needle, and mikunis can fix that, while amals cant.

but the point of a motorcycle is to ride it, not to treat it it as a monument to some vendor manufacturer. id run jikovs if they worked.

you clearly are sensitive to your motors performance. for people like you it doesnt matter what kind of carb you run. whatever you use, youll get it right.

just sayin

When I acquired the Mikunis (from MAP Cycle) back in 1983, the original Monoblocs on my '67 L were clapped out, new Amals were not readily available, I didn't know about Lund's, and Mikunis were cheap ($150 for the pair, with manifold adapters). Without asking, MAP sent me VM32s, I think because that was the hot setup for British flattrackers at the time. If I had known what I know now, I would have asked for VM30s. The rest is history.

Thanks for the vote of confidence. In the vein of "what did you do to your bike today?":

The carb body on the left is untouched. The carb body on the right was done with Brasso. The float bowl on the left was done with Mother's Mag Polish, and the float bowl on the right was done with Brasso followed by Mother's. Conclusion: Mother's is better than Brasso.

AMAL is original , Minuki not so much

everything else is technical ... depending obviously on what you are trying to achieve

Kevin. Do you have a lighting and a Thunderbolt? Im after a seat of the pants comparison between the 2 for low end to midrange performance. Not interested in top end as i will be using it on 1/4 lose speedway tracks, i like the idea of one carb and simplicity, but also looking for low end performance.

no both of mine are Tbolts. i dont have a lot of experience with single carb motors. theyre certainly easy to set an idle on, but other than that i cant compare.

In my experience there's not a huge difference until you approach 5k rpms. Assuming stock gearing and equal states of tune. Above 5k a big difference.

In my experience there's not a huge difference until you approach 5k rpms. Assuming stock gearing and porting and equal states of tune. Above 5k a big difference.

My '71 had 200 mains when I bought it. Those proved to be too large, stock air filters, but different exhaust.
I ran VM 32's for most of the time I had the bike. Gas mileage dropped from 45 to 35 MPG, and I got very tired of rejetting to try to get the mileage up. The VM is a large instrument, as mentioned it's difficult to mount and find air filters. Unless you can find a 4-way fuel line adapter you must turn on both petcocks and have no reserve (90 degree aftermarket petcocks are required for clearance.) I guess they can work, but I cant recommend them. I was really looking for some Mk2 Amals when I bought the Miks.
Later I found a set of new Concentric bodies and reverted to them. The bike was running quite well when I sold it.

Its funny with the twin carb bikes that the main jets for both Lightning and Firebird models changed for the OIF. For the Thunderbolt, carb settings remained the same.

Bacon gives for the Lightning 30mm carb main jet:
'68 190
'69/'70 180
'71/'72 200

The Thunderbolt 28mm carb main jet:
'68 230
'69/'72 230

The '68 Hornet and '69/'70 Firebird had the same mains as the Lightning but the '71 Firebird Scrambler have 220 mains.
Possibly the '71/'72 Lighting had larger mains to reduce warranty claims due to more WOT riding.