Struggling with MK11 carbs

Gents, I’m struggling with the mk11 carbs on this T140 I have just bought.

The carbs are brand new and fitted with 20 pilot jets, 106 needle jets, number 3 slides, 200 mains. These are the ‘standard’ settings for this bike according to Amal.

I have ensured the enriching circuit plunger is sitting properly with slack on the cables.

It is very rich. Currently, I’m only talking about low revs / small throttle openings cos it’s a newly rebuilt motor, when opened up briefly it it seems to clear, but the low end is very rich.

So I started reading up and learned that carb settings for the late model T140s (mines late 1980 so probably a 1981 model year, but kinda unsure) are a bit of a nightmare with EPA spec bikes vs none EPA spec bikes etc.

The carbs will be coming off today to see if I can find anything amiss. Float height is my first port of call.

I have a brand new pair of mark 1.5 concentrics in stock and am sorely tempted to fit these at the moment (I had a Harris Bonnie with these and it was fantastic).

But I’ve also had Bonnies in the past with MK11s that ran perfectly, so something is obviously amiss, and I’d like to know what.

I’ve read the John Healey notes posted elsewhere about the difference between EPA and none EPA settings but am confused as to which is best regarding overall performance with today’s fuel. I’m also confused because I had thought that EPA settings would be weaker, but some jets are richer?

As you can tell, I’m somewhat discombobulated over this. Does anyone have any sound advice as to the best way to get these MK11 carbs running as Amal / Triumph intended?

I have a 79 T140 D with Mk2's...When I bought the bike it had the stock EPA jetting but was very rich at idle because of worn cold start plungers.. Changed to the so called European jetting listed on online and new cold start plungers...The listed number 3.5 slides were too lean for my bike and had to use 3.0 . The engine responds cleanly at all speeds .....An overly rich mixture, especially on a new engine,can glaze the cylinder walls..

Absolutely look at the rubber tips of the enrichers, they dry/harden and cause rich low speed running.

I will look at the enrichener rubbers, but as I said, they’re brand new carbs, so I’m doubtful they’re my issue.

Fast Eddie said:

... they’re brand new carbs...

Click to expand...

New "old stock", or recently manufactured?

Ha, fair question!

From Burlens, so I assume they’re newly manufactured, but I guess it’s impossible to know for sure.

As most of their Mk2's are showing as out of stock then what stock they have must be old.

One thing to check for sure about the cold start ....if the carbs have the linked flat bar linkage, little parts get bent and one or both plungers may not seat when supposedly in the off position..Push down on each plunger...

Well, we have progress.

Cold start plungers are all fine, they’re on a cable, so no bent rod issues etc either.

Basically, Burlen supplied the carbs with a curious mix of EPA and ‘standard’ (none EPA) Jets and some from neither !

The most obvious was they fitted 2C3 needles but 106 needle jets (whereas it should be either 2C3 with 105s or 2A1 with 106s).

They fitted #3 slides, but if I’m going for none EPA, I need #3.5.

They fitted #20 pilot jets, which is correct for neither, and I need [HASHTAG]#15s[/HASHTAG] for none EPA.

So, all in all, it was rich everywhere on the settings they provided. I think this is very poor, it’s a stock bike, and all it needs is carbs with stock settings, it ain’t rocket science!

Probably a deliberate rouse to sell more jets and needles !!

If you generally use E10 fuel the #15 pilots might be a bit lean....The #3 slides may cause a slight hesitation off idle. 106 needle jet and 2A1 needle are ok....this also depends on the airfilter type and exhaust...Just saying, it may be perfect, let us know....
I believe I have new 3.5 slides if you want to try them..Live in the USA? Opps, I see you live in the UK, forget the E10 gas and slides, lol

Well, I have number 3 slides for the mk11s presently, but am going to order some 3.5 chromed brass ones to try, along with the correct needles and pilots jets to set the carbs up to correct none EPA spec.

They’ll take a couple of days to arrive, and, being a bit of an impatient type, I dug out the new mk1.5s I had ‘in stock’ and have got them fitted now. I had some of these on a Harris Bonnie years ago and they worked perfectly, that bike was fantastic. So I’ll try them out tomorrow (way too late to be firing up a T140 on peashooters now)!

I live in the U.K. Hillbilly, and we’re blessed with quite good petrol in my locality with big brands like BP, ESSO and my favourite Shell, all doing good high octane petrols that are low (some say zero) ethanol.

We’ll see what tomorrow brings!

Keep y’all posted...

Ok, I fitted the new mk 1.5s but replaced the #3 slides with #3.5s...

I tested it today... it was s*it...!

So, I put the #3 slides back in and it’s was much better, fiddled with the air screws a bit and bingo, it’s great!

I took it for a decent test ride and it feels nice, but, there’s an occasional stutter that I think corresponds with the needle, so I’ll try lowering the needle next, it’s not got a 100 miles on the newly built motor yet, so I’m being conservative with the throttle hand!

I’m staggered at how much difference there was between 3 and 3.5 slides. When I were a lad I cobbled carbs together out of a tin of old bits and generally, they worked!

Anyway, at least we’re making progress and it’s starting to feel like a Bonnie at last!

Guys, can someone help me understand the ‘air jet’ on MK11s?

It’s the little top hat affair directly below the intake and held in place by the intake bell mouth.

This little critter is something I’ve never noticed, or played around with before at all.

But according to John Healey, it’s part of the EPA package of settings. So, if I want to go to none EPA settings, do I just remove the air jet altogether ?

I can’t work out in my head just what it does, or what effect removing it will have.

Fast Eddie said:

But according to John Healey, it’s part of the EPA package of settings. So, if I want to go to none EPA settings, do I just remove the air jet altogether ?

Click to expand...

http://www.britbike.com/forums/ubbthreads.php/topics/121757/re-carb-data-1978-t140e

John Healy
"The MKII carbs came with two basic set-ups for T140s:
European (non-EPA)
124/026 #15 pilot jet
124/012 #35 choke jet
2928/122 .106 needle jet
2928/030 #3 1/2 slide (leaner than US equivelent)
2622/124 2A1 needle
No air jet fitted

US EPA
124/026 #25 pilot jet
124/026 #50 choke jet
2928/031 .105 (special stainless steel)
2926/060 #3 slide
2928/030 2C3 needle
An air jet was fitted to the central air inlet on US EPA models."

Thanks Lab, I’ve actually already read that from John, and I’ve just found, and read, his extended notes, in which he says:

“Note: Non-EPA Jetting refers to the jetting Amal would supply if the carburetor was not being set-up to meet U.S. EPA emmisions.
Note: You will have to remove the air jet from behind the velocity stack. It is the small aluminum disc used to restrict the flow of air through the center hole in the face of the carburetor. Removing the air jet reduces the vacuum signal on the needle/main jet leaning out the overall mixture, as the slide approaches wide open operation”.

So, I understand that I should remove the air jet completely, and John does explain its function a little, but it still remains a tad mysterious to me.

The trouble is, Amal show a number of different air jets on their web site and show it as standard in mk11 carbs, even in mk11 smoothbores which are only for racing, so I’m struggling to believe that they were only for EPA market bikes.

Hopefully, removing them is all I have to do, so the finer details of their function will be a moot point.

Hopefully...

https://web.opendrive.com/api/v1/download/file.json/NDlfNDk4NDMyX0JNOGlK?inline=1

"Air jet
This controls the amount of air which pre-atomises the fuel before it enters the mixing chamber body.
Normally the air jet fitted as standard for the particular size of carburetter should be correct, but
it is a component that can be changed should the depression on the main jet need to be influenced.
Fitting a smaller jet will increase the depression while a larger jet will reduce the depression."

"4 stroke
Air jet 3.5"

That’s another good article LAB, thanks again. And it’s another article saying to remove the air jet completely, so that seems to be the consensus. Sadly I’m gonna be out of the shed for a couple of weeks, but at least I can get the bits ordered and ready!

A restriction in the centre drilling can make the mixture at high rpm richer than the mixture at lower rpm.

They may have found that the mixture was getting weak at high rpm with that environmental jetting and so fitted the air jet.

I’m just conjecturing though.

According to John Healey, you’re right TT:

“Removing the air jet reduces the vacuum signal on the needle/main jet leaning out the overall mixture, as the slide approaches wide open operation”.

So, with it fitted, it must be enrichening ?

Also, when you look at the specs given by John, the pilot jet, starter jet and slide are also richer in the EPA carbs, which, as we know, are supposed to run leaner...

I’m assuming that the 2C3 needle WITH the #105 needle jet is so weak, other aspects have to be enrichened to compensate? Although, I also know that doesn’t really make sense as the circuits are different, and the needle/ needle jet shouldn’t really effect the pilot or starter jets.

If the compensation theory is right, that might explain why mine seemed so rich with a 2C3 needle and 106 needle jet?

The needle and needle jet do have the biggest impact on 1/4 to 3/4 throttle opening... so that doesn’t really explain why the pilot and starter jets were richer in the EPA carbs.

I’m stabbing wildly in the dark here though, and I rather doubt I’ll ever really understand, but am hoping I don’t have to!

A further point of interest / confusion is that the genuine Triumph workshop manual specifies jetting that is different to both of Johns lists and doesn’t even mention the air jet at all, or show it in the exploded diagram of the mk11 carb. Amal have fitted the same jets as shown in the manual, plus an air jet.

I’m ordering the jets and slides to put the carbs to full ‘Healey non EPA spec’ and see how we go from there.

Keep y’all posted in a couple of weeks.

So, with it fitted, it must be enrichening ?

Click to expand...

I reckon that’s what I said.