Amal mk2 tuning

Status
Not open for further replies.
Old Bloke

Old Bloke

Joined
Feb 14, 2013
Messages
293
My 850 mk1 is being a witch to start after its rebuild, maybe it was before, I don't remember!

It has a single Amal mk2 fitted which I've scrupulously cleaned out iaw the Bushmans web information. I've also stripped and cleaned/checked the cold start plunger assy as per threads on this forum.

The symptoms are that even when cold, it does not seem to like the cold start lever on, and will start (after a dose of kicking) without it on, then idle ok. An observation is that the air screw is screwed nearly all the way in(less than 1/2 turn out) to get stable tickover, does that indicate that it needs a bigger or smaller Pilot jet, it has a "15" fitted at the moment?

I've checked for airleaks, made sure the cold start plunger operates ok, static timing is setup as per boyer instructions(still to be strobed), and the ignition Sparks look bright.

Anyone recognise these symptoms?

I'll not mention the smoking engine when it gets hot yet :P
 
go up to a 25 pilot jet and check the float height it may be a little low ,also have you got an air filter fitted? best to have one,when you start the bike try kicking it through ignition off 2 o 3 times before staring it ,,,,,,,,,,,,,,,,,baz
 
The amals with chokle levers , dont rate them for old machines, i have found that commandos like plenty of fuel to start them, a MK1 amal with a tickler after getting loads of petrol dripping soon burst into life. my mates 62 bonnie starts first prod with monoblocks , but is your problem fuel? Chuck a nice dripping carb on..its better.
 
john robert bould said:
The amals with chokle levers , dont rate them for old machines, i have found that commandos like plenty of fuel to start them, a MK1 amal with a tickler after getting loads of petrol dripping soon burst into life. my mates 62 bonnie starts first prod with monoblocks , but is your problem fuel? Chuck a nice dripping carb on..its better.
Click to expand...

I know, something reassuring about the petrol spilling out the carbs :)

Choke lever seems a bit "girly" in comparison. Is the problem fuel? hoping to answer that one.........
 
I think it's something to do with old two valve design, lever type choke methods just dont get the gas in, kick,kick and kick again..pull
the plug and they are dry...no doubt some lever "owners" will disagree.
 
Not sure about the single carb setup, much less the 850, however if you've your idle screw is screwed in, I'd raise the float level. My 750 seems to like a good bit of fuel to start from both carbs, like dripping out of the bowl onto the top of the crank. When I rebuilt mine I went by the book on the float level and it really didn't like that, could hardly start it. Raised the level and now 1-2 kicks with Pazon SureFire, even after sitting all winter. Granted avgas works wonders too.
 
As you cant raise the Float Level , to enrinchen it . Needs the Ckoke On ( cold ) pushing the cylinders through to prime them . Or it wont .

Then likely can flick it OFF before starting . Damned annoying with NO Choke Lever/s fitted , as you have to block intake to pull fuel into the cylinders .
Was supposed to be a cable choke set up available for them when they were introduced . Are we still waiting .
 
Valve clearances were set when I refitted the head, but I will check.

I'll try the bigger pilot jet first, have to buy one first of course, seems the easiest first step, and then have a look at float levels. I did read some about setting that up, but theres many opinions on what to set it at, and makes it difficult to get a definitive answer :(

Wonder how easy it would be to retrofit a tickler onto the mk2 :twisted:
 
john robert bould said:
I think it's something to do with old two valve design, lever type choke methods just dont get the gas in, kick,kick and kick again..pull
the plug and they are dry...no doubt some lever "owners" will disagree.
Click to expand...

I think that is fair, I see plenty of people on the forum happy with the single mk2/mikuni etc and report easy starting, and others with issues.
 
john robert bould said:
If all else fails......try a slide with a smaller cut-away, this will put a bit more "Suck" on the fuel.
Click to expand...
Would that be like a 3 instead of the 3 1/2 that is fitted?
 
Try a 3,
Just a thought.....doe's this modern fuel "light" up the same? which if not, do we require a "bit more"? anyway try a richer slide[3]...it will make a differance. the needle jet gets more suck.
 
Amal Mk 2 has an enrichment port rather than a choke. There are two options for fitting the pilot jet. one is in the bottom of the carb body and the other is in a deep hole in the bottom of the float bowl, the preferred position I think if I remember correctly Check you haven't got a jet in each. Get a set of pilot jets 20, 25, 30, 35 and fiddle until it idles and picks up cleanly of the throttle stop. My 750 went like hot snot with one of these and a velocity stack fitted but would freeze up at about 4000rpm. The mikuni is a better piece of kit with similar performance. For starting keep the throttle closed to make the enrichment circuit work properly after kicking over ign off a couple of times.
 
Thanks for advice, I've a 25 pilot jet on order, see what difference it will make. Only has the one pilot jet fitted in the float bowl, bottom of the carb body is plugged. Wish I had a big pile of amal parts to play with, have you seen the price of a chromed brass slide.....

What was causing it to stop revving at 4000, if that's what you meant?
 
I tried the single 34mm Amal MK2 setup a few years ago on my 750. I found it to be quite weak to say the least and unable to deliver at higher RPM's. It started good though.
 
gripper said:
For starting keep the throttle closed to make the enrichment circuit work properly...
Click to expand...
Most definitely. Unlike Amal carburetors that have a true "choke", the new Amals and Mikunis run a "fuel enrichment" system, which is basically a small carb in parallel to the main bore. Once the twist grip is opened ever so slightly, the manifold vacuum drops to virtually zereo (at least, at kickstarter speeds), and air/fuel throug the enrichment system ceases. You have to leave the twist grip alone to get these to start cold. Of course, that's assuming everything's right once the engine's at temp. You're definitely on the right track by going after the pilot jet.
Nathan
 
Nater_Potater said:
gripper said:
For starting keep the throttle closed to make the enrichment circuit work properly...
Click to expand...
Most definitely. Unlike Amal carburetors that have a true "choke", the new Amals and Mikunis run a "fuel enrichment" system, which is basically a small carb in parallel to the main bore. Once the twist grip is opened ever so slightly, the manifold vacuum drops to virtually zereo (at least, at kickstarter speeds), and air/fuel throug the enrichment system ceases. You have to leave the twist grip alone to get these to start cold. Of course, that's assuming everything's right once the engine's at temp. You're definitely on the right track by going after the pilot jet.
Nathan
Click to expand...
Gripper/Nathan: You made a good point there, I've a natural tendency to crack the throttle when kicking the bike, especially when it does not start after a couple kicks. As you say that is probably the thing not to do! I'll make sure I keep it closed next time and see how that works.
Peter
 
Old Bloke said:
My 850 mk1 is being a witch to start after its rebuild, maybe it was before, I don't remember!


The symptoms are that even when cold, it does not seem to like the cold start lever on, and will start (after a dose of kicking) without it on, then idle ok. An observation is that the air screw is screwed nearly all the way in(less than 1/2 turn out) to get stable tickover, does that indicate that it needs a bigger or smaller Pilot jet, it has a "15" fitted at the moment?
I'll not mention the smoking engine when it gets hot yet :P
Click to expand...


Re; less than ½ turn out.
Most of these types of carbs work best when the pilot air screw is around 1/12 turns out. (+ or - 1/4 turn)
½ turn out suggest it too rich if the mixture is correct, could the pilot jet in the carb have dropped out :?:
 
Status
Not open for further replies.

Similar threads

robs ss
FCR 35 tuning
Replies
16
Views
1,035
BrianK
B
D
Tuning the mikuni low speed fuel circuit with air screw
Replies
6
Views
664
acotrel
acotrel
mean gene
Advice tuning flat slide Mikuni
2
Replies
28
Views
1,779
Torontonian
Torontonian
D
Can you give me advice about tuning/jets etc on my single Mikuni VM34
2 3
Replies
47
Views
2,884
DennisMo
D
baz
Amal MK2 carbs
Replies
7
Views
1,007
Matchless
Matchless
Back
Top