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Misc: Ignition Need Advice, can't keep an idle (intermittently) approaching or at a stop with New Amal 276's

Mark Stephenson

Well Known and Active Website User
VOC Member
Hello VOC Experts--need some advice

Problem:
Warm or cold idle is not reliable as I come to a stop. On a fast stop even I can experience bike just going dead-but easily re-started thereater. Sitting, the bike will idle generally fine. Trying to set up a reliable idle with New Amal 276's from Coventry

Background:
Series B (2501) with Alton 12V, BTH Mag both <3rs; new replacement 276's with new dual throttle set up and cables. I do notice that my BTH lead cases have become somewhat melted on exhaust fin (cables were to close)...I put electrical tape around the casing damage--could this be the issue as has happened around the time of my new carb install?

What I have tried:
1. Adjust mixture, 2. set throttle stops 3. new spark plugs. (B6's w .16 gap)

What should I try?

Thx so much

Mark
 

Gene Nehring

Well Known and Active Website User
VOC Member
Hi Eddy,

I am contemplating fitting a 28 or 30mm Mikuni to my Comet but have NO experience with setting them up. Are you able to share with me the configuration, jets selection etc?

Many Thanks,

Martyn
Martyn,

I have a vm28 set up for comet sitting on shelf. It was supplied by Mikuni set up for a comet with jets etc. I could also supply the right manifold. DM me and we can arrange a price and shipping to aus if you’re interested.

Best,

Gene.
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
Hi Eddy,

I am contemplating fitting a 28 or 30mm Mikuni to my Comet but have NO experience with setting them up. Are you able to share with me the configuration, jets selection etc?

Many Thanks,

Martyn
Dear Martyn,
When I first built my 1948 Rapide from a pile of mostly worn parts I fitted two second-hand Amal Monobloc carburetors. (I was on a tight budget back then.) The bike ran quite well on these and once I'd got the bike registered and on the road (late 2008) I completed a few thousand miles. Although it started easily and ran well the slides would often stick if If went over half throttle. I tried a lot of "fettling" but never really got it sorted. This is not a condemnation of the Monoblocs, they were very second-hand but served the purpose of getting the bike up and running. Stuck open throttles can lead to interesting experiences that are probably best avoided! ;) I thought about having the Monoblocs sleeved but when I looked at how much this would cost it was as cheap to get two new Mikuni VM28 carburetors. Originality is not important to me so I've used the Mikuni carburetors for most of the 50,000 miles I have put on the clock.
The settings as supplied by Motocarb were:
Main Jet 190
Needle Jet P-6 (182)
Needle 5FL 14
Slide 2.5
Pilot Jet 30
Air Jet 2.0

That is what I used and suggest that these would be a good starting point that you could test and adjust to the engine configuration on your Comet. I put the needle in the middle slot and experimented from there. There are lots of different needle types for Mikuni carburetors but for a Rapide it doesn't seem critical. I reduced the main jet to a 160 but now that I am now running 8:1 pistons and a straight-through Armours silencer I will probably increase the main jet size once the Covid 19 restrictions allow us to get out on the road again to do some testing.

An observation on the Mikuni carburetors is that they seem to be very well made. The aluminium body casting is of good quality, possibly LM24. The slides feel quite weighty so are probably brass and they are chrome plated. There doesn't seem to be any significant wear.
I've no recent experience of recently made Amal carburetors but would think (hope) that the materials used now would be better than the built down to a price of the originals.
I did have a new Norton Commando in the early 70's that had two Concentric carburetors that worked OK. As a teenager, I had a BSA Gold Star with an Amal GP carburetor and my brother used Amal TT carburetors at times when we were vintage racing. The GP and TT types seemed well made with better materials.
 

stu spalding

Well Known and Active Website User
VOC Member
Hi Martyn, my VM28s came from Allens over 20 years ago with the following settings and I haven't adjusted them since.
Slide no 2
Main jet 160
Needle 5 F 21
Clip 2nd from top
Emulsion block 169 0-8
Pilot jet 30
The standard mounting rubbers are poor, as Ed suggests, radiator hose is much better, Cheers, Stu.
 

Peter Holmes

Well Known and Active Website User
VOC Member
According to "Richardson" Post War Comet and Shadow Carb Settings
Comet:- Main: 200, Slide: 29/3, Needle: 3, Needle Jet: Std
Shadow:- Main: 180, Slide: 29/4, Needle: 3, Needle Jet: Std
If those variations relate to other carbs like Mikuni etc. I have no idea.
I have a pair of VM28s waiting to be fitted, they were supplied and jetted by Steve at Motocarb I seem to remember, my bike runs so well on the original Type 29s I can't bring myself to upset the applecart, maybe one day, but the years are rolling by!
 

greg brillus

Well Known and Active Website User
VOC Member
There is a huge difference in the slide from a 3 to a 4 That just shows how much more fuel the single needs to be happy........I'm a big fan of the original carb's though some on here might disagree........apart from the odd drips of fuel, they are easy to setup, they last a long time, and they don't seem to block up in their low speed circuits like all the newer carby's do........Most concentrics, Mikuni's and the like tend to block up their pilot jets if left for 3 months or so. Well in our hot climate at any rate.......
 

greg brillus

Well Known and Active Website User
VOC Member
Absolutely Peter, they look great.......I can see why owners gave up on them if they were badly worn and too hard to keep in tune.......I use them on all the resto's that I do, but the cost of them has gone though the roof, so buying them now is a big question mark......The Shadow ones especially.......
 

Bill Thomas

Well Known and Active Website User
VOC Member
For every day running I am a big fan of Amal Mk2,
Made much better than Mk1, Maybe some slide problem, But I will put up with that.
The 30mm went straight on my Comet, Flash spec, And went and started well, Amal settings,
For 30mm.
 

hadronuk

Well Known and Active Website User
VOC Member
An observation about the earlier suggestions to move the 276 float chamber forwards/backwards to counteract fuel surge.
When measured carefully, I found the mounting faces of my standard manifolds are not accurately vertical. This results in the bore of the rear carburettor sloping down a fair bit towards the engine, whereas the front carburettor bore slope slightly upward.
Consequently moving the float chamber alters the fuel height at the main jet quite appreciably, especially on the rear carburettor. Its also more pronounced if you have the long arm float chambers. I use the short arm ones and have fitted a sort of capstan to the mixture screws so they can be adjusted from the side when they are behind the float chambers.
 

kerry

Well Known and Active Website User
VOC Member
May be worth mentioning . About 4-5 years ago a batch of shadow 276 carbs were made wrong, from memory I think I had to buy a .80mm drill and drill an internal hole, a phone call to Amal may help as they told me to do that at the time as my bike would not run off chokes !
 

Chris.R

Well Known and Active Website User
VOC Member
My Comet with a rebuilt remote float 29 slide 3.5, needle in slot 3 middle pilot at 1-1/4 and slide set at 1/16 will not idle satisfactorily in fact a couple of days ago she did a ten mile run with some spitting back, but ran. Yesterday chocked on anything other than full choke ease off and she stalled, stalling twice on a run round the block so unable to get her up to working temp to try what I had been advised to do test for air leaks with WD40 around carb mounts. What next?
 

vibrac

Well Known and Active Website User
VOC Member
Double cxhack everything especially the inlet tract sealing if all else fails I would go Mikuni, the original 276 were fine I worry about the following designed Amals and later remakes
Having said that a good solution is a Mikuni on the shelf that seems to have sorted my Monoblock out I swear it knows what will happen if it goes wrong!
 

delboy

Well Known and Active Website User
VOC Member
My Comet with a rebuilt remote float 29 slide 3.5, needle in slot 3 middle pilot at 1-1/4 and slide set at 1/16 will not idle satisfactorily in fact a couple of days ago she did a ten mile run with some spitting back, but ran. Yesterday chocked on anything other than full choke ease off and she stalled, stalling twice on a run round the block so unable to get her up to working temp to try what I had been advised to do test for air leaks with WD40 around carb mounts. What next?
Sounds like classic case of cr*p in the pilot jet if it ran before okay.
Cheers,
Delboy.
 

hadronuk

Well Known and Active Website User
VOC Member
Might I also suggest that as you try different carb settings, you frequently replace the spark plug?
I have found that after only a short period of a rich tickover, the plug is sooted enough to cause a misfire even though it only looks slightly mucky.
Modern plugs don't seem to be burnt clean by a full throttle blast, as used to be normal practice. Replacement is the only option.
 

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