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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
 

Bill Thomas

Well Known and Active Website User
VOC Member
I m not good with standard carb's , But I have found Vincent's are not good at long time tick over ?,
I have always thought the heat of the engine goes up quick , So altering the mixture,
I used to ride with a friend who fiddled every time He stopped, Tony Wheatley !!, West London,
Drove me Mad !, He wanted the tick over SOO low, And stalled often !,
Sounds to me like you are running a little weak ?.
The other thing when braking, It's possible to get petrol surge ?,
Maybe moving the float chamber forward or back may help,
Setting the tick over a small bit higher may help ?.
Cheers Bill.
I know my engines are race tuned but I would blip the throttle after a few seconds in traffic,
Also when stopping.
 

Mark Stephenson

Well Known and Active Website User
VOC Member
Gents,

I will try to locate the float bowls at rt angle to the carb body on both sides. Some background for further sleuthing please: <3yr old Alton 12V, BHT Mag, New plugs B6's gapped at .16 running new VOC cables, new dual throttle (good movement and "snap back") and new 276's from Coventry. I have the original 276's. I. think my local expert Charlie Taylor (9 blocks away in SF) is doing a great job helping me along, we might be trying for a tick over that is "too minimalist". Thx Mark
 

passenger0_0

Well Known and Active Website User
VOC Member
Something to bear in mind Mark is that these bikes are unlike the fuel injected and electronically managed machinery of today so they do have their behavioural quirks.
I've been trying to get a rock solid reliable idle out of my Rapide for 21 years, and to date I'm close but no cigar.
 

Mark Stephenson

Well Known and Active Website User
VOC Member
Something to bear in mind Mark is that these bikes are unlike the fuel injected and electronically managed machinery of today so they do have their behavioural quirks.
I've been trying to get a rock solid reliable idle out of my Rapide for 21 years, and to date I'm close but no cigar.
Good point for sure. I actually have never road a modern bike..have a vintage BMW and Triumph...should be able to keep an idle. Tuning and tweaking part of the fun:). BEst Mark
 

Bill Thomas

Well Known and Active Website User
VOC Member
I wonder if uneven firing of a Vincent Twin makes it harder ?,
At one time with Mk1 concentrics , I soldered the head of a key into the screw head,
Of the slow speed screw, Adjusts the slide, And if in traffic for a long time I could just
Turn it up or down a touch on the front carb', Worked well !,
As I said the heat of the big lump of a Twin goes up quick, More so on a hot day.
To me it's a wonder these things run at all :D .
 

greg brillus

Well Known and Active Website User
VOC Member
The bikes are like people.......very temperamental at times........Funny how you can throw something together and it will Idle all day long happy as the cat........On the next one you meticulously set everything up, dial in the cams spot on, get the ignition and carbs set up like an all out race bike, and it will idle like a pig.......But sometimes you have to think outside the square........My bike has big cams, bigger than Mk 2's and 8:1 pistons and it goes very well........It use to idle lovely all day long........Then I removed the BTH mag and installed a new Grosset electronic ignition set up........all turned to shite........The ignition works a treat, but the wasted spark set up does not go well with the big cams, at idle it will tick over for a couple of seconds, then gives a spit back through the front carby often this will stall the engine.........as of today I have removed the ignition and gone back to the BTH........This also shows just how mild our so called "Hot cams" are as my engine when warm would normally idle all day long. Ok, so getting back to Mark's issue........I mentioned the manifolds because sometimes the carb's are quite a sloppy fit on the stubs, and simply heaving on the pinch bolt may not be enough to get a good air tight seal........an air leak here will give a poor idle on account of the lean mixture.......It is possible it could be in the new carbs themselves but it is easier to eliminate all the outside factors first.........Ok I'll go back to my Bourbon now...........Cheers.
 

brian gains

Well Known and Active Website User
VOC Member
if they are out of the box new carbs and you've not already done it, clean carb thoroughly. Not just carb spray cleaner but ensure all jets are cleaned out particularly idle jet with fuse wire, fine drill, guitar string etc, but avoid scratching. Read multiple times of new carbs still having swarf debris.
 

Bill Thomas

Well Known and Active Website User
VOC Member
When I was a young lad, I was, I was, :D
I could get the 3 carb's on a jaguar Mk10 or E/type to tick over soo smooth, In the garage !,
Using a Sun machine from USA, But as soon as I got on the road , It was Rubbish !, Too weak,
It hurt me to run them a bit more rich, But that is what was needed.
You have to adapt !.
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
Air leaks as mentioned above by Greg can give some strange tick-over problems because the vacuum when the slides are closed will likely pull in air if there is a leaky joint. A small leak isn't so likely to show up as a problem when running with the throttles open as there is less restriction so lower vacuum.
Re: heat build-up. I don't have a problem with the tick over long idling periods. I use a new type BT-H magneto but have VM28 Mikuni carburetors fitted so not a direct comparison. I have the carburetors mounted using rubber radiator hose so I'm wondering if that reduces heat being conducted back to the carburetors.
There must be lots of people using standard carburetors that don't have a tick-over problem so likely checking all the little things will improve things.
 

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