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ET: Engine (Twin) Occasionally missing a beat



Sakura

Well Known and Active Website User
VOC Member
#1
Just fitted a new set of 276's to my Rapide. They're properly balanced but the pilot mixture may not be 100% correct yet. Only done a few miles but when pulling up a hill at 30mph in 3rd it occasionally missed a beat. Nothing dramatic, just slightly annoying. It ticks over slowly and reliably but I'm thinking the pilot mixture maybe a bit rich. I know I can play with it but has anyone got any thoughts?
 

chrislaun

Well Known and Active Website User
VOC Member
#2
If you've just fitted them new they are going to need setting up to suit your bike, every one is different and the settings provided are only a rough guide.
 

brian gains

Well Known and Active Website User
Non-VOC Member
#3
I understood that under certain riding conditions there can be a noticeable transition between pilot / needle / throttle body cutaway/ main jet. This can be reduced by altering needle position and/or amount of cutaway in throttle body but have been told that usually you just end up moving the 'blip' around.

Sure someone will chime in with more technical description of adjustment finesse.
 

brian gains

Well Known and Active Website User
Non-VOC Member
#5
second thoughts, ignore the cutaway as a solution if that is incorrect it just runs generally poorly at that throttle opening.
 

greg brillus

Well Known and Active Website User
VOC Member
#6
Have you changed or adjusted the settings at all after installing the new carb's..........?..........It is not uncommon for the clip fitting carb's to not be a snug fit on the manifold stubs and may leak air. The mixture screws should be around one to one and a quarter turns out from fully seated. Replace the plugs with two new correctly gaped ones and see what plug readings you get............Generally I have found the new 276 carb's to work very well straight out of the box with minor adjustments.
 

Sakura

Well Known and Active Website User
VOC Member
#7
Have you changed or adjusted the settings at all after installing the new carb's..........?..........It is not uncommon for the clip fitting carb's to not be a snug fit on the manifold stubs and may leak air. The mixture screws should be around one to one and a quarter turns out from fully seated. Replace the plugs with two new correctly gaped ones and see what plug readings you get............Generally I have found the new 276 carb's to work very well straight out of the box with minor adjustments.
I'm generally well pleased with the carbs. They carburate well throughout the range and tick over reliably. Plugs are new and the colour looks good. Carbs are a good fit in the stubs. I'm nitpicking, it only misses a beat every now and again and obviously I can keep experimenting with the pilot mixtures or even cutaway. I'm pretty sure it's in the range of pilot to cutaway and you can usually cover the transition with pilot mixture. I was just hoping to save time and effort using someone's experience, like Greg, who has trod this exact path before. My feeling is it's a bit rich and I'll just ease the screw out a little.
 

Sakura

Well Known and Active Website User
VOC Member
#9
30 in 3rd uphill?
I would be in 2nd, much less stress on engine and transmission...
Peter B
Fairly gentle slopes and it seemed happy enough to me. On the level it will pull down to 30 in top but 3rd feels better. Surely the whole point of the Vincent engine is torque?
 

greg brillus

Well Known and Active Website User
VOC Member
#10
The slight miss could be just the fact it is cold............An engine that performs perfectly when cold is generally too rich in the carburation. My Rapide is a bit like that, but I run the VOCS foam filters which I feel have very little drag/restriction to the air flow, the plugs show a rich though not absolutely sooty black colour.
 

Nigel Spaxman

Well Known and Active Website User
VOC Member
#11
My thoughts are you are going way too slow! At 30 MPH I am in second gear. I have a tachometer so I know how slow my engine is going. It does depend on the sprockets you have though. I never go less than 40 MPH and I don't shift into fourth until I am going about 50 MPH. Sometimes I take it to 100 mph in third just to make sure it can. I have on friend who says he never uses fourth gear. One time I did a trip 100 miles on back roads of southern Vancouver Island and I never used fourth gear. But seriously the Vincent is an amazing motorcycle that is pretty happy in heavy slow traffic and also good on the open road. Usually when I finally settle on my carburetor settings I have tried up and down from every setting possible, from pilots to mains. It takes a long time to get it right.
 

MartynG

Well Known and Active Website User
VOC Member
#12
Just fitted a new set of 276's to my Rapide. They're properly balanced but the pilot mixture may not be 100% correct yet. Only done a few miles but when pulling up a hill at 30mph in 3rd it occasionally missed a beat. Nothing dramatic, just slightly annoying. It ticks over slowly and reliably but I'm thinking the pilot mixture maybe a bit rich. I know I can play with it but has anyone got any thoughts?
Possibly your spark plug leads are "leaking". Run the motor at night or in a dark shed and look for any tell tale arcing from anywhere along the spark plug leads - if there is any it will be intermittent and is just the thing to cause a strange miss. If they need replacing use the high quality silicon rubber stuff - any good ignition shop can make them up for you.

M
 

MartynG

Well Known and Active Website User
VOC Member
#14
If you are running a BTH then it is advised to NOT use silicon leads.
Just to be clear. I am totally AGAINST the use of silicon cored, sometimes called suppressor HT leads.

What I am advocating and use is multi strand copper cored leads with silicon rubber insulation that have NO ignition 'noise' suppression properties, but great HT insulation - just like is used in international motor sport. You need special tools to make up these leads, that is why you need to get a specialist ignition shop to make them for you.

And if you think these leads are expensive - just check out the market value of your Vincent!

M
 

Chris S

Well Known and Active Website User
VOC Member
#15
Might be worth checking your ignition timing. At 30 mph in 3rd you are pulling very few engine rpm and the ignition is probably not fully advanced. You can find that loading the engine in these circumstance can lead to occasional splutter as fuel struggles to fully burn early enough.
Chris
 

Albervin

Well Known and Active Website User
VOC Member
#16
I made a big mistake about 35 years ago of running up a hill in top gear at 30 mph on my Comet. A very slight seizure ensued. Vincent engines need more revs than most 1950s engines to run optimally. If you are using more than half throttle and not accelerating then change down a gear. I have a 48T rear sprocket on my otherwise standard Rapide and it is 25, 35, 55 and 75 mph in gears for normal riding. If I am angry it is 30, 55, 70 and XXX mph. The other issue is you won't be balancing the ammeter at 30 mph in 3rd if you have lights on.
 

vibrac

Well Known and Active Website User
VOC Member
#18
how interesting! my 2020 Comet just up to the testing stage has an occasional interruption (28mm Mikuni and BTH) the first runs showed a weak mixture on 160 main jet so I went to 180 but it still occurs under various loads. I have the feeling that main jets on Mikuni cover and influence a wider range of openings that the Amal crowd but I have no source for that intuition. as with all the running in period its change one thing at a time.
 

SteveW

Active Website User
VOC Member
#19
Check the mag and points for oil contamination. Had similar symptoms happen to my Rapide.
I had the same symptoms, skipping a beat once in awhile. I found it was oil contamination in the mag coming from a loose oil line banjo bolt from front heads inlet valve. It dripped down and worked it's way forward to the HT leads and directly into the mag.
 


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