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Misc: Everything Else Misfire

nigsey

Well Known and Active Website User
VOC Member
Have a slight misfire, mostly when accelerating on my C twin, wondering if it could be a blocked carb jet. Carbs are Amal 276’s with Vokes air filters, currently running on Esso Supreme 99. Ticks over fine, new NGK plugs fitted, see photo, only done 90 miles on them. Any ideas please as to what might be causing the misfire.
 

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vibrac

Well Known and Active Website User
VOC Member
I have been playing with Vincents for soon to be 60 years when I think of all the hours spent in the hassel of trying to devine what the carbs or ignition settings should be for a particular bike compared to this last couple of years of simplicity. Take this year as an example, huge new GP Carbs 3 changes of exhaust systems ignition settings all done in a hour at the rolling road with a lamba connections in the exhaust pipes. No guessing no worry. (But admittedly you need a good operator who understands old bikes!)
 

greg brillus

Well Known and Active Website User
VOC Member
Most common causes of rich running on these old girls........Too high a float level........worn needle jets.......incorrect grade of fuel.......incorrect heat range of spark plug. I've never seen a blocked jet that caused rich running, unless the pilot air bleed passageway is blocked.......and I've never seen that ever....... Put some new plugs in NGK B 6 es OR BP 6 es or Champion N5C set at 0.018" gap, lower the float bowls by rotating the carb's slightly on their manifolds by say 3 mm........remove the air filters.......Take the bike for good run, and see what the plugs look like after.
 

Bill Thomas

Well Known and Active Website User
VOC Member
If it runs spot on when cold , It will be too rich when hot,
That's why we have chokes or why we flood to start from cold.
The H.T. leads I am talking about , By solid I mean A bit of wire in the middle,
Years ago there was a Carbon track stuff for suppression, And they used to break up.
If you have the new type BTH they say keep the earthing wire away from the H.T. Leads ?.

Years ago my Dad had a V8 Rover and he just drove it local,
So when I went to Classic Club night, He would say , Take the Rover and give it a clear out !!,
He ment give it some rev's !, Up the A 40 to London !!.
It did go !.
 
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David Wardingley

Website User
VOC Member
Hello,
Interesting thing a misfire and often terribly frustrating. My Norvin is similar in behaviour except my plugs on the rear cylinder ( twin plug heads) are black and very wet, the front cylinder the perfect mid brown. I had to do 20 miles on one cylinder as it simply stopped firing on the cyl. Once the plugs were cleaned it ran perfectly again until I came to an uphill drag where it started again to misfire on the rear cylinder. Once over the crest it came back onto two and ran very well. Someone told me to run hotter plugs in the rear cylinder. I don’t know yet if that has worked as I haven’t tested it but worth a thought. My carbs are Dellorto, ignition is Pazon.
Regards
David
 

Bill Thomas

Well Known and Active Website User
VOC Member
Hello David, Have your carb's got the pumps in them ?.
I have taken the arms out that work the pumps,
It all worked fine when running 12 to 1 pistons and doing a bit of racing and road work,
But after I dropped to 9 to 1, because of no 100 octane petrol,
The pumps had to go,
And a few years later with the ethanol in the petrol, I had to use weaker slides,
Bigger cutaways, I think I cut them by hand ?.
Dellorto's are tricky to tune and cost a lot for jets etc.
Cheers Bill.
 

SteveW

Well Known and Active Website User
VOC Member
Go down a step on the jets and see if it improves. I recently sold a guy from Colorado a Rapide and he changes his jets "in situ" when at higher elevations.
 

David Wardingley

Website User
VOC Member
Hello all,
I actually didn’t build my Norvin so am very much the apprentice here. It’s a 1200cc, twin plugged heads etc , I’ve no idea of the compression etc but the motor was a new build as were the dellorto carbs. The weather is nice today so a run this morning will confirm if the hotter plugs on the rear cylinder have actually improved it although this is very much a starting point for me. I will investigate the carbs by a bit of stripping and looking as I know zip all about dellorto carbs but am ok with Amal’s and also Mikuni carbs.
Dave
 

Bill Thomas

Well Known and Active Website User
VOC Member
Morning Dave , Can we have a photo ?,
The pumper carb's Have a triangle bit above the bell mouth,
This is mine.
Cheers Bill.

P.S. What plugs are you now going to use ?.
Don't go too soft !.
 

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