250 Dianna Missfire

**bevel bob** · 18 Feb 08, 08:26 PM

My mark3 has a bad miss at 6-7k, i think i've tried everything, begining to think its not going to ever be right,any ideas anyone???

**stevel** · 18 Feb 08, 08:34 PM

Bob/ Given the hard-to-pinpoint nature of a misfire, it might be helpful if you added a bit of detail about what you have / haven't done already.....would help narrow things down maybe? And a pic or two!

**bevel bob** · 19 Feb 08, 12:09 PM

bought the little beast in 2002!!,It had been rebuilt by a M/c shop (the professionals!!) sold to a duc racer who fettled it for his own use, It looks like a Mach 1 but i think its a NC Dianna,It had a half completed 12v conversion.I ride it occasionally ,SSI29d carb,10.2 cr, grey white cam,40mm inlet.Loads of compression,pulls hard up to 6k then misses badly.I rebuilt the carb ,new exhaust,ignition sort out, good battery,new slide,jets,needle in different positions,the problem has not changed.My next move is to look inside,I have found a few daft and dangerous errors so its the wise thing to do.I'll probably change to a rubber mounted racing float .I can play with fuel levels then.Ignition is Pazon Points assist unit Total loss.

**Dan** · 19 Feb 08, 12:10 PM

I'd love to see a picture of it Bob, if you have time.... 8)

**bevel bob** · 19 Feb 08, 02:37 PM

Not much good with fotos, I'll give it a go, went out for a test ride after fiddling with carb,all ok till it warmed up then 3miles away--- wont run at less than 6k-----and wont run over 6500!! now I know what a narrow power band feels like. Interesting journey home in the traffic.Think i'll take the Norton out next time.

**bevel bob** · 19 Feb 08, 02:54 PM

As reqested piccy of tempramental Itallian dalliance.

**bevel bob** · 19 Feb 08, 03:14 PM

****ed off with the Ducati, this is another of my rides, as you can see

good for long rides.

**Mr.R** · 19 Feb 08, 06:25 PM

Hi Bob,
It's a Norton but which one? Where was the picture taken and when?
It looks to have been taken quite a while ago, somewhere in the Med.
Here's a couple of thoughts on your problem.....
Weak points spring causing the points to bounce/flutter,
Coil breaking down, although these normally just fail completly,
Condenser starting to give up the ghost.
Vibration causing the fuel to froth in the float chamber resulting in a weak mixture,
Vibration causing the float neadle valve not to seal resulting in a rich mixture,
Fuel starvation,
Wrong size jets, causing weak or rich missfire,
Jet holder loose or not sealing,
Too soft a grade of spark plug,
Bad earth on the ignition,
Dodgy ignition switch....Fit a Honda one!
That should be enough to be getting on with.
From your use of the abriviation 'Duc' and Mk3 for a narrow case motor, my guess is your not from round here are you?
North American continent?
Are you Bob Hartman?

Steve

PS. I have a "74 250 Desmo, my wife has named 'Bertie', fitted with a rubber mounted 32mm Smoothbore Mk1 Amal concentric carb, large 1/4in Id fuel taps, (from a pre-unit Triumph) Lucas Rita ignition and 12volt conversion.
This does away with most of the possible problems listed above.

**bevel bob** · 20 Feb 08, 06:57 AM

HI Mr R, thanks for your timely reply,my pathetic call for help did the trick!,I'm usually good at diagnosis and scathing when others bounce around looking for complicated solutions to simple problems,slow methodical logic always is best.The problem is electrics and most likely the ignition switch,what was i thinking?.The bike has a vibration period at 6k and the switch feels loose inside,time for a hotwre .Bye the way i'm called Bertie sometimes!, not a Yank,I must be infected by spending too much time on thier sites.The norton is a 600 featherbed,in sunny Spain in the 1960s,it morphed into a sport bike but is now back to a full dress tourer with avonaire and dolomite panniers.You cant do this stuff with a 748.Another test run today,let you know what happens. Bob.

**bevel bob** · 21 Feb 08, 09:34 AM

Blast!!! its not the switch,however getting better at starting,After 2 locked wheel slides the 3rd attempt worked.Even managed to get the beast to the MOT it passed!! (your not going to ride this thing in the dark now are you?? Nah not likeley, Ok forget the lights!!)--Can I have a push please? !! Tank off next to reset the needle into 2nd notch, 1st notch has turned my new exhaust a pretty shade of yellow.

**bostrom998** · 22 Feb 08, 10:42 PM

Hi Bob.

Sorry to jump in like this but have you tried fitting a foam filter as a temporary measure?
This will prevent the carb from freezing up due to the cold outside temp & when fuel/air mixture runs through the carb it will freeze up and feel like fuel starvation.

The other thing I do agree with Mr R the the condensor may break down under load & you may get the odd back fire.

Cheers & good luck.

**bevel bob** · 24 Feb 08, 08:12 AM

Hi all, taking the little beast out again today, after I reset the carb ,we can rule out carb icing(sunny day) and the points condenser,it doesent use one. I'm going to do a rough check on the fuel flow and float level before I go Off into sunny Kent lanes.The plug may also be a grade too soft.A bit too much oil is a possibility , difficult to see on the n/c motor + the rubber cam screening is missing possibly a mod from its racing days?and too much oil could be getting to the valves.

**Imola Duke** · 24 Feb 08, 06:04 PM

Quote:

Originally Posted by Mr.R

Hi Bob,
It's a Norton but which one? Where was the picture taken and when?
It looks to have been taken quite a while ago, somewhere in the Med.
Here's a couple of thoughts on your problem.....
Weak points spring causing the points to bounce/flutter,
Coil breaking down, although these normally just fail completly,
Condenser starting to give up the ghost.
Vibration causing the fuel to froth in the float chamber resulting in a weak mixture,
Vibration causing the float neadle valve not to seal resulting in a rich mixture,
Fuel starvation,
Wrong size jets, causing weak or rich missfire,
Jet holder loose or not sealing,
Too soft a grade of spark plug,
Bad earth on the ignition,
Dodgy ignition switch....Fit a Honda one!
That should be enough to be getting on with.
From your use of the abriviation 'Duc' and Mk3 for a narrow case motor, my guess is your not from round here are you?
North American continent?
Are you Bob Hartman?

Steve

PS. I have a "74 250 Desmo, my wife has named 'Bertie', fitted with a rubber mounted 32mm Smoothbore Mk1 Amal concentric carb, large 1/4in Id fuel taps, (from a pre-unit Triumph) Lucas Rita ignition and 12volt conversion.
This does away with most of the possible problems listed above.

Steve
I just love reading your posts

Any Chance of publishing a book 8)

**hughdg** · 24 Feb 08, 06:43 PM

I agree with Mr R, it is a Norton. A 250 Jubilee or a 350 which I think was called a Navigator. Of course somebody may know better than I.

**Mr.R** · 24 Feb 08, 06:59 PM

Steve ID,
Anymore of that and I'll be

, the plain truth is that I'm far to laid back (read lazy here) to be ars*d to get around to it.
I've had a couple of mates who've written in the past 'How To' books only for me to think........
He's got that wrong, he should have said this/that whatever, I'm sure I could do better myself.
But the fact is they DID do it, fair play to them I say.
I post on here to amuse myself, trying to help out fellow enthusiasts/Ducatisti's along the way.
If I've ever helped just one person, then that'll make me a happy bunny/man.

Steve