Carb or Sparks?

[Old Cog]

Hi

Off on our hols and it’s raining! So I’m looking for some advice on rules of thumb.
My 250 Mk3 was running pretty well: it was a bit on/off around the town, but it’s a highly tuned engine - non-standard, high compression piston - and the only real fault was a LOUD bang in the exhaust when on the over-run - snapping the throttle shut quickly generally provoked it. So I put in a new exhaust gasket - to very little effect. So on to address the other cause (I’ve always believed) of such a back fire - idle mixture.
As I tried adjusting the throttle stop and mixture screws, the backfire could be improved but the on/off effect round town was worsened and eventually it refused to pull away smoothly: now, as the throttle is opened, the engine misfires two, three or even more times in rapid succession. As the the throttle is opened further and the revs increase the problem disappears.
The misfire ‘feels’ electrical - completely missing a few beats - rather than the flat or gutless couple of seconds I’d associate with a carburettor problem (at least on the bikes I was more used to, in a softer state of tune than this one). And of course it could be an ignition issue (though not routine like points ‘cos it has none) but its onset certainly coincided with me messing about with the throttle stop and idle mixture.
Of course a properly methodical approach would have me strip the carb right down and rebuild it - the operation of the throttle stop and idle mixture screws is unconvincing, but there again it’s a 40 year old carb and we all wear out in time. I’ll try and do that this winter, but there should (surely!) be a few more weeks of decent riding weather before this summer ends and what I’m looking for now is advice/guidance from those who know these motorcycles far better than I do about what might most likely be the problem and any fairly simple tests or fixes worth trying short of the full-scale strip-down.

Many thanks

Brian

[graeme]

Faulty spark plug breaking down?
What carb?
What ignition?

Graeme

[Eldert]

Hi Brian

could be to lean on the idle jet . did you try a bigger one ?

Eldert

[Old Cog]

Thanks both.

New plug - and new spare no better.
Dell'Orto VHB29AD
Ducati Elettrotecnica

I haven't tried a bigger idle jet. Good idea. Sounds like a winter job though. Would raising the needle a notch have a similar effect, even if it made the mix a bit richer throughout?

Cheers

Brian

[Bevel bob]

Sounds too rich producing a dirty plug which needs a belting to clear it.A big bang in the exhaust can be unburnt fuel from a missfire.usually needs a bad joint with the silencer. Very common on these bikes .If like me you are running a racy cam you will also be getting reversion with the air going back and forth picking up extra fuel but only at certain revs. Not something you can cure with carb setting changes I should put in a sensible cam and a compression plate, but i probably won't. I just enjoy the challenge.

[Old Cog]

Thanks Bb.

Guess I'll have to get some gum for the exhaust/silencer join. Any recommendations?

I'm sure I should de-tune the engine. I'm not sure what cam it has in it but I believe the engine was built for racing so I can see how it might not be worth it. If I fitted a lower comp piston I guess I'd need a softer cam. But how difficult would it be to get the bevel drive just right again if I fitted a compression plate?

The bang in the exhaust is just embarrassing, but the misfire on pulling away feels like it puts extra strain on everything from the con rod to the back wheel. And it's the same whether the engine's just started or well warmed up so there's no relief for either of us .

I might not be able to live with it!

Brian

[Bevel bob]

HI Brian, High comp pistons don't always realise the quoted figure,especially in the Ducati. A compression plate (if needed ) does not affect the Bevel clearances as its accomodated in the stepped joint . I would say 9 to 1 would be a good target, I dont think the 10 to 1 Borgo realises much more than 9 to 1. To determine whether you have a radical cam in there will need you to record the valve timing figures ,not simple!.I hear that ordinary household silicon is used by some mechanics to seal ex joints , take appart and smear in the joint. What is the state of the plug.?

[Old Cog]

That's good to know about the compression plate, Bob. Thanks. The guy who built the engine says he thinks the piston might be even higher than 10:1 since he recalls machining it down a bit to lower it!

I'n pretty sure the plug's not the issue. When I've had the bike warmed up and out of the town for a few miles the plug seems fine : if I've just been pottering round the town it'll be a little bit oily round the rim and sooty on the side electrode but the ceramic round the central electrode is a nice light brown colour. I've swapped the plug for a new one and meticulously cleaned and re-gapped both old and new and it doesn't seem to make any difference at all to the misfire. (By the way, a previous difficulty over starting was cured when, on the advice of others on this forum, I reduced the plug gap to 20-25 thou instead of the 35-40 specified by Haynes. But that was resolved a few months before this misfire appeared.

I'll give the silicone a go in the next day or two, but investigating the valve timing and identifying the camshaft may have to wait a bit!

Thanks again for your help.

Brian

[machten]

Another thing that can cause backfiring on overrun is leaking on the intake side. You throttle down and close the slide, but it's still getting air in that combusts in the exhaust. Have you checked your intake manifold gasket is sealing properly? If it is leaking badly enough, all sorts of strange things happen. One indicator is the carb is not very responsive to idle mixture changes.

Kev

[Old Cog]

Sigh....

So after cleaning scrupulously, replacing seals, filters, jets, atomiser, throttle needle, the damn thing's very little different!
The only part I've not replaced is the slide - can't get one - and maybe it's leaking enough to mask the slight effects of the changes I made.

Maybe there's some crucial passage in the carb still blocked.

Or maybe it is an ignition issue after all and the fact that it appeared just after I fiddled with the idle screw is only a co-incidence.

Thanks to all who offered advice, but I think now I need some flesh-and-blood hands-on help.

oh and btw if it is a carb problem but I can't sort it, which Mikuni should I buy?

Thanks

Brian