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Mantra DB3 erratic running below 4000rpm

 
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Sylvester



Joined: 14 Feb 2018
Posts: 5
Location: Belgium

PostPosted: Sun May 27, 2018 6:35 pm    Post subject: Mantra DB3 erratic running below 4000rpm Reply with quote

Hi everyone

It's probably the first time that I post something on this forum but it has been so helpful to me in the past numerous times, but now I need some assistance! Smile

My 1998 DB3 is not running well. I bought this bike last year and the previous owner took it to a Ducati workshop recently, it was running like a pig (only on choke), don't know what they did to the bike but I'm guessing not much besides oil and cambelt change.

Anyway I cleaned the carbs (original Mikuni 38mm) and set it back to standard settings. It ran ok but the horizontal cylinder kept receiving too much fuel. I adjusted the idle mixture screw and had to set the horizontal cilinder to 1/2 turn out only, while the vertical one is perfect at 3,5 turns out. That alone seems strange to me already...
Now when running well on the bench without load, so picks up revs well, steady tickover, plug doesn't foul anymore.
But when I testride it, it runs erratic on the small throttle openings and low revs but runs great above 4000rpm. I can't get it to run better...

Has anyone some advice for me? Should I fettle with the carbs further (I don't have vacuum gauges, yet), what should I do? Could it be the coil or ???

Thanks in advance for your help!
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Gavin944888



Joined: 29 Jan 2014
Posts: 334
Location: Essex....way South of Scotland

PostPosted: Mon May 28, 2018 7:56 am    Post subject: Reply with quote

Sylvester

Hello and welcome...with regards to the Ducati engine issue here is my ususual routine

Having had many 2 and 4 valve engines although mainly with injection, when faced with your dilemma, it is totally back to basics

Remove the spark plug caps one by one, place in the cap a spare plug and ensure that sparks are a plenty.
.....should help eliminate any coil question.

Then, gap, clean and ideally change the plugs.
..if it has Champion written on them ....select the nearest bin and dispose of them

Next check your float levels on both carbs before doing anything else
....a sticking needle valve will have to put the excess fuel somewhere.....worth a try

Just keep saying .......air, fuel, spark......
if all these are there then (other than a manifold leak) the issue must be the setting on the carbs

Check even the air vacuum feed on the manifolds....it has been known for the air feed tube to the vacuum pump to split after going brittle

Good luck and where in the world are you?

Regards

Gavin
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On an Island in the River Thames (sounds wonderfull...doesn't it)

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Sylvester



Joined: 14 Feb 2018
Posts: 5
Location: Belgium

PostPosted: Mon May 28, 2018 9:26 am    Post subject: Reply with quote

Hi Gavin

Thanks for the help. I am located in Belgium Smile
I will check the air vacuum feeds on the manifolds. Worth a try.
I will keep you updated!
Kind regards // Sylvester
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2bims



Joined: 03 Apr 2010
Posts: 5332

PostPosted: Mon May 28, 2018 7:17 pm    Post subject: Reply with quote

The Mantra shares the same "fuel pump" as the DB4....its vaccum operated rubber diaphragm sucking from a tube on the carb intake tube.......they are know to perish the rubber flap valve that causes the suck and blow....but normally that affects High speed running as the bike will run well at low speed...and only when it needs more fuel is it a problem.....they are only around 6 off ebay delivered....search GoKart 2 stroke fuel pump....as go kart boys use them...

I'd also unscrew both HT leads and check the state of the copper core at the plug caps and the coils.....snip a bit back and rescrew them on at both ends....I've recently ressucutated a 1997 Ducati 900ss and the vertical cylinder had no spark.....it was simple....tugged on the HT lead and it fell away from the coil....the 2 strands of wire left were green with corrosion...remade all ends and fired and ran well on both cylinders...

Poor low running but fine above 5thou says low idle in carbs....Do you have fresh fuel? Clean fuel filter??Clean air filter???...check the wiring to the HT ignition packs also and clean the terminals.....as gav says...its all old skool so simple checks........
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brian



Joined: 22 Aug 2011
Posts: 2617
Location: QLD, Australia

PostPosted: Tue May 29, 2018 12:18 am    Post subject: Reply with quote

http://www.bimotaforum.co.uk/viewtopic.php?t=1885&highlight=db4
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Current motorcycles - '99 Bimota DB4, '96 Bimota SB6, '08 VTX1800, '93 ZXR750R M1, '95 ZXR750, '95 ZXR750 Race Bike, 94 CBR400rr NC29 Race Bike, '94 CB250, '49 BSA C10 250, 61 BSA A10 650
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brian



Joined: 22 Aug 2011
Posts: 2617
Location: QLD, Australia

PostPosted: Tue May 29, 2018 12:21 am    Post subject: Reply with quote

brian wrote:
http://www.bimotaforum.co.uk/viewtopic.php?t=1885&highlight=db4


Sylvester, you may find something of interest in this thread of mine from a few years ago. This is on a DB4 but essentially the same as DB3.

I ended up replacing just about EVERYTHING. Still not 100% sure, but the problem may have been the coil wires in the harness
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Current motorcycles - '99 Bimota DB4, '96 Bimota SB6, '08 VTX1800, '93 ZXR750R M1, '95 ZXR750, '95 ZXR750 Race Bike, 94 CBR400rr NC29 Race Bike, '94 CB250, '49 BSA C10 250, 61 BSA A10 650
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Sylvester



Joined: 14 Feb 2018
Posts: 5
Location: Belgium

PostPosted: Sat Jun 16, 2018 2:57 pm    Post subject: Reply with quote

Finally the beast is roaring again Smile

It was a combination of dirty carburettors and faulty wiring.
Now it is time for her to find a new home, as I remember why I sold my first Mantra: I am too tall for it (1m95) and I am not really a speed freak Smile
I have listed an add in the Bikes for sale forum.

Thanks for your help anyway!
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