Bikesure_adrianflux

Misfire

motomartin11

motomartin11

Registered
Joined
Sep 4, 2010
Messages
388
Location
stourbridge
Bike
M750
96MY 750 carb. About 3 weeks ago my bike started to misfire. I replaced the plugs and plug caps (cutting back the HT leads) having noted the plugs were black and sooty. This cured things until yesterday. The bike had been running well until this time when the misfire returned while out on a local ride. I managed to limp the 10 miles back home and whipped out the plugs which were mainly white in colour with a small chocolate brown stripe down a part of each insulator. The battery was showing 12.4 volts across the terminals without the engine running. One other thing which may or may not be connected : the oil appears to becoming emulsified at the sight glass window - this doesn't usually occur until the colder weather. The bike is rarely used for short stop start journeys .Any clues where i should start looking for the cause please? Thanks in anticipation.
 
Well for what it's worth from your description I'd be thinking carbs & floats (Inc their flow pin seals) in particular.
 
Thanks Bitza for your response.

Well I've drained the float bowls in preparation today and was vey surprised that about 300ml came out of one of the carbs whereas the other resulted in about a tenth of that amount.

Anyone have a view on whether this could be connected to the misfire please?
 
slob said:
12.4 V sounds a bit low on the low side, 2.2V/cell = 13.2V
Click to expand...

The battery or charging system could well be another connected (no pun intended) issue. I re-checked the battery voltage yesterday, not having used the bike for the past 10 days or so, and it was only showing around 12 volts across the battery terminals. I charged up the battery with my Accumate and it's now showing around 13.2. The green charged lights lit up only after a few hours on the charger. I'll re-check in a few days to see what it's dropped to.
 
I've now had my bike vanned over to my mate who is investigating the cause(s) of the misfire.
He's been in touch with his preliminary investigations and it seems the carbs require a service and ultra sonic cleaning. The carb set up is standard despite running an aftermarket air filter and Sil Moto carbon cans (now changed for standard 620i cans due to wear and tear of the Sils). The electrics are in something of a state apparently due to age ,the usual corrosion and various bodges. A load of the wiring and connectors have had to be replaced. The charging system was on its last legs and the master fuse nearly melted through .The CDI pick up connectors were breaking down and it now appears the one of the pick ups is causing an intermittent misfire on tickover so both will be replaced with Electrex replacements if they have them. That's all for now-will report his further findings as and when I get them.
 
Update: mate has sent me a vid of the plug firing on tickover but then not firing when the engine is revved up?
Looks as though I'm going to need a set of new after market pick up colis from Electrex.
Also the rotor is loose on the crank damaging the end of the crank.
My mate found that the crank nut was loose.
He reckons he can effect a repair of this however.
Carbs are cleaned up and re-assembled.
 
I am having a similar problem. Looking at getting my carbs cleaned with Markus at Rosso Corsa. Thanks for that update too. Could be helpful
 
motomartin11 said:
Also the rotor is loose on the crank damaging the end of the crank.
My mate found that the crank nut was loose.
Click to expand...

Hopefully caught in time? Because, certainly on 916 model bikes (same basic crankcase/shaft design) there have been a few instances of the nut loosening and the resultant vibration destroying main bearings, cranks and even crankcases...
 
Further Update : the rotor is (hopefully) secured back on the end of the crank.
I have a photo but struggle to post on here / not tech savvy old etc...
Have yet to speak further to my mate but he did explain what he was going to do before executing the repair. I haven't had the chance to speak to him today but I'm inclined to think he would have contacted me had there been any serious issues.
He's been running up the engine regularly to check for misfire etc so I'm sure he wold have mentioned further more serious damage to the crank-he certainly knows what he's doing-been working on jobs I can't do on my bikes for over 25 years so I totally trust his judgement.
The engine did sound like the proverbial bag of nails after I fitted the much quieter 620i cans but I put this down to the clutch and now of course the loose crank rotor. No rumblings or untoward vibration that might indicate to my inexperienced ear serious crank problems but you have me worried Dukedesmo!
I'm now awaiting the CDI pick ups from Electrex which I ordered yesterday but they are caught up in our delightful ongoing Royal Mail's industrial action. These are being fitted purely as a precautionary measure as the motor is stripped as the misfire could have been due to the loose crank rotor as part of the ignition came away with the loose rotor I understand!
Hopefully it will all be re-assembled next week ready for road testing weather permitting.
 
Have spoken to my mate this evening and he's fairly confident what he's done will NOT result in the rotor nut coming undone again. He's used a special sealer / filler/or threadlock or bearing securing medium. Sorry he did tell me what it was called but I've forgotten. He also peened over a (Belleville?) washer/ spacer at the back of the rotor nut -like you do with some gearbox sprockets as well as other securing methods including using an air ratchet up to I think he said about 157 nm. He did say if this doesn't work the area where the woodruff key is worn can be built up with special weld but hopefully this won't be required.
 
motomartin11 said:
the misfire could have been due to the loose crank rotor as part of the ignition came away with the loose rotor I understand!
Click to expand...

Quite possible, the pickups need to be very close to the flywheel (0.4mm IIRC) to get a strong/consistent reading so any variation wouldn't help.

Hopefully all will be well, I only mentioned it because it can be a big deal, mainly on Desmoquattro models IIRC? In fact, Ducati recommend checking the flywheel torque as a service thing and there are numerous fixes, including higher torque settings, strong Loctite and even double nuts to jam one against the other.

But I think you'd know if there was significant damage as there would be play in the crankshaft both visible and audible.
 
Dukedesmo said:
Quite possible, the pickups need to be very close to the flywheel (0.4mm IIRC) to get a strong/consistent reading so any variation wouldn't help.

Hopefully all will be well, I only mentioned it because it can be a big deal, mainly on Desmoquattro models IIRC? In fact, Ducati recommend checking the flywheel torque as a service thing and there are numerous fixes, including higher torque settings, strong Loctite and even double nuts to jam one against the other.

But I think you'd know if there was significant damage as there would be play in the crankshaft both visible and audible.
Click to expand...

Thanks. I'm sure my mate would have definitely flagged this up had it been a problem.

He's on to other issues now with the fuel sender as the fuel light has never worked. He traced through the loom to find a PO has cut off the feed wire, taped it up and taped over within the loom. Unfortunately -see in my wanted post-the 2 senders I have-one plastic / one metal are of no use so I'm looking for a new one.
 
Update: have now received a s/h sender via Fleabay and it had to be stripped to make good the wiring and carp from fuel residue.
The ignition pick ups have arrived from Electrex - quality is said to be good.
Engine is being re-assembled but I'm told there's more carp from the fuel system so carbs are having to be stripped and cleaned again.
New fuel filter (the 4th I have bought in 2 years of ownership) is on order as well as another Mikuni fuel pump rebuild kit (2nd one in 3 years) and as previously recommended by some on her a manual fuel tap is being plumbed in the the cheap little OE vacuum one cannot be trusted. Well I say cheap -they are not if you price up a new one!
Bike is however reported to be running on both pots again at last.
Going to speak to my mate later for the full details.
 
Further update-fuel system cleaned again, carbs ultrasonically cleaned again, carbs full of silt (again), idle jet on rear cylinder carb blocked (again),pump contaminated shutting off pressure relief valve.
Carbs were put back on today-misfire returned on rear pot. Coils were swapped to no avail. Remote tank set up but misfire still persisted on one pot. Further checks threw up possible CDI black box issues. Luckily a set off a 600 were sourced on a loan basis and both boxes were swapped as I understand one faulty black box can result in the other being affected. Engine then ran reliably on BOTH pots. Had to order a pair of new CDI boxes off Fleabay Germany...Now awaiting delivery of fuel pump hit, manaul fuel tap, fuel filter and new CDI boxes..........
 
check ur plugs are correct I had problems with misfire on rear pot using Dpr9 after a rebuild which i had used on M750 for years put in a set of DPR 8 and problem dissapeared ???? but 2 of my 750 run ok on Dpr 9 ,,,

I had cleaned and rebuit carbs twice and changed ignighters coils and pickups so plug was last option its weird ,, that bike had run well on DPr9 for 11 years ???
 
You mention earlier that you are on your 4th fuel filter and second fuel pump rebuild in three or four years ... and then you report that your carbs are full of silt again and an idle jet is blocked.
Therefore I'm thinking that your tank must contain rather a lot of debris.
Have you ever drained and flushed it ?

For comparison, my 750 has done over 30,000 miles and is still on only its second fuel filter (original plus one change) and has never had its fuel pump rebuilt.

Sorry to hear that your's is giving you so much trouble.
Hope you get it sorted soon.
They are a lovely bike when they're running well.
 
Thanks.
I did renew the plugs and caps when the first misfire occurred and this cured it for a few miles.
I must admit to buying another (resprayed )tank off Ebay as the original has begun to leak around the prop area and NOT (foolishly cleaning it out before fitting it to the bike). BE WARNED AND NOT LAZY like me.
Yes the tank has now been flushed out several times.
I'm told it runs really well now and is extremely mechanically quiet -well it did with the borrowed igniter boxes. Hopefully the German ones when they arrive will be fine.
https://www.ebay.co.uk/itm/173945178487
 
German igniter boxes have been shipped apparently but guess which UK (not so Royal) mail outfit is tasked with delivering them...Yes they have gone into the black hole that was formerly known as Royal Mail .Been waiting for 2 weeks or so now for these items after waiting countless weeks for the clutch. Post Brexit, post pandemic but just no post any more! Such is life. Bike could be ready for the road come spring.
 
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