900SS missfire at 5500 rpm?? - Ducati.ms

900SSDesmo · May 30th, 2010, 3:14 pm

I have an 81' 900SS that begins when missing rolling to full throttle at about 5500 rpm in the higher gears. I have rebuilt the carbs, checked and cleaned wiring, grounds, replaced plug caps and so forth. I am running 40's with 152 mains and read where someone suggested leaning the jetting for modern fuels? As it sits the bike will clear out and run pass the miss, yet it is a bit down on power - even then. Any suggestions are welcome. I have tried the usual and replaced the plugs which read dark until used at higher rpm for a bit. It is a standard 900SS with Conti's and 40's running standard jetting currently.

Thanks in advance

wdietz186 · May 30th, 2010, 5:53 pm

Before you start changing things make absolutely sure the float levels are correct and you have good fuel flow.The difference in the gas really isn't enough to cause a misfire unless it is also old and stale. Another thing to check is that the battery is fully charged and the voltage to the coils is over about 11v.the plug gap makes a difference too, much over .025" can give you a miss.

900SSDesmo · May 30th, 2010, 10:33 pm

Thanks WDIETZ. The plug gaps are close to .029 to I will close them up and give it a try. Not sure how to check coil voltage - but I will figure it out.

Duccout · May 31st, 2010, 1:15 am

First, are you certain that the valve clearances are correct? if the opening clearance is too tight it will cause a mis-fire. I would not change any of the jetting from standard, it will confuse things; these bikes always run a bit rich at lower RPMs. What is the condition of the ruber manifold spacers? they are prone to ageing and splitting, giving an air leak. Another possibility is the ignition wiring breaking down in the clutch cover. My first check would be valve clearances though.

900SSDesmo · May 31st, 2010, 9:39 am

Thanks Duccout. I have replaced the intake rubber spacers (and they were trashed) and the ignition wiring in the clutch cover - after reading about that on the bevel heaven site. The bike supposedly had the valves done by an 'expert' - right before my purchase, so that will be another item to check. I have always set them myself on my other bikes, and never experienced a miss, so that is a new one for me. I will report back my findings as this has been a very sticky problem.

wdietz186 · May 31st, 2010, 12:33 pm

You just need to be sure the battery is good. Check the voltage with the ign. on to get an idea if it drops below 11 or so it is suspect.Another thought! I had a similar problem on my racebike and it turned out that the ignition switch contacts were corroded and at certain rpms[vibration induced] it would break contact.Drove me nuts for more than a few weekends.Might be worth a look.

Duccout · May 31st, 2010, 1:20 pm

Well done wdietz186, you reminded me that my ignition switch failed once and in the run-up the bike had begun to mis-fire. Luckily, when the breakdown man came out to me, I mentioned that I thought the ignition switch may be on the way out and the genius of a man simply bypassed the switch at the fusebox and I was on my way again.

shanetbird · Jun 1st, 2010, 11:52 pm

1/ battery got rid of my now fitted with deko "yes they fit "
low battery level caused misfire , back fire, you name it , gave me hell!

2/ check coils having change numerous coils on everything that moves
i found this to be a constant cause worse in cars same symptoms

3/ air leaks do a search around everything

4/ check all connections to those damn Bosch magic box's and clean all
connections with electronic cleaner on re connection

hope this helps my 82 SS had similar problems but the battery and plugs
and lack of use and carbie are now rebuilt this caused most misfiring
drama's Ive just replaced the coils to Dyna's plus 7mm leads
( bevel heaven)

All in a lasting hope of smooth riding

shanetbird

900SSDesmo · Jun 5th, 2010, 11:36 pm

Duccout Thanks for the suggestion. Seems that the front intake opener was a tight .002" and the front exhaust is even tighter! Weird that the closers appear to be set correctly because the openers are the easy part of the job. I figure if he botched the openers then the closers would be a mess. I hope it does not have a burned exhaust valve. Also it was supposed to have been ridden sparingly after the 'service'. Seems like every Ducati I find has had a hack go after it. It's probably why they are for sale in the first place! A friend just bought an 860 Gt and had me over to see it. I noticed the front cylinder had a plug in it labeled for use in Briggs and Stratton mowers! Wrong reach and way too hot. Previous owner claimed to have been a motorcycle mechanic too. Here's to always doing it myself. I may be slow but...

Thanks everybody for the responses. They are greatly appreciated!

Duccout · Jun 6th, 2010, 1:00 am

I do all my own maintenance and have done for 35 years, but the jobs that I can't do (splitting flywheels, machining valve guides) has left me with a pathalogical distrust of 'experts'. Almost every 'expert' who I have entrusted a job to, has f***** up the parts. I have had to threaten legal action to some of the most well-known Ducati people in the UK. Someone here took their desmo to an 'expert' who boasts of their skill, to have a service and on the ride home it dropped a valve....

If I have to have any engineering work done I prefer to go to a general engineering company, tell them what I need and let them do the work. At least then they do as I ask and with great skill. A Ducati shop manager told me once, when I was buying a valve shim, "It helps if you don't check the clearances." I think that the reason why your closing clearances are alright, is because the closers tend to loosen up, which generally will not damage anything, unless you are caning the engine, but the openers tighten up. I once bought a 750 SS inj from a Ducati main dealer, low mileage and full service history and when I checked the valve clearances, they were so tight that all the chrome on the rocker feet had worn away.