1979 Ducati 900SS Backfire

I’ve recently located and imported a beautiful 1979 Ducati 900SS. It has 40s & Contis. It starts with 1 or 2 kicks and ticks over perfectly. Above 3000rpm, it’s smooth but below that it spits, pops and backfires. Sometimes flames spit out if the bell mouth and often I can see a blue flame igniting in the mufflers.
Interesting, the plugs fitted when I got the bike were Champion L86C but my manual says they should be L87YC. I have bought a set but notice the tip protrudes slightly further.
I have fitted them and the bike still starts easily and idles well but still the pops, spits, backfires and flames persist.
Is this just a function if the overfuelling 40s that I’m gonna have to live with ?!!!
Appreciate any advice/knowledge

Hi I probably dont think you’ll get much response on here which is disappointing.
I dont know much about the 40’s but having dealt with a few carbs ( race ones to ) they should have a set up for low revs , tick over , they do sound like there over fueling a little to much

Thank you for your response :+1:t3:

Hi David, obviously your bike should not be doing this, and properly set up, a 900SS will run very well through the rev range on 40s. One way to get to the bottom of it would be to put it on a dyno. I can recommend BSD in Peterborough and RPM in Northampton, both know Bevel Ducatis well.
I suspect that your ignition timing is out, and if the flames only come from one carb/pipe in particular, your carbs may be a long way out of balance.
These bikes are sensitive to carb balance and run very much better if the balance is correct.

Also, there are a number of groups on Facebook dedicated to Bevel Ducatis with lots of help available in the UK.

Many thanks for your response. In the meantime I’m about to replace the carb rubbers (soft, mushy and split !) and jets to see if air ingress might be the issue.
Thanks again

You may have hit the nail on the head with that one , any air getting in will mess up the carburortion , try that first yourself before spending a shed load of money .

Just a little thought, float level , does you’re bike flood fuel at all

I had sim ilar problems. Old fuel can clog up the carbs. It sounds like the fuel feed jet above the float is not shutting off. clean the float chamber and while you are there replace the fuel feed needle and jet. Use the best fuel with less organic chemicals. If laying the bike up add a fuel additive to your last gallon or less, supplied by your local lawn mowwer shop.
In mine it even had different sizes of main jet. whether this was to aid cooler running on the rear Pot I am not sure. Please take the usual precautions, becaul fuel is a nasty fire hazzard. Som sports riders drain off fuel to a can remove tank empty it and sore it safely away with the cap open. before that run carbs dry. Just a few possible ideas for you. good luck withe the old dog

Dtanglewood I think that is all good advice ,
I think the problem may be simpler, just poorly set up carbs or carbs that need a clean .
I hope davidf lets us know the problem when he finds it

There is also a lot of basic information in the Technical Literature section on the website, workshop manual, parts book and dellorto carb manual for the basic stuff. But has already been said, sort the basics, rubbers, valves, jets, diaphrams, floats and balancing before anything else.

1 Like

Good advice again . Cleanliness is next to Godliness with you’re carbies

all good suggestions and hopefully it’s all fixed now. When my '81 Darmah was (almost) new, and running on its Bosch ignition system, it used to backfire on startup when the spark plugs needed replacing - my neighbour at the time even had a visit from the Police as they’d received a report that he’d shot his Wife . . .
I’ve drastically upgraded the ignition system since, as well as moving to the opposite end of the country - but my point is that if all else has failed so far then start on the HT ignition side.

Hi all
It was the inlet rubber ! I took the opportunity of renewing ALL jets, sliders…pretty much all that remained were the bodies !! Runs very sweetly now and has done a 200 mile trip recently to Sammy Miller’s Italian Day :+1:t3:
Carb parts courtesy of Eurocarb…fantastic advice, great service & quick delivery.

Thanks to everyone who responded