1989 750 Sport idle issue

discussion about the 750 Sport and '89/'90 900SS, which share many mechanicals with the Paso series
deswoodau
Posts: 53
Joined: Fri May 02, 2014 10:08 pm
model: 750 Sport
year: 1988
Location: Australia

1989 750 Sport idle issue

Post by deswoodau »

Hi All,

Seems like this forum has gone a quiet however I'll put a post out and see if anyone can assist. I've owned this bike for 10 years and in most part has run very well. About eight years ago I changed the Aux Venturi from 3.5 to 4.5 and stepped up the 'main' and 'AC' jets on the DCNF44 Weber to alleviate the low range flat spot and it's been a pleasure to ride. Bike is 100% original except for a pair of open mufflers for sound. About 17,000kms travelled. It's running original coils, Marelli ecu, airbox and Weber DCNF 44.

Issue: Once fully warmed up and a few kms away from home it refuses to idle when stopped at an intersection. This is a recent and sudden issue. I recently replaced the engine oil & filter, belts and applied new spark plugs(NGK DPR8EA-9). Same plugs and oil as always when serviced, nothing new. Initially it ran perfect when I took it out a month ago post service. Last few days I tried on two occasions\rides and it's cannot maintain idle under 3000rpm when hot. I inspected the plugs and they both appear sooty and a little wet. Seems to be fouling up.

Status:
- Drained the fuel and added fresh 95 octane fuel
- New sealed style battery 210 CCA (12 months ago maintained on trickle charger)
- When stopped, I need to apply continued throttle and keep rpm above 3000rpm or it will stall and become difficult to restart
- Cold start with choke is perfect and quick. Initial cold idle is smooth and warms up without issues.
- On open throttle, it maintains good power through the higher rev range without issue and changes gears well. No issue under load.

The only issue I can detect is I may have over filled it slightly on the last oil change but I wouldn't suspect that to be an issue.

Any opinions on where to start would be appreciated.
Last edited by deswoodau on Mon Jan 15, 2024 3:54 am, edited 3 times in total.
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higgy
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Re: 1989 750 Sport idle issue

Post by higgy »

Over fill on the oil can cause issues..........
Air leaks can also be an issue.
valve adjustment

No easy answers. Sudden problems can be electrical,corrosion is common on older bikes.

Where to start ?
First I think I would check the voltage at the coils. Anything less than 12 is a problem.
General carb condition....air leaks,stuck or incorrect float level.
Spark plug gap or broken insulator tip
Ducati,making mechanics out of riders since 1946
There's no problem so bad that a little fixing can't make it worse! : )
If it ain't broke keep fixin it till it is
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deswoodau
Posts: 53
Joined: Fri May 02, 2014 10:08 pm
model: 750 Sport
year: 1988
Location: Australia

Re: 1989 750 Sport idle issue

Post by deswoodau »

Thanks Higgy.

Things are starting to unravel.

The good:
- 13volts + supplied to each coil (Battery is 13v plus too)
- Plugs are new (still sooty), checked, cleaned and refitted as they're new
- Weber\Carb, all the jets appeared clean so I blasted them out anyway and refitted
- Carb float, float valve all clean and functioning okay
- Pipes, breathers and manifolds all looked good
- Coil wiring looks good and teminate as required

The bad
- Drained some excess engine oil out and discovered it's contaminated with petrol (quite a lot). I didn't over fill it upon a recent oil change after all, petrol contamination is the issue.
- Upon going through the carb, I noticed a lot of wetness between the fibre gaskets and I suspect they're leaking down the manifold. Carb was re jetted and these gaskets were changed about 8 years ago so they may be due for a change. Could be something else but thats my first impression.

To do or thoughts?

- Firstly I'm considering a compression test to ensure that side of things is okay.
- If compression is okay, perhaps a full carburretor rebuild? Or new gaskets at a minimum.

Oil pics attached (new vs used). Watery consistency and smells like petrol, has darkened and I've only done about 50kms since the oil change.
Oil Sport.jpg
Oil Sport.jpg (127.31 KiB) Viewed 20586 times
750 Sport 2023 v2.jpg
750 Sport 2023 v2.jpg (150.27 KiB) Viewed 20580 times
Last edited by deswoodau on Fri Dec 29, 2023 10:41 pm, edited 2 times in total.
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higgy
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Re: 1989 750 Sport idle issue

Post by higgy »

Flooding of that degree is as you stated either the top gasket leaking or the float not closing. There is also the possibility of a crack in the carb.
Ducati,making mechanics out of riders since 1946
There's no problem so bad that a little fixing can't make it worse! : )
If it ain't broke keep fixin it till it is
88 750
90 906
92 907ie
deswoodau
Posts: 53
Joined: Fri May 02, 2014 10:08 pm
model: 750 Sport
year: 1988
Location: Australia

Re: 1989 750 Sport idle issue

Post by deswoodau »

Thanks Higgy.

Updates:
- Compression test (cold) is 130psi front and rear.
- Applied new engine oil and filter therefore I'll await the Weber service parts before a hot compression test
- Float offset was at 42mm. Adjusted to 46mm.
- Pressure tested the needle float valve and it may have a slight leak too. I'll renew it to be sure.
- Pump diaphragm looks okay but I'll swap it out regardless

I'll purchase a full service kit next week...... hopefully it'll be back on the road and running as it was.
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higgy
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Re: 1989 750 Sport idle issue

Post by higgy »

:thumbup: :thumbup: :thumbup: sounds like you have a good handle on the issues
Ducati,making mechanics out of riders since 1946
There's no problem so bad that a little fixing can't make it worse! : )
If it ain't broke keep fixin it till it is
88 750
90 906
92 907ie
deswoodau
Posts: 53
Joined: Fri May 02, 2014 10:08 pm
model: 750 Sport
year: 1988
Location: Australia

Re: 1989 750 Sport idle issue

Post by deswoodau »

After a refresher course \ read through the Weber thread last night I've concluded there's no silver bullet with the setup. We all need to factor in climate, riding styles, air filters, exhausts, fuel from different parts of the world etc.....

The bike ran fine with the previous set up but my float level was at 43mm. Once the workshops re-open after the summer break I'll fit a full service kit and proceed with the following setup and hopefully it'll ride as it did prior to leaking fuel. It was by no means perfect, but overall I was happy...........after all it is an 89 Ducati. There's a reason you don't see many around anymore :)

Setup: (The 4.5 aux venturi, main and AC jet step up improved things eight years ago and I didn't need to touch it for years)
- Aux venturi 4.5 (from 3.5)
- Main 150 (from 145)
- AC 175 (from 170)
- Idle Jet 60 (Dealer fix years ago with a previous owner - Ducati paperwork and old 55's came in the tool bag as spares). A desperate solution at the time I guess.
- Float level 46mm (from 43mm)

Other:
- Stock air filter and box
- Open exhaust
- No cam shaping just yet (that's a maybe at the moment)
Last edited by deswoodau on Mon Jan 01, 2024 4:59 am, edited 2 times in total.
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higgy
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Re: 1989 750 Sport idle issue

Post by higgy »

:thumbup: :banghead: yes you are correct,there is no magic bullet
Ducati,making mechanics out of riders since 1946
There's no problem so bad that a little fixing can't make it worse! : )
If it ain't broke keep fixin it till it is
88 750
90 906
92 907ie
deswoodau
Posts: 53
Joined: Fri May 02, 2014 10:08 pm
model: 750 Sport
year: 1988
Location: Australia

Re: 1989 750 Sport idle issue

Post by deswoodau »

FAILED.

Applied a service kit with a new needle valve and set the float to 46mm........it was worse than before.

Therefore a second batch of brand new oil&filter was ruined within a couple of minutes of running and attempted tuning. It stalled after turning the choke off and I left it alone.

I opened everything up and discovered the following:
- Siphoned 250ml of petrol from the rear cylinder inlet manifold
- Siphoned excess petrol from the front cylinder and inlet manifold
- Drained the engine oil again as it was ruined\contaminated. Approximately 4.5 litres drained out from the sump when it's a 3 litre refill. I couldn't even detect an oil level from the inspection window because it rose too high.

Essentially I'm back at square one. I spoke briefly spoke to a Weber specialist and he mentioned the float and needle settings are the likely cause.

Anyone got an idea where to go from here? I reverted the float back to 42mm \ on level with the top lid as per factory specifications.

I'm yet to buy more oil and try again.....just wondering whether it's seeming like the float setting was the likely cause. Nothing else has changed except for the needle valve and some gaskets.
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Re: 1989 750 Sport idle issue

Post by higgy »

level with the top lid as per factory specifications

should work so the question becomes does the needle prevent gas from flowing :truck:
Ducati,making mechanics out of riders since 1946
There's no problem so bad that a little fixing can't make it worse! : )
If it ain't broke keep fixin it till it is
88 750
90 906
92 907ie
deswoodau
Posts: 53
Joined: Fri May 02, 2014 10:08 pm
model: 750 Sport
year: 1988
Location: Australia

Re: 1989 750 Sport idle issue

Post by deswoodau »

With the float level reverted to 42mm........I tried these bench tests.

Test 1:
I manually tested the float valve by blowing air through the inlet while tilting the float down (gravity only) and the air valve closed. I do believe the valve works fine but I don't have adequate tooling to pressure test.

Test 2:
I connected the fuel lines and tank\pump to the Weber on a stool adjacent to the bike in a stainless bowl to observe. Initially it remained dry and it was holding pressure without leaking. I then applied a few pumps and fuel squirted into the chokes as expected. I opened the choke and things remained stable. I tilted it gently left and right by hand and it started to spill excess fuel internally and via the choke screen......slowly getting worse.

I'm thinking it's a failed float. Perhaps it's getting old and cannot maintain enough buoyancy. Being 30+ year old plastic it's possibly becoming porous and not maintaining the required valve pressure anymore. The float does rise when I tested in a jug of fluid but thats different to whilst riding with movement.

At this stage I'm considering a full overhaul, pressure test etc with a local Weber technician. He's been in the business for 30+ years and he'll be able to ascertain any fatal flaws one would hope. It could have a crack in the casting, bad float......the mind ponders.
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Re: 1989 750 Sport idle issue

Post by higgy »

floats do eventually take on fuel and become less buoyant or even not at all buoyant

Might also be a good idea to check your fuel pressure as it may be too much pressure for the float to overcome
Ducati,making mechanics out of riders since 1946
There's no problem so bad that a little fixing can't make it worse! : )
If it ain't broke keep fixin it till it is
88 750
90 906
92 907ie
deswoodau
Posts: 53
Joined: Fri May 02, 2014 10:08 pm
model: 750 Sport
year: 1988
Location: Australia

Re: 1989 750 Sport idle issue

Post by deswoodau »

Fuel pump hasn't changed and should be fine.

The Weber has been dropped off to a local specialist for a full check up, pressure test and service. I briefed him on what's going on so hopefully he finds the issue.

My hope is post service it's back on the road, if not I'm selling!!
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higgy
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Re: 1989 750 Sport idle issue

Post by higgy »

Fuel pump hasn't changed and should be fine.
Famous last words :phone: :banghead:
Ducati,making mechanics out of riders since 1946
There's no problem so bad that a little fixing can't make it worse! : )
If it ain't broke keep fixin it till it is
88 750
90 906
92 907ie
Mc tool
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Re: 1989 750 Sport idle issue

Post by Mc tool »

wouldnt be the 1st weber Ive seen with a crook float . The black plastic float in my Capri ( 32/36 dgav ) took on fuel and was a real bother to sort . Wasnt untill I parked the car facing steep uphill and I couldnt start it at all.....I could see fuel welling up out of the air correctors and straight down the venturi. I bought a brass float ..... which requires a different float level setting
I wish I was young again............Id be heaps smarter than last time
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