Darmah Front cylinder running rich

Post your 'Carburettor & Fuel System' FAQs, comments & questions regarding Ducati motorcycles here.
mdal8798
Cucciolo - the Lil Pup
Posts: 10
Joined: Sun Feb 03, 2019 2:52 am
Location: Mackay, Qld, Australia

Darmah Front cylinder running rich

Post by mdal8798 »

Hi,
Having issues trying to get reliable idle.
Carbs (PHF32) have had overhaul kits fitted and spent a lot of time ensuring passages clear, float levels correct etc.
Carbs have been synched as per Steve's instructions.
Motor will rev out with no hestitation and sounds good. Accelerator pumps are visibly working.
Front cylinder is running noticably rich at idle speed (1100rpm) with exhaust smelling rich and exhaust popping. Adjusting mixture screw makes no noticable difference until it is wound in (currently 1 turn out).
Front cylinder exhaust is noticably warmer than rear cylinder (hand over exhaust).
Have clear plastic in-line fuel filters, front cylinder fuel flow appears to be excessive compared to rear cylinder at idle (1100rpm)
It appears more fuel is being delivered apart from mixture circuit.

Have rechecked float level and seating of choke piston.

Any suggestions?
User avatar
Craig in France
Paso 750
Posts: 917
Joined: Tue Mar 23, 2004 3:58 pm
Location: Montpellier, France

Re: Darmah Front cylinder running rich

Post by Craig in France »

Initial quick thoughts/questions:

1. Have you checked for air leaks around the intake manifold, both where it bolts to the head - did you fit a new gasket? - and where the carb clamps on? (Don't over tighten the clamp). Hold the engine at a low idle and get someone to spray something inflammable like WD40, carb cleaner etc around the area - not much, just a little, in short bursts. If there's a leak, the engine revs will briefly pick up.

2. Have you renewed the O ring at the base of the mixture screw? They quickly get flattened. Are the accompanying spring and washer installed? Is there an old O ring or washer left in the carb (not uncommon)? Is the tip of the mixture screw undamaged? Have you got the correct mixture screw - they're NOT all the same.

3. How did you check the seating of the choke piston? Visually only? Much the best way is to remove the float bowl, attach a length of tube to the end of the uptake pipe and see if you can blow up it, past the piston seal - but only lightly, no great pressure.

3. Did you use a gauge to set the float height? Were you scrupulous in ensuring that you took the measurement at the point at which the needle valve has just closed? Any later and the float bowl will be too full. Again an accurate way to gauge this is by blowing, lightly and constantly, down a tube connected to a fuel banjo at the same time as taking the measurement. At the very moment that you can't blow past the valve, that's where you need to have the 18mm.

Think that will do for starters ... :)
mdal8798
Cucciolo - the Lil Pup
Posts: 10
Joined: Sun Feb 03, 2019 2:52 am
Location: Mackay, Qld, Australia

Re: Darmah Front cylinder running rich

Post by mdal8798 »

Craig in France wrote: Tue Jul 14, 2020 1:02 am Initial quick thoughts/questions:

1. Have you checked for air leaks around the intake manifold, both where it bolts to the head - did you fit a new gasket? - and where the carb clamps on? (Don't over tighten the clamp). Hold the engine at a low idle and get someone to spray something inflammable like WD40, carb cleaner etc around the area - not much, just a little, in short bursts. If there's a leak, the engine revs will briefly pick up.

Will check again.

2. Have you renewed the O ring at the base of the mixture screw? They quickly get flattened. Are the accompanying spring and washer installed? Is there an old O ring or washer left in the carb (not uncommon)? Is the tip of the mixture screw undamaged? Have you got the correct mixture screw - they're NOT all the same.

New O ring, all old components accounted for as new installed.

3. How did you check the seating of the choke piston? Visually only? Much the best way is to remove the float bowl, attach a length of tube to the end of the uptake pipe and see if you can blow up it, past the piston seal - but only lightly, no great pressure.

Measured distance and felt for spring loading up as installing. Will try your method.

3. Did you use a gauge to set the float height? Were you scrupulous in ensuring that you took the measurement at the point at which the needle valve has just closed? Any later and the float bowl will be too full. Again an accurate way to gauge this is by blowing, lightly and constantly, down a tube connected to a fuel banjo at the same time as taking the measurement. At the very moment that you can't blow past the valve, that's where you need to have the 18mm.

Used a gauge but will check again using method suggested.

Think that will do for starters ... :)
User avatar
Craig in France
Paso 750
Posts: 917
Joined: Tue Mar 23, 2004 3:58 pm
Location: Montpellier, France

Re: Darmah Front cylinder running rich

Post by Craig in France »

Further thoughts:

1. The idle on a PHF depends on a clean air feed as shown below.
Idle air passage.jpg
Now, as you can see, that drilling narrows within the carb body. As you can imagine, it can easily get partially blocked at this point - and stay so, even after an ultra-sonic clean. So be very fussy.

Here's a method of checking that all the galleries are clear, given me by someone with decades of professional experience of working with these carbs. These days, I follow it every time I have a carb in my hand:

"Checking the galleries with carb cleaner.

The easy way to check the galleries with the carbs off or on the bike is:

1. Remove the pilot jet and the mixture screw.
2. If the carb is still on the bike, open the throttle. Using a can of carb cleaner with an extension tube fitted, put the tube up the hole where the pilot jet goes. Cover the mixture screw hole with finger and squirt cleaner while looking for a stream coming up into the venturi, just in front of the back edge of the slide.
3. Then put the tube in the mixture screw hole. This time, cover the pilot jet hole with your finger and squirt cleaner whilst looking for a stream coming out between the slide and the engine.
4. Then put the tube into the little air hole under the main air intake (not the "middle" one: the one on the side). Squirt, and look for cleaner in venturi."


2. If none of the above work, here's something else you might like to consider ...

Because "All carburetor faults are ignition faults ... and vice versa", it might be worth swapping the carbs over. It's a bit tedious to do, but if the fault transfers with the carb, then you have at least confirmed that it IS the carb and not an ignition fault (which it could be).

And naturally, if the fault doesn't transfer with the carb, you'll know you've been on a wild goose chase .. :-D
User avatar
Craig in France
Paso 750
Posts: 917
Joined: Tue Mar 23, 2004 3:58 pm
Location: Montpellier, France

Re: Darmah Front cylinder running rich

Post by Craig in France »

Oh, and maybe this will help.

If your choke piston looks like this, change it.
Worn enricher seal.jpg
mdal8798
Cucciolo - the Lil Pup
Posts: 10
Joined: Sun Feb 03, 2019 2:52 am
Location: Mackay, Qld, Australia

Re: Darmah Front cylinder running rich

Post by mdal8798 »

Thanks Craig for your help.

Have been away and got back to working on bike today.

Rechecked float level and choke piston as suggested - all good.

What i did notice was when comparing mixture screws was that the taper on the front carb mixture screw was noticably shorter than the rear! One would then assume that this would explain why screw setting improved idle with only one turn out.

On restarting bike, and trying to warm up bike, the front cylinder started backfiring and spitting flames from exhaust. History over last few weeks has shown that this has occurred intermittently.

As suggested i then swapped carbs front to rear. Still backfiring on front cylinder.

I am now thinking I have an intermittent ignition issue.

The pickups are original and when inspected at motor overhaul, mechanic believed wiring was in good condition. (This was 6 years ago but motor has never been running)
I have already replaced coils with Dynatech units and swapped ignitor boxes.

Will pull off sidecover and inspect pickup wiring next.

Thanks again.

Mark
User avatar
Craig in France
Paso 750
Posts: 917
Joined: Tue Mar 23, 2004 3:58 pm
Location: Montpellier, France

Re: Darmah Front cylinder running rich

Post by Craig in France »

mdal8798 wrote: Sat Jul 18, 2020 1:44 am As suggested i then swapped carbs front to rear. Still backfiring on front cylinder.
Good! That's progress, Mark :)

You probably know this already, BUT DON'T DISTURB the pick-ups unless you absolutely have to. If you do, they will need to be re-positioned using a special tool, and even then it's not that simple.

It will be the pick-up - or rather its wiring. And not just the wiring. When you get there, you'll see that the pick-ups themselves will be covered in a grey metallic sludge ( it's the magnetism of the pic-ups that's responsible). This sludge creates loose signals that make the Bosch box fire all over the place - as you know only too well ;) . Just clean it off.

1. Checking the performance of the pick-up itself is easy. Stick a meter across the terminals and you should have 220 Ohms.

2. If the insulation has come off the wiring, see if you cut the wires, not close to the pick-up; but far enough back upstream to be able to slide a piece of heat shrink over the exposed wiring. And then solder the two ends of the wire back together. All without disturbing the pick-up retaining screws ...

Good Luck!

Craig
900tlc
Mariana
Posts: 32
Joined: Fri Feb 24, 2012 12:00 pm
Location: North Dorset,UK

Re: Darmah Front cylinder running rich

Post by 900tlc »

When you say the mixture screw is longer.you mean physically.If that is the case you have the wrong mixture screw.I would say the shorter one is correct.I raised this on the forum before.
mdal8798
Cucciolo - the Lil Pup
Posts: 10
Joined: Sun Feb 03, 2019 2:52 am
Location: Mackay, Qld, Australia

Re: Darmah Front cylinder running rich

Post by mdal8798 »

Yes, the taper section is visibly longer. (Estimate 0.5mm.) Inspected both under magnifying light thinking that shorter one may have tampered with but appears to be original.

Correct me if i am wrong but the mixture screw on these carbs adjusts fuel flow.

Searching forum suggests that screws should be wound out between 1.5 & 2 turns as a starting point. The original rear carb is happy within this range, the front carb with shorter screw (which appears to be running rich) will presumably require less turns out.
User avatar
Craig in France
Paso 750
Posts: 917
Joined: Tue Mar 23, 2004 3:58 pm
Location: Montpellier, France

Re: Darmah Front cylinder running rich

Post by Craig in France »

mdal8798 wrote: Mon Jul 20, 2020 12:53 am Yes, the taper section is visibly longer. (Estimate 0.5mm.) Inspected both under magnifying light thinking that shorter one may have tampered with but appears to be original.
There are at least 3 versions of the PHF/PHM mixture screw, 2 of which you can find on Bevels - see the picture below, with thanks to Steve A for the info on the top two.

I'm reasonably confident in saying that if you have AD/AS carbs, you should have the shorter ones. If you have CD/CS carbs, it should be the longer ones.

Mixture screws.jpg

Me, I'd want to have a pair of the correct ones. For you in Oz, Mark, Phil Hitchcock at Road and Race in Sydney (amongst others) should be able to supply.
mdal8798 wrote: Mon Jul 20, 2020 12:53 amCorrect me if i am wrong, but the mixture screw on these carbs adjusts fuel flow.
Correct. The strength/weakness of the air-fuel mixture is determined by the air drilling at the front of the carb and the size of the idle jet. The screw regulates the amount of this mixture.
mdal8798 wrote: Mon Jul 20, 2020 12:53 am Searching forum suggests that screws should be wound out between 1.5 & 2 turns as a starting point.
Correct. That should get the motor running. After that, a bit of carb balancing/tuning may - or may not - alter it slightly.
mdal8798
Cucciolo - the Lil Pup
Posts: 10
Joined: Sun Feb 03, 2019 2:52 am
Location: Mackay, Qld, Australia

Re: Darmah Front cylinder running rich

Post by mdal8798 »

Thanks again Craig,

Will break out cheap and nasty verniers and check taper lengths on screws. :-D

I am just waiting on getting a 22mm bolt turned down so i can remove engine side cover to inspect pickups.
User avatar
Craig in France
Paso 750
Posts: 917
Joined: Tue Mar 23, 2004 3:58 pm
Location: Montpellier, France

Re: Darmah Front cylinder running rich

Post by Craig in France »

mdal8798 wrote: Mon Jul 20, 2020 4:40 pm I am just waiting on getting a 22mm bolt turned down so i can remove engine side cover to inspect pickups.
In practice, you don't need to fab up a puller, Mark. Once all the screws around the cover are out, some gentle taps with a rubber mallet all around the casing will release it. (I guess you know to first release the starter chain - it's all in the w/shop manual. Watch for the shims).

Don't forget to order up a new gasket* - again, ask Phil. And also:

There is a poxy little O-ring at the bottom of the cover which me, I almost always forget to order at the same time :doh: . 'Push comes to shove', you can re-use it. But for the few extra cents, it ain't worth it. Part number 0150.22.260.

Dramah O ring.jpg

* Fit the new gasket using just a thin layer of regular grease on each side. No gasket glue. And apologies if I'm telling my grandmother how to suck eggs :shock:
mdal8798
Cucciolo - the Lil Pup
Posts: 10
Joined: Sun Feb 03, 2019 2:52 am
Location: Mackay, Qld, Australia

Re: Darmah Front cylinder running rich

Post by mdal8798 »

Hi Craig,

Did as suggested and used rubber mallet, cover came off easy.

Pickups and wiring are are in good condition, I suspect they may have been replaced at some point in time. Resistance measurements are within specification. I will probably just sleeve wiring and put back together after I polish sidecover.

This of course has left me with a bit of a dilemma, what to do next.

I have contacted Gowanlochs in Sydney (Ducati Bevel drive specialists in opposition to Phil at Road & Race) and spoke with workshop foreman at length and discussed my progress to date. When I made it apparent that bike hasn't been on the road for 11 years and only been run in shed he is of the belief that i just need to get bike on the road and basically blow the cobwebs out of it first then look at idle setup later.

Will keep you posted of progress.
900tlc
Mariana
Posts: 32
Joined: Fri Feb 24, 2012 12:00 pm
Location: North Dorset,UK

Re: Darmah Front cylinder running rich

Post by 900tlc »

Yes,I would tend to agree with the foreman.Get out there,and give it a shake down,you could have a worn exhaust valve guide or the guide seal is not happy. Go for a ride,stop the engine,leave it for a while(fuel stop say),and fire the engine and look for smoke from the right hand side.
If you have the left hand side disassembled ,I personally would put a Sasche ignition system on it.Remember all the electrics are 40 years plus,and change the coils as well.I had the same problem as you,running rich. I changed the carb seals,replaced with new ignition coils,set them up....Still no joy !
Replaced the front exhaust valve and guide.Stopped the smoking,but the tickover still not good.Changed the ignition system to Sasche,starts first time and tick over positive. Bottom line is,I think the electrics are old,the tolerances to set the pulsers are very tight.Need to spend money really,unless you enjoy spending hours in the garage and not miles on the road
Refer to a good write up on this forum," NEW IGNITION SYSTEM:NEED ADVICE" by 81mhr900ss.
User avatar
81mhr900ss
Parallel Twin
Posts: 143
Joined: Tue May 31, 2016 3:49 pm

Re: Darmah Front cylinder running rich

Post by 81mhr900ss »

Sorry for joining in so late. Lots of great advice already. Last year, following 40mm (tickler) carb rebuilds and many other things on my 81 MHR, I got the bike running on the 4th or 5th kick after 19 years of dormancy but something wasn't quite right on the rear cylinder. I strongly suspected the ignition system so rather than screw around, I bought a Sachse system, Dynatek coils, new HT leads and caps and a Shendingen Regulator to protect it all. I also bought MotionPro vac gauges! Never used them before with my Ducatis; always set up the carbs by feel but figured I'd be a little more pro this time. This delayed things about three weeks. With the new goodies installed, this is what I did:

- I swapped the slides. After years of dropping the slides onto the throttle stop screws, the little slanted land on the slide develops a dimple. Swapping the slides exposes virgin lands for contacting the throttle stop screws.
- I loosened the carb top adjusters and removed the bellmouths so I could stick my finger inside and feel the slides. I unscrewed the throttle stops all the way out and then back in until they just touched the slides. I then adjusted the carb top adjusters so that the slides lifted simultaneously when the throttle was turned. Easy done by ear or feel/sight.
- I screwed the idle screws in all the way then out 1.5 turns then put the bellmouths back on.
- 2 blips on the accelerator pumps and one good kick and the bike fired up easily but needed help to keep it running as the throttle stops had not yet been set for tick-over. I tweaked each throttle stop screw in a quarter of a turn at a time until the bike was ticking-over at around 1800rpm. I was not able to ride the bike as it was not insured yet so I continued annoying the neighbours for about 5 minutes to warm it up a little then shut the motor off.
- I had to balance the vac gauges by feeding two from the same carb and tweaking some screws on the gauge block so fired up the motor again for about a minute to do this then shut down and connected each carb to a dedicated vac gauge.
- Fired up again and tweaked the throttle stops to balance the gauges then tweaked the idle mix screws, one at a time to get the strongest vac and best sounding "PUT, PUT" out of each exhaust at 1800rpm. Then backed off the throttle screws a little at a time to a balance vac at 1200rpm and played with the idle screws again. Repeated one more time to just over 1000rpm idle. I tried going lower but the bike was a little lumpy no matter what I did. I've read of 900SS bevels idling at 900rpm. Not mine.
-Once the idle was happy at just over 1000 rpm, I tweaked the carb top adjusters to synch the lift and ensure there was a little play in the throttle; move the handlebars full lock both ways when doing this to see if either of the cables tightens.

It's been happy ever since. I had to tweak once more after I got it insured and was able to give it a good run; surprisingly the tick-over was just a little higher after a good warm-up. Good luck

According to the Internet (so it must be true) the smaller carbs are much easy to set-up. This was certainly the case with my old 860GT and it's 32mm carbs.
Post Reply

Return to “Carburettor & Fuel System Shop”