PHM40 Synching Query

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Humphrey Smith
Cucciolo - the Lil Pup
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PHM40 Synching Query

Post by Humphrey Smith »

I rebuilt the PHMs for my '76 900SS over the winter and I am now trying to get them properly tuned and synched. I have synched them using vacuum gauges running from each manifold and both slides are lifting in unison. All appears good and the engine idles nicely.

One trick that I learned some years back was to fit extensions to the spark plug terminals. I make them out of old spokes with an alloy terminal soldered onto each end - this enables me to briefly short out each cylinder, with a screwdriver, to compare the running of each cylinder individually.

When I short out the front cylinder the rear cylinder stumbles but does not die. However, when I short out the rear cylinder, the front one dies almost immediately. I have played with the mixture settings to no avail. I'm now wondering whether I should raise the front slide slightly as this should allow the cylinder to stumble but continue running, as per the rear one. But it will also mean that the slides will be slightly out of synch, though lifting together. Is this acceptable? Can one slide be slightly higher than the other when resting on the adjusting screws? This will, of course, mean a discrepancy in readings on my two vacuum gauges...

Hopefully someone will be able to elucidate.
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Craig in France
Paso 906
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Re: PHM40 Synching Query

Post by Craig in France »

Humphrey Smith wrote: Wed Jun 07, 2023 7:57 am <snip> I'm now wondering whether I should raise the front slide slightly .... (which) will also mean that the slides will be slightly out of synch, though lifting together.

Can one slide be slightly higher than the other when resting on the adjusting screws? This will, of course, mean a discrepancy in readings on my two vacuum gauges...
Let me take your questions in reverse order.

1. Yes, one slide can be at a different height from the other. In a perfect world, they would be the same. But in practice they may need to be different in order to compensate for differences in carb wear, in valve timing, in cylinder compression, in ignition timing or performance - for all sorts of things, individually or combined.

This is where the vacuum gauges come into play for the first time. As you know, they tell you whether each cylinder is pulling the same amount of air (and therefore fuel). This is desirable so that neither cylinder is doing more work than the other - which makes for a smoother engine performance.

So with the engine at idle, you use the idle screws to arrive at even vacuum readings. Which adjustment process may of course end up with the slides not being at the same heights ;) .

2. "Synchronising" the carbs means making sure the slides rise at the same time. Same reason as before: to avoid having one cylinder pulling harder than the other as the engine speeds up.

But this is independent of the starting position of the slides.

So back to the gauges. Open up the throttle to around 3,000 and this time adjust the cable adjusters to obtain equal vacuum readings. Don't touch the idle screws.

Does that help?
Humphrey Smith
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Re: PHM40 Synching Query

Post by Humphrey Smith »

Firstly, thank you very much for your reply to my query, Craig - much appreciated.

Everything you stated makes perfect sense to me until the last sentence:
Craig in France wrote: Sun Jun 11, 2023 6:20 am So back to the gauges. Open up the throttle to around 3,000 and this time adjust the cable adjusters to obtain equal vacuum readings. Don't touch the idle screws.
Surely if I do this - with one slide being higher than the other whilst resting on the adjuster screws at idle - won't one slide end up "hanging" on its throttle cable?
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Craig in France
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Re: PHM40 Synching Query

Post by Craig in France »

Not sure what you mean by "hanging".

If you mean, might one cable be taut while the other has some slack in it, then yes. That's what you take out with the cable adjusters, with the aim being that both slides start to rise at the same time, regardless of their respective start positions.

P.s It doesn't matter much if at full open throttle the slides don't clear the carb bores by the same amount/at the same time. At FOT, the engine isn't sensitive to such a minor variation. But it is at idle and mid-throttle.

Trust the gauges.
Humphrey Smith
Cucciolo - the Lil Pup
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Re: PHM40 Synching Query

Post by Humphrey Smith »

Yes, that is what I meant by "hanging" Craig. Many thanks for your input.

I would seem that getting two cylinders to run evenly through large-bore 40mm carbs is truly a dark art. The concept is simple enough but achieving it is no easy task. I'm in awe of the veterans who do it by ear... That and shimming the desmodromic rockers are the two mystifying aspects of these incredible engines IMO. if anyone on this forum can point me towards an online "for dummies" resource for either of these tasks it'd be much appreciated.
Humphrey Smith
Cucciolo - the Lil Pup
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Re: PHM40 Synching Query

Post by Humphrey Smith »

I have decided to post the doc that I have been putting together on this topic. Any comments from learned members would be appreciated. Posting it warts 'n' all; I'm sure there're lots of errors that I need setting straight on. The bike is a 1976 900SS with an engine rebuilt by VeeTwo (some 20 years ago but hardly run since then). I have rebuilt the PHMs; replaced the slides and changed out all worn/consumable parts without cutting corners. Running on Silent Hektik ignition system. The rockers were last shimmed some three years ago.


Synching Dellorto PHM 40 Carburetors - 1976 Ducati 900SS

Initially:

Prior to undertaking this procedure, exhaust leaks need to be eliminated. This can be done by smearing some paste into the grooves where the pipes join (with the clamps on).

Screw out the slide stop screws (so the slides bottom-out on the carb body) and slacken off the cable adjusters. Now take up any slack in both the cables, leaving a bit of free play (1 - 2mm) to ensure that they lift the slides approximately together.

Determine the initial slide stop contact starting point (where the slide stop just touches the slide).

This can be achieved by inserting a pencil into the slide cutaway and watch the end (of the pencil) for movement as you wind in the throttle stop screw.

Once both slides appear to open at exactly the same time, open the throttle until the slide cutaway on one carb is flush with the top of the carb bore. Then hold the throttle and check that the other carb slide is in exactly the same position. This will mean that both slides are synched through the full throttle range.

Check that the cables still exhibit the required 1mm free play. If they don’t then adjust as necessary from the beginning. Both slides should “click” together when they fall.

Now wind in in each slide stop 3 full turns

Screw the mixture screws in all the way and back them out 1.5 turns.

Now switch on the fan, start the engine and warm it up to operating temperature.

Raise the slides equally (on the stops) to enable the engine to run without resorting to using the throttle. This might require turning the mixture screws out a little. Set them so that the engine idles a little higher than recommended - at (or slightly over) 1,200rpm.

Now turn the mixture screw, on each carb, in and then out to find the point where the engine idles the fastest. Once found, turning the screw either way should result in a noticeable decrease in firing rhythm. Err on the side of richness. On the PHM, the mixture screw reduces fuel when you screw it in - turning counter-clockwise richens.

The idle might need to be readjusted to compensate for this. Do this in equal amounts on the stop screws. Listen to the exhaust note on each cylinder/feel the pulses with hands cupped behind the silencers - they need to be running evenly.

Synching

Remove the screws on the manifolds and fit the vacuum gauges - these can be bungeed to the tank (as per pic). Now adjust the slide stop screws until both gauges register the same. Intake on both carbs will now be equal.
Or just crack the throttle (less than a millimetre) and if one cylinder is leading (ahead of the other) it’ll be apparent to the ear. One stop will then need to be adjusted to compensate for this.
Or wind the stops in and out and listen for the point when they’re firing evenly. Back one stop off and the cylinder won’t fire in time. Bring it up and it’ll chime back in. get them as even as you can.

Both cylinders should now climb in unison. The running of each cylinder might still be different though - this could be down to a variety of reasons/slight discrepancies between the two cylinders. To check for this, use spark plug terminal extensions and short out each plug to see whether the cylinders are truly running identically. Adjust the slide stops accordingly. Inevitably one slide will end up further up than the other; they’re rarely the same.

Once this has been achieved then the stops can be lowered, by equal amounts, to the desired idling speed of 1,000rpm.

Now that the engine is running closer to optimal, the mixture can be checked again; it’s easier to assess at slower rpm. Find the spot where the cylinder fires fastest. You might have to re-adjust the idle again.

Perform a final check by shorting out each spark plug to ensure both cylinders are running identically.

Next tighten the cable adjusters leaving 1mm play before the cables lift the slides. sync the slide lift by taking up the slack in the cables. Ensure that neither slide should be “hanging” on a cable.

> The carbs are now balanced and the mixture set.

> The carb slides are synched.

> The idle is set.

> The cables are set.

The bike can now be ridden.

When riding, pay attention to how responsive the engine is under different situations/load. If performance/response seems inadequate you can use this trial & error method:

From a high gear, and not going too fast, open the “taps”. If it falters then stop and adjust the mixture screw on each carb equally (either richer/weaker) - then go for another ride and see whether it’s improved or worse. If it’s worsened then turn the screws back to where they were and turn them the other way and see whether that eliminates the problem.

The spark plug electrodes can now be checked for high-speed running by performing a “plug chop” by running under load and cutting the ignition and pulling the clutch in simultaneously. Third gear, up a long hill, using 3/4 throttle should provide the right situation to get an accurate reading. If the result is unsatisfactory then it might be necessary to drop/raise a needle a notch.
Humphrey Smith
Cucciolo - the Lil Pup
Posts: 8
Joined: Wed Jun 29, 2022 6:17 am

Re: PHM40 Synching Query

Post by Humphrey Smith »

A shot of the spark plug terminal extenders that I use. These enable me to short out each cylinder in quick succession (with a screwdriver) to check for even running.
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Humphrey Smith
Cucciolo - the Lil Pup
Posts: 8
Joined: Wed Jun 29, 2022 6:17 am

Re: PHM40 Synching Query

Post by Humphrey Smith »

Gauge setup bungee'd to bike.
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IMG_4082.jpeg
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