Honda-SOHC

Practical advice for OP to check nothing simple is catching you out on this set.

No carbs should "hang" on the throttle pull cable at closed position. This is ONLY facilitated by throttle stop screw next to cylinder #4 in providing absolute slide base position.  If there's any tightness in pull  cable at this position, then it needs adjustment to provide slack such that it can't interfere with complete "drop" onto that stop when you just let the twistgrip go.
Make absolutely sure there's no conflict with secondary cable throughout full twistgrip movement.  There's usually only adjustment on pull, with return being fixed.
You can remove the return cable while you try to diagnose fault, but advise to use when ultimately riding it though.

Idle mixture screws are set from a start point of 1 & 3/4 turns out from fully closed on the F carb set. Anything significant below that will likely give the symptoms described, effectively lean idle, with revs slowly drifting down and stalling. Make sure they are in the starting position if you don't know how they've been set.

Worth verification of these points to see if there's anything basic running you ragged.

Quote from: Bryanj on April 01, 2024, 11:29:55 PM

PD carbs are notorious for clogging and need complete and thourough cleaning by somebody who understands them, i wont touch them at all

Are there later carbs available that will fit to replace the PD carbs Bryan - they must be bad for you not to touch them?

There's general record of people's dislike for working on them throughout the forum, being very fiddly the primary observation. But then so are many intricate device in reality.

They are though an absolutely excellent carburettor design in metering and accuracy terms.  Viewed as "lean" running, well that's only when referenced to their predecessor types which are technically too "rich" by design and common faults in that fuelling strategy.

These are one of the nearest to fuel injection in the optimisation of air fuel ratio from that era in slide carbs on Honda bike, in my view.

They run close to optimum for A/F ratio for constant demand to give more ideal fuel burn without much in the way of excess .... the excess inherent in predecessor being primary reason for short oil change interval by running mixture sufficient to accelerate all the time, whether needed or not. These PD 41/42 series run much more optimum steady state mixture, supplemented by accelerator pump only when required to stoke it into fast response ..... exactly what a fuel injection system does.

Cold blooded, yep, that too until they get combustion up to temp, facilitated by convenient choke pull up by instrument console with full rpm holding range to give competent low temp, running.
This is why FI stuff ordinarily use 88C thermostat in water-cooled installation, to bring the engine temp up to target as fast as possible, then to reach normal (non enriched ) mapping and run near to stoichiomeetric combustion.

Its all there in these carb, highly advanced when you look at them now. The design is no excuse to having them work competently, there's no inherent "fault" with them.

Cleanliness and diligence absolutely required, but they do work well if clean and set properly.

Thanks for this. I will test the throttle tomorrow and start there. I'll also look into the idle circuit set up...

They are similar to your garden machinery in some effect. The adjustment screw on these carb is engine side of throttle slide and controls fuel (unlike predecessor, with air adjustment) winding out/unscrewing should bring more fuel to low speed.

Changing each one in isolation should bring about obvious shift in running for that cylinder, no response suggests impairment as way of assessment.

The aim in Honda setup routine is to bring them to borderline lean point to give combustion reference, then lift the mixture a little toward richer ( they indicate  100 rpm drop from that richer state) to get them as good for further evaluation. 

If left too lean on this circuit, that forces you to then run the slides lower than ideal in controlling the tickover aim point. That can give some of the inconsistencies you've currently got.

Some considerations.

The throttle cables need to have a prescribed amount of slack, usually the "pull" one is adjusted only. A rudimentary test is to turn steering from lock to lock and blip throttle (while running) at each extreme .... any change at either end, then it needs more room to play with. I can get the spec from manual at twistgrip if you need it.

There's concise method to setup idle circuit jetting if they're been apart before, that's in manual, to get them on aim for mixture. This can get them "hanging " as described if not fully understood and implemented.

Choke should give up to about 2,750 rpm fully out, but obviously when fully home shouldn't be lifting the throttle slides at all. Also data in manual to set that correctly.

I can speak front experience that the good alloy wheel cleaners like Wonder Wheels that work great also corrode wheels. Merc will not cover alloy wheels under warranty that have had such cleaners used on them. The original W124 alloy wheels were Anodised for the first few years of production before they used paint & lacquer like most others

My experience too Ted, with that brand (I've dome stock  ) containing Hydrochloric Acid in my sample.  You can smell it on bare aluminium based alloy, like a battery charging.

Think SteveW has sensibly included a time of immersion in anecdotal usage, which I think is wise precaution.

It is one of the only solution I've tried that removed brake dust and specifically contaminated with iron from disc brake rotor "outwash" on car alloys that haven't been kept clean.

On the Guzsi Mondello axis, I'd encourage a test ride of Ducati 821 Monster if available ..... a very different bike but along the same vein in being very different.

If youve not ridden similar, they've a very different character engine wise to many others, they just feel like much les friction in engine response, really eager torque, plus high rev hit if you want it, all in one engine. Very well balanced too with narrow bicycle like feeling to weight distribution.

Go on, you know you need another one 

I use clock cleaning fluid on the brassware. I pour some into a jar (you have to dilute it) and leave the brassware to soak for a couple of hours. Once  rinsed I put them into the ultrasonic bath with carb cleaning fluid from Allendale.
I got the tip for clock cleaning fluid from the Repair Shop on TV and after watching an Allan Millyard video where he used it on some Z1 carbs.

Interesting lateral thinking there Dave, seems appropriate to the materials in the jets themselves.

List of ingredients here

 3. COMPOSITION/INFORMATION ON INGREDIENTS
Component CAS Number Percentage
WATER 7732-18-5 10 - 25
ISOPROPYL ALCOHOL 67-63-0 20 - 25
OLEIC ACID 112-80-1 5 - 10
PINE OIL 8002-09-3 5 - 10
AMMONIUM HYDROXIDE 1336-21-6 30 – 40
The exact concentration of composition has been withheld as a trade secret

Ph listed as 11.5 on msds sheet, for the concentrate.

Camera technical often use "lighter fluid" for old and gummy parts, quite effective too.

The steam suggestion comes from analytical chemical laboratory in which the glass vessels are routinely steamed to remove residue, sometimes "tarring" that's similar to that whichbwe get in carbs etc.

A number of different solutions to try.