Dellorto PHM40s

[peakrock]

My Type 1, which I've just finished rebuilding, came with a pair of Dellorto PHM40s that had been ultrasonically cleaned and reassembed with Dellorto gasket/service kits. I started it up yesterday and it ran ok, albeit with the tickover needing fine tuning. I duly set the tickover up this morning to about 1,200 rpm and all seemed fine, although it did need a lot of cranking over to fire up, even with the float tickler operated.

I took it out for a run and for no apparent reason the revs shot up to 5,000 rpm and stayed there, making things rather awkward to say the least. Still, it was a good excuse to bed the brakes in trying to keep the speed down so I could get it home.

I checked and the throttle slides are not stuck open and move freely, I reset the idle and mixture screws back to roughly where they were but as soon as I start it up the revs shoot up to 5,000 rpm. Has anyone ever had this happen to them before?

The obvious thing I guess is to strip the carbs down and check the float chamber heights and that nothing has come undone inside since the carb rebuild - I haven't touched them as yet (being more of a fuel injection man ) Ant other suggestions gratefully received.

Thanks Dave

[Weg]

One thing that springs to mind is about the throttle cable.
If you have a 2-1 splitter, has the outer cable of the primary dislodged form it's ferrule thus holding both slides open. Even 1/4" would be enough to rev up to 5000.

[peakrock]

That's a good thing to check Tony, although I actually ran two new cables back to the accelerator pedal rather than use a splitter. It would definitely be sensible to check both cables still have some free play in them though. I'd kind of discounted them as I was very careful setting them up but best to start with the simple checks first.

[peakrock]

I checked out the throttle cables and they were both fine. However, when I took the wire mesh trumpets off I noticed that the RH carb throttle slide is sitting about 3mm higher than the left one, even with slack in the cables. This looks a good place to start fault finding so I'll investigate further when we get back from a short break.

[Oleg]

Hi Dave,
Our T2 has PHM 40s as does the T1 we owned previously. My thoughts.
To add to Tony’s comment about throttle cables also check that the choke cables have a small amount of slack and the choke sliders are not sticking holding the chokes partly closed.
PHMs, as most Dellorto’s, are fussy about fuel levels so worth checking the float height settings and adding anti-surge washers does aid flow stability. .
You mentioned starting, I find that from cold that choke is needed briefly and as PHM’s are pumper carbs no harm in giving a couple of throttle pedal depressions to add fuel.
Congratulations on bringing your T1 back to life!

[peakrock]

Thanks for the advice about the carburettors and I'll definitely check the float levels when I'm back. These carbs don't have a choke operated by a cable, instead there is a button to depress the float and flood the carburettor. A bit awkward scrambling in and out if it doesn't fire up first time! The short drive I've had in it so far was good, it seems to ride well over the Norfolk potholes.

[peakrock]

I stripped the carbs down and on one of them the float needle valve assembly was approx 2.5 turns loose (I didn’t rebuild these carbs). I tightened it up and checked the other jets etc for tightness too. I also checked the needle valve (K4), main jet (AB265) and idle jet (152) were the same on both carbs. It’s fixed the over revving problem although now one cylinder is running much hotter than the other when trying a quick set up of the idle. So I will balance the carbs properly and go through all the settings from scratch again and maybe use my old colortune to see where we’re at.

A question: where does the anti surge washer go in the carb?

Thanks Dave

[Weg]

Under the main jet holder, dish down.

[peakrock]

I have the Dellortos set up nicely with a good idle speed and responsive throttle. Today, on a test run I was in some light start stop traffic when the engine started running roughly and backfiring. This got worse and it would die and then pickup with really loud backfiring. In the end I had to pull over and stop. I checked for something obvious but to no avail. It wouldn't restart so in the end I had to recover it with my trailer.

I'll try it again from cold tomorrow but I've already checked the timing which is ok ( it has the Sachs unit) I'll give it a compression test just to make sure but I'm suspecting fuel vaporisation. Are these carbs prone to vaporisation and would the fuel surge washer help with that?

Cheers Dave

[Throwley]

The Guzzi I've been running for twenty or so years is fitted with PHM 40s. A different application I know, but I've never had any issue with vapourisation, even in slow-moving summer traffic following a fast ride.

Your symptoms sound like both cylinders are being affected simultaneously by the same issue. The systems common to both cylinders (assuming you have individual air filters fitted) are ignition and fuel feed. You've checked ignition, so is your fuel pump still flowing OK? Is the tank cap vent free? Which pump are you using, and has the vehicle successfully run with that fuel pump fitted?