Slide cutaway

[Boxprod]

At the risk of what younger people call "oversharing", I thought it might be useful to tell the tale of my recent challenge with the Dellorto PHF32 that sits on my late-model 350.
Very early on in my ownership - about 1979, I think - I acquired an engine built by John Wittman, which had a properly smoothed 32 mm inlet tract. I replaced the cam (which has 250SCR stamped on it, so maybe not the best for a fast road bike) with a green and white from Mick Walker, and then had to replace the VHB29 with something to fit the port.
Back then there wasn't much expertise available unless you actually bumped into somebody who knew what they were talking about, so carburettor choice was limited. I'd had concentric Amals on a T120 Bonneville, and they were fine but not easy to get, there weren't many Mikunis around that weren't still attached to Japanese bikes, so you were faced with exciting unknowns like the Gardner or perhaps half a twin-choke Weber DCOE. Yes, I did see that on a British single.
Then I found the importer of Dellortos in a private house in Reading. The firm still exists, and is flourishing, trading as Eurocarb, and still importing top-quality Dellortos and parts. From him I bought a PHF32, which he set up with jets which were likely to work.
And it was great. I did have to tweak the pilot jet a little to get it to idle properly, but that was about all. I tolerated any "uncertainty" below half-throttle because traffic wasn't really a problem in those days, and by the early 80s my daily ride was 52 miles each way up the M1.
Reviving the old thing this year has led me to work on that "uncertainty". It helped when I realised that the first 100 miles of shakedown had slightly loosened the inlet manifold nuts, so I retightened them and eliminated that air leak. Riding gently - as befits my age and the rural Cornish roads - allowed me to concentrate on getting the fuelling as good as I can throughout the range. Although I won't be bothering the full-throttle end too much, honestly officer.
Raising and lowering the needle provided good evidence, and it helps to have a choke lever which can richen the mixture to check what's going on. What it came down to was a hesitation just off idle, which I worked out must be caused by the transition from pilot jet to needle jet, and that means the slide cutaway.
Dellorto can supply slides with different cutaways and different accelerator pump ramps, but I reckoned that the combination I needed wasn't quite available. Research across the internet revealed that keen engineering types like the Vincent Owners' Club have been modifying cutaways for years.
My cutaway (a 60/1) started with a gap of 6.0 mm. (The /1 indicates the accelarator pump ramp's shape.) Road testing suggested that it's too rich, so in order to slow down the airflow across the atomizer tube, and thus diminish the amount of fuel sucked up, thereby weakening the mixture, the cutaway needs to be enlarged a bit. More research, and I set about lapping the slide very carefully with wet-and-dry on a glass plate. That way the angle of the cutaway remained pretty much the same. I knew that if I took off too much I could always take a bit off the bottom face of the slide - lowering it so that the cutaway gap became smaller again - and that might need some work on the needle position, but it should be ok if I was cautious.
I took the gap to 6.6 mm in a couple of stages, with road tests after each session, and I'm delighted to say it has worked very well indeed.
All this waffle is by way of encouragement if anyone else is scratching their head about such issues. What it is to be retired(-ish) and have enough time to do these things!
All the best,
Pete.

[Duccout]

Well done Pete. I have modified cutaways myself over the years, including those fitted to a 500 Pantah that came from the factory with cutaways that were too small. One problem with Dell'Orto is the price of the slides, making it an expensive business to try a different cutaway. Same goes for Mikuni. Amal are a lot cheaper (in both senses of the word).


Cheers,

Colin