SF1 Del Orto’s 36 PHB advice

S

smlav

Senior member
Location
Shropshire UK
My SF1 has 36mm PHB’s, standardise air filters. Carbs were cleaned and refurbed by Paddy back in 2019. They have
Mains 140
Pilots 50
Slides 50
Atomisers 265AB
Needles K1

I had a problem of hesitant pick up at very low throttle opening, seemed worse with poor fuel (95 octane rather than 98/99), which responded well when I opened the mixture screw to 1.75 turns out, and would probably improve a bit more to being 2.0 turns out. So I’ve ordered 55 pilots which I think is what is needed, as I understand 1.5 turns on mixture screw is optimum.

My question is that reading the forum shows standard needles should be K2. Should I fit these as well - obviously I’d do the pilots first, test ride, then needles; one thing at a time etc. My understanding is that the needle affects mixture throughout the throttle range, and K2 is richer than K1. The needles in the carbs at the moment are on their middle notch. Is there much of a difference between K1 and K2?

Thanks
Sean
 
Would surely take the K2s, yes. Good idea to try the 55 idle jets, with mixture screws at 1,5 to 2 turns out you should be fine.

Marnix
 
No, 1.5 turns is not optimum. Optimum is where you get the strongest idle and can be anywhere between about one and two and a half turns out, more or less and I would change the jets. In my experience PHF seem to give best results being about an eighth of a turn out from strongest idle, just a tad rich. Change to K2 needles before you do anything else. The needle/needle jet affect mainly from quarter to three quarter throttle. Pick-up from closed throttle is heavily affected by slide cut-away interacting with idle mixture, and your 50 slides have the correct cutaway. I also do any of these low speed adjustments without air filters, and if things change with filters fitted I address the filters. There is ignition timing and all that also in the picture so make sure that is correct. On my SF2 it makes absolutely no difference to how it runs if I use 98 or 95 fuel. I have 45 pilot jets and it ran fine on any fuel between Italy and Norway last summer.
 
My ignition is electronic and bang on with strobe. Float levels are good. Fuel flow is good. The fuel issue was real, over 10 days of riding in Spain and about 1500 miles, the 95 octane fuel guaranteed splutter, some 98 octane had a bit of splutter, most 98 had very little splutter. Screwing mixture out to about 1.75 all but eliminated splutter with 98. And filling up in UK with Shell premium which is 99 gave smoothest of all.

I’ve read elsewhere on forum that if you get to 2 turns out on mixture you likely need to look elsewhere. I may as well go the 55 pilot first, that seems to be fairly standard. Then I’ll go K2.

The splutter is/was at very low throttle opening, almost but not quite closed, just a bit of hesitation that cleared once past about an eight of throttle.

Cheers
Sean
 
Why faff around with jets and stuff if the needles are so bleeding obviously incorrect?

First logical step is to fit the correct stuff, then ascertain if anything requires additional fettling...

My Lavs all run nicely on 95 octane E5 or E10. 98 or more octane only used for high-speed track use for its safety margin.

piet
 
sfcpiet said:
Why faff around with jets and stuff if the needles are so bleeding obviously incorrect?

First logical step is to fit the correct stuff, then ascertain if anything requires additional fettling...

My Lavs all run nicely on 95 octane E5 or E10. 98 or more octane only used for high-speed track use for its safety margin.

piet
Click to expand...
Same here. They run fine on all types of fuel.
Contrary rumours are to sell "specialised" magazines and additives to people who'se bikes don't run very well because they don't ride them much.
Paul
 
Paul Marx said:
Same here. They run fine on all types of fuel.
Contrary rumours are to sell "specialised" magazines and additives to people who'se bikes don't run very well because they don't ride them much.
Paul
Click to expand...
In UK, on Shell premium, bike runs fine. In Spain, it was farting on E95, less so on E98. Soon as I filled up with Shell premium in Plymouth, running great again. Very reproducible behaviour. I've done >3000 miles on it this summer, and it was only the Spanish fuel which made it fart at low throttle. I guess that is different to the experience of others.
 
Sounds like the fuel was compensating for slight mismatch in setup. Def K2s. Don't be afraid to wind the mixture screws out ... as Tippie says, setting is more about achieving smooth pickup and idle - I also have always run them slightly richer than highest idle to achieve best pick uip and quick return to idle. Often highest idle will see revs hang slightly before dropping back. if you're widning the mixture screws out past 2 1/2 then fitting larger jets is how you compensate and move things back into normal parameters.

As Piet says, no.1 job is fitting K2s.
 
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if the bike ran fine from 2019 when Paddy rebuilt the carbs, what has changed recently to create this stutter? or have you had the stutter for six years?
CLEM
 
Last edited:
CLEMTOG said:
if the bike ran fine from 2019 when Paddy rebuilt the carbs, what has changed recentky to create this stutter? or have you had the stutter for six years?
CLEM
Click to expand...
I’ve only had the bike six months Clem. It’s fine on Shell premium (my local garage), the stutter only became apparent on a trip to Spain when it tasted their petrol, and that gave me cause to look through the paperwork which came with the bike, which is when I saw it had 50 pilots and K1 needles.
Sean
 
I think your timing will need retarding, maybe 2 or 3 degrees, check it out and if stock, retard it anyway, modern fuels mean that the factory settings may not be apropriate for ignition or carbs, and you probably dont know if it has stock compression ratio, very often following a rebore this changes.
CLEM
 
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