Carb novice requests help

[gmlaverda]

Hoping the knowledgeable folks here can help with my carb issue. I have been rebuilding a 1969 350 SCR that I purchased a few years back. After a long sleep of probably 20 years it roared to life today. But I need some help with the carburetion. The original carb was missing so I purchased a new Dellorto PHBH26. The bike is running very rich, idling for a while but very rough and lumpy above idle. Not sure about the high RPMs.The plug is black, sooty but backing out the air mixture screw does not help. I also moved the needle clip up a notch which I believe leans out the mixture but no appreciable difference was noticed. Anyway I am excited to get it running but have not enough knowledge of carburetors. The carb was supplied with the following parts:
40 slide,X6 needle,268T atomizer, 120 main jet, 50 idle jet.

[Jon Pegler]

Looking at the jetting you have got in your carb, I would say that the atomiser is very big, which would cause richness.
I would think a 262 or a 264 would be a better size.
The rest of the jets look to be a reasonable starting point.
Are you sure that screwing the mixture screw out is not richnening up things too?
The Dellorto PH carbs do not have a pilot air screw like an Amal carb, that admits more air, but a mixture screw that admits a fuel/air mixture.

Jon

[Ventodue]

(Apart from the very first models), The standard Dell'Orto carb for the 350 SCR was a VHB29. So yours is neither the 'right' size nor the 'right' type . Which risks making things a little more complicated ...

FWIW, the spec for the VHB carb was:
Atomizer: 265M
Mains: 110
Idle: 45
Needle: V13
Slide: 50.
(Source: owners h/book and factory parts manual).

How these specs might change for a smaller PH carb, I can't say. Somebody at Eurocarbs might - ?
But, if it's faulting on transition, as Jon says: first suspect the atomizer and/or the needle.

[gmlaverda]

Thanks for the input gentlemen. Regarding the mixture screw, it does idle better and at a higher RPM with the screw completely turned in. But the throttle response is still the same as before.

[Jordan]

Above idle, the system that comes into affect next is the the slide cutaway.
The smaller it is, the richer. Yours is #40.

For a 250 with PHB carb, these settings were suggested by Phil Hitchcock of Road & Race:

#60 slide, #60 idle jet, #265 needle jet, K5 needle, and 114 -118 main jet.
If the PHB is a pumper, disable the pump by removing the lever pinned to the carb top.

[Ventodue]

Above idle, the system that comes into affect next is the the slide cutaway. The smaller it is, the richer. Yours is #40. <snip>

Good point, Jordan - thanks. Forgot/overlooked that ...

FWIW, here's the Dell'Orto explanation, in pictures and words:

[ducwiz]

An old thread http://www.motoscrubs.com/forum/viewtopic.php?f=3&t=1269&p=8628#p8628 might be useful for you. It'sabout a PHBH30, which for me seems a more natural replacement for the VHB29.

cheers Hans

[gmlaverda]

Thanks again for all of this great information. I have been reading here and elsewhere about carbs and came to the conclusion that I should start by changing the idle jet first and second experiment with a larger slide cutaway. My question now would be, how different from my #50 idle jet do I change to? I suppose small steps are in order here, try the next smallest jet first. And if I need a new slide how far should I deviate from my #40? The jets are inexpensive but the slides are not. And seeing the settings for a 250 engine I am wondering how different these settings would vary for a 350.