Carb Question

oehlin

oehlin

Junior member
Location
Sweden
Soon it will be time to start up the engine in my project. 750 SF 1973 (#11332) according my reg.papers
The engine is an SF0 (# 9430) which from the beginning probably had Dellorto 30mm carburetors. Now there are a pair of PHF36 carburetors mounted and my question is, will these work? and if so, what jets should I start with. Don`t think there are any other modifications to the engine.
/Göran Sweden
 

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That is a big valve head very likely in which case standard is 55 pilots and 140 mains. 36mm carbs will not fit a small valve head without extensive removal of metal.
Bellmouths will probably need 5 points more on the mains.
Check.

Paul
 
Paul Marx said:
That is a big valve head very likely in which case standard is 55 pilots and 140 mains. 36mm carbs will not fit a small valve head without extensive removal of metal.
Bellmouths will probably need 5 points more on the mains.
Check.

Paul
Click to expand...
Hi Paul
Are you sure it`s a big valve head. The headers are 35mm.
Should not the big valve head have 38mm headers ?
Maybe the extensive removal of metal in the inlet has been done as there appear to be grinding marks in the channel
 

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The inlet stubs are for a big-valve head. These will not fit a small-valve head without reworking the head. By that, I do not mean the port itself.

All the different headers will fit on all heads... :rolleyes:

Perhaps you should review your carb balance hose, while you're at it.

piet
 
oehlin said:
Hi Paul
Are you sure it`s a big valve head. The headers are 35mm.
Should not the big valve head have 38mm headers ?
Maybe the extensive removal of metal in the inlet has been done as there appear to be grinding marks in the channel
Click to expand...
To be more explicit, this is a stub for 36mm carbs on a small valve head. Even the stud holes for the stub don't line up.
Intake in the head is anywhere between 27 and 30mm on a small valve head.

Conclusion: somewhere in it's long life, the head on 9430 has been changed.

Paul
head.jpg
 
Paul Marx said:
To be more explicit, this is a stub for 36mm carbs on a small valve head. Even the stud holes for the stub don't line up.
Intake in the head is anywhere between 27 and 30mm on a small valve head.

Conclusion: somewhere in it's long life, the head on 9430 has been changed.

Paul
View attachment 62032
Click to expand...
You mean, I do have a big valve head ? My studs line up fine
 
oehlin said:
Hi Paul
Are you sure it`s a big valve head. The headers are 35mm.
Should not the big valve head have 38mm headers ?
Maybe the extensive removal of metal in the inlet has been done as there appear to be grinding marks in the channel
Click to expand...

Yes, I too have an original big head (the one with one less fin) that is unwelded and definitely only has a 35mm inlet port.
The cylinder type, which also had one cooling fin less, was also mounted on this engine.
The studs for the intake manifold have the correct spacing of the large heads. Either these heads are from a transitional period or Giovanni had a lunch break/after-work break in the middle of grinding. After that, he forgot that he hadn't finished grinding that head yet.

Thomas aus LAU
 
Thomas aus LAU said:
Yes, I too have an original big head (the one with one less fin) that is unwelded and definitely only has a 35mm inlet port.
The cylinder type, which also had one cooling fin less, was also mounted on this engine.
The studs for the intake manifold have the correct spacing of the large heads. Either these heads are from a transitional period or Giovanni had a lunch break/after-work break in the middle of grinding. After that, he forgot that he hadn't finished grinding that head yet.

Thomas aus LAU
Click to expand...
A 35mm inlet port is normal for a big valve head. I don't understand your comment?
Heads rarely correspond to nominal inlet diameter.

Paul
 
sfcpiet said:
The inlet stubs are for a big-valve head. These will not fit a small-valve head without reworking the head. By that, I do not mean the port itself.

All the different headers will fit on all heads... :rolleyes:

Perhaps you should review your carb balance hose, while you're at it.

piet
Click to expand...
What differens will a review of balance hose do :unsure:
 
Paul Marx said:
A 35mm inlet port is normal for a big valve head. I don't understand your comment?
Heads rarely correspond to nominal inlet diameter.

Paul
Click to expand...
Apparently not all. I have two heads (from different previous owners) here with d=36 mm on the head and the socket has 36 mm throughout.
Subsequent modified inlets?
Another head has the approx. 34.5 mm at the inlet.

oehlin: Please exchange the connecting hose for a stable fabric connecting hose with the right diameter! This must be temperature and fuel resistant. Gasoline or oil hoses from the automotive sector are optimal.​


Thomas aus LAU
 
Last edited:
Thomas aus LAU said:
Apparently not all. I have two heads (from different previous owners) here with d=36 mm on the head and the socket has 36 mm throughout.
Subsequent modified inlets?
Another head has the approx. 34.5 mm at the inlet.

oehlin: Please exchange the connecting hose for a stable fabric connecting hose! This must be temperature and fuel resistant. Gasoline or oil hoses from the automotive sector are optimal.​


Thomas aus LAU
Click to expand...

exchange the connecting hose​

DONE (y):cool:​

 
And your pistons will be happier if you run 145 or bigger main jets if you do any full throttle stuff. My bike runs perfectly with 45 pilots, but ideal pilot jets size seems to differ from bike to bike, you will find out when you set the idle mixture.
 
Aren't small valve head inlet angles different as well (so the carbs sit closer to parallel to the ground)? My SF1 had 34.5 at the head but opened back out as it went deeper - apparently a 'double venturi design 36 carb tapering to 34.5 in the inlet stub then back out to 36 beyond. I ported mine to 36mm straight through which I think is std SFC practice.

The exhaust port on the small valve head I used as the basis for the SFQ had a deeper recess for the stepped headers - I had to machine and press in a 'ring' to allow use of std SF1/2/3/C headers. No idea what model the small valve head came off - bought it from Wolfgang.
 
Smaller ID headers will affect carburation and you may find there's an ugly step down from the big valve exhaust port to the smaller ID headers - but maybe not.
 
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