Carb Question

piranha-bro2 said:
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.
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Port position is slightly different and stud centres are closer. The actual sealing surface to the inlet stub is the same, the carb angle is given by the stubs.

piet
 
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.
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That's what I wrote Thomas. You'll find inlets on big valve heads from 34 mm or less, to 36. On small valve heads, you'll find 27mm to 30, more often 29 mm.

Paul
 
sfcpiet said:
Port position is slightly different and stud centres are closer. The actual sealing surface to the inlet stub is the same, the carb angle is given by the stubs.

piet
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So inlet stubs have angles, Piet? I went steeper inlets ports and longer inlet tracts for FCR carbs - the frame was very much in the way! But I've built in a good air filter system with silicone tubes 65mm at the intake stack to 80mm filter end.

I could possibly have modified Red's angled triple inlets for Mikunis (or Red may have CNC patterns for 750s) but this ended up working fine.
 
Now 55 pilots and 140 mains in the carbs.
Compression on both cylinders 10,5 psi
Easy start with half choke, and while warming up I can decrease the choke to maintain 2000 rpm.
When engine is warm and choke is of it will run stabile from 2000rpm and above .
If I get under 2000rpm it will start splutter and backfire true the carbs and stop.
Then I must start it again with some choke.
Suggestions ?
 
Idle circuit is blocked or partly restricted. Don't imagine it is clear just because the idle jets themselves are clear - the idle circuit delivery is quite a tortuous route and can be blocked somewhere along its length.

HTH, Paul
 
Haggis said:
Idle circuit is blocked or partly restricted. Don't imagine it is clear just because the idle jets themselves are clear - the idle circuit delivery is quite a tortuous route and can be blocked somewhere along its length.

HTH, Paul
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On both carbs ?
 
Before I assemble the carbs I cleaned them in my ultrasonic cleaner and sprayed them with carb cleaner but I suppose I will have to do it again
 
You misunderstood. You use the carb cleaner to check if the drillings are clear. This means removing the mixture screw, holding the slide fully open and blocking the small venturi intake hole. Then reverse. You should see a fine jet of carb cleaner fluid squirt up from the tiny hole just in front of the needle jet (atomiser) , where the slide sits.

Check the Dell'Orto manual in the library... it has a drawing that shows the pilot circuit path for air and fuel

Also see https://laverdaforum.com/xf/threads/j55154/post-102290
 
You are certain that ignition timing is correct and valve clearances are within spec? Check also that the choke plungers are fully seated with the choke off, as well as freeplay in the cables. What happens if you try different settings on the mixture screws, try a quarter turn at a time until it is almost all the way in and up to three full turns out and see if that affects running, as well as ensuring that the passages are clear as Rob has explained. Spitting back at almost closed throttle is also a symptom of sucking air from somewhere between the carb and the head. O rings and insulators are good?, the tube between the manifolds is good?, the screws for vacuum gauge plugs are sealing?
 
If ignition, static and full advance, and valve clearance is ok, then either as mentioned your idle circuits are blocked, or.....your float levels are too low.

Paul
 
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Is it possible you have your cam timing one tooth out? It appears this is the first startup since the big valve head went on? I know it's a hassle to check but are you absolutely sure it's correct? Do the carb clean thing first but if nothing else works I'd be looking at cam timing.
 
I have had SFs with timing a tooth out that have been running for years and the owners had no idea. An easy check is to remove the plugs (cold engine) and cover the plug hole with your finger while turning the motor over from before TDC compression. As it compresses it will build up pressure, you can feel it, and after TDC it will try to suck vacuum - if you remove your finger it will sort of pop - if that vacuum isn't there it is a tooth out.
 
I spent months and many hours of work and pondering to get my 750S carbureting properly. It was overly rich.
I went as far as tilting engine and checking cam timing. All was ok.

It turned out that it needed leaning out the pilot jets. Due to unleaded fuel, or my having opened out the inlets from 27 to a more logical 30mm? Who knows.
It ended up running fine a full 10 points down on the pilots, from 55 to 45. I now run 47 or 48 pilots, I'd rather be a tad rich. There's just a hint of richness with those.

Paul
 
Paul Marx said:
I spent months and many hours of work and pondering to get my 750S carbureting properly. It was overly rich.
I went as far as tilting engine and checking cam timing. All was ok.

It turned out that it needed leaning out the pilot jets. Due to unleaded fuel, or my having opened out the inlets from 27 to a more logical 30mm? Who knows.
It ended up running fine a full 10 points down on the pilots, from 55 to 45. I now run 47 or 48 pilots, I'd rather be a tad rich. There's just a hint of richness with those.

Paul
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My GT750 used to occasionally “pop” just off idle and the tickover would feel a little rich when the engine was really hot at idle . The last time I rode it it lost a spark and thudded home on one cylinder. A fifty year old coil seemed to be the problem so two new bosch blue coils were fitted, two new condensers, new plugs.Whilst I was at it I stripped and cleaned the carbs and went down to 50 pilot jets.On firing it up - coughing, spluttering and barely able to run. Switched back to 55 and everything’s hunky dory. I think a weak spark caused by the dying coils made the tickover erratic and fooling me into thinking it was carburation- very difficult when symptoms are so similar.
Must have all the basics in place before looking for a problem .
 
Exmoor beast said:
My GT750 used to occasionally “pop” just off idle and the tickover would feel a little rich when the engine was really hot at idle . The last time I rode it it lost a spark and thudded home on one cylinder. A fifty year old coil seemed to be the problem so two new bosch blue coils were fitted, two new condensers, new plugs.Whilst I was at it I stripped and cleaned the carbs and went down to 50 pilot jets.On firing it up - coughing, spluttering and barely able to run. Switched back to 55 and everything’s hunky dory. I think a weak spark caused by the dying coils made the tickover erratic and fooling me into thinking it was carburation- very difficult when symptoms are so similar.
Must have all the basics in place before looking for a problem .
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Yes, everything needs to be checked.
In the case of my S, coils were new as was wiring to and from, as was the cam timing!

Paul
 
Back in startup again and still problems with low RPM

The carburetors, a PFH36AD is equipped with
pilotjet = 60
mainjet = 145
Needle =?

Throttle slide = 60/2
which indicates that the carburetors come from a triple machine.
750SF is supposed to have a 50/3 throttle slide?

Could this be the cause of the low speed problems and will the replacement of the throttle slide solve the problem.

Valve clearance, cam chain and ignition are checked and are still OK
 
oehlin said:
Throttle slide = 60/2
which indicates that the carburetors come from a triple machine.
750SF is supposed to have a 50/3 throttle slide?
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Most big valve Lav750 prefer 50 cutaway slides in their PHF or PHB 36mm carbs, certainly.

Marnix
 
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