SOHC.co.uk Forums > CB750
K7 carb help.
K2-K6:
My understanding of these carb sets is as follows.
These PD series were specifically designed to run as lean a mixture as possible through lower and mid range throttle positions, to a degree that the motor will be too lean to accelerate cleanly. This is what is facilitated by the "accelerator pump" function to get it going just as you are winding the throttle slides open.
The advantages of running lean are that under low or light load the mixture is burnt more completely, giving less unburnt fuel to pass the piston rings or into the exhaust and out as emissions. This compared to a normal jetted carb that has to accelerate cleanly without additional fuel so in effect running slightly rich all the time. Benefits of less oil contamination and less unburnt fuel in the exhaust system will just make the components last longer.
I'd estimate that with the bike use it would see more throttle openings than in a car just because of having five closer gear ratios, so more times the fuel fuel is supplemented with the pump as you close then open the throttles more frequently.
The Jetting at wide open throttle has to be more close to ideal (not lean) as when held wide open under maximum load it can't be supplemented by the pump, so you get the throttle slide cutaway and needle taper designed to give you a fuel gradient that goes from pretty lean at bottom to middle, up to fully supplied at the top end. It's this relationship I don't believe you can readily change when trying to set these carbs.
What people do is respond to the lean running by lifting the needle or fitting a bigger mainjet which increases fueling across the range. So you may get the bottom and mid somewhere near to what you want, but then the top is overfuelled and runs rich.
I don't know if you can resolve this, or certainly not without a dyno run with exhaust gas temp and analysis to see where you are.
If it were me making a decesion, I'd go for that choice that Hairy has given above with the CB900F carbs as a much better starting point.
It's a much closer starting point I feel regarding engine capacity and not a bad match to the Imp motor being able to flow for that capacity (what size are you running?) as they I think originally had a rev range to about 10,000rpm with bhp around 96.
They came from the development program of Honda's RCB endurance racers and where specifically targeted at good starting over any heat range, economy and particularly well resolved response over the whole rev range. You may even find they are, even with no adjustment, very close to the demands the Imp motor requires.
royhall:
Crikey, that's the most complete answer I have ever seen on a forum. Nice one!! Also helps us F2 boys understand the system a little better. Thanks.
K2-K6:
Yeh Roy, I don't think you'd want to get stuck talking to me at a party! :) specialised interest, lean burn carbs fitted to Honda motorcycles of mid to late seventies. ;D
I do think they are very good carbs though if they are properly taken care of.
zippy!:
Absolutely invaluable information. All makes more sense now.
The engine was originally 875cc, but I have used a larger Talbot sunbeam 930cc unit for the slightly bigger capacity and stronger block. The next steps are boring out to 998 or 1040 so the 930 is a happy (ish) medium from standard 875.
I will try to add a photo of the set up. At first, the intake side of the carbs was so close to the offside chassis rail that I could only get some mesh (cut up sieve) jubilee clipped over each carb inlet. After reading some forum advice regarding the extra air flow, I lifted the needles on position on the clips.
Over winter I had the manifold modified some more to allow room for better filters. All I could fibd was some cheapo k and n style copy things on eBay. They do the job but one jubilee clip has already breached its rubber connector. I also put in different main jets. Will have to check out the size.
I will look at the 900 carbs as an option. So thanks again for that advice.
The main issue is my air leaks. I have just dropped the manifold off with the welder, he's hopefully sorting it Monday, then I can get the face of the flange skinned. It is warped by about 1mm at either end.
I must admit, as much as these things annoy I us and cause you to tear your hair out, it keeps things interesting!
Johnwebley:
--- Quote from: K2-K6 on April 29, 2017, 12:13:35 PM ---Yeh Roy, I don't think you'd want to get stuck talking to me at a party! :) specialised interest, lean burn carbs fitted to Honda motorcycles of mid to late seventies. ;D
I do think they are very good carbs though if they are properly taken care of.
--- End quote ---
I think that would be a subject on MASTERMIND ,
with no passes !!
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