Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: StealthDJ on July 01, 2022, 09:52:48 AM
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So after my F1's 20 year sleep, I've decided to give the carbs some attention - new airbox and engine rubbers, new boots, new jets and seals, plus the whole lot going through the ultrasonic cleaner (multiple times - the previous owner was pretty liberal with WD40 as a 'protective agent', so they're caked in crud.)
But it occurs to me that I still have, from my previous ownership, a full set of F2 carbs and the accompanying airbox.
Is it worth me fitting the F2 setup instead? I figure its newer, and slightly more technologically advanced, not to mention not having all that exposed linkage that the F1 items do. Not sure if it'd give any power advantage, but I wouldn't complain if it did.
Or will I just end up chasing bizarre mixture problems?
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The F1/K carbs and F2 carbs are completely different, even the fitment is different. There is a way somewhere on the internet where you can swap F2 for K carbs but have never seen anything about the reverse. Put simply they don't fit.
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F1 & F2 carbs are identical, Keihin 069a's
Forget that........ I thought you were talking CB550 🙄🙄🙄🙄🙄 That's my senior moment of the day.... 😂😂😂😂
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Plus the F2 has the godawful pd carbs that are an utter nightmare.
DO NOT fit new jets just ultrasonic all the old stuff, aftermarket brass is not always accurate
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The F1/K carbs and F2 carbs are completely different, even the fitment is different. There is a way somewhere on the internet where you can swap F2 for K carbs but have never seen anything about the reverse. Put simply they don't fit.
Interesting - my old F2 had F1 carbs fitted (which is why I have the spare set). It did have pod filters fitted though, so that may have made the conversion easier...
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Pretty sure the inlets are bigger on the F2.
Cheers
Dennis
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Your F2 had F1 carbs cos the F2 PD type are abysmal
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Plus the F2 has the godawful pd carbs that are an utter nightmare.
DO NOT fit new jets just ultrasonic all the old stuff, aftermarket brass is not always accurate
Unfortunately, I do need to replace the main jets at the very least - it's been fitted with 115s instead of the standard 105s. I'm not sure why (to compensate for an air leak, perhaps?) but I don't have the originals anymore, either way.
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Your F2 had F1 carbs cos the F2 PD type are abysmal
Really that bad? What makes them so unpopular, out of interest?
Irritatingly, I actually have *two* sets of F2 carbs - one built up, one in pieces. Guess that ruins my plan of trying to sell them... ;D
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i'll give you a tenner for the lot 8)
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PD carb problems
1 accelerator pump( only on 750)
2 pilot jets are press in and block for a pastime
3 pilot screws very easily damaged and getting O rings and washers seperately only from canada
4 choke mech insanly complicated
5 to split the carbs from bank you have to remove choke flaps, which have staked screws
6 only 1 groove on needles so adjustment is by shims
7 airbox to carb rubbers dont exist
8 carb to head rubbers different and expensive
9 if you leave them more than 10 days without running they gum up
10 they were used cos of the yanks ideas to lower emissions
Thats the first 10, need i go on
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You really don’t like them do you Bryan.
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Spent too much time in Mead And Tomkinson trying to get them to work as they should Steve.
Would sooner do a cam chain mod on a CX 500 than walk past PD carbs mate
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Good to know, thanks - I'll stick with the original F1 carbs and airbox.
On a related note, are most people running standard (105) main jets? Previous owner has told me that the reason for the 115s being fitted was because the originals ran too lean for EPA reasons and also because of the Dunstall exhaust. I've fitted a Delkevic exhaust which claims to use standard jetting and replaced all the intake rubbers which were either loose, hard as nails, or both and so feel like I should start from standard. But that said, I don't want to toast the engine, or waste time if no-one uses the 105s anyway...
(Yes, I know jetting is bike and situation specific, but a pointer to a sensible starting point would be good)
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Mine is bog standard
Honda CB750F – F1 Carburettor
Main jets #105
Idle/slow jets #40
Air screw opening 1 turn +- 3/8th turn
Cutaway number 2.5
Valve seat diameter 2mm
Fuel level 26mm
Jet needle setting 3rd Notch
Cheers
Dennis
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Thanks - I'll start with the standard settings then and work from there.
And a couple of last stupid questions (they're back up on the bike, I'll stop soon, I promise!)
- How do you all route your fuel lines? To me it looks like there are two holes in the mounting plate perfectly positioned for the lines to go through and then run across the top of the inlet rubbers before linking up. Only other way I can see is to loop round underneath the carbs, or go round the end of the carbs and then underneath one inlet rubber to come up and join the fuel tap (as mine was, and it looked ugly). Main problem is that I can't find any pictures of existing setups.
- The carb rebuild kits I got (I'm not using the jets, just the needle, gaskets and float assemblies) contained a small rubber o-ring which I can't identify and I definitely haven't removed any similar ones from the carbs. Anyone hazard a guess as to where it should locate? Did wonder if it was related to the air mixture screw, but I'm not sure how that'd work, or ever be retrievable.
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Here's mine ready to go.
(https://imagizer.imageshack.com/img922/5225/DakyfY.jpg)
Could the O rings be for the carb tops.
Cheers
Dennis
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…very pretty.
The small neoprene O rings go around the brass carb drain screws to seal them to the bowl…
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Stealth… the routing fuel lines up to you, best route with shortest route / least twists & turns / clear of all moving parts inc the choke mechanism. Here’s a pic of my rack as it came off the bike pre-resto; indeed the lines go through the holes you mention.
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Thanks all.
re. the drain plugs - I guess that explains it. I left them in place in the sonic cleaner after a previous bad experience when one sheared and left me with a continuously draining float bowl until I could find a replacement. I tend to just remove the float bowl if I want to drain it.
Looks like there are some differences in the mounting plates - in mine below (throttle stop on the opposite side for a start) you can see two holes in the plate in direct line the with fuel unions. I had the idea to run both pipes through these and then link up behind the carbs before going to the fuel tap. My main concern was how close to the engine block it'd all be...
[attachimg=1]
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Best not use aftermarket carb needles and needle jets, they may leave you with a horrible flat spot and hesitation as the needle profile is not usually accurate enough.
(Been there, done that and wasted cash)
Better to do the job once and do it right, use original Honda needle jet kits:- part number 16012 392 004.
Good luck
Skoti