1978 CB750 F2

[blackeagles]

I have been trying to sort out a problem with my CB750F2 carbs and was recently told they are PD42a but I have just noticed mine are stamped PD41B.  Anyone know what, if anything, is the difference and will it affect setting them up as per book i.e main=105, slow running=35 and a setting of 1 3/4 turns out etc??

[Bryanj]

Dont think the settings vary, needles and adjustments do.
Both are a real PITA unless bike used at least weekly, the press in pilot jets block for a pastime.

[blackeagles]

"the press in pilot jets block for a pastime" - I guess I am lucky as when I took the carbs off and 'fired' cleaning fluid into all the jets they were clear!!

[K2-K6]

The A version is F2 fitment,  B is for K7.

They are paired in one manual from Honda but obviously the engines differ in spec.  Valve size,  cam,  compression ratio differ between the two so they seem to respond to that spec by slight carb differences.

Mains are 105 idle screw 1 3/4 out (F2 PD42A) and 110 for K7 B fitment with 1 1/2 out for idles.

Unsure of what they changed between A and B spec but thought it was different cutaway on throttle slide to go with lower rev / torque curve on K7 versus the F2.

They don't state in their manual the idle jet size,  or if different needle profile is fitted to them.

[blackeagles]

ok, so I seem to have the wrong carbs on the bike.  I will have to take them off again and check the sizes of the jets (which I did not do the first time) when I have time

[blackeagles]

ok, a sort of update.

since the pd41b and 42a bodies are the same (and given that I seem to have the wrong one on the bike) I have taken them off again and main jet is 110 and not 105 as its supposed to be (but I guess its not enough to matter) and the slow running is correct at 35 - I cannot see any numbers of anything else. 

yes, k2-k6 was correct about a possible air leak, I sprayed some carb cleaner around the carb to head rubbers and the engine speed up and slowed when the liquid evaporated (although I do not know if it was one or all rubbers!!)  any chance there is a sealant I can use or are we talking a new set - any suggestions from where as they seem to be quite expensive ?

the other thing which is more of a problem is that I notice when I pull out the choke knob the flaps close but the little screw that is supposed to contact a cam and raise the revs does not even touch the cam!! even with the screw in a far as it will go.  either the cam is worn or the head of the screw that contacts the can is badly worn - I have to say they look ok but I am not that sure as I do not know what a new one looks like for comparison.

I am toying with the idea of drilling out the bit which contacts the can and fixing a small screw into it to close the gap 'hopefully' make it work 

[K2-K6]

Looks like you've just got a straight swap to K7 carbs complete with jets then.

It doesn't look much of a problem as Honda were chasing horsepower figures on the F2 in competition with other manufacturer bikes that were coming onto the market at the time.  It's setup to run higher rpm peak than K7 which was orientated towards slower cruise style riding.  You can assess the running once you get it going competently.

Carb rubbers,  I'd not be encouraged that there is much of any choice regarding replacement and if not fixed they will run you raggedy with setup problems.  Maybe some of the F2 owners on here could point out any other route for this.

I know what the choke / throttle lift does but don't have example to look at right now.  Manual drawings don't show exact details of that screw but do request greasing of that part for assembly and maintenance.  Presumably the screw replacement would reinstate the lift function, so worth considering.
It's not a show stopper if the throttle doesn't lift,  it just means it won't reliably run during choke phase while cold starting so you'll just have to modulate revs yourself to avoid stalling.
It should ride fine otherwise, as choke flaps will make it respond ok to riding.  In reality when they were new you'd want to get it off choke as soon as possible as riding at slow speed the elevated rpm use wanted to push it along too fast.

[blackeagles]

Looks like you've just got a straight swap to K7 carbs complete with jets then.

It doesn't look much of a problem as Honda were chasing horsepower figures on the F2 in competition with other manufacturer bikes that were coming onto the market at the time.  It's setup to run higher rpm peak than K7 which was orientated towards slower cruise style riding.  You can assess the running once you get it going competently.

Carb rubbers,  I'd not be encouraged that there is much of any choice regarding replacement and if not fixed they will run you raggedy with setup problems.  Maybe some of the F2 owners on here could point out any other route for this.

I know what the choke / throttle lift does but don't have example to look at right now.  Manual drawings don't show exact details of that screw but do request greasing of that part for assembly and maintenance.  Presumably the screw replacement would reinstate the lift function, so worth considering.
It's not a show stopper if the throttle doesn't lift,  it just means it won't reliably run during choke phase while cold starting so you'll just have to modulate revs yourself to avoid stalling.
It should ride fine otherwise, as choke flaps will make it respond ok to riding.  In reality when they were new you'd want to get it off choke as soon as possible as riding at slow speed the elevated rpm use wanted to push it along too fast.

there seem to be plenty of carb/head rubbers advertised for the sohc k series including the k7 but I don't seem to be able to see many for the sohc f2 (most seem to be the wrong shape so may be the later f2)  but since the 1977/78 k7 and f2 are effectively the same bike I will assume they use the same rubbers - need to check the part numbers!!

as far as the choke screw is concerned, I have sent off for one although I have to say the picture of it looks the same as the one I took off (a close inspection seems to show the screw is almost like new and although the mounting was bent and I have put it back straight but the rest looks correct) but I will compare otherwise I will be fitting a screw to the adjuster screw to bridge the gap - i'm going to make this work if it kills me!!

[K2-K6]

The listing for K series may be inaccurate.  Up to K6 there is a match I think,  K7 and F2 should be the same and not interchangeable with previous K and F1 types.

So you are definitely looking for dedicated K7 and F2 rubbers.  Also they "modelled " them into K8 F3 certainly in USA and possibly Europe for reference.

There is,  as you found out,  the later F2 that's completely different engine and carbs.  Deriving from CBX 750 of early 80s it evolved into F2 with change to twin shock frame,  also Nighthawk model too.  No use to you at all regarding any spares.

As a temporary running solution you can make a set to get it going from car radiator top hose cut to length and alignment.  Could give you breathing space while you search correct items.
1991 Range Rover 3.9 V8 is one long hose with various bends formed in it that could be of use.

[blackeagles]

thanks again.  I have found a couple of charts that show a '78 f2 frame number should start from 2k and the '77 one will have a frame number starting 1k, as mine so it must be a 77 model (there are bits of information on a frame sticker, one of which starts 77-nnnn), i'm mentioning this as another list I have found shows the 77 f2 had a 41b carb with the 42a introduced for 78 so other then the main jet being a bit larger than original i'm 'ok' - but from what I've read the records were not well kept back in the day so who really knows .

as far as the rubbers are concerned, this project was supposed to be my 'winter' project !!!    so i'm not in a great rush to find a set - I notice my ones have 410 stamped on them (same as the middle number of the part) along with the pot number so I will just keep looking until I either find a set at a 'good' price or use a k7 set which I guess will have 405 stamped on them if original.

[blackeagles]

I took off the carbs yet again along with the rubbers, which were not as hard as I had expected, and gave them a good clean internally and refitted it all and there seems to be no air problem now.  I have tried to sync. them but my 'kit' is one of those very cheap ones and only 3 seem to work (one just goes back to zero when adjusted) so when I sort this out then maybe I can get it finished before winter!! - which may mean a little honda to play with over winter - not telling the wife of course.