Topic: K1 engine with F1 carbs - what jets?

[nancy]

Ok the CB750 is a mixed bag of K's and F's. Late K2 frame number and a K1 engine number. I've got it running nicely enough,..but have been vexed in trying to improve fuel economy. After a(nother) carb clean I rejetted up from 105 to 120 and it ran like a rocket on a short blast. I did a bigger ride today and did another plug check. Oh oh. 1 & 2 are lean and white on centre core area and 3 & 4 are black sooty rich. I figure I have a fuel routing issue with 1 & 2 as the fuel line is not dropping down on an angle nicely - it's running level across before I drop down. So - I hope I can resolve that leanness to 1 & 2.
But what the heck is it with 3 & 4? So I looked closer and found my carbs are a later spec,..F1 generation. So if my engine (K1) is spec'd for 120/40 jetting and the Keihen carbs are spec'd 105/40,....what is the correct answer? Is there one? Do I choose the jets as spec'd for the carb,...is there any other internal diff to these carbs between K1,K2....F1, apart from the jets and needle position? Is the carb's spec the meaning of life?
Needle clip is 3rd position DOWN. Should I leave the 120 mains in and drop the needle down 1 posi (nasty job)? I'd love to save the planet and not waste fuel and see brown/grey on all plugs..but 105's on 3rd clip were not doing it for me. I resorted to illicitly backing my idle screws out 1.5 turns and got a reasonable plug - but a poorer idle/start. Foo.
Mark

[nancy]

Forgot to mention: the carbs are stamped as 064a. So according to carb table - should have 105mains, 38 slow, needle position 2. Does anyone have any comment as to whether I should stay exact to the carb spec - when the carb to actually fitted to a K1 engine and not an F1?
Cheers
mark

[BeSeeingYou]

I don't think the engine matters that much for jetting.  What matters more is what exhaust and airbox are you running.  The stock jetting for the F0/1 is 105/40 but the F carb has a slight difference in the emulsion tube and the airbox is different from the early K bikes. It also runs a 4 into 1 exhaust.  If running the stock K airbox and exhaust I would jet it for the K1 spec.  The improvement in the emulsion tube of the F0/1 carb tube was to get rid of a flat spot off idle from what I have read.

[Terry in Australia]

I'm not sure why mate, but most of the CB750K bikes delivered to Oz had 110 mains, I've got several sets here and when ratting them for the jets, the vast majority had 110's fitted. If I was you I'd (apart from migrating to Oz, of course.... ) re-route your fuel line for 1 and 2, and install some 110's, 120's seem a tad too rich for a stock engine with stock (?) airbox. Have a play with it at both city and highway speeds to make sure it will run smoothly right across the rev range, do a plug chop and adjust as necessary. Cheers, Terry.

[nancy]

Thanks guys. Yes - I have stock airbox from K1/K2 and 4 x HM341's. The bike arrived with a 4-into-1 piece of junk exhaust and the standard airbox. I swapped the pipes out right away but left the carbs as received - wit 105 mains, 40 idle and clip no 3.
It has run too rich and gets about 32-33mpg on USA measure - according to Andy750's calculator.
I have re-routed the fuel line already and cleaned out the tank again - pretty good really. Fuelcock was clean enough.
So, I thought I'd run it today without any changes so I get the same plug condition across. Assuming these are all sooty, I thought I'd pull the rack and alter the clip setting to number 2 - to lean it that way. If still rich, I can replace the 105 mains and lean it again for top range.
Comments? I'm about to go "shed" for a couple of hours.
PS: Terry - I'd love to move over there,...weather here is stink!

[nancy]

Re the airbox - while I have a stock airbox - it is labelled 750F - is this indicative of a later generation Honda airbox for the F1. I had observed this before and dismissed it as some wierd way of Honda marking these - not realising that I had F1 carbs at that time...

[Terry in Australia]

G'Day Mate, every airbox I've got in my garage is marked CB750F, and most of them came off K's, so I think that's just a Honda idiosyncrasy. The house next door to mine is up for rent, do you want me to call the realtor? Cheers, Terry.

[BeSeeingYou]

I wonder if the air box housing is the same thus the labeling but if you look on the bikebandit website at the parts diagram you will see there are some differences.  You could pinpoint which one you have for sure.  Check your float level too as this will have some bearing on this.  Pollution control was starting up around 1975/6 so I think the later bikes run a bit leaner than the early ones.  It seems odd that yours runs rich with the 105/40 jets.  What plugs are you running?  Make sure all the little holes in the emulsion tubers were clear during your cleaning process?

[Markcb750]

I have a set of '75F0 carbs on my 76K6.  he mains on my K6 where marked 110. the mains on the F0 carbs are marked 105.

I also have an Airbox from a K7/K8 with the breather and water separator connected to the valve cover, which functions as a positive crankcase ventilation system. K&N filter.

My bike runs well on the 105s, slightly less pull then the 110s when I get on it.

With the 105s in the plugs are a nice light brown. much better then the 110s which consistently made the plugs carbon black.

All Airboxes I have have 750F on them, even the original box on my K6

[nancy]

OK - I have a K series airbox - no breather-hose connector on top.
I have just pulled the rack and reset the clips to position 2 down. Checked all emulsion tubes are in tip-top shape. Did a float bowl contents check B4 I pulled the rack off and pulled each cover off and tipped fuel into a shot glass and marked it with a marker pen. Found pots 1,2 & 4 to be within 1mm but pot 3 was down by about 3mm. Adjusted float tangs again - found that 3 was out. DANG & BLAST!
I've left the 120 mains in and will refit and test run tomorrow.

[nancy]

Thanks for all the feedback guys - I have this licked. Pulled the carb throttle slides and whipped out the needles...and bugger me - all the retaining screws came out easily! Reset the clips to 2nd notch from top to drop the needles. Left the 120 mains jets in. Found 1 throttle slide had a scrape - brass showing through the chrome. So swapped in a newer one. All back together and test ride 60km - plug chop of a sort - did a 110 to 120kph stretch of 5km up to my front gate and shut down at the front gate. Found all 4 plugs nicely toasted grey/brown on the electrode area. Will do another sunny ride today and try and get a measure on MPG to see if it's using less fuel. I seem to have stuffed up the throttle cables and had to force the handgrip back solidly to get rpm to drop. Damned tricky when cornering and changing gears. Back to the shed today for an hour. Was OK till I put the tank on - so I guess it's a routing issue. Comes with age.
Regards
Mark.

[Terry in Australia]

Well done Mark, congrats on your self education achievement!