cb750 carbs

[Bitsa (Ralph Wright - RIP)]

Hi all,
I still have not got her right and do not really what to do next suggestions please.
Starts like a normal 750 full choke knock it off quick runs fine.Get it hot ticks over OK rev the engine idles nicely.The thing is I can not get the air screws out past 1/2 turn.If I turn them in one at a time it starts revving as Its riching up turn back out 1/2 turn idles nicely again but the plug colour is white,lean I think.I have checked float heights numerous times and can not get it any better.Should I try 42 slow jets, that's if I can find some?The motor is all stock including carbs jets etc.I would live with it but after reading topics here and elsewhere I am paranoid about the colour of the plugs.It seems no matter what I do I can not get it to run on the rich side.
Cheers
Bitsa

[Waggles]

Tis easy to get paranoid isn't it? must admit I haven't even checked my plugs yet, I just set all jets as per manual, balanced and left it as it runs fine, might pull the plugs at the weekend and let you know what its like.

My first thoughts are:

Is the exhaust standard? are all the plugs the same? what model do you have?

Have you tried screwing the air screws in a bit, then as the revs rise, back off the throttle stop?

1/2 turn out doesn't sound quite right to me, you may have a slight blockage of the idle circuit. Do you have Hondaman's book? lotta good stuff in there about carb tuning / cleaning etc

[Bitsa (Ralph Wright - RIP)]

Hi Waggles,
Yes stock exhaust,plugs the same and she is a k4.If I turn the screws in yes I can slow her down with throttle stop but then all the screws would be fully closed.I have had them sonic cleaned and cleaned with carb cleaner several times no change.I do not have Honda mans book but have checked all topics that he has replied to on sister site and followed.
Bitsa

[Seamus]

I guess you have a 341 exhaust on the bike. Have a look at the sealing of the exhaust to the head and also the inlet rubbers. If all the carb settings are correct, then I would suspect an air leak somewhere.

Also, check rubbers from filrter box to carbs. Is the air filter a standard one or an after market jobbie (K and N) that needs to be oiled?.

Hope that helps.

Good luck

Seamus

Another thought, does it have Kehin jets and needles? as the Keyster ones are reputed to produce erratic running

[Bitsa (Ralph Wright - RIP)]

Seamus.
You guessed right 341s needles and filter oems.I think its getting air from somewhere but after wasting 2 tins of various sprays can not find a f in thing.The tubes from box to carb new.All the stuff new is DS and no aftermarket crap.
Thanks
Bitsa

[Waggles]

Point is, if, with screws fully in, plug colour and mixture is OK and she runs OK at least you know the problem is with the idle circuit.

Seem to recall from the book there are jet differences in emulsion tubes, can't remember why or what difference this makes, will try and remember to check the book this evening.

Can you take her for a brisk run then pull a plug in a lay by or something to see if the needle etc is OK? might eliminate some bits

[Bitsa (Ralph Wright - RIP)]

Yo Waggles,
Appreciate this as for a run wont get around to that until Thursday because of commitments will do what you say and pull outside plugs down the road.I have managed to find some 42 slow jets might try that after results on Thursday.
Cheers
Bitsa

[Waggles]

OK this is interesting. I always believed there are 2 types of adjuster screw on carbs, one type controls the supply of fuel to the pilot jet, the other type controls the air supply. I know the 750 carbs use air supply screw. I would have therefore thought that screwing in the screw restricts the amount of air and richens the mixture, much the same as you if I read your posts correctly. Just read Hondaman's book, he states screwing it in WEAKENS the mixture. Apparently, increasing the amount of air ( by screwing the screw out ) pulls more fuel out of the jet and forcing it into the venturi so richening the mixture.

I have only ever 'twiddled' until it runs best! The advice, therefore, seems to be turn your screws OUT by another half a turn as the standard setting is around 1 turn out. Give it a try, thats where mine are set.

I was right in that there are differences with ikkle holes, not in the emulsion tube but in the actual air screw, some have solid tips, others have small holes in the tips, you could also try changing them. The book has nothing to say regarding which type is best or what the effect is.

Hope this helps. I really recommend the book, expensive but worth it!

[Seamus]

That makes sense Waggles. If opening the screws tends to drop the idle, then the only option is to adjust on the throttle stop till it works.

I am also guessing that the carbs have been balanced and therefor not being led or hindered by one or two in the group.

Keep us posted

[BigAl (Alan)]

Hi Bitsa,

I have assumed you have re-built the carbs with 'Keyster kits' ? the needle & needle jet in these kits are for the  early CB's (size 120s)  for your model K4 they are 105.
I had a similar problem, but once I fitted the 105's from DS it sloved the problem.
Have you set the needle clip plates on the middle position?

Alan

[Bitsa (Ralph Wright - RIP)]

Alan,
Yes I rebuilt the carbs but with all from DS not keyster.Hopefully tommorrow I will give her a thrashing down the road and hit the kill switch and check the outside two plugs.I will take a pic of the plugs and post and maybe everyones comments will be aired.Again stock manifolds,stock 341s,stock airbox and filter,gas 98 octane in fact There is no pattern parts at all.The only thing maybe is the plugs in France they recommend same as the USA D8EA.The engine to touch is red hot but if you take the dipstick out and splash the oil on your hand it is Luke warm no bloody hell that's hot.
Regards
Bitsa
Will post results Thursday if all goes well.

[Bitsa (Ralph Wright - RIP)]

Hi All.
Well today I did give her a run out over 30 miles.I started off with air screws out 1/2 turn.Ran OK warmed up OK but started to jerk about at low revs pull away etc.Gave it flat out hit kill coasted to stop pulled outside plugs.Photos 4 and 5.Looked lean to me and reset screws to Honda mans advice on k4s 7/8 out.Started off all over again screamed her stop and did not seem much difference photo 11.Good thing nice and smooth now with no jerking about good idle steady but still not happy with colour or as I said before paranoid?They are a close resemblance to the Haynes colours photo 13.According to a chart I saw on here my carbs are 657a main jet 110 years 69 to 76 don't know what year my k4 is.After that its F's with 105 mains.Got home ticked over perfect same after a handfull Had a bloody good time on it really nice.Question is do I stop worrying about it and just ride her?One other thing I did hear the odd burble at the back of the exhausts nice sounding burble not what you hear on modern bikes with go faster exhausts.Any comments Ladies and Gentlemen?
Cheers
Bitsa

[Spitfire]

Looks a bit sooty (rich) to me, also what throttle opening did you have before doing the plug chop, were you wide open (on the main) ?
Here's some interesting stuff.
http://forums.sohc4.net/index.php?topic=5410.0
This is what I am getting at the relationship between jets and throttle openings.

More stuff


Cheers

Den

[BigAl (Alan)]

Hi Bitsa,

Looks a little sooty to me, bit if its running as you like it after 20/30 miles  why worry!

Anyhow, the Haynes manual is based on the early CB750's upto K1...possibly K2 the exhausts are the (HM300) pattern and carburation jetting is 120/110
The K3 onwards there where a number of changes due to emission controls in the USA, carburation jetting size (105) advance and needle clip adjustment,also exhausts changes (HM341) style with 3 or 5 or 7 baffles...?
My CB750 book by Mark Paris page IV-8 the modifications through the K2,3,4,5,6 are well worth a read.
Question, are you sure your bike is a K4? the 657A carbs are the earlyer type as fitted to the K1 with HM300 exhausts?

Alan

[Bitsa (Ralph Wright - RIP)]

Spitfire,Big Al,
Thanks for your replies yes flat out for plug chop.I have checked engine number and it is a k4 motor but the bike was a k6 but the cylinders were shot.It was cheaper to buy a low milage motor than re boring in France.The carbs that I am using came with the bike on a k6.Maybe PO changed them don't know.As Al says I think I will live with it as she goes very well made me feel 19 again.Last thing to do in winter is to fix a poxy tiny oil leak in between 2 and 3 cylinders.
Thanks again
Bitsa