Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: blackeagles on July 10, 2020, 12:42:39 PM
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reading on this site and elsewhere i know that the annoying rattle these bikes have tend to be associated with the carbs being out of sync. and the primary drive chain thrashing about at tickover.
i had this with the cb400f2 i have but most of the (worst of the) rattle has gone after i crudely synced them using a cheap set of gauges from ebay (but they are not very accurate and i suspect th carbs are not spot on or even that near if i am truthful !!).
having set the 750 up - tappets, ignition etc i am still having trouble getting it right and the rattle tis more than i can put up with. the main problem is that the engine 'hunts' so its prob. a mixture of sync and pilot (which is out 2 turns)
anyway, does anyone know of a mechanic or someone who specialises in the older bikes (plenty in my area who do the new stuff) in the n/w kent and se london areas- i'm in orpington so the nearer the better.
thanks
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I've no experience of mechanics local to you able to work on your bike, not so much help there :D
But from a start point you need 1 3/4 turns out for low speed mixture screws (they're opposite on F2 carbs to earlier K type) with 2 turns being too rich. Suspect you'll end up between one and a half to one and three quarters ultimately, that's if all standard setup.
I don't mind looking at it with you if you can't find anyone commercially that is knowledgeable with it. By all means PM me if you're stuck.
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I would say bring it to gloucester but it has those god awfull PD carbs on it so i dont want to know, seriously i would happily put my vac gauges on it but i will not do anything else to the carbs
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Have a word with 21st Moto in Swanley. Won’t necessarily be cheap but they’ve got some good people there.
Rob the boss has a collection for of older Honda’s including a K0 750 and last I heard a K0 Goldwing. Dave in sales has been around them for years. And if you stick your head in, you might even get to talk to Keith from the workshop. He’s done a bunch of my ‘70s stuff and given the choice, he’s the one I’d want working on anything like this.
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ok, thanks all.
k2-k6. i did start at 1 3/4 out as per your suggestion in an earlier post but i was under the impression that with these carbs you then went in or out until you had the fastest tickover (rather like most carbs) then went back weaker by maybe a 1/8 turn. doing this i get about 2 turns out! so maybe since its 'hunting' a bit i need to go back in another 1/8 turn and reset the tickover - i have to say i have tried this and to my befuddled mind the engine seemed to run rough rather than 'hunting' but that may just be me (since my hearing is not what it was)!!!! .
i will give it one more go so may be in touch (this is really driving me mad)!!
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the more i look at it the more uncertain i get.
if i remove the 3 bolts holding the cam chain adjuster housing to the engine will it come away along with the tensioner push bar spring i.e as a complete unit? - i'm fairly certain mine is not moving as a while back i undid the adjustment bolt and nut and put a rod into the end and gave it a light tap and it sounded solid so left it.
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Yes , you can remove the adjuster block from the engine. It just presses against the tensioning unit inside the barrels. They are effectively discrete components.
If you remove, free and lubricate the adjuster, then set it by squeezing the spring and locking off with security bolt, reassemble into motor, and then perform adjustment as per manual.
If it's stuck as you suspect, then it's likely the cam chain will be running loose.
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If you leave the pinch bolt done up the rod will stay in the casting, two things can happen apart from soezing
1 cam chain worn out and adjuster rod is hitting clamp screw, there is a flat milled on the rod with a vertical "wall" at each end which hits the clamp bolt when chain worn out
2 the clamp screw has been done up too tight and deformed the rod a bit making it tight in bore.
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If most of the rattle goes away when you pull in the clutch then it's probably not your cam chain that's the problem.
You could also check the primary chain wear tolerance by removing the sump plate and measuring between the tensioner bracket and sump plate mating surface.
But you need to complete your carb tune up first because that may silence the rattle.
Good luck!
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no change if i pull in the clutch.
i'm supposed to be looking after my wife but i could not resist having a look. it was slightly sticking (slight bur due to the bolt being tightened with the shaft off centre) but a bit of emery and some copper slip made it slide nicely. the bolt marks on the shaft show it touches about half way so seems there is some adjustment left.
so rattle can only be the primary chain and i still think its due to uneven tickover (you can hear a slight change in the tickover and see the tacho needle moving slightly), so i'm still looking at the gauges not reading correctly - this is very similar to the poor initial running of the cb400f i did 2 years ago and it turned out to be the slow running jets being 'cleaned' and made oversize, a new set and it runs lovely!!
may have to beg, borrow or buy a decent set of gauges just to make sure the sync is ok before i yet again remove the carbs.
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Analogue gauges can go off if they've been handled roughly, I use Morgan Carbtune which have always provided an accurate reading. Bought mine in the 1980's when they used mercury filled tubes.
https://www.carbtune.com/
Another thought is to make sure your rubber manifold stubs are in good nick and tightly secured, drawing air from that area would probably also give you a rough idle.
Hope this helps
regards
Skoti
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"this is very similar to the poor initial running of the cb400f i did 2 years ago and it turned out to be the slow running jets being 'cleaned' and made oversize, a new set and it runs lovely!! "
Definitely part and parcel of this scale of engine, just having one carb per cylinder takes us toward a very fine metering component set to run profficiently.
Could be that previous ownership has compromised the jet bores, making it uncertain for you to adjust.
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ok, thanks. well i am going to give it a little bit more thought before i give up and call for help
i have ordered a carbtune pro so i can, at the very least, check the setting the analogue ones are giving me.
this is going to sound a bit like i'm a simpleton but given that the factory used to churn out parts for carbs by the 1000's so they are likely to be almost identical...well i notice the adjustment screw/nut on the top of carb 1 has the screw head flush with the upper nut surface whereas 3 and 4 the screw is about 1/2 turn or so showing above the nut!! i'm grasping at straws but this surely can't be right??
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That is exactly what you change while synchronizing, but while you wait for the new set, why don't you put them all at the same height and see how it runs?
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That is exactly what you change while synchronizing, but while you wait for the new set, why don't you put them all at the same height and see how it runs?
yes, i did that last thing, did it help?, well about just over 3/4 of a turn out so they all look level(ish) by eye and yes the tach went up by maybe 100rpm but it still has the very slight wavering effect. i know its an analogue tach so that may be the effect of the cable but the engine still sounds like its wavering (maybe that is standard).
oh, slow running needle is 1 1/2 out. 1 1/4 revs drop maybe 100 rpm but hard to tell (tried the multi meter but the reading is up and down so much its unreadable), 1 3/4 out and revs increase maybe 100rpm, 2 out revs increase a little bit more but seems to be 'hunting' and beyond this they drop so 1 1/2 it is!!
oh, oh, i see that no3 has a very occasional fire back into the carb - hope it goes away when i adjust them.
soon find out!!
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That #3 condition may be significant.
With the mixture screws, winding one out individually should give you a response from lean to rich if you go completely outside their range, say 1/2 turn out to 2 1/2 out, which is what you'd expect to see.
What you are looking for is one of them that doesn't have that linear response, indicating that the carb is not clear for that low speed metering. If one is impaired when tested like this then it won't matter what the clean ones do as they'll not overcome the irregularities from one poor performing carb.
That #3 would be the initial suspect.
The revs so unstable suggest that one cylinder is erratic in firing, which would give you that pulsing as it chimes in/ out.
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That #3 condition may be significant.
With the mixture screws, winding one out individually should give you a response from lean to rich if you go completely outside their range, say 1/2 turn out to 2 1/2 out, which is what you'd expect to see.
What you are looking for is one of them that doesn't have that linear response, indicating that the carb is not clear for that low speed metering. If one is impaired when tested like this then it won't matter what the clean ones do as they'll not overcome the irregularities from one poor performing carb.
That #3 would be the initial suspect.
The revs so unstable suggest that one cylinder is erratic in firing, which would give you that pulsing as it chimes in/ out.
when i alter all 4 screws in/out individually the revs change up and down so that seems to be working but i should explain that the fire via 3 is very intermittent and only happens when the setting is very rich but only on this carb!
as far as the 'wavering' of the revs is concerned, i may be just expecting it to run ultra smooth since its 4 cylinders but i will (hopefully) know what is going on when the gauges arrive.
i have connected a strobe light to each plug and the fireing looks even to me.
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finally got round to sync'ing the carbs again with a carb tune.
i tried setting it at 3k rpm but its not what i would call reasonable at tick over so did it again at 1500 rpm and it's a little bit better but still not what i would like although i don't know what is reasonable for these bikes. still the revs die away even with the choke out initially but will tick over when slightly warm and it quite happy (revs do seem to raise and lower slightly) with no choke when warm but maybe this is normal!! set the warm tick over at 1200 rpm with very little chain rattle so i may have to live with it.
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My F2 does exactly the same. They run badly until you have covered at least 5 miles to fully warm them up, a bit of revving in the garage won't do it. You need to take it for a good run then do the settings with everything hot. Mind you don't burn yourself doing it though. Seriously though you cannot set these PD carbs up unless the bike is hot.
On a better note, after warming up mine runs great but it will hunt on tickover before then. It's the nature of the beast.
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thanks for the info.