Author Topic: primary drive train rattle - CB750F2  (Read 712 times)

Offline K2-K6

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Re: primary drive train rattle - CB750F2
« Reply #15 on: July 19, 2020, 12:42:45 PM »
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.

Offline blackeagles

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Re: primary drive train rattle - CB750F2
« Reply #16 on: July 20, 2020, 11:22:48 AM »
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.

Offline blackeagles

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Re: primary drive train rattle - CB750F2
« Reply #17 on: September 12, 2020, 10:17:38 AM »
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.

Offline royhall

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Re: primary drive train rattle - CB750F2
« Reply #18 on: September 12, 2020, 10:49:29 AM »
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.
Current bikes:
TriBsa CCM 350 Twin
CB350F in Candy Bacchus Olive
CB750F2 in Candy Apple Red
CB1300SA in Black
Triumph T100C
Suzuki GS1000HC
CB450K0 Black Bomber (current project)

Offline blackeagles

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Re: primary drive train rattle - CB750F2
« Reply #19 on: September 14, 2020, 03:21:47 PM »
thanks for the info.

 

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