Author Topic: CB400F Re jetting and carb sync but still running bad. Advise please.  (Read 6912 times)

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Offline flybox1

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compression check measures the max pressure inside the cylinder as the piston is rising before the ex valve opens.
No/low pressure = no/weak bang bang
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline Jimbo

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compression check measures the max pressure inside the cylinder as the piston is rising before the ex valve opens.
No/low pressure = no/weak bang bang

Got it...

Ill have to invest and check all cylinders

Stay tuned 
"If you've never owned one you'll never understand"

CB400F

Offline flybox1

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Seeing your black plugs earlier makes me think compression is good enough. . .
IIWM, i'd verify all plug caps, connectors and plugs were getting juice and sparking properly, then ensure carbs setting on 1/4 are similar to 2/3.  Check for blockages.
Any chance pilot jets on 1/4 are of a different brand than 2/3?  overall length of the pilot jets the same? 
Go back and clear tube 1/4 to be sure....
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline Jimbo

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Will do fly

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"If you've never owned one you'll never understand"

CB400F

Offline Jimbo

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Hey guys just a quick update before i go to work

Put the needles back to stock today and ran the bike, cylinders 1,4 jumped into life as i reeved the hell out of it ???????
did a plug chop and found that all my hard work getting them nice and chocolatey brown went down the toilet as they all came out rich and slightly carbon fouled (gutted). My plan tomorrow is to return them where they were and start again with dialling it all in hopefully 1,4 will fire so i can get them all up to scratch.

Ill keep you all posted
"If you've never owned one you'll never understand"

CB400F