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Motorcycle Repair/1971 Honda CB350 K3 12K miles

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Question
Bike has free flow ehaust with OEM air cleaners with a bad stumble between 1800- 2500 RRM, high 3000RPM idle after warmmed-up would settle back to 1000 RPM. I figured CV slides were sticking and it needed an entire cleaning. Dipped carbs/parts, CV diaphram is not torn-no holes, new gasket rebuid kit, larger main and secondary jets for free flow exhaust and velocity stacks were installed. This has corrected the previous 2K RPM stumble with OEM air cleaners. Bike runs strong off bottom to 9K. Short amount of miles after these upgrades-after bike is  warmed up, LH piston does not seem to be firing- more quiet at exhaust tip and exhaust pressure much less than RH side. Compression is 128 LH and 142 RH. Lightly emery cloth cleaned points, set points at .012" have nice spark there no arching, RH plug dark with strong spark, LH plug wet/dark weak spark. Coil- Ohms at primary wires about <1.5 Ohms or less. At plug wire about >6K Ohms or less. Have I missed anything? Have the coils gone bad? Please help!

Answer
Kevin,

Compression readings should be in the 175 psi range with throttle held wide open. Honda books show 153 to 187 psi range as standard. Readjust the valve clearances and check again. If even and low on both sides, either the motor is worn out or the cam timing is off a tooth.

Ignition coil primary resistance readings should be about 4.5 ohms. I have started seeing a lot of failing coils on 350-450s in the recent past, so I suspect the LH coil is dying.

I would set the point gaps up to .014-.016"  When the points gaps are down too low, there is the possibility of the coils sharing current draw from both coils momentarily. They need to take turns, so ensure that the gaps are wider than narrower to prevent current sharing from happening. If that is happening the engine will run poorly due to lowered coil voltage.

Intermittent high idle often comes from sticking spark advancer weights, so pull the point plate, remove the advancer and clean/lube the points cam and ensure that the return springs are doing their job.

Bill Silver

Motorcycle Repair

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Bill Silver

Expertise

Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.

Experience

I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

Organizations
VJMC (Vintage Japanese Motorcycle Club) of North America

Publications
VJMC newsletter, as editor for two years and as contributing editor currently.

Education/Credentials
3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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