1977 cb750f1 misfire on cylinder 3. HELP

[CJGasMan]

Hey guys,

I'm new to forums and really am in need of help with this one.

Last week I purchased a 1977 cb750 F1 from a chap. It seems to have a misfire on cylinder 3 causing a pop from the exhaust.

I've thermal camera checked and that exhaust pipe is significantly cooler than the other 3 when this is happening.

I have:

- Checked compression which is 137psi
- New plugs.
- New coil pack.
- Checked plug caps resistance which were fine.
- Cleaned carbs thoroughly.
- Drained/cleaned tank and refilled with fresh E5 fuel.
- New fuel filter.
- Check air filter (basically brand new).

I'm now at a point of asking for help

Thanks,
Chris

 

[dirtdigger]

is your miss just at idle or throughout the rev range? I would almost bet you still have a restricted idle jet if it is doing it at idle

 

[CJGasMan]

is your miss just at idle or throughout the rev range? I would almost bet you still have a restricted idle jet if it is doing it at idle

It's through out the rev range.

It pops when riding also

 

[UK K6]

Could be the float level?

 

[Rjay]

The carby get deposits internally from particulate matter in the fuel. Residues , and oxidation from water can build up internally in the pilot jets and air jets . Also the float valve needle and seat may have a strainer screen which can block up. This can cause low fuel level in the Carby bowl.

Try cleaning the carbs internally first and see if that works .Then diagnose if the 3 Rd carby has low float level .

I’ve found the floats set at @ 26 mm height to be sufficient. There should be similar volume of fuel each bowl when you drain them .

 

[hinksy]

Given you have changed and cleaned most of it , have you checked the valve clearences ? Then I would try a leakdown test , you may have a bad valve . 137psi isnt bad , but it could be better , what were the other cylinders ?