Honda BF 5A wierd problem

[bobcube]

The engine starts and runs OK except that:
1. If the cover is on, the engine noticeably slows down.
2. the engine runs badly - misfires and splutters - at high revs, with or without the cover.

I have checked the valve gaps, cleaned the carb, changed the spark plug and the fuel 'pump' seems OK.
I speculated that the motor didn't like low grade fuel so tried it with 99 octane, which made the high revs misfire worse.
The plug seems to indicate a correct fuel mixture - light grey/brown deposit.

As regards the slowing down when the cover is on, there's no indication the cover is fouling anything. The obvious cause of restricted airflow seems not to be the case, as I can mostly cover the carb air intake without causing the same effect.

Any ideas please?

 

[dac31]

Maybe a vibration/loose connection problem? Try wiggling all connectors to see if anything changes.

 

[Milgrip]

I'm not familiar with your particular engine but engine revs dropping with the cover on may be a result of piston blow by where exhaust fumes are escaping past the piston rings into the crankcase, out through the breather and mixing with the air being drawn unto the carb upsetting the air/fuel ratio and combustion. When he cover is off, the air fuel ratio is back to normal. Generally the sign of a worn engine or sticking piston rings. A compression test may confirm this.

 

[VicS]

The engine starts and runs OK except that:
1. If the cover is on, the engine noticeably slows down.
2. the engine runs badly - misfires and splutters - at high revs, with or without the cover.

Any ideas please?

This often indicates an exhaust gas leakage under the cover .

.

 

[bobcube]

Interesting!
Thanks. I must admit I'm not sure how the motor deals with the exhaust. Presumably it's pumped down the shaft?
Would an exhaust leakage be a problem with the head gasket, or something to do with the exhaust manifold... if such exists?

 

[penfold]

Have you adjusted the plug gap? I agree these are very sensitive to sucking in exhaust; the thing would barely run in the engine well of an Impala with the lid on, so we had to listen to it rattling away if we wanted to go anywhere under power.

 

[bobcube]

To conclude this thread, the issue turns out not to be worn piston rings etc. but a hole in the exhaust manifold. Seems this part is a common point of failure in these engines