Skip to main content
Collection, Preservation and Display of Old Lawn Mowers

Suffolk engine misfire under load

Hello  

My 75cc Suffolk engine misfires underload. 

I have changed the spark plug, unless I have 2 duff plugs, that's not the issue.

I have cleaned the points and have a strong blue spark 

I also cleaned out the carburettor to rule that out.

The misfire under load does get better if I adjust the main jet for a leaner mix.

I have also replaced the air filter.

Do you think it is likely to the coil? Or condenser?

The engine starts and runs fine. The problem only starts under load.

I just wonder people's thoughts as a new coil is quite expensive.

Could it be a build up of carbon in the engine ?

Thank you for your help. 

Forums

wristpin Wed, 18/05/2022

“Do you think it is likely to the coil? Or condenser?

The engine starts and runs fine. The problem only starts under load.”

Could be either but unless they are given a proper test , it will be guess work.  Have you checked the valve clearances  ?.  7thou inlet . 15thou exhaust

Series3boy Wed, 18/05/2022

Thank you again for your help.

I have check the valve clearances and they were ok. But might check them again to be sure.

I think the coil may be playing up at the engine has started cutting out when hot.

George at Villiers part sells exchange wipac ignition units. 

The misfire makes a heavy note down the exhaust if that is something you have come across before . Wondered if that would point to the problem?