1968 40HP Lark spark plugs

[jai070404]

I have read in these pages that I should use J4C plugs. I assume these are Champion plugs and I will not use Champion. Is there a NGK equivilent plug?<br />Thanks.

 

[Paul Moir]

Re: 1968 40HP Lark spark plugs

I know. I hate Champion too. But I use them in my Johnson.<br /><br />NGK B7S is the equivalant, but it's near impossible to find. You can use the very common B6S which will also help your low speed a bit, since it's a hotter plug. I have a set of these as well.<br />The (very common) B7HS has too long a reach (threaded portion), but I have seen those in a few big twins. Wouldn't do it myself though.

 

[cblguy4u]

Re: 1968 40HP Lark spark plugs

whats wrong with champion plugs?

 

[jai070404]

Re: 1968 40HP Lark spark plugs

They are considered not much better than bin fillers by many over here.

 

[jai070404]

Re: 1968 40HP Lark spark plugs

Sorry, should have elaborated. They are known to cause back firing and occasionaly split in two. Incidentally, the plugs I removed after I got the motor were NGK B4LM. Does anyone know how these would have performed?

 

[Paul Moir]

Re: 1968 40HP Lark spark plugs

It's way too hot. Could have caused pre-ignition. You might want to shine a flash light down the spark plug hole and make sure the piston crown doesn't have a crater in it.<br />The reach is also too far (7/16"). The B6S length is 3/8". Not as bad as the B7HS which is almost 1/2" though.

 

[jai070404]

Re: 1968 40HP Lark spark plugs

How about Bosch WR9EC or W7EC?

 

[Paul Moir]

Re: 1968 40HP Lark spark plugs

Bosch W7EC seems a just little hot to me. You can try it though, just read the plug after running wide open for a minute. Kill the engine without slowing down. If it's white it's too hot. A W5EC (if available) seems better to me.<br />I'd stay away from the WR9EC or any resistor plug with the OMC magneto system. That little bit of resistance from the EMI supressor has caused mid-range misfires for some. It's for the same reason you want to stay away from automotive 'supressor' spark plug wire.<br />I have no experience at all with these plugs. Tell us what you find out!

 

[jai070404]

Re: 1968 40HP Lark spark plugs

Went to the nearest dealer. He had Champion J4C and NGK B7S both in stock. He said I was very lucky. Got the NGK B7S. Thanks to all who helped.