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More Jetting ...


Jetting a dual-plugged 1000cc CR9.7 engine

So, here I am with my bitsa engine that has the following specs:

The first 12,000km were done in 4 weeks on two big trips and the plugs looked like this:

LH plugs, bottom plug slightly wet (oil fouled?), no gas smells

RH plugs, RH carb needle was set wrong at 3rd, both plugs smell of gas as a result, lower plug seems oil foiled (?)

The left top plug has the best appearance: heat range is correct, good tan look on the isolator, no considerable break away of porcelan, but the lower plugs are a lot darker. They seem oil fouled but oil consumption of the engine is at about 0.2l/1000km. However, when I remove the valve covers for valve adjustments I think there is a lot more oil in the covers then there was in the R80G/S or R100T I owned.

I guess I have to have a look into re-jetting and first put the information together for the different carb setting of the high compression engines:

Main Jet
Needle Jet
Idle Jet
Needle position
Bing P/NComment
'77 R100S
170
2.66
45
3rd
94/40/103 & 104Clymer data
'77 R100RS
170
2.68
45
2nd
94/40/105 & 106Clymer data
'78 R100S/RS
160
2.64
45
2nd
94/40/107 & 108Clymer data
'79 R100T
170
2.66
45
3rd
94/40/103 & 104Clymer data
'79 R100S/RS/RT
170
2.68
45
2nd
94/40/105 & 106Clymer data
'80 R100S/T/RS/RT
160
2.66
45
2nd
94/40/109 & 110Clymer data
81-'83 R100CS/RS/RT
160
2.66
45
2nd
94/40/113 & 114Clymer data

These setting obviously don't take dual plugging into consideration and I am disputing if there was a necessity to re-jet, if it wasn't for the R100R exhaust that normally works in a 1000cc engine with a CR of 8.2.

I think the engine runs rich and the grey tan of the top plug is fooling me when I look at the lower plugs (D6HA) whih are fouling. I decided that trial and error was the best way to go to get rid of the fouling. Bought a variety of NGK plugs and experimented. In the end, I decided to run NGK BP6ES plugs in the top and (10mm) NGK D8HA plugs in the bottom. The bottom plugs are just a tad too hot but there are no suitable NGK Dx7xxx available.

Resulting spark plug colors: top plugs near perfect tan, bottom plugs are light gray ...

I left the carb jetting untouched.