Plug check...

[JoshKeller84]

Running a 1984 70 hp johnson jet with a 75 hp head and carburetors. This is pulling the killswitch while running wot. The 70 calls for 52d jets, while the 75 calls for 56d. Is these plugs running rich, or good enough? Should i drop down a jet or two?

 

[emdsapmgr]

Hard to tell. So, you are trying to do a high speed throttle "chop" in order to see what the plugs look like at WOT. Am wondering if your throttle "chop" also includes immediate shift to neutral? They look dark. Normally, a high rpm throttle "chop" will show the plugs to be a very light tan.

 

[JoshKeller84]

This is a jet motor - there is no neutral, as the driveshaft is directly attached to the crankshaft and impeller. Neutral is simply a gate to deflect the water stream

 

[JoshKeller84]

Heres a clearer pic in the daylight

 

[emdsapmgr]

Without the ability to "freeze" the crank at WOT (keep any fresh fuel/air from getting to the cyls-it's difficult to really get to good read on the plugs on this engine. From looking at yours, they show the engine is running rich (ok.) These crossflows plugs run black, and wet at idle. They actually overfuel slightly at idle. They run very light tan/dry at WOT. That's normal. The other way to tell if the engine is running lean (without using the throttle chop method, is to pull the plugs and look at the face of the pistons. If they look aluminum colored or have pits in the head face, the engine is lean. If your test run shows black pistons crowns, you should be ok.

 

[F_R]

Those plugs look fine. For whatever that is worth. Which is not much. Agree with the statement concerning too lean. But if it is too lean, you are on dangerous territory. That can destroy pistons in a few seconds.

 

[JoshKeller84]

Biggest thing im trying to figure out, is should i be running the 75 carb jets or the 70 with a 70 block, 75 head and carbs. Using my micrometer, i cant detect any difference between the 2 heads volume wise. Im just wondering if im dumping too much fuel into the cylinders at wot and causing me to lose a few rpm on the top end. I know that it starts running at 5450 rpm, and after a few minutes of running, its dropped to 5300-5350. Would running without the air box for a minute to see if i gain rpm tell me if im running rich?

 

[JoshKeller84]