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The importance of using correct spark plug

Started by Swissqp, February 01, 2021, 04:45:46 PM

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February 01, 2021, 04:45:46 PM Last Edit: February 01, 2021, 10:55:17 PM by Swissqp
So I've restored this 35 year old Honda HR21-5 and replace the damaged crank (oil leak due to multiple gouges in the shaft right where the seal sits). Sonic cleaned the carb and stripped it down, resprayed with OEM paint, etc. She started first pull after rebuild. Gotta love Hondas

But in typical fashion and before I get too big for my boots, i have to admit she hunted and surged a little bit. It happened when I took the revs down but calmed down when i got to idle. If I then slowly added thottle it was fine.

I first thought the fuel/air mixture was lean so I fettled with the pilot jet screw and set it to spec (all the way in and back it off 1 and 3/4 turns). This is not a two minute job as to access the screw you need to undo the intake manifold. Anyway made no difference. So I took the carb off and cleaned it again. That's an hours job as the fuel tank has to come off as do all the linkages. A right pain. Anyway got it back together. Started hunting again.

So I then thought about the spark. I had put a new one on and it was gapped fine. But i noticed i had used a BMR6A rather than a BM6A.....could a resistor really make all the difference? Well the answer is yes it can and it did. The resistor is for non-CDI models with points and condenser. Apparently the spark is less powerful.

So it just goes to show always use the exact spark plug!   


Hiya, I need to try this on my HR21-5. As it hunts and surges too. I have cleaned the carb and have been trying the governor arm adjustment to no avail.

I previously had a vapor lock issue too due to a split in the muffler sending heat to the carb!

Anyway I think mine is about 35 years old too. Had it for 10 years. But it is a bit like Triggers broom. A lot of second hand parts.

Love Honda's.