Carb problems on 1970 20 hp Johnson.

[oldboat1]

Tap in the center will do it (hex bolt works well). Some use a little nail polish or something similar to make sure of the seal.

 

[heypawpaw]

While cleaning carb I noticed one of the holes in carb throat was plugged. After putting carb back on motor I checked spark and one cylinder has 5/16" spark but other will not jump more than 1/8". Took boat to the bayou for test run and after adjusting low idle rode around a couple miles and motor pushes boat 21 mph, same as the FD20, but this motor doesn't run as smooth at top end. I'm thinking the weak spark on one cylinder may be causing this. How do I diagnose cause of weak spark?

 

[oldboat1]

I would first pull the flywheel and look at the coils. If the cases are cracking, they need to be replaced. New ones are often lime green in color (but not always). Anyway, I would start there.

While the flywheel is off, check to see if the magnets are doing their job -- should be a strong pull on a screwdriver dangled in front of them.

If all OK on the face of it, I would clean and reset the points (.020) then check spark again.

(good sleuthing on the carb.)

 

[heypawpaw]

Thanks oldboat1 opening carb passages makes it easier see and clean everything.. Coil prices vary 17 to 23 dollars for OMC, Sierra, and CDI. Should I use OMC or does it matter? Not concerned with price difference as much as product performance and how long it lasts. My guess would be OMC or CDI but I don't know. Can I buy a prop bushing for this motor? I didn't seen any on parts diagrams on I Boats or Marine Engine.

 

[oldboat1]

I've had good luck with both oem and aftermarket coils. Given a choice, just buy the OMC. Don't have a good answer on a prop bushing. Guessing you might mean a thrust washer to fit up behind the prop (outside the gear case), and don't think your model uses one -- could be wrong, but would show up on a parts diagram.

 

[heypawpaw]

On first attempt to run motor after carb rebuild prop started slipping at 1/2 throttle or above. After returning boat to trailer I put engine in gear and could turn prop by hand without prop shaft turning. I'm assuming there is a rubber bushing slipping in prop because shear pin was OK when prop removed. I have two 9x10 props for FD20 and 20RL70 both have shear pins and look the same just one slips. Removed flywheel and coils look good but points on same side as starter appear to be more like .030 than .020 like other points. The round feeler gauge I use for plugs doesn't fit in points, I'll have to get a folding set. Is the key way top dead center?

 

[oldboat1]

Ah. misunderstood re prop. If the hub is slipping a prop shop could rework the prop, although not sure about cost. Used replacement prop might be cheaper. A matchbook cover might work for the points. Good to clean them at the same time.