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  1974 Johnson 70 hp, seems sluggish

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Author Topic:   1974 Johnson 70 hp, seems sluggish
blueboat posted 04-22-2003 03:54 PM ET (US)   Profile for blueboat   Send Email to blueboat  
1974 70hp johnson. seems to be very low on top end power. boat is a 17' with a 14X17 prop it didnt seem to have any power. I put a 13 3/4X 13 prop and it seemed to have much more pep. Is this most likely a carb issue? compression is great, starts good, idles smooth, just no ump when you nail the throttle. I dont think it is getting near 5000 rpm's i'd say more like 33500. I have a tachometer on the way but for now I am guessing.
JBCornwell posted 04-22-2003 05:45 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Hi, Blueboat,

Your boat should run in the mid 30s at 5500-6000rpm with that engine, in good tune and properly rigged and propped. My guess would be that a 17" prop is correct.

You have an engine problem. Could be running on 2 cylinders with only 3500 @ WOT (33,500rpm!!??). That engine will pretend to be running great on 2 unless you put a load on it.

Some troubleshooting is in order.

Check each plug wire at idle for a hot, blue spark to the block and note engine speed while doing so.

A cylinder that has spark but doesn't cause rpm to drop when the lead is off is a dead cylinder. See what happens if you squirt a bit of fuel mix into its carb. If that picks it up overhaul that carb.

Next, see what happens if you pump your primer bulb when under power. Picks up? Fuel pump or filter.

Let us know what you find.

Red sky at night.. .
JB

blueboat posted 04-23-2003 01:12 AM ET (US)     Profile for blueboat  Send Email to blueboat     
Thanks for the input. when I purchased it I did the plug wire test. all three had a distinct fire. I didnít look at the spark color a(I just tried not to get zapped to many times. The guy mentioned that it had been "tuned for high altitude" and from what I can tell and hope is that it's in the carbs. The gas lines are brittle, carb hoses are not what I would keep on it. change those and check the jets #'s to make sure there not the high atl ones. Any other targets? Thanks Scott

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