'84 Johnson 70 high WOT 8k rpms

Socak

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Oct 24, 2012
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Just got a 15' fiberglass skiff that has a 84' Johnson 70 on a jackplate running a 13.5x17 prop. I installed a tiny tach on the motor and now when I am running WOT the rpms were getting up to 8k and the boat wants to idle at ~1500-1700 rpms in gear. I have messed with the idle screw and if the rpms drop lower at idle it just cuts off. From what I can gather the motor might be running a bit lean causing the bad idle. Do I also need to run a higher pitched prop to cut down the rpms? The boat also wants to porpoise sometimes, can this be compensated by raising the motor on the jack plate some? Thanks and let me know if you need more information.
 

boobie

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Re: '84 Johnson 70 high WOT 8k rpms

IMHO, I don't trust tiny tachs. 8k???????????
 

nwcove

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Re: '84 Johnson 70 high WOT 8k rpms

sounds like the tach is showing close to double the actual rpms......or i hope it is!!!! yikes!
 

LongLine

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Re: '84 Johnson 70 high WOT 8k rpms

A 70 doing 8 grand won't last very long. i.e seconds...maybe just one race
 

SparkieBoat

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Re: '84 Johnson 70 high WOT 8k rpms

also make sure you have the right pole setting on the tach.
 
Joined
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Re: '84 Johnson 70 high WOT 8k rpms

I'll add a vote to everyone elses, I don't believe those numbers. I don't think your engine is capable of getting anywhere near 8000 rpm, at least not without some very serious modification.
 

mark1961

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Re: '84 Johnson 70 high WOT 8k rpms

The tiny tach is most likely fine but currently setup for a 4 stroke instead of 2 stroke....fear not, you are probably actually doing about half those numbers.....the tach needs to be set @ 360deg which equates to one firing event per revolution per cyl.

If you suspect any lean condition.....time to park it and rebuild the carbs, sooner the better.
 

Socak

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Re: '84 Johnson 70 high WOT 8k rpms

The tiny tach was worthless, I took that off. Got a real tach not reading 4500 at WOT and have gotten the idle under control to 750. I am thinking the high speed jets need to be adjusted also to increase the rpms at WOT. I also did a compression test which showed the bottom cylinder being lower than the rest (125, 120, 95). What could be causing this low compression (head gasket, piston, rings)? The air silencer cover is also not on the carbs, does this affect anything?
 

retiredfornow

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Re: '84 Johnson 70 high WOT 8k rpms

Sounds better, now. Your WOT is adjusted by prop pitch, so you'll need a lower pitch prop. I'd hold off on re-propping figures until I figured out the low compression on the bottom cylinder. You might pull the head and check head gasket leak, look for scoring in the cylinder from a broken ring. The air silencer difference would be insignificant. Good luck.
 

Socak

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Re: '84 Johnson 70 high WOT 8k rpms

I plan on replacing the head gasket and looking inside to see how everything is. It also knocks/runs really rough in the lower rpms is this a symptom of the low compression?
 

Faztbullet

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Re: '84 Johnson 70 high WOT 8k rpms

The air silencer cover is also not on the carbs, does this affect anything?
This missing can make the engine actually run leaner.....
 

Socak

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Re: '84 Johnson 70 high WOT 8k rpms

That makes sense because it seamed to be running lean. I'll have to be on the look out for one. Thanks.
 
Joined
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Re: '84 Johnson 70 high WOT 8k rpms

I plan on replacing the head gasket and looking inside to see how everything is. It also knocks/runs really rough in the lower rpms is this a symptom of the low compression?

Yes, with one cylinder so much lower that the others there's something wrong. The engine will never run right until you get that fixed. Pull the head and see what's going on. Look for a broken ring, or who knows what. No sense in playing around with changing prop pitch 'till you get the engine right.
 

Socak

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Re: '84 Johnson 70 high WOT 8k rpms

I took the carbs off this weekend and they looked to be in great shape. Then I pulled the cylinder head cover off and found all the piston heads to be fairly dirty. The cylinder walls look to be in good shape though. The bottom piston has a damaged head and top land they look nicked up from some metal possibly getting in there(I will add pictures later). What else needs to be taken off to check for broken rings?
 

retiredfornow

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Re: '84 Johnson 70 high WOT 8k rpms

The bottom piston has a damaged head and top land they look nicked up from some metal possibly getting in there(I will add pictures later). What else needs to be taken off to check for broken rings?
That damage is probably from a piece of broken ring bouncing around in there. Please post the pics.
 
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