1975 Johnson 70hp

[Neil1982]

I just bought this boat, and it dies when shifting gears. It seems to lose a cylinder at low RPM and then clears out just above half throttle. I checked the compression, and I hope my gauge is wrong because it shows 75 psi on all three cylinders. Would it even run with that low of compression? Would running a hotter plug like a QL86C help? It likes to Idle around 1000rpm, much less, and it will eventually die.

 

[tphoyt]

You need to try another gauge and hope yours is wrong. Keep us posted.

 

[Crosbyman]

did you do a trial run ? was it ok did you do any service on it like... carbs , clean out old fuel, filters etc...etc... change gear oil.... impeller

check the low speed idle circuit on the carb (s) at 1000rpm it likely feeds off the hs nozzle

check the saddle getting on a new horse !

 

[Neil1982]

One thing I have done while waiting for a new compression gauge is spray carb cleaner into the carbs. The bottom two carbs showed significant change, but the top carburetor showed little to no change in the engine when I sprayed it while it was running. What could cause that?

 

[Neil1982]

did you do a trial run ? was it ok did you do any service on it like... carbs , clean out old fuel, filters etc...etc... change gear oil.... impeller

check the low speed idle circuit on the carb (s) at 1000rpm it likely feeds off the hs nozzle

check the saddle getting on a new horse !

I have just changed the gear oil and spark plugs, I used Quicksilver Power Tune on it to try and clean out any carbon buildup and that's about all I have done to it.

 

[DeepCMark58A]

Put some transmission fluid in the cylinders, let it marinate and hope for the best.

 

[tphoyt]

Check for spark on the top cylinder to be sure it’s firing.

 

[Crosbyman]

One thing I have done while waiting for a new compression gauge is spray carb cleaner into the carbs. The bottom two carbs showed significant change, but the top carburetor showed little to no change in the engine when I sprayed it while it was running. What could cause that?

carb cleaner does nothing but clean the core air passage of the carb and thin out the critical oil coating on the engine innards...

you should do a carb take apart clean all internal passages and blow dry passages. THe small side passage feeds fuel to the upper carb drip chamber for feeding fuel while idling...clean HS jet , side channel to upper chamber etc... lots of utubes on these old style carbs

jmho if this was never done after 50 years

 

[airshot]

I would use Sea Foam instead of carb cleaner, SF has oil in it along with crud cutters and won't strip oil off critical parts.

 

[jimmbo]

I used to use Lacquer Thinner for Cleaning Carbs, it removed Dirt/Gum/Varnish and if they were left on the Carb, Plastic Parts. SeaFoam is mostly Oil and Naphtha. I have found just about anything, including Pine-Sol to be a better Cleaner. Oil in a Carb, well the Carb gets lots of Oil, as it in the Fuel,

A Semi Recommended(in other words, Do at your own Risk) way to clean a Carb, is to add a couple of Cans of Fuel System Cleaner(I used to use Combustion Chamber Cleaner) to 2 1/2 - 3 Gallons of Fuel. And run the engine at various Speeds, with a several not running Periods. I found this to dissolve most Gums and Varnishes in the various Fuel Circuits. It also runs the Risk that a Clump of Dirt/Gum/Varnish breaks loose, and clogs a Passage, starving the Engine of Fuel. No Fuel = No Oil, = Engine Damage

 

[tphoyt]

Not sure what happend to rest of my last post.
Aside from checking for spark I was also going to suggest checking the idle circuit. No fuel no oil.
When you get into higher rpm’s you may start getting fuel then. Confirming your theory of losing the top cylinder at low rpm’s.

 

[Neil1982]

Ok, so the compression wasn't as bad as I thought. New gauge readings are 110, 109, 110. Spark is good, but I have one coil pack with a crack, so I'm replacing all the coils. My adjustments to the throttle and gear cables seemed off, so I fixed those, and things appear to be better, but I won't know for sure until I can test in the water this weekend. After that, I will probably order carb rebuild kits, rebuild the carbs, and clean them out.

 

[tphoyt]

Just keep in mind if your going to run it no fuel means no oil in that top cylinder. If it were me I would just do the carbs and be done with it before running it again.

 

[Neil1982]

Just keep in mind if your going to run it no fuel means no oil in that top cylinder. If it were me I would just do the carbs and be done with it before running it again.

Fair point.