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TOPIC: 1975 70 HP Evinrude----need help

1975 70 HP Evinrude----need help 2 months 3 days ago #140931

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Problem solving guru's!! My 75 70 HP Evinrude seems to have issues with idle. I replaced 2 cracked coils. Replaced the water pump impeller. The previous owner said the carburetors were rebuilt. It is as though the motor is loading up due to loss of spark at idle. Never stalls. Starts easily. Please watch the videos and chime in! I just do not want to chase too many rabbits on this old motor. Cream puff. Never seen salt. Hopefully its nothing major

www.youtube.com/playlist?list=PLOPfsvBhKM5WSnm9L-xjEDiJ1kGTb_Iuo

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1975 70 HP Evinrude----need help 2 months 2 days ago #140933

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Check for spark at idle. You can use a timing light for that. It'll be bright enough to see on the water. You should be idling in gear to simulate the load. Or idle in a barrel if you have the facilities, but in the drink is better.

Don't assume the P.O. (or whoever serviced the motor) knew how to work on the carbs, they may have been improperly assembled. Here's a diagram:

www.marineengine.com/parts/johnson-evinrude-parts.php?year=1975&hp=70&model=70572B&manufacturer=Evinrude§ion=Carburetor

#27 in the diagram is the idle jet (aka "orifice"), .030". #28 is the optional size, .033", which I'd assume would give a richer mixture. You can pull the idle passage screw (#26) and washer, then the idle jet just unscrews all the way out of the carb. Take care to use the correct-sized flat-blade screwdriver and use caution when unscrewing, so as not to damage the brass jet.

Pull the airbox lid and spray pre-mix into a carb throat while idling in gear, and see what happens. If that cyl picks up, it's not getting enough fuel. If it dies out, then recovers, it's probably ok. If it does nothing, you got problems!

You may have to pull the carbs and check for proper assembly; leaking float needle, bad float height, missing gaskets (like #39 "nozzle" gasket; seals the float bowl & emulsion tube and if missing will cause Big Time running problems!), etc.

Leaking or bad reed valves can cause issues, place a mirror (or equivalent bright shiny object) near the carb throat and if the carb is spitting fuel on the mirror you'll see it. That cyl has a reed valve sealing problem. You can pull the carbs & remove the entire reed block assy on this powerhead. Lucky it's not an old Merc!!

These don't idle well if cylinders are worn. Check compression but better yet do a leakdown test. Relatively inexpensive leakdown testers are available or you can make your own.

www.amazon.com/OTC-5609-Cylinder-Leakage-Tester/dp/B0030EVL60?SubscriptionId=AKIAILSHYYTFIVPWUY6Q&tag=duckduckgo-d-20&linkCode=xm2&camp=2025&creative=165953&creativeASIN=B0030EVL60

If a compression check looks good (all cylinders within 5 psi of each other would give good confidence), I wouldn't spring for a leakdown tester unless you've exhausted all other possibilities. If the compression is off quite a bit, it's not gonna idle well regardless.

Hope that helps, keep us posted...............ed

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1975 70 HP Evinrude----need help 2 months 2 days ago #140934

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Thank you Ed! I will check all. I do have a leak down checker.

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1975 70 HP Evinrude----need help 2 months 2 days ago #140936

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I unlocked the rest of the videos. youtube sort of crashed on me last night. 7 vid's in all

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1975 70 HP Evinrude----need help 1 month 3 weeks ago #141017

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I think the top carb is the problem. After it gets warm it seems like it wants to lean out
Spark seems consistent with a timing light check all three cylinders when I hold a mirror if from of each carb throat there is no spitting of fuel. Only the foos out of the top carb when it appears to lean out. I still have to check compression. The cylinders are consistent with one another meaning when you crank it is not rolling over a weakened cylinder
I apologize for taking so long here but y’all know how life goes!! Any added insight is appreciated and I do appreciate the reply already received
Thank you
Jim

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1975 70 HP Evinrude----need help 1 month 3 weeks ago #141019

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Much better now! Lake test tomorrow

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1975 70 HP Evinrude----need help 1 month 3 weeks ago #141020

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It sounds better, what did you do to it??

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1975 70 HP Evinrude----need help 1 month 3 weeks ago #141021

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I started with the top carb pulling the idle jet screw. I inserted a nozzle (tiny red straw) into the jet sprayed carb cleaner and let it sit for about 30 minutes. Sprayed again let it sit for another 15 then replaced the screw and fired the motor up. Let it warm up then put it in gear. I was much better. Farted once. So after running for a bit I pulled the hose off and let it run out of gas as I would do end of day. Then I repeated with the top carb and for yucks I did the same on the middle and bottom. Let it soak for a while then fired it up again letting it warm up then running it in gear for a while. That’s was it really. Hopefully on the water she acts as she did here in the barrel. I hope that does the trick. It’s a real clean motor. I just wish it had trim and tilt my shoulders are shot. Someone said put a cmc plate on the boat another guy says that head will fit on s newer mid that utilizes the better trim and tilt.
Let’s see what tomorrow brings!!

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1975 70 HP Evinrude----need help 1 month 3 weeks ago #141022

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It's nice when the "quick and dirty" carb clean works! I can't count how many I've done like that, sometimes it works, sometimes it don't. Always nice when it does!

G'luck on your Sea Triials...........ed

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