1978 70 hp evinrude overcharging and cuts out at high rpm

[tomkat]

was running great for 1st ten minutes then all at once just dropped to 2800 rpm and had no power,then it surged a couple of times then bogged down again.Now what don't make sense is that i can tilt the motor up and starts to run good for a little bit then drops again.motor idles fine and starts fine.compression is 125,125,126h yeah charging at 15v

 

[jimdd810]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

well first 15 volts is not overcharging. I would dissassemble the carbs and rebuild them. should get you on your way. Might need new floats.

 

[jtexas]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

Agree, that's an unregulated alternator, volts will get on up there. Solution is to use a good quality marine cranking or dual-use battery (I use interstate deepcycle), it can handle the voltage.

When you say "tilt the motor up", how high?
Is the fuel line holding pressure?

Carbs is a good guess, and chances are, you're going to end up rebuilding 'em.

But, couple other things you should rule out first:

1. make sure the fuel tank vent is open and clear
2. check for water/debris in the fuel
3. make sure the fuel is fresh
4. spark plugs should be champion QL77JC4 at .030, in good condition
5. Spark test (not a "spark plug" test). get an inline spark tester (couple bucks at autozone), set the gap for 7/16", clip it to the engine block, attach the plug leads (one at a time) and look for a strong blue spark while cranking. If it can't jump a half-inch gap in the open air without a spark plug, it's not firing in the hole under heat & compression.

If all that checks out, then rebuild the carbs.

 

[tomkat]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

my boat doesn't have a gage on the tilt and trim i would guess by eye around 10 degrees or so.Bulb will go soft while driving,had wife drive and i pumped it did change performance at all.Took air box off and overfill plug was barely screwed in on top carb.How hard are hese carbs to rebuild i am a machinist and have worked on cars alot but not boats.Is this something i can do myself or should i fork out the money?

 

[jimdd810]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

not hard at all take them off and mark them so you know which one come from where. Take pics of linkages and such so you know where they go. Dissassemble, clean and reassemble one at a time. i would recommend you get a factory service manual and youll be good. Most of you machinist types are anal about details any way

 

[tomkat]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

yeah we are but we have to be.well have to do spark test picked tester up today if it checks out going to pull them wish me luck.where can i find a manual.

 

[jimdd810]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

Some one should post a link to somewhere you can get them.
I think there is a way to get them through iboats

 

[Daviet]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

Look in The Top Secret File, there are links posted for manuals.
Get the OEM manual.

 

[jtexas]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

my boat doesn't have a gage on the tilt and trim i would guess by eye around 10 degrees or so.Bulb will go soft while driving,had wife drive and i pumped it did change performance at all.Took air box off and overfill plug was barely screwed in on top carb.How hard are hese carbs to rebuild i am a machinist and have worked on cars alot but not boats.Is this something i can do myself or should i fork out the money?

outboard carbs, amazingly simple devices. That plug at the bottom of the carb bowl? behind it is your hi-speed jet. The slow speed jet is behind the screw plug at the top of the venturi. that's it.

Follow the "carb cleaning" link in the Top Secret thread.

Fuel line should hold pressure all day. Tighten the #1 carb, might be all you need.

factory shop manuals available at marineengine.com and kencook.com

 

[tomkat]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

found the top flat head screw under air box on top carb completely loose and also the fuel line clamp on same carb wasn't tight could move line and it leaked a little.It had just rained here before i started now kill switch won't let it crank can you bypass these.

 

[jimdd810]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

should be able to unplug it

 

[jtexas]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

if by "won't crank", you mean the motor's not turning over, the kill switch has nothing to do with that.

If you mean the starter motor is spinning the flywheel but there's no spark on any cylinder, the kill switch is a likely suspect; disconnecting and taping up the black ground wire will fix it.

 

[tomkat]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

yeah read last night after previous post kill switch just grounds power pac.Thinking it my be neutral switch because it clicks at motor and battery is charged and has good connections.Heading out to start on it again.thanks to everyone for all the help hope i can return the favor someday.

 

[jimdd810]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

might be the solenoid

 

[jtexas]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

neutral safety switch interrupts 12V feed from keyswitch to starter solenoid.

Try again with a known-good fully charged battery, and clean & tight connections battery to solenoid to starter (including ground connections for battery and solenoid). About a 93% probability this will fix it.

Have voltmeter?
If the solenoid "clicks", measure voltage at starter motor positive terminal (keyswitch turned to "start"), if 12V or more, rebuild or replace starter. No sound from solenoid or bad volts at starter, measure voltage at solenoid input from keyswitch (yellow/red wire, maybe white on that model), keyswitch turned to "start". Should be very close to battery voltage, if so replace solenoid, otherwise troubleshoot keyswitch, neutral safety switch and wiring harness.

Note that you can't check hi-RPM operation without putting the boat in the water (unless you have a test tank with the proper test wheel). In the first place, on the muffs the motor is completely unloaded...it'll sound great even if it's still running crappy, but you won't know until you get beyond the no-wake zone and put the hammer down. In the second place, revving up an unloaded engine is really bad for it.

 

[tomkat]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

had bad battery connection fixed it .going to water to see if loose fuel line was the problem whe bogging down at wot.did not exceed 2000 rpm on muffs hope that wasn't too much.

 

[tomkat]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

well cleaned carbs and replaced all fuel lines ran great for couple minutes then same old problem power surges then will bog down to 2800 rpm.came back home cleaned carbs again clean as can be not trash whatsoever,took it back to lake again ran good for a few minutes then bogged down now idle is a little rough.Where do i go from here i am determined to fix this thing.

 

[jtexas]

Re: 1978 70 hp evinrude overcharging and cuts out at high rpm

1. make sure the fuel tank vent is open and clear
2. check for water/debris in the fuel
3. make sure the fuel is fresh
4. spark plugs should be champion QL77JC4 at .030, in good condition
5. Spark test (not a "spark plug" test). get an inline spark tester (couple bucks at autozone), set the gap for 7/16", clip it to the engine block, attach the plug leads (one at a time) and look for a strong blue spark while cranking. If it can't jump a half-inch gap in the open air without a spark plug, it's not firing in the hole under heat & compression.

sounds like the tank vent is closed.