1976 Evinrude 70hp running problem

[Battered&Fried]

We've been having a peculiar problem with this old engine. It cranks up and idles normally, then as we increase throttle it gets a throatier sound, and right before we get on plane it drops in pitch and bogs on down, sometimes shutting off. Now the odd part about this probelm is that it doesn't do it consistently. We'll crank it up and speed around, with it running perfectly, and after 1 minute or 30 it will evintually start dropping and we'll fall off plane. In the bogged down state it sounds like a completely different engine with a mild vibration, any attempt to increase the throttle only bogs it down more, and it also WILL NOT choke dead in this state, very wierd. The carb has been rebuilt, fuel mixture is fine, and running a bottle of fuel cleaner through it sort of fixed the problem for one outing only. Any tips or help will be appreciated.

 

[OBJ]

Re: 1976 Evinrude 70hp running problem

Welcome aboard Battered&Fried!!

Gonna' axe a few basic questions.

Compresson good?
Good spark to the holes?

When you rebuilt carbs, did you use new OEM kits and soak the carbs out, followed up with choke and carb cleaner in a can or compressed air?

Float levels set level with the carb inverted?

Fuel lines good....still plyable? Hard fuel lines lose their grip and suck air.

Carb/ignition link and sync done after carb rebuild?

Fuel tank vented?

Seeing that it won't choke out, just off the top of my head it sounds like fuel delivery from the tank to the carb....in between maybe.

Lotta questions I know but it'll help us better to help you.

 

[jtexas]

Re: 1976 Evinrude 70hp running problem

just a nuther couple questions - after this happens, does it start back up and maybe run good for another minute or so, or is it pretty much done for the day?

how old is the fuel? any chance of some water in there? or some other debris that might block the inlet tube then fall away after she shuts off?

is the fuel line still pressurized after it bogs or do ya have to pump it back up?

can you get the rpms back up by pumping the primer bulb while she's bogging?

 

[OBJ]

Re: 1976 Evinrude 70hp running problem

Think we got all the bases are covered jtexas...8)

 

[Battered&Fried]

Re: 1976 Evinrude 70hp running problem

As for the carb rebuild, not sure of the technicalities because it was handled by a local dealer who took it to a local river, and ran it after he finished. He said it ran fine, we got it back and after about halfanhour the problem started again.

'Most' of the fuel lines have been replaced within the last couple years, using champion plugs with good spark, pumping the bulb while running has almost no effect on the problem (tried it on the water yesterday), and the bulb wasn't very firm at all when we tried it while running. As I said earlier, the problem seems completely random.

We ran for 8 minutes to our favorite hole yesterday, completely fine. 5 hours of fishing we cranked up, it idled normaly, and when we opened on up the bog just kicked in and we fell off plane. It ran good one more time that day and fell off again in a sharp turn at wot, never ran the same after that. We always unhook the gas line and run the fuel out of it in case it's schallack (sp) or something. We're still gonna check a different gas in a different tank, and maybe more of that cleaner, but I don't know...

 

[OBJ]

Re: 1976 Evinrude 70hp running problem

You may have "something" floating around in one or more of the float bowls. Had a similar thing a few years back. Fellow with a late 70hp. Would bog similar to yours on occasion. Got into a long conversation with him and found that while on clean water, no problem. Soon as the water would dirty up (chop, wakes and/or waves) the engine would bog.

In the bottom carb, I found a splinter of a core plug. It would get sucked up against the high speed jet and restrict fuel flow. The boat bouncing around would stir up the gas in the bowl and finally the splinter would suck up into the jet.

Have you noticed what the rpms are before and after the bog?

The fuel lines that have not been replaced, are they hard and dry...not plyable?

Something you can do.....get a spray bottle with some premix fuel in it. Remove the carb cover to expose the carb throats. Take the rig out and when it bogs, spray some premix into each carb one at a time and see which causes the engine to pick back up. Have a friend along to drive.

 

[Battered&Fried]

Re: 1976 Evinrude 70hp running problem

Fuel lines, for the most part should be fine, I'll fill them over tomorrow. For the rpm's, I don't have a tach, but from the sound it's a very noticible drop in engine speed, ( it isn't anything like prop slip and the like) Next time we'll have a new tank with fresh gas for sure, and I'll try spraying the fuel mix in the carbs themselves. Sadly, it will be awhile until the next time I can get out on the water anyway. Thanks for the help. Any other tips or solutions will be appreciated.

 

[OBJ]

Re: 1976 Evinrude 70hp running problem

I can understand BF....winter can really bite. While mulling over the problem, consider a tach for your rig. RPM's tell a whole lot about an engine. Keep in touch.

 

[xtraham]

Re: 1976 Evinrude 70hp running problem

check the fuel line on the bottom carb, the air silencer snout will pinch off the fuel line, not saying thats your problem, but I have the same engine and had the same problem, I had to cut the silencer to make clearence for the fuel line to the bottom carb............

 

[willamettejeff]

Re: 1976 Evinrude 70hp running problem

Had similiar problem with my 1978 70hp Johnson and it turned out to be the power pack. Would run fine for the first 45 minutes then the engine would just quit or if I caught it soon enough it would idle. Would start right back up, but would only then go to 3000 rpm and quit right away. Keep using restarting and then would only go to 2000 rpm. Shut the engine off for 5 minutes or so and it would run fine again for 30 minutes or so. Replaced the power pack and the motor has been fine now for 2 years. Just something else to consider if no problem found with the carbs.

 

[mikesea]

Re: 1976 Evinrude 70hp running problem

i go with your dropping a cylinder,so many possibilities, could be among the already listed ,a coil going bad,as it heats up,could be leaking,or the primary (the small wires to the coils) dirty contacts,or the secondary,which is your plug wire,may have rust on an end or more,plug wire might leak spark ,grounding out every now an then,check all the connections for clean and tight,spray some contact cleaner in the plugs for coils ,looking for corrosion or rust,also be sure the grounds to coils are good,if not that,I go for the carb problem,bowl might not be refilling at times for some reason,dirt,bad needle or seat,which can be bought and replaced.the wire checks should be done and its free if you do the work

 

[OBJ]

Re: 1976 Evinrude 70hp running problem

Your right Mike....so many possibilities...

Add a timing light to the tool list next out B&F. Good way to see if it's hitting on all 3 holes at all throttle settings.

Lotta times troubleshooting is finding out what it ain't. Cancel out all the "what it ain'ts'" and what's left is "what it is"........right on!!8)

 

[Battered&Fried]

Re: 1976 Evinrude 70hp running problem

I'm deffinately appreciative of all your help, and your recommendations will be considered. Glad I found this sight, Merry Christmas.