Motor idles fine but no power...

[lucabrazi]

I've got a 74' Evinrude, starts right up and idles fine, but when I give it some throttle it struggles and I get very little RPM increase. In fact I can give it full throttle and it chugs and tries, but only increases the RPMs a little. Doesn't stall or quit, just no power.

I'm guessing maybe it's not getting enough fuel? I checked the little screen in the filter and it looked clear. Any thoughts would be appreciated. Thanks!

 

[rjezuit]

Re: Motor idles fine but no power...

Could be ignition related. Are you getting a good spark on all cylinders? Points, coils, condenser, wires, plugs could all be suspect. Rick

 

[F_R]

Re: Motor idles fine but no power...

It almost certainly is not running on all cylinders. You didn't say what model it is, or even how many cylinders it has, so it is hard to give an intelligent diagnosis beyond that.

 

[lucabrazi]

Re: Motor idles fine but no power...

The motor is an 1974 Evinrude 70 hp 3 cylinder. Model# 70473M

New Spark plugs and wires. An inline spark tester shows spark running strong to all 3 cyclinders.

With water muffs in the driveway the RPM's seem to increase fine, engine doesn't seem to bog down. I realize this is without resistance...so does that suggest wrong prop angle? Do I need to play with the air/fuel mixture?

Thanks all

 

[jtexas]

Re: Motor idles fine but no power...

on muffs with no load on the engine, even a crappy running motor will sound great.

Never rev a motor that's running on muffs--read here about thermal runaway:
http://forums.iboats.com/showthread.php?t=430070
and here:
http://forums.iboats.com/showthread.php?p=2871136

you are experiencing a classic symptom of dirty carbs. But before spending any money on parts, do a compression test first, and report your results.

compression should be about the same on all 3 cylinders, with the lowest being within 10% or so of the highest.

Make sure the fuel is fresh, no water/debris in the tank, the fuel line holds pressure and the tank vent is open & clear.

If all of the above checks out, rebuild the carbs with new carb kits. follow the "carb cleaning and rebuilding" link from the "top secret file" at the top of this forum.

Get yourself a service manual, I highly recommend the factory shop manual, availabe at outboardbooks.com and marineengine.com has saved me literally $thousands.

 

[bmstang]

Re: Motor idles fine but no power...

I had the exact same problem on my 235 did a compression test and found 2 dead cylinders. It ended up being a blown head gasket replaced it and it runs great. Mine would not go above 1900 under load at idle it sounded great good luck.

 

[lucabrazi]

Re: Motor idles fine but no power...

Thanks for the thoughts...one more piece of data:

Took it out on the lake just now. I moved the choke from the "auto" position to the "off" position. The engine runs better...not great; it takes about 30 seconds to plane (and the exact right throtle position)...but this is a marked improvement over the trolling I was doing before.

Allegedly it had a clean comprehsion test a month ago- although this was performed by the guy I bought it from, so who knows

BTW, I am in Colorado at 5500 ft. Does the improved performance with my choke adjustment perhaps suggest it is running too rich?

Thanks again-

 

[bmstang]

Re: Motor idles fine but no power...

I would still do a compression test just to eliminate that thought that was the last thing i tried when it should have been the first now i know. It's not hard to do and it doesn't take much time give yourself a piece of mind then move on.

 

[bigpoppakdog]

Re: Motor idles fine but no power...

This past friday my motor(1970 115 evinrude) suddenly lost power at full throttle while pulling a couple tubes. Thought it was spark plugs. Changed those out(needed it after three years of not changing them) still no change. Thought it could be electrical. Checked my fuel lines and saw the line leading to the carb was leaking. Cut about 1/4 inch off and reconnected. Took it out today and back to FULL THROTTLE!!!

In other words...check basic small things before getting to big things. I've learned my lesson on that years ago.

 

[jtexas]

Re: Motor idles fine but no power...

If it's ever run correctly since you bought it, then any test done prior to the arrival of your symptoms is irrelevant. Something has changed and it might be compression.

Did you buy this motor locally? I mean, do the carbs have hi-altitude main jets? If not, you need 'em.

What do you mean by "exact right throttle position?

Check to be sure your throttle valves are all parallel, closed at idle and perfectly horizontal (not beyond) at WOT.

There is about a zero percent chance of getting a boat on plane with only 2 cylinders firing.

Yeah the automatic choke flooding the motor is not at all unlikely. They abandoned that concept not long after your motor was built -- do you have a choke switch or "push-to-choke" keyswitch?

You need the choke plates closed for a cold start and open at all other times.

 

[lucabrazi]

Re: Motor idles fine but no power...

Thanks Jtexas-

To clarify- I bought the boat from a guy on Craigslist who said it had been brought up from Sea Level, but that he had given it a "high-altitude tuning". Whatever is wrong with the motor has been wrong since I picked up the motor.

My comment of "exact right throttle position": If I give it too little throttle it doesn't plane (obviously because it is too slow) and if I give it WOT it bogs...but if I back down from WOT I seem to be able to hit a sweet spot where the motor stops bogging and begins to run smoothly...

Perhaps re-checking comprehesion is a good idea just to eliminate that.

I'll check the throttle valves.

It has a choke lever on the motor...maybe I can adjust this a bit to lean the mix?

Thanks for any other help.

 

[Ironsides]

gone...

 

[jtexas]

Re: Motor idles fine but no power...

Looking for fuel leaks is one thing, but there's no point guessing at stuff like "loose hi-speed jet"...you can spend days checking on one random occurance after another without ever accomplishing anything. A methodical troubleshooting approach will get you there -- that's my opinion anyway.

There's no adjusting the choke, it's closed during cold starts, and open otherwise.
There is a simple procedure for disabling the "automatic" feature of the choke, so you can use it when needed. I don't know it though, I read it on this forum -- try the search feature.

Bogging like you described (even with the choke turned off) is a classic symptom of dirty carbs, but there are other possible causes (spark and compression for example).

Usually, slow to plane but you can nurse it up to full throttle, sounds to me like it needs a link & sync. For the full step by step you'd need a service manual (factory shop manual is your best bet, available at marineengine.com and outboardbooks.com) but in the meantime, see the photo below (later model than yours, but the important parts are all there); the cam needs to hit the roller right on the mark.

In your case though I would check to see if I had the right main jets. Follow the "carb cleaning and rebuilding" link from the "top secret file" at the top of this forum. There's instructions on fabricating a jet removal tool -- don't try using a regular screwdriver on 'em, too much chance of ruining it by marring the surface.

The main jet is behind the carb bowl drain screw. Look for the size stamped on the jet -- OEM size for sea level operation is 61D. I don't know what you'd need at 5500, maybe you can get some local advice, or somebody here on iboats will know.

If all of the above checks out, you'll want to rebuild the carbs, with new kits.