1979 25 hp runs rough at idle, please help

[spacemonkey77]

Hey guys, thanks in advance for any help you can give me! I have a 1979 25 hp model 25904r evinrude, pull start.

This past summer it was running flawlessly. Now that it’s gotten colder (I use it for duck hunting) im having some issues. When I cold crank it it starts pretty easilly, but bumps and idles really rough and wants to die. When I put it in gear it just sounds funny, kind of like it’s bogging down. As I accelerate it kind of surges a bit and hesitates to take off. After a few moments it will kick in and run great at mid throttle or wot. Once warmed up it seems to run fine at higher speeds. When I drop back to idle it will run pretty good for a while, then after a while at idle it will start showing the same symptoms over again ( mostly bogging down). If I kill the motor and it cools off then the symptoms start over again.

I have tried richening the low speed idle and it seems to have no effect. I recently cleaned the carb and still no changes. It has fresh gas, fairly new fuel pump, new power pack and coil packs. Il

im at a loss! It ran rough last year when it was cold, then when summer came around I had zero issues. This winter however is far worse than last winter. Even letting it idle for a long time to warm up seems to not do much. Any ideas would be greatly appreciated!

 

[oldboat1]

Wouldn't hurt to do some basic testing first -- spark and compression. Use an adjustable spark tester, both plugs removed -- look for something around a half inch or a little less, and sharp. Compression should be even between cylinders.


There's always potential for carb issues. Might check fuel pump operation first -- may need diaphragm replacement. Additionally, the motor could be running cold -- easy to pull the t.stat and check it for operation (should open at approx. 140F).

 

[spacemonkey77]

Well now that you mention it I’m willing to bet the thermostat is the original in the motor. I prob should replace it regardless. I put a new fuel pump on about a year ago.

ill check compression and spark as well.

 

[spacemonkey77]

I just picked up an adjustable ignition spark tester but I am unsure of what to set the gap for on it? Would it be 17 to 20?

 

[spacemonkey77]

Never mind you said half an inch or less. Thanks

 

[Joe Reeves]

I just picked up an adjustable ignition spark tester but I am unsure of what to set the gap for on it? Would it be 17 to 20?

To be exact (Factory Specifications) for that engine... 7/16" air gap.

The spark should be a strong blue lightning like flame... a real SNAP!

 

[spacemonkey77]

Ok so an update. The compression was pretty much strait up 90 psi on both cylinders. I tested the spark at about a half inch for both cylinders! It seemed in my opinion to be a crisp blue snap like described. I couldn’t tell any difference in one or the other cylinder. I think it’s fine.

I went ahead and ordered a new thermostat and the seals to go with. I figure it’s prob not a bad idea to change out since it may very well be the original 1979 thermostat in there! I have yet to pull the old one out to check it. I’ll do it tomorrow and post what I find on that.

the fuel pump is only about a year old. How can I check it for pinholes? Is there an easy method?

thanks again for the help. Any other thoughts of what to inspect in the meantime, I’m all ears!

 

[Joe Reeves]

You asked "The fuel pump is only about a year old. How can I check it for pinholes? Is there an easy method?"

********************
(Fuel Pump Diaphragm Test)
(Two Hose Type Only)
(J. Reeves)

Note: This pertains only to the regular Two Hose type fuel pumps.

Leave the hoses attached to the fuel pump. Remove only the two screws that attach the fuel pump to the powerhead. Re-insert those two screws and install a nut to each of them so that the fuel pump is securely clamped together. With the fuel line attached to the engine, pump the primer bulb while observing the pressure/vacuum operating hole on the back side of the fuel pump. Should any fuel leak out that hole, the diaphragm is faulty.

If the diaphragm is faulty, the fuel pump will require rebuilding or replacing. Note that repair kits are not available for all fuel pumps, in which case replacing the pump would be necessary.

 

[spacemonkey77]

Awesome thanks so much

 

[racerone]

I am sure you can find a new diaphragm if it proves to be holy.----Have to think motor temperature may be the issue here.----Compression numbers are low !---Test with another gauge or pull the cylinder head to inspect pistons / cylinder walls.

 

[spacemonkey77]

What should the compression numbers be?

 

[spacemonkey77]

So I rechecked the compression with a new gauge! They both showed 120 psi! Didn’t realize my guage was bad!

I pulled the thermostat and it has a bit of corrosion but wasn’t as bad as I thought it might be. I could open and close it by depressing it. I’m still going to replace with the new one when it comes in! I’ll check the fuel pump next!

here is a pic of the thermostat!

 

[oldboat1]

Just for grins, test it out. I dip them in a cup of hot water -- local shop owner here touches a torch to the sensor to see if the mechanism is stuck. Better part of wisdom is probably to just replace it, like you are doing, but fun to see how they work.

 

[spacemonkey77]

I had not thought of testing in hot water! Good idea! I actually did that and the thermostat seemed to open fine and then fully reclosed when done. So I guess I’m back to square one now. I checked the fuel pump as described above and it appears to be fine as well.

I’m at a loss as to why this engine runs so rough and bogs down/ surges until it clears out and gets up to speed. Then runs fine after that.

i just pulled the low speed needle valve out of the carb and it looks fine. No wear. I sprayed some carb cleaner in case it had any debris in there! I replaced it back to the same setting which is about 1 1/4 turn out! I cant run it to check yet because I’m waiting on the thermostat parts. Should be in tomorrow.

any other ideas to check?

 

[Bosunsmate]

Hang in there, these are the interesting cases.
Do a drop test, run it in a barrel and pull one spark lead off and then do this with the other after replacing the first, you should get an even drop in rpm between cylinders, if you dont then one is weaker than the other and may be the sole problem, if they both drop evenly then its something that affects both cylinders.
You may have to do this on muffs if its too hard to run in a barrel as is.
You should also check that your timing is correct by checking that when the top cylinder (number 1) is at top dead centre that the flywheel pointer is at 0tdc. And also check spark with a timing gun too is in time if you can

 

[Bosunsmate]

Double posted on this new site

 

[Bosunsmate]

Also when it is running fine and drops back to idle and then it eventually starts going bad again, well put your hand on the head then and see what the temperature is, if its cold then the spring on the thermostat may be tired and its not closing off the flow adequately.
I think that this is still probably the cause of its issue

 

[spacemonkey77]

Ok my parts came in and installed! No change really. I did reuse the spring. I should have gotten a new one.

ok I did the drop test. You may be onto the problem now! When pulling the bottom plug there is a bit of drop in rpm but it continues running. When pulling the top there is def more rpm loss and the engine dies.

what should I look into now?

here is a video.

 

[Bosunsmate]

So since the top one is taking much more of the load it could be a few things.
One is that the top cylinder will always be warmer due to its higher position so you may still need to rule out thermostat issue. By just putting your hand on the head and feeling around it you will be able to check that its all getting hot but not scorching, mine i can easily keep my hand on. About the same as a cup of tea after about 8 mins thereabouts. You do need to rule out this overcooling possibility.

If its not temperature related then that bottom cylinder has something wrong with fuel or spark as your compression is good.
What colour are the sparkplugs? That often tells a lot.
A cause to still consider at the back of mind is whether the bottom crank seal has gone.
I am too laxadasical after a late night to look this up but i suspect that this motor only has one carb so since its running well on the top you can rule out a carb issue.

 

[oldboat1]

An infrared tester is really useful (and pretty cheap), but the old heel-of-hand test can work. If it's too hot to touch, it's running too hot. If you can hold your hand on the top of the head for a few seconds, that's about right. It should be warm to the touch -- harder to judge the low end. My 9.9s idle at about 120-130F in a barrel (hose running -- hose water temp probably about 40 degrees F.) Technically, should operate over 140F, but no hotter than 160.

not sure what follows from your drop cylinder test. Sounds like it's running on both -- could hook up a timing light and see if spark is erratic (erratic flashes).

Might pull the flywheel and check condition. Look for damage to key, indicating flywheel has slipped on shaft due to improper torque (affects timing). If you find that, clean the shaft and torque to specs -- use a new factory key. Check wiring for abrasions, and make sure the sensor and charge coil are installed correctly.