Possible fuel problem, can't go past trolling speed

[bhughes]

I have a 1974 25 hp johnson. Last week it ran fine. This friday I put it on the water and it wouldn't rev up high enough to go past a slow trolling speed. I'm thinking it must be a fuel problem but I just don't know where to start.

Can anyone give me a few ideas of some basic things to check to narrow down what the problem is? I've haven't checked the spark plugs or anything but I figure since it cranks up fine and was running great just last week that there must be either something clogging the fuel line or something.

I had a friend that told me that I may need a water/fuel separator if there isn't one already but I have no idea.

Help wanted! Thanks!

 

[OptsyEagle]

Re: Possible fuel problem, can't go past trolling speed

You say it won't rev up enough to go more than a trolling speed. My question to you, is whether it is stalling out on you as well. If it can't get the revs to go past a troll, it should be stalling and cutting out.

Now, if you are saying that it is reving up fine, but you still can't get more than a trolling speed, then I would say that you have spun your propeller.

Let us know.

 

[BAproject]

Re: Possible fuel problem, can't go past trolling speed

bad cooling coils can cause slugish engines....so can bad plugs.

Just cuz it starts doesn't meant the plugs are any good

 

[bhughes]

Re: Possible fuel problem, can't go past trolling speed

In gear, when I pull the throttle, it increases in rpms about a quarter of the way up the throttle, but it doesn't rev up any higher or go any faster past there which is at trolling speed. It doesn't stall or cut off or sputter or anything like that. The rpms stay exactly the same once it reaches that speed and it won't go any faster although I'm pulling the throttle all the way to full speed.

It's not the prop because the engine itself isn't going faster. It even revs up slower than normal in neutral, although it does go higher then in gear.

Still don't know about the plugs. Will try replacing them but I just don't think that's it.

 

[bhughes]

Re: Possible fuel problem, can't go past trolling speed

One other thing, I don't know much about carbs, but there is an adjustment knob that screws into the top of my carb. When I was trying to see why it wouldn't rev up, I screwed the adjustment know just about all the way in and nothing happened, then I screwed it all the way out till it was completely out and it still have no affect on the motor whatsoever. The motor continued to run but at the slow speed. Could it be that the carb is clogged and just needs to be cleaned?

 

[iwombat]

Re: Possible fuel problem, can't go past trolling speed

Could be a dirty carb, or just running on one cylinder. Time to cover the basics. Compression check and spark test. Go on from there.

 

[wwcj101]

Re: Possible fuel problem, can't go past trolling speed

check and make sure when you throttle it up that the throttle plate is opening the same amount. if the throttle goes wide open and the engine doesn't rev up it should cough sputter or something. try reving it in neutral to see if it will gain the rpms that way. also be sure to check the lube in the lower unit.

 

[bhughes]

Re: Possible fuel problem, can't go past trolling speed

I believe the throttle is working fine because it revs up higher in neutral, but still not as fast as it should I think. But I can see the throttle cable thing moving and I have tried to push the throttle thing attached to the motor and it doesn't go any further.

I haven't checked the lower unit lube, but why would that affect the engine reving up?

In order to check the plugs, can I just pull the wire off the plug while it's cranked and see if it cuts off or slows down in order to test it?

 

[iwombat]

Re: Possible fuel problem, can't go past trolling speed

That will tell you if it has any possibility of sparking, but won't tell you if it's got spark under load. To do that you need to see if a spark will jump a 7/16" gap through the air.

See the attachment for an example. You can also make one with a board and some nails.

 

[bhughes]

Re: Possible fuel problem, can't go past trolling speed

If there was a problem with the spark under load, wouldn't it sputter or stall when I gave it more throttle? It seems like it's more of a carb or fuel problem since the engine continues to run smooth, just doesn't increase in speed or rpms. I'm probably just going to change spark plugs and clean the carb this weekend and see if that does anything. If not, then I'll look into some of these more complex solutions. I just don't think the coils or spark just stopped working after working fine last week.

 

[Johnny Too Bad]

Re: Possible fuel problem, can't go past trolling speed

Hey BH, I had a '98 Yamaha 15hp that did exactly what you're talking about. It drove me crazy. I rebuilt the carbs and replaced fuel lines and all connections. Nothing worked. The irritating part was that it ran fine one day, and the next day I couldn't get it to run more than about 5 mph. It revved fine in nuetral, but I couldn't get it to turn any rpms under load.

After checking everything else, I decided to change the coils. It turned out it to just be a loose coil. Tightened it up and no more problems. Could be worth checking.

 

[Fl_Richard]

Re: Possible fuel problem, can't go past trolling speed

Yes you probably have good spark. The issue at hand is just going methodically through the troubleshooting/diagnostic procedure. Yes people will tell you to test things you believe to be OK. But if you go through each step methodically by the time you get to the end you will know exctly what the problem(s) are. And you can fix the proper component.


I know it seems like your testing things you shouldnt be but spend the extra ten minutes it takes to test everything and you'll be rewarded in the end.

 

[jbjennings]

Re: Possible fuel problem, can't go past trolling speed

I believe the throttle is working fine because it revs up higher in neutral, but still not as fast as it should I think. But I can see the throttle cable thing moving and I have tried to push the throttle thing attached to the motor and it doesn't go any further.

I haven't checked the lower unit lube, but why would that affect the engine reving up?

In order to check the plugs, can I just pull the wire off the plug while it's cranked and see if it cuts off or slows down in order to test it?

I agree with iwombat, which shouldn't account for much since he's a real mechanic and I'm not. You need to get a $6 spark tester at autozone and see if your spark will jump a 1/4 inch gap at LEAST with a hot blue spark. You could try grounding a plug wire and see if it has an effect on running, and if so, replace it and try the other the same way. If you ground one wire and it doesn't change the way the engine runs, then there's not enough spark on that wire. I'd get the spark tester, it's a much more reliable test and will usually leave no doubt as to the problem.

You definitely can have good spark one day and bad spark the next. That's the problem with electrical stuff, it's hard to predict when they will go bad. Some electrical parts go bad in a week, some last 10 years. There's no telling.
Anyway, the symptom you're describing is one in which 1 cylinder doesn't have good enough spark, keeping that cylinder from making any power. That's my bet, anyway.
Good luck,
JBJ

 

[iwombat]

Re: Possible fuel problem, can't go past trolling speed

Sort of depends on how you define "real mechanic", but I'll accept the moniker begrudgingly at least.

Obsessed internal-combustion hobbyist is a better title.


Anyway, I suspect you have a failing or failed ignition component causing one cylinder not to fire. It'll seem fine at low speeds, but once you start giving it throttle it simply can't generate any power.

Pulling off a plug while running will tell you if something failed outright. Spark test will tell you if something is failing under load.

 

[bhughes]

Re: Possible fuel problem, can't go past trolling speed

I guess it won't hurt to try the spark plug tester. Here's a question. Can I test the spark myself or do I need someone's help for that. Just wondering cause I'm hoping I don't have to involve my wife in this, she already hates my boat and fishing hobby as it is.

Also, in case it is a spark problem, how expensive is the potential solutions for this?

 

[jbjennings]

Re: Possible fuel problem, can't go past trolling speed

It is much easier to get someone to help pull it over while you look at the tester. I suspect it will cost less than $80 to fix. A '74 johnson 25hp is a really good motor. Unless it has been abused, it has many years of regular use left in it most likely. The main problem I would worry about, as long as compression is good, is the lower unit holding gear. If so, the rest of the parts are fairly cheap.
Don't sweat it. If you can't identify the problem with the spark tester, we can help you find it elsewhere. Those motors are very easy to work on, and easy to LEARN how to work on as well.
Later,
JBJ

 

[BAproject]

Re: Possible fuel problem, can't go past trolling speed

I would listen to myself and Johnny Too Bad!

My buddy has and 85HP outboard on his boat and the engine sounded like it was running fine but had no power.

He had a mechanic/friend look at it and one of the coils was toast. I would start there....they're probably $60/ea or so to replace.

Also, pull the transom plug and make sure you don't have water in the bilge! It may look clear but you never know what might be hiding in there. 1 gallon weighs appx. 8.35lbs....so 10 gallons hiding in there is 84lbs of dead weight.

-BAproject

 

[bhughes]

Re: Possible fuel problem, can't go past trolling speed

Update on 1974 johnson 25 hp low rpm issue:

Here is the list of steps I took today in order to diagnose/solve my issue:

1. Obtained a loaner (free) compression tester from autozone, both cylinders had 100 psi. (relief)

2. Purchases a spark tester from harbor freight ($3) and tested spark on both cylinders. Both plugs had a very fast, consistant flickering light on the tester. (more relief)

3. Purchased a carb kit from the local johnson dealer ($30, I know it's cheaper online but I didn't want to wait 9 days for delivery), carb cleaner spray from autozone ($3), plus 2 replacement fuel lines that were basically falling apart when I was taking off the carburetor ($5).

4. To get the carb off to clean was harder than I thought simply because I had to take the electric starter off in order to get to the carb. After taking off the starter and carb, I did my best to clean the carb as this was my first time. I do not have the "special tool" to get to the high speed jet. I tried to file down a scredriver, got it close enough to reach the jet, tried to unscrew the jet, then start stripping the jet so I said screw it and just sprayed more cleaner on it. I don't think the jet for slow speed is the same. I think it's the long need thing that you screw out at the beginning. If there is a smaller jet deep inside the low speed hole, then I do not know how to get it out. When done, I sprayed the carb with water, air compressor, then replaced gaskets and put back together.

5. From advice I got from the mechanic at the johnson dealer, I emptied out the fuel I had in my gas tank b/c he said I may have had water in my gas. He said to empty out all that fuel from the tank and replace with new fuel. Then, before I put the carb back on, I pumped the new gas through the motor for a few pumps to get out any old fuel.

6. Also replaced the spark plugs that were on there (not sure if they were the recommended ones for johnson) and put on JC4's ($5).

7. Put everything back together.

Result: I haven't tested in gear on the water yet, but in a big trash can in neutral, when I started her up she definitely had a lot more power in neutral and sounded like she used to before this issue. It definitely rev's up way to high when sitting in neutral (I think the carb's low speed needle is screwed up), but this was the case before my issue. Basically I crank her in neutral, but to avoid slamming her into gear while in really high rpm's, I kill the engine, put it in gear, then restart (I disabled the no-start-in-gear thing). Even though it idles high as crap in nuetral (sounds like it's about to take off into outerspace), once in gear it idles great.

Now I just have to take her on the river and see if the fruits of my labor are good. Maybe tomorrow if I'm lucky.

Thanks for the help and I'll post another update soon.

 

[bhughes]

Re: Possible fuel problem, can't go past trolling speed

Took the boat out Friday night and she opened right up. I think I can do about 20mph with just me in the boat. Today I decarbed it with sea foam using TD's method. I took the prop off and performed the decarb in the driveway with a big trash can of water.

I also got my hands on a 1993 3 hp johnson that I had on a jon boat that was at my in-laws place in storage. It hadn't run in couple of years and I remember it didn't run the last time we tried to use it. Today, I took it out and looked it over and found that when I pulled the throttle, the cable wasn't attached to the little hole under the flywheel used to give the motor gas. Had to rig it up to stay in the whole as the piece was broken, but once I got it in place she cranked right up to my surprise. Oh, I also emptied out the old fuel that was in her first and replaced with some new fuel.

Now I've got a 25hp with a 3hp kicker motor on the back of my 14' fiberglass. I don't think the 3hp will push it faster than a troll, but it's better than paddling if it came down to it.