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Engine struggles at low rpm

O

operationchevy

New member
Hi all,

This is a long one but here is the story,

Ok so I've been looking at posts on here for the past couple years and most of all have helped to fix and get my boat back on the water but now I've came across a new problem that is odd and I decided to just ask what yall think it is.

Fist of all the motor is a 1976 Johnson Stinger 75 hp, I dont know the pitch on the prop but it's new from the other owner and the motor worked and ran its ass off once 2 years ago and then a series of problems.

Ok so i have a 2007 Bass tracker pro 175 boat, i got this motor and put it on and rewired everything so it would work with my controls, sense the boat was wired for a merc and I changed to a johnson, took forever but I got it all right.

So when I got everything right and running good in the yard I took it to the water and it idled good and so we slowly made it to the bay and when I went to wot it had no power, felt like clogged carbs but I know I checked them in the yard so i knew they were good, so p pulled off the cover and touched the coil wires and found that #3 had a bad wire from the coil, about fell off the boat when I found that one. So I turned off the motor and twisted the wire a little and pushed it into the coil and fired it back up and went to wot again and she ran good for about 10 minutes and the wire came off again and dogged out again, so went home and replaced all 3 coils and new plugs and tried in the water again and again no power, I changed all the fuel lines and rebuilt the carbs just so I knew everything was good and again in the water it would just bog down and wouldn't go over 2k rpm.

So I replaced the fuel pump this time and in the water at first she got up to 2200 this time and I just held it at wot and she kinda felt like she wanted to go but just bogged the whole time so I was headed back to the boat ramp and still at wot the motor felt like it was pulsing a little so o watched the rpms and it went up to 2500 and when I almost got to the ramp it just started to raise rpms and at 3k she took off. Wot for 25 minutes top speed 45mph, I could slow down a little not lower then 3k rpm and then slam it and the response was good but if I let it go under 3k rpms it would bog down again for a few and it would struggle to get up to 3k and then would take off again.

So I'm lost at what to do now, it struggles untill 3k rpm and then all top end is good. I know this was long but I wanted to give the full history of what it's doing. Any ideas? Thanks in advance
 
When the bogging happens does the motor want to die quickly and throttling back makes it recover, or does it burble and smoke?

Wanting to die quickly is the sign of a lean condition and burbling means you're too rich or not enough spark.

What max RPM did you reach when it did finally get up and run?

First check the easy things and report back.

- Spark: Does it jump 7/16" gap?
- Compression: Around 100PSI + and no more than 10% variance?
- Timing Advance: Is the arm moving properly with the throttle linkage?
- Spark plugs: Do you have Champion QL77JC4 plugs gapped to 0.030"?
- Prop Pitch: As Racerone said, you need to post your prop pitch. 13 1/4" X 17P is a pretty common size for the 3-cylinder engines.
- Trim: Do you have hydraulic trim and tilt or is this lift assist only and you have a stationary pin? Is the engine all the way in the down position when you're trying this or is it tilted up?
 
When it does the bogging it just burbles and smokes, I can pull it back to neutral and it idles just fine.

The compression is good on all 3 cylinders

The plugs are champion L78V and spark jumps to the side good.

Timing advances all the way and carbs open fully with wot

I'll have to get the prop size tonight after work.

It has a cmc tilt and trim, I've tried both positions all the way down and a little tilted up.
 
The sparkplugs are NOT USED when you test maximum output of your ignition system !-----So does spark jump a gap of 3/8" or more , yes or no ?----Compression should read about 130 or higher too.
 
Ok well I'm not sure the size of the end of the plug but the spark jumps to the edge good and strong. If its 3/8 from the tip to edge then yes. The compression is about 138-140.
 
Hurry and test the spark with a testing device.----The plugs are not used when testing the peak output of your ignition system.
 
racerone said:
Hurry and test the spark with a testing device.----The plugs are not used when testing the peak output of your ignition system.
Click to expand...
Exactly. you need to use an air gap tester WITHOUT PLUGS to determine if the ignition system is working properly. If you have a bad plug and you're using that to test spark it will give you false results.

You need to use something like this Lisle 50580 inline spark tester.
lisle-spark-tester.JPG
lisle-spark-tester2.JPG

Also, according to the reference on this site, you should be using QL77JC4 plugs, not L78V. The L78V surface gap plugs may not be giving you enough of an arc to ignite the rich fuel mixture when you try to take off. The protruding electrode of the QL77JC4 should help with better ignition as those lower RPM.

http://www.marineengine.com/parts/champion/outboard/johnson-outboard-spark-plug-guide.html
 
Ok so I ordered the spark tester and I'll check it when it comes in, also I went ahead and got the other plugs QL77JC4 just to be safe. I'll update on the spark tester test soon. I'm waiting on my pops to get time to pull the prop off to get the size. I've got the boat parked on his property, it's an hour south or I would of had it by now.

Thanks for the input for now and when I have more results I'll let yall know.
 
Prop size has nothing to do with it if it intermittently runs at full power. You said it ran 45 mph for awhile.

If the plugs don't help, I suggest you check it for water getting in a cylinder(s) before going any further.
 
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