1979 Johnson 100hp surges at WOT

[Kymc3000@gmail.com]

Having a random problem with my Johnson. When taking off across the lake it will start surging at full throttle, if I back it off to 3/4 it goes away and runs good. For example yesterday and the first 3 runs to different fishing spots it would surge and on the last run to the launch it was perfect with no surging. I have replaced all my fuel lines, cleaned screen in fuel pump, replaced fuel filter, rebuilt both carbs(twice now, first kit wasn’t treated for oil and took a ****) and replaced the spark plugs. Just wondering if anyone has an idea what to try next? I was thinking maybe coil packs or stator? Since I can’t duplicate the problem on the trailer it’s more of a replace parts and hope for the best.
thanks for your input

 

[racerone]

All this stuff can be TESTED.------One step at a time.-----Start with a compression test and post your numbers.----Other help may follow.

 

[Kymc3000@gmail.com]

Compression is
1= 126
2= 127
3= 124
4= 128

forgot to mention that in the original post.

 

[racerone]

Is it gradually surging or instant power changes ?------Instant changes would be electrical.----Test run with a separate / different fuel tank and hose.----Test run with a timing light hooked up to #1 followed with the other 3 cylinders.----Observe the flashing light.-----Might be as simple as a sparkplug wire shorting to cowling latches !!

 

[Kymc3000@gmail.com]

It’s very gradual, when it wants to surge you will be at WOT for 30 sec to a min and the it will surge from 500-1000 rpms. There’s probably about 2-3 seconds between surges. I also blew through the tank breather to make sure the check valve on the breather wasn’t plugged causing a vacuum on the tank. It does seem to do it more in the beginning of the day. Also this last time I was out I checked the primer bulb and it was still hard and had not lost prime

 

[racerone]

Test run with another tank and hose.----Test run with timing light !----Remove cover from air box.----Use a STRONG flashlight and look into the carburetors bores with the STRONG flashlight.---Observe / compare fuel coming up the main nozzles.----This problem should be elegantly simple to figure out.

 

[Kymc3000@gmail.com]

I will do that, thanks for your help!

 

[Kymc3000@gmail.com]

Did some testing this afternoon, I re checked compression with a new gauge and all cylinder are 140psi except #2 which is 142. Checked spark with a test light and a inline spark tester and had good spark on all 4. Looked down the carbs while running and didn’t notice anything odd except there’s a ton of fuel built up right before the bore neck down to the throttle plates. There 2 brass ports sticking toward the front of the boat and that’s were the fuel is sitting. Is this normal or maybe a stuck float? What are those ports for?