1974 Johnson 25 hp backfire issue and other

[clemsonfor]

Hello all,
I have a 1974 Johnson 25 hp motor pull start, Model number 25R76E
It is a long shaft. first thing i did was pull the spark plugs and sprayed oil in cylinders. check for spark fixed the points, re gapped them, ran some 800 sandpaer through them till i got a good streak on paper. took carb apart, the low idle speed was clogged, i took it all apat and cleaned it with carb cleaner, did not put it into the ultrasonic cleaner yet, choke was sticky, but i got it to function some, will need more attention. took plugs out did compression test both cylinders got 140.
Took the line off the fuel pump pumped the bulb a few times till i thought the fuel that was in the lines was gone. it pushed out fuel so it was not cologged. put it back together, will change the fuel lines later. put the choke on gave it a pull nothing first pull, on the 3rd pull it popped loud, so i switched the wires going to the coils. tried it again. it ran for a 4-5 seconds then backfired, so it runs yea....... any idea what would cause the back fire. if people say carb i knew i should have ultrasonic cleaned it.
thank you all

Dale

yes i am the same person who posted about the evinrude 25 1974... are these the same motors, they look identitical ??????

Yes a 1974 Johnson 25 HP and 1974 Evinrude 25 HP are the same motor.

 

[Dalejam]

I could rebuild your entire motor without glancing at a manual and I have many years of simple trouble shooting experience as well..----There could be lots of issues causing poor idle.----Worn out pistons / rings, weak spark, poor crankcase sealing, carburetor flooding, issues with low speed mixture needle , fuel pump leaking., blown head gasket , thermostat missing or stuck open, reed valves stuck open ( sea gull feather ) perhaps.---Broken carbon seal on bottom of crankshaft.---Up to you to do the inspections.

as for pistons i did a compression test pictures are above, i did let it sit holding the pressure for a bit and no leak. as for spark i did replace everything to do with the ignition system, accept the driver coil. so i orderd one and the RING to set the driver properly. i have taken the carb apart, removed welch plug for low idle cleaned carb in ultrasonic for about 2 hours, then cleaned with carb cleaner. put carb back together. replaced the fuel pump and put towel under it to see if i had a leak when i replaced it dry.
as for head gasket i checked all the bolts for any loosen ones all tight,( if head gasket was blown wouldnt it show a leak in the cylinder)? replaced thermostat and gasket and engine runs cool not hot at all. as for the reeds i cant say, that would be the last thing i take apart. please dont let that be the issue. as for carbon seal ? you got me on that one ... never heard of that.
as for a possible leak in a gasket i sprayed carb cleaner all around the head to see if the idle would pick up, for if it did i have a bad gasket, but no change in motor speed at all. i cleaned this motor to the hill, paid attention to what i cleaned so nothing would fall apart or if i knocked something off. it is clean looking runs but slow idle iss my issue ????
i will be taking the carb off again and double checking everything.
thanks for the assistance .
thank you
Dale

 

[racerone]

The gauge shows the compression.-----Holding the gauge there would show you nothing as the gauge captures the pressure.----The gauge has a check valve.-----These motors are a simple , 2 stage air pumping machine.-----Air / fuel / oil mix is drawn into the crankcase.------That is first compressed in the crankcase ( stage 1 ) .-----It is then transferred to the cylinder and compressed again.-----Then ignited to produce the power.------There is a carbon seal on the bottom of the crankshaft.----Part of the sealing of stage 1 process.-----I am not saying that carbon seal is the issue.-----But if you have great spark , fuel in the clean carburetor and compression is good in the cylinder you have to look for simple issues.-----The trouble shooting and inspection is up to you.

 

[clemsonfor]

The gauge shows the compression.-----Holding the gauge there would show you nothing as the gauge captures the pressure.----The gauge has a check valve.-----These motors are a simple , 2 stage air pumping machine.-----Air / fuel / oil mix is drawn into the crankcase.------That is first compressed in the crankcase ( stage 1 ) .-----It is then transferred to the cylinder and compressed again.-----Then ignited to produce the power.------There is a carbon seal on the bottom of the crankshaft.----Part of the sealing of stage 1 process.-----I am not saying that carbon seal is the issue.-----But if you have great spark , fuel in the clean carburetor and compression is good in the cylinder you have to look for simple issues.-----The trouble shooting and inspection is up to you.

What he said. You can unscrew the gauge and it still will say what it did in the cylinder. That's why they have that relief button to release the pressure on the gauge for next test.