85 HP Johnson Randomly Backfiring Through Exhaustt

[jiju1943]

I have a 1976 85 HP Johnson on our 24 foot pontoon, motor had been sitting for a while so I cleaned the carbs checked the foot oil checked the plugs. Put the muffs on and it would not slow idle and it would and still does backfire through the exhaust at no certain time. Went to the lake, and the only way to make the engine stay running from idle to shift into gear, was shut the fast idle down and very quickly push into gear before engine died (hard to do but no gear grinding).

The engine would never open all the way up, it seemed to run a little past 1/2 WOT. The engine would backfire but stay running as we ran a little over wake speed, no need to try open it up, pontoons don't go fast, at least this one didn't and won't. It was tough getting back on the trailer because the engine just didn't have enough power to shove the boat on. Burned 5 gallons gas in 4 or 5 miles, black oily junk dripped from prop area, wasn't gear oil.

Ran a compression check 92 PSI on all 4 cylinders, changed plugs, replaced the carbs, changed out two coils, changed one plug wire end as it was extremely rusty. Motor will slow idle and runs much better now but it will still backfire through the exhaust. I plan to replace the power pack today and see if that does any good, also will make sure plug wire ends are letting spark get to plugs correctly.

I have read for several hours here on Iboats trying to find out about this problem, hopefully someone will know what the problem is.

 

[daselbee]

When checking plugs, you probably put the wrong plug wire back on the wrong plug....swapped two somewhere.
My bet.

 

[daselbee]

Oh I see also changed coils.....you could have swapped two of the coil primaries also.

 

[jiju1943]

Actually there is no way to cross the plug wires, the ones at the top are way to short to reach the bottom plugs.

I don't understand what you mean "coil primaries", are each of the coils specifically for a certain plug and will not work randomly?

 

[emdsapmgr]

Backfiring is a violent event. That means the engine is firing out of time. Could be an ignition problem or the flywheel key could be sheared. Or are you trying to describe a "lean sneeze" a sort of non-violent missfire at idle? You will want to determine if the engine is firing on all 4 cyls or not. The compression on that engine was in the 125 range-new from the factory.

 

[jiju1943]

When the idle is low, or just a hair above low idle, and it "backfires" or sneezes, it will kill the engine, it makes a fairly loud pop through the exhaust. I can run the idle up about half way and it runs smooth (after it warms up) and you can not detect it doing anything, unless you are at the motor and have your hands on it. With my hands on the motor I can feel it skip at random, or what ever it is doing, but it doesn't make a backfire sound idled up high The skip or misfire is almost like a plug wire is jumping fire randomly.

When we were on the lake and running just above no wake speed, it would backfire, skip, sneeze or what ever it did, every once in a while, there was no noise but it felt like the prop hit something big, there was no certain rhythm to the skip. It may not skip for 20 or 30 seconds, then it may skip at 5 or 6 seconds but only one skip at a time.

I pulled the new plugs again today, the ones on the left side of the motor were clean, just a little damp. The plugs on the right side were different, the top one was black but not damp, the bottom one was really wet and black. Could it be the reed valves are cruddy?

 

[emdsapmgr]

That misfire can occur when the engine is out of time. Check the timer base under the flywheel. It should move freely with the movement of the spark advance lever. If the timer base is dragging or stuck in the wrong location, it can cause odd firing. That timer base should move easily back and forth by hand. If the original factory grease ever dries out, the timer base can stick or move very slowly through a limited range-and that can cause timing problems. Based on the condition of the plugs, you should verify that all 4 of them are firing when the engine is running. That engine will idle fast when sitting on the trailer. Different story when the prop exhaust is 20" underwater.

 

[daselbee]

He was screwing around with the plugs and coils. He has odd looking plugs on one side. Now he has a backfire.
I told you to check the coil primaries.
Make sure the proper primary wire is going to the proper coil. Geezzzz all you gotta do is LOOK! Free!

 

[Faztbullet]

Poster says it backfiring from exhaust which is a ignition/timing problem(to early/late/out of time), backfire/sneeze from carbs is a fuel/mixture problem.

 

[jiju1943]

That misfire can occur when the engine is out of time. Check the timer base under the flywheel. It should move freely with the movement of the spark advance lever. If the timer base is dragging or stuck in the wrong location, it can cause odd firing. That timer base should move easily back and forth by hand. If the original factory grease ever dries out, the timer base can stick or move very slowly through a limited range-and that can cause timing problems. Based on the condition of the plugs, you should verify that all 4 of them are firing when the engine is running. That engine will idle fast when sitting on the trailer. Different story when the prop exhaust is 20" underwater.

I will pull the flywheel Monday and check that out, I really do appreciate your patience and information. I will let you know what I find Monday.

Jim

 

[jiju1943]

He was screwing around with the plugs and coils. He has odd looking plugs on one side. Now he has a backfire.
I told you to check the coil primaries.
Make sure the proper primary wire is going to the proper coil. Geezzzz all you gotta do is LOOK! Free!

daselbee, the motor has had the backfire/sneeze from day one since I have owned this motor (one month), I did change the spark plugs hoping that would help the situation. I did indeed change out one of the coils because the wire end clip was so rusty it was disintegrating. There is absolutely no way the plug wires could have been crossed because the top wires are way to short to reach the lower plugs on each side. There is no way to get the wires going to the coli from the power pack crossed as only one wire was removed, one wire from the coil replaced in same spot on the power pack.

I have an identical engine to this one, it did run great when I parked it, when I replaced the coil I took it from the exact location as the one I removed, being an old 1960s mechanic I do understand the importance of placing the plug wires and all wires in the proper place. As for the primary wire going to the proper coil, there was only one screw removed, one wire removed and one wire replaced in the vacant space and one screw replaced.

I can understand your frustrations trying to help people with their motors, I appreciate your patience and understanding, have a fantastic Farther's Day.

 

[jiju1943]

Poster says it backfiring from exhaust which is a ignition/timing problem(to early/late/out of time), backfire/sneeze from carbs is a fuel/mixture problem.

Could the reed valves cause this if they are dirty? I will check the key way and timer base Monday, thank you. I do appreciate your help.

Jim

 

[racerone]

Reed valves only see clean fuel / air going through them.-Not likely a reed valve problem

 

[emdsapmgr]

As long as you are checking the ignition system, check the output voltage on the stator. If it is low, it won't fire every plug on every rpm. A different plug can miss every rpm or two.

 

[jiju1943]

Thanks racerone, that does make sense.

emdsapmgr where do I find the specs on checking the stator? I appreciate it.

 

[emdsapmgr]

Ignition troubleshooting tips and test values can be found at this website: cdielectronics.com

 

[Faztbullet]

Four Cylinder Engines (Except Quick Start Models)

No Fire at All: Disconnect the black yellow kill wire and retest. If the engines now has fire, the kill circuit has a fault-possibly the keyswitch, harness or shift switch. Disconnect the yellow wires from the stator to the rectifier and retest. If the engine fires, replace the rectifier. Check the stator resistance. You should read approximately 500 ohms from the brown wire to the brown/yellow wire. Check the DVA output from the stator. You should have a reading of at least 150V or more from the brown wire to the brown/yellow wire (while connected to the pack). Check the timer bases resistance from the white wire to the blue, green, pink and purple wires. Reading should be 38-42 ohms. Check the DVA output from the timer base. A reading of at least 0.5V or more from the white wire to the blue, green, pink and purple wires (while connected to the pack) is needed to fire the pack. Check the cranking RPM. A cranking speed of less than 250-RPM will not allow the system to fire properly.

 

[racerone]

Step # 1 is to determine if this is an ignition fault or a fuel system fault.----Run with a timing light and observe the light when motor falters.

 

[jiju1943]

I can't thank you fellows enough for bearing with me on this problem. I will try each and everything all of you have suggested. This is some fantastic information. I didn't do anything yesterday, it was so hot and I was just wore out. Just a thought I had, since the bottom right hand side plug is soaked with fuel and none of the other three are soaked, maybe that one isn't firing at all most of the time then it will fire sporadically causing the pop through the exhaust. Anyway, the first thing I plan to do is to try the test light on that cylinder to see if it is firing most of the time or not firing but once in a while.

Another thing I remembered, when running for a while, the left bank heads are a little warm, the right bank are cold, almost like it wasn't firing enough to get the head warm. The clean plugs are on the warm side, the flooded plug and black plug are on the cold side.

emdsapmgr, thank you for the link, I will check that out.
Fastbullet, I have that great information copied and saved, I will try that.
racerone, that is the first thing I will try today.

Thank you so much, I will let you know what I find today.

Jim

 

[jiju1943]

Step # 1 is to determine if this is an ignition fault or a fuel system fault.----Run with a timing light and observe the light when motor falters.

Racer, I did that today, all the plugs are firing and while it is better there is still that slight pop, you can actually see the test light skip when it pops. I took the flywheel off today and the trigger, timing deal is not easy to turn, I took it off and there is grit in it so tomorrow I will clean it well and replace it. Is it a good idea to use grease on the brass part to let it operate easier? It does need some kind of lube to make it turn smoother.

The inside of the flywheel is clean and rust free so that is a good deal.