79 johnson 140 backfires thru prop

[mnw001]

I just bought a 79 140 johnson at an auction so I know no history on it except previous owner said he thinks it may need a tune up. It runs and starts well except at idle. When I first started the motor I found that it was idling way too fast around 15-1800 rpm's I guess. Compression is good on all cylinders. The motor was runing fast becasuse the slow timing was moved up to the point that it doesn't backfire. If I close the choke 3/4-7/8 of the way I can back the timing back down and it will idle great. Thinking I had an idle circuit problem I torn down carbs and they look as if they have been recently cleaned as they look great. I have inspected all passages and they seem to be clear. Float levels are both the same but still need to get a gauge to check for sure. Idle jets are size 30 which I'm told are stock. I'm at 5000 feet elevation and I believe this motor had been at this altitude for at least several years. I've metered all electronics, and installed new plugs and motor runs fine at higher rpm. Old plugs didn't look bad. fuel line clamp was a little loose at output of fuel pump but problem is a low speed not high so I think that was not the problem. There is no spitting out of carbs and reeds look fine. This motor does not seem to have a great deal of hours on it maybe 3-500 and is good shape otherwise. I see no visible signs of crank seal leakage, is there a way to diagnose them? I was running it on new premium fuel. not sure where to look now and still have carbs apart. There is slight movement in throttle shafts. I wonder if that could be problem. Both jets are fixed. Thinking about trying larger idle jets but I think stock size should be fine at this altitude. I still have carbs apart. Any ideas anyone?

 

[Joe Reeves]

Re: 79 johnson 140 backfires thru prop

If the engine is actually firing out the exhaust, it is out of time. On the 1979 model, being out of time can only be accomplished by having the flywheel key sheared, or even slightly damaged.<br /><br />The flywheel nut must be torqued to exactly 105 foot pounds. Anything less will result in a sheared flywheel key and usually damaged flywheel and crankshaft tapers.

 

[mnw001]

Re: 79 johnson 140 backfires thru prop

Joe <br />Thankx for your help<br />I removed the flywheel nut which was not real tight but the keyway seems to be intact although I have not used the puller to remove the flywheel yet. How do I hold the flywheel when I tork it to the 105. Could the keyway be bad and still look good from the top. I was told usually they won't start if key is bad..This motor starts fine

 

[DHPMARINE]

Re: 79 johnson 140 backfires thru prop

The part of the key you can see before pulling the flywheel will look fine,so you must pull it to check.I use a Snapon automotive flywheel holder,but if you have a lifting bracket just to the rear of the flywheel,apply a large screwdriver between it and a tooth on the flywheel.If the flywheel is only off a bit,it will run,just not well.<br /><br />DHP

 

[mnw001]

Re: 79 johnson 140 backfires thru prop

Tried using a puller on the flywheel. Its so tight that the center shaft of my puller started bending as I was trying to tighten it. I would like to think that if the key had slipped it would not be on the shaft so tight. Its a fresh water motor and there is no corrosion on the crank or flywheel. I thought about heating the flywheel to see if it would pop loose or using a penetrating oil. Any ideas I still find it hard to believe that the key is bad.

 

[ledgefinder]

Re: 79 johnson 140 backfires thru prop

Use a good quality puller, and adjust the side bolts so the center shaft is short. Once the puller is tight, tap LIGHTLY underneath the flywheel with a hammer in a few widely-spaced places. <br /><br />Usually pops right off. If not, please post back.

 

[Joe Reeves]

Re: 79 johnson 140 backfires thru prop

You must use a flywheel puller that incorporates bolts that screw into the three threaded holes surrounding the flywheel nut. DO NOT attempt to pull the flywheel using a puller that grips the side of the flywheel.<br /><br />Yes, we know the flywheel is tight (105' pounds) but DO NOT use heat of any nature unless you want to replace the flywheel!

 

[DHPMARINE]

Re: 79 johnson 140 backfires thru prop

Fishslayer,Because you said the flywheel nut was not real tight,I feel you should pull the flywheel and check the key.Are you using a three bolt puller like Joe Reeves said? It's the only way I know.<br /><br />Make sure the bolts are screwed in evenly and apply a good amount of pressure,but not enough to be distorting anything.Let it sit that way for awhile,you're not going anywhere,anyhow.Apply a little more pressure.Now sharply 'rap' the center puller bolt squarely with a hammer.Usually works.<br /><br />DHP

 

[R.Johnson]

Re: 79 johnson 140 backfires thru prop

A back-fire through the exhaust is normal when running on muff's, at least on first start up, The engine is running lean do to the lack of back pressure, as being submerged in water. I beg to differ with this sheared , or distorted flywheel key. The only way that can happen is with a severe back-fire, or a high impact to the gearcase. A loose flywheel nut will show up as a build up of rust between the tapers on the flywheel, and crankshaft tapers. In machine work! this is called "fretting". In normal running, a sheared , or distorted key is not likely. As I stated in an earlier post, you need a proper puller, using a shouldered stripper bolt to remove the flywheel. Another thing to look for with a backfire is a slipped magnet on the flywheel hub. This is held in place with epoxy. Just as!, or much more likely than a sheared, or distorted flywheel key.

 

[Joe Reeves]

Re: 79 johnson 140 backfires thru prop

The 1979 140 Evinrude/Johnson has permanent magnets which are imbedded within the flywheel. The magnets cannot slip. This engine does not incorporate the later model 35 ampere flywheel which has the magnets epoxied to it.<br /><br />The owner states that the flywheel nut was not very tight, indicating that it was not torqued properly, and this factor in the past usually has resulted in a sheared key.<br /><br />There have been times (very few) whereas the key did not shear completely but instead looked like one side had been chiseled away slightly which allowed the engine to run but not run properly, and a backfiring effect would occur at various rpms.

 

[mnw001]

Re: 79 johnson 140 backfires thru prop

I have done all of the things suggested to remove the flywheel including using the right kind of puller, the center screw on the puller I used was a 1/2" bolt and I turned it so hard that it started to bend. I had it close to the flywheel and used 5/16 bolts screwed all the way in to the flywheel. The flywheel has not been damaged at this point. I did tap on the center screw after It was under pressure but I did not tap on outside of fly wheel. I did let it sit under pressure for a while. Nut was tight but not real tight I got the feeling that someone else had attempted to remove flywheel and had not torqed it properly. There was no sign of prop damage and engine does not backfire once engine is accelerated. I know about back pressure so I ran motor in a very large wash tub as well as on muff. The cavitation plate was under water and I could see the backfires shoot out of prop when I would back the timing off enough for engine to idle normally. The interval between backfires was about 2-3 seconds apart and seemed consistent when timming advance was slowed. Sometimes the backfire would actully stop the motor running. As I said it ran very good except to get it not to backfire at idle I'd have to give it a lot of choke. I'm going to look for a better puller but I still wonder if the key could be bad I get the feeling the flywheel had never been off. This has cd4 ignition so normally there should be no reason for it to have been off. Thanks a lot for all the help

 

[R.Johnson]

Re: 79 johnson 140 backfires thru prop

Are the plug wires going to the correct plug?

 

[mnw001]

Re: 79 johnson 140 backfires thru prop

Just wanted to report that I got he flywheel off finally. I uesd a heavier puller and it was still really tuff to get it loose. I had the puller extremely tight and when it finally popped the whole thing popped about a foot in the air. <br /> The Keyway looked fine I'm sure that flywheel hadn't been moved in about 26 years. Anyway retorked it to 105 and reinstalled carbs and did best I could to sink. Will test run as soon as I'm able.