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Backfire!

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Goommer

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I've got a 1979 85HP Evinrude that I completely re-built over the winter (Bored .030 0ver, Rings, Pistons,Bearings Carb kits etc... etc.... I've timed it using "Joe Reeves" procedure and everything seems to be right; I don't have a test wheel so I cannot time it under load. I've done the syncronization procedure per the repair manual specs. This thing will start up with a little more than just a "Tap" of the key. I mean it starts right up... However I'm getting an intermittent "Backfire" at Idle. It'll be idling along fine and the out of the blue POW! through the exhaust (sometimes it'll keep running after the backfire, and sometimes it'll kill it). I was just wondering if anyone here could tell me where to start? Should I replace the Coils or Power packs or do you think it's a timing problem or maybe carbs? Any help would be greatly appreciated. I read somewhere that maybe it's a coil leaking through the insulation, or possibly a plug wire grounding out. Does that sound like that could be the problem? If so how do you isolate which coil or wire? Again... any help would be greatly appreciated. Thanks...
 
I'm gonna drop it in the lake tomorrow to see what happens when the exhaust has got back pressure on it. I'll relay my experience in the water tomorrow. I don't know if there's much more that I can do with the Carbs... I gotta be honest in that I really don't know that much about them. The kits that I bought to rebuild them didn't come with a lot of parts but I replaced all the ones offered in the kits. Like I said, I'll drop it in the water tomorrow and let you guys know what I find. Thanks for your help...
 
Sounds leaned out, Try turning the carb air/fuel screw(s) in about 1/4-1/2 turn, wait 40secs. and adjust more if needed. Or better yet run the adjust screws all the way in, then turn each one out 1-3/4 turns and adjust out from there 1/4 turn @time until it sounds right, again waiting about 30-40sec in between adjustments.
If was misfiring from the coil it would normally only do when warmed up. You could take a spark board or inline tester and hook it up real quick(when it acts up,or reproduce the situation) and see if its either not firing or misfiring one of the cylinders.
best of luck..
 
There are no adjustable idle mixture screws on that particular motor !-----------What I have done in the past is install a set of carburetors with low speed mixture screws.-----------Proved that this type of miss is a lean condition.------------See how it runs in the lake.----If it still misses then you may have to remove the idle jets and open them up a wee bit using the next size # drill.
 
Thanks Racerone... I was a little confused... I didn't remember seeing any idle mixture screws on those carbs but didn't know for sure. I don't have another set of carbs with idle screws to try, maybe I can find a set used? Do you have any suggestions? and if so, can I just use them instead of the ones I have on there now? One thing I did fail to mention in my earlier post is that I have not had this motor running long enough to graduate from "Break In" Oil/Fuel mix gas. I don't know if maybe that has something to do with it. I'm gonna drop this thing in the water today per your suggestion and see what happens. I'll let you guys know the outcome... Thanks again for everyones advise.
 
Goommer said:
Thanks Racerone... I'm gonna drop this thing in the water today per your suggestion and see what happens. I'll let you guys know the outcome... Thanks again for everyones advise.
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Well... I put this thing in the water today and I'm still having the same problem but of coarse when it "Backfires" it's not a loud POW! it's sounds a little more like a intermittent Phist! through the carbs. Sometimes it would die and sometimes it would keep running after the "Backfire". I was able to take it for a short spin and learned that it would never plane the boat out a W.O.T. Now this is a little 16' whaler and (I would think) it ought to plane out fast. The throttle response was SLUGGISH at best and it used a BUNCH! of fuel! I mean a BUNCH! I went through about 3 gallons in I guess was maybe a 3 to 4 mile (or less) trek. Something is definitely wrong, but like I stated in an earlier post, this thing will start with just a tap of the key. Anyone got any ideas? I'm still using the "Break In" Oil/Fuel mix gas. I don't know if maybe that has something to do with it.
 
Are the spark plug wires crossed or close together? Old wires can leak and crossfire. You can check them for leakage by taking a spray bottle of tap water and mist the plug wires when it is idling on a garden hose. If it begins to miss w/the water replace the plug wires.

I have had some success cleaning out hidden stubborn fuel varnish deposits in a carb by first disconnecting the fuel supply and running them until they are empty. Then just remove the fuel hose from each carb and spray Seafoam into the carb via the fuel nipple. Fill each carb up and reinstall the fuel hose. Let it sit overnight and then start it up and let it idle. It will smoke heavily until the Seafoam burns off. You'll find the carbs will have a smoother idle and quicker throttle response.
 
kimcrwbr1 said:
There are alot of factors to getting the boat to plane if the bow of the boat points us trim the motor in meaning move the prop forward and if the bow noses into the water move the prop backwards. The 85 should make your boat fly hitting the high spots on the water try putting some injector cleaner in the fuel tank and see if it makes the jets open up a little bit. What size and pitch is your prop it may need less of a pitch both to get it out of the hole and for WOT rpm. Hull design, Trim, is the hull clean and how much weight are all factors to consider when sizing the prop you should get 35mph plus with that motor maybe 50 on a smooth lake. For the break in on a new motor you dont want to run WOT for extended periods anyway 1/2 to 3/4 up and down for at least 8 hrs to seat the rings then use the standard fuel mix. I say for now work on dialing in the proper trim get it to lift the bow up high and move the prop forward until it planes out good if your still not reaching proper WOT rpm consider a different prop to bring the Rs up. Your never gonna get real good fuel ecomomy but when your not lugging the motor it will get better.
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This boat and motor belonged to a friend of mine, I have been in it a couple of years ago before the #3 (OEM) piston blew to pieces (I guess the old OEM pistons were notorious for that) and it would jump right out of the water and plane out almost instantly with 3 fat guys in it. Nothing has changed on the trim or prop on the motor. I tried triming the motor today while I did my little test run but I don't think the trim or prop is the problem. This thing acted as if it were getting too much fuel. It was SLUGGISH! at WOT I'm thinking... "Is that it"? "Is that the best you can do"? Something's wrong here. after my little trip is when I noticed how much fuel I went through. My 6 gallon tank was 1/2 gone... we use to tool around all day on 12 gallons. Carbs and motor are freshly re-built so I know it cannot be a carbon problem.
 
guyjg said:
Are the spark plug wires crossed or close together? Old wires can leak and crossfire. You can check them for leakage by taking a spray bottle of tap water and mist the plug wires when it is idling on a garden hose. If it begins to miss w/the water replace the plug wires.

I have had some success cleaning out hidden stubborn fuel varnish deposits in a carb by first disconnecting the fuel supply and running them until they are empty. Then just remove the fuel hose from each carb and spray Seafoam into the carb via the fuel nipple. Fill each carb up and reinstall the fuel hose. Let it sit overnight and then start it up and let it idle. It will smoke heavily until the Seafoam burns off. You'll find the carbs will have a smoother idle and quicker throttle response.
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I had read somewhere that it could be the plug wires leaking or the coils or power packs leaking through their insulation. Carbs are freshly re-built so I'm thinking it's not a varnish or carbon build up problem. Leaking plug wires or coils or power packs might explain the backfire, but would that affect the performance and fuel consumption? I'm thinking about just buying 2 new power packs and 4 new coils but wanted to get some input from some of you experts on here. I don't want to buy a bunch of parts if you guys don't think I need to. This is my first 2 stroke and I'm learning as I go...
 
Do you have spark on all 4 cylinders that will jump a gap of 7/16" with a snap you can hear ?-------------Has the flywheel key been checked ?--------Does the primer bulb go hard after a few pumps ?----Look into the carburetor bores ( strong flashlight required ) at full throttle out on the lake and compare amount of fuel coming up the 4 main jet nozzles.--Buying new parts can get expensive.-------Testing and good trouble shooting is the path forward !!
 
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racerone said:
Do you have spark on all 4 cylinders that will jump a gap of 7/16" with a snap you can hear ?-------------Has the flywheel key been checked ?--------Does the primer bulb go hard after a few pumps ?----Look into the carburetor bores ( strong flashlight required ) at full throttle out on the lake and compare amount of fuel coming up the 4 main jet nozzles.--Buying new parts can get expensive.-------Testing and good trouble shooting is the path forward !!
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Yes, I do have spark on all four wires and the spark will jump 7/16". How I checked that was by drilling and tapping a hole in a peice of 1" square mechanical tubing and made a washer thick enough to space the bottom of the spark plug electrode 7/16" from the opposite of the tubing. Spark jumped the full gap. I'm not too sure about the loud "Snap" my hearing is not that keen. I also looked at the flywheel key, and it didn't appear to be damaged (nothing distinct anyways...) I actually though about changing the woodruff key but had some trouble getting the old one out of the crankshaft so I quit. I was afraid I'd open up a whole new can of worms. Looking down the carbs at full throttle would be kind of tough because I'm out there by myself. I was actually thinkin about maybe trying to find a used "Testwheel" on ebay and then selling it after I used it.
 
I am not sure why you would consider spending $400.00 on parts to maybe fix it.----------------Trouble shooting is a must , but it seems that is not very common anymore.----------Get a helper to drive the boat !!-------You are sure that you have the plug wires going to the correct cylinder, absolutely sure !
 
kimcrwbr1 said:
Timing and sync between the timing and fuel delivery are criticle for any outboard also we are questioning the condition of the high and low speed orifices in the carbs, Did you soak them in carb cleaner overnite and blow them out good with air. You just need to dial it in good sounds to me.
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I did not soak the carbs in carb cleaner over night. I read somewhere not to do that because of some kind of coating on marine carbs and carb cleaner eats that coating off. When I re-built these carbs I was thinking to myself that the re-build kiys sure didn't have many parts. If I remember correctly just some gaskets new needle valve and seat, new float. I've never done a marine carb before so I didn't know what to look for so I just replaced all the parts that were provided in the kit. Did I miss something?
 
The high speed jets ( orifice ) are in the bottom of the float bowl.-----Did you remove them for cleaning ?------You read about a coating on the castings ?----------------I soak those carburetors in a cleaner , never a problem doing that.
 
My 6 gallon tank was 1/2 gone...
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Did you install the frost plugs back over the jets? I think you need to revisit the carbs and clean the fuel circuits completely. If you are concerned about carb cleaner use spray Seafoam in the fuel passages. It cleans extremely well but isn't cheap like carb cleaner. The above video is informative.
 
racerone said:
The high speed jets ( orifice ) are in the bottom of the float bowl.-----Did you remove them for cleaning ?------You read about a coating on the castings ?----------------I soak those carburetors in a cleaner , never a problem doing that.
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To tell you the truth, I don't remember if I removed the high speed jets or not. I'm thinking because I don't remember, probably not... I read in a service manual (not a factory manual) to not place the carbs in either a hot tank or in carb cleaner vat because it said that a "sealing compound" is used around the the metering tubes and on the casting to eliminate porosity problems. It said that putting them in a hot tank or carb cleaner that it could remove that sealing compound. So I did not soak them. I guess maybe I need to. These are my first 2 stroke carbs and I don't know too much about them.
 
guyjg said:
Did you install the frost plugs back over the jets? I think you need to revisit the carbs and clean the fuel circuits completely. If you are concerned about carb cleaner use spray Seafoam in the fuel passages. It cleans extremely well but isn't cheap like carb cleaner. The above video is informative.
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I did not install the new frost plugs... Was a bit afraid to go there but after watching the video kimcrwbr1 suggested I'm not too scared to do it now. Yes... I think I need to go back through the carbs, I'm gonna take them off this afternoon and go through them again. I'll let you guys know what I find. Thanks to everyone for your help.
 
I think we are starting to look at a classic ---" I fixed the damage, but not the cause of the damage "----------and hopefully no damage was done while running this rebuilt motor.---------------These motors have carburetors that are also the oil pump so to speak.--------------No fuel or lean on fuel also means less oil going into a particular cylinder.------------Go over the carburetors carefully, remove the high speed jets ( 5/8" long or so ) and clean everything !
 
I have seen this before. When assembling after the rebuild, it is very common to swap the coils on the heads. This makes the #2 and #4 cylinder fire out of time.
One way to absolutely verify, is to index the flywheel and look for spark at TDC for each cylinder.

But, if it were me, I would swap the spark plug wires on #2 and #4 and try it. It will not cost you a dime, and you will know immediately if that was the problem. By the way, if this is correct, I bet #2 and #4 are running cold.

If after swapping the plug wires, it runs right, then fix the proper coil locations/ignition primary colors so that the engine is firing properly.

Seen it too many times.....always after a rebuild, when the coils are removed at teardown, but the wiring is left alone...nothing is actually disconnected.....they always put the wrong coil on the wrong cylinder.
 
a coil swapped will shear the flywheel key pretty quick ..... i think i would pull the flywheel and replace the key and make sure the wheel is torqued to specs...verify correct wiring....the wiring may be correct now but could have been swapped sometime in the process....
 
Well I took the carbs off last night. What I found was (you know that little wire that holds the needle valve against the float?) that thing must have come off during assembly or something. It was lodged in the bottom of the float bowl. I'm thinking maybe it was restricting the fuel circut. Took all the jets out (they were clean by the way...) and made sure they were clean, and sprayed carb cleaner thoughout carbs to make sure all orfices were clean. Also blasted with air these carbs are clean! I'm gonna put them back on tonight. Check the timing using "Joe Reeves Procedure" re-check the syncronization and see where it takes me. If you guys have any other suggestions I'd sure like to hear them. Oh... The wiring is/was correct, no crossed wires or anything.
 
Sorry to leave y'all hangin... I just haven't had a chance to work on it this week. I may get some time tomorrow to get back on it. Stay tuned...
 
Well... I still haven't had a chance to stick this thing in the water, so I started with muffs and still had a backfire or maybe it's a preignition problem. So I took the carbs off again just to make sure that everything was still clean and they were. So I put them back on and of coarse still had the same problem. So, I got pissed off and went ahead and bought 4 new coils because I thought maybe one on them was somehow leaking current or grounding through a wire or possibly through the coil itself. Changed them all out today and started it up... no change. I thought the new coils cured the problem at first but it started doing the same thing after it warmed up. Is it possible there's a "hot Spot" in one of the combustion chambers that heats up after it warms up? And then every 10th or whatever revolution heats up enough to cause a pre-ignition? I'm posting 2 short video's, maybe some of you guys have seen this before, notice in one of the video's the backfire/pre-ignition is violent enough to stop the water flow on the tattle tail.
I'm about to pull my hair out. Like I said... This is my 1st 2 stroke and I'm at my wits end. Any help would be greatly appreciated.
 
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