Intermittent Backfire at Idle

[WhiteRice]

97 Evinrude 25HP 3Cyl. Runs and starts great. Recent history of motor: Carbs have been rebuilt within the last 60 days, all gaskets are new and replaced manifold gasket, replaced all fuel lines with new ones and new clamps within the last 4 months, new PowerPack in the last 4 months, carb idle adjustments are to Evinrude Factory Manual specs, all re-circ lines have been replaced and re-clamped, new fuel bulb, tank vents properly, good blue spark from all cylinders, flywheel key inspected, added manual primer to assist in cold starts but motor was backfiring prior to this install.

When the motor is running at WOT or even at 1/4 and 1/2 throttle it runs great. At Idle, no wake speed or in my test barrel I will get an occassional backfire with a slight puff from my exhaust. I added a clear fuel filter when I changed the lines so I can keep an eye on debris accumulations and when the motor backfires, I can see a pop of air in the fuel filter itself when the motor backfires. Obviously I am getting an influx of air somewhere but I cannot locate where or what would be causing the backfires. They are not violent backfires but they are present and very inconsistent. Ideas on where they may be originating or what could be causing them?

 

[Scott Danforth]

Lean sneeze. Look at your carbs

 

[boobie]

Do a cylinder drop test on it to see if you can narrow it down to any one cylinder.

 

[WhiteRice]

I've looked at the carbs pretty closely. When they were rebuilt in Dec last year the jets looked clear. Anything particular besides the highspeed jets that I may need to look at more closely?

 

[jbuote]

Idle needle adjustments perhaps?
You said you adjusted to factory spec, but sounds like maybe a bit too lean at factory specs...

 

[WhiteRice]

Could be jboute. I guess I need to break down and do an adjustment while underway. I know it's done regularly, just a little tough to hold a steady hand on the bay making 1/4" adjustments.

 

[jbuote]

LOL!! Yeah....

Not sure what your manual says, but mine for my 71 50hp is to adjust on water, in gear...
End goal is to idle at 700rpm in reverse gear... (per my manual)

But yeah.. 1/4 turns on a rocking boat is fun!!
Unfortunately, unless you have the proper test prop (to do it in a test tank), then on the water is the only real way to do it..

Edit: And to the point boobie made.... I think he's right too... if I understood his suggestion correctly, do a drop test, and if pulling the plug boot from any 1 cylinder makes the backfire/lean sneeze go away, then that's the cylinder you most likely need to adjust the carb on...

 

[WhiteRice]

Thanks again jboute, boobie and Scott. I did a cylinder drop test a few months back when I was concluding the timing issue. All three cylinders had a noticeable impact on the RPMs. When the shop that replaced the powerpack took off the head cover to rework the water seals, they indicated the pistons, rings, cylinders and exhaust ports looked pretty good.


I'm taking her out fishing tomorrow but the idle adjustments will have to wait for another day. Forecast on Galveston Bay is a SSE wind at 15mph. I may see if I can get my friend to steer while I am still coming out of the canals. That's a little wind protected but I'm not sure that will give me enough time to adjust three carbs. I guess we will see.

Is there a particular RPM range (or speed) I should aim for when attempting to make the adjustments?

 

[oldboat1]

rpm range of 650-700 in gear is typical. You should be able to make those idle adjustments in a barrel or at the dock -- no need for a test wheel. Make sure the engine is at temp.

 

[Silvertip]

Tie the boat to the dock, shift into gear and make the adjustments. It's an "idle mixture" adjustment. Not an adjustment that needs to be made at any higher rpm. Those adjustments are made in very fine increments. In until engine begins to bog. Out noting the rotation until the engine again bogs. Somewhere between those two settings is correct. You do this alternately on each carb. You also may need to readjust idle SPEED during these adjustments. Be aware that you repeat this process at least twice.

 

[WhiteRice]

This may be a dumb question oldboat1 but I've tried putting it in gear while in my test barrel and I end up with water going everywhere and my hose can't keep up with what it splashing out. If I do this at the dock, is it better to put it in a slip bow first and then place in Forward gear (idle speed of course) with the nose pushing into the bulkhead or tie off the bow and place it in Reverse at idle speed? I have a Carolina Skiff with a rub rail all the way around so I'm not overly concerned about damaging the fiberglass.


*Edit* Thanks Silvertip. Should I start from scratch and bog down (IN) the idle screws in on all three carbs at the same time and then adjust out one at a time or adjust one at a time from #1 - #3 individually?

 

[jbuote]

Joe Reeves has a sticky about adjusting carbs..
Don't have a direct link to the most recent update to it I've seen him post, but this is copied from another post I read....

Maybe this'll help?

****************************************************************************************************
(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)

Initial setting is: Slow speed = seat gently then open 1-1/2 turns.
Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.

Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

Note 1: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.

Note 2: If the engine should be a three (3) cylinder engine with three (3) carburetors, start the adjustment sequence with the center carburetor.

When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.
******************************************

 

[WhiteRice]

Great! I've seen that sticky and I just was curious why it says "this is for early models" but then I am just now seeing "or smaller HP Outboards"... This will be very helpful... now. LoL

 

[jbuote]

I can't imagine a problem with bow first into slip in forward (with bumpers or rub rail if you're truly not concerned) or tie off bow and use reverse...
Though I can't say for sure..

My local lakes don't have docks, so I'm stuck rocking with the boat while someone steers me around at idle... LOL

 

[WhiteRice]

Great! Maybe I'll take a few minutes before I load back up and make some adjustments. Thanks again!

 

[oldboat1]

It sounds like your throttle idle stop might need to be adjusted -- throttle it down to a comfortable (i.e., lower) idle, then do the carb idle mix adjustments. (See Silvertip's reference to speed adjustment.)

 

[WhiteRice]

Makes sense. Then dial it back up a little once the adjustments are completed?

 

[oldboat1]

Depends. Want a baseline idle of 650-700 rpm in gear (proper operating temp, and in the water -- not on muffs). Set the carb needle(s) at the initial setting for your model -- should allow you to start the engine with the throttle in the "start" range. That's a high idle. Throttle down to a low idle if you can, and start carb needle adjustment. Leaning out the mix (clockwise) increases rpm. Throttle the engine down and continue carb adjustment. When you get a lean sneeze, back the needle out about 1/4 turn -- usually gives you the right setting, or close to it.

Final setting for the throttle idle shouldn't be higher than 650-700. It could be set lower, if, for example, you want to stall the engine by throttling down. On smaller motors, throttle idle is sometimes temporarily set at a desired trolling speed, which could be above 700 rpm. Permanent setting at dock buster speed, though, is a tad too high!

Hope this helps. Easier to do than to describe.

 

[WhiteRice]

Thanks oldboat1. Fishing was a bust yesterday but I was able to come back in, tie off the bow and put it in reverse at about 700RPM. I basically started from scratch (as I am just now seeing you suggested) and just took my time with 1/4 turns. I got it to a point where it didn't sneeze for about 5 minutes. Then I put it back in neutral and 10 seconds later I got one sneeze. I then let it run for another 5 minutes and I got one more :-/

Odd... It wasn't sneezing in gear but seemed to do it at idle. No mind you the idle was much smoother at neutral this time and the sneezes were much more spread out but still present.

Much better than before though so thanks again!

 

[lmuss53]

If you are still getting a sneeze turn the idle screw or screws out another 1/8 to 1/4 turn.

I also take the throttle cable off at the beginning of this process, so it is easy to keep the idle against the stop. If you make significant changes to the idle stop screw you may have to adjust the cable a little to keep the idle against the stop when you shift to neutral.