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Tan
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10621
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I have a 1989 evinrude 120 v4 and it's trying to backfire

Customer Question

I have a 1989 evinrude 120 v4 and it's trying to backfire out the top left carb if your looking at thee carbs. Replaced the cdi/powerpack and still doing it. Is this a timing issue?
JA: Sometimes things that you think will be really complicated end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Is there anything else important you think the Marine Mechanic should know?
Customer: Just backfiring issue
Submitted: 4 months ago.
Category: Boat
Expert:  Jason replied 4 months ago.

Good morning, my name is Jason.

Popping out of the carbs is typically a lean issue. Does adjusting the idle mixture screw for that carb have any effect as to the way the engine runs?

Expert:  Jason replied 4 months ago.

Are you still with me?

Customer: replied 4 months ago.
Do I screw it in or out?
Expert:  Jason replied 4 months ago.

Either way, what I am curious to know is if the engines rpms change at all when you turn the screw in or out.

Customer: replied 4 months ago.
Is there a way for me to send you a video of what is doing?
Expert:  Jason replied 4 months ago.

Sure, just upload it to youtube and copy a link over to here.

Customer: replied 4 months ago.
https://youtu.be/AecxPFgnyK0Let me know if that works not the best on technology
Expert:  Jason replied 4 months ago.

In your video the engine was popping out of the exhaust, not out of the carb. Is it popping out of the carb as well?

Customer: replied 4 months ago.
If it's dark you can see it between the carb and where it mounts too.
Expert:  Jason replied 4 months ago.

What is the story on this engine, have you ever personally seen it run correctly?

Customer: replied 4 months ago.
Had it for several years. Had rebuild and ever since never ran the same.
Customer: replied 4 months ago.
Some days it runs good some days are running like crap
Customer: replied 4 months ago.
I bough a power pack/ cdi cause a few weekends ago it would cut out like something was killing power. I tested kill switch and it was good. Then only had half throttle all the way home with it cutting in and out. Sometimes just die like I pulled kill switch.
Expert:  Tan replied 4 months ago.

Hello. My name is Tan.

What you have is a lean idle pop. Have you checked all the recirculation valves and hoses?

The valves are in the intake manifold and should flow in one direction only out. If any of the check valves are not working correctly this will lean out cylinders causing your condition.

Here is a diagram of the recirculation system.http://continuouswave.com/whaler/reference/graphics/OMC_V4HoseRouting.jpg

You have 4 of these valves # ***** one for each cylinder. Check each valve. Make sure they flow one way. http://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1989&hp=120&model=E120TLCEM&manufacturer=Evinrude&section=Cylinder+%26amp%3B+Crankcase

Post back with questions and findings.

Customer: replied 4 months ago.
I got 3 of the 4 out. Should there be gas in them?
Expert:  Tan replied 4 months ago.

Yes these recirculate the puddled fuel behind the reeds. The excess fuel is sucked into a different cylinder. If the valve does check it will blow air in to the wrong cylinder causing a lean fuel condition and the popping through the carb.

Customer: replied 4 months ago.
The cylinder that is acting lean the valve is working. Did you see the video I uploaded. Sometimes the pop is random and goes away with acceleration
Expert:  Tan replied 4 months ago.

Here is the car that corresponds to it's cylinder.

Customer: replied 4 months ago.
The hardest one to get to is bad. Is there anything else I need to check while I'm waiting on parts.
Expert:  Tan replied 4 months ago.

Not at this point. I have seen this condition hundreds of times and this has always been the cure. You should clean the other valves and make sure they work correctly after cleaning. I always replaced all the valves when one went bad. I had to do this as if another valve failed I had to do the labor for free. So this is something you can choose not to do if you like. The valves are pretty expensive.

Customer: replied 4 months ago.
Thanks I got one ordered and if happens again I will replace others. It says I should get it by the 30th and will let you know what happens.
Expert:  Tan replied 4 months ago.

Ok. Please don't forget to positively rate my help as that is the only way I get credit for helping you.

Follow up questions are always welcome at no extra chage.

Thank you...........Tan

Customer: replied 4 months ago.
Will those valves also effect wide open throttle
Expert:  Tan replied 4 months ago.

No

Customer: replied 4 months ago.
Ok cause I ran it today and ran really good then started hearing that popping noise and didn't have enough power to get up on plane. Then kept dying when I tryed to load it
Expert:  Tan replied 4 months ago.

Ok we need to do a spark and compression test. We need to make sure we don't have another issue.

Customer: replied 4 months ago.
Compression is the first thing I checked last time and it's 120 on all 4. I still have to buy one of the spark gap tester tools. But did use spark plug to test for spark
Expert:  Tan replied 4 months ago.

Spark plug won't tell us the strenght of the spark. If the spark is weak it won't fire under compression. That is why we have to check with a spark gap tester gapped at 7/16" to know it will fire under compression.

Try this with engine running pull each spark plug wire off at the spark plug does the engine drop rpms on all cylinders? Use insulated pliers it will shock.

Customer: replied 4 months ago.
Is hard to tell but with it idling rough but I did notice it drop on a few cylinders
Expert:  Tan replied 4 months ago.

A few cylinders this is a 4 cylinder. That tells me it dropped on all cylinders?

Do you hear the spark arc when you remove the plug wire at each cylinder?

Customer: replied 4 months ago.
I did not notice a sound
Expert:  Tan replied 4 months ago.

Ok we are back to using a spark gap tester. If you have one cylinder not firing it will cause your symptoms.

Customer: replied 4 months ago.
Ok I'll try to pick one up tomorrow
Customer: replied 4 months ago.
Yea I heard about them in the service manual. Been meaning to get one but kept forgetting
Expert:  Tan replied 4 months ago.

Ok keep me posted.

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