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Jeff G.
Jeff G., Marine Mechanic
Category: Boat
Satisfied Customers: 4728
Experience:  42 years experience, Certified Master Technician, All Major Outboards, Sterndrives, Hi-Performance
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I have a 2004 90hp mercury 4 stroke that has a back fire

Customer Question

Hi, I have a 2004 90hp mercury 4 stroke that has a back fire issue, it has new plugs, leads ,coils,voltage regulator, stator, triggers, throttle pos sensor, and water temp sensor
Submitted: 2 years ago.
Category: Boat
Expert:  Jeff G. replied 2 years ago.

That's a lot of stuff that has been replaced. What is the history on this engine? Why was all of this done?

When does it backfire idle or all speeds?

Customer: replied 2 years ago.
The motor has done about 800hrs and this is the first time it has ever given any trouble. Originally it had a miss/back fire and because of the hrs I decided to change the leads, plugs and coils more for a piece of mind thing. Changed these components and still missing, did all of the resistance checks and found the voltage regulator and throttle pos sensor faulty so I ordered them and while I was waiting for them took the carbs off to check the fuel bowls for crap but they were very clean , changed the other components and still missing so I checked the timing with a timing light and found that if you advance the timing 5 degs the back fire went away but it still had a miss.. Did the resistance checks on the stator and triggers again and they come up all right again so I bit the bullet and found a second hand CDI on the net and bought that, no change, getting desperate, only one thing left changed the stator/triggers and the miss has gone but still got a backfire again moved the timing forward 5 degs miss goes away but this is against mercury's throttle pos adjustment procedure.Anyway thats the story. Cheers jon
Customer: replied 2 years ago.
I forgot, it will spit at any rev range in about 30 second intervals
Expert:  Jeff G. replied 2 years ago.

When you did the carbs did you change the intake gaskets and carb gaskets?

Have you checked the timing belt that it hasn't jumped a tooth? What you are describing fits the timing belt being off.

Customer: replied 2 years ago.
yep did all that, lost count of the amount of times I checked the timing, any way took the plastic intake before the carbs off and found that it is only spitting through # ***** so I swapped plugs leads and coils step by step and each time got the same result a pop back through # ***** If I move the tps and advance the timing a little it stops. Starting the suspect the head gasket.
Expert:  Jeff G. replied 2 years ago.

I am experiencing internet problems so please hang in there with me.

Before changing the head gasket we need to do a compression test. This will give us an indication of the condition of the head. Preferably we would like to do a leak down test. This will put air into the system and help pinpoint where there are any leaks.

Yu could very well have a problem in the valve train. A boroscope inspection of the valves and cylinder are warranted. You may be able to borrow these tools from a local auto parts store like Auto Zone. You could have a valve that is hanging open slightly.

What we want to do is to look at the valves while rotating the engine by hand, spark plugs out, ignition off. See if one is not closing all the way.

Post back your results of the compression, leak down test.

Customer: replied 2 years ago.
1 step ahead of you done all that except the leak down test, compression is 190-195 psi across the board checked all valve clearances and all are within spec also cant see any sticking also borrowed a boroscope from work and couldn't see anything either. took it out for a run on saturday so I could test it under load and it goes like it always has at WOT top speed is still the same runs real rough between 2-3000 rpm but as it approaches 3 grand it starts to pick up and clear. next step swap 2 carbs probably 1 with 3 to see if the back fire moves with the carb.let me know what you think. Cheers
Expert:  Jeff G. replied 2 years ago.

The carbs are set up differently so you can't swap them as you want. Have you checked the primer/starter. It could be leaking which would screw up lower speed operation and not affect high speed operation as much. Do you have a service manual or do you need help checking this? It is the round white electrical part on the top carb.

Customer: replied 2 years ago.
yep swapped carbs because 1 & 3 are the same part number and both have the enricheners on them and the back fire has moved with the carb so I swapped the enricheners and no change, must have an air leak in that carb causing it to lean out especially at low rpm when vacuum is high and running on the idle circuit. what do ya reckon?
Expert:  Jeff G. replied 2 years ago.

There being a problem in that carb would make sense. Swapping the carbs depends on the serial number of the engine, sorry about that.

There is an idle mixture screw, it is behind a plug on the to, side of the carb. At this point I would drill and remove the plug and see if you can richen the mixture a little bit. Also check the plate over the idle mixture area on top of the carb. That gasket must seal.

Customer: replied 2 years ago.
i'll give it a go, but it won't be till next week, just started my swing which goes for 8 days of 12 hr shifts so it's dark going and coming to work. I'll let you know how it pans out.Thanks Jon
Expert:  Jeff G. replied 2 years ago.

No hurry, let me know Jon, thanks.