Mercruiser 5.0L backfiring after 3k RPMs

[TampaBoater]

Hello.
So, slowly but surely, Im getting there. I am currently waiting for the rebuild kit for my mercarb 2bbl. If you were following my last thread, I finally got the engine to quit stalling after throttling down to neutral. I guess I was easing down way to slow and was causing the switch by the gear to stall the engine, and i also adjusted the idle mixture screw in water and in gear as the manual suggests. Anyways, that is fixed. New problem now. The engine is shooting out of the hole decently now, but after about 3100 rpms, it starts to back fire frequently. Any thoughts?
To answer some commong questions:
No, I have not changed the spark plugs, but 2 of them.
I have not rebuild the carb since I am waiting for the kit ( but it runs great under 31-3200 rpms.
Thanks a million in advance.
Les.

 

[alldodge]

Re: Mercruiser 5.0L backfiring after 3k RPMs

Hello.
So, slowly but surely, Im getting there. I am currently waiting for the rebuild kit for my mercarb 2bbl. If you were following my last thread, I finally got the engine to quit stalling after throttling down to neutral. I guess I was easing down way to slow and was causing the switch by the gear to stall the engine, and i also adjusted the idle mixture screw in water and in gear as the manual suggests. Anyways, that is fixed. New problem now. The engine is shooting out of the hole decently now, but after about 3100 rpms, it starts to back fire frequently. Any thoughts?
To answer some commong questions:
No, I have not changed the spark plugs, but 2 of them.
I have not rebuild the carb since I am waiting for the kit ( but it runs great under 31-3200 rpms.
Thanks a million in advance.
Les.

If it's back firing that's either timing or valves, valve springs or guides. Don't remember your last post and could go back but make it a bit easier if you fill me in.

 

[TampaBoater]

Re: Mercruiser 5.0L backfiring after 3k RPMs

If it's back firing that's either timing or valves, valve springs or guides. Don't remember your last post and could go back but make it a bit easier if you fill me in.

I my last post, I was having problems with the engine stalling when quickly accelerating, or quickly deccelerating into neutral.
I also researched other forums and seems like other folk are saying water in the fuel?

It runs good under 3k rpms though...

 

[alldodge]

Re: Mercruiser 5.0L backfiring after 3k RPMs

I my last post, I was having problems with the engine stalling when quickly accelerating, or quickly deccelerating into neutral.
I also researched other forums and seems like other folk are saying water in the fuel?

As you probably know but just to state the obvious, backfiring is a spark is happening when a valve is open. If it is coming through the intake it's intake valves, and out the exhaust is the exhaust valves. Now if the engine has an intermittent shutter or stumble followed by some kind of pop, this could be a fuel problem. The fuel pump is not supplying enough fuel, float level not set correctly.

Edit: fuel line restriction

 

[Bondo]

Re: Mercruiser 5.0L backfiring after 3k RPMs

Hello.
So, slowly but surely, Im getting there. I am currently waiting for the rebuild kit for my mercarb 2bbl. If you were following my last thread, I finally got the engine to quit stalling after throttling down to neutral. I guess I was easing down way to slow and was causing the switch by the gear to stall the engine, and i also adjusted the idle mixture screw in water and in gear as the manual suggests. Anyways, that is fixed. New problem now. The engine is shooting out of the hole decently now, but after about 3100 rpms, it starts to back fire frequently. Any thoughts?
To answer some commong questions:
No, I have not changed the spark plugs, but 2 of them.
I have not rebuild the carb since I am waiting for the kit ( but it runs great under 31-3200 rpms.
Thanks a million in advance.
Les.

Ayuh,... A Lean condition, for Whatever reason is a usual suspect in a backfire at higher rpms,....

My guess at this point is yer carb really needs that Kit yer waitin' on,...

Anything, besides gas in the fuel filter,..??

Is the fuel pump, pumpin',..??

 

[fishrdan]

Re: Mercruiser 5.0L backfiring after 3k RPMs

The engine is shooting out of the hole decently now, but after about 3100 rpms, it starts to back fire frequently. Any thoughts?

Lean backfire?...

I'd rebuild the carb and then see if the problem still happens. You may need to do some diagnostics/testing on the fuel system beyond rebuilding the carb, fuel pressure, replace fuel filters, etc.

(looks like Bond-o beat me to it............ )

 

[TampaBoater]

Re: Mercruiser 5.0L backfiring after 3k RPMs

Ayuh,... A Lean condition, for Whatever reason is a usual suspect in a backfire at higher rpms,....

My guess at this point is yer carb really needs that Kit yer waitin' on,...

Anything, besides gas in the fuel filter,..??

Is the fuel pump, pumpin',..??

I just pulled the fuel water separator and there was small particles of debris, and the fuel pump works as I can go at 3k rpm all day. i adjusted the carb mixture as per the manual but I can't tell when the carb is running lean or rich. As to AllDodge comment, is does feel like the back fire is coming up the carb, not the exhaust.

 

[dan t.]

Re: Mercruiser 5.0L backfiring after 3k RPMs

Yep, sounds like a lean condition to me.

 

[dingdongs]

Re: Mercruiser 5.0L backfiring after 3k RPMs

have you done a compression check?
What happens if you give it full throttle getting out of the water and onto the plane?
Remove all 8 plugs to see if their colour is black or straw colour or wet etc.

 

[harleyman1975]

Re: Mercruiser 5.0L backfiring after 3k RPMs

That carb is not like a chain saw. there is no adj for high speed. if it is popping thru carb at top end it is a dirty or partially plugged jet or the float is out of adjustment...or water in the carb...or weak fuel pump. I will bet that the main jet/s (one or both) is/are plugged or just partly restricted.

 

[TampaBoater]

Re: Mercruiser 5.0L backfiring after 3k RPMs

have you done a compression check?
What happens if you give it full throttle getting out of the water and onto the plane?
Remove all 8 plugs to see if their colour is black or straw colour or wet etc.

I have not done a compression check. The spark plugs are somewhat of a dark color like from smoke build up but clean, no sludge or anything. The boat does fine when quickly throttling up now, but can't get past 3000 rpms before it starts backfiring through carb.

 

[dingdongs]

Re: Mercruiser 5.0L backfiring after 3k RPMs

are you sure the firing order is correct as on V 8's many people incorrectly cross wires but the HT leads also breaking down is also a fault that's often overlooked. If you have access try running the engine up in complete darkness to see if the wires are arcing to earth or ground in USA terms.

 

[TampaBoater]

Re: Mercruiser 5.0L backfiring after 3k RPMs

are you sure the firing order is correct as on V 8's many people incorrectly cross wires but the HT View attachment 221889leads also breaking down is also a fault that's often overlooked. If you have access try running the engine up in complete darkness to see if the wires are arcing to earth or ground in USA terms.

DingDongs you are the man…I did cross the 3 and 5 wires. after checking on google for distributor numbers, i realized that when i changed the 2 sparks, i crossed the wires. Thank you so much. I have to say i like this forum and the people in it a lot. Thanks for all your help guys.

 

[TampaBoater]

Re: Mercruiser 5.0L backfiring after 3k RPMs

Update.
I correctly replaced the spark wires and it now cranks up fast as lightning. Can't wait to take it for run. It also sounds at idle like it did before, like a hot rod Thanks all.

 

[harleyman1975]

Re: Mercruiser 5.0L backfiring after 3k RPMs

Thanks for reporting the cure for your ailment!

 

[dingdongs]

Re: Mercruiser 5.0L backfiring after 3k RPMs

Have fun,:joyous:

 

[Walt T]

Re: Mercruiser 5.0L backfiring after 3k RPMs

I think Alldodge is right. This is a classic too far advanced timing symptom. If it were a lean condition then that means the jets and/or power valves are plugged or damaged and it would become lean as soon as it went out of idle/accelerator mode and the backfiring would happen way sooner, around 2000-2500 rpms. With timing, initial timing is still okay at lower rpms but 3100-3300 rpm is exactly where the top of the advance curve should be. That's why it kicks in there. That's my story and I'm sticking to it.

 

[Grub54891]

Re: Mercruiser 5.0L backfiring after 3k RPMs

Did the same thing to myself before. Spent three hours trying to figure it out. My brother pulls in,says mabe you crossed the wires? Well he is not a mechanic,I say no way,I've checked em 3 times. So he checks my checks.....dam! Easy to do in tight places....:embarassed:

 

[TampaBoater]

Re: Mercruiser 5.0L backfiring after 3k RPMs

I think Alldodge is right. This is a classic too far advanced timing symptom. If it were a lean condition then that means the jets and/or power valves are plugged or damaged and it would become lean as soon as it went out of idle/accelerator mode and the backfiring would happen way sooner, around 2000-2500 rpms. With timing, initial timing is still okay at lower rpms but 3100-3300 rpm is exactly where the top of the advance curve should be. That's why it kicks in there. That's my story and I'm sticking to it.

Sorry man, but DinDongs was right, I had the wires crossed, ran it on Sat, and it ran like a champ at 4700 rpms. no problems.

 

[Walt T]

Re: Mercruiser 5.0L backfiring after 3k RPMs

Awesome!