Need Advice- Carb Back fire

[BigJoeB]

Hello Everyone,

This is my first post but been a member a bit and buy through IBoats.

I have a 7.4 Merc Inboard closed cooled with about 625hrs.
After we splashed for this season we took her out for a run and when she hit 2000 rpm start backfiring out of carb.this never happened last years. So, I changed fuel filters again, rotor,cap,wires and plugs, adjusted carb. Fuel was good, this is on port motor, starboard fine. The motor starts fine, idles fine, runs at 1900 rpm like nothing is wrong. Hit 2000 rpm and pop pop. I had a compression test done by marina mechanic a few weeks ago. found all cylinders at 145psi except #4 was 90#psi.

I didnt make them go further yet, I am gonna pull rocker covers off soon and check rocker arms and give a look. thinking valve problem thats the obvious.

lets here some thoughts and worst case

Thanks
Joe

 

[alldodge]

Howdy

Idle to 1500 to 1900 is using the idle circuits of the carb, above that its getting into the main jets. With more info I'm leaning toward needing a carb cleaning currently.

You say fuel is was good, but do you know the pressure (4 to 6 psi)?
The 90 psi cylinder could be bad ring, but if it's a valve this could cause a backfire. Need a leak down test to verify.
I wouldn't touch the rockers just yet

Probably to late but did you look at what came out of the old filter?

What year and serial number is the motor?

 

[BigJoeB]

filters were clean, didnt check fuel pressure. or the marina didnt - would a bad ring cause a backfire? the motor was a rebuilt in 2008. I have to look for serial number but It is the 92-92 Bluewater with Hurth Trans

 

[alldodge]

No rings won't cause a backfire. A backfire is caused from an open or leaky valve, incorrect timing, build up inside distributer cap, or lean fuel condition.

Being a 92, you should have 4 bbl weber carb, good carbs if kept clean inside. Should be good without the serial number, but its always good to know just in case.

I would start with a compression test to check engine health

 

[Pete104]

POP POP POP I think is different than a back fire. Look close at the firing order.

But I think you might be looking at cam/lifter issues.

 

[BigJoeB]

No rings won't cause a backfire. A backfire is caused from an open or leaky valve, incorrect timing, build up inside distributer cap, or lean fuel condition.

Being a 92, you should have 4 bbl weber carb, good carbs if kept clean inside. Should be good without the serial number, but its always good to know just in case.

I would start with a compression test to check engine health

Compression test was done ,,,, #4 cylinder was 95 psi, all others were 145 I was told. marina mechanics did it last week. and it was timed before the compression test. I went through a bunch of trouble shooting before the compression test on the cylinders. I was sending carb out to be rebuilt and still may just because. the boat is a 92 but the motor is a 2008. I have the paperwork it was dropped in july of 2008.Just don't have the serial numbers just have other paperwork and old survey from 08 when it was installed.

I am just trying to figure out if I should install new heads and thats that and be done with it problem solved.

 

[Pete104]

#4 bothers me. I'd pull that valve cover & look at rockers.

 

[BigJoeB]

#4 bothers me. I'd pull that valve cover & look at rockers.

I am gonna do that again, I had them adjusted also same time as the timing was done. They changed the rocker arm nuts also when adjusting but havent opened it up again which I plan too

 

[alldodge]

Sorry for asking about the compression test again, blew right past your comment.

To find out if it's the valves or rings do a leak down test. On the weak cylinder do it as is, then squirt some oil in the cylinder. If it's the rings the oil will reduce the percentage of leak

 

[BigJoeB]

AllDodge, thanks for the input,

So, you are saying to test the cylinder as is and do it again adding some oil and see if there is a difference? and if the test comes out to be less restricted in pressure its the rings?

 

[alldodge]

You want to do it at TDC for that cylinder. Then listen for air escaping and it may tell you straight away that it is the valve(s). You could put oil in straight away but its best to get an over all picture of the cylinder. You could be leaking from more then one area.

 

[BigJoeB]

ok I see, thanks for the input