Backfire out of carb. Looking for opinions before I go further than what was done

[BigJoeB]

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 year. So, I changed fuel filters again, rotor,cap,wires and plugs, adjusted carb had the timing adjusted and had the valve cover taken off to check rocker arms and adjusted them or valve adjustment done, 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 back fire out of car.. 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

 

[K9Deuce]

Could be many things that cause this such as a lean condition in carb ( off idle circuit plugged) incorrect firing order ( plugs wires in wrong spot) etc. given your compression test I would be looking at the cylinder with 90 psi. A leak down test would tell you a lot about that cylinder such as a worn valve, valve seat , bent pushrods, rocker issues , piston ring issue ( would not cause backfire). Note , a rocker that's adjusted to tight could cause the valve to stay open / not seat correctly and this would cause lower compression and would be easily detected with leak down test. I would start with the leak down test and let us know what you obtain.

 

[SCORPIO]

I'd be looking for a sticky intake valve on that low cylinder. Do a leak down test and let us know results.

 

[Lazy Daze]

I had a similar backfire issue on my Sundancer. It would happen at 3,700 RPM, but the scenario was the same. I could run at 3,600 all day long but once I hit 3,700 (although it didn't happen right away - it would take a little time for things to warm/heat up) I would start getting a pop-pop-pop. I would back down to 3,600 and everything was fine - throttle up a bit, and pop-pop-pop. In my case, it turned out to be the ignition sensor that is located under the distributor cap. I can't say whether this is your problem, or not, but in your case you can simply swap port for starboard and see if the problem follows. If it does, great - problem solved. If not, well, at least it was a freebie and you've also eliminated a variable.

 

[ttmott]

You might swap the ign. coils. The coil may not be saturating; not uncommon. Also in the distributor there is a wire that runs between the pickup and distributor body to the harness outside of the distributor. The ignition advance rotates the pickup and consequently the wire also moves. I've seen these wires wear out and either intermittently open up or ground out. Two simple and easy things to check.

 

[scoflaw]

This guy has a cylinder that's 55 lbs short of the other 7. Would it not be prudent to investigate the gun that has the smoke coming out of it first?

 

[SCORPIO]

I'd pull the proper valve cover and see if the valves on the affected cylinder are operating normally. Agree it could be a coil or a pickup but that low compression number has me thinking valve train issue. Coils and pickups are easy and cheap to rule out so it's not a bad place to start. A full tune up couldn't hurt either.

 

[BigJoeB]

I'd pull the proper valve cover and see if the valves on the affected cylinder are operating normally. Agree it could be a coil or a pickup but that low compression number has me thinking valve train issue. Coils and pickups are easy and cheap to rule out so it's not a bad place to start. A full tune up couldn't hurt either.

I did new wires/cap/rotor/plugs a few weeks ago before compression test. Also the fuel filters again to rule out them things. Ignition coil I did not check but will now. I have to pull valve covers and check the rockers/springs and all that before I go further.

 

[Lazy Daze]

This guy has a cylinder that's 55 lbs short of the other 7. Would it not be prudent to investigate the gun that has the smoke coming out of it first?

Agree that that's a likely cause... maybe a tuliped valve (along with the other stuff mentioned above). But it could also just be a coincidence that there's low compression and another issue causing the backfiring. Just thinking that doing some of the easy things first would either find the problem or at least narrow the search.

 

[BigJoeB]

well, it was a bad valve, I took the head off and on was rusted and other carbonated and also found besides that a nice chunk taken out of the piston, now motor shopping lol