1996 mercruiser backfire through carb.

[Century Arabian]

Hey guys, I am new to this forum so bare with me. I have owned this boat for four years with no issues. It's a 1996 Century Arabian 21 with a 5.0 ltr. mercruiser. Last season I started to hear a little whine when I would turn the wheel pass center and sometimes the starter would make a little grinding sound. Anyway over the winter I decided to change the Gimble bearing and rebuild the stern drive. I also replaced the starter. This afternoon when I tried to start the motor, it appeared to bring up in mid rotation. I turned off the key and tried again to start it, it was very difficult to start and backfired through the carb. I finally got it started by blocking the choke open but it is popping through the carb with every revolution. The engine starts fine but continues to make a sharp popping sound through the carb. I have not done a compression test as of yet, just wondering if anyone had any ideas.
Clyde

 

[merc120_81]

Start with the simple stuff, check the ignition timing. Also was there any change to carb settings ?

 

[Century Arabian]

merc120, there was no change to anything, I ran her on the trailer last fall and the motor ran fantastic.

 

[Bt Doctur]

backfiring back thru a carb is usually a open/damaged/stuck intake valve if the cap/points/rotor have been changed

 

[Century Arabian]

Thanks guys. I will check all of the ignition system and do a compression test and get back to you.

 

[kenny nunez]

Like the Doc said, you have a stuck valve that may have pulled a rocker arm stud or the valve is just stuck in the open position. Sometimes can be caused by an exhaust valve not opening because the rocker arm stud pulled out. Usually caused by the valve stem being gummed up. A compression test will tell what cylinder is the cause.

 

[nola mike]

Did you take off any plug wires? Reversed/out of order plug wires could do it.

 

[porscheguy]

+1 on checking plug wires. This can definitely cause rough running and backfire, and is free and very simple to check.

 

[Century Arabian]

okall cylinders range from 140 psi to 160 psi. I will pick up a cap, rotor button, plugs and wires tomorrow. I checked for top dead center today and realized someone has been into the engine, as the distributor is in 180 degree out. or at least out from what I am use to. TDC, rotor button should be pointing forward towards #1 cylinder, this one is pointing aft. No big deal I guess, but will probably change it tomorrow. Keep u posted.

 

[Century Arabian]

Nola mike, issue is nothing should have changed since last fall. The motor was working fine when I put it on the trailer. Changing everything out tomorrow, a couple of plugs looked corroded pretty bad.

 

[Century Arabian]

Did you take off any plug wires? Reversed/out of order plug wires could do it.

Not before all this started, I have removed them now to replace them.

 

[Century Arabian]

Well, I found the source of my backfire, sometimes it's the simple things, and sometimes it's not. This time it'd not. guess I had a bad cylinder head gasket as well as a bad upper manifold gasket, combined I ended up with salt water in the #6 exhaust port and on the # 6 cylinder seizing the valve solid. When I tried to start it this season it broke the #6 exhaust push rod. \\looks like total head rebuild. Thanks for all the advise. Off to the machine shop tomorrow.

Broken pushrod:


Number 2 bank, excessive rust on # 4 and # 6 cylinders.


# 6 exhaust valve stem.


Upper manifold gasket failure.


When this boat was trailered last fall there was no signs of pending failure. Anyone with older boats may want to pull the exhaust manifold and take a quick look in the exhaust ports. \any signs of rust may indicate pending failure. I removed the manifold from number 1 bank and all looks as it should, just exhaust soot.
Happy boating.

 

[nola mike]

I had almost the exact situation a few years back. Got lucky, bent a valve and pushrod but no other damage to the head. Yours looks ugly though. ..
any damage to the pistons? If that water got in there you may be dealing with more than the head

 

[Century Arabian]

It appears this whole issue was started from a bad gasket between the riser and exhaust manifold, but also looks like the cylinder head gasket between 4 and 6 and a water jacket was also deteriorated. Anyway, the valve was seized so solid in the valve guide that the guy at the machine shop is not sure if he can get it out or not. I am at work now for three weeks. Let's just hop i can get away with rebuilding the heads. The piston looks good, no water in the base. let's keep our fingers crossed.