Backire through carb
 

Could really use some help.. I have a 1987 27' Mach 1 Concorde. Powerplant is a 454 mag 330 horse coupled with an alpha outdrive. Having wot problems. Wants to fall on its face at wot. This will not allow me to reach max rpm. Have changed plugs, prop,rebuilt carb, changed secondary metering rods. Now it will reach 4400 rpms and 49 mph. Since changing all of this it is now backfiring through the carb at wot. I am going crazy trying to figure this out. Any help would be appreciated. Thanks

A compression test and check for water in fuel. Would be the first things I would check.

Already checked for water in the fuel twice this is not the problem. I agree a compression test is in order I am trying to find the time to get it done. thanks

Could have a bad cam lobe.

if the cam lobe and or lobes are wore will this show up during a compression test.

Backfire through the carb usually indicates a lean mixture, which I don't think is your problem. The second thing I'd look for would be burned valves, which will show up on a compession check. As far as losing lobe on a cam, I doubt it. Will usually happen before 21 years. 49 with that power plant on a 27 foot boat ain't bad. Btw, it's not a 454 Mag, it's just a regular 330 horse 454. That was the only year that Merc ever mated a BBC to an Alpha drive. It just has a sticker on the drive that says 454 Mag. I have one of those drives in my garage. Just a standard Alpha drive.

Best of luck!

I was thinking the exact same thing.

I had this problem with a truck one time and it was the cam that was bad. good luckm .02:evilb::cool-smiley-027:

Thanks for all of the input. Hopefully it will quit raining here sometime this week and I will be able to perform the compression test. Hopefully this will tell the tale. Much appreciated.

Sounds like burnt intake valve. My 289 '66 Mustang would do that.

If compression test show nothing, check for leaks or rot at intake manifold gasket.
That would make a cylinder run lean, but then that should show on the spark plug.

When was the last time you changed the plug wires, cap and rotor?
Also...Double check the firing order.

The falling on its face at wot has been going on since I purchassed the boat 2 seasons ago. I replaced the cap rotor button plugs and plug wires last season. This did not effect the issue. Also replaced fuel pump and and check valve and the water seperator twice. Before the last round of changes ie: metering rods plugs and prop I could only pull about 4000 rpms. With the change of these things I was able to achieve 4400 rpm but thst is when the backfire started.

My first thought was plug wires on the backfire issue. On the falling flat, I'm thinking tired valve springs. Don't know what your total hours are but 21 years is an eternity for a boat motor, even sitting. May be time for a top-end job.

A compression test will tell you if you have a problem, but it won't tell you where. You need to do a leakdown before disassembly. No reason to do the heads on a motor with a cylinder problem.

I think in terms of likeliness. The exhaust has probably been leaking and now the valve seats are rusted and pitted. when you changed the spark plugs last season did any have any rust on them. maybe one or two of the center ones? When you do your comp test pay attention to see if there is any signs of rust or water on the plugs.

Chris is right the leak test is more telling but a compression test will help you find direction.

I replaced the plugs last year and this year. All appeared to be normal. Looked almost new when removed. The engine hour meter has been replaced and reads 265. Don't really know why it was replaced if it was due to rebuild or repower. Or could have just been to lower the hours. I really don't know much about what happened to to the boat before I purchased it. What really makes me crazy is that at all other rpms the boat purrs like a kitten. Theres no ticking or anything. I almost wish something would break so I could figure it out.

Check distributer,main engine, and amplifier grounds. Some early motors did not have a ground from dist. housing to motor. Also check main engine plug. Spread prongs and spray with CRC and push in completely. You can try with alligator clips 12 volts to coil.
After you try these things and if compression is good I'd pull valve covers to check springs and valve movement as others have mentioned.
Good luck

For the hell of it...change the coil out.

I've done that more times than I care to remember, and not once was it bad coil. Usually, when someone claims to have fixed a problem by changing out the coil, it's because they changed out the bad connection at the coil.

If I remember correctly 4400 is about the redline for that engine, and with 21 year old springs, you could be getting valve float.

The mercruiser manual suggests 4400-4800rpm @ wot

I had the same problem in a 1989 454 mag at 400 hrs. Went through the same replacement pattern you did. Finally pulled the heads and found a few cracked valves, intake & exhaust.

Compression test did not show the problem. Did not do a leak down though but it might show the problem. In fact the problem got worse. Like you only poped once a while at first above 4000 rpm. Then got worse down to 3500 rpm.

I can't guarantee you have the same problem, but sure seems the same. If you do get the heads done, be sure a shop does it right for a marine engine with Inconel or other correct valves. Good luck.

Double check your work here since this is when the backfiring started.

You said it has been falling on it's face at wot ever since you have owned the boat. Correct? I had this happen to me right after a total rebuild. The lobe on the cam for the fuel pump wore off in less than 30min of run time after 1hour and 45min. of break in. My motor had the same symptoms you are talking about. I don't know how to tell you to check this maybe mercman can help in checking this.

Install a in line, fuel pressure gauge at the carb.
Run it without the hatch if possible and have someone monitor it at WOT. If it starts falling, ya know what it is.


Darrell.

If the cam lobe for the fuel pump is worn wouldnt this condition exist at all rpms? Also, does anyone know what kind of pressure I should have on the fuel line at the carb?

5-7 psi, preferably the 7psi.

My suggestion on the first thing to do is pull the valve covers and check the pushrods and rockers. Unfortunatly, I've got alot of experience with this.:rolleyes::(:(:rolleyes:

If the cam "pump" lobe is worn, it will cause a reduced movement in the fuel pump, reducing volume. At lower rpm, it may be "enough" flow for the motor. As demand increases, the pump simply cannot keep up. Just one possible cause.
I chased a backfire last year in my ski boat. Went through the same checks as you. It ended up being ONE plugged pasageway in the carb. After about a month of pulling my hair out, I was able to blow it out with compressed air....and was good to go. Total PITA.


Darrell.