898 MerCruiser Backfiring

[ESM-James]

I have a 1980 898 MerCruiser I/O with a Rochester 2 Barrel on it engine serial number 5496339. Last fall the boat ran good. The end of April before launch the distributor and coil were replaced to update the ignition system. Went from points to HEI after that change the boat ran great for about two months. Last fall the spin on fuel filter was replaced and the gas is fresh. Now all of a sudden the engine will idle ok but will not run above around 2000 rpms and also back fires on hard acceleration. I have eliminated the boats gas tank temporally and hooked up a good outboard tank to eliminated problems like a bad anti siphon valve that changed nothing. Also while someone else drove the boat I sprayed carburetor cleaner in the carburetor to see if the engine was running lean. When that was done the engine only ran worse. The accelerator pump is spitting out gas when the throttle is advanced and the carburetor does not seem to be flooding over. The base timing is at 8 degrees before top dead center and is advancing when the rpm increases. Even though the engine ran good for two months I did double check the firing order 18436572 clockwise rotor rotation twice because others have been in the boat and the firing order is correct. Someone else did change the fuel pump it is pumping gas so I guess the new pump was installed correctly. Any ideas on why this engine is back firing and will not run over 2000 rpms under a load.

 

[NHGuy]

Check your plug wires. Check your plugs. Unscrew the distributor cap hold downs and check the the rotor is on the shaft tightly. Also inspect the rotor and cap for burned contacts.
Check that the manifold and carb bolts are snug. Disconnect the battery and snug up every connection you can find. Don't rip off the bolts just snug them. It can improve electrical conductivity.
My 5.7 would kick back when the ignition wires were old, did you keep the old wires when you did the ignition?
Have you tried very gradually working up the rpms. That can pull water out of the float chamber in the carburetor. But it could take some time.
With some carburetors, when the carburetor is not running at high load the fuel in the top of the chamber gets used. But at larger throttle openings the bottom of the float chamber gets accessed. I don't know the whole set up of yours, but it's possible.

 

[ESM-James]

I am helping someone out with this boat and wanted to change the plug wires and plugs at the time of the distributor change but was told they were new last year. So I do not have any old plug wires to try. I did try bringing the rpms up slow but still could not get over 2000 rpms. The carburetor is a Rochester 2 Barrel and we did run on a different fuel supply to eliminate the change of water.

 

[Bondo]

Last fall the spin on fuel filter was replaced

Ayuh,..... Welcome Aboard,..... So pull the filter, 'n check it's Contents for water, 'n crud,...... then replace it,...

I'm guessin' it's goin' lean, as ya say the ignition is up to snuff,.....

Where did this Hei distributor come from,..?? is it Marine, or outa a car,..??

 

[ESM-James]

The new distributor was a PerTronix D200800 which is listed as a marine distributor by PerTronix I also installed a new coil from PerTronix that is recommended for that distributor. Also because the new coil has an internal resistor a new wire was added to the coil positive connection to bypass the resistor wire because this ignition system can handle full battery voltage and does not need a ballast resistor.
Could I have a mechanical problem the engine is from 1981?

 

[ESM-James]

I will check the fuel filter

 

[Bondo]

Could I have a mechanical problem the engine is from 1981?

Ayuh,..... There can be mechanical problems with a 2015 motor, but so far, the diagnostics here says it's a question of motor tune, 'n/ or fuel supply,.....

If yer concerned 'bout it's general overall health,.... do a compression test,.....

 

[ESM-James]

I did what to post what corrected the problem with this boat hopefully it will help someone. The fuel pump had failed before the engine started backfiring and the owner replaced the fuel pump himself. Also the filter in the carburetor was messed up and installed wrong. I do not know if the problem with the filter happened before or after the fuel pump failure but when I disassembled the carburetor there was a lot of fine metal in the float blow.

So I cleaned the carburetor thoroughly made sure the power valve was clear and working properly and the float setting correct. I rebuilt the carburetor after that the engine ran good no backfiring and WOT of 3700 rpm. Next we will address the other problem of a propeller which is too large but that is another post. (Note per the MerCruiser Service manual WOT range for this engine is 3800 to 4200 rpm.)

​Thank you everyone for your assistance.