What will cause an old OMC V8 to surge?

[kimswang]

Last time I went out I had a surge on one engine while running at around 6000 RPM. The surge was about +/- 500RPM. What could possible cause this? I am leaning towards a battery, but would love to hear other suggestions before I replace the batteries. Having 6 of them, and will replace non or all, it is quite the expense.....

[baja200merk]

Batts won't do it. Look for air in that fuel line using clear hose then check vacuum

[kimswang]

Batts won't do it. Look for air in that fuel line using clear hose then check vacuum

Only one of three motors did it, they are all hooked up to the same fuel tank. Although it was an outer motor, the other outer was running just fine. Fuel pressure is identical as I have gauges on that. No vacuum on my motors as they are all holley pump fed, premix and all.

[PanRonnie]

going out with a fresh set of spark plugs might give an idea on which cylinder to focus for troubleshooting
or none at all offcourse if it comes from your ignition
powerpack swap?

[V-bottom]

Only one of three motors did it, they are all hooked up to the same fuel tank. Although it was an outer motor, the other outer was running just fine. Fuel pressure is identical as I have gauges on that. No vacuum on my motors as they are all holley pump fed, premix and all.

Try running the problem engine on a 5 gallon jerry can to eliminate the fuel tank pick up. Maybe there is some dirt clogged on that tube pick up. Also check the inline filter or filter in the fuel pump. By any chance were your high speed jets drilled out to suit?

[XstreamVking]

Hate to say reeds, don't know much about the omc's. But had one of my 3.0 efi mercs doing the same surge and it was a broken reed.

[baja200merk]

This one might piss of the neighbors but fire up that motor on the hose at night look for a spark leak. If you can't see one U can use a spray bottle with some water to try to make it more obvious.

[racer]

Kevin brings up good points found many engines with spark leak at night and salt will cause issues with this, I think I might have told him that trick a few years back. Coat the engine with wd-40 and see if it runs correct then, this is another way to check a spark leak. You may also have a carb leaking, a weak stator.

[STV_Keith]

Let me know if you figure it out Kim. One of my motors does that if I turn them up above 6400. It's only about 200rpm surge for me though, and it's not the limiter.

[flabum1017]

It is a looper..... do a compression test and look at the cylinders for scuffing. That said, those plastic carbs a troublesome, could have a cracked emulsion tube or warped float bowl..........

[flabum1017]

Electrically, pull all the coils and look for and clean any corrosion between the coils and block......... also take a peek at the flywheel magnets for cracks or movement

[baja200merk]

Racer, teach me? Ha!

Only if he's speaking :leaving:

[kimswang]

I will definitely do some of these test and try to eliminate it. The engines are all Monty engines and have a bout 7 hours of run time, including the testing and dialing in by Monty. That being said, we are dealing with salt water and some may not have been washed out. Today I got a new trim/tilt unit so its time for some test and tune again after I also get my set of 26" props back from a rehub. Thanks a million for all good suggestions and advice, I will keep you posted on the progress.

I know my project has been dragging out but if you have watched the weather channel during the last few months you realize that Florida has had nothing but heat records and record rain, hence the project got a rest for a few months. Good weather ahead so back to wrenching and getting this thing all dialed in. I will run the 26" Rakers for a few hours, then mount the new hydraulics for the jackplates and my set of 28" Bravo 1's and hack at it again.

[lokinutz]

Racer, teach me? Ha!

Only if he's speaking :leaving:

Or writing. :thumbsup:

[rude tim]

I had a coil that was bad on one cylinder on my V8. It idled great but started dropping that cylinder intermediately above 5000 rpm. I found it by luck, i noticed the potting on the end of the coil was burnt. Its a long shot.

[perfmarine1]

What happens if you tap the key(choke) when it surges?

[kimswang]

What happens if you tap the key(choke) when it surges?


Did not do that as it only happened on one motor only. While breaking in/dialing in we did that all the time though. I will definitely check all the coils as my suspicion is there now. But thanks for the suggestion, it will definitely be tried too.

[perfmarine1]

Well you have 3 keys so see what happens,if it stops surging you know it is fuel then. If it falls on its face it could be ignition.