Backfire on Acceleration

[jeverse]

My 1995 230 OV 5.7 has a problem that has gradually worsened over two years. When I quickly throttle up the engine backfires, now to the point it will not get on plane. With a very s-l-o-w throttle application it will ultimately plane, but it takes a deft throttle hand. Maintenance items like plugs, cap, rotor, filters, etc are all new and have been replaced yearly since I bought the boat in 1999. The carb has also been rebuilt, then replaced and the fuel pump is new - all proving to be no help. My mechanic thinks the timing chain is stretched, but w/low hours, fanatical maintenance and a non-aggressive driving style (crusing speeds of 3500 RPM and under) I'm doubtful about the timing chain theory. Also, when a timing light is used and the RPM is stepped upward in 100 RPM increments from idle, the timing marks "scatter" around 1100 RPM in neutral. I don't want to pull the motor for a "maybe" timing chain replacement. Anyone had a similar experience and find a solution? Thanks.

 

[sdarc]

Worn out distributor. I'd check there. Could be worn advance springs causing the scatter.

 

[CliffA]

i assume your engine has electronic ignition so it could be a faulty control module or maybe a coil or ignition sensor in the distributor going bad.....have you checked the anti-siphon valve in the fuel line at the fuel tank?.....they can get partially clogged and restrict the flow of fuel to the engine.....

cliff

 

[jeverse]

Thanks for your input!

I've experienced the worn (actually rusted) advance weight springs w/an old Chrysler powered boat, but its my understanding (and my mechanic confirms) the advance in my boat is controlled electronically - so no springs. In fact, I pulled the cap when the problem first started and looked for the springs only to find none.

When you say "worn distributor" what exactly wears?

 

[jeverse]

Thanks for the input, Cliff!

I've always suspected the problem was electrical, but rather than throw a bunch of expensive parts at it I was trying to diagnois the symptoms. Any idea if/how the module, etc can be checked? Is there a good mechanic-level repair manual w/diagnosic procedures or on-line source that you know of?

Haven't checked the anti-siphon valve, but since it does backfire through the carb I've always assumed it was due to a lean condition if the problem is indeed fuel related.

 

[CliffA]

Thanks for the input, Cliff!

I've always suspected the problem was electrical, but rather than throw a bunch of expensive parts at it I was trying to diagnois the symptoms. Any idea if/how the module, etc can be checked? Is there a good mechanic-level repair manual w/diagnosic procedures or on-line source that you know of?

Haven't checked the anti-siphon valve, but since it does backfire through the carb I've always assumed it was due to a lean condition if the problem is indeed fuel related.

if the timing is set properly it should remain steady at idle....the fact that it is jumping all over the place at idle suggests to me a problem with the control module....on my Thunderbolt 5 ignition system the control module is supposed to maintain the timimg throughout various running and load conditions on the engine....unfortunately i don't know of a foolproof troubleshooting procedure for the control module and they can be quite expensive (around $500) to replace......i don't like to just throw parts at a problem until i get lucky....but in some cases where a problem only manifest intermittently making 'educated guesses' is about all you can do and replace suspect parts....a new ignition coil and ignition sensor will only cost you around $100 total so you may want to start by replacing these two items....if the problem continues the ignition control module may be suspect...a marine mechanic told me the best way to check the control module when you have an intermittent problem is to leave a timing light connected to the plug wires all the time...then run the boat until the problem ocurrs....check the timing during the time the engine is acting up...if the timing is jumping all over the place the most likely cause is a faulty control module....

a partially clogged anti-siphon valve (or any fuel line obstruction) can cause a lean fuel/air mixture in the carb and cause backfiring and/or limit the RPM's the engine can reach.....

good luck...

cliff

 

[sdarc]

Thanks for your input!

I've experienced the worn (actually rusted) advance weight springs w/an old Chrysler powered boat, but its my understanding (and my mechanic confirms) the advance in my boat is controlled electronically - so no springs. In fact, I pulled the cap when the problem first started and looked for the springs only to find none.

When you say "worn distributor" what exactly wears?

First go with what Cliff suggests.

The distribtor shaft sits in bearings, these can wear and cause inconsistant timing.

 

[AKBASSKING]

Would you have access to a Mercruiser manual with a troubleshooting tree? Seems you are poking around in the dark.

 

[jeverse]

I don't but am investigating the purchase of one.

 

[jeverse]

Good counsel - thanks for the detailed info. It gives me something to go one. I love my boat, but am at wits ends w/this issue.

 

[jeverse]

Got it. Thanks everyone!