Engine Backfiring Help

nateo

Petty Officer 1st Class
Joined
Apr 13, 2014
Messages
352
Hello there. just bought and 1986 craigslist special with a mercruiser 305. During my sea trial I heard the engine backfire a few times so the first thing I wanted to start with is a compression test. I've also been having a problem with the engine dying when I put in reverse and then it won't start back up for another couple of hours. Anyone know if it might be a case of a bad carburetor? Well anywho here the test results. I read on the innerweb that any variance below 15% is ok but I still can't help but wonder why I have one high one and one low. On the low one I put in a little oil into that cylinder and it came back with same results. Are these numbers acceptable?

stern

150 - 150
150 - 142
145 - 150
165 - 150

bow
 
Last edited:

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: Compression test result help

Re: Compression test result help

Hello there. just bought and 1986 craigslist special with a mercruiser 305. During my sea trial I heard the engine backfire a few times so the first thing I wanted to start with is a compression test. I've also been having a problem with the engine dying when I put in reverse and then it won't start back up for another couple of hours. Anyone know if it might be a case of a bad carburetor? Well anywho here the test results. I read on the innerweb that any variance below 15% is ok but I still can't help but wonder why I have one high one and one low. On the low one I put in a little oil into that cylinder and it came back with same results. Are these numbers acceptable?

stern

150 - 150
150 - 142
145 - 150
165 - 150

bow

yes.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,582
Re: Compression test result help

Re: Compression test result help

Hello there. just bought and 1986 craigslist special with a mercruiser 305. During my sea trial I heard the engine backfire a few times so the first thing I wanted to start with is a compression test. I've also been having a problem with the engine dying when I put in reverse and then it won't start back up for another couple of hours. Anyone know if it might be a case of a bad carburetor? Well anywho here the test results. I read on the innerweb that any variance below 15% is ok but I still can't help but wonder why I have one high one and one low. On the low one I put in a little oil into that cylinder and it came back with same results. Are these numbers acceptable?

stern

150 - 150
150 - 142
145 - 150
165 - 150

bow

Howdy Bow
I don't see an issue with your compression numbers. Sure could use an engine serial number and what kind of outdrive is on it, I'm guessing an Alpha?

Backfiring, first thing to check is the timing and dwell angle. What do the plugs look like? With it dying when going into reverse I would start looking at the shift interrupter and cable adjustment.
 

nateo

Petty Officer 1st Class
Joined
Apr 13, 2014
Messages
352
Re: Engine Backfiring Help

This is a mercruiser 230hp with mvm 305. Thunderbolt IV iginition.

Engine backfiring issue. Compression test good, spart plugs looked good (replaced anyway). Inspected wires (didn't find any nicks or anything). Took distributor cap off and it looked new (no pitting on those metal contacts or anything).

Here is question I have about timing. I'm about to run out and buy a timing light and wondering which one I should get (as in with advance or without) Do I need to adjust for the total timing or just the initial timing? 8 btdc is on factory sticker on engine.
 

UncleWillie

Captain
Joined
Oct 18, 2011
Messages
3,995
Re: Engine Backfiring Help

Backfiring means two different things.
Is the "Boom" out the Exhaust (Afterfire) or out the Carburetor (Backfire).

It is the difference between misfire and a leaking Intake valve.
 
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Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,468
Re: Engine Backfiring Help

This is a mercruiser 230hp with mvm 305. Thunderbolt IV iginition.

Engine backfiring issue. Compression test good, spart plugs looked good (replaced anyway). Inspected wires (didn't find any nicks or anything). Took distributor cap off and it looked new (no pitting on those metal contacts or anything).

Here is question I have about timing. I'm about to run out and buy a timing light and wondering which one I should get (as in with advance or without) Do I need to adjust for the total timing or just the initial timing? 8 btdc is on factory sticker on engine.

Ayuh,.... The T-Bolt IV is a point, 'n shoot timin' job,...

Clean the indicator, 'n marks,...
Set the timin' to Spec, at Idle,...

If it's Backfirin' through the carb, that's usually a sign of a Lean condition,....
Lean Sneeze,...
 
Last edited:

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,582
Re: Engine Backfiring Help

This is a mercruiser 230hp with mvm 305. Thunderbolt IV iginition.

Engine backfiring issue. Compression test good, spart plugs looked good (replaced anyway). Inspected wires (didn't find any nicks or anything). Took distributor cap off and it looked new (no pitting on those metal contacts or anything).

Here is question I have about timing. I'm about to run out and buy a timing light and wondering which one I should get (as in with advance or without) Do I need to adjust for the total timing or just the initial timing? 8 btdc is on factory sticker on engine.

Cheap point in shot timing light as Bondo mentioned, get one with an inductive pickup for the spark plug lead
 

nateo

Petty Officer 1st Class
Joined
Apr 13, 2014
Messages
352
Re: Compression test result help

Re: Compression test result help

That was it. I adjusted the timing and all is well now. The hold down bracket was barely on there. I suspect it loosed up. Nice and tight now. Thanks
 
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