MerCruiser 140 Backfire

[Jacalore]

Had acceleration problem, replaced fuel pump, now good, but backfires. Q: does idle mix adj have an effect on backfires?

Electronic ignition, timing good. Thanks.

 

[alldodge]

Re: MerCruiser 140 Backfire

Had acceleration problem, replaced fuel pump, now good, but backfires. Q: does idle mix adj have an effect on backfires?

Electronic ignition, timing good. Thanks.

If your getting a backfire and your sure your timming is correct, need to do a compression test. You have a valve not seating correctly and it's partially open. The slight bit of opening will cause a backfire.

No, idle screws cannot cause a backfire.

 

[Jacalore]

Re: MerCruiser 140 Backfire

Thanks, timing is good, a couple degrees more advanced than spec (8 vs 6), but shouldnt cause backfire. Plugs and wires are a year old. Problem?

 

[alldodge]

Re: MerCruiser 140 Backfire

If it were mine I would do a compression test

 

[Jacalore]

Re: MerCruiser 140 Backfire

Compression tested last summer, all good. Boat has sat since. Taking it out on the water tomorrow to see what's what after fuel pump replacement. Trying to get a handle on what I might expect and any adj that can be made while out there. Thanks.

 

[thejeep007]

Re: MerCruiser 140 Backfire

That spark is before tdc right? Not advanced 8?

 

[Jacalore]

Re: MerCruiser 140 Backfire

That spark is before tdc right? Not advanced 8?

Sorry, yes that's BTDC

 

[bonzoscott]

Re: MerCruiser 140 Backfire

Make sure your fire extinguisher is handy when you go.

 

[Jacalore]

Re: MerCruiser 140 Backfire

Yes, sir!

 

[Jacalore]

Re: MerCruiser 140 Backfire

Took it out on the bay today, no backfires. But the acceleration/stalls problem remains. Seem with new fuel pump, revs up in neutral ok, but in gear stalls around 1500rpm. Does this sound like it's still a fuel issue?

 

[alldodge]

Re: MerCruiser 140 Backfire

Took it out on the bay today, no backfires. But the acceleration/stalls problem remains. Seem with new fuel pump, revs up in neutral ok, but in gear stalls around 1500rpm. Does this sound like it's still a fuel issue?

Sounds like fuel, but that is if it boughs down but you can keep it runnnig at 1500. An engine uses next to no fuel in netural, needs a whole lot under load. Could still be the other issue I mentioned previously

 

[Jacalore]

Re: MerCruiser 140 Backfire

Thanks, Dodge. Not sure which direction to go now... I can take it back to the shop, have compression readings taken again... Im leani.g towards a fuel starvation prob, but what the hell do I know! Tell me, should you smell gas when you take the boat cover off?

 

[alldodge]

Re: MerCruiser 140 Backfire

Smelling fuel under the cover doesn't sound good, probably want to check into that. Do you have a fuel pressure gauge? If so put it in line and measure your pressure, should be around 4 to 6 psi. Should have asked sooner, but have you replaced your fuel filter and made sure your lines are clear?

 

[Jacalore]

Re: MerCruiser 140 Backfire

Replaced all fuel lines and fittings from tank to engine and from fuel pump to carb. All filters changed (in-line, fuel pump, carb). Fuel tank emptied and fresh fuel added.

 

[alldodge]

Re: MerCruiser 140 Backfire

Replaced all fuel lines and fittings from tank to engine and from fuel pump to carb. All filters changed (in-line, fuel pump, carb). Fuel tank emptied and fresh fuel added.

Starting to look like we are circling the wagons around the problem and may come up with it before long. Fuel pressure, carb and compression is all there is left

 

[Jacalore]

Re: MerCruiser 140 Backfire

Dodge, that actually makes me feel better... At least there are targets... Fuel pressure was low, fixed by new fuel pump, but that doesnt eliminate fuel delivery from tank to engine. Carb was swapped with identical part number unit, no change. Previous owner claims equal and good compression in all cylinders.

So, fuel pickup, carb, compression?
What's the logical attack here?

 

[alldodge]

Re: MerCruiser 140 Backfire

From someone outside looking in your telling me that everything is OK because your were told it was OK, it was done last time and was OK, or someone else was telling you it's OK, but I have seen nothing that says it is OK.

Did you check the fuel pressure after the new pump was installed, if so what was the pressure?
Who changed the carb and verified they were the same?
The previous owner said the compression on all cylinders were equal. I have never seen all cylinders equal, there are always variations and minor vairations are OK, major ones are not. You need to do a compression test and know the readings are valid and accomplished correctly. Otherwise we are chasing ghosts.

Something is wrong and your engine and the engine is not complicated. Finding out is actually happened has become complicated. We need some real verified numbers, so if you did not do them yourself think of them as they did not happen

I suggest
Fuel pump pressure test
Fuel flow test at the carb (how much fuel will it pump into a container in 60 seconds)
Compression test

 

[Jacalore]

Re: MerCruiser 140 Backfire

Yes, you are absolutely correct. I was trying to explain how I go to this point. I went after the unknown stuff first. Swapped carbs with same results, so I put that to the end of the list. Same for compression (assumed good). Fuel pressure (2-3 before new fuel pump, 6-8 after). As the symptoms/culprits are narrowed down, I understand I have to revisit some of the preliminary assumptions. Like compression, which was hearsay, you are correct. I'll start work on first hand observations. Thanks, Dodge!

 

[bonzoscott]

Re: MerCruiser 140 Backfire

Title of thread includes "back fire". Provided the timing is good, compression test is in order. It may reveal a bad valve or two. You mentioned taking to the shop to verify compression, but it's not hard with about a $40 tester. It'll cost you double that to go to the shop.

 

[Jacalore]

Re: MerCruiser 140 Backfire

Thanks, Scott, I'll look into that.