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Backfire at full throttle


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I think I may have a bad plug wire.  I'm down in South Padre and had an opportunity to go wide open.  At full throttle I'm getting an after fire in the exhaust, which coincides with a miss.  If I pull a little throttle it stops backfiring.  Occasionally a miss develops at lower speeds but no backfire.  Until today I thought I had a carburetor issue.  But the popping at full throttle makes me think it's an ignition issue.

thoughts?

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I had that problem. Chugging around at low speed, all seemed fine, until I started to "give her the gas", then the popping through the carb would start. Very nerve wracking when it happened.

It was the fuel pump not supplying enough pressure / flow to overcome the floats and keep the fuel bowl filled under acceleration and heavy fuel consumption. Changed the fuel pump and problem went away.

Running too lean makes it sound just like an ignition problem. 

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I had the same problem a couple weeks ago after getting my spark plugs replaced. Couldn't get on a plane and it started to backfire around 3k rpms. I took it back to the mechanic and they said it was a bad ignition cable. They put one on my boat temporarily that's not factory and it runs great. I have the new spark plug wires and ignition cable kit that I ordered from Cecils that I plan on installing saturday.

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There is a way to find out if all the cylinders are running lean or a random spark in the ignition system.  Check the spark plugs. Hopefully after a wot run of about 30 seconds & a VERY quick stop & shut off the engine. Then drop the anchor. You can cool down completely. Then check each plug. Odd color would be a nice problem.  All off color. VERY WHITE ?  Definitely lean at WOT . Older engines do get a rare sticking valve action. It can drive everyone nuts. VERY RARE.  1950 Oldsmobile V 8. Running the engine with the valve covers off gave the  " squeaky "  rocker arm away.

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On 4/6/2017 at 9:59 AM, Phillbo said:

Time for a full tune up.

I don't think a full tuneup is the answer.  I did a full new engine just just last year.  I suspect a bad plug, cable or possibly even cap/rotor.  Everything is new except the plug wires, which are possibly original.

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I had this problem two seasons ago.... right after a full engine rebuild.  It ended up being the (original) rotor.  I believe mechanic replaced cap and rotor to fix. Also tweeked the timing a bit as I recall and all was good.

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Just to rule it out you may want to check total ignition timing with a light, no more than 34 degrees, also a fuel pressure check under full throttle and also float level setting. Maybe you have something half plugging off a main jet. Secondary ignition mis fires tend  to be most noticeable under hard acceleration.

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On 4/8/2017 at 11:59 PM, Duane2135 said:

I don't think a full tuneup is the answer.  I did a full new engine just just last year.  I suspect a bad plug, cable or possibly even cap/rotor.  Everything is new except the plug wires, which are possibly original.

Could be a cap and rotor but your vintage Merc has the preferred conventional cap, and not the problematic crab cap design. Shepherd may respond, but there is an issue with that particular ignition module and he may be able to tell us which one was prone to failure. The only other thing that comes to mind is a weak intake valve spring and I'd be interested to learn if your replacement engine had new springs, or shimmed to old ones. She could be running out of fuel too and a quick fuel pressure check at the carb inlet at WOT will tell that story. That's the easy one as it's a go/no go black and white test. W

http://www.marinepartsplus.com/catalog/mercruiser/serial/5.7L_2_BBL._GM_350_V-8_1988-1995/816596/1014-110

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Interesting,

Before I took her out of the water, I ran her up to 3500 rpm in neutral.  I can hear the popping on the Starboard side.  I can see a little smoke from the starboard exhaust when it pops.  

Btw, the springs and valves were all brand new.

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39 minutes ago, Duane2135 said:

Interesting,

Before I took her out of the water, I ran her up to 3500 rpm in neutral.  I can hear the popping on the Starboard side.  I can see a little smoke from the starboard exhaust when it pops.  

Btw, the springs and valves were all brand new.

That is also a symptom of a burnt exhaust valve. Wouldn't that be something 

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If its new springs and valves a spring could have broken too.  Pulling the valve cover and looking at the springs it should be easy to to see a broken spring if that's it.

You could also take it back to who did your top end and let them look at it. 

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15 hours ago, Duane2135 said:

10deg 

 

Bad distributor or ignition coil? Either that or it could be fuel starved like I alluded to. Did you check the anti siphon valve? My 2139 would lose power over 3500 rpms before I replaced it.

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Lots of real plausibles listed earlier IMHO. If the engine was a MerCruiser EFI version from that period, than I would go straight to the shift interrupt switch probably messing with the engine timing.

Carb'd engine with Thunderbolt IV or V ignition? If so, recommend replacing the ignition sensor under the distributor if you have the old style that used ring terminals to connect to the sensor, even it that isn't the problem. Old style will fail leaving you down and out sooner or later.

New part # is 87-892150K02 or Q02 (Mercury/Quicksilver) If you replace this part, almost certain you'll need a new V8 rotor 13524T1 or Q1 as they get bent during removal.

Typically though I find that a faulty ignition sensor won't allow the engine to run. (cranks over only)

Here is a picture of an older style ignition sensor with the ring terminals, and below it, the newer style ignition sensor.

Image result for thunderbolt iv ignitionImage result for 892150k02

 

Image result for thunderbolt ignition

Image result for thunderbolt iv ignition

 

Image result for thunderbolt ignition

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Spark plug wires will have a date code if they are MerCruiser.   Will probably read 94 for 1994 if original. The picture below for instance reads 1Q12 on the wires for a date code.

NOTE: red or blue MerCruiser spark pl ug wire sets were designed to handle more engine room heat than the standard black spark plug wires.

OEM-Mercruiser-ignition-spark-plug-RED-w

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I'm still waiting on new wires and plugs.  I'll probably see them tomorrow or Tuesday.  I took the boat out this weekend and I'm certain I lost another cylinder, but the second is in the port side.  This weekend I found raw fuel signs from both captains call exhaust ports.  

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