Mercruiser 4.3 backfires when reving

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pluto383

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Apr 29, 2004
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13487
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Please can someone help me. 2 years ago my engine seized. This year I bought a long block along with an engine that had done 10 hours then when taken out of the water, the water froze inside the engine, cracking the manifolds, the ancillary parts were fine and pretty much new.

I have used the doner engine to transfer all the ancillaries onto the new 4.3 block. I have had to replace the wiring loom in the boat and the steering ram for the quick release fittings and new style electrical plug.

I have now installed everything and started it up today. I checked all the timing by getting to TDC after checking compression on no.1 cylinder and lining up marks. I made sure the rotor of the distributor was pointing at no.1 lead and that it worked round clockwise 1-6-5-4-3-2. Everything started up fine, it idled nicely so I did the timing advance to 10 degrees with strobe light (although the revs did tend to drop at the 10 degree mark). All set to give some revs yes? NO!, it just kept backfiring at both ends at about 2500 rpm (mainly through the carb). I read some forums which pointed towards the carb, so I swapped over to my spare carb - did the same thing. I then swapped over the distributor and leads, which were only a year old when the old engine seized. This did improve things a bit as I was able to rev through the backiring range to get a clear rev range between 4000 - 5000 rpm. I swapped over the coil for my old spare one, but it did the same. I haven't checked plugs yet but they were new as they came with the long block. Is there an obvious problem or any ideas of what it could be?

If it idles fine and revs fine at the top range, would this mean that it's unlikely to be (flat cam problem? (I have no idea what that means but a thread was talking about it).

I really hope someone can help as this engineis just driving me crazy.

Thanks
 
Check and re-check the spark plug wires. Have to make absolutely sure the wires are correct before going any further.

Reving in neutral really doesn't tell you much.
 
What Charlie said. Recheck firing order at both ends of the cables and confirm they are tight.

What ignition system is yours? If by any chance it involves the Thunderbolt V system , I believe it is necessary to configure for Base Timing Mode by grounding out a lead to prevent advance before setting base timing, then back to Normal/ Run mode after the distributor is tightened back down and after timing rechecked . I think TB IV does not require any Timing Mode, not sure.
 
This is probably too easy, but check Distributor Cap and Rotor for Cracks and/or Carbon Traces. I believe you said these are used parts. Any chance one of them got cracked or contaminated with something that might cause the spark to arc across it? Doesn't take much, but it can drive you crazy.
 
Please can someone help me. 2 years ago my engine seized. This year I bought a long block along with an engine that had done 10 hours then when taken out of the water, the water froze inside the engine, cracking the manifolds, the ancillary parts were fine and pretty much new.

So was the newer block dissasembled then Magnafluxed to make sure there was no internal damage? This sounds ignition related hopefully....
Bill
 
yes recheck base timing ( easy to be one tooth off), dist cap and firing order. if it had points I would look there.
 
quote:
Originally posted by Billylll

Please can someone help me. 2 years ago my engine seized. This year I bought a long block along with an engine that had done 10 hours then when taken out of the water, the water froze inside the engine, cracking the manifolds, the ancillary parts were fine and pretty much new.

So was the newer block dissasembled then Magnafluxed to make sure there was no internal damage? This sounds ignition related hopefully....
Bill

Click to expand...


The way I read it is he bought both a new long block and a second doaner engine for the accessories. This does raise the question where they both the same rotation? Firing order is different for opposite rotation and distributed advance is different I believe.
 
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I'm curious about what Billy brought up. The engine that froze, is that what's in the boat right now? Was it ever checked for freeze damage?
 
quote:
Originally posted by jtybt15

I'm curious about what Billy brought up. The engine that froze, is that what's in the boat right now? Was it ever checked for freeze damage?

Click to expand...


Charlie - As I read it at least, the frozen-water engine was simply the donor of ancillary parts to the new?/reman?/just used? long block installed in the boat this year.
 
Sandy,
I read thru the posts a little too fast these days. I'll have to slow down a little.
 
Hi, Sorry, had time away from the engine today. The new long block is in the boat. It has all the ancillary parts from the donor (frozen) engine. I have got all the leads to the correct spark plugs from the distributor and the timing seems correct. The marks line up with the strobe and I advance it by 10 degrees as per manual. Quite a few posts say that if the backfire is in the carb, then it tends to be a fuel delivery problem, could this be correct? Should I change my fuel pump next?
 
A no load lean condition backfire would be a surprise. Can you see fuel spray in the carb when you open the throttle while not running.
Does it have points??
Dou have a vacuum gauge??
 
quote:
Originally posted by pluto383

If it idles fine and revs fine at the top range, would this mean that it's unlikely to be (flat cam problem? (I have no idea what that means but a thread was talking about it).

Click to expand...

What they are referring to is cam where the lobe(s) are worn down. A long block should have a new cam in it. Have you done a compression check on all clyinders? A tight intake valve will cause it backfire thru the carb.
 
Your description doesn't necessarily point to a lean condition yet. A new engine can have lots of bugs to work out. Even if the carb was running lean, there are other things that can cause a backfire thru the carb...as mentioned, valve adjustment.
 
I thought about a stuck valve etc. but that should be independent of RPM
 
I will do some test this evening, like the carb squirting fuel. It's a TKS ignition, so don't think there are point, just a rotor and distributor cap. It is only backfiring when reving over about 2500rpm, with the Thunderbolt IV distributor in, I can kind of break through the backfiring and it will rev happily between 4000-5000 rpm, but it is hard work to get it through the backfire range.
 
I hope you're reving to 4000-5000 while underway. Doing that in neutral for any period of time can blow the engine. You'll also be creating an uneven wear pattern on all rotating parts. Under load, the bearings will load a set section of the bearings. Under no-load, especially if held at one RPM, it's possible the bearings will 'rattle' between the load and no load bearing surface. Also happens to the pistons. When an engine is broken in, what you are trying to do is establish a wear pattern. No-load breaks up that pattern.
 
Have you done the basics and checked the wires and plugs? I had the same problem on a 5.0 this spring. Did all the stuff that you did, then while I was checking compression, I checked the wires and plugs. I had three out of eight spark plugs that would not spark. These were NGK less than two year old plugs. I put in an old set of spare plugs and it started and ran fine with no back fires.

So, before you waste more time (like I did), check the wires and plugs.
 
You might consider putting it in the water and checking it under load. If it pops under load you can remove and GROUND one plug wire at a time to isolate the cylinder. If its not a lean condition, which is hard to determine with no load, take another look at the distributor cap for tracking. A sticking intake valve will cause intermitent popping while a burnt valve will cause a steady pop. In a regular ignition system a failing condenser will cause random backfiring thru the carb.

If you isolate it to 1 cylinder remove the rocker cover and check for bent or loose push rod.

A real outside possibility is a lifter pumping up but this is usuallya high RPM problem.

If push comes to shove check the valve lash adjustment. You might have one too tight.

Two other things to consider. Leaking intake manifold gasket and compression check.

Compression check eleminate burnt valve as a possibility.

Heed what Charlie said - High REVS at no load, on a fresh build, can scatter an engine all over the bottom of the boat.
 
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