Boat engine backfire, with sound clip.

[SimonTHK]

Hello
In an engine, we have alot of electronics. What can cause this to happen, kinda backfire into carburator!

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Does anyone have any knowledge of this?
Thank you in advance

 

[JimB]

Best guesses:

Bad ignition timing
Bad valve timing
Inlet valve not sealing correctly

JimB

 

[SimonTHK]

Do you know if an inlet valve can be check for bad sealing, with a kompression test tool?

 

[Nigel Goodwin]

Do you know if an inlet valve can be check for bad sealing, with a kompression test tool?

I would imagine so, as good compression requires both outlet and inlet valves to be well sealed.

 

[ClydeCrashKop]

Make sure the accelerator pump is squirting. NOT with the engine running!

 

[Pommie]

When you say a backfire into the carb, is it more like a popping noise than a gunshot noise? If so, it maybe a faulty coil feeding the spark plugs. Another symptom is it'll run fine at higher revs.

Mike.

 

[SimonTHK]

Make sure the accelerator pump is squirting. NOT with the engine running!

Ive read up on this. But how do I do that? How do I check if it squirts?

 

[SimonTHK]

When you say a backfire into the carb, is it more like a popping noise than a gunshot noise? If so, it maybe a faulty coil feeding the spark plugs. Another symptom is it'll run fine at higher revs.

Mike.

The coil has been changed. There is a video with sound in first post.
Maybe more a popping noise than a gunshot

 

[ClydeCrashKop]

Looking down into the carburetor, toward the front center, there are 2 little nozzles. When you accelerate from idle, they should squirt a stream of gas into the venturis. (Center rings in the primary barrels)

And don’t run it without the flame arrester.

 

[SimonTHK]

Ok so without ignition on, it should still squirt?

 

[SimonTHK]

Does anyone know if you can measure hz from spark plugs with multimeter that has a hz measuring setting?

 

[Pommie]

Do engines still have accelerator jets? Isn't it now all controlled by the engine management unit?

Mike.

 

[ClydeCrashKop]

Isn't it now all controlled by the engine management unit?

The new ones are but that looks like a Rochester 4 barrel Quadrajet.
There are videos on YouTube on Quadrajet accelerator pump and rebuilding the carbs.
I didn’t find one that showed it operating. I may look again later.

 

[shortbus=]

Best guesses:

Bad ignition timing
Bad valve timing
Inlet valve not sealing correctly

JimB

I think Jim has it, ignition usually doesn't pop back through the carb. If diagnosing this myself I'd pull the valvecovers off and make sure everything(intake valves) are moving correctly. It could be a broken valve spring, a wiped out cam lobe or burnt valve. But ingition back fires are usually in the exhaust not the intake.

 

[ClydeCrashKop]

I don’t know if that is the problem but that and the timing are easy to check before disassembling the engine.

 

[SimonTHK]

Both the valve head (the head the valves are placed in) and the valves are completely new.
It runs smooth when it has high revs, and from high to super high revs, no backfire occours.
It is only at the acceleration from low revs.
I noticed (I check this boat for a friend), that they have disabled the choking or choker or what it is called in english (the metal clap, that chokes the carburator to suck in fuel). I believe that if somehow, this might am being used at low revs to draw in fuel automatically, like a choking works, but automatically. Not manually choking. So somehow it should choke itself at startup or early revs or similar. Does anyone have any experience with this?

 

[SimonTHK]

I don’t know if that is the problem but that and the timing are easy to check before disassembling the engine.

View attachment 126794

I chekced for fuel. It spray in alot of fuel through two holes (nozzles) as you pictured. So I beleive this works fine.

 

[ClydeCrashKop]

If that popping is only when the engine is cold, it could be the choke.

 

[SimonTHK]

Not cold, but allways from low revs to medium revs. BANG
But I was told that they have uninstalled the choke, and I just dont like that, because it seems that more parts has a signinficance to the choke, more than only the driver manually using the choke.!

 

[Nigel Goodwin]

I think Jim has it, ignition usually doesn't pop back through the carb.

I used to have a 1954 Velocette 350cc motorbike, a single cylinder four stoke engine with magneto ignition.

Anyway, the petrol tank had a small leak at the rear, but so small I never did anything about it.

There was no air filter on the carb, just a bell mouth, and no ignition key either - you just turned the petrol tap on, kicked it up, and away you went.

So one morning, ready to go to work, I kicked the bike over and it back fired - THROUGH the carb and bell mouth, setting fire to the leaking petrol at the rear of the tank (and rather close to bits you were fond of)

Luckily there was a guy close by cutting the grass, so I ran and grabbed handfuls of grass cuttings and used those to extinguish the fire.

I then tried again, and merrily continued to work.