Carburetor Popping

[boatn]

I have a 3.0 mercruiser and I can start the engine with no problem but the Carburetor is making a popping sound every second or so all the time it did not do this at first trial run at the lake happened within 10 min of running the boat the carb is rebuilt it has 1 set screw could it be the choke is to tight? bad rebuild, this is a reman engine I just put in wires look right its a distributorless system

[cougarman]

Hi Boatn,
Did you do a compresstion test? Sounds to me that you might have a stuck intake valve? and who rebuilt the carb? does it look right? and what screw are you talking about????? and if you bring up #1 cyl on compresstion stroke and then remove the dist cap and see where the rotor is, it should be at #1 sparkplug wire, then bump the key to see witch way the rotor is turning and check to make sure the rest of the firing order is correct, The firing order should be stamped on the intake manifold, Best of luck, I hope this helps you, Scott

[boatcrasher07]

sounds like you have a leaking manifold gasket take carbys of and replace gaskets

[Poida]

Yep unfortunately Cougar man might be right with the intake valve not seating, but way off with taking the distributor cap off an engine without a distributor. Sorry Cougar couldn't help that.

But of course try the less expensive causes first.

I don't know about a leaky manifold gasket? Unless you are hearing leaking exhaust gas.

Now you said it started 10 min after you started the engine. There is an automatic advancement on an engine that is operated by a vacuum tube that runs to a diaphram on the carby. It could gave fallen off or missing, blocked or has a hole in it. If it is that it should run OK in idle but miss fire at higher revs.

Good luck

Poida

[TerryKing]

The Pop is a disturbance of some kind in the intake manifold, I think... (This is why we have flame arrestors: it's usually a quick fire in the intake manifold / carb, quickly sucked into the engine... )

Usual causes are:


Bad / stuck intake valve or lifter, bent pushrod etc.
Firing while intake valve is open (bad ignition of some kind)
possibly intake manifold gasket, if that causes too-lean mixture


Just my Guesses Of The Day...

[Frosty]

Re adjust the carb mixture screw.

[Bergalia]

Check ignition retard/advance.Too far either way will give popping and 'flat spot' ignition. Cause possibly with vacuum auto retard/advance. As Poida suggests: Blockage, split diaphram or just damn ineffective. Leave the mixture control until you've checked ignition questions then get back to us...:)

[redtech]

hate to ad to all the maybes but just so i can spend my two cents
start with the basics dwell if you have pionts
then timing only takes a light
then make sure plugs are in good shape
then start looking into carb and manifold
check for a vacuum leak with a little carb cleaner spayed around the base of the carb and the intake manifold you will hear the engine change speed if you have a vacuum leak
next if you have a popping sound you are most likely lean not rich so back out the idle mix screw 1/4 of a turn a time with about 20-30 seconds between adjustments if this fixes it great if not time for a compession test

[alan white]

Rebuild... one tooth off between crank and cam will make popping occur. Check that marks are lined up.

A.

[Frosty]

If the carb has had a rebuild then it needs to be set up for compatability for your engine. Set the mixture screws to the fastes/smoothest idle.

If it doesnt do it on choke then its too weak when the choke comes off.

The popping is because the weak mixture is still burning even after exhaust stroke and even still when the inlet valve opens to intake then you get the pop.

Do the easy stuff first. I cant understand why people are so afraid of turning the mixture screws, theres no magical setting that can be re attained.

The engine will tell you when it likes the setting, it will run faster and smoother. You may need to re-adjust the slow running after.

[longliner45]

easy stuff first like jack said .if it is a sticky valve pour a little transmission fliud in the carb when it is hot ,if that dont help ,then your talking about the hard work.longliner,,,,by the way wasnt jack an ugly baby?