5.7 backfiring thru carb

[98bryant214]

New to the board, was hoping I could get some advice here..

I have a 98 mercruiser 5.7, thunderbolt ignition, serial # 0L033404, MerCarb 2bbl.

I have had the boat for 4 seasons now, it's always ran flawless, the last time I took it out last year it started backfiring thru the carb, so I thought I would deal with it later, which is now.
I have changed the plugs, plug wires, fuel filters, distributor cap, rotor, put fresh gas in the tank, set the timing, new anti-siphon valve.
It runs perfect on the trailer, zero backfire, from 650 RPMs to however high I rev it, sounds amazing, and idles good.
Took it out 2 times so far, same result, backfiring after 1800 RPMs, and @ WOT, but not as bad when wide open. It also backfires pulling the throttle back as well, until it gets under 1800.
Why on Earth would it run so great in the driveway, but runs like poop in the water?

Thank you for any help/advice,
Ricky

 

[Bondo]

Ayuh,.... Welcome Aboard,.... It uses little gas on the wagon, 'n is goin' Lean when pushed harder,.....

Sounds like it's time to pull the carb, 'n rebuild it,....

 

[alldodge]

Sounds like it's time to pull the carb, 'n rebuild it,..

Agree, also check fuel pressure 3 to 7 psi

 

[98bryant214]

Thanks Bondo, so you would rebuild vs buying a new carburetor?

Someone also suggested getting a Holley 4bbl & Intake to bring the engine to life, but $$$ ugh

 

[Scott Danforth]

Spent the $35 and rebuild it. Will take about 4 hours to properly clean and chase each passage

a holley wont make any more power than the Q-jet or its clone

 

[98bryant214]

Where is the serial # on the carburetor? Or, are the rebuild kits universal, doing a "Mercarb 2bbl rebuild kit"?

 

[Bondo]

Ayuh,..... Use the serial number of the motor, to find the right parts for it,......

A 4bbl. up-grade ain't a bad idea, but Holley ain't the way to go,.....

Merc uses the Merc/ Weber AFB, which is a carbon copy of the Edelbrock Performer carbs,......
Factory cables, 'n brackets fit perfectly, No fab work to be done,....

 

[Maclin]

If the carb rebuild does not do the trick then I would do a compression test, and if any cylinders are down do a leakdown test on them, you may have an intake valve not sealing as well as it should.

Welcome to a fellow Bryant owner ​​​​​​​

 

[QBhoy]

Did you totally drain the fuel tank ? Sounds like there might be water getting in there. Filter clear ?
not getting water ingress from the risers or manifolds ?
just trying to think simple.

 

[98bryant214]

I haven't drained it yet, it has like 45 gallons in the tank, not sure how I'd dispose of all of that gas lol.
I did order a new carburetor, so it would probably be a good idea.
Although it went into storage with only about 10 gallons, with Stabil in the gas. I'd still prefer to drain it before installing the new carb.

 

[98bryant214]

I did take the fuel filter off, poured the gas in it into a clear glass, didn't notice anything unusual, although I'm no expert when it comes to analyzing gas

 

[98bryant214]

How do I check for water entering in the risers/manifolds?

 

[alldodge]

Don't think you have a gas issue as yet, but you can run the motor from a gas can for testing to be sure.

How do I check for water entering in the risers/manifolds?

Are you wanting to find out how much water is going thru them because of a cooling issue or wondering if water is making it into the motor?

 

[98bryant214]

The boat runs cool, but worn out exhaust flappers wouldn't be the reason it's backfiring correct?

 

[alldodge]

The boat runs cool, but worn out exhaust flappers wouldn't be the reason it's backfiring correct?

This would not cause backfiring. If they broke off and dropped into the Y pipe they would cause some loss of power

 

[kenny nunez]

You may have a broken or weak valve spring.

 

[dollar7499]

I had the same issue a couple of weeks ago, I had set the timing to 0* instead of 8* BTDC, so I put it to 8* and changed the fuel filter at the same time, so I'm not sure which fixed it! As for fuel analysis... I dumped my old filter into a mason jar and had 3" of orange ruined fuel floating on 2" of water! Search gasoline and water in a jar for image examples.

 

[98bryant214]

Bought a new carburetor, still doing the same thing.
Compression test up next :-/

 

[alldodge]

Hope its an easy fix

 

[Lou C]

When you do the comp test check for water in the cyls, this can cause a misfire similar to what you describe. Usually if there is water in a cyl you will see it expelled out of the spark plug holes while cranking the engine with the ignition disabled and the plugs out. There will also be some rust on the plug electrode from the cyl that is getting water. Sometimes water in a cyl = corroded intake valve stem = valve not closing all the way = backfire through carb. I had this from blown head gaskets but there's other causes. It does not always show up on a comp test because they can loosen just enough for the comp test results to me normal. However if you take off the valve covers and crank the engine over by hand you can see if the valve does not close all the way, the stem will be sitting lower than the others.