5.7 Mercruiser Thunderbolt surge at high rpm

[Iamjtm]

How’s everyone doing, I’ll try to be as detailed and quick as possible. I’ve read just about every topic related to my problem, but am not finding the EXACT problem I’m having.

i have a 94 wellcraft 236 5.7 MerCruiser thunderbolt alpha one boat.
When the engine is above 3k rpm I get a surge/bogging issue. It revs in neutral, has plenty of power, and will get to 3k with no hesitation, it will also run past 3k frequently to WOT for a few minutes before it acts up. When it does start to surge it doesn’t shut off it just loses power and I HAVE to put the boat in neutral to continue. Just letting off the throttle and going again will cause it to surge full-time. After it surges and I go to neutral it takes roght off full power. Here’s a list of what I’ve done, I’d like to fix it myself but am kind of lost and figured this would be a place to start before I called a mechanic.

I replaced today;
the distributor cap and rotor
Plugs and wires
fuel filter/water seperator
disconnectes the fuel tank vent and blew thru it, was effortless
checked vent screen on the hull while filling up with 40gal of fresh fuel

its a Quadrajet carb, PLEASE HELP, thanks in advance.

 

[Scott Danforth]

Time to rebuild the carb

 

[achris]

Pull the carb, strip it and give it a very thorough cleaning. Set it back up as you re-assemble.

Chris.......

 

[Iamjtm]

Is that something fairly easy to do? I’ve seen the rebuild kits that just come with what looks like just gaskets...
kinda seemed like gaskets isn’t enough to be considered a rebuild? If I do the carb tomorrow can I soak it in gasoline instead of the carb cleaner bucket I’ve read about?

 

[Iamjtm]

I’d there any area I should focus on given the symptoms off only at high rpms? I’ll pull the carb first thing in the morning

 

[achris]

Quadrajet carbs are particularly fiddly, and quite finicky. There's a lot of setting up to do. Soaking in petrol doesn't help, it's soaked in petrol the whole times it's being run! No, use carb-cleaner, and DRY compressed air.

Take your time, and read the manual VERY carefully and thoroughly. Follow each and every step, in order and exactly...Inspect each and every part, with a magnifying glass if necessary. Some of the needles and jets can be damaged/worn and only very close inspection will reveal that...

What I would be tempted to do is buy a new Edlebrock 1409 carb and just drop that in place.

Chris...

 

[alldodge]

Suggest checking fuel pump pressure before getting into the carb (4 to 7 psi from idle to WOT)

 

[Scott Danforth]

99% of the time a rebuild is a good cleaning...... So all you need is new gaskets, seals, needle and seat

 

[Iamjtm]

Alright today I’m going to pull the carb replace it with the Edelbrock, I hate to just throw parts at it but I’m not qualified or patient enough to rebuild a carb. Are you guys somewhat confident that could be the problem even though it has full power most of the time and will actually get above 3k? Zero hesitation if I give it full throttle it’s the constant rpms above 3k when the problem arises. Plan to get everything done and test drive today...

 

[Iamjtm]

Any particular way I can check the fuel pump pressure? The line from the pump to the carb is hard lined

 

[Bondo]

Alright today I’m going to pull the carb replace it with the Edelbrock,

Ayuh,..... The 1409 carb is the one for yer motor,.....
You'll need a late model Merc carb cable mounting bracket, as the late model Merc/ Weber carb is a carbon copy of the 1409,.....
You'll also need to bend up a new hardline from the fuel pump to the carb,... I use 3/8" automotive brake line,....

AD has posted the fitting needed to check fuel pressure,....
When he sees this, maybe he'll post it again,....

 

[muc]

Any particular way I can check the fuel pump pressure? The line from the pump to the carb is hard lined

MerCruiser sells an adapter.

Might want to check the air windup spring for the secondary butterfly’s.

 

[alldodge]

Any particular way I can check the fuel pump pressure? The line from the pump to the carb is hard lined

Rubber fuel line, Tee fitting and barbed fittings for connection to the carb and pump
Or get this and attacch at carb end 91-18078



Then need a vacuum fuel pump tester, could probably get a loaner from auto store

https://www.harborfreight.com/fuel-pump-and-vacuum-tester-62637.html?cid=paid_google|*PLA+-+All+Products+-+Lower+Sales+Items|New+Products+-+%282%29+Price+%2410-30|62637&utm_source=google&utm_medium=cpc&mkwid=sE r1CMISE|pcrid|318476002941|pkw||pmt||pdv|c|slid||p roduct|62637|&pgrid=63088204546&ptaid=pla-298162056658&pcid=1654049980&gclid=Cj0KCQjwnpXmBRD UARIsAEo71tS5x_nQKG8MInqO1A5qJtZrxTF6HTbLS_KANzj1l 3uj8usJC7R_nBgaArfqEALw_wcB

 

[Iamjtm]

Alright thanks, I’ll do all the above and report back

 

[Iamjtm]

Looking down in the top of the carb, I see the bottom 4 holes. The top has the choke plate I’d assume, in the front, and the back has another set. That are about half closed and are moved freely, not connected to anything that I can tell. Does that seem right?

 

[muc]

no

google

quadrajet air windup spring

 

[Iamjtm]

How do I upload a photo to show you? It tells me a maximum of 2mb or something I’m on an IPhone. I also looked up that spring, but can’t find a location explanation or see that spring on the carb...

 

[muc]

How do I upload a photo to show you? It tells me a maximum of 2mb or something I’m on an IPhone. I also looked up that spring, but can’t find a location explanation or see that spring on the carb...

Don’t know about pictures, never done it. But I think there is something in the sticky’s about how to do that.

Don’t mess around with the carb without the MerCruiser manual. You have good chance to make things worse. Post your engine serial number and we can tell you what manual you need. Your questions are in the manual.

 

[Iamjtm]

OF144823 is the serial number on the cover

 

[Iamjtm]

I’ve got the manual, I see all the breakdowns and diagrams for my specific carb but nothing that shows that specific butterfly right above the secondaries or a spring connected to it, I also don’t see an Allen key with a flat blade screwdriver adjustment spot either. That thing just flaps back and forth with no effort