1988 Quadrajet and gasket issues

[keviwe]

Mercruiser 5.0LX - 230 HP. My OG 1988 Quadrajet started acting up. I bought a new 4-barrel (Quadrajet knock-off) from Marine Carburetors. OG Rochester had an Open base gasket that is about 1/4" thick. New carb came with a 4-hole gasket about 1/16" thick.
new carb seemed to "overheat" (vapor lock) and wouldn't accelerate. Carb worked fine after it cooled down. I'm thinking the thicker OG gasket was a heat insulator.
QUESTIONS:
1. will a thicker gasket solve my vapor lock issue?
2. OG gasket is "open style" one large hole (not individual "barrel holes"). What is the benefit of "Open Hole" style base gasket?
3. how is engine performance different when each intake bore is not sealed (with 4-hole gasket) but each intake bore is "open" to the adjacent bores? (I ask because new carb with 4-hole gasket kicks in the secondaries very aggressively where the OG Rochester with thick open gasket accelerated gradually, smoothly when secondaries begin to kick in.) (I also had to adjust secondary butterfly spring tension/metering rods (knock-off new carb) to an extremely light tension to prevent bogging. OG carb secondary butterfly spring tension was much "stiffer")
Thanks for your experienced replies

 

[Davetowz]

https://cliffshighperformance.com/ This is the guy to call with all your quadrajet questions. He is the master.

 

[Scott06]

Mercruiser 5.0LX - 230 HP. My OG 1988 Quadrajet started acting up. I bought a new 4-barrel (Quadrajet knock-off) from Marine Carburetors. OG Rochester had an Open base gasket that is about 1/4" thick. New carb came with a 4-hole gasket about 1/16" thick.
new carb seemed to "overheat" (vapor lock) and wouldn't accelerate. Carb worked fine after it cooled down. I'm thinking the thicker OG gasket was a heat insulator.
QUESTIONS:
1. will a thicker gasket solve my vapor lock issue?
2. OG gasket is "open style" one large hole (not individual "barrel holes"). What is the benefit of "Open Hole" style base gasket?
3. how is engine performance different when each intake bore is not sealed (with 4-hole gasket) but each intake bore is "open" to the adjacent bores? (I ask because new carb with 4-hole gasket kicks in the secondaries very aggressively where the OG Rochester with thick open gasket accelerated gradually, smoothly when secondaries begin to kick in.) (I also had to adjust secondary butterfly spring tension/metering rods (knock-off new carb) to an extremely light tension to prevent bogging. OG carb secondary butterfly spring tension was much "stiffer")
Thanks for your experienced replies

thicker gasket is often used to insulate the carb from heat soak and associated issues. Typically on intakes and spacers and open hole will give some plenum volume and shift power band higher in rpm but with a 1/4" spacer gasket I wouldn't worry about it, not practical difference.

the secondary opening is related to the spring tension, probably what you are seeing is the difference between a OEM carb and the aftermarkets. These cheap carbs seem to be hit of miss in the reviews.

Id start with a thicker base gasket like you had - not sure if it was stock look at cliffs or summit racing, or jegs.

What runability issues did yo have that made you replace the carb?

If you can use a place like cliffs and his forum or mikes carb parts you are likely better just rebuilding the OEM carb as long as the core is good.

 

[Lou C]

When you say it started acting up what was it doing (or not doing as the case may be?)
I agree with the idea that your original equipment carb (if that's what it is) most times will be better matched to the engine you have. It is fairly complex with different jets & metering rods used for different applications. The Quadrajet was made from 1966 or so to 1990 or so, and used in just about everything GM ever made, from small blocks to big blocks and was even used by Chrysler and Ford at times. So there are many variations of jets, metering rods, accelerator pumps, etc to add to the difficulty of getting an unknown spec carb to work.
The tough part is the cleaning, the idle air tubes are difficult to get clean, also it is a bit more complex to take apart than let's say a Weber or Edelbrock.
I have one and have always used the thick open base gasket. The gasket should match the intake you are using and all the Merc and OMC/Volvo cast iron marine intakes I have ever seen have an open plenum design with a divider in the middle that goes up part of the way. Some auto applications used a 4 hole spread bore intake and on those you could use the 4 hole spread bore gasket. The rebuild kits I have gotten from Cliff's all come with that open thicker gasket.

 

[Scott Danforth]

I would rebuild your original carb.

 

[keviwe]

Lou - This OG Mercruiser - genuine GM Quadrajet ran excellent for decades. The fall before last, I used Starbrite Tron Enzyme fuel stabilizer when winterizing. I have always used Stabil in past decades. That following spring (2025), the engine acted like it had water in the fuel (sometimes sputtering, coughing, poor acceleration). So I added HEET and ran that gas tank out. No improvement. Then changed the fuel/water separator filter canister two times, 2 tanks of gas, no improvement. Pulled the carb that fall, rebuilt it (over winter). Re-build improved performance (still coughed but less, never could reach full rpm) but still did not run the way it used to. I was told to replace the carb (and I didn't have time to re-build again (since I store boat in KY but live in CO now, and use the boat 2 times/season) so 1-1/2 months ago bought a carb from I-5 Components - Marine Carburetors which turned out to be a Quadrajet knock-off. (I needed to get the boat running quickly because my grown-up daughter was borrowing it). I messed with the boat just before I had to go home and Daughter and family arrived. I had a horrible time getting it to run without bogging during acceleration. It came with a 1/16" thick gasket. Just a couple weeks ago, playing with kids behind boat, carb seemed to overheat, which I think was because of the thin base gasket, not insulating carb from heat. OG base gasket was a large single hole and 1/4" thick (GM cast iron manifold is bore-partitioned all the way up to carb mounting surface.) I will consult with Cliffs and definitely buy a thicker base gasket.
Thanks for all the comments!
I would really like to just return this knock-off carb, but I think I'm stuck with it

 

[jimmbo]

I too recommend rebuilding the OEM Carb, using a Mercruiser Rebuild Kit. You say the previous rebuild didn't restore its performance, all I can say, is you didn't clean all the fuel delivery circuits, adjust all the linkages that could have been affected by slightly different gaskets on between the Throttle Body-Fuel Bowl-Cover. Float Adjustment, is another item that gets messed up in a lot of Rebuilds. Is the Power Piston free and not sticking?
As for buying a Knock-off, those are truly 'A Pig in a Poke'.
The thicker Gasket between the Carb and the Block might make a slight difference.
Most Heat-Fuel issues these days are the result of the gasoline formulas refiners make these days, They are refined for use in a Fuel Injected Engines where the Fuel is under far higher pressure than atmospheric(carb). Lower Pressure and the Gas boils away at lower Temps. One of the Main Culprits for that, is all the damn Alcohol(s) they put in the Gasoline. Even non-ethanol Gas does contain Alcohol...

 

[Lou C]

Agreed I would convert to a different carb (new) vs buying any “rebuilt” Quadrajet. It’s more work and expense but if your OEM Quadrajet can’t be rebuilt it’s the best solution for reliability. With a Merc engine switching to the Edelbrock 1409 shouldn’t be too hard because Merc used the essentially identical Weber (after GM quit making Quadrajets in 89 or 90) the brackets etc shouldn’t be too hard to find. With OMC/Volvo the Holley 4160 would be an easier conversion because that’s what they switched to after GM quit making the Qjets.
I would try to clean it again esp the idle air tubes, make sure the primary power piston isn’t sticking (with the engine off it will be in the up position but as soon as it starts it will get sucked down by engine vacuum). Also pay attention to proper float adjustment. Lastly don’t forget other fuel system problems like a weak, failing fuel pump or clogged/stuck anti siphon valve or clogged fuel
Pick up screen or fuel tank vent.

 

[jimmbo]

I have used a Holly Marine 650cfm Spreadbore Carb #0-80552


If you go the Edelbrock route, get the Genuine Mercruiser Adapter, while not the Best, it is far better than many after market ones

 

[Lou C]

Edelbrock says to use their 4 hole adapter which is different from all the aftermarket ones I have seen.
That Holley above was also called the 4175, correct? It was their spread bore replacement for the Quadrajet, even had the same fuel inlet design so you could use the same steel fuel line.

 

[jimmbo]

Edelbrock's adapter is one of the Worst. Chokes the larger Primaries severely


The Mercruiser adapter, allows the higher flow from the Primaries to divert to the Secondaries inlets




Yes, it is a 4175, the Double Pumper versions of the Spreadbores are 4165s.
I had replaced the Holley 2 barrel 500cfm(378cfm if flow rated like a 4bbl) and the Inlet was certainly different. Volvo was so kind to have used a very well designed 4bbl Intake on their motors, using a plate to mount the 2bbl.

Ignore the arrow, as I had used the pic to show somebody where the Idle Mixture Screws were

 

[Lou C]

Interesting that's the first time I have ever seen that Merc adapter.
That fuel inlet on the 4175 looks like it was designed to mate up perfectly to the Quadrajet steel fuel line.
OMC/Volvo had their own adapter for using the Holley 4bbls on the Spread bore intake.
Looking at some of the turnkey engines sold by Michigan Motorz, many come with the Edelbrock on the standard cast iron spread bore intake, but what they appear to be using is the thin Edelbrock 2732 adapter, that is basically a thin plate with a square bore style gasket on each side. Not the 4 hole one that Edelbrock themselves said to use but I'm thinking maybe that was intended for automotive applications where some of the GM spread bore intakes had an opening with 4 holes like that adapter, not the open style which is what nearly all marine 4 bbl intakes use.
Any conversion leads to confusion, that's why if you can cleaning and rebuilding your original carb is the simplest!

 

[Scott06]

Interesting that's the first time I have ever seen that Merc adapter.
That fuel inlet on the 4175 looks like it was designed to mate up perfectly to the Quadrajet steel fuel line.
OMC/Volvo had their own adapter for using the Holley 4bbls on the Spread bore intake.
Looking at some of the turnkey engines sold by Michigan Motorz, many come with the Edelbrock on the standard cast iron spread bore intake, but what they appear to be using is the thin Edelbrock 2732 adapter, that is basically a thin plate with a square bore style gasket on each side. Not the 4 hole one that Edelbrock themselves said to use but I'm thinking maybe that was intended for automotive applications where some of the GM spread bore intakes had an opening with 4 holes like that adapter, not the open style which is what nearly all marine 4 bbl intakes use.
Any conversion leads to confusion, that's why if you can cleaning and rebuilding your original carb is the simplest!

MM uses a cast iron manifold with that fits bore sq and spreadbore carbs with no adpater. The eddy 1409 bolts up with a std gasket. I think this is similar to eddy manifolds that fit both as well. zero issues with vacuum leaks since 2016

 

[Lou C]

Looking at their site in the parts section I can see that is true for the V8s, but for the V6 which is what I have the only option is the original GM style intake unless you go to an Edelbrock alu intake.
I am still thinking of replacing the old Quadrajet with a brand new Edelbrock 1409 I have in the garage so this is something I'm researching now. I looked on their site under the section where they sell turn key engines and all the 4.3s I saw, seemed to use that thin Edelbrock plate. Probably because there is no dual pattern cast iron intake for these as far as I know.


older pic, here's a newer one that's on their site now:

if you look closely you can see that plate under the carb....

 

[Scott06]

Looking at their site in the parts section I can see that is true for the V8s, but for the V6 which is what I have the only option is the original GM style intake unless you go to an Edelbrock alu intake.
I am still thinking of replacing the old Quadrajet with a brand new Edelbrock 1409 I have in the garage so this is something I'm researching now. I looked on their site under the section where they sell turn key engines and all the 4.3s I saw, seemed to use that thin Edelbrock plate. Probably because there is no dual pattern cast iron intake for these as far as I know.

View attachment 418621
older pic, here's a newer one that's on their site now:
View attachment 418623
if you look closely you can see that plate under the carb....

for vortec this intake has the same dual pattern carb as my cast iron

Edelbrock 2519 Marine Intake Manifold, 1996-present Chevy 4.3L V6

www.speedwaymotors.com

I bet yours is not a vortec though... VP uses a similar manifold for 4.3s but the only ones i can find are vortec not sure if their older ones are universal carb base.

I thought your q jet was running well- whats the impetus for the swap?

 

[Lou C]

Well there's something wrong with either it, or the fuel pump because it's been giving me fits this year. I finally put together all the fittings needed to measure fuel pump pressure and volume (it has the OEM steel line from the filter to the carb). I measured vacuum at the inlet side of the fuel filter and it was fine, also measured vacuum in the fuel line from the tank with it running and is supposed to be low (below 4"Hg) that was fine too, no sign of an air leak, so I am thinking the pump is a more likely fault because of how it is acting. Starts, then stalls immediately. Never did this before even when the carb needed a rebuild. Funny thing is this mechanical pump is only about years old, the OEM one on it originally lasted about 30 years, this is a Sierra, so who knows. I have an OEM Carter in case it's bad.
The Quadrajet is 38 years old, it has run OK but I would love to have a modern carb that is easy to work on vs one you'd keep on a collectable car.

 

[Lou C]

About the intake yes it is a Pre-Vortec engine (1988) 12 bolt intake, there is an Edelbrock intake that would have worked similar to that one but it won't fit with the OMC/Volvo high mounted alternator. So I am limited to the old style cast iron spread bore intakes.
If I switch to the 1409 I have to use a bracket to move the OMC shift linkage bracket which bolts to the 2 last manifold bolts on the starboard side. The Q Jet fits fine and even the Holley 4160 will fit but the Edelbrock is just too wide. I tried that Holley 4160 a few years ago but it gave me fits too.

 

[Lou C]

View attachment 418631Here’s what I have this is a spare intake I had stashed in the garagetest fit the Edelbrock with the 2 different adapters:
View attachment 418631
Thin plate


Thick alu 4 hole one recommended by Edelbrock

 

[Lou C]

I really think that thick 4 hole one is for vintage GM automotive intakes that used the 4 hole spread bore manifold. Doesn’t make sense on the open plenum design of the GM marine intake.

 

[Lou C]

Doesn’t make sense on the open plenum design of the GM marine intake.
It looks as if it would really change the flow pattern for the primary bores.