Thermostat installation / full throttle hestiation question 5.0LX

Badfish95

Petty Officer 2nd Class
Joined
Jun 30, 2012
Messages
118
1995 Mercruiser 5.0LX 4 barrel carb

I noticed that my temp gauge never got more than a hair over 100 degrees. I just assumed the thermostat had been pulled. The boat runs great at all rpms but, when going from idle to full throttle the boat shutters or hesitates slightly. Almost like it is geting too much fuel. The plugs look good, cap and rotor too. The carb could use a rebuild being that it is 17+ but it runs too well for it to be a clogged carb.

I checked the thermostat housing and I was right about the thermostat being pulled. Made a phone call to marina and just picked up these parts:
All Quicksilver
27-41812 gasket
807252T 1 thermostat. 140 degree
23-47508T sleeve

The thermostat and sleeve had been yanked so I only have my manual to refer to. My Manual shows the sleeve having cutouts in it, but this one is a solid brass ring with on end rounded over and flattened. I installed the thermostat spring down with and the sleeve's rounded edge down.

I am concerned that the solid ring won't allow flow to the upper passages in the thermostat housing. My dealer said this ring phased out the old one. Anyone have experience with this?


Housing. Notice the ports at the top of the housing.


Brass Sleeve. No cutouts to allow flow to upper ports.


Here is how I installed them.


I really dont' want to get out on the lake and find out that I am starving a portion of the engine sufficient flow. Thanks in advance for any help you can provide.

Badfish
 

Badfish95

Petty Officer 2nd Class
Joined
Jun 30, 2012
Messages
118
Re: Thermostat installation / full throttle hestiation question 5.0LX

Thanks for posting that diagram. It looks like they are installed correctly, but I'm missing part 6. It looks like an o ring type gasket between the sleeve and the thermostat. I'll have to track one down.

Do you think this could be affecting throttle response/hesitation? My thought is that the engine never gets up to temp which is causing the stumbling. Or possibly the choke isn't opening fully?
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,470
Re: Thermostat installation / full throttle hestiation question 5.0LX

Thanks for posting that diagram. It looks like they are installed correctly, but I'm missing part 6. It looks like an o ring type gasket between the sleeve and the thermostat. I'll have to track one down.

Do you think this could be affecting throttle response/hesitation? My thought is that the engine never gets up to temp which is causing the stumbling. Or possibly the choke isn't opening fully?

Ayuh,... I'd sooner think rebuildin' the carb might fix That issue,...

It does that from a Lack of fuel, not too much,...
 

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
Re: Thermostat installation / full throttle hestiation question 5.0LX

That hesitation is the infamous Quadrajet bog. It's what happens when your secondary fuel supply or secondary air doors aren't right.
I just rebuilt my Quadrajet and it came out really well. I will look up what my book recommends and come back with some possible info.
 

NHGuy

Captain
Joined
May 21, 2009
Messages
3,631
Re: Thermostat installation / full throttle hestiation question 5.0LX

Can you slowly roll on the power? if so that's a workaround til you can service the carb.
The service procedure to set the air door spring is easy to botch and will make your engine unusable if it goes wrong so I am going to suggest rebuilding instead of taking a shot and losing parts. Get the book "How to Rebuild and Modify Rochester Quadrajet Carburetors', by Cliff Ruggles. Don't worry about the modifying, just do the rebuild. Mine came out better than new. I use to drive dealer prep new cars that came with these carbs back in the 70's. I'm serious, my boat runs better.

There are also some tiny holes leading from the fuel float chamber to the secondary fuel accelerating wells. These are small fuel storage spaces between the primary bores and the secondary ones. If those little holes are clogged or if any of the other accelerating tubes are junked up you could have the issue you described.
On the other hand it's June, you need quick turnaround. Take your carb to a rebuilder or buy a new one. Don't get one from Ebay. Those generic quadrajets are set up for cars and they won't work well on your boat. Plus a car carburetor comes with all sort of other wear issues that boats don't see.
Rebuilding the factory carb is your best move. It has the right jets and settings for your motor.
If you decide to do the rebuild yourself, get the parts from Cliff. The Mercruiser kit is missing some of the stuff you need. Cliff's reasonably priced and will guide you to success. He takes phone calls and has an internet forum for Q&A. cliffshighperformance.com.
 

Badfish95

Petty Officer 2nd Class
Joined
Jun 30, 2012
Messages
118
Re: Thermostat installation / full throttle hestiation question 5.0LX

Yes, I can roll on the throttle and it accelerates just fine. The reason I thought it was an engine temp thing is because it seems to get better if I have been running under plane for awhile and then hit it. It really isn't that bad, but when pulling a slalom skier up its nice to have full consistent power. Sounds like a carb rebuild is in my future. Thanks for the reply.

Oh, and I figured out I have the wrong parts for my thermostat. I was given the standard cooling system design I parts rather than the design II. I'll get them on order and hopefully get back on the water this weekend.

What is the consensus about not running a thermostat? Is it ok? Can it do any harm?
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,583
Re: Thermostat installation / full throttle hestiation question 5.0LX

Running without a thermostat will not hurt anything, it can only cause issues with performance. An engine runs best when its hot, fuel vaporizes better but not so hot the engine is damaged by the heat. I fuel injected engine (my 502) must have a 160 degree to operate correctly, but a carb engine does not have this issue. You may burn a slight bit more fuel and performance can suffer.
 

Badfish95

Petty Officer 2nd Class
Joined
Jun 30, 2012
Messages
118
Re: Thermostat installation / full throttle hestiation question 5.0LX

Thanks for all the input guys.

It seems that pulling the thermostat is a normal operating procedure with many boat mechanics. They probably think that a stuck thermostat will do more harm than an engine running a little cooler than optimal. I was hoping this would fix my problem as it is the least expensive and invasive repair. Plus it puts the boat back to how it was intended to operate.

I'll let you know if it works but I am not as confident that it'll solve my hesitation problem. Thanks again for the help!
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,470
Re: Thermostat installation / full throttle hestiation question 5.0LX

Thanks for all the input guys.

It seems that pulling the thermostat is a normal operating procedure with many boat mechanics. They probably think that a stuck thermostat will do more harm than an engine running a little cooler than optimal. I was hoping this would fix my problem as it is the least expensive and invasive repair. Plus it puts the boat back to how it was intended to operate.

I'll let you know if it works but I am not as confident that it'll solve my hesitation problem. Thanks again for the help!

Ayuh,.... Those are the guys that are Just Mechanics, 'n not Boat Mechanics,...

There's alot more goin' on in a marine t-stat housin', than on a car,...

That brass collar is part of the plumbin' that directs the raw water,...
Runnin' No t-stat can, 'n does cause localized overheatin' from disrupted flows,...

It ain't just a valve to the radiator, it directs flows to Either the motor, or the manifolds,...
Water takes the path of least resistance,...
If it's Easier to flow out the risers,.... how much water flow do ya think the block is gettin',..?? or vis-versa,..??
 

Badfish95

Petty Officer 2nd Class
Joined
Jun 30, 2012
Messages
118
Re: Thermostat installation / full throttle hestiation question 5.0LX

That makes a lot of sense. Even when fully opened the thermostat would restrict flow and create more pressure where it's intended.

If sea foam and a thermostat doesn't help the hesitation, I'll attempt to rebuild the carb myself. It'll have to wait for the off season. Thanks for the help.
 

Badfish95

Petty Officer 2nd Class
Joined
Jun 30, 2012
Messages
118
Re: Thermostat installation / full throttle hestiation question 5.0LX

Finally got the 140degree thermostat installed and no more hesitation! However, I did put about 15 gallons of 93 octane in before the test ride. I had been using 89 octane which I'm sure is high in ethanol content.

Could the 89 octane be the reason for the stumble? It never dieseled or pinged that I could tell.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
40,583
Re: Thermostat installation / full throttle hestiation question 5.0LX

Could the 89 octane be the reason for the stumble? .

No, higher octane fuel is need for higher compression engines only. High compression engines build up more heat during the compression stroke which can cause the fuel to start buring before the spark plugs fire. A 425HP 502 requires only 87 octane, your 5.0 will do fine with 87 unless you bore it out and put a blower on it.
 

Badfish95

Petty Officer 2nd Class
Joined
Jun 30, 2012
Messages
118
Re: Thermostat installation / full throttle hestiation question 5.0LX

I figured I would follow up with what fixes my issue. I'm pretty sure my first fuel up of the season was bad gas straight from the gas station. My tank had been removed and thoroughly cleaned prior to filling up. I will no longer purchase fuel from the local gas station that rhymes with Lacey's General Store.... I have filled up twice since and dropped to mid grade fuel and the hesitation is still completely gone. Also put in plugs, cap and rotor and that solved a slight stumble at 1100 rpms. She Runs like a top and I'm glad I didn't have to rebuild the carb. The thermostat probably had nothing to do with the fix, but at least it's back to Original. Thanks for all the replies.
 
Top