View Full Version : 350 running weak after altitude drop

09-24-2012, 11:19 PM
Hey everyone I wanted to see if anyone else has had this same experience as I am having and if there is anything I can do to fix it.
I recently took my boat up to around 6000 ft above sea level to cruise around. I ran it for 3 days without any problems except the expected sloppy idling and 20 percent or so loss in power. Well since I have brought the boat back to where I live which is sea level, I have noticed a definite difference in exhaust/engine sound. It sounds a bit more muffled and less "crackly" if you will then it used to. I also think Ive noticed a slight power loss.

Does anyone know if there is some sort of altitude sensor on my 87 350 mag that may have gotten stuck when i was 6000 feet? Sounds a bit technical for such an old engine.

Another observation, I am noticing a large amount of exhaust residue on my transom now. I had it quite a bit at altitude where it would accumulate at the water line and above and I have never had this before. Ever since, I still have quite a bit of this black residue now on my transom, does this have something to do with it or am I imagining things?

thank you all!!

09-24-2012, 11:49 PM
When you ran your boat at a very high altitude you were no doubt running an unusually very rich fuel/air mixture there with that very thin air at 6000 feet which created the soot out your exhaust tailpipes and on your transom.

You may have also carboned-up your engine's combustion chambers, piston tops, & sparkplugs.

The very first thing I would do is change-out your sparkplugs~~or at the very least clean them to remove that carbon build-up.
A product called Valve -Tech sold at marine & auto parts stores will also aid in removing the engine carbon build-up if used per spray can directions.
I have also used a fine mist spray bottle to spray a fine mist of water into the air intake at a fast idle to aid in combustion chamber, valve, and piston top, carbon removal.

Caution: Be sure not to spray very large amounts of water in a running engine or serious engine damage may & will result.

09-25-2012, 09:32 AM
did you buy fuel up there?

09-25-2012, 10:10 AM
Ok guys I will start with the spark plugs then. thanks for the insight as I am by no means a mechanic.
-Buizilla, I have been running 91 octane lately as that is what is sold at my home marina. I purchased two tanks of fuel up there and they only had 87. I have since ran through the stuff and am now running 91 again. what are your thoughts on the fuel from up there?

much appreciated folks, will post results when I clean/swap out plugs. Any suggestion on special spark plugs while I'm at it?


Dr. David Fleming
09-25-2012, 12:44 PM
Mercury Fuel Injection uses a MAPS sensor in the intake manifold to determine Manifold Air Pressure - this would record different manifold pressure at altitude than at seal level. Some of these sensors like the IAC - Idle Air Control can get fouled with fuel residue and stick in one position or another. This usually requires a session with carb cleaner.

09-25-2012, 12:54 PM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

Dr., Is this true even for the older, carb. versions of the 350 mag like mine?


09-25-2012, 12:58 PM
Dr, a '87 350 Mag has a carb... no IAC or MAP

09-27-2012, 10:41 AM
Hye folks reporting back......mrfixx and silverghost were correct, I replaced the spark plugs and sure enough they were carbon fouled to hell. Popped the new ones in and she sounded like her old self again. STILL have aproblem though!:banghead:

ran her for about an hour or so around the river yesterday and I noticed several things still:
-exhaust smelled gasoline-y as well as inside of the pipes after I took her home.
-Black soot on transom still
-could be my imagination, but I swear I was getting crappy fuel efficiency as I could practiacally watch my gauge dropping.
-Once again could be my imagination but slight drop in performance toward end of day when I should have picked up more speed with just myself and 1/8 of a tank of gas.

Once again I am not even close to being a mechanic, but this may all tell me that there is way too much fuel getting poured into my engine right? If this is the case what could be causing this?? I also hope that I dont have to change my plugs agian as it was a total pain in the a$$.

that's the last time I take my boat to tahoe (6,000 ft alt)

Thanks everyone!

09-27-2012, 11:13 AM
You were running way rich at altitude. Not only were your plugs fouled but everything else is sooted up as well (combustion chamber, exhaust, etc). It will probably blow out with time. We always had to re-jet and lean out carbureted snowmobiles when we ran them at altitude. Aircraft typically lean out at 3000 feet.

09-27-2012, 11:27 AM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

Ok, but does it sound like for some reason I am STILL running too rich? Now that I am at lower altitude it should've gone back to normal right?


Morgan's Cloud
09-27-2012, 11:57 AM
Not that this is my forte , because it isn't , but aren't the stock Merc engines supposed to be quite happy with a diet of 87octane ..anything more is overkill ? ?

09-27-2012, 12:23 PM
make sure you are firing on all cyl, when you changed the plugs you might not have gotten all the wires back on snuggly. also check your choke that it is working and opening up. might be a good time to read the plugs and see what they look like. keep an eye on your oil has well fuel could wind up in the oil if it is running real rich

09-27-2012, 12:37 PM
What brand of carburetor do you have on this engine ?
If it is a Holley it might be a ruptured power valve diaphram that is causing excess fuel to dump into the engine's carb & intake.

I would run it for a bit at high speed to try to blow-out the old carbon inside the combustion chambers, on the piston tops, & on valves.

Check for gasoline in your motor's lube oil as Matty suggested.

09-27-2012, 01:28 PM
Ok, but does it sound like for some reason I am STILL running too rich? Now that I am at lower altitude it should've gone back to normal right?


Everything should be normal back at lower altitude. You might want to try running some SeaFoam in the tank. I would think a few hard runs would do a pretty good job of cleaning it out. Here's a pretty good article on carbon deposit removal.


09-27-2012, 01:51 PM
Thanks guys I will try all of the above and report back.

-Matty, Ran about 58 mph turning 4800 RPM yesterday for quite some time, thos enumebrs would be substantially less if is wasnt firing on all 8 I would think yeah? Ran hard for a while maybe the soot on the transom is all the carbon being blown out and I am imagining the lsos of performance and bad fuel economy? What are your thoughts ont this?

-Silverghost, I just have the regular stock one that the engine came with...

-Sandog, nice article very informative, really sheds some light on the internal happenings of htis carbon deal thank you.

Any ideas about maybe I am jsut blowing off all that built up carbon still? I probably have only 2 hrs on it since ive been back from altitude.....But would the exhaust pipes have a gasoline-y odor if it was simply carbon being blown off?

thanks all

09-27-2012, 02:20 PM

if the exhaust smells like gas the motor is not burning everything getting dumped into it. get a glass jar or use your hands see if any fuel is in the exhaust water. it does sound like the motor is making decent power, i had the power valve let go on a holley and it ran like crap in the mid range . take a pic of the carb maybe we can id it that would help. again a plug reading would help to

if all the cyl are firing then the carb is dumping to much fuel that the cyl can't burn it all, which could be a tuning thing or a fuel pressure thing, but that might be too far down the road , look for the simple things first.

if it is a holley there are some simple tests to see if the power valve is blown.

09-27-2012, 02:47 PM
OK Matty excellent start, I will do this and get back thank you for the guidance.....What is so dam strange is that none of this was ever a problem before my trip up to elevation I dont get what could have happened up there.


09-27-2012, 03:10 PM
Seafoam in the tank, seafoam down the carb. Great stuff!

09-27-2012, 03:14 PM
If you really do have a rich mixture fuel problem it might be a high fuel pressure issue pushing the carb's float needle valves off their seats~~~
But I doubt it since your engine was running well before you made that trip to a high altitude area.

If you find you do have a Holley carb I am betting on a ruptured power valve diaphram.
The power valve is used to regulate the amount of fuel going to the carb's jets.

A ruptured power valve diaphram will cause poor performance, rough running, excess fuel consumption, overly rich mixtures, soot comming out your exhaust tailpipes, and fuel washng past your piston rings and into your engine's lube oil.

Engine vacuum has a direct effect on the carb's power valve enrichment circuit.
High altitude & low atmospheric pressure may have stressed the old power valve's rubber diaphram.
Also if you used fuel with any ethanol mixed in while there it may have damaged the old power valve's rubber diaphram.
This is now a common problem issue with the newer ethanol mixture gasolines. Newer power valves are now made with special new rubber compound diaphrams designed to run the newer gasoline & 10%-15% ethanol mixture fuels the government has now mandated by law.

A Holley power valve can also be damaged by a single simple engine backfire throuh your carb.
More modern Holley carbs have a simple small built-in check valve to help prevent a backfire blow-out.
You can also buy a simple kit to retrofit older Holey carbs with this small simple pressure check valve.
These check valves do not always react fast enough to prevent all power valve blow-outs however.

if it is indeed a damaged power valve as Matty stated you will see a real perfomance issue at some engine speeds.

All the above assumes that you do indeed have a Holley carb~~~which you may, or may not have.

As Matty also suggests post a photo of your carb with the flame arrestor , and any plastic engine & carb cover removed so we can see what type of carb you really have there.

The Holley carb will have it's part number stamped vertically on it's main body near the fuel bowl or near it's lower mounting base.

09-27-2012, 03:56 PM
an original 350-270 Mag of this era, will have a Quadrajet on it, not a Holley...

58 @ 4800 is dead nuts right for a decent runner, doubt you would ever see 60-61 with it if't is a stock 350-270

09-27-2012, 04:15 PM
If as Jim stated it does in fact have a Quadrajet carb~~~you have no power valve at all to worry about.
The GM Quadrajet uses jet step-up rods in it's jet enrichment circuit.

09-27-2012, 05:12 PM
I will inspect my Carb here in a bit and post what I find...I will also post some pics of the old spark plugs.

thank you all

09-27-2012, 05:42 PM
another place to look for extra fuel to enter the carb is to check the clear or yellow hose. the yellow hose comes up from the fuel pump to the carb . if the fuel pump diaphram is ruptured there will be fuel in the line and dumping into the carb.

09-27-2012, 07:20 PM
Sea Foam is a very good product but it will not hold a candle to Mercury Engine Tuner, It is the only off the shelf product that will clean like a demon. I do each and every engine I own on an annual basis and have for over 30 years. Note: MET should only be used on the hose it is toxic, you will be more than amazed at what comes out of the pipes!!!!!!!!!!


09-27-2012, 08:34 PM
MOP.... does it have ammonia in it?

09-28-2012, 07:48 AM
a Qjet does have a power enrichment circuit....

09-28-2012, 10:26 AM
OK fellers, here we go:

-First the Spark plug monitoring. Found that the spark plugs seemed to look a tad worn or fouled a touch with carbon already for only having about an hour of useage on em since replacing but I dont know:
Old one:
New Ones after 1 hr of useage:

-Carb Identification/condition pictures
Holley or quadrajet? Looks like a quadrajet yes?

-Clear tube from carb:
dark and dirty inside see pics

-Dipstick test:
smelt kind of smoky but not obvious gasoline smell...Would the gasoline smell be obvious?

-Exhaust water in glass test:
No gasoline found visually, water had an exhaust odor.


09-28-2012, 10:46 AM

I am like you no real motor head , and I can read tea leaves but not spark plugs, i leave that to the guys that know.

with the oil was it clear even if it was dirty as long as it was not cloudy/milky . oil that has been invaded by fuel is not as oily or it's viscosity is gone. if you rub some between you thumb and finger it rubs away very easy and is not slippery.

other thing i see is the carb is pretty sooty and dirty.

this is what gas in the oil looks like from a motor not firing on all cyl

I don't think this is your issue if all the plugs looked close and none of them was wet, I think the carb might need just a little attention.

09-28-2012, 11:15 AM
^thanks Matty, do you think the soot deposits on my transom are from all the crap being blown through this Carb? Someone also stated that it's from using high octane gas, what are your thoughts on this? Should I back down to 87?

My water was crystal clear and I tried several times to see a layer of gasoline or something and did not.

Maybe all this stuff in the carb and on my transom are residuals left over from that high altitude run.......

Oh and the dipstick looked like oil and felt like oil should.

09-28-2012, 11:45 AM
Not trying to be a smart azz but did you gap the plugs or at least check the gap on the new ones?

That carb is very dirty. At the very least a good cleaning is in order, or having it removed and sent to a carb shop for a cleaning and rebuild.


09-28-2012, 12:17 PM
the old spark plug ceramic has a crack down the side, did this happen upon removal?

the new plug shows a clear visual color change on the strap, which is the engine timing talking to you...

never read a plug from the top of the ceramic, except if your looking for detonation splatter, get a GOOD light and look at the very bottom inside for a color change area, or *color ring*

it appears your idle air jets are pretty dirty in the primary bore venturi area, these jets are specifically drilled for air mixture sizing within the carb base/body bores, the dirtier or more restricted the air jet bore, the richer it will run at idle and off idle..

I would get the carb overhauled by a competent source, and verify the engine idle timing is at 10-12*

09-28-2012, 12:42 PM
thanks you guys I will do that then, so carb overhaul and timing check and you guys think my problem is solved?


09-28-2012, 12:46 PM
From your carb photos you do not have a Holley carb.

You have a GM Rochester Quadrajet carb.

They are very good carbs~~~

I also agree it looks pretty dirty and you should have it professionally cleaned and a new rebuilding kit installed...

09-28-2012, 12:53 PM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

Brad do you think this solves my problem(s)? Also, do you think I am losing a decent amount of hP form this dirty carb?


09-28-2012, 01:17 PM
Patricke ~
I would think that should get you fixed-up.

Make sure that the carb mechanic totally takes your carb completely apart and properly solvent tanks & cleans it; & blows all passages out with air etc..
Do not just clean the outside of the carb.
Then have him install a new carb rebuilding kit and make all necessary adjustments such as adjusting the float level & float drop etc.

Also as Jim suggests check your ignition timing and general condition of your ignition system .

Then you should be good to go.

09-28-2012, 02:27 PM
Sweet will do. You guys are awesome. So to wrap everything up common opinion is that :

After fowling up my system at altitude, the internal soot has negatively effected my performance and fuel efficiency and caused residual carbon build up in my carb and is being blown out of my exhaust onto my transom.....In addition my problems could be further remedied by making sure my timing is on.

Oh and sorry Buizilla...Yes I cracked that one trying to get the dang thing out. It is well documented how difficult it is to get to the spark plugs on that engine lol. After haveing a carb overhaul, should I swap out the plugs again Buizilla?

I will post results and new figures after this is completed.

09-28-2012, 06:34 PM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]

sounds good mrfixx, I will do this tonight or tomorrow morning but let me ask you can I simply hook the hose up and do the whole process on the trailer?

thank you

09-28-2012, 08:04 PM

Here are some thoughts and ideas. Keep in mind that I don't know what you have had done to the boat in the past.

Clean the flame arrester as fixx said.

If you plan on not removing the carb and having it overhauled, clean it as fixx said. But, I would do it in the water. Running it at elevated rpms on a hose has its own issues and drawbacks. If you are in the water those issues, well, are not an issue. Just squirt carb cleaner in the carb as it is running. It will start to bog down as you do this. The trick is to keep it running. If you plan on rebuilding the carb, don't worry too much about cleaning it now.

You already replaced the plugs. I would not worry about those. They have a little carbon on them, but not enough to be an issue now.

Since you have gone this far, you might as well go further. Pull the cap and look at the underside. If the terminals have build up on them and the rotor looks a little burnt, replace those.

In the process of working on the cap and rotor, take a solid look at your plug wires. Look closely for cracks, breaks, wear, dry rot, etc. Depending on how old and what shape they are in, replacement can do you some good. Yes, you can test them with a meter, but the easiest way may be just to replace them. They can "leak" but the best time to see that is with the boat running in the dark. Also, if you do this yourself and you haven't any experience doing it, replace them one at a time so as not to mix up the wires.

Lastly, for the end of your tune-up, replace the fuel filter. No much else to do and these things don't cost a lot of money and you can do it yourself.

Oh, yes, and as Jim said, check your timing.

Best of luck.


PS. These are strictly my thoughts and how I would approach the problem. Other opinions may vary.:shocking:

09-30-2012, 12:06 AM
Ok a little before and after on this flame arrestor....was pretty bad:


Ok totally cleaned now, and ran the motor at 2k rpm and did about 6-7 x 3 second bursts into the carb.......thanks for the instruction guys.

09-30-2012, 10:49 AM
[ QUOTE=$originalposter]{$pagetext}[/QUOTE]
Theres so much build up on there can i basically remove it with a rag? I am however afraid to cause chunks to come off an fall into the carb......

09-30-2012, 11:44 AM
I have never seen that much gunk in and around a carb caused by carbon. Looks like that came after the fact. Based on the looks of the flame arrestor and the front of the carb, is it possible you have an engine belt going bad? That would cause the engine to be choked off and then the carbon would come into play. Just seems like alot of crud. Will still need to rebuild the carb at this point, but I would check the source of all that dirt on the engine.


09-30-2012, 03:48 PM

From what I am seeing in your photos you really need to take this carb to a profssional carb rebuilder who really knows what they are doing.
The carb needs to be taken apart and completely tanked & internally, & externally, cleaned along a new carb rebuiding kit installed along wih the proper adjustments made.
I have done this myself countless times on many many carbs and the results will work wonders.

Just taking a few cans of carb cleaner and cleaning the outside & carb throat would do very little here in my opinion.

You are only kidding yourself in thinkng an external carb, & carb throat, cleaning will completely solve your dirty & malfunctiong carb's overly rich mixture & excessive fuel consumption issues.

The cleaning & rebuilding of this carb seems long long overdue.

09-30-2012, 04:21 PM
www.thecarbshop.com (http://www.thecarbshop.com)

09-30-2012, 10:57 PM
Thanks guys....yes indeed the Carb will be coming off asap and being completely rebuilt, etc as suggested here. I have no idea why there is so much crap under my hatch, all belts are in great shape, I just think that possibly nothing has been cleaned under there for several years and now it is time! I will clean under the hatch as well....

Once again you all are the best thanks again,