165hp gm 250 motor woes

[bigguy6f4]

hey guys. Old faithful started giving me fits yesterday. Ill try to give a run down on the situation and ill let you guys decide what the best course of action would be as im sorta stumped at the moment.


i have a 71 19ft trihull boat with a wake tower. it has a alpha 1 outdrive installed with matching gear ratio. and,, ive had these symptoms happen before with the original drive. it has the 165 inline 6cyl 250cu 4.1liter.

New carb rebuild.

new alternator.

new balancer and belts.


Boat ran awsome! got up to speed faster than ever after i got the carb rebuilt. turns out it has the wrong accelator pump installed.


anyways 2 days ago we went out wakeboarding with 5 people in the boat and i noticed the power started falling off twards the end of the day. at the end of the day it was taking about 4 times as long to get on plane. And would only run to about 3000rpms in gear. it would rev freely out of gear. and wot is around 4100 ish depending on the how many friend are with me when the boat is running correctly.

anyways the next day we took it out and it ran better for the first 10 minutes. then everyone showed up and i had 6 big people including myself in the boat and could barely pull a wakeboarder up. took along time to even get past 15mph once the boat would plane then the power was fine.
It got worse and worse till the boat would only do 15mph tops and wouldnt plane out.

all the guages were showing ok numbers. motor didnt seem overly hot. although i could be wrong. however there was no hot smell in the air.

It did do this once before last season and went away on its own. i could never find the problem cuz it ran fine after that horrible day of idling 10 miles to the boat ramp.


i put the boat in neutral and reved it up and it would free rev to 5000 but seemed sluggish. the water hose before and after the thermostat was cold. which i thought was wierd. but figured it meant the water pump was flowing water good.

um... about the only other things ive done to the boat is new cap and rotor and wires and plugs last season.

i did cut the resistor wire going to the coil when i first bought the boat not knowing what a resistor wire was. however i soldered and shrinktubed it back together. dont know if that would play into anything or not.

i installed a new fuel filter last month.

And off and on the boat has had a wierd problem of kinda surging. the tach would be real jumpy. you could feel the boat run good, then drop off then pick back up. sometimes it would drop for only a second. othertimes it would be for 10 secs were the power would cut to like half.. then instantly pick back up.


I almost feel like its a coil issue, a mechinacal advance failure, or possibly a fuel related issue.

however i dont have a service manual and dont know where to buy one locally.
if i could buy one online in pdf format that would be great.
Or if someone wanted to send one to me would be awsome.


any thoughts?

many thanks, Drew

 

[Silvertip]

Re: 165hp gm 250 motor woes

You have taken a guess at what "might" be wrong but you have done zero from a diagnostic standpoint so lets get started. Your symptoms do not sound at all like ignition but they DO sound fuel related. Specifically, check fuel pressure after the pump (4-6 PSI) would do nicely). If you don't have it, the engine can't deliver power. Look for fuel restriction (kinked lines, clogged pickup in the tank, stuck anti-siphon valve if you have one, or a bad fuel pump).

Lastly, stop free reving this engine unless you want a serious hole in the bottom of the boat as parts from inside the engine make their way outward. It is unnecessary and proves nothing as an engine running on half its cylinders will free rev. Do you actually know what a dead cylinder feels like? If not you should do a spark check to make sure all cylinders have spark. Pull one spark plug wire at a time with the engine running. Each time you do that the engine should die off a little and miss! If you pull a wire that causes no change you found a dead cylinder which may be due to a bad plug wire, plug, or distributor cap. A compression check would also be suggested but if the engine runs good when it runs, compression is likely not a problem but you should at least have the numbers for reference anyway. No hot smell in the air is a terrible way to tell if an engine is or is not overheating. Look at the guage. If it doesn't work it would be a good idea to fix it. If the boat doesn't have one, (or an oil pressure gauge) get one. Much cheaper than an engine. As for service manuals, they are as close as your public library. Even a Chevy car service manual (Chiltons or Motor Manual) for a 60's Chevy 6 cylinder will work for most engine issues.

 

[MikDee]

Re: 165hp gm 250 motor woes

A 19' Trihull with a tower, & 6 people, it sounds like a lot of weight! and drive coupling overload failure to start with! Thats why when it's a light load, & cool to begin with it doesn't slip as much.

 

[bigguy6f4]

Re: 165hp gm 250 motor woes

thank you for your responce. Ill go see if i can find my compression tester and my leak down tester. i used to race a supercharged truck on the 1/4 mile track and blew a few motors in my time. so yes i know what a dead cylinder sounds like. and feels like. and what rods poking out of the block looks like too! lol. however i only delt with fuel injection, programable pcms using laptops and other guages, and individual coils per cylinder. never had to mess with distriberters, carbs, setting timing via timing lights and turning distributers, etc. the list goes on.

So while i understand the fundamentals of how everything works, i was hoping for some real person advice on how to approach the old girl. lol

Thanks again and ill try to get some numbers.

 

[Don S]

Re: 165hp gm 250 motor woes

Here is a link to the OEM service manual for your engine and drive.
It's over 1GB in size (HUGE FILE).

http://www-alt.mercurymarine.com/mnetdata/SERVICE/CRUISER/Servmanl/01/cover.pdf

 

[bigguy6f4]

Re: 165hp gm 250 motor woes

Thanks for the link Don! You surely are the Man with the Plan.
I got it downloading over to my desktop since i have plenty of gigs free on that one.



Ok guys here are the numbers ive got so far.

I dunno which cylinder is which so im starting from the waterpump side of the motor working back on a 4.1 liter 250 cubic inch 165hp Gm motor. Straight 6.

1. 92 psi

2. 101psi

3. 91psi

4. 90psi

5. 90psi

6. 90psi


I dunno with the older engines if they are low compression or what.. so ill leave it to you guys to decide. however my 5.3 liter needed around 135psi
however my motor still cranked very very smoothly and quickly.


Fuel Pressure after the fuel filter but before the carb was exactly 4.5 psi

Out of curisoty i pulled the cold water hose that hooks below the thermo stat and hooked up some muffs.

I turned on the hose to full blast.. nothing came out.

I cranked up the motor and a steady stream of water came out kinda like straight out of a garden hose. So im hoping that 2 part test shows that my impeller is working fine.

I then pulled my thermostat to do a water test on it. Here are my findings.

I Heated a pot of water to 140* and removed the pot from heat and dropped the stat in. Didnt budge.

150* Cracked open slightly.

here is a pic of it at 190*

And the pic below it is at boiling temp next to a tape measure.

Ive never seen a thermostat work before. so i dont know what im showing is considered wide open or not. doesnt seem like open very much to me though.

 

[bigguy6f4]

Re: 165hp gm 250 motor woes

for some reason my pics didnt show up.. here is a link.


http://cid-a95937a5c3695d24.skydrive.live.com/self.aspx/Public/picture2.jpg

http://cid-a95937a5c3695d24.skydrive.live.com/self.aspx/Public/picture1.jpg

 

[Don S]

Re: 165hp gm 250 motor woes

With compression like that, no wonder it won't do anything.
Should be seeing 140 PSI (look at the specs in that link I posted)
But before we say your compression readings were bad, make sure you did them properly so the numbers mean something. Read this Mercruiser service bulletin on how to do the compression test properly.

http://www.boatfix.com/merc/Bullet/97/97_25.pdf

Oh, and stop playing with an iffy thermostat and get a new one.

 

[bigguy6f4]

Re: 165hp gm 250 motor woes

I guess i try squirting a lil oil in there to see if it raises.

however Im gonna be stubborn and say i fail to see how it could run fine one moment and fall on its butt the next. i doubt ever cylinder lost its compression at once like that. the change was night and day. and happened very quickly.

Of course im gonna replace the stat, and ill re try the test. however my stubborness tells me this isnt the problem im tring to find. (although i very well know low psi IS a problem in itself.)

and thanks for the compression test link. never heard of the oil test.

 

[Don S]

Re: 165hp gm 250 motor woes

You could also have some ignition or fuel problems along with the compression. It was probably just on the ragged edge of starting and not starting.

 

[bigguy6f4]

Re: 165hp gm 250 motor woes

whats wierd is it starts up faster than every vehicle ive ever owned. i bet it doesnt even turn over a full turn and its already running. everyone who has heard it run says they are amazed at how fast it starts. lol. however your prolly right.


I cant possibly see how all could lose compression so fast. And nothing would make since except that its just an old engine that is worn out. that i can except.


Ok im showing 4.5 psi on the pump at idle. however im not at the lake today so i dont know how to test it at wot under a load without the lake. so im clueless if its still a lack of fuel on the topend.


And the engine will still run with several spark plug wires pulled. So i feel my sudden lack of power is something electrical.

However that narrows down to the coil, distributer and mechanical advance.

I dont know really know how to test either of them. i have a brand new coil that says requires external resistance written on the side. andi think it would work. however i havent tried that yet. plus id have to be on the lake to see.

i do have a timing light. so i guess i could at the very least check timing. however i dont really know what id be looking for.

And remember. It did the exact same thing last year. It then Dissapeared.


On a different angle of view.... Could it possible be anything outdrive related?
Because it looks like could possibly have some outdrive issues i saw while wakeboarding.

When driving i noticed a roost coming about 2 feet out of the water at full trim down. the roose was comeing almost straight up like i was trimmed up. which ive never seen before.

And when i was in the water fixing to wakeboard i noticed there was ALOT of water and possibly exaust coming up right next to the transom. And im worried i might have a gasket failure where the outdrive mounts to the transome plate.

No air was coming out of the water tube next to the thermostat so im gonna guess i have a good seal on the oring on the outdrive. however maybe the gasket blew out between the exaust and bellows area????

Anythoughts Don?

 

[bigguy6f4]

Re: 165hp gm 250 motor woes

OK might of found something.

It seams it might be EXACTLY what i thought it could of been.. electrical

I did the pull the spark plug wire while the engine is running trick and found a plug not firing. a few that seemed to go in and out. it was number 5 from the front.

I hooked a spare plug into the spark plug wire and tested it against ground.. no spark. i then traces the wire to the distributer and pushed in on the wire to the cap and when i pushed hard the motor about died! the distributer doest move or twist.. however i push it a lil one way the engine bout dies. sounds like something in there is out of whack.. i tried to wiggle it a lil and even got a shock through the dead wire.


So now i guess i need to figure out how in the heck to get a distributer all set up and the clearances set and points set. etc.

I dunno.. seems as if the old gal has everything wrong with her but she still wont die. lol

 

[bigguy6f4]

Re: 165hp gm 250 motor woes

i did another spark test and every cylinder is getting spark except the 2nd from the rear.

Also the spark is yellow. dunno if that means anything or not.

I tested by pulling a wire, inserting a new plug, and holding it to ground. everyone would fire except the 2nd from the rear.


Im unsure on how to procede. And the manual isnt downloaded yet.

 

[Maclin]

Re: 165hp gm 250 motor woes

You mentioned that one of the symptoms was the tach jumping around. They can short out and cause problems because they are connected to the negative wire on the coil. You might try unhooking the tach wire on the coil minus terminal as one thing to eliminate.

Also, I think I would get a new coil ballast resistor wire as well just to be sure you are in spec regarding coil loading.

 

[bigguy6f4]

Re: 165hp gm 250 motor woes

i wonder how much a new resistor wire is? or if i should just get a coil with a built in resistor.

however im confident its not the tach. because it did this last year, and i replaced my entire guage cluster with new guages. still does it. only 3.5 hours on brand new setup.


looks like i just need to figure out my distributer problems. im tring to descide if its worth converting to eletronic ignition. just replacing the entire distributer, or new points and condenser stuff. i have no clue at the moment.

 

[starsnstripers]

Re: 165hp gm 250 motor woes

try a different wire on the # 5 cyl. if it fires it's that wire. also inspect the wires where they enter the dist. for cracks.

 

[bigguy6f4]

Re: 165hp gm 250 motor woes

Already did. I swapped wires and its dead at the distributer. Thanks tho!


Also Don... the first link to the 1gb Manual doesnt work. Any suggestions?
I noticed if you type in http://www-alt.mercurymarine.com it tells you to click the link to be redirected to the new site. i dunno if that affects things or not.

 

[fishrdan]

Re: 165hp gm 250 motor woes

the first link to the 1gb Manual doesnt work. Any suggestions?

Grap a cup,,, I mean a pot of coffee and let it download. I did it earlier today and it took hoooours to load. 1GB file and probably a lot of people hitting it at the same time.

71 Mercruiser, has anything been done to the distributor except for plugs, wires, cap rotor? Have you changed the points/condenser and adjusted the dwell, then timing, if not this should be done irregardless if it cures your problem. While your changing the points/condenser, remove the plate these mount to and you will see the mechanical advance. The advance should move freely and snap back smoothly with the springs, if the weights are hanging up and binding or the springs too loose (rusted and weak) this could cause poor performance,,, (something an electronic conversion won't fix) clean and oil the mechanical advance as needed. Since you have already identified a dead wire, might as well change all the ignition parts (plugs wires, cap,,,,,) not going to cost all that much.

If you have an ol'school mechanic in your town, see if they have points distributor testing machine. Years ago my local mechanic had a Sun-Tune machine and he could set the dwell and check/adjust the timing curve, swap springs and weights. Though I guess it's probably a lost art...

Beyond tune-up parts, the distributor could have worn out shaft bushings, if these are bad you won't be able to adjust dwell correctly as the shaft will be moving around and changing the dwell randomly. I had to replace the distributor in my 73 Mercruiser because of this. An electronic conversion might get you past this problem, but I would replace the distributor if there is a lot of play in the shaft.

 

[bigguy6f4]

Re: 165hp gm 250 motor woes

ok.. took the distributor off.

Here are my findings. the shaft bushings feel very tight and no noticible shaft play. which is good. althou i called the parts store and a new distributar is only 50 bucks.

however im 90% sure i found the problem after playing with it in my hands.

now keep in mind this is my first good look at the points and condenser.

i rotated the shaft and watched the movment of the rotor and shaft and i remember reading about setting the dwell. and i know you can use a feeler guage to do a basic dwell set but without a dwell meter you can get it close but not exact. And i noticed there was nowhere to check with a feeler guage. the part that rotates agains the shaft wasnt touching. and the points stayed closed! i took a screwdriver and adjusted the points so the feeler arm was striking the shaft and the points staring clacking open and close! Now im not exactly sure where to stick the feeler guage. inbetween the points clackers? or between the arm and shaft?

I did however buy all new everything. even a new coil. cost me under 20 bucks total. and the new cap compared to the new cap was night and day.
the old one looked burned almost.

im about 40% thru the download for the manual so unless someone pipes in how to set my points ill just have to wait a few hours.

Anythoughts on any further action i should pursue?

 

[MikDee]

Re: 165hp gm 250 motor woes

I think it might be a bad Dist cap myself, anyway, with the hold down screw on the igntion points, a little loose, put the rubbing block of the ignition points on any one of high spots of the distrbutor cam (there's 6 of them), and then set the gap to .017" (a standard generic setting), then just tighten the hold down screw, recheck after cranking abit, reset if need be, and you're done!