Mercury 2.0 liter fuel pressure question

[tombrown]

How much fuel pressure should I have between the carbs and the fuel pump on a 1978 Mercury 1750? My Seloc manual tells me to pull one of the hoses off and watch the fuel stream fill a container. It's purely subjective.

I don't have a tester, but I'm confident there is a fuel supply problem. It idles perfectly but starts to stumble as soon as the carbs start to open. When I can coax it to get up on plane, it will run for about 10 seconds and then die completely.

It has a rather small looking Yamaha fuel bulb that I will replace with the big QuickSilver one.


Thanks,

Tom Brown

[Rusrog]

Next time the motor dies, simply pull on of the main jets out of the carbs and see if fuel drips out. You should get several ounces out of the float bowl.
I have seen both the bulb and the twist style fuel connectors give trouble and starve a motor.
Even after all of that though you mught still have one of the check valves in the mechanical fuel pump giving you trouble.

[Techno]

Your book probably has a misprint, it's supposed to tell you how much fuel will pump in a given amount of time. Like 1 Qt. in 1 minute. I don't know the specifics. This is volume, you can have pressure but not the volume. Or the opposite.
I had a problem once where I had flow with the hose disconnected. The hose had delaminated internally and the pressure in the hose made an anurism that throttled the hose down when it was hooked up. There was a very slight bubble on the hose, when I cut through I found the problem.

[MadMat]

Try getting a mate to squeeze the bulb as you open up the throttle. If it continues to run then that confirms your suspicions that it's probably fuel related.

The bulb won't be rock solid when the engines running of course, like it is when you prime the engine without it runing, so don't get mislead like I did.

[us1]

I like the bulb squeezing idea. We used to do that to see if the fuel pump was at fault when our straight 6s would act up.

[Raceman]

Fuel pressure isn't really critical on a carb motor as long as you don't have too much, which will blow the needles off the seats, and have enough volume to keep the float bowls full. Remember, the jets are the demand side and the float bowl is just a small gas tank supplying the jets. I'm assuming you've checked to make sure it's not restricted on the vent side of the tank. If it's a vent side restriction it should suck the bulb at least partially flat. The small primer bulb won't make a difference either, as long as it's not restricted in some way. If you're sure it's fuel and don't want to diagnose it, just stick an electric pump on it and regulate it to about 4 - 5 psi and do away with the stock pump. That'll do away with the need for a primer bulb also. If you take the stock pump off you'll have to plug the vacuum supply to it also. Again, volume is what's important on a carb motor. Theoretically you could have 1 psi with proper volume and it'd work fine.

[Edited by Raceman on 07-26-2001 at 08:23 PM]

[tombrown]

> I'm assuming you've checked to make sure it's not
> restricted on the vent side of the tank.

Yes. I even took the hose off the fuel tank barb in the splash well and stuck it into a bottle of fuel. No difference. The bulb doesn't go flat. Of course, it doesn't stay completely firm. If I pump it during the problem times, it doesn't help.


> The small primer bulb won't make a difference either...

Thanks. It didn't. It's nice to have a new bulb on there, though.


> If you're sure it's fuel...

Well, I was sure but now...

The engine idles beautifully. It's really smooth. When I put it in gear and advance the throttle, it pulls nicely until the throttle bodies begin to open. Even when they just open a tiny bit, it starts to sound throaty and bogs down. I can't find an enrichment valve.

When it's cold, it goes much better, but not well. Usually, it will come right up on plane. I haven't seen over 4000 RPM and it stumbles periodically. Eventually, it will start to really stumble and then die completely. At that point, it will be hard to start for a couple of minutes. After a brief rest though, it will fire right up and idle smoothly.

A guy I know has an almost identical Glastron with a 78 - 1750 and his is rev happy with the little propellers (19p). The way it feels and sounds, as well as anecdotal evidence of this other boat, make me confident that it is way, way down on power.

Would anyone care to offer a course of action? I'm starting to think it's a leak somewhere in the intake system, but I don't know how to test for that.


Thanks,

Tom Brown

[Edited by tombrown on 07-27-2001 at 01:38 AM]

[us1]

Hi Tom, It’s probably not a leak in the intake. It sounds almost like the timing is not advancing from the idle position. Check to make sure every thing is hooked up and timed properly from the throttle arm to the trigger. Then put timing light on each cylinder one at a time and rev it up until it dies checking for lack of spark at RPM. I don’t know what rpm your hitting befor it dies but it could be the high-speed windings on the stator, the last test will test this. If squeezing the ball doesn’t fix it, it probable isn’t fuel. It would be unlikely that all the carbs would be malfunctioning at the same time. With one bad carb the motor will still rev up.

[tombrown]

Thanks for the help, John.

> Check to make sure every thing is hooked up and timed properly from the throttle arm to the trigger.

When I checked it, it looked good. Also, I can manually move the spark advance lever and the RPM will increase with no movement at the carbs. One thing though, the idle adjustment on the bottom of the spark advance lever is out of it's range. The rubber cap on it never touches it's rest. The boat idles at 900 - 1000 or so and I can see no way of lowering it since the carbs are closed and the timing looks to be in the fully retarded position.


> Then put timing light on each cylinder one at a time
> and rev it up until it dies checking for lack of spark at
> RPM.

Will do.


> I don’t know what rpm your hitting befor it dies...

It gets to about 1500 on the tach before it starts to bog down. At that point, it starts to stumble heavily (the revs won't increase, but they don't decrease significantly). That is precisely the point at which the carbs begin to open. The more gas I give it from there, the slower it will run and the more throaty it will sound. Full throttle will certainly kill it when it's doing this. When the engine is cold I can sometimes coax it to get past that phase and the boat will get up on plane and rev to 3000 - 3500. It will only hit 4000 when it's first run after sitting for a long time. Even when I can get it up to higher RPM, it misses here and there. It's never run properly in the two days I've owned it.


[Edited by tombrown on 07-27-2001 at 01:56 PM]

[us1]

It does sound like no fuel, but check the spark on the cylinder just to be sure. Look in the carbs when you are revving it up and you should see fuel start to dump out of the emulation tubes, if not, something is wrong with the carbs supply of fuel. I would also try manually filling the float bowls and revving it up to see if there is some kind of major restriction in the fuel lines, pump or filter, that even when squeezing the ball wont let enough fuel through.

[Instigator]

I had an in line one time with the same symptons that I could'nt figure out.
Went through and checked everything to no avail.
Then one of the old timer mechanics from a shop across the street from us told me that sometimes the main seals on the crank (among other things) wear enough with age that it loses vacum thorugh them which robs the vacum needed to operate the pumps at speed.
We installed the cheapest electric pump we could find as an experiment and it worked like a bitch!!!
Funny thing??
It was on a boat very similar to what your parents have!!
Very clean but their was no way the owner was going to pay me to rebuild his ancient in-line motor.
Think we charged the guy $75.??
Needless to say he was a happy camper!!
Good Luck!

[tombrown]

The timing light test looked OK to me. At 1500 RPM when the engine bogs down the flashing looks normal.

How would I manually fill the bowls? Should I get some new fittings and hook an electric fuel pump up to the barbs on the carb bowls? Instigator, is that what you're suggesting I try?

I couldn't see fuel coming out of the tubes. I could feel a decent pull at the throat of each carburetor, although putting my fingers in front of the carbs didn't seem to effect the engine at all.


Thanks for the help guys,

Tom Brown

[Unloaded Racing]

Tom,
Try checking the Bowls and Jets on the carbs for Trash. We had an older 200 V-6 and had many of the same problems you described. We finally pulled the bowls off the carbs and there was trash under the seats and the mains were almost totally clogged. It would Start to come on plane and then Die. Just a suggestion.
Tim

[us1]

Yes you could use Instigators method or a small funnel or metal turkey baster or something like that. At least you know you have a fuel delivery problem now. It’s just a question of what’s causing it. If you do buy an electric pump you mite as well get a good one because you may end up using it permanently. You need a pump with good volume to run a Merc.

[Edited by us1 on 07-28-2001 at 08:51 AM]

[sosmerc]

...been that all V-6 Mercs will benefit from the addition of an electric fuel pump that is regulated at 4-5 PSI. The whimpy diaphram pump on your motor needs to be rebuilt prior to every season to keep it in top shape. (diaphrams get streched, and I have seen tiny holes develop in the diaphram) You should also consider the addition of a small inline filter between the pump outlet and the carbs to keep trash out of the carbs.
I would highly recommend you purchase a factory manual for your engine. Due to its age, I would not be surprised if your stator is contributing to your problem. Coils, plug wires, plugs (are you running NGK BU8H?), all wires, switchbox grounds, etc.
Proper synch of timing to carbs is important as well. Lots to consider beyond just fuel problem.
Hope this helps.

[us1]

It is always a good idea to check everything but don’t get to side tracked, when all six emulsion tubes are not squirting fuel you have a fuel problem.

[Instigator]

If you put your hand over the front of the carbs while its running it should puke fuel all over!! If not, you definately have fuel starvation problem.
Open the carbs as suggested before replacing parts.
If it sat fro a while, varnish will also do this.

[Glenn]

Hey Tom. I was sort of having the same problem with my own 78 1750 above 6000 RPM a couple of weeks ago. A few years back before I owned the motor the owner at the time was having trouble with the stock diaghram pumps. Seem to be changing them all the time. He put in a no name electric fuel pump and pluged the holes for the stock ones. This worked good for him. Worked good for me 'till I raised the motor. Engine would loose about a 1000 RPM after about a 1/4 mile stretch over 6 grand only. On the advise of US-1 and a couple others I instaled a Holley Red fuel pump. Easy to install, didn't need a regulator. Problem solved. Runs great. Not a question if it will run out of fuel flat out now. Will I run out of nerve!

Glenn

[Markus]

I would suggest something completely different: Check the reeds.

I had a problem on my Yamaha V6 a month ago. It sounds like what you describe: Engine getting weak at 1700 rpm and sounding rich. Turned out a reed was loose. The engine builder forgotten to put thread lock on the screws.