74 Mercury 500 50 hp

[kembry]

Once I get the motor started and in gear it will run for about 45 seconds to a minute and then sounds like it runs out of fuel. I have replaced the fuel lines from the quick connect at the motor all the way up to the carbs, replaced gaskets, diaphram, and check valves in fuel pump, cleaned and put a new kit in each carb. I know that the fuel line from the tank to the motor is good and the vent on the tank is all the way open. The primer bulb gets softer once the motor begins running, but that seems like it would be normal. Out of the water the problem seems to be about the same, will run for a period of time, maybe longer than when its in the water, and then cut out. Also when I advance the throttle lever to wide open, the motor does not sound like its at its highest rpms.Thanks for any help.

 

[tashasdaddy]

Re: 74 Mercury 500 50 hp

sounds like floats are not opening the needle to allow the gas to flow to the carbs. thus just burning the fuel forced into the bowl when you pump the bulb. try pumping the bulb when is running. this will tell you that it's either the float, or something is wrong in the fuel pump.

 

[kembry]

Re: 74 Mercury 500 50 hp

I did try to pump the bulb when the motor started to die and it seemed to work sometimes but sometimes it did not. Pumping the bulb seemed to work more often at lower rpms. When I put the kits in the carbs the floats were in good shape. I did notice that the new needle had problems falling down to contact the levers. There was no problem with the needle going up into the new seat, just when its coming back out.

 

[Texasmark]

Re: 74 Mercury 500 50 hp

Well pilgrim (as John Wayne used to say), when it's up is when it shuts off the fuel. You just nailed your culprit.

Mark

 

[kembry]

Re: 74 Mercury 500 50 hp

Since the needle and seat are both new, can I use some emery cloth and gently take off a little bit of the sides of the needle?

 

[tashasdaddy]

Re: 74 Mercury 500 50 hp

yopu probably still have some sticky crud in there. keeping it from moving.

 

[kembry]

Re: 74 Mercury 500 50 hp

OK, i took the top of the carbs back off and checked each needle and seat, both are moving freely. I think the needle was sticking before because everything was new and the carbs had been cleaned and were completely dry. Now that some fuel has run through them all seems to be fine. Could my problem have anything to do with the idle mixture screw? I put them back in by turning untill lightly seated and back out 1 1/2 turns. The gas tank is very old and I can pick up a new one for 11 bucks and maybe new quick connect fittings. I have checked the fuel line from the tank to the motor by disconnecting from the motor and pumping the bulb untill it was hard, everything checked out fine. Thanks for the help everyone.

 

[tashasdaddy]

Re: 74 Mercury 500 50 hp

if the inside of the tank is clean and the pickup is clean, should be no reason for a new tank. i'm at a loss right now let me think about it over night, wish Clams or Tim would chime in.

 

[kembry]

Re: 74 Mercury 500 50 hp

When I replaced the fuel lines from quick connect to fuel pump and from pump to the carbs I had to use 1/4 I.D. hose because of the fittings. I could tell that the lines had been replaced at some point but were in bad shape, so I kept everything the same size. How critical is it to have 5/16 or 3/8 I.D.? The fuel flows freely right now but do you think I'm not getting enough volume? If I need to change, do the fittings have to exactly the same or can use fittings with hose barbs and clamps since psi is relatively low? When I was working on the carbs I found that adjustment on the butterflies were not exactly the same. With the motor off the top butterfly has a small gap between it and the wall of the carb, the lower carbs butterfly was shut completely so I adjusted the linkage to the bottom carb to match them up. Did not get a chance to fire it up after that so I was not sure if that would have anything to do with my problem. Thanks again for the help.

 

[kembry]

Re: 74 Mercury 500 50 hp

One thing that I forgot to mention was that I have installed an inline fuel filter between the fuel tank and the primer bulb. It made sense to me to put it there so it would stop anything from plugging up from the bulb on, but I haven't thought about it pressure wise.

 

[kembry]

Re: 74 Mercury 500 50 hp

started it up today after taking off the fuel filter that was between the tank and the bulb. bulb no longer gets firm, but the motor runs longer without dying. Now it sputters more and surges like its still not getting enough fuel. Took rpms with tach from the top of the flywheel-- surging between 1100 - 1600 rpms in neutral and throttle lever in the idle position. When I try to pump the bulb I get fuel coming out of the throat before it ever firms up. Its seems to act different from day to day.:/ Thanks to all.

 

[tashasdaddy]

Re: 74 Mercury 500 50 hp

it's pushing the the needle and that is possible as the bulb puts more pressure than the fuel pump. you can use barb fittings and clamps i do.

 

[tashasdaddy]

Re: 74 Mercury 500 50 hp

hopefully, the real mechs will be back tomorrow.

 

[Texasmark]

Re: 74 Mercury 500 50 hp

If fuel comes out of the throat of the carb it says your needle is not seating correctly. You don't want to sand that cone as it has to completely seal off gas and gas has a very low surface tension meaning it moves around at the molecular level.....seeps everythere; will seep by the minute groves you cut in the needle with your sandpaper.

Now that you have a little oil on your needle and it works without binding fine.

You may not have adjusted your float correctly as your needle should be able to resist the pressure you can gererate from the squeeze bulb unless you go nuts squeezing it.

The 1 1/2 turn sounds great to me. And you said the right thing when you said you "gently" seated it and then backed off. Once things clean up if that is a bit rich you may want to screw them in 1/4 turn......but wait and see.

The fuel filter between the tank and the fuel pump can cause vapor lock. Vapor lock is caused by gasoline boiling due to the low pressure caused by the fuel pump suction (pulling against the filter). It can't pull fuel freely through it so it creates a negative pressure which boils the fuel. Boiling fuel won't run your engine; not enough volume. Best place for the filter is a mesh screen (no pressure drop) in the tank (comes with the tank) and an OEM approved plastic inline after the pump. That way the pump can suck all the fuel it wants (within reason) and "push" it through the filter. BTDT

On the fuel line diameter a 1/4 line should be plenty for a 50 hp engine.

Don't remember the ignition setup on the '74 engine but you could be chasing an ignition problem along with your fuel issue. Since it does the same in or out of the water, put it on muffs and get a timing light. Check all the cylinders when it's running right to get a reference point and then check again when it acts up. May find something there.

Mark

 

[kembry]

Re: 74 Mercury 500 50 hp

Thanks for the reply Mark. When you say adjusted the floats correctly are you talking about the spring on top that is supposed to be a certain length? When I took the carbs apart they had different floats. I wasn't sure where to take my measurement from to determine the specified length of 3/32". New floats did not come with the kit from Oldmercs. Can the primary and secondary lever under the needle cause this problem if they are not completely accurate? I'm not a mechanic by any means but I'm learning, however this problem is becoming annoying. Thanks again for all the help.

 

[Texasmark]

Re: 74 Mercury 500 50 hp

I can't answer that. Probably Clams Casino (Spelling) or Rodbolt or one of the other engine mechanics.

Your floats may be different because of different mfgr's, like one is OEM and one after market. Some are soldered brass and some are plastic.

Don't think it matters as long as the do as they are supposed to do.

Butttttttt don't overlook the possibility of a saturated float; i.e. a float that doesn't float. Might get a container of gasoline and set them in it and insure that they float. It happens.

Usually the correct float level is somewhere around the body of the float being parallel to the body of the carb with the carb turned upside down and no weight on the float other than gravity.

If you are using Tillotson carbs, it would be parallel to the aluminum flange where the bowl mounts......as I recall on what Tillotson looks like......been a long time.

HTH.

Mark

 

[kembry]

Re: 74 Mercury 500 50 hp

I didn't have time to pull everything apart today and check the floats. I did however check the fuel line from the motor to the tank, again to verify that the bulb is o.k. Once again bulb gets rock hard. Plugged it back into the motor after releasing the pressure off the bulb, primed the motor and the bulb actually stayed hard this time before any fuel spilled out of the carbs. I decided I would try to start it up and after a few cranks the bulb was soft again, so I gave it a few pumps and sure enough fuel started leaking out of the throat of the bottom carb. I don't know if this gives you any better idea, or pinpoints a different problem, but thought I would post it anyway. Tommorow I plan on checking the floats out and needle and seats. Thanks again.

Kyle

 

[tashasdaddy]

Re: 74 Mercury 500 50 hp

points to floats.

 

[kembry]

Re: 74 Mercury 500 50 hp

I opened her up today to check the floats. The bottom carb was where fuel was coming out of the throat, but that float checks out fine. I had to take both of the caps off where the screen goes on the carbs to get to the bottom bowl. I took the fuel pump off and there it was a slight tear in the brand new diaphram. My question is can you pump the bulb hard enough to tear these diaphrams?

 

[tashasdaddy]

Re: 74 Mercury 500 50 hp

i don't believe so. as when i'm cold starting my 71 1350, i put the bulb on the motor well and push down hard on it.