1978 Mercury 500 carb questions

[MPrimeaux]

Ok guys, in my frustration from working on my '63 Merc 650, I have gone off and bought a fishing boat that came equipped with.... you guessed it.... another old Merc. :faint2: I guess I'm just a sucker for pain! It's a 1978 Mercury 500 (serial # 4963671 fitted with the carbs that have the fuel pumps built on the side of the carbs) In reality, the motor is in really good shape and does run pretty well. I ran into a small issue where... sometimes... when it would sit for a while (as in I was fishing for a period of time) it would crank up fine, but when I went to give it throttle to get on plane it would act like it was running out of gas and bog. When the issue started, I could get out of the throttle and get back in it a couple of times and it would go on, but eventually got to where I would have to go pump the primer bulb on the tank to get it to straighten out. Last time I went, that didn't work either. I suspected fuel a fuel pump issue. I figured if I was going to pull the carbs, I would go ahead and get carb gasket kits too. I did find a small tear on the fuel pump check valve diaphragm. But my main question is I noticed in the carb gasket kit (carbs were squeaky clean by the way. Not even a tiny bit of varnish or grit in bowl) that it only came with one fiber gasket for the bowl. I did order a service manual for this motor and it states that it needs 2 fiber gaskets for the bowl screw, one inside the bowl and one outside on the screw. I see in the exploded view of carb on multiple parts sites that it does indeed show a small gasket inside and outside of the bowl for the screw in the bottom, but doesn't list part numbers. My local parts guy said that you can only get that gasket in the kit..... which I got 2 of. Each kit only had ONE fiber gasket that size and ONE metal washer of the same size. I put the metal washer on the inside of the bowl and the fiber on the bowl screw on the outside (figured the metal one would leak if on the outside??). When I got everything put back together, it fired right up and purred like a kitten.... except it idled a good bit higher. Ya'll please forgive me... I'm a maintenance electrician and a details nut on top of that, I'm just trying to give as much info as I can to try to get some help!!. I tried to idle it down using the idle mixture screws on carbs to richen it up and it didn't seem to do much. At that point, it was my last day home and I HAD to start putting things up and getting ready to go to work, so that's where I left it. Ok, so my questions are:

1. can anyone tell me for sure if there is supposed to be a gasket on the inside of the bowl around the bowl screw? I've never seen one on any other carb and there wasn't one on these when I pulled them apart, but it does say that there should be in the manual.... If I'm reading right.

2. my logic says that IF fuel is leaking by the metal washer on the inside of the bowl that the carb would suck that fuel up instead of going through the main jet and that may cause the high idle. Does that theory hold water???

3. I have not had it back on the water since the fuel pump rebuild to see if the bogging issue is fixed, but would a small tear in the check valve gasket cause that or am I barking up the wrong tree?

couple pics to try to help....

 

[racerone]

What does it mean----" motor is in really good shape and does run pretty well "----Nice coat of paint and decals ?-----Do you have compression numbers ?----Did you install a new impeller ?

 

[MPrimeaux]

No, not necessarily the paint or decals. Exterior of motor, although not bad, does show signs of age. Decals are faded, there are a few scratches, etc. BUT... Compression was 1 = 100, 2= 110, 3 = 107, 4 = 110. Wiring is still in good shape, haven't found any insulation that is crumbling at all, choke solenoid still works great, plug wires are in good condition, even the controls are smooth as silk and shifts into gear with ease and the choke button isn't dry rotten or missing. No corrosion, etc. found. It cranks right up and idles great and it RAN great the first few times I took it out and would still run great if you could get past the "bogging" issue. The power tilt and trim still works although a new motor is coming because it's got a little motor noise that I don't like and drags sometime. I haven't replaced the impeller yet, but I will. It does have a good stream right now, but I will change just to have a starting point. It does look like the switch box and rectifier have been replaced recently. All in all, it's in "pretty good" physical and running shape and I went fishing the morning after I picked it up and several times in the following couple of weeks, which is a lot more than I can say about the '63 that I bought that had magneto bearings, points, and condensers that were covered in rust and corrosion and next to NO insulation of any of the wiring. This fuel/carb issue came about after a few trips to the lake so I went through the fuel pumps and removed the bowls and main nozzles to inspect. That's when I found the bowl screw gasket was in bad shape and judging by the fuel residue on the outside of the carburetors when I removed them, decided to change the bowl screw gasket. My local guy said that gasket was ONLY available in a Quicksilver carburetor gasket kit. I bought 2 of them. When I got them home, I read the service manual and noticed it does indeed call for 2 fiber gaskets for the bowl. I also noticed that the kit came with 2 different kinds of fuel pump diaphragms also. One regular black, thin rubber and one a little thicker and kind of a copper color. I have yet to find out the difference in the two and opted to go with the thin black one..... the engine fired right up after replacing and idled great, but idles noticeably higher than before even after adjusting the idle mixture screws and noticing that the idle mixture screws didn't seem to effect the idle as much as they did before I replaced the before mentioned parts. Although I didn't have a lot of time to troubleshoot before I had to go to work for a month, I have thought about what might be causing it and came up with the bowl gasket question. Really, the only thing that I can think of that I did different is added a metal washer between the tube in the carb that has the main jet fitted in it and the inside of the bowl. I was just wondering if someone with more experience than me could tell me if they remember having 2 gaskets for the bowl screw and if my logic seemed legit..... or am I just way off?

 

[MPrimeaux]

some pics of what I consider pretty good condition for a 41 year old motor.

 

[The Force power]

two washers on the bottom bolt and one gasket on each side of the plate at the fuel-pump

Nice motor by the way:joyous:

 

[The Force power]

I also noticed that the kit came with 2 different kinds of fuel pump diaphragms also. One regular black, thin rubber and one a little thicker and kind of a copper color. I have yet to find out the difference in the two and opted to go with the thin black one.

IF....I remember correctly the copper-color one is to be used on the outer part

 

[The Force power]

Make sure the gasket between the carb. and the block is in good condition, verify the hole that lets the pulsation air through into the carb.

 

[racerone]

Use both diaphragms.-----The rubber one on the holes for a better seal.----The fiber backs up the rubber.

 

[MPrimeaux]

Use both diaphragms.-----The rubber one on the holes for a better seal.----The fiber backs up the rubber.

ok racer. I will pull the pumps and add the copper colored fiber “diaphragm “?? between the rubber diaphragm closest to the carb and the gasket. Thanks!

 

[MPrimeaux]

two washers on the bottom bolt and one gasket on each side of the plate at the fuel-pump

I’m not sure I follow Force. Are you talking about on the carburator bowl screw? That’s where I’m talking about.

 

[MPrimeaux]

Make sure the gasket between the carb. and the block is in good condition, verify the hole that lets the pulsation air through into the carb.

I did replace these with new ones and made sure the holes were clear. I didn’t necessarily turn the engine over and feel the pulses, but she fired right up and idled really smooth...just noticeably faster.

 

[MPrimeaux]

I also noticed that the kit came with 2 different kinds of fuel pump diaphragms also. One regular black, thin rubber and one a little thicker and kind of a copper color. I have yet to find out the difference in the two and opted to go with the thin black one.

IF....I remember correctly the copper-color one is to be used on the outer part

I think I see what you’re saying. Like racer mentioned... the thicker copper colored one backs up the thin rubber. They are shaped identical.

 

[The Force power]

[QUOTE

I’m not sure I follow Force. Are you talking about on the carburator bowl screw? That’s where I’m talking about.

[/QUOTE]

Yes, sorry I was being confusing. One washer on each side of bowl

 

[Faztbullet]

I did order a service manual for this motor and it states that it needs 2 fiber gaskets for the bowl screw, one inside the bowl and one outside on the screw. I see in the exploded view of carb on multiple parts sites that it does indeed​

Yes it needs both... The one inside seals the emulsion tube to bowl cover. If missing the motor will not pull fuel thru main jet and possible run rich at WOT

 

[MPrimeaux]

Yes it needs both... The one inside seals the emulsion tube to bowl cover. If missing the motor will not pull fuel thru main jet and possible run rich at WOT

That's what I was thinking..... and thanks for the term!! emulsion tube.... I was struggling with that one! Thanks. Funny that the Quicksilver gasket kit doesn't come with 2 gaskets and you evidently can't order just a bowl screw gasket. That's frustrating.