Logo
FULL SITE MENU

1978 Mercury 500 thunderbolt - changed trigger, cleaned carbs - now revving up and down at idle

M

MaxMalmgren

New member
Hi,

It seems my 1978 500 was ready to fall apart just after I bought it - got a good season out of it, then at the end of the next season it started to act up. Next year it wouldn't start.

I identified the trigger as being faulty, swapped it out for a new one and I got spark on all cylinders now. I tried to start it, but it wouldn't, so I checked the fuel - lines were stiff and broken, so I bought new ones.

At the same time I took home the two carbs and cleaned them - lots of gunk in them, no wonder it didn't start. Took lots of pictures and notes, so I should have assembled it correctly. Didn't seem like too many mistakes were possible, except the order of the membranes / gaskets in the fuel pump. I think I got it right, but how do I tell?

When I put the carbs back in it fired right up after a second of using the choke, but it idles horribly. It revs up, to what I would say is more than idle speed, then almost dies, then revs upp again, then repeats. Sometimes it does die completely, but fires up with a bit of choke.

What could cause this?

Haven't been able to test it much since the impeller is shot, I think I need to buy a new housing and not just the impeller since the gaskets are falling apart.

So, two questions:

1. How do I tell how to put the fuel pump back together?
2. What could cause revving up, then almost shutting down completely, repeatedly while idling?

Thanks for any help :)
 
fastjeff said:
A vacuum leak at the pump would, yes. Did you use a manual when assembling the pump?

Jeff
Click to expand...

Thanks, that gives me hope. I've bought a manual that seems to be a scan of many different motors, so its not always easy to find the right section. But in this case, I thought I'd just assemble it the way it was, but I mustve gotten it wrong. I'll try to find it in the manual :).

Now ordering two carb kits and a water pump kit.
 
One question about the water pump though - the lower pump housing part seems to be fused to the lower unit, or is it possibly not part of the water pump?

After removing the upper part of the pump housing, impeller, rubber gasket and metal housing, there is a 3/4 inch or so of of a plastic part, directly above the gears. It seems to have fused onto the lower unit with some sealant of some sort.

It's not included in the Sierra kit I ordered (https://www.amazon.com/gp/product/B00027H0WM/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1)

Is this the gear seal which I shouldn't touch, or what am I missing?
 
Max- Take a look at Sierra part #3320. It has all the parts and should include what I think you are talking about. The lower half of the pump doesn't come out of the pump housing easily so at times I would think someone would think it may be fused. (Consider that another human owned that motor before you got it).
 
Interesting - that part is similar but not exact to what I'm seeing on the lower unit. Unfortunately the motor is a few hours away, but I'll update this post with a picture when I can.

Do you think I could leave it in there and just change the other parts? It seemed dirty but intact.
 
I will ask a question before answering yours. Was your lower unit taking on water? If so, change that also. A waterpump for another motor is somewhat like comparing apples to oranges. Without clear pictures of what you have at present, installed, we can't really determine what your dillema is. The only real difference between Sierra Parts and OEM is usually the color There may be some other slight differences in the features but essentially the same. I own a 1977 Merc 500 and it takes the same parts as yours if you have a 1978.
 
I don't think it taking in water, last time I changed gear oil it came out black without any hint of water damage.

Seems I bought the wrong sierra kit, that piece in the upper right of the kit you linked to is definitely the part that is mounted in the lower unit now. I did read the Q&A and someone asked:

[FONT=&quot]Will this fit my 1979 Mercury outboard 50 horsepower 4 cylinder 2 stroke?
[/FONT]
The answer was yes, so I thought it was the right one.

I'm a bit confused though, it seems the kit you linked is the same parts plus the two additional ones. Is is the same kit, just that the one I ordered was incomplete?

If the answer is yes, I think I'll keep the original large rubber gasket and large plastic piece, as I haven't had problems with water leaks. Would you agree that's okay?

Thanks for the help.
 
You should be good. The bottom of the pump housing has a seal installed in it. That seal wears out over time . Keep all your spares so You aren't grounded waiting for shipping to perform. Marine Engine.com has the easiest parts catalog to surf, in my opinion. Spend some time looking up parts and their availability and you may end up agreeing. Boat Motors are tough for many of us since we were raised on Ford and Chevy.
 
After changing fuel pump gaskets and membranes, the motor idles pretty ok, but theres still an issue with a grinding noise. I guess i screwed up the flywheel installation somehow? Havent run the motor for more than 5 minutes total, so hopefully its not damaged.

The noise isnt immediate, it seems to build up over 20 seconds or so. Water pump is still not working, but the issue doesnt seem related to overheating. The motor is just a bit warm to the touch in the video.

Please check the video below.

https://youtu.be/6YytWj0gK5U
 
Facts for you.-----You do not have nearly enuff water in your test bucket.--------Water should be 8" above the pump.-----You may have ruined the new impeller.-----Others will confirm this mistake !!
 
IF you had a hopes in there keeping the water level that high (it doesn't show in the video) you're okay.

Need a longer video.

Jeff
 
Hey giys, thanks for the help.

In the meantime I disassembled the waterpump again - getting to be real quick with getting the lower unit off. Impeller was still good, propped the test bucket up with some bricks, blew compressed air through pee hole and now the motor temp is good. So it was user error, as usual. Bucket is a little too small, didnt get water to the impeller without the bricks beneath. Well spotted, racerone :).

Also, grinding noise was from the flywheel area, so after making sure the cooling was good I pulled it off, and sure enough, it was chafing the stator. Just barely, it didnt even hinder movement at all when rotating flywheel by hand.

I filed down a very very small burr on the metal surface below the stator and made sure to wiggle the stator into the most seated position possible before tightening the bolts (which were tightened properly before as well).

No more grinding noise, it idles very well, and cooling is good. Going to try it in the water next, hoping that it does well under load.

Thanks for the help!
 
I have a '92 Merc 40 4 cylinder 2 stroke; yours idles smoother than mine (when mine was running). I never run with muffs; only with a huuuuge Rubbermaid plastic waste barrel with water well above the water intake; that way you can put it in gear with the prop spinning introducing load and adjust the carb idle screws so that it will idle under load.

It is also good to see exhaust and water escaping from the relief exhaust ports just below the power head; if these are clogged with carbon, there will be no idle.
 
Last edited:
Back
Top