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Thunderbolt 500 1970 crank but doesn’t start

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bastienlaf

Member
Hi Everyone I am new to this forum and I’m looking for help to repair my mercury outboard engine it is a thunderbolt 500 1970. I picked it up in a barn it as not run for about 10 years that my uncle said . First thing I did was I pulled the plug and shoot some lube in all four cylinders and let it sit for a few days before I crank it.Then I pulled in clean both carburetors filled a tank of fresh premium gas And tried to start it. the engine did fire for a few seconds but dies immediately after.

I installed a new fuel pump kit
plugs are brand new
sparks looks good on all plugs
carburators and strainers are clean

i tried to spray some starting fluid in the carb but the engine only fire for one second and does not rune more than that. Tried to spray some fuel in the cylinders but same result.

i’m out of idea now ! I need help i’m pretty new to this kind of mechanic but i’m used to all other small engines

can it be the timing?
can the lower unit affects the engine starting behavior?
can it be realy fouled by all the starting fluid

Sorry for my poor English I’m a french Canadian! doing my best ;)
thanks everyone
 
First thing you should do is check the compression. Then get back to me with your results.
Also... don't ever use quick start in a 2 stroke.your going to damage the piston rings.
As for the lower unit. Drain the gear lube. Notice the colour of the gear lube coming out and let me know.
 
mechanicman said:
First thing you should do is check the compression. Then get back to me with your results.
Also... don't ever use quick start in a 2 stroke.your going to damage the piston rings.
As for the lower unit. Drain the gear lube. Notice the colour of the gear lube coming out and let me know.
Click to expand...

Forgot to write it but compression is at 125 on all 4 (full throttle with the other 3 sparkplugs in place)
i will drain the gearbox oil and tell you what it looks like when its done
 
I suspect timing. You can check it with a timing light while spinning the motor over, plugs out. Look for WOT timing only.

Jeff
 
fastjeff said:
I suspect timing. You can check it with a timing light while spinning the motor over, plugs out. Look for WOT timing only.

Jeff
Click to expand...

Thanks for the info! It just makes me think, the timing belt looks in pretty bad shape.
I will look for the timing mark on the flywheel and distributor just in case the belt would have slip or something!
I will look at this and change the timing belt as soon as I can. Is there a tension adjustment on this belt?

Thanks guys for the help!
 
I had a look yesterday, the timing belt seems so worn i think it skipped. the timing was off a lot. I tried to put it back in place at the good timing but when I screw the top plate of the distributor pulley it skipped again. I will order a new one. Does anyone have the timing belt number ? The marine shop gave me #20140, is that good?

One more question for the timing,
I was able to place the mark on the ring aligned with the arrow on the pulley, but i'm not sure i'm at TDC on piston 1. how can I confirm that I am?



NB: Oil in the lower unit was a bit brown but nothing crazy.

Thanks!!
 
The distributors on mercury outboards all use the same belt.--------Only one belt from about 1954 to the last ones around 1978.-------So yes that is the correct belt.
 
racerone said:
The distributors on mercury outboards all use the same belt.--------Only one belt from about 1954 to the last ones around 1978.-------So yes that is the correct belt.
Click to expand...

Thanks for the info!

If anyone can help me to figure out the TDC that would be awesome :D

I'm confused all the manuals say that when the line on the ring is aligned with the arrow, engine is a TDC. Does TDC means that the piston 1 is at the top of the cylinder (highest point near the spark plug) if that's so then mine is not.


What does the line on the cover TDC piston 1 means? (where the switch box is bolted to)

I'm very confused and I can't seem to find good clear information in the manual.

Thanks everyone for the precious help!
Timing-1.jpg
View attachment Timing.pdf
 
Pull plug no.1.Turn flywheel until piston comes to top of cyl.,I use a pencil to feel for it.That's TDC.
When you fit the belt,the arrow on the pulley cover should point at the white line on the flywheel.
If by some chance the white line is not in line with the center of the distributor at TDC,someone has removed the flywheel by the eight bolts and not put it back in the right place.
If that's the case,undo the bolts and rotate the flywheel until the white line lines up with the center of the dissy,keeping no.1 piston at TDC.
 
haventaclue said:
Pull plug no.1.Turn flywheel until piston comes to top of cyl.,I use a pencil to feel for it.That's TDC.
When you fit the belt,the arrow on the pulley cover should point at the white line on the flywheel.
If by some chance the white line is not in line with the center of the distributor at TDC,someone has removed the flywheel by the eight bolts and not put it back in the right place.
If that's the case,undo the bolts and rotate the flywheel until the white line lines up with the center of the dissy,keeping no.1 piston at TDC.
Click to expand...

Can anybody look at this photo and confirm that these are the 2 timing marks i'm supose too look for?


attachment.php


Also what does the TDC line on the cover means? that's what confuse me the most..


mercury-500-50hp-cover-_57.jpg


Thanks everyone for your time
 

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Yes those are your timing marks.
When the support bracket is bolted in place and the mark on the flywheel is in line with that line,no 1cyl is at TDC,supposed to be..
 
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haventaclue said:
Yes those are your timing marks.
When the support bracket is bolted in place and the mark on the flywheel is in line with that line,no 1cyl is at TDC.
Click to expand...

Sorry not sure I get it yet :(

If my understanding is good there is 2 lines:

one on the rim of the fly wheel

one on the case (the white decal with CYL #1 T.D.C on it)

and a arrow on the distributor gear

From what i understand the #1 piston must be at TDC when the arrow is aligned with the mark on the RIM of the flywheel (if not the flywheel must be reinstalled)

If that is true, then what is the mark on the decal for ?

I feel so dumb :confused: I just want to make sure I do the things properly.

Thanks!
 
It indicates TDC and the degrees Before Top Dead Center (BTDC) when checking the timing with a timing light when setting spark advance.It shows where TDC is in relation to where the cyl is firing. ie: When engine is idling the cyl should fire 7-9 degrees BTDC, or in this case where the pick up line is.
So paint a small white blob on the timing mark on the flywheel so as you can see it in the timing light strobe as it passes the pick up line while idling at.
Now have fun with this,'cause the outboard needs to be in water and in fwd. gear.I hope you have a very big test tank or able to put it in the water and tie the boat up tight.
 
Thanks a lot! very useful info and videos. I received my timing belt this morning I will install it this week and do the proper timing. I'll keep you in touch!

I think I know why I was so confused, on the first video you can see that there's 2 timing mark on the flywheel (one pointing at the distributor and one at the case). On my flywheel I have only one mark. I will try to figure out a way to know it's at the right place with only one timing mark.

Thanks again!
 
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Your distributor and flywheel are slightly different,the one in the video is probably newer.
Timing is fixed on these motors and dependent on the correct fitment of the belt.
Take an imaginary line through the center of the flywheel,the timing mark,the arrow and the center of the distributor.Now it might not line up perfectly,in that case fit the belt as near as possible clockwise from exact.
This is where the Seloc manuals let you down as they cover numerous outboards and the factory specific rule. It might be worth your while to check the woodruff key is still in place.
 
haventaclue said:
Your distributor and flywheel are slightly different,the one in the video is probably newer.
Timing is fixed on these motors and dependent on the correct fitment of the belt.
Take an imaginary line through the center of the flywheel,the timing mark,the arrow and the center of the distributor.Now it might not line up perfectly,in that case fit the belt as near as possible clockwise from exact.
This is where the Seloc manuals let you down as they cover numerous outboards and the factory specific rule. It might be worth your while to check the woodruff key is still in place.
Click to expand...

I will order the puller tool for the flywheel and have a look at the woodruff key while changing the belt. The tech at my boat repair shop told me not to bother buying the tool and just knock on the crankshaft end (with the nut on) to remove the flywheel. Not to sure about this technique, I'm not a big fan of hammering hardened steel ahah!
 
Knock on the nut is the way I do it on my auld '65 and '74 Seagulls,it's the only way to get the flywheel off them.Wouldn't be a big fan of doing it to my auld '69 Merc800 though,anyway I wouldn't be able to lift the Merc with one hand:D.Although many people do hit the nut,I just know my kinda luck.:D
Anyway the tool is reasonable about $17-$20
 
Shipping to canada for the puller tool was too long so I made my own puller yesterday, I pulled the flywheel off installed the new timing belt aligned everything and bolted everything back up. The engine fire right away, idle was very good, I didn't let it run for too long. I wanted to do a better checkup before I test it thoroughly.

Dumb question now: I ran the engine with muffs, hooked to my garden hose. Where does the water get out of the engine? Is there a pee hole on this engine?

I have heard of water pump impeller replacement on old engine that haven't run for a long time, do you think I should change the impeller before trying it ?
 
There is no warning horn or shutdown system on your motor.-----In my opinion you should change the impeller.-----If it survives your first outing with old impeller you should buy a lottery ticket.
 
Do i need to change the impeller only? kit with gasket? complete kit with housing?

Do you think there's other things I need to change/look before the first run on water?
-Carbs have been cleaned
-Plugs are new
-New timing belt
-Compression checked
-Fresh mixed fuel
-Water pump rebuilt
-Lower unit oil changed



Thanks!
 
Definitely change the impeller,pity it got missed when rebuilding the water pump.If you have already rebuilt the water pump,there should be no need to do the gaskets and housing again.
There should be a pee hole on the right,standing at the prop.Poke a wee bit of wire around,might be a spider nest or put some compressed air through when you have the lower unit off when you're changing the impeller.Good practice is to change every two years,after ten,a must :D.
 
haventaclue said:
Definitely change the impeller,pity it got missed when rebuilding the water pump.If you have already rebuilt the water pump,there should be no need to do the gaskets and housing again.
There should be a pee hole on the right,standing at the prop.Poke a wee bit of wire around,might be a spider nest or put some compressed air through when you have the lower unit off when you're changing the impeller.Good practice is to change every two years,after ten,a must :D.
Click to expand...

I installed a new impeller kit with new gasket, i also ran compressed air in the copper tube when the lower unit was off. I started it hooked with mufs. Ran great water was coming out of the pee hole after a few seconds! When it was at idle the water is not flowing that much from the pee hole is that normal?

Thanks!
 
When you installed the impeller,did you turn the flywheel c/w to bed the vanes in the correct direction? It could still be a minor blockage on the tell tail. Run the engine until it warms up,put your hand on the top of the block,if you can hold it there without having to scream to high heavens:D,go boating.
 
haventaclue said:
When you installed the impeller,did you turn the flywheel c/w to bed the vanes in the correct direction? It could still be a minor blockage on the tell tail. Run the engine until it warms up,put your hand on the top of the block,if you can hold it there without having to scream to high heavens:D,go boating.
Click to expand...

I did turn it to have the vanes in the correct direction, i'll give it a try and see if the engine stays at a reasonable temperature after warmup. :D
 
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