1972 Mercury 800 80hp Timing/Fuel Issues

[Super Dragpack]

Hi all,

I have just completed a rebuild of a 1972 Mercury Thunderbolt Blueband 800 80hp 4cyl outboard.

I have one remaining issue I wanted to run past you guys.

I can only get the motor started if I run over 30 degrees of timing (BTDC). It will then idle at 4500rpm with the carbs CLOSED. i.e. extremely high idle. As I try and run the timing back I can get it down to 10 degrees BTDC and 2000rpm but it will then become rough and cut out if I try any less. It is then a pig to start again even at 30 degrees. If I leave it at 30 degrees it will start first flick everytime.

I have tried lots of timing adjustments (ranging from the factory setting to the above extremes) and this is the only way I can get the motor to run at all.

I have a very strong even spark to all cylinders and have manually checked #1 TDC and marked it appropriately so I know the timing mark is right. I have tested this with two timing lights (the later being a Crypton tuning machine) and no difference.

The only thing left is the fuel mixture or other fuel issue. I assume a weak (or is it rich) mixture would give these symptoms. Any ideas?

Thanks in advance for any help.

Mick

 

[coolguy147]

Re: 1972 Mercury 800 80hp Timing/Fuel Issues

air leak?

im thinking that some seal has gone bad? never heard this happening before

 

[Super Dragpack]

Re: 1972 Mercury 800 80hp Timing/Fuel Issues

air leak?

im thinking that some seal has gone bad? never heard this happening before

I suppose its possible. I have just rebuilt every thing (except the carbs) so I assume I sealed everything up right, but I'll check again.

I have a spare set of carbs, so I will rebuild these and switch them over.

How many of you guys think this has to be a fuel/mixture related issue? Basically, I am thinking that the mixture is so far out that it requires this extreme timing to run.

 

[Gomer50]

Re: 1972 Mercury 800 80hp Timing/Fuel Issues

I have a 72 merc. 80 hp and Nr.1 are you sure about the 30 degrees BTDC and 4500 RPM.It shouldnt even be close to that,definatly not good for the motor.As stated on previous post check for air leaks.WOW thats a new one on me as well.I am also leaning towards a major fuel issue,maybe your reeds are stuck open but then the possibility would be a backfire as well.I still have a hard time picturing 30 Degree's BTDC.Check your pointer on your distributor gear so that it is pointing directly at your crankshaft when nr.1 piston is at TDC.Keep us posted this is a good one.

 

[Super Dragpack]

Re: 1972 Mercury 800 80hp Timing/Fuel Issues

I have a 72 merc. 80 hp and Nr.1 are you sure about the 30 degrees BTDC and 4500 RPM.It shouldnt even be close to that,definatly not good for the motor.As stated on previous post check for air leaks.WOW thats a new one on me as well.I am also leaning towards a major fuel issue,maybe your reeds are stuck open but then the possibility would be a backfire as well.I still have a hard time picturing 30 Degree's BTDC.Check your pointer on your distributor gear so that it is pointing directly at your crankshaft when nr.1 piston is at TDC.Keep us posted this is a good one.

If I time it correctly, it just won't run (with an occasional splutter/fire on one).

As I bring the timing back round to where it should be, the revs do drop, but only down to 2000rpm (10-15degrees) before it dies.

I'll rebuild my spare carbs today and swap them over I think.

I don't recall this engine having reeds. Where should I be looking for them?

 

[Gomer50]

Re: 1972 Mercury 800 80hp Timing/Fuel Issues

The reeds are inside the engine and thet look like these

 

[Super Dragpack]

Re: 1972 Mercury 800 80hp Timing/Fuel Issues

The reeds are inside the engine and thet look like these

Those are around the main bearings arn't they? What are they supposed to do then please?

 

[Gomer50]

Re: 1972 Mercury 800 80hp Timing/Fuel Issues

Reed valves are like a one way check valve. They allow the fuel/air mixture to be drawn into the crank case when the piston moves towards top dead center. The reed valves then close as the piston moves back towards bottom dead center, which forces the air fuel mixture in the crank case, through transfer ports, and into the cylinder.

If you have a bad reed, you will usually see fuel spitting back at you through the carburetor. There is no real way to test them, you just have to look.They are located on yor crankshaft right behind your two carbs.

 

[Fuzzytbay]

Re: 1972 Mercury 800 80hp Timing/Fuel Issues

Did you remove the flywheel? If so is it back on correctly?

 

[Super Dragpack]

Re: 1972 Mercury 800 80hp Timing/Fuel Issues

Did you remove the flywheel? If so is it back on correctly?

Yes I did, but I have located and marked TDC with a bore DTI guage so I know the mark is a true TDC mark.

 

[Fuzzytbay]

Re: 1972 Mercury 800 80hp Timing/Fuel Issues

I haven't worked on that year of motor, but on some of the older ones, you can put the flywheel back on incorrectly. then the magnets are not in the correct location, which could account for the "wierd" timing.

 

[Super Dragpack]

Re: 1972 Mercury 800 80hp Timing/Fuel Issues

I haven't worked on that year of motor, but on some of the older ones, you can put the flywheel back on incorrectly. then the magnets are not in the correct location, which could account for the "wierd" timing.

On this model the flywheel has just the charging stator under it. The ignition is belt run to a distributor with a standard pickup and rotor with cap etc. Hard to get it wrong it really.

The stator and ignition are two seperate units and not attached in any way.