1974 Mercury 850 Timing Decal

I have scoured the forums and have found different answers regarding timing marks on an 850. Attached is a picture of my timing decal. I replaced the trigger and the flywheel was originally set at the three vertical marks being TDC. However, there is a larger mark to the left of it that says TDC. Which marks should actually be set at TDC?

Thanks,

Dan

Re: 1974 Mercury 850 Timing Decal

The distributor cast arrow should point to the three punch marks when the timing mark indicates TDC. A '74 may have timing mark on cowling support. it could also have a separate timing pointer that is adjustable. If you have this, you can adjust it for best accuracy, using the .464" measurement.

Re: 1974 Mercury 850 Timing Decal

Chris1956 said:

The distributor cast arrow should point to the three punch marks when the timing mark indicates TDC. A '74 may have timing mark on cowling support. it could also have a separate timing pointer that is adjustable. If you have this, you can adjust it for best accuracy, using the .464" measurement.

Click to expand...

Sure glad this subject came up!!!! I broke the belt on my 1975 850, S/N 429xxxx. Don't ask how, S*&% happens. Attached are two photos of my 850. There are three (3) dots to the extreme right side of the decal. Are these the marks to align with the arrow on the pulley and the center of the crankshaft?

The second picture shows the timing pointer, which doesn't line up to TDC when the three dots are aligned with the pulley/crankshaft center.

Thanks for any help!

Tom

Re: 1974 Mercury 850 Timing Decal

Chris1956 said:

The distributor cast arrow should point to the three punch marks when the timing mark indicates TDC. A '74 may have timing mark on cowling support. it could also have a separate timing pointer that is adjustable. If you have this, you can adjust it for best accuracy, using the .464" measurement.

Click to expand...

Thanks Chris. I believe my timing might be set up wrong. To further clarify: When my timing pointer is pointed at the three vertical dots, cylinder #1 is at TDC. However, when my timing pointer is pointed at the large dot marked TDC, the distributor pulley arrow points at the three dots like you described.

I ran the engine with the timing set-up like this and my timing light read directly between the two black lines which is 3-4 degrees BTDC which I believe is what the engine should idle at and read. But, now I am questioning whether everything is slightly off based on what TDC actually is vs what the timing pointer reads.

Thanks so much for your help.

Dan

Re: 1974 Mercury 850 Timing Decal

Tom. You need to see if the timing pointer is set properly. Set the motor to TDC using a screwdriver. Measure from spark plug hole to top of piston. Now turn flywheel CCW, past .464 mark. Add .464 to your measurement and insert ruler into spark plug hole. Turn flywheel CW until you hit ruler. Now piston should be .464 from the top on the upstroke, and you should set timing pointer to .464 mark on decal.

Dan. Carbs should just be starting to open at 4-6* BTDC (idle pickup timing). Actual Idle timing can be less than 4-6*, because you will likely set idle speed when carbs are closed.

Re: 1974 Mercury 850 Timing Decal

Chris, Thanks so much for your help. I used my dial indicator to establish TDC. Measured from spark plug seat to piston with dial calipers, added the .464, rotated crankshaft to deminson established by adding the .464 and the dot was right on. The belt is back on now and it is time to move onto the carbs. Someone has been in there already, the hinge linkage for the needle/seat/float is a mess. Looks like they tried to open the hinge hole instead of removing the pins to replace the needle and seat. I do have two new carb kits on the work bench.

Have another question about plugs. I'll start a new thread for that.