1974 85hp 850 thunderbolt idles too fast, idle stop all the way out

[b0horman]

i have a 74 mercury thunderbolt 85hp 850 that i am having some issues with. i just recently rebuilt the carbs and was cleaning the distributor cap and rotor, the main pickup on the distributor broke on me. boy that part is expensive $450 new. so i found a 1972 80hp donor bought the whole motor for cheap (power head was ciezed and lower unit done) i took the distributor out and replace for mine. i am trying to sync the timing right now. she starts up great and is smooth running. the problem that i am running into is getting the timing correct for lower idle. at idle the timing is around 10 deg above TDC. According to the manual i need to get the timing to 2-4 deg BTDC then adjust the color on the distributor to "touch" the throttle valve. Then run the engine up to 21 deg BTDC and adjust the max spark advance, and the secondary adjustor should be "touching" the enrichment valve. Finally WOT and set the throttle stop to have carb valves wide open. Before i can get to this point i cannot get the idle below 1400 rpm. the stop adjustor is not even touching the distributor. i can turn the bottom of the cap to get it lower but as soon as i let go it springs back.

i made sure the distributor and flywheel marks were lined up with the timing belt. Saw in a previous post someone changed the switch box and it fixed his problem. i am not sure how that could change the idle speed but i do have an extra box from the 80hp engine to try.

What is the proper timing at idle? before i rebuilt the carb and had the distributor problem i could use the idle adjustor to raise / low my idle, now i cannot?

first thing is first i need to get the idle down to start syncing. Any other areas i should be looking at?

 

[Chris1956]

Re: 1974 85hp 850 thunderbolt idles too fast, idle stop all the way out

Does the throttle cable push the throttle closed? If not adjust it until it does. That motor has timing marks on upper cowl support. Make sure upper cowl support is in correct place.

Sometimes cast arrow on distributor is 1/2 tooth off. If you have this issue, retard distributor 1/2 tooth. That may help

Idle timing is as you suggested...4-6* BTDC.

 

[Faztbullet]

Re: 1974 85hp 850 thunderbolt idles too fast, idle stop all the way out

Sometimes cast arrow on distributor is 1/2 tooth off. If you have this issue, retard distributor 1/2 tooth. That may help

Most likely the problem....

 

[b0horman]

Re: 1974 85hp 850 thunderbolt idles too fast, idle stop all the way out

i am setting the timing and idle with the throttle disconnected first. To retard 1/2 tooth which way should i move the distributor gear? to the right or left when facing engine from the stern.

I must be way off somehow because my idle timing is around 7 deg above TDC.

the lower part of the distributor only gets to a certain point and will no longer retard the timing which is cause the idle to be to high. should the distributor be able to turn past the idle stop screw?

 

[Chris1956]

Re: 1974 85hp 850 thunderbolt idles too fast, idle stop all the way out

A high idle at 7* ATDC is unusual. Are you sure it is not 7* BTDC?

 

[b0horman]

Re: 1974 85hp 850 thunderbolt idles too fast, idle stop all the way out

it is to the right of the timing marks. timing marks being at 2-4 deg Below top dead center.

 

[Chris1956]

Re: 1974 85hp 850 thunderbolt idles too fast, idle stop all the way out

OK, you are reading it correct. It is time ot make sure the outer flywheel is installed properly. Set the top cylinder to TDC with a screwdriver. Now see if the timing mark on the cowl support reads just about zero. If so, that timing/idle combination is very confusing....

 

[merc850]

Re: 1974 85hp 850 thunderbolt idles too fast, idle stop all the way out

I had the same problem with an aftermarket trigger I set it to factory specs and it idled high so I moved the belt one tooth CCW on the dis. pulley.That solved the problem of hi idle and the max spark fell into range.

 

[b0horman]

Re: 1974 85hp 850 thunderbolt idles too fast, idle stop all the way out

so i moved my timing gear 1 tooth to the right (clock wise) and i can now adjust idle but my timing at idle is still around 8 deg above TDC (to the right of TDC mark on flywheel)