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70s mercury 40 4hp ignition timing (thunderbolt)

FstaRockr Burns

FstaRockr Burns

Regular Contributor
Hi there - bought a gr8 freshwater merc 40 single last week. Rewired the head/coil wires (they were blistered). set points gap at 0.020 (it was closed!)..

Now im curious what the general timing map for small 2 strokes is?

I read somewhere usually all advance shd be in by about 1/3 throttle - and what about base? I have marked TDC using a piston stop tool.

Otherwise the motor runs on the first pull pretty much - just want her running as best as she can..

(fyi i usually dial the advance back a bit from 40 yr old documentation - it gives a little added safety :)

Im also about 1 turn out on the carb idle mixture - should this be set to gain max rpm? Cos when I turn it in the revs go up but im not sure if thats because its lean?
 
The points should be around .020 inches. Get that right and it takes care of the timing for you.

Look for best idle, then add another 1/4 turn OUT (CCW).

Jeff
 
Hi jeff, I set points exactly at 0.020". The cover says thunderbolt, however finding points seems contrary? When I opened it at first, the points didnt even open up! I rotated till the lobe was at max lift and set it at 0.020 there, lucky they were in good shape!

Not to question your info - but in the manual it states to turn the idle mixture CCW for a lean condition (CW for 69 models which mine is not - mine is the 40 model single)

idle-speed-merc40-4hp.jpg

Is it possible I have a different carb?

Heres a pic of the motor:

merc-40-1.jpg
 
What is the serial number? Does it have external coils? That is the tillotson carb the proper way to adjust the slow needle is to slowly turn it out until the engine loads up not that position then slowly in until it cuts out then split the difference. Always ru it a tad rich it helps on cold starts and the hole shot. It also protects the engine from running too lean if you do a lot of slow speed operation. When adjusting the needle tut it 1/8 turn at a time and wait about ten seconds for the crankcase to stabilize.
 
Phase Maker ignition? Aaargh! Actually, they seem to work fairly well on single cylinder Mercs without a kill switch.

The carb leans out as you screw the idle mixture IN (CW). Since there's no accelerator pump on carbs like this, you need to 'help her5 out' by making the idle a tad rich.

Jeff
 
ok gr8 thanks. So it the phasemaker NOT thunderbolt? confused! also - surely adjusting the front arm that moves the advance affects the timing / something to check ESP since i adjusted the points gap?

I read somewhere some ignition types fire when points close? Ill verify with a continuity test to gnd and coil - wire while slowly turning the flywheel..
 
Thunderbolt is a label that Mercury put on many different ignition styles. Your ignition fires when the points are closed I believe.
 
Heres a short vid I made this evening showing some timing on the motor as is:



Here is one showing the idle mixture adjustment on the carb:



Comments welcome!
 
Last edited:
Timing is not adjustable on that ignition just get the points as close as possible. Adjust the throttle pickup to around 5 degrees after top dead center. Set your flywheel at 5 degrees ATDC And the turn the handle to where the points jus close at that point you want the throttle cam to start opening the throttle butterfly in the carb. The only other adjustment is max spark advance. Put the piston at TDC and you measure the stroke I just use a caliper put the piston .198 inches BTDC and make sure it don’t move advance the throttle until the points just close at that point set the stop screw to just touching the tab.
 
Just a side note those impellers are real sensitive to turning the flywheel counterclockwise once they flip over backwards the only way to get it right is to pull it apart and redo it. I would recommend just replacing it now and once the impeller is in never turn the flywheel backwards.
 
kimcrwbr1 said:
Timing is not adjustable on that ignition just get the points as close as possible. Adjust the throttle pickup to around 5 degrees after top dead center. Set your flywheel at 5 degrees ATDC And the turn the handle to where the points jus close at that point you want the throttle cam to start opening the throttle butterfly in the carb. The only other adjustment is max spark advance. Put the piston at TDC and you measure the stroke I just use a caliper put the piston .198 inches BTDC and make sure it don’t move advance the throttle until the points just close at that point set the stop screw to just touching the tab.
Click to expand...


Great info thanks - will have a look.. I think the throttle pickup is currently around 5 btdc.. Ill play around and see where I get.
 
One thing to check after you get the throttle pickup and max spark advance correct at WOT measure the gap between the throttle cam and the pickup lever you should have around .03 mm gap when you open the carb all the way with your finger. If it is binding just back off so you don wear out the throttle shaft in the carb.t
 
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