1979 40 hp high idle timing

Hi, I'am having a problem diagnosing a 1979 40 hp mercury I bought for my whaler to repalce a blown 35 Evinrude. It has spark on both cylinders verified with a timing light and 180 volts to the coils along with 150 psi compression on both cylinders.

However, it idles very high, estimated well over 1000 rpm's (no tach) with carb idle adjustment screw turned 1 3/4 from lightly seated. No adjustment can get it to idle down. I also discovered a misfire from mid range to high range.

Further diagnostics found at idle the timeing is just over 29 BTDC, increasing idle brings it way beyond this point and this is when the misfire begins. I checked the stator red wire to ground 54.0 ohms and red to blue 58 k ohms. The triger magnets and centering hub magnet appear good. I removed the two yellow wires from the rectifieer and conditon remaind the same. I belive I need a new stator but I do not understand how this will cause the high idle and advanced timing. I can get the timing down if I retard the timing lever by hand at idle to around 0 TDC. Is it posible that I also need the switch box or am I mising something here?

Any insight would be greatly appreciated.

Rick M

Re: 40 hp high idle timing

something is not right, sounds like the timing base/trigger is stuck or the controls are jammed. on the left is the idle speed, it is turned out more than 1 3/4, that sounds more like the idle mixture adjustment.

On the right is the mar rpm and max timing, dont mess with em. There is a link and sync page in the book, you need to read and follow. There is also a carb pick-up cam on the carb that might be out but its part of the link and sync procedure Base timing is 3-5 degrees at idle and max 29

ps, thats my 83 40hp, solid little engine....jb

Re: 40 hp high idle timing

I should have indicated that I have both cables disconnected and the speed screw backed off, I also verified that the throttle plate is closed.

I have performed the sink and link check.....ok. The trigger is not stuck and advances properly.

I did find and repair some cracked wire insulation on the triger coils on the stator. I supect I may need a stator but at this point I am further suspecting a possible vacuum leak, I just don't see what else could coause such a high idle. However I have not found any visible indicators of vacuum leaks. Very frustrating. Is there any way I could test for a vacuum leak such as a vacuum pump or gauge? I do not want to just through parts at this engien without knowing what the actual problem is.

Any advise would be greatly appreciated at this point.

Rick M

Re: 40 hp high idle timing

you can use propane from a plumbers torch and wave it around the engine to test for leaks. I don't know of any stator problem that causes high rpm/timing. Did you check the flywheel key. Here for ign specks and a troubleshooting guide
http://www.maxrules.com/fixignitionspecs.html

Still think you have something bound up...jb

just re-read, why is the timing lever advanced, it should be at almost zero untill the lever is advanced

edit 2, saw one once with the trigger linkage on backwards causing the timing to be way off like yours

Re: 40 hp high idle timing

Thanks for the advise jb, I'll recheck all the linkage. I did check the flywheel key and it is ok. I even placed #1 piston at TDC and verified the flywheel to pointer was properly indexed. I have done an extensive google search for causes of unusualy high idle and came up with nothing substantial. This is kind of which came first the chicken or the egg, is it the advanced timing causing the high idle or the other way around? Does your stator wire harness exit directly at the right side of the engine? I was actually thinking of rotating the stator 1 hole positon to see what happens.

The more I dig in to this engine the more I can tell that some one has had there hands in this prior to my purchace, this would explain the low purchace price. What is amazing is the engine starts right up, does not misfire but idles so high (at least over 1000 rpm) which I have also visually verified the throttle plate is closed within the carb. At this point the timing is at 29 BTDC Then when you manually open the throttle (cable disconnected) It will increase in rpm but the engine begins to misfire and at this point the timing is way beyond 29 BTDC.

Clearly there is some type of timing issue that is causing it to be so far advanced. I'll take a look at the trigger linkage to se if it is on backwards but I have manually advanced and retard the timeing and fully retard is around 0 degrees (TDC).

By the way nice clean lookng 40 hp.

Anyway thanks Rick M

Re: 40 hp high idle timing

The sw. box bias could also be bad. Can only be tested with a another one. The idle would shure go high, yeah.
All black stators were ~34K ohms lo and 140 hi.

It might be possible that the box is the wrong one, too. Some of those later models (25HP) had a fixed trigger and module advance instead of a spark linkage. You don't need both.

Re: 40 hp high idle timing

I have rulled out a vacuum leak tonight. I was able to retard the timing to apoximalty 0 degrees and it brought the idle right down to normal, then I increased the throttle lever (cable disconnected) and teh timing advanced to 29 degrees BTDC this is when it misfires. Then return linkage to idle positon, idles very high still timing at 29 degrees BTDC. At this point I manually retard timing by pushing against spring/lever after holding it here for approximatly 5 seconds, idle drops and returns to 0 degrees. Clearly an igntion/timing related issue. I'll test the trigger tommorow but I'm now leaning towards at least the stator which I know is bad, and possibly a switch box also. I suspect the stator may have wiped out the switch box. MAy require replacment of all three. However clearly advanced timing clearly increases idle rpm.

Rick M

Re: 40 hp high idle timing

Time Bandit, I have the black stator and my stator test readings are out of spec, I had to make repairs to wire insulation on the stator leads, I think this is what causes the misfire and possibly fried the switch box. I'm just trying to wrap my mind around how it could cause advanced timing. As I understand it it is the advancment of the trigger in relation to the inner flywheel hub/magnet that advances and retards the timing.

I am fairly new to merc igntion sytems, could you please explain what you mean by switch box bias?

Thanks
Rick M

Re: 40 hp high idle timing

I'm not an electronics expert but can try to give you a rundown of how Merc's T3/T4 ignitions work.

The stator is a dual function unit. It has ignition windings (bobbins) and charge coils (to charge a battery). Most "ignition" windings on Merc's are further broken into high speed and low speed windings.

The low speed side produces (high) voltage at all rpms - so they always contribute to the ignition. The high speed windings produce very low (unusable) voltage at lower rpms but are there to augment the low speed side at higher rpms when the ignition requires "more power, more often".

While these windings are located on the same (plate) the components are somewhat independant.

All these stator windings produce alternating current (AC) which is changed to DC at a follow-on component for use to either power the ignition or charge the battery.

The trigger, depending on the exact design will either have one bobbin shared by 2 cylinders or a single bobbin per cylinder. Likewise, it produces AC.

If it's a single winding shared, the positive portion of the AC signal is "chopped off" by the trigger and that "pulse" is sent to the switchbox to tell it to fire cylinder 1, then on the next pass of the magnet (normally two magnets are located on the outer ring of the flywheel so the trigger bobbin "gets passed" twice per rpm or every 180 degrees of rotation), the negative portion is cut off and sent to fire cylinder 2. If it's a single cylinder only half the signal would be used for the single cylinder.

If the trigger uses one winding per cylinder, the windings are oriented around the crankshaft at a location equivalent to the degrees of separation between cylinders, and the magnet that powers them is normally located on the inner ring of the flywheel. So on a 3 cylinder it would be every 120 degrees.

The power generated by the stators low and high speed coils is sent to the switchbox.

The switchbox contains no "servicable parts" but does contain a number of components.

There is a rectifier (to convert the AC produced by the stator into the DC that the sparkplug ultimately needs), a silicon controlled relay (SCR) - often (mistakenly) just called a switch, a Bias switch, capacitors (between 1 and 3 dependant on the number of cylinders - so a 6 cylinder would have two switchboxes configured with 3 capacitors) and blocking diodes (to make sure the power doesn't flow back to the stator). There is also a "stop circuit" which will ground all the ignition power available at the switch box if you hit the kill switch, turn off the key etc. I have probably missed a component or two but those are the major ones.

(and I apologise to the electronically inclined if my explanation is a little off - just a laymans understanding of what is going on).

So, the stator (ignition side) sends the AC to the switch box - at the switch box it passes the blocking diode to "keep it in the box" and goes to the rectifier which converts it to DC and stores it in the capacitor(s).

When the SCR get's it's AC pulse from the trigger, it "relays" to the switch telling it which cylinder needs "fire". The switch turns on the path between the capacitor and the plug coil and the bias (kinda) regulates/conditions power to keep it consistant between all cylinders. The bias is capable of regulating the power to keep it within about a 20 volt window so the power to the plugs is within a similar range so that no one cylinder gets a significantly "different power feed" than the others.

The sparkplug coil up-converts the voltage into the 45-50,000 volt range and fires the plug.

The timing is advanced or retarded (during normal operation) by mechanically moving the trigger plate by moving the windings themself to different degrees in relation to top dead center.

The AC produced by the charge side of the stator is fed to either a rectifier (which gives you un-regulated DC normally in the 13-16 volt range) or a regulator rectifier which would give you somewhat stable power for charging a battery.

That's my understanding of what is going on with the ignition on Merc's Thunderbolt 3 and 4 outboard ignitions.

(should have added)

You can tell if you have a toasted capacitor because you will have "no spark" (all else being good). The bias test will tell you if something is "sick in the box" even though it may appear to be functioning (you have spark)...

Re: 40 hp high idle timing

hello, I am having the EXACT same problem with my 1976 mercury 402 40hp. I just replaced my stator prior to this problem. My trigger is less than 2years old. Before I shell out another $200.00, I was wondering if you ever determined whether or not it was the switchbox?

Update - Changed all three coils and plug wires - no change. I'll keep trying.