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V8 Mercruiser will not rpm above 2700.

2

2sloboat

New member
'91 mercruuiser Alpha 1 V8 (350)...has thunderbolt 4 ignition (module on exhaust manifold NOT box). Problem is not a fuel issue. Starts ok..runs up to 26-2800 rpm ok then stays there....correct prop. Runs all day at this rpm...replaced coil with new, no help. Could the distributor sensor not be advancing the ignition? Thanks
 
The ignition timing is controlled in the TB-IV module, not the sensor....I'd suggest getting a timing light and checking the advance....not having it will definitely limit the RPMs, especially under load..
 
makomark said:
The ignition timing is controlled in the TB-IV module, not the sensor....I'd suggest getting a timing light and checking the advance....not having it will definitely limit the RPMs, especially under load..
Click to expand...

Yes...but doesn't the sensor signal the module how fast it is operating (distributor rpm)?
 
Tests forThunderbolt IV Ignition:
W/ignition key on AND BILGE WELL VENTILATED OF GAS FUMES!!!

#1 - Connect your voltmeter neg. lead to the engine ground and pos. lead to thewhite/red wire terminal at the dist. It should read 12 volts.

#2 - If 12 volts is present, remove the coil spark wire from the distributorand connect it to a spark gap tester to ground. Remove the white/green leadfrom the dist. terminal. Turn the ign. key to on and strike the white/greenlead to ground. If there is spark, replace the ignition sensor in the dist.

#3 - If there is no spark, substitute a new coil and repeat test #2. Now if youget spark, install a new coil. NO SPARK, replace ign. amp.

#4 - In #2, if there is No voltage present, disconnect the white/red wire and checkit again for 12 volts. If 12 volts is present, replace the ignition sensorinside the dist. cap. If no 12 volts present w/it disconnected--ignitionamplifier module is shot and must be replaced.
 
Indirectly, yes...

The primary purpose of the sensor is to tell the module when to "fire".

It is the frequency of these "fire" signals that the module uses to adjust the timing...if the sensor is intermittent, the engine will miss when the connection goes bad...
 
The original sensor (optic trigger) under the rotor was subject to corrosion on the bottom side...fine wires corrode and can contribute intermittent firing as makomark stated. The upgraded model is sealed to prevent corrosion. Perform the TB IV ignition test above. If it passes the test pull the rotor and inspect the sensor for corrosion. If the rotor seems like it is glued in place, it was originally loctited. Grab it w/a channel lock pliers and twist and pull or split it gently w/a chisel to remove it and replace it.
 
Post back after you try the fix.

There are other options and we can explore them together.

A all in one unit with the coil built in mechanical advance might be in your future.

One wire in one wire out and one wire to the tach.

After you try the fix and if it doesn't work disconnect your tach. and try running it again.

One thing at a time so we can narrow down the problem.

Post back.
 
2sloboat said:
'91 mercruuiser Alpha 1 V8 (350)...has thunderbolt 4 ignition (module on exhaust manifold NOT box). Problem is not a fuel issue. Starts ok..runs up to 26-2800 rpm ok then stays there....correct prop. Runs all day at this rpm...replaced coil with new, no help. Could the distributor sensor not be advancing the ignition? Thanks
Click to expand...
See Mark's response.

makomark said:
The ignition timing is controlled in the TB-IV module, not the sensor....I'd suggest getting a timing light and checking the advance....not having it will definitely limit the RPMs, especially under load..
Click to expand...
Ditto Mark's suggestion.

The TB photo-eye system is supplying a "timed" TA triggering event to the module.
The module then progressively delays this event, giving the engine a progressive ignition advance as RPM increase from BASE advance on.


Pick up a Mr. Gasket decal (or one of the many others) for your harmonic balancer diameter.
The decal must be correct for the balancer's diameter!!!

Clean the surface well, and apply the decal.

With your TB ignition system in standard run mode, strobe your timing marks while the timing light is also in "Standard" mode.

Jot down what you see at an array of RPM (let's say in incriments of 250 rpm), and up to approx 3,500 RPM.
These numbers will represent the progressive curve and TA (total advance).

Compare your numbers to the Mercruiser ignition advance curve that you will find here.

If your numbers are not very close, then you have an advancing issue.
Baring no other issues...... any Marine gasser who's ignition system is not delivering the correct progressive advance, will suffer from loss of power.


Worst case would be a progressive that is delivering too much advance/too early.



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