Baffled with misfirerough running

"Here's the background: 4

"Here's the background: 454/330hp mercruiser alpha1. After long layup, (2 years) I brought it out this summer. With a new battery, it started right up, but ran very rough with a misfire. As the gas smelled bad, I drained the tank, changed the fuel filter, no change. Carb rebuilt. No change. I found oil leaking from coil, so went ahead with full tune up parts including coil, plugs, wires, cap, rotor, and ignition sensor. I could then get it to run smooth at times, thought I might have had a shift interuptswitch problem,as I could wiggle the inturpt shift and get it to run fine sometimes. I found a bad shift cable bellows, so changed cable and bellows. Now I cannot get it to routinely run smooth even with the shift interupt disconnected. It is a very rythmic misfire, it runs smooth sometimes, just not all the time.

Any thoughts? Could the Thunderbolt IV (I think) be inconsistent in producing an amplified spark. I put a timing light on it last night, and at times get a consistent pulse/light, but at other times seem to get a double firing of the timing light. Seems odd. I have went over wiring looking for perhaps a chaffed or pinched harness, possibly grounding out, but so far have found none."

Take a look at the optic trigg

Take a look at the optic trigger under the rotor in the dist. If it is heavily rusted it could be part of the problem. Pull the connector off of the amplifier module and clean the contacts.

"Guy, is that not the ignition

"Guy, is that not the ignition sensor? I changed the ignition sensor, with the cap and rotor, the old one was rusty."

"B Bordin:

Sorry, I missed

"B Bordin:

Sorry, I missed that item. What did the spark plugs that you replaced look like in color and carbon deposits? Any of them rusty on the electrode? Remove the plug from the amp. module and clean the contact areas of the plug and module. I've used a fine brass brush and WD-40. Blow dry w/compressed air. Remove any brass brush hairs.

With engine running, wiggle and bend the amp. module wires to see if it affects the engine.

Since you are getting a double strobe sometimes from the timing light, see if you can borrow one to verify that yours is not producing an error pulse."

"My Thunderbolt does not have

"My Thunderbolt does not have a plug on the module, just 5 wires coming out: Black to ground; purple and grey to coil; white/green white and white/red to ignition sensor in distributor. I've wiggled all the wires, checked/cleaned ground wires, looked good, but cleaned anyway and snugged down bolts holding grounds real good.

I also disconnected the keyed ignition/main harness with blue plug to engine, and started it with a jumper from bat+ to coil+ in place, with no performance change noted. (I read about a bad tach causing ignition issues, doing this should rule that out)"

"Bump for more ideas. I have

"Bump for more ideas. I have gone through the thunderbolt iv diagnostics, all check out ok. Can the module die a slow death? As I can ground out the green/white wire, then let up, and get it to run smooth, for a time, I believe it has to be electrical. In my small mind, I am wondering if there is a bit of a voltage spike when I unground the green/white wire that allows the module get an extra charge and run for a bit. It seems to be getting worse, which makes me think it is the module."

"Also, the tach is very errati

"Also, the tach is very erratic, like it is only getting pulsed signal part of the time. Could be the tach itself. Can't remember if the tach smooths out when the engine is running right. I guess I think the module is only working like 1/2 the time. Is that possible?"

"Pretty good detective work so

"Pretty good detective work so far. A few more items to consider.

You may want to verify each plug lead has continuity. (I've seen new OEM wires with breaks, right off the shipping truck.) Also, verify each lead is tight on the plugs. A thin layer of silicone dielectic compound in both boots will minimize any HV leaks and minimize corrosion, too. Also, verify the correct firing order....

Do you have the amp on the distributor or off on the inside of the exhaust elbow? If on the elbow, I believe there are two ground wires that go under the mounting screw. If on the distributor, is there heat conducting 'grease' on the back of the amp?

Another item that may help is a good visual inspection when its dark outside....much easier to see any 'leak' that's occuring. A final check (which isn't an "approved" approach) is to pull one plug wire at a time with the engine under load. RPM should drop with the wire off and increase with the wire back on. No drop means that cylinder is working. Big blocks, especially at idle, are great at masking misfires.

It could also be a mechanical issue (sticking or rusted valve); a vacuum gauge may be beneficial in verifying the 'health' of the engine. A compression test may help, too. (I never saw in the thread where it has run right after the layup and maybe something is rusted/sticking/etc due to the storage period.)"

">>>>Also, the

">>>>Also, the tach is very erratic,<<<<


Hook a vacuum guage up and see what it tells you.

Make sure you have a good base gasket under the carb.

Stab each wire on the dist cap with a very sharp test light (other end connected to ground) and see if it causes a change in running.
NO CHANGE = bad wire/plug or both. If there is a pattern of more than one with no change then it is a fuel/carb issue.

Hook a timimg light up and watch the mark and see what it does when held steady at 2000 rpms..."

"Update for anybody wondering.

"Update for anybody wondering...(always frustrating to me when someone posts a problem, but never posts the result)

I replaced the module, and all is good."