Thunderbolt IV ignition question

[Striker36]

I'm working my way through troubleshooting a misfiring, poor running 1991 5.7, 4 barrel Rochester with Thunderbolt IV. I'm trying to do the troubleshooting procedure for the ignition, but I have the newer style ignition sensor in the distributor that doesnt have any posts on it, just the three wires going into the resin filled sealed unit. Is there a way to meter test this one, since I cant get at the terminals inside? Is it safe to rule out the ignition sensor if the boat runs, although poorly?

 

[Striker36]

Update, pulled all my plugs, and my #5 is visibly different than all the others (see pic), the one in the middle between two normal ones. All plugs are recently new.
Checked compression all down the port side, and all cylinders are between 165-170. engine was warm but not up to full operating temperature.
Distributor cap and rotor are new.
Could it be anything other than bad plug or bad plug wire?

 

[alldodge]

I'm working my way through troubleshooting a misfiring, poor running 1991 5.7, 4 barrel Rochester with Thunderbolt IV. I'm trying to do the troubleshooting procedure for the ignition, but I have the newer style ignition sensor in the distributor that doesnt have any posts on it, just the three wires going into the resin filled sealed unit. Is there a way to meter test this one, since I cant get at the terminals inside? Is it safe to rule out the ignition sensor if the boat runs, although poorly?

Got a pic of the module?

Update, pulled all my plugs, and my #5 is visibly different than all the others (see pic), the one in the middle between two normal ones. All plugs are recently new.
Checked compression all down the port side, and all cylinders are between 165-170. engine was warm but not up to full operating temperature.
Distributor cap and rotor are new.
Could it be anything other than bad plug or bad plug wire?

The one plugs has me leaning toward a leaky intake valve guide seal.

One on left looks good, one on far right is either not firing or running lean
How old are the wires?

 

[alldodge]

Also looks like you have the wrong plugs in the motor, I'm seeing RV9Y something
Should be
AC MR43T
NGK BR6FS
Champion RV15YC4

 

[Striker36]

Also looks like you have the wrong plugs in the motor, I'm seeing RV9Y something
Should be
AC MR43T
NGK BR6FS
Champion RV15YC4

Boat is new to me and Im just going through it to see what previous owner did and didnt do.
Plugs that are in at Champion RV9YC. Is that much of a difference between the RV15YC4, and would I have noticed anything from their use?
Are the plugs you show above the right ones for the year, 1991, and carbed non-vortec?

 

[alldodge]

Are the plugs you show above the right ones for the year, 1991, and carbed non-vortec?

You bring up a good point and my bit of oversight. Your motor shows in Manual 15 it should have
Champion RV8C
AC MR43T
and does not mention NGK, but that said MR43T crosses to the NGK BR6FS

So your RV9YC is one heat range higher then listed, and crosses to R43TS

 

[Striker36]

Got a pic of the module?

The one plugs has me leaning toward a leaky intake valve guide seal.

One on left looks good, one on far right is either not firing or running lean
How old are the wires?

Would the valve seal not result in a lower compression reading? If thats what it is, can it be changed out from the top or does the head have to come off? How do you check for it?

 

[alldodge]

Would the valve seal not result in a lower compression reading? If thats what it is, can it be changed out from the top or does the head have to come off? How do you check for it?

No, is just a bit of oil. The seals can be replaced without pulling the head. Need a compressor and a fitting to screw into the cylinder. Rotate the motor around to TDC on the cylinder to be worked on. Hook up an air line to the cylinder and pressurize. Take a valve spring compressor like this one


Then install it on the valve spring and compress


The two half moon pieces on each side of the vale steam are removed, then the spring can be lifted off the valve.
Next remove the valve seal (either rubber or plastic umbrella type), then install the new one. Can do this for every valve seal, just bring each cylinder to TDC for that cylinder

 

[Striker36]

Got a pic of the module?

New style modules look like this;

No exposed terminals, just the three wires going inside the resin filled unit. The two holes are just for the hold down bolts going to the distributor, nothing attaches to them.

 

[Striker36]

Old style looked like this, had attachment terminals for the wires, and allowed for the Thunderbolt IV ignition tests as laid out in the manual.

 

[alldodge]

Old


New


The black wire is ground and is bolted to some point on the motor. The other two wires and the just like the ones coming off the old TB-IV.

Here is the troubleshooting guide

 

[achris]

Another trick I learnt from another member on this website to hold valves up while removing their spring is to pull the spark plug, push as much soft 1/4" rope into the cylinder as you can, then turn the engine by hand until the rope locks the valves in place. When you're done, just turn the engine back a little and pull the rope out.

Never been a great fan of champion plugs, been know to cause misfire problems. Stick with AC or NGK.

Chris. .....

 

[Striker36]

Im putting a new set of NGK's in in the morning. We'll see if that makes a difference.

 

[Striker36]

Put another new set of wires in and swapped out the plugs. Seems to have solved the problem pending an on the water trial to confirm vs. muffs in the driveway. I suspect it was a bad plug wire despite being only a few months old. Also had a bit of crud in the carburetor inline filter but not enough to cause any serious restriction. Fuel pickup in tank was clean.