Ignition system common failures? 1994 7.4GL

[vroom ZOOM]

Hi, this one has me stumped. Seems like im better at diagnosing efi motors than the carbed ones. I don't know the exact model number, because the sticker is long gone, but it is a 1994 7.4GL. The ignition system is NOT the EST, or the EEM version. Here is a pic of the motor:


This thing backfires like an AK-47 as soon as you give it any throttle. When cold, it isn't so bad, but it still stalls with a loud pop out the carb. Sometimes it pops and stalls, then it pulls smoothly for a second or two, then starts popping again. I checked the carb and float etc thinking it was a lean pop, but everything checked out ok, and all jets were clean. After putting back together no change. It will idle forever, no pops or anything, although it does seem to miss a tad. One time when it started popping, it shut it off with the key and drained the carb bowl - it had quite a bit of fuel, so therefore the lift pump is working ok. After taking out one of the plugs (others were hard to get to), I saw it was all sooty and black... definitely not lean looking.

Fuel wise, I think I checked everything that could be problematic. The thing is, I'm not too familiar with this ignition system, so I could use some advice. I looked at the cap for carbon trails, but that was impossible to find because it is black. any advice on what to look for? I'm starting to think valve springs? I really don't want to pull the valve cover if I don't have to.

Here is a video of it running at idle:

video when it backfires:

 

[muc]

Google "how to tell if my holley 4 barrel power valve is bad"
Guessing you have the Prestolite B.I.D. distributor. The weak points, leaving the key on when engine isn't running (it causes issues with the ignition module), air gap out of adjustment or excessive wear in the shaft bushings. And yes you really should use a brass feeler gauge when you check the air gap.

There should be a tag on the engine block near the starter with your serial number. Use your phone to snap a pic. Going by year of the boat is a bad way to I.D. an boat engine.

 

[alldodge]

Do a compression test, the valves might not be fully closing

 

[vroom ZOOM]

Google "how to tell if my holley 4 barrel power valve is bad"
Guessing you have the Prestolite B.I.D. distributor. The weak points, leaving the key on when engine isn't running (it causes issues with the ignition module), air gap out of adjustment or excessive wear in the shaft bushings. And yes you really should use a brass feeler gauge when you check the air gap.

There should be a tag on the engine block near the starter with your serial number. Use your phone to snap a pic. Going by year of the boat is a bad way to I.D. an boat engine.

Definetly the BID system. If it wasn't for the flame arrestor the boat would have definitely blown up by now. IMHO not the power valve because if it was blown the engine would be idling really rich, right now it idles fine with the screws 1.5 turns out.

Is there a procedure for testing the pickup, module, and coil? Does the BID system have a a coil with an internal, or external resistor? What resistance should I be seeing? I forgot to check the timing, but I will next time. Will a bad module cause the timing to bounce around? Or should I just rip it all out and throw a delco EST system in it, or maybe a pertronix?

 

[vroom ZOOM]

Do a compression test, the valves might not be fully closing

will do, but wouldn't it backfire at idle if the intake valve wasn't closing?

 

[muc]

Definetly the BID system. If it wasn't for the flame arrestor the boat would have definitely blown up by now. IMHO not the power valve because if it was blown the engine would be idling really rich, right now it idles fine with the screws 1.5 turns out.

Will it idle with the screws turned in?

Is there a procedure for testing the pickup, module, and coil? Does the BID system have a a coil with an internal, or external resistor? What resistance should I be seeing? I forgot to check the timing, but I will next time. Will a bad module cause the timing to bounce around? Or should I just rip it all out and throw a delco EST system in it, or maybe a pertronix?

Most of the testing is for a "no spark" issue.
I'm a fix what you got (if you can) kind of guy.
Serial Number?

 

[Scott Danforth]

bad cap and rotor

 

[alldodge]

will do, but wouldn't it backfire at idle if the intake valve wasn't closing?

Spark advances more as rpm's increase

 

[vroom ZOOM]

Spark advances more as rpm's increase

Ah yes, ok, that makes sense.

 

[vroom ZOOM]

Will it idle with the screws turned in?


Most of the testing is for a "no spark" issue.
I'm a fix what you got (if you can) kind of guy.
Serial Number?

Stalls if I lean it out. Sorry I don't have the SN yet, will have it in a few days hopefully. Problem is that a lot of those prestolite bid parts are NLA. Now I recall the last time out the tach was bouncing a fair bit. Maybe related, maybe not.

 

[vroom ZOOM]

bad cap and rotor

Yes, that is what im thinking, a cracked cap causing flashover to another cyl and making it backfire. will probably replace wires and cap and such.

Ive got lots of good things to test now, thanks everyone!

 

[Scott Danforth]

you will find green contacts in your cap and you will find arc tracing.

 

[vroom ZOOM]

its now fixed, dizzy was turned about a degree and now it runs good. I also found out that it has something called Xact steering, what is that, what is it supposed to do, and what are the components of it?