JonnyD757

I have a 454 Mag in my 22' Velocity. I am a new owner of the boat. First few times out the boat ran flawlessly. 73 MPH at 5200 RPM. The last few times not so much. I notices a miss when cruising at 3500 - 4000 RPM's. The motor starts to cut out a few times then recovers. The last time it cut out and only allowed me to idle at 1000 rpms. If I feed it any throttle it stumbled. It doesn't back fire, just stumbles. I idled back for 35 minutes and tried to get it on plane again and it ran up to 3500 then started to miss again. Got it home, rebuilt the carb (it was dirty), new plugs, cap, rotor, plug wires, fresh fuel. Checked the fuel pressure. All good. Ran it on the hose and sounded good. Could rev to high rpms with no issue. Took it to the river last night and couldn't get it over 1000 rpm's. I am running the Merc Thunderbolt IV ignition. I've seen where that could be bad but I don't know if a bad module would cause this type of miss.

Looking for suggestions.

SB

iTrader: (1)

Unfortunately, could be fuel, carb, or ignition.

Weber or Quadrajet ?

Whose rebuild kit ? Change needle and seat too ? Blow out all passages with compressed air while cleaning ?

What do you have for good spare parts , if any ? Or borrow wuickly from friends good running boat ? Coil ? Tbolt amplifier/module ? Etc

You getting 12 volts to coil ? Ground wire on distributor hooked up ? And clean grounding point both sides of wire ?

Uf I had to bet $25 now I’d say carb or coil. Meaning, I’d attack, diagnose those first. Yeh I know you cleaned carb but it won’t be millionth time someone would have to redo.

For whatever reason, I’ve dealt with 4 coil issues this past year (diff boats) where I have dealt with maybe 2-3 others in past 25-30 years . Lol. Granted the handful of boats that come my way are late 80’s to late 90’s, so it’s that time I guess for these.

Unlimited jd

iTrader: (5)

Same here, never saw tbolt module fail, the. All of a sudden one year several. Certain parts only last so long.

JonnyD757

It had an Edelbrock. I put on a Holley 750 off my 57 Chevy that i know is a good carb. Unfortunately i don't have a lot of buddies that are into boats, mostly muscle cars. I checked all the connections. They are good. No corrosion (boat has been garage kept its whole life).

Would a bad coil cause the mis firing?

SB

iTrader: (1)

Yup. Cool can do that.
Had one completely fail. Stopped running, no start until replacement. Coil oil had also made it’s way into the wire tower of coil.

Had one run fine for 5-10 minutes and either die or reduce power to idle speeds only.

Had one after about 35 minutes running fine would start misfire over approx 2500rpm.

One wouldn’t start engine first fire up this spring.

JonnyD757

So a bad module may let the boat idle but not rev? I've seen a lot about replacing the TB IV with a DUI. They say it gets rid of all the modules, coil, and wiring. Before i drop more $$ i want to make sure its not something stupid.

F-2 Speedy

iTrader: (6)

yes, the module is what advances the timing as the rpm comes up......did it ever run right with the new carb

SB

iTrader: (1)

Why is a DUI better than a tbolt ? It’s not.

It’s just because it’s offered as a complete unit.

OP - where sre you located in case someone here is nearby with spare parts to borrow for a day or two ?

JonnyD757

The new carb didn't help. Tried to get it up on plan and 1500 rpms it missed. It should still run with retarded timing just no power correct?

SB

iTrader: (1)

What coil does your 57 have ?