1993 Mercruiser 7.4 - Is my Problem Carb or Thunderbolt IV module

[farscape102]

Hi, I had a problem all last summer that plagued me. I have a 1993 Formula 252 with 454 carb 7.4 Merc. It started and idled fine and ran at 4300 WOT at 55 mph just fine. Problem was in the cruising range of 3100 to 3800 rpm which just like clockwork it would consistently surge. By that I mean it would go up 200 rpm and back down then up 200 rpm and down. It would do that every second or so and just keep running like that. One time when I was out it stumbled a bit at low speed in BIG waves, but I chaulk that up to an anomoly and not related.
Steps I took to fix:
- New plugs, wires, distributor cap, replaced module under distributor cap, new ignition coil
- Ran like this all summer so not bad gas
- I replaced the gas tank pickup tube
- Replaced mechanical fuel pump
- Checked for blockage in gas vent, ran with gas cap off
My mechanic doesn't think it is the carburetor and thinks it is a bad Thunderbolt IV module not doing a proper ignition curve and he recommends to install a Michigan Motors Delco Voyager EST kit which he has done on some other Mercs. I would appreciate any thoughts? Thanks Phil

 

[Scott Danforth]

Check fuel pressure

 

[04fxdwgi25]

My past boat (about the same as yours) , a 2550 Chaparral, with that same power plant did the same thing.

Was the carb

 

[dubs283]

Check fuel pressure

And vacuum

Will say if fuel pressure/vacuum tests good its way cheaper to rebuild a carburetor than update the entire ignition system

 

[Bondo]

Ayuh,..... 'n the T-Bolt ignition either works,..... or it don't,......
Surgin' sounds more like a fuel delivery issue,....

 

[Lou C]

Did the mechanic actually test the timing advance with an advance timing light at different rpm? Between an electronic ignition system & a carb, a carb is more likely to cause running problems.

 

[farscape102]

Thanks for the replies. No my mechanic never did look at it because it was end of the season. The boat did sit over a season prior due to me rupturing achilles but it fired right off last spring. Prior to talking to mechanic I thought it was the carb as well, I have never had it rebuilt in 8 years of ownership. Any recommended places to send for rebuild? Looks like quadrajetparts is a 4 month wait. Place called SMIcarburetor I could but a rebuild one and send core back. Thanks Phil

 

[farscape102]

Another one I see some recommended is flying fish carburetors.

 

[Lou C]

I wouldn’t exchange an original marine Q-jet for anything you get via internet because you might wind up with a Franken carb that will never work right. I used to think a QJet was too hard for a novice to rebuild but when I took a crack at my original one I found that it’s no harder than any other 4 bbl. Yes there are a few tricks ti taking off the air horn but at least the gaskets don’t get glued on like Holley float bowls. So far I've had it apart 2x, and used better quality rebuild parts available from Cliff's High Performance Quadrajets, the next thing I have to do is adjust the secondary air door or replace the spring, to eliminate a slight hesitation that is has always had.
Suggestion:
Ask your question on Cliffs High Performance Quadrajet forum he’s one of the QJet experts
For a rebuild a shop that restores classic muscle cars should be very familiar with the QJet they came on everything GM made from 1965-1985 or so. Only the highest performance variants came with Holley 4bbls.
The main thing is the cleaning, that's what takes the most time. The actual rebuild part, is not hard at all, actually.

 

[Lou C]

See it's not rocket science!

 

[Scott Danforth]

Another one I see some recommended is flying fish carburetors.

No

 

[04fxdwgi25]

My past boat (about the same as yours) , a 2550 Chaparral, with that same power plant did the same thing.

Was the carb

The replacement carb came from Flying Fish and was still operating perfectly when I sold the boat 4 years later.

 

[todhunter]

I second Cliff's High Performance. They sell great kits (and a great book) if you feel like diving in yourself, and I think you can also send in your carb and have them do the rebuild, but I'm not sure on lead time.

 

[farscape102]

Thanks for replies everyone. I will check out the shop that did my girlfriend's carb in her 70's mustang. Also I find a shop in Minneapolis that only rebuilds performance carbs.

 

[Lou C]

If you have to use E-10 gas there are better quality accelerator pumps available from Cliff's High Performance, make sure you tell the shop the carb is for a boat.

 

[kenny nunez]

If you can use the boat before sending the carburetor to get rebuilt try this.
With the flame arrestor removed have someone watch the secondary air valve when the engine starts surging. The vacuum unit that holds the valve closed may have a internal leak allowing the valve to raise the secondary metering rods just enough to bleed extra fuel on the secondary butterflies.
You also can test the unit with a vacuum/brake bleeder tester sourced from any auto store.
Since it idles smooth and will turn up to 4300 rpm that is the only thing I can guess at.

 

[Lmschwartz04]

Was this a carb issue?