1995 350 5.7L Mercruiser Engine Bogging and Backfiring Under WOT

[ThndrSki]

Hello,

I have a 1995 Mercruiser 350 Mag 5.7L that is bogging/stalling on the muffs on the trailer. It has a Weber 4bbl carbeurator, whichh if I take the flame arrestor off, I can see the secondary arms turning, but the butterflies inside with the weights just barely move.

I also cannot see any fuel shooting down from the opening like I can with the primaries. Am I supposed to see fuel shooting down these as well?

 

[Bt Doctur]

I believe most carbs need to have the choke plate fully open to have the secondaries open.

 

[ThndrSki]

The choke plate is fully open

 

[Scott Danforth]

most carbs also do not have a secondary accelerator pump. so you would not see fuel shooting down the secondaries. the secondary throttles rely solely on air flow to siphon the correct amount of fuel. the exception is Holley double pumper carbs - hence the name.

also, you should not be trying to get into the secondaries while on muffs on a trailer for two reasons. first is you will never load it enough for the secondaries to need to open. the second is that you will quickly run out of cooling water flow and burn up your raw water impeller. normally muffs are good for 1500rpm max.

 

[MikDee]

When the secondarys open, they lift the center metering rods out of the rear main jets to supply the extra fuel, I doubt you can see anything visibly, besides the secondary metering rod arms moving. As was mentioned by Scott, the engine needs to be running under a load normally for the secondarys to open.

 

[Bondo]

Hello,

I have a 1995 Mercruiser 350 Mag 5.7L that is bogging/stalling on the muffs on the trailer. It has a Weber 4bbl carbeurator, whichh if I take the flame arrestor off, I can see the secondary arms turning, but the butterflies inside with the weights just barely move.

I also cannot see any fuel shooting down from the opening like I can with the primaries. Am I supposed to see fuel shooting down these as well?

Ayuh,.... As noted, the 2ndaries Won't open on the muffs, ya need a Load for that to happen,.....

If it's bogging/stalling, I guess the carb needs a Rebuild,....

 

[NHGuy]

Is the carb adjusted right? Any crud in the filters? Fresh wires, proper timing? Good fuel? I'd look at those items before removing the carburetor. But once the fire & fuel is right it's time to look at the carb.

 

[ThndrSki]

I believe I have it fixed - It appears to be the coil so I wanted to post a resolution. I tried adjusting the carb using the guidelines (and it ran great the last time I used it a week ago) to no avail. Couldn't seem to dial anything in. I had just replaced the plugs and wires, so I didn't believe that would be the cause. Checked the distributor and cap, and all looked good.

That's when I moved to the coil.

According to the Mercruiser Service Manual these are the readings I should get for a Thunderbolt IV coil:

Description Specification
Coil Part Number 392-7803A4
Primary Resistance .60-.80 Ohms
Secondary Resistance 9,400-11,700 Ohms


I pulled the coil ran a test of primary and secondary resistance readings and this is what mine read:

Primary (test between the + and - terminals on the coil): 2.0 Ohms
Secondary (test between + and coil tower to distributor): 7,750 ohms

I happened to have an old spare coil from when I first got the boat 4 years ago and put that in and VROOOM! Started right up and no popping/backfire. So I think it was doing just enough job to start, but not run at higher RPMs where higher resistance would be bad.

This coil lasted about 4 years, but it was a NAPA marine coil crossed referenced specifically. I'm ordering a new Mercruiser coil as we speak. If anyone has any suggestions for true OEM parts, please let me know.

Thanks for all those who gave suggestions.