Shallow Minded

Hey guys,
I'm running a 454 .30 over with a roots-style B&M 174, about 5 lbs of boost. The boat has ran fine all summer (3rd summer on it) until the last couple of trips out.

It will run normal for the first 20 minutes or so (cold) and then it develops a backfire, sputter and just sounds terrible. I lose power and hate to even idle it. The previous trip, I was on vacation and stuck miles from home so I let it cool down and ran it again. I had no problems the rest of the weekend. Today, similiar situation... hit a sandbar and let it cool down. It then ran fine for the first 20 minutes back to the dock before the backfire started again.

I changed the plugs last time and they looked normal. It seems to happen mainly under load and it will usually idle fine. A couple of thoughts below...

Bad gas - it would do it all the time, right? Plus, I burned most of the tank on my vacation.

Weak valve spring - it would happen all the time?

"Chunk" of carbon build-up inside the cylinder head that once it gets hot enough "glows" and actually causes a misfire?

I'm lost on this one... I'd really appeciate any ideas. Thanks in advance.

SM

Payton

Bad coil?

GOODT

could be a bad coil, when the problem starts feel coil will be hot if bad ( most of the time)......

bad gas or water in gas would cause this also....check water separator


is it poppping though carb or exaust or is it a loud " back fire "

SB

iTrader: (1)

Fist things to check are the coil as stated, and then the fuel supply system including needle + seats in carb.

What are you fluid temperatures when this happens ?

Does it happen faster if you run it harder ? Take longer to happen if you run it easier ?

Shallow Minded

I've changed the water separator and had the same issue the first time back out, so I don't think it's gas.

I had not thought of the coil, it's a full MSD set-up... sounds like a good place to start.

As for the backfire, it really sounds like it's coming back up through the manifold & blower... not really a popping, but more of a backfire.

Fluid temps all appear normal... I don't have an oil temp guage so I can't be too specific.

It seems like it happens about the same time whether I'm on the gas or babying it...

I wondered about the carb, too. Maybe running lean? Holley 850, 3rd summer on it from new and I've never touched it.

Thanks guys, I really appreciate the ideas.

ghittner

Start with the coil, I've seen this exact problem more than once.

Griff

Start with the coil. I had an MSD coil go bad after less than 2 years. I put a 10 year old merc spare one I had back on and it was fine.

Being that it is intermittent, I would lean toward something in the ignition or fuel system. Could have a float sticking in the carb. I would also change the powervalves if you have had it backfire through the carb. A blown pv will make it idle and run like crap at idle and low rpm. Its just not something that will happen intermittently.

bobkatz

Check your blower bolts for proper torque, and look for leaks around the blower to intake and blower to carb. Even if that's not the problem, had a gasket blow out from the backfire. Check your timing, and see if the distributor is corroded. Was the problem there from the start or did it get progressively worse.

Griff

He's been running the same engine for a couple years.

Shallow Minded

Ordered a new coil today... should be here to test out this weekend. My mechanic spoke to several sources (including MSD direct) and even they stated it is not uncommon for their coils to fail "prematurely" due to the voltage their running through them.

I'll let you know how it goes, hopefully this is a simple fix. Thanks for the tips, guys.