Help - Backfiring Issue on Blown 454
 

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. :mad:

SM

Re: Help - Backfiring Issue on Blown 454
 

Bad coil?

Re: Help - Backfiring Issue on Blown 454
 

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 "

Re: Help - Backfiring Issue on Blown 454
 

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 ?

Re: Help - Backfiring Issue on Blown 454
 

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.

Re: Help - Backfiring Issue on Blown 454
 

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

Re: Help - Backfiring Issue on Blown 454
 

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.

Re: Help - Backfiring Issue on Blown 454
 

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.

Re: Help - Backfiring Issue on Blown 454
 

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

Re: Help - Backfiring Issue on Blown 454
 

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.

Re: Help - Backfiring Issue on Blown 454
 

Well, took the boat out today with the new coil and damn it, same thing happened. Although it did seem to run for a longer time before the first "sneeze", something is still not right. I'm thinking it's electrical in nature. Going to check wires, cap & rotor.

Anything else I'm missing? This intermittent stuff sucks!

Re: Help - Backfiring Issue on Blown 454
 

I think everyone here thought you already checked the cap and rotor ....( should of been one of the first things to check )...


ussually after 30min running can be filter or coil


what carbs, did you ck carb filter ???

Re: Help - Backfiring Issue on Blown 454
 

On cars a backfire is typically caused by too much fuel.. maybe you're running rich?
Maybe lean it out a tad.

Re: Help - Backfiring Issue on Blown 454
 

no offence but did yo miss the after 20 min part

or are you suggesting choke sticking

Re: Help - Backfiring Issue on Blown 454
 

no offense taken.

too much fuel.
it ain't gettin' burnt.
simply too much or lack of spark. thats my uneducated opinion.

M

Re: Help - Backfiring Issue on Blown 454
 

Sorry, should have probably mentioned that. Going to check cap & rotor, plug wires now. Got to looking and everything (including single Holley 850) was purchased in Jan '04 to give you an idea of age. Probably 150 hours on the set-up.

Dumb question, but you lost me on the carb filter part...?

It doesn't appear to be running rich, if anything, I'd rather add a little more flow to it.

Thanks guys for the quick response, this thing is keeping me up at night. :mad:

Re: Help - Backfiring Issue on Blown 454
 

I had the same thing happen on a 525sc which is basically a 454 with a 177 blower on it. The fuel pump was going bad and it would start to back fire when it was starving for fuel. Its kinda the same principal as a torch set. When you turn off the gas before the oxygen you get a pop :D Do you have a fuel pressure guage?

Re: Help - Backfiring Issue on Blown 454
 

Yep, I'm measuring fuel pressure right at the carb and it stays rock solid at about 7 PSI.

But my carb does seem to have a large dead-spot / hesitation in it, like the secondaries are slow to open up or something. I wonder if this could have something to do with it.

Re: Help - Backfiring Issue on Blown 454
 

did you check the filters in the carb itself

Re: Help - Backfiring Issue on Blown 454
 

Watch your pressure as you bring up the RPM's If you have a restriction in the supply or if the pump is bad you will see the pressure drop once you loose the volume of fuel in the system. If it stays then you know your good to the carb. I think GOODT is refering to the stone filter on the carb where the fuel line goes in.

Re: Help - Backfiring Issue on Blown 454
 

Cap and Rotor have seen better days (ie, they look TERRIBLE). Hoping that's the problem, try to get out on it this week to see if it helps.

Re: Help - Backfiring Issue on Blown 454
 

I had the same setup only with the 250's.
It seems the Merc Tbolt ignition would from time to time, fart and retard the crap out of the timing and backfire. I could hear a stumble below 2000RPM, but a backfire over 4000. I swapped the port module with the strbrd one and the problem went with the module, then module went into garbage.
Also, are you boost referencing the power valve or just over jetting? If not mapped right with your timing, you could get some lean popping.

Re: Help - Backfiring Issue on Blown 454
 

I had a relative new MSD cap and wires on a motor that must have had some moisture under the cap....got corroded and acted up, took a while to find since I thought they were still new.
Let us know how it goes.

Re: Help - Backfiring Issue on Blown 454
 

magnetic pickup in dist. moisture gets to the old style mercs all the time.

Re: Help - Backfiring Issue on Blown 454
 

It's all MSD... Distributor, Ignition, Coil and the carb is boost referenced.

Having my shop look at the carb and hope to get out on it this week with the new cap/rotor. I'll let you know how it goes...

Thanks for all the ideas, I think we're getting there.

Re: Help - Backfiring Issue on Blown 454
 

Roller cam or flat tappet if not roller check cam sometime exchust lobes where off..take valve cover of and see if all rocker arms are moving the same amount but look at the exchust..
chuck

Re: Help - Backfiring Issue on Blown 454
 

Men,
I just wanted to let you know I finally got some time on the boat this past weekend. Ran it pretty hard on Saturday and Sunday with no issues. Guess it was the cap & rotor. I could not believe how bad they looked when I finally worked on it.. tracking inside the cap, o-zone build-up on most of the contact points, etc.

Thanks for the all the tips. Now I'm ready to sip a beer and watch the LOTO Shootout...

Quint

Re: Help - Backfiring Issue on Blown 454
 

Well you must be in St. Louis because it rained in Omaha Sunday. :(

Re: Help - Backfiring Issue on Blown 454
 

Glad it was nothing serious. See you guys at the Shootout.

Re: Help - Backfiring Issue on Blown 454
 

Sip? That's a new one - but seriously, glad we got that boat running good.