350 mag mpi backfire at WOT

gherkin

Hi all.  Just got my newly rebuilt 350 mag mpi back in the boat, it’s a 2000 270 FV. The engine is mostly running great.  But it seems to take longer to get up on plane.  I thought it was just me but after a few hours of running it in I tried a quick WOT run.  It backfired at WOT and only got to around 4200 rpm.  Pull the throttle back just a little and it clears right up and runs great.  I think this is related to the slow planing now.  Does this sound like a lack of fuel issue?  Almost everything is new on this thing.  Plugs, wires, cap, rotor, distributor pickup, coil, fuel pump, regulator, injectors, filters, and anti siphon valve is temporarily removed.  Timing set to 8 BTDC.  Fuel pressure jumps to 32 when key is turned on and maintains 25 when running.  From what I’ve read that fuel pressure is correct. Fuel in the tank is from the beginning of summer. Im gonna keep running it out and then get some fresh fuel. But I doubt that is the problem.   Any other thoughts on how to diagnose this.  This is tough because virtually everything is new so I’m not sure where to go.   Any help is appreciated. 

LaRea

Have you checked for water in the fuel filter?  

reneechris14

Is this mpi or throttle body, mpi should be at 43psi and stay there even when on the throttle,  try to check while under way. My 350mag did the same thing and it was the injectors on both motors at different times. Maybe wrong injectors?

gherkin

As far as water in the fuel. It’s a spin on water separator so cant see inside it.  Certainly a potential.  That’s why I plan to continue to run this tank of fuel through it then put in fresh fuel and filter.  This is the MPI engine.  I have read many places that the fuel pressure should be 43.  But for this specific serial number range it calls for the regulator with the pink mark on it which runs 32 with no vacuum and about 25 with.  I have tried running it with the vacuum disconnected and it still back fires but it seems to be a little better.  I wish there was an easy way to bump up the fuel pressure to see if it would help. But all I can think of to try would be to order the newer style regulator and install it.  Now from what I’ve read even the newer regulator only gives you 43 without the vaccuum line installed and comes down to about 35 with the vacuum line installed.  This week I’m gonna try to get someone to go with me and I’ll put the gauge on it and see what my pressure is while under way.  Any other ideas?

LaRea

Remove the water separator and dump the contents into a clear container to look for water.  Coughing under heavy load is a common symptom of water in the fuel.  

Aqua_Aura

Water in fuel is definitely a symptom. Speaking from experience. It is very easy to see the water in your fuel when you dump out the filter. 

gherkin

I will definitely check on the water and like I said I want to run some of this older fuel out of it and get fresh fuel.  But I’m still leaning toward some sort of supply issue.  I’m running ethanol fuel which will typically absorb the water unless you get too much water and in which case I certainly wouldn’t get on plane and run great except at WOT. I’m also gonna have a friend take me out so I can monitor fuel pressure. I ordered the higher pressure 43 psi fuel regulator.  Worst case if fresh fuel and filter doesn’t fix it I may throw that one on and see what happens.  Do I need to be concerned about trying that higher pressure regulator?  I can’t imagine it would hurt anything.  

Aqua_Aura

If your running ethanol fuel be sure to put treatment in it ALWAYS. I use Star Bil marine and it has proven to me to break down the water in the fuel and make it easier to burn out. 

212rowboat

43psi is correct for mercruiser? It is for a lot of automobiles, and they use smaller injectors- usually around 21-23#, to get the right fuel trim... i 'thought' mercruiser was around 35psi and used 35# injectors to make up for lessor pressure? ... but even said, it should not drop one bit no matter the throttle position or load. 

You've an obstruction or a weak pump.  Maybe its electrical and you don't have a great connection anymore, and the lack of juice to the pump causes it to struggle.... an obstruction could be a dirty filter with ease... and that could very well be because ots wet (water, not fuel) and swelled the filter medium.  

Another just as likely issue is timing chain slop... the cam is clocked with the crank, and the dizzy is driven by the teeth on the cams nose... even a little slop causes issues..  the chain stretches, and it throws off timing.  You say 8* but you should be at 10-12* without advance.  To set base timing you have to shunt the ignition... dumbest of dumb questions: did you release the shunt after checking timing?  

For certain: you dont want to lean out to the point you get backfire, which in that condition is certainly predetonation too... and can kill an engine... stay off WOT until you figure this out!!! 

All said, my money is on the filter... if not that, I'd be pulling the dizzy cap and checking for slop in the timing chain.  

gherkin

I highly doubt it’s timing chain. The chain and sprockets are brand new.  As far as timing yes I put the adapter on.  Set rpm around 1200 and then set to 8 BTDC. Then I removed the adapter. I did not check the timing without the adapter to see what it was.  I may double check the timing when I have someone to help me.  I’ve ordered a new fuel cooler as I’ve read about problems with them blocking up.  Also ordered the 43 psi regulator. For now my plan is to keep running the old fuel out of it. Then fill and change filter.  If I can get someone out with me I will check fuel pressure on the fly. to see if I’m getting any variance.  Then eventually replace the cooler and maybe try the higher psi regulator if I’ve still got problems. Will let y’all know what I find.  Thanks. 

dalek

If you think there is the possibility of bad fuel or a pick-up problem, you can do a neat job of fitting a 5 gallon temporary tank on a 270. Lift the waste basket out. Remove the 10 self tappers that hold the cooler box moulding. Put a clear plastic tank in there, plenty of space. Holes to engine bay are already accessible for fuel lines. Put the cooler holder back. It only takes 20 minutes. You can keep an eye on your fuel level by just opening the waste basket lid. Worked for me.

LaRea

@gherkin you're going to check for water in the fuel filter before doing any of that other stuff, right?  Also check the rubber o-ring on your fuel fill cap.  If it's cracked or missing, that's a way you can get water in the fuel.  

ruggerole

I had the same problem. It turned out to be a very small crack in the cylinder head. The crack was on cylinder #7 between the exhaust and intake valves. It ran great except for a certain RPM. Backfired around 4000 RPM and took a long time to get on plane. Check the compression on the cylinders.

gherkin

This is a fresh rebuild and machine shop did the heads so i doubt I have any issues with the heads. Yes my first thing Is to run the tank low on fuel and then check and replace the fuel filter.  Just need the weather to cooperate with me.  Will post back once I know more. 

212rowboat

gherkin said:

I highly doubt it’s timing chain. The chain and sprockets are brand new.  As far as timing yes I put the adapter on.  Set rpm around 1200 and then set to 8 BTDC. Then I removed the adapter. I did not check the timing without the adapter to see what it was.  I may double check the timing when I have someone to help me.  I’ve ordered a new fuel cooler as I’ve read about problems with them blocking up.  Also ordered the 43 psi regulator. For now my plan is to keep running the old fuel out of it. Then fill and change filter.  If I can get someone out with me I will check fuel pressure on the fly. to see if I’m getting any variance.  Then eventually replace the cooler and maybe try the higher psi regulator if I’ve still got problems. Will let y’all know what I find.  Thanks. 

With that information, I highly recommend checking the timing.  

Chains stretch after initial use when the little nylon ring thingies installed to make them "quiet" compress... which is why I refuse to use a chain so equipped, but which also means I have to go out of my way to find them.  

gherkin

So finally this weekend I got her down to about an 1/8th of a tank and filled it up and changed the filter. Old filter had no sign of water which I didn’t expect due to the ethanol fuel. But she is now running great with fresh fuel and filter.   Thank you all for your help.  

aero3113

Good news @gherkin !!