HELP!!! Monterey 256 Sport Cruiser 1996

[richardldemers]

Not sure if this is the right place to post, but here goes.
I have a ford 351 Windsor marinized by Volvo Penta. I developed a light back fire coming from the intake manifold. When you quickly push forward throttle it makes one loud back fire . I just had the engine completely rebuilt, along with new distributor, new plugs twice, wires, coil is good, rebuilt carb.
The mechanic eliminated / bypassed 12 volt system, so its not electrical problem. When you pull the #8 plug wire, the engine back fire noise stops and engine seems like it run smoother. Yes ,the wires are in the correct order. Keep in mind this is on land (ear muffs) and not in the water.
I bought a cheap smoke machine to try and find a vacuum / exhaust leak, but I'm not sure where to hook up to the right entry point.
I don't want to give up on this , We love boating. (working on this issue for 2 seasons) but I feel I may be beating a dead horse and I can't afford to keep paying a Marine mechanic $160.00 per hour anymore. Any suggestions may help. Thanks, Richard.

 

[ESGWheel]

There are a lot of smart folks on this forum, and I am sure they can help solve this. Let’s see if we can get the conversion started with some of the thoughts I have regarding this frustrating issue.

First off, it may be useful to post a short video of the situation.

For backfiring: there are some basic causes which are outlined in this short description link.

While not intuitive given the recent complete motor rebuild the fact you get this backfire when rapidly accelerating the engine – which causes the fuel mixture to go rich with the accelerator pump – but will NOT backfire with plug 8 disconnected – leads one to believe you may have a non-sealing intake valve on cylinder 8. Just curious: how did you come up with unplugging no. 8?

As a suggestion I would start with validating the ign. timing, and if good, check the valve adjustment on No. 8. Others would know better, but I think this has hyd lifters and the adjustment is (with the piston in the correct position) to remove the lash, and then either tighten an additional ¼ turn or some torque value. If this is over tightened, its possible the valve will not fully seat. Note: if you do find this one out of proper adjustment, check all of them. If it is properly adjusted and still have issue, its possible the lifter is bad.

Anyone else got ideas?

 

[kenny nunez]

Just as ESG posted you need to look at the #8 cylinder. Remove the valve cover and watch that both valves move the same amount when turning the engine by hand with the bolt holding on the harmonic balancer with a 15/16” socket.
Your engine probably has positive stop rocker arm bolts so there is no adjustment.

 

[richardldemers]

Just as ESG posted you need to look at the #8 cylinder. Remove the valve cover and watch that both valves move the same amount when turning the engine by hand with the bolt holding on the harmonic balancer with a 15/16” socket.
Your engine probably has positive stop rocker arm bolts so there is no adjustment.

Thanks you guys for getting back to me. The back fire happens when idling in the intake manifold, its not real loud and almost sounds like a knocking noise. When you throttle up it makes on big pop then the noise seems to settle down some. At idle when i use a stethoscope and touch the intake manifold you can hear it some BUT when you touch the side of the carburetor Man is it loud. I had the carb rebuilt, had the heads rebuilt once before when I was told by another marine mechanic that that would solve it and turns out that was not the issue. Now I had a complete motor rebuilt with heads by a local reputable engine rebuilder. Still doing it. That mechanic suggested I look for a exhaust and or an intake leak. So I bought a cheap Smoke machine to try and find a leak somewhere hopefully this weekend.. after doing some more research I am going replace the mechanical fuel pump , its the original from 1996. Early on it was suggested to pull one wire off the plugs at a time to see if any changes. When I got to #8 after doing all 7 one at a time that's when the backfire would stop. It has all of us scratching our heads. I did also do some adjustments on the heads, no change.

 

[alldodge]

Fresh rebuilt motor and backfire on 8, so to follow ESG post

I would remove the valve cover over 8 (Port side rear)

Rotate crank until cylinder starts coming up on compression stroke or until exhaust valve starts opening.

Mark the rocker nut and stud so you can see if it comes back up to same spot when readjusting. Back off the nut until there is up/down play on the #8 intake push rod.

With thumb and index finger holding push rod and you should be able to move rod up/down slightly. Doesn't matter if you can rotate the rod only that it moves up/down. In small increments tighten rocker nut and keep checking for up/down movement. Once the nut is tightened "just" barely enough that all up/down movement has stopped. Now tighten nut exactly 1/2 turn more.

The valve is now adjusted

Edit: port side rear

 

[Bt Doctur]

turning the engine by hand with the bolt holding on the harmonic balancer with a 15/16” socket.

Not really recommended ,your taking a chance of breaking the bolt or stripping out the crankshaft threads.
Most of what you describe is a intake valve not seating all the way or a bad valve grind or bad seat

 

[ESGWheel]

Thank you for the follow up and the explanation on the cylinder 8. Pulling plug wires one by one is a good and long-standing troubleshooting practice. I am not sure what I was thinking asking you this question > I should have known better, so please excuse my question.

And you are probably already there, but best to pull all the plugs prior to turning the engine to make it easier and lower the stress if you decide to turn it by the crank bolt.

For the louder at the carb vs intake manifold: I have no explanation other then knowing that the entire intake system is akin to a brass instrument, like a trumpet: the pulses of air will resonate at different points as well as these two units are made of different metals which also impacts sound travel and levels. I would not consider this difference as a clue of the issue.

Another thought occurred to me as well: the ‘stack up of tolerances’. Its sounds like the heads have been rebuilt twice. Based on that i am discounting a ‘sticky’ valve but possible. Instead, if they took of ten or so thou on the head and also on the deck (block) and / or ground down the valve seat too much it’s entirely possible that with a push rod at the high end of the tolerance for length it all adds up to the inability for the valve to close. There are specs for all of this and may have to do disassembly to validate if the above suggestions do not fix.

Question: did this backfiring occur from day one post the rebuild?

While I am not an expert in these matters, I do know this: this problem is solvable. Keep trying and posting and that horse will get up and run and you will have great stories to tell as you cross the finish line!