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?