Depends on how quick you are with a Fire Extinguisher. LOL

Seriously speaking...with marine Flame Arrestors your biggest risk is float bowl overflow and flooding the bilge with fuel.

How is is coming along now ? Certainly wish you well with this.

imo,650 is too small.i would go with 850,you really can,t over carb a blower motor.

Two 850's??

yes.

X2. You definitely need more carb regardless. I normally have a tendency to under carb however on blown application like Mike said. Also if you do end up with larger carbs go with DP.. Btw you went over everything very closely and have no vacuum leaks? You need to make a list and go through everything one by one.
Back to the carbs I wouldn't go anything smaller than 800's amd make sure their boost referenced. Just don't let the black plug let you think you have plenty of carb. That's just telling you something is way wring and not firing properly and carbs out of tune/timing etc.

Do a research on Google or even here on oso for similar builds and ck for normal carb set up. I'm going to experiment with a tunnel ram and two 700's on 496'. Set up is for 565 builds but...

Thanks for the input guys. I'm listening. But I think the carb size is not related to this problem. I think it would run on 600's or 850's. The holley 600 blower carbs came in. Installed them. No difference. Changed the plugs out to AC Delco R44T gapped at .045. No change. Plugs still black, backfiring out exhaust, and backfiring out carbs when given throttle. We're gonna change out the distributor and coil from right side motor and see if anything changes.

Or let me ask you guys this, don't you think they'd run on 600's? Or do you think that is part of this problem?

2 600's on each engine? Should be enough. Do a compression test, put a vacuum guage on it. Is this same issue with both engines?

Yes, two 600's. We just swapped the distributors and coils. Same thing. We're only running the left motor right now. These engines only have prolly 2-3 hours on them. All plugs are equally black. They ran strong in november. We havent run the starboard motor yet with a blower. I think we're gonna pull the blowers off and put the air gap intakes and single 750's back on and run it for a while. We're supposed to take it to the beach this weekend. It's getting down to the wire. I wonder if the cam is so far off for a blower setup that it just wont run??

I was going to suggest going back to naturally aspirated since you're in a tight time wise for your trip. If they run ok natural then you know for sure there is a bug in the blower set ups some where.