small issues help needed

[76 searay]

1973 merc, ford 302, 188hp, 21' searay, 19 pitch prop.

I am able to cruise around at WOT which i belive is low at about 3500, for a good 5mins before she backfires and drops about 500-1000rpm. then a few seconds later she does some more backfireing and resumes to 3500rpm with out me ever touching the throttle arm. I am not sure what this means. it just dosent run well at all. She hesitates and some times stall out of the whole as well. I did notice all gauges are reading good but the vacumm gauge reads almost nothing at WOT opposed to the 16-20 at idle. Thanks for any advice you may have for me. I am looking forward to the ocean but just cant trust the boat yet. Thanks...

 

[gcboat]

Re: small issues help needed

Couple of things come to mind. Coil going bad and possibly the timing advance is sticking. That is unless you have installed electronic dist.
Remove distributor cap and just move the advance manually. Should be free.
If not a little grease works wonders. I'd just replace the coil and keep the old one as a spare - just in case. My $.02

 

[Maclin]

Re: small issues help needed

Zero vacuum at WOT is normal.

When did you get this boat and what all have you done to it so far regarding electrical tuneup, fuel filters, checking fuel quality, etc.

 

[Haut Medoc]

Re: small issues help needed

You need to switch props......
You are over propped, you should have a 14 x 17 on there.....
You will gain 200 rpms for every degree of pitch, which will put you at 3800-4200 WOT which is where you want to be........
You are only pushing 188 HP on a 21 footer.....
The backfire is coming from your engine lugging & probably some water in your fuel..........
You will destroy your engine in short order if you continue operating it with that prop.....
Your hole shot will improve drastically as well....
Change the prop & see what it does for performance, then go with the full tune up.....
That includes changing the fuel water separater, checking the screen at the carb inlet & cleaning your flame arrestor.....
Zero vacuum at WOT is normal, btw......​