NEW BOATER WITH ISSUES.... NEED PROP OPINIONS, MAYBE MORE

[IIslow66]

I'm new to boating and now to this site, although I've already searched and learned a lot! So, I picked up my first boat 18' open bow bayliner with a 2 barrel 4.3/ alpha one and a 17p prop. Issues first were misfiring/backfiring at 3/4 throttle. The marine shop the previous owner took it to replaced;fuel filter, plugs, rebuilt carb and it still had issues. I purchased the boat as is and I did a little visual inspection and found cylinder 4 plug wire really corroded. Installed a cap, rotor, wires, new plugs gapped to.035, set idle to 650rpm and timed it to 8? btdc and fully advance hits 26? now it runs like a champ but will over Rev heavily at full throttle. I can hammer it and get from 4500-5500rpm in a second and the engine pops /backfires at higher rpms. I know max rpm is 4300-4800. The engine never backfires/pops/misfires between 650-4800rpm after my tune up. At 4800 I can only get 30 mph on the GPS. Questions; does the thunderbolt IV have a Rev limiter causing this popping/backfire? Any opinions on prop or anything else to get this thing working more efficiently?

 

[CV16]

I can't answer your rev limiter question, but I'd try to borrow a 21 pitch prop and see how that works out for you. Keep the 17 for pulling rubes or skiers but watch the tach and don't over rev the motor. There are reasons they have a max rpm.

 

[steelespike]

It appears to me that a 2 barrel 4.3 would have a 1.81 ratio.With a 1.81 there is about a 20% slip and at 1.62 slip would be about 30% at 4800 30 mph.
The numbers make me suspect the tach is not accurate.

 

[SeaDooSam]

It appears to me that a 2 barrel 4.3 would have a 1.81 ratio.With a 1.81 there is about a 20% slip and at 1.62 slip would be about 30% at 4800 30 mph.
The numbers make me suspect the tach is not accurate.

+1 on that. The tach in my boat is off as well. I would suggest trying to find a prop shop near you that will let you experiment with different props. If you can't find a shop I would try to find a 19 pitch or a 21 pitch as CV16 said. Plus a higher pitch will make it go a bit faster. Just make sure the Rpms don't drop below the correct range as this will put strain on the motor.

 

[IIslow66]

Thanks for the reply guys! The tachometer is surprisingly 100% accurate. I checked it against the snap-on digital timing light from idle to WOT. The prop on it is in rough shape, chips gouges etc. anyone experience prop hub slip? Could this be occurring when I get the miss?

 

[SkiDad]

i would say that prop is too low of pitch - i would try a 21p. I doubt you have a rev limiter so you want to get it within the range.

but first, if you want to check for prop hub slip you take the prop off and mark the inner and outer hub (bottom of prop) with a permanent marker - go for a spin and see if the lines stays lined up.

http://www.boatingmag.com/how-to-tell-if-youve-spun-your-propeller-hub

 

[Sea Rider]

Waht's the factory wot min-max rpm range for that engine ? when saying that engine over revs, which was the rpm achieved, assume full hammerr down, right ? were you boating lightly, medium or fully loaded when that happened ?

Backfires has nothing to do with props, it's an engine issue, probably carb and timing synch is not spot on synched, incorrect plugs ?

Happy Boating

 

[IIslow66]

Waht's the factory wot min-max rpm range for that engine ? when saying that engine over revs, which was the rpm achieved, assume full hammerr down, right ? were you boating lightly, medium or fully loaded when that happened ?

Backfires has nothing to do with props, it's an engine issue, probably carb and timing synch is not spot on synched, incorrect plugs ?

Happy Boating

Thanks for the responses!

Factory rpm range is 4300-4800. If I hammer down with 2 people in the boat not pulling anything the engine would hit 5500-5600 and stay there if I didn't let off. The engine literally couldn't be timed any better. 8? btdc idle and 26? fully advanced. I just picked up a 21p prop and I'll give that a shot.

 

[Sea Rider]

Lets hope that with a 21 pitch as previously loaded, wot revs doesn't descend towards the 4300 lugging zone, probably a 19 pitch would have been better than a 21. Tach will tell...post wot results for the record..

Happy Boating

 

[theBrownskull]

I had the same engine in my other boat. I ran a 21 pitch aluminum and a 19 pitch stainless. I tried a 17 but it would over rev.