berkeley 12jc-more speed

**kcccsryan** · 09-15-2006, 04:57 PM

What is the BEST way to get more speed(top end) from a Berkeley 12-jc w/A2 impeller. has old single piece bowl and no jetovator.Running a Ford 460 4bbl. Changed to 750cfm and have no problem getting over 5000rpm's (before you know it-try not to run that high very often).
Before carb change ran 4200rpm right at 60mph gps. The engine had work done before I purchased the boat. Have not checked speed w/new carb.
Family will not even go with me(say too fast already) to the lake -More fun for me.
I have no problems w/control. the boat is very stable and handles very well(also have 21' formula i/o 350 mag).
I am trying to get as much SPEED as possible. Want it to scare me too!

Any info will be helpful.
Live in E. Tenn and ther are not many jets around here.
Boat is a 19' (1979) Spectra
Thanks

**nitrousdog** · 09-15-2006, 07:50 PM

Mount a guage on the intake side of the pump and see what kind of intake pressure you have. With that big of an impellor I bet you are running out of water turning that kind of rpm. A different kind af grate might help. You need 10 to 15 psi to load the pump properly. If the pressure is not there the motor turns to many rpms and waste power. My boat runs the same impellor and turns 6000 rpm at 105mph. 700HP

nitrousdog

**kcccsryan** · 09-16-2006, 03:36 PM

Thank-you for that info. I will check and go from there. Will a split bowl make any difference(currently use single piece [old bowl]? I have not checked speed WOT since new carb.
Thank-you again

**nitrousdog** · 09-16-2006, 07:05 PM

A split bowl will allow you to use a droopsnoot to get the thrust lower, closer to the water. Tunnel hulls especially benifit from this. A scoop grate may help you as long as you are not scooping too much water. An intake pressure gauge will tell everything. Do you have a ride plate and what angle is it set at?

Nitrousdog

**sleekcrafter** · 09-17-2006, 08:33 AM

Over 5000 rpm seems a bit high for a 460 turning an AA impeller. First off verify the tach reading is correct, most likely your running an A impeller. Secondly, Check the impeller and wearring for propper clearances, as the impeller is susecptable to damage from ingestation. thirdly, check the bottom of the hull, for hook. The hook is a big killer in speed, it forces the nose of the boat into the water more, the faster you try to go.

**kcccsryan** · 09-17-2006, 11:24 AM

Thanks to both of you for the info.
The jet intake is stock to the best of my knowledge with no ride plate. Had the bowl off this previous spring and the impeller shaft seemed to have some looseness-about 1/16 inch. Should the bearing and wear ring need replacement? Pretty sure about impeller size .Has A-2 stamped on it.
Also, the keel looks straight and has great handling at any speed Hope to get some pics soon. Boating season is over here except for the die hards-we get the lake to ourselves-no kids w/jet skies. the lake is run by tennessee valley authoriy is is lowered 40' in the winter months.
Thank-you again for the info!!!
Kevin

**sleekcrafter** · 09-17-2006, 11:44 AM

to check for hook, use a 6 ft straight edge, check the entire back of the boat from the keel out to the sides, holding the straight edge fore and aft . Lots of boats had hook built in, to help the handling of the boats. Note most Jetboats had no trim to adjust the ride, so hook was built in to improve the ride, but sacrafised the performance on the top end. Play on the shaft, there should be none fore and aft, a little up/down and side to side os ok. The wearring does wear, as does the impeller wearring surface, eccessive wear results in poor holeshot performance, and the sensation of slippage when you hammer the peddle, and most of all top end performance suffers too, most niticeably is the rpms's are sowly increasing over the years, and the speed is dropping. The impelller may say A2, but there is a good possiblity it's been cut down, from it's original size, you can go to the cpperformance website and see the Berkeley impeller size chart, to get an idea of what to measure to check the size.

**kcccsryan** · 09-19-2006, 03:18 PM

The tach was not reading correctly -it was actually 4600 rpm.
The shaft looseness was up/dwn & side to side-none front to back. I was kind of worried about the play and am relieved with the knowledge of that info.

Thanks again
Try to add picture today

**sleekcrafter** · 09-19-2006, 06:40 PM

if your looking for more speed, you will need to cut down that impeller, to an A or A/B cut. You will get about 250 rpm per size cut, on an impeller. If your boat is not too heavy, go with the A/B cut, other wise don't go less than an A cut, as it will cavitate out of the hole.

sleek