Thread: Need help with bucking problem
08-28-2006, 11:19 PM #1
Need help with bucking problem
Well, I need the pros to help me figure out what happened to my handling. I was running consistent 73+ mph runs with 800 grate and a skegless rideplate and was playing with a skegged rideplate to get the correct angle to match the skegless speed. At the same time I took off the pump shoe and raised the rear about 30 thousandths. It now steps up about 65 thousandths in the front and 50 thousandths clearance in the rear. It was originally around .055" front and .020 in the rear for clearance from the botttom of the hull... I also modified the 800 grate by sharpening the leading edges and cutting the rear ramps back a bit, but not completly like a 1200 grate. Now I have a nasty buck at about 72 mph and am unable to decide where to go next. I've tried running stock trim tabs "the fastest", riva moded trim tabs, and shimmed riva trim tabs (2 washers in front and 3 washers in back) They were running similiar speeds, but I had the buck regardless of trim tab shimming. The ride plates are between 4-5 degrees of angle. I didn't have this problem until the pump shoe mod and intake grate mod. Please help me tame the bronco....
Last edited by jgustman; 08-28-2006 at 11:21 PM.
08-28-2006, 11:25 PM #2
"I didn't have this problem until the pump shoe mod and intake grate mod."
you answered your own question - try a new grate 1st
Last edited by philip_gpr; 08-28-2006 at 11:37 PM.
08-29-2006, 12:49 AM #3
Well I modded the 800 intake by just sharpening the leading wing egdes and verticle divider edge and then cutting back the rear ramps, I cut the outside ramps back about 7 mm. and making all the ramps square across the front instead of curved to let more water in. I didn't make it look like a 1200 grate completly, looks like I need to, but didn't think that would cause a bucking issue...
I remeasured the pump shoe, and it's sitting up in the hull approx .070" front and .050" to .060" in the rear (left rear corner has less clearance). Originally it was .060" front and .030" in the rear.
08-29-2006, 07:39 AM #4
I cut the leading wings of an 800 grate also and had a real bad time with the buck. Don't think it's your shoe I would agree with philip I would put a diffrent 800 grate on and see what happens, But I think your buck is coming from cutting back of wings on the grate.
08-29-2006, 08:03 AM #5
I would not do any modifications to any grate.
08-29-2006, 10:55 AM #6
I need to clarify that I didn't cut the leading edge or tips back I just sharpened them. I only cut the rear ramp that meets the pump shoe back, but not as far as a 1200 ramp is...
08-29-2006, 11:41 AM #7
use a standard grate with no modifications.
9 times out of 10 the buck is caused by the pump loossing suction/feed. this is mainly due to the angle of the hull in relation to the pump inlet and waterline. however, in your case (as this issue surfaced after the modification), when you sharpen the grate you are slightly redirecting the waterflow at the impeller. depending on if you took material off the bottom or top of the grate (or both) you effected the waterflow to the impeller causing the buck from either over flow or lack there of.
if you didnt have the buck before you sharpened the grate and modded the shoe...well, you answered your question yourself.
you can get close to fixing the buck by moving the grate back tight to the shoe,,,however, i dont recommend that as you will only be masking the real issue at hand.
i used untouched grates on my 81mph single. whenever i started to sharpen the leading edge i found other "issues" pop up. de-bur and gently "clean-up" the castings to make it flow as it should IF it came from a "PERFECT" casting process. 300 grit the surface in the direction of the flow.
08-29-2006, 11:42 AM #8Originally Posted by jgustman
08-29-2006, 12:23 PM #9
Thanks for the info, the grate has been sanded in the direction of flow down to 400 grit and wd-40. Question is what will happen if I re-dull the leading edges if that's what caused the issue. I'm wondering if it's the ramp at the rear with the leading edges squared off and sharpened, and the outside ramps not even with the center (like a 1200).
08-29-2006, 01:29 PM #10Originally Posted by jgustman
it will be very difficult to "dull" the leading edge as it was. especially since material has been removed, which effectively removes (or lessens) the amount of leading edge (of top loader blade) penetrating the water for load.
your best off getting a new grate and starting fresh.
the only reason we cut the side wings back was to help elleviate the buck (if it occurred). since then we found the buck was caused by one of the following:
-shoe plug pushing the shoe away from the hull (shoe not flush to hull)
-shoe castings AND boat moldings slightly differ from one to the next. some shoes may need to be shimed or cut to obtain the correct flow to the pump.
-intake grate variations in castings can cause the intake grate to be to deep in the water, not deep enough, or to far forward in the intake tunnel. 9-times out of 10 the grate needs to be moved slightly back and centered in the tunnel.
there is other reasons for the buck....but the above is the main culprits to be checked first.
some boats buck at 68, some at 71, some at 75, others at 80+. I had a buck at 71.8 on my stage 2. cut the wings back and it went away but 71.8was it (or somewhere in that area). i "thought" once i could get the boat to settle at 71.8 it would peak .5 to 1 mph higher. it never did.
i went 86mm lamey big bore single and had to up pitch the prop. hit 74 and had the buck. cut more of the wings back and lost speed and the buck. pulled the shoe off and found the PPK pushing the shoe down/not allowing it to sit flush against the hull. i ditched the PPK and went back out. hit 74 with no buck and the trim all the way up. got a new grate and hit 75.something (this was 4 years ago). this was with a island full angle mod with two shim on the shoe side.
went to triple pipes....had to adjust the prop to get the rpms right and had a brutal buck at 79-80. removed one washer from the ride plate and the buck went away (decreased hull angle at a higher speed)
back when i was cutting wings (2001-early 2002) i also tried to shim the front of the intake grate down by as little as .005 to as much as .050. shimming the front down got ride of the buck AND also scrubbed speed. shimming the front of the grate down backed up my theory on the grate loosing suction causing the buck....also backed up by a carl at island and bob morgan @ shred...hence his plate attempting to get the hull to ride at a neutral position....
with all this said,
-IF the grate is loosing suction at high speeds with the boat at full angle/trim
-IF the boat looses speed but the buck goes away with the grate front shimmed down
move the grate closer to the rear of the hull (tight to shoe, grate ramp to shoe ramp surface) to further plant the grate in the water at full speed/full trim.
with all this said, as i give away my specs, my 81.8 (multiple) runs on my single last year was with the SAME island plate i ran on my single big bore back in 02 (double washers in the front @ 75mph) and the same as when i ran triple in 03 (with a single washer@ 81-82mph)..the difference? my 81.8 single runs where with both front washers installed, the pump shoe tight to the transome and the grate (untouched, sanded 300 grit) tight to the shoe....oh yeah, and an unplugged/sealed pump (imho, is a money making gimmick for those who sell ppk kits).
so, i have learned that a standard untouched grate can run 81+ mph so long as everything behind it is in harmony.
and...for those that say why was your 86mm big bore so much slower in 02 vs 05? for one, when i first went out last year my boat ran 76 (very similar pump, ride plate, and grate configuration). lowells motor had a solid mph gain over my lamey ported motor. the rest was all hull mods and aligning the pump pieces as i described above.
Last edited by HopatcongGPR; 08-29-2006 at 01:32 PM.
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