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  1. #1

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    GP1200R fuel delivery issues under load

    This one has me stumped, hopefully someone out there might be able to chime in...I got a 2001 GP1200R at the end of last season and just put it on the water this year. It had been converted to premix and looks like it has a stinger 2 on it. It also has a primer. First time I took it out it started right up, idles fine but won't rev past 1/4 throttle. It will take off if I manually pump the primer while working the throttle but as soon as that fuel is used up I'm back to 1/4 throttle max. It will run at this speed as long as there is gas in the tank. I have since checked everything between the fuel tank and the carbs, fuel filter, fuel lines, fuel cock, pickup was already missing the flapper in the bottom but screen was there so someone already pulled it off, temporarily bypassed everything and went straight from the tank to the carb with no change. Carb rebuild time.

    Got it back home and pulled the engine, took the carbs apart and rebuilt all three with Mikuni kits. All three had corrosion in the pump bodies and didn't look like the diaphragms were sealing not to mention #2 carb didn't even have a clear diaphragm but had 2 gaskets so at this point i'm like wtf?? but at least I'm feeling good that i got the fuel delivery issue taken care of. The carbs had stock jetting as well with accel pump but then aftermarket exhaust and stock intake so when i rebuilt the carbs i rejetted to 125 mains, 110 lows, 1.5 n/s with 95g spring and added a riva powerfilter to help out the intake. 1 1/4 turns on the highs and 1 turn on the lows to start. Removed the accelerator pump at this point and checked the pop-off pressures. They were all right at 32psi which seemed low to me for the 95g spring but all 3 were consistent...i ordered 95g and the package said 95g on it.

    I put this all back together and replaced all 3 spark plugs and fired it up on the trailer, starts right away sounds good, idles good, revs up real nice. Took it down to the lake and dropped it in and it starts right up but won't give me more than 1/4 throttle again, take it out of the water and fire it up and it will rev right up no problem. Put it in the water but kept it on the trailer with the seat off and same thing happens, it won't rev up. I noticed some water dripping from the bottom of the exhaust manifold but didn't have a mirror handy to see the source. Hose clamps were are checked and tightened during reassembly so now I'm thinking a crack in the exhaust chamber somewhere?

    So with this long post does anyone have any suggestions or ideas on why it behaves poorly when it is in the water? What under load would cause a fuel delivery issue? At any time I can use the primer manually to get it to rev all the way up but it won't stay for more than a second or two. Could the low pop-off pressure cause this? Is the pop-off pressure even too low? Maybe the package contained the wrong springs?? No idea at this point so any help would be greatly appreciated.


  2. #2
    TX21's Avatar
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    Thought the pop off was supposed to be 45 or so. What does osides stickey say?

  3. #3

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    the sticky said with that setup it should be around 42-45 which made me wonder why i'm getting so low but consistent. If anything i'd be running rich right?...but i'd be running...i'm burning some plates now to do a leak test on the engine, that's the only thing i can think of at this point.

  4. #4
    TX21's Avatar
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    Quote Originally Posted by Lexington View Post
    the sticky said with that setup it should be around 42-45 which made me wonder why i'm getting so low but consistent. If anything i'd be running rich right?...but i'd be running...i'm burning some plates now to do a leak test on the engine, that's the only thing i can think of at this point.
    Well yeah would explain why it runs fine on the trailer but crap under load. Its flooded with gas so much it cant get going. Change the pop off to 43-45 and I bet it will go away. Mine is at 42 and it hesitates just a bit before it hits very hard.

  5. #5

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    I'll work to get the pop-off pressure correct but does that mean i have to go to 115g springs since i just put in brand new 95g springs and I'm only at 32psi? Also if it was just flooded and couldn't get started wouldn't it stay running hard once i got it going with the primer? I literally have to pump the primer to get enough fuel into it to get it to rev up higher.

  6. #6

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    Have you run it with line straight to carbs since you have rebuilt them? Also, 32 pop-off is a bit low for stock but will NOT cause this issue. Low pop off would make it boggy on take off and middle range accel. You could have a big air leak somewhere, and I have seen ignition problems cause similar issues, but the pump primer/run good thing kinda makes that unlikely. You could put the original 1.2 N/S back in to raise pop off, or try the original springs, or stretch the ones you have to fine tune the PSI. Another thing to check would be the fuel tank vent, if it is blocked it could allow the tank to build a vacuum when fuel is used, and slow fuel delivery.

  7. #7
    TX21's Avatar
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    Try turning the lows in a half turn and see if it improves at all

  8. #8

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    So far i've found 3 corroded carbs, a missing pump diaphragm on carb 2, 3 different spark plugs, bent electrode on spark plug 1, and power valve 1 was in 3 pieces....on top of that i may have a cracked exhaust chamber but that hasn't been determined yet! She's gonna need a lot more love than i thought so i already pulled apart the top end...time to order some parts.

    The exhaust chamber is a little sketchy at the moment, i saw water dripping around the small hose fitting on the bottom when it was running but the hose was on securely so i thought it was leaking from somewhere else and just running down to the low point before dripping...after getting it off i was able to pull that small fitting right out with my fingers...is that just supposed to be pressed in like that? Also there looked like a bunch of bondo or something smoothed down between the mount and the middle of the exhaust chamber then painted black to match which i figured was a previous crack and my leak but i chipped that crap off and wire brushed it down to bare metal and can't find a crack or any defect under that stuff for the life of me...anyone seen this before?

  9. #9

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    May have been cracked if water was not flushed out at end of season and it froze/cracked pipe. Isn't there a plastic pipe manifold on bottom? that may be cracked or the o-ring not seated right. I have seen the middle chamber cracked and had to buy a used one of ebay. Sounds like yours may be and they tried to cover it up, everything else was half azz done it sounds.

  10. #10

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    Wow.....The previous owner must have been a real genius....maintaining his equipment like that. It's almost always been my experience that it would never break down for the dummy that that executed all that backyard engineering. The unlucky guy that buys it thinking he got a real steal is always in for a hard row to hoe. I feel for you. ...been right where you are on more than one occasion.

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