06-08-2009, 12:31 PM #1
- Join Date
- Oct 2008
gp1200 fuel pump issues and solutions... long.
I realize im not a frequent poster here... but Id like some advice or comments.
I've got a 99 GP1200 and i am annoyed with the fuel pump issue that is apparently common but not well documented or described. the fuel pump check valve on the inlet side of the pump is creased/crushed.
here is the story... so i noticed i had some dribblers.... carbs 1 and 2 were dribbling outta the venturi's... so i replaced the needle/seats because this is the common thing right? well i test the pop off and for leakers and everything is AOK.... so i slap them back on the ski.
now they are dripping again... the #2 is dripping as it was before, but #1 is now litterally pissing little jets of fuel out the venturi! WTF!
i tear the carbs apart again and check the pop off, looks ok... so something else has to be screwed... i stop and think and it must be building too much pressure, i think for a sec that the restrictor in the carb must be plugged and over pressurizing the carb... i rip the carbs apart and no the restrictors are clean, but now i notice the inlet checkvalves are all damaged............
I then proceed to spend the rest of the night (5 hours) searching the web, here, sbt, pwctoday, riva ect... as to the cause and correct solution of which i never found.
i've read many different opinions on why this happens but many are just "ideas" or myths based on groundless facts. Im gonna try and solve this atleast for me, permanently.... no more bullshit, i want solid reliability with no "every 50 hrs rebuild the carbs" as every other part on my carbs was perfect... machine has 110 hours.
What i see is the check valve is crushed.... by what appears is ALOT of pressure.... people claim its the choke that causes this during starting... i doubt this because the choke provides suction to the fuel holes, which is suction to the regulator section causing the N/S to open applying suction to the fuel pump which would cause it to open both checkvalves and SUCK FUEL, not force them closed with great pressure.
Others say that it occurs when there is alot of pulse pressure from the crankcase that causes this backfires, filling the engine with fluid ect...... probable but that would require that some other means to cause a pressure build up in the pump.... ideally if the pump pumps hard, the excess fuel is dumped to the return line.. so is the return restriction the fault?
I noticed that the check valve sealing surface has not alot of supporting material for the check valve disk. which looks like it would easily be crushed. but again this would require substantial pressure to crush the disks.
this all keeps leading back to a extreme buildup of pressure in the pump... why?
working this out in my head it seems if the diaphragm regulator chamber is full of fuel, the needle/seat is closed.... due to the high stock pop off of over 50 psi this means it requires alot of pressure to force more fuel in that direction.... but look at the fuel return restrictor.... its tiny... smaller then 30 thou... the smallest drillbit i have. very small.
looking again at the whole setup... we have 3 large carburators, one per cylinder. originally these carb designs were for 2 cylinder motors, one per cylinder. so it seems to me the fuel pump is designed for ALOT of fuel flow, with minimal returned excess....
so it seems that there is alot of fuel pump capacity since we have 1 pump per cylinder.... and it also seems we have periods of very little fuel usage.... like idling, and slowing down from full speed.
so working this all out in my head its almost like the fuel pump is trying to force more fuel then the engine needs either through high pressure pulse spikes from the crankcase, or whatever, but cannot get rid of that excess fuel fast enough so it causes the pump checkvalve (the inlet one) to fail... because that is the only way to low pressure since the return restrictor and needle/seat are in the way at the other doors and they are far stronger.
so what should we do?
i hear some double up on the inlet check valves to strengthen them. seems like a good idea but does not remove the cause... seems to me that this is a bandaid approach. fix the symptom, not the problem.
i hear others get rid of the internal pumps and use an external triple outlet pump, still the pulse style pump. seems to me this could work, but again this might still have pump damage again. causing now all three carbs to loose pressure instead of one or two.
how are either of these solutions? are they reliable?
if the cause of the damage is due to pressure spikes in the pulse lines... then why bother sticking with a pulse signal in the first place. has anyone fitted an electric fuel pump for the carbs? easily and cheaply had and will allow fuel charging even with the engine completely off, making starting far easier.
Id imagine this would yield much more consistent fuel pressure across the board aswell..
anyways... im looking for advice, comments, and help with this issue as id like to have a bullet proof ski, that wont blow a piston or something due to a 1 cent part failing.
06-08-2009, 07:27 PM #2
Rebuild the carbs with genuine Mikuni kits.
Remove carbs during off season to inspect check "valves" and drain fuel out of them.
I could get at least 40 hours on a set. How many hours do you ride during a season?
Keep it simple.
06-08-2009, 08:54 PM #3
When the Mikuni I-body carburetor came out this issue was addressed, They used a larger check valve (20mm) compared to the older SBN's (18mm) the size of the sealing surface is the same on both units. I tried to use the 20mm check valves in the older carbs but they would require modification to make them work.
To address the dribbling issue which has nothing to do with the fuel pump, I have done the following, I run 1.5 N&S with a gold 115gr spring, I double up on the check valve under the jet cover block. This has helped quite a bit. The 1200 motors that I do this on are known for their violent shaking especially at idle. I also do a mod to the fuel pump which helps prolong the life of the check valves, I drill the returns out and add an inline restrictor.
06-08-2009, 08:58 PM #4
- Join Date
- Jul 2008
- Chesterfield Virginia USA ~ On the James River~
Or run a external fuel pump so that when the check valves crease you dont have to remove the carbs.
06-08-2009, 09:04 PM #5
there was a valve body update later on ..a new checkvalve body with more or wider webs. I have a carb rack with them in my spares bin..they were only 20 bucks each.
With 3 gp12's i didn't get a lot of hrs on any of themj..not over 60-70 before i sold them.
I found after using a primer I didnt have the problem after 50hrs..didn't keep the ski's long enough to check after that..
But i also did a few of the things Bill does and they work great.
06-08-2009, 09:39 PM #6
- Join Date
- Oct 2008
ok well here is the scoop to my frustration.... last year,i bought the ski with a blown motor (engine blew up due to a screw injestion not carb related). i did a full top end on the motor new gaskets ect and i rebuilt the carbs. this was late august.. it ran decently. i rebuilt the carbs with everything except the needle and seats....they were not supplied in the kits. when i did rebuild the carbs, the original pump valves were damaged.
this year i take it out and it begins to idle poorly and rough after about 4 -6 rides.... so being the good boy i am, i look in the carbs to see the venturi's dribbling.... i read up and find its a n/s problem... this equates to what i did not replace in the carbs during the rebuild.... following what was common practice for people, i replaced the n/s with a 1.5 and a gold spring. i checked the popoff before and after.... before it was high 60's to almost 70 psi... with the new 1.5 n/s and gold spring im hovering around 58-60psi.
so i reassemble... put them on the ski, take it out. runs like balls... completely over fueled at low speed. look at the carbs again and like i said before. carb #2 is a dribbler.... carb #1 is litterally squirting a jet of fuel from the venturi... this is no high speed jet checkvalve failure... it is seriously over pressurized in the regulator section.
i take the carbs off the ski again, and like i said, i go through the pump section to find all the inlet valves crushed... i check the pop off on all the carbs and they all seem ok no problem, no leakers.
from last year to this year, id estimate the ski saw less then 15 hours usage. this is what frustrates me, it crushed the valve so soon and so badly.
im thinking about what i should do, if i rebuild them again thats 40 bux per kit, only to have them get ruined again at a moments notice. i just rebuilt the motor and i do not want it to blow up because of these valves, im seriously considering running an external pump.
the ski runs very well when it was working so i know there is no engine problems, im seeing 120ish psi across the board on compression tests.
06-08-2009, 09:47 PM #7
Sounds to me like the squirter is an arm height issue, possibly the same on the dribbler. what year ski is this on?
06-09-2009, 06:36 AM #8
- Join Date
- Oct 2008
1999 gp1200. standard mikuni SBN carbs, not i-bodies, no accelerator pump.
06-09-2009, 09:18 AM #9
I am fairly familiar with that model.
06-09-2009, 09:32 AM #10
- Join Date
- Oct 2008
what would you consider an effective, lasting repair?
im hesitant about just rebuilding the pumps and putting it back together if its just going to ruin the inlet checkvalves again since reading up alot it seems this model has that tendancy.
i figured the 1.5 n/s with the gold 115 spring would have cured the dribbling issue.
do you feel that adding an extra anti siphon valve in the jet/metering block would cure this dribbling issue?
i guess what im saying is, despite the dribbling issue im more concerned with the fuel pumps dying out again and possibly ruining the motor.
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