Thread: 99gp1200 carb rebuild/tune
04-17-2007, 06:52 AM #1
99gp1200 carb rebuild/tune
The engine is stock, carbs never been done, choke plates removed, primer kit, premix. The ski runs fine now but i figure its time to rebuild the carbs.
I have 3 mikuni rebuild kits, 1.5 n/s, a pop-off tester (made from a regulator, 60 psi gauge, valve, etc) and the mikuni manual. In conjunction with this i am going to be doing riva flame arrestors.
I do not have any problem taking the carbs out and actually rebuilding them, i am mechanically inclined. My main concerns are pop off pressure, what spring to use, high/low settings..etc.
Should i rebuild the carbs first before installing the flame arrestors and leave all the settings stock?
Also what should the pop off pressure be on this ski? should i check it before i rebuild the carbs for a benchmark, then make it lower for the flame arrestors(manual says)?
i just dont want to rebuild the carbs and go from a pefectly good running ski to crappy running, but all in all its better than a blown motor in the long run..
any help would be appreciated.
04-17-2007, 07:31 AM #2
The pop off is high on the stock setup, I want to say its around 40 or higher. On my Group K motor, I run the stock needle and seat with one of the silver springs,(this is their carb setup, not mine) I am not sure what gram spring it is. If you get the pop off too low on the GP1200, you will have sluggish low end and mid range performance. These motors vibrate alot and with a low pop off it causes issues that you will not be able to tune out with the adjustment screws. I also have Riva flame arrestors. I guess what I am trying to say is that I would leave the needle, seat, and springs alone.
As far as the tuning after you install the flame arrestors, I would open the highs and lows about a 1/2 to 3/4 of a turn. This will probably be rich, but you can always start coming back in on the screws.
04-18-2007, 12:52 PM #3
ive decided to skip the flame arrestors and just rebuild the carbs with original needle and seats and springs, and leaving all high/low screws as they are.....
what should the screws be with just the choke plates removed?
stock or like an 1/8th out on all...
04-18-2007, 03:45 PM #4
You can still do the flame arrestors, adjusting the pop off just is not necessary. Maybe an 1/8 of a turn with the choke plates removed.
04-24-2007, 05:50 AM #5
well i just got my order and they shipped the flame arrestors anyway even after i had riva take them off the order...........now its very tempting to keep them
i guess i gota decide. What do ya think?
04-24-2007, 07:02 AM #6
Keep them and put them on. There is no need to worry about messing with the pop-off pressure. Before I sent my motor to Group K, I had Riva air filters, with stock pop off. In my opinion adjusting the pop off is over rated, especially on the GP1200. Like I have said before, these motors vibrate so much that too low of pop off causes sluggish performance off idle and will cause the low speed circuit to dribble.
04-24-2007, 09:43 PM #7
yeah i hear ya. i just am new to this but i think i am understanding them more and more. just for fun after i rebuilt the first carb, i played with the pop off pressure springs and with the 1.5 n&s just to see what the numbers were for a referance.
riva tech said to keep the oem spring, n&s size, keep stock mains and pilots and suggested to open lows to 1.25 and highs to 1 turn. also said pop off should be in the upper 50's
i made a pop off tester and with the stock n&s and spring i got a few tests of 62 then after a few more i was geting close to 70 psi before the needle started leaking air.
i dont know why it was all over the ball park but my fuel return plugs that were hose clamped were definally leaking a little so that could be the problem.
with a new 1.5 n&s and stock spring it went way down to 37 psi consistently
then for fun i tried 1.5 and the gold 115g spring and the popoff went back up to 54 psi......acceptable
however after all the playing i still think that i should keep the 1.2 needle and seat with the original spring
going to be rebuilding the rest tomm and then painting
p.s. the f/a's look nice
04-25-2007, 08:16 AM #8
I am glad you decided to keep the F/A's. Going that far out on the adjustment screws will be plenty safe. Do you have an aftermarket tach? If so, you could safely dial the carbs in and go a little leaner.
04-25-2007, 06:25 PM #9
yes i have a briggs and statton small tach. its a little slow reacting but i think it should do the trick
for the highs you just want the highest rpms right?
how about the lows? best holeshot performance? or keep them at 1
04-25-2007, 08:56 PM #10
You are going to have to be in glass conditions to get a good RPM reading. Your going to have to stretch it out for atleast 1/4 to 1/2 a mile at topspeed without coming unhooked to really know how your dialed in. I have a tiny tach and it is also slow (updates every 2.5 seconds). Have you tried the tach on the ski yet? I would be suprised if it works if it is designed for a lawn mower engine. Most tachs are model specific for a two stroke or a four stroke and whether or not it is a single, twin or triple cylinder.
You want the highest RPM's, but you want them substained. If it starts dropping RPM's while your at top speed it is running lean. You may lose 20-30 rpms after a long substained run, this is O.K. If it starts falling off fast, get out of the throttle. If it seems to take for ever for the RPM's to max out you are running rich. If the longer you run the RPM's are slowly starting to climb, you are rich. Remember, rich is better than lean. The difference between a perfectly tuned ski and one that is a little on the rich side is less than a mph.
On the lows, pay attention to how the motor responds when you grab a handful of throttle from idle. If is hesitates and almost dies, it is lean. If it bogs and sounds like it is gurgling, it is rich. Alot of people confuse a hesitation and a bog. A lean hesitation is when you pull the throttle and there is nothing there, it almost dies. A bog is just when it sounds and feels like it is struggling to clear out. Also know that your idle adjustment works hand and hand with the low speed circuit right off of idle. Some times you can cure a slight hesitation by adjusting turning the idle adjustment to the in (to the right). This opens the throttle plates more at idle.
Take your time and you will get it.
Last edited by 97GPSLEEPER; 04-25-2007 at 08:59 PM.
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