05-12-2011, 09:20 PM #1
Bogging down once warmed up good... but cause is not the VR?
1996 GTI with 720 dual carbs. Towards the end of last summer it began cutting out at 1/2 throttle or greater. The first 10-15 minutes of riding is fine, then the problem occurs and does not go away until the engine is completely cool again. I know the Voltage Regulator (VR) is usually the culprit with this symptom... so I replaced it late last summer. The ski ran fine the very next time I had it out. Then it started doing its bogging down again. I thought I might have fried the VR due to a bad coil, so I winterized it and put the ski up since the riding season was over. So today I had it on the water for the 1st time this year. Just wanted to see how it ran. Same thing- 10 minutes of running great it starts to bog. So this time I unhooked the red wire on the VR and ran it some more. Still had the problem. So I can rule the VR out now, right? It is likely working fine.
I just rebuilt the top end at the start of last year, and compression is still good. Gray fuel lines have been swapped out. I cleaned and rebuilt the carbs at that time with genuine Mikuni parts... but I thinking I'll check the little filter screens again just for grins. (But it runs at low and mid range just fine, and high range just fine before it gets hot. So I don't really think those carb filters are my problem?)
Is the MPEM possibly screwing up? I've got an 1997 XP that I use for parts. I think the MPEM on that one is compatible with this GTI. I'll have to check & I might swap those out. I'm just racking my brain this evening trying to think of what to check next!
Comments and ideas are appreciated.
05-12-2011, 09:43 PM #2
check the carb screens again. when I did my carbs I found debris from cutting new lines in the screens. The MPEM is not compatible with a 720.
05-13-2011, 09:14 PM #3
Feel the underside of the rubber hose between the pipe and waterbox. Sounds like it could have gotten weak with age and blown a small hole.
05-15-2011, 04:09 PM #4
I've checked the rubber hose between the pipe and water box. Very good condition- with no tears. (This is a super clean ski. Inside still looks basically new.)
I tore into the two carbs. Both screen filters were perfectly clean. However, I rechecked pop off pressure. They both pop off within specifications at 50 psi... but I found that the Mag carb (1st carb) will not hold pressure on its own. (Supposed to hold a minimum of 10 psi for at least 30 seconds. This one drops 2-3 psi every second with all other fuel ports blocked off.) A little fogging oil around the needle and seat while under pressure gives air bubbles too. I swapped the needle and seat from the PTO carb into this Mag side carb... and it would still leak around the needle and seat! I tried a couple of new Mikuni gaskets on the seat too. Has me puzzled for the moment for why it still leaks? (No... the spring and arm are not lifting the needle up.) I'm wonder if the seat hole is wobbled out?? Maybe a slightly bigger gasket will work.
Anybody know the size Needle and Seat to use on a stock 1996 GTI? (Carb is Super BN-38I) I can't find that info in the Owners Manual.
05-15-2011, 04:24 PM #5
Well... a little more internet searching and found it: 2.0mm is the stock needle size for the Super BN-38I.
~ 3 minutes later ~
Dang.... just found another site that said it was 1.2 mm!!! Guess I'll keep looking.
05-15-2011, 08:40 PM #6
Does it do it on "res" and "on"? Does it do it with the gas cap off or loose? Maybe your tank vent is plugged .
05-16-2011, 12:21 PM #7
I'm not hip on the 720 mtors but my 787 had the same symptoms & I swapped everything that I could think of but still had the issue. It ran fine for a few minutes till it warmed up & then it was down 500 rpm... Turned out to be the pickup mount was cracked & when heated up it would act stupid & throw the timing out of whack. I don't know if the 720 motors have a timing pickup or not but it's worth looking into
05-16-2011, 04:06 PM #8
Yes... the 720 has a pickup mount coming off the coil. Now you're back into where I originally thought the problem could be too. It is just a pain in the butt to get to the generating coil. (Nearly easier to pull the engine and place it on a bench than try to pull the flywheel off in the craft. I've got a flywheel removal tool and maybe this one will not be stuck on too badly. Some other engines I have rebuilt were definitely not the case.) But it has to be done! Not much fun at all trying to ride it and have it bog down like this.
05-16-2011, 04:35 PM #9
I was getting ready to sa Eff it and sell it. I replaced everything you could think of and spent a lot of money only to get pissed off on the next ride..The only reason I found the problem was because the ski just stopped running at WOT & was dead. Got home & took off the mag cover to check the fly wheel & there was the pickup stuck to the cup.. it was the single bar mount and you could see where it was cracked & rubbed smooth then the end that was roughed up. Replaced it with the newer style mount & BAM! It's been 4yrs and still running strong. A big help would be to warm the ski up on the hose & use timing tape on the pto & see if the timing gun jumps all over the place at 5+k rpm, that will tell you right off the bat!
This problem like any electrical problems are the worst to trace & you only find the solution by accident.. LOL
05-17-2011, 09:16 AM #10
Well... I was wrong. The 720 does not have a pickup from the coil. It simply has a magneto / generating coil. (I had pictured my SPX's 787 engine in my mind when stating that earlier.) I got the flywheel on this 720 engine pulled off last night. Did it in the craft and did not have to remove the engine. There is not an ounce of rust or residue in this little 1996 ski! Super clean inside and out. Everything "looked" fine... but apparently something wrong electrically is still going on. I'm debating swapping out the coil, but the cheap side of me just doesn't want to start replacing parts for the sake of replacing parts!
Other thing I remembered while working on it last night... about 2 years ago on this ski I swapped out the MPEM. The old one was instantly cranking the ski anytime the battery was connected. I absolutely could not make it stop doing that. Swapped the MPEM out with what was supposed to be a compatiable one from an old XP. Guess I'll spend some more time making sure all the MPEM numbers cross referenced correctly. The ski ran great after swapping the MPEM out that time. It wasn't until many rides later the bogging down probelm started to appear.
My son asked me last night... "Dad, why don't you sell all these older ski's and buy us 2 used Honda Aquatraxs?". My reply.... "I've got less money tied up in all 5 of these old SeaDoo's than I do a single Aquatrax." But this frustrating electrical problem does make me think twice! (Still have to tackle seeing what going on with my other 1996 GTI too. Just rebuilt that engine and it wouldn't stay running well or handle any throttle while in the water. But I'm hoping it just has fouled the plugs. Kind of looked like it after getting it home. I was running 32:1 pre-mix in it to break it in.)
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