97 GTI high speed check valve

[BrianMoore]

Carburetor question about the high speed check valve.

Background. 1997 GTI. Two years ago while camping with the family, son-in-law says what about a family jetski? All the men say hell yes, women say hell no. So my son-in law got this ski and trailer from the original owners for $1. They take really good care of their stuff and have only dealers work on it. This boat had no major work done on it, only maintenance. But our family knows that grandpa, me, can fix anything. It didn’t start 7 years before and was sitting since. The fuel filter was completely clogged. Rebuilt with an OEM kit. Changed out the fuel lines. There was low compression, 110 PSI, on the original motor. But it started and we ran it last summer. Only issue was a bog at ¼ throttle. That was solved by opening up the throttle a lot and wait a second or so for it to rip.

But I didn’t trust that really worn engine. Last fall I picked up a SeaDoo factory rebuilt short block. I was really surprised how well it had been running with rusty cylinders, pistons knocking around, blown crank seals, and .020” rotary valve clearance. I never touched the carb. I broke it in for a few hours and the bog was intermittently there. Last week we took it out on the Columbia and started riding it like normal. The bog came back at times. Other than that it ran really well. With the rotary valve fixed, it starts really well when cold. Starts instantaneously when warm.

I was coming back in for the last time to load it up and it died about 10 feet from the dock. Wind is about 35 MPH pushing me back into the channel and the ski refused to start. It would fire for a few cycles with the choke on but wouldn’t run. Then the rescue adventure began as I floated downstream. Back at home I tore back into the carb. I really expected clogged transfer ports or a clogged filter. All those were good. Finally I noticed the high speed mylar check valve was curled and not sitting flush with the block. The little piece of stuff under it was a bit of Cristolube. It is a silicone grease I put on a couple o-rings to make it easier to get apart if I was ever into the carb again. I am never using that again since it never deteriorates in fuel and even using it very sparingly it ended up somewhere else. I gently got the speck of grease out and the check valve was still curled wher it had not been curled after the carb rebuild. Removed and reinstalled the check valve and it was still curled. I got the original check valve I had saved and it went on flat as it should. Ski fires right up now. I have not taken it out on the water yet so I don’t know if the bog is gone.



Question one is why did the mylar strip curl? It is genuine Mikuni so I don't think it is a quality issue.

Second question is about accelerator pumps. Seadoo started using them in 1998 from what I see. Was that to overcome the bog I am seeing on this boat with no accelerator pump?

Brian

 

[mikidymac]

Not sure as I haven't seen one curl like that.

The reason for the accelerator pump was for emissions. They could jet them leaner at idle and still make them run.

There is no way to eliminate all bog with a carb as it takes a minute for the air velocity to increase and fuel to start being drawn in. Acceleration will actually be faster if you progressively increase throttle as opposed to just hammering it wide open from low rpm.

 

[BrianMoore]

Thank you mikidymac. Few words but words that made me think. I had to go back and read the Mikuni water craft owner’s manual. Accelerator pump carbs the year after mine increased the pop off pressure and increased the low jet (leaner) and leaned out the high adjuster ( zero turns out). At least that’s the way I think I understand it. During break in I wasn’t moving the throttle fast so that’s why the bog was not noticeable most of the time.

I am still thinking that warped mylar had something to do with the silicone grease I used. It is just squishy grease but the mylar has almost zero strength to flatten it out. I’ll get it back out on the water and see how it does and report back. Mostly I want the bog to go away the majority of the time. The owner’s manual said the check valve is to prevent the low speed circuit from sucking air from the high speed circuit at throttle openings less than 3/8. The constant bog may have been a sign that the first rebuild I did was never right. The old check valve makes it start and run on the hose. I want to make sure it keeps doing its job.

I also read your carb rebuild post. Well written, good pictures.

Brian

 

[BrianMoore]

I took it out and it runs same as before. Most importantly is that it is running.

I spent time messing with the bog. Bring the throttle on slowly, it is a pretty long bog. Hold throttle at ¼ for some time, then nail it, the bog is barely there. It seems like the bog is right around the ¼ open position. Seems lean in the transition. The transition holes in the carb are clear as I can get them and they all look to flow freely.

I am a retired engineer so I try to get into minds of the guys sitting 40 hours a week at desks working on SeaDoo carbs 26 years ago. I made this sheet to see how they changed settings. I read 1999 was the years the EPA started mandating emissions on PWC and 1998 on outboards. Mine is a 97 so the first year of this single BN-40I on the 717.




1998 they fattened up the low speed from 67.5 to 70 and opened up the adjuster ¼ turn. Also they added the accelerator pump. They leaned the high speed from 175 to 167.5 and closed the high adjuster. My thinking is they worked on the lean bog down low and also leaned the high end for emissions.

1999 they went a little further on the low end by going from a 70 to a 75 and took out ¼ on the low adjuster. They kept it adjusted this way until the end.

I really thought about what I could do to this carb to bring it up to the later tune to see if the bog would go away. Two issues with that. One is the lack of an accelerator pump. Second is mine has a broken choke flange on the the back. The head to flame arrestor bracket wasn’t bent just right and there was a little preload in it. I didn’t do anything about it first time I pulled the carb since everything had been good since the factory installed it. That was a bad decision. After I found the broken flange during engine removal, I tweaked the bracket so there is now zero preload. Then a little JB weld on the flange would keep it in place, right? Wrong. JB weld doesn’t fix it. So I have a carb supported by only one bolt on top. This is the point where I have decided go down a new path and I bought a new carb from OSD.

Here’s the question. The OSD carb comes with the 99 specs, 75 pilot and 167.5 main, and an accelerator pump. Does it make sense to open up the main adjuster slightly or go to a bigger main jet to keep it off the ragged edge of lean? Maybe a better question would be are the later specs really close to being too lean on the high end?

Brian

 

[mikidymac]

If you have the newer carb then run the newer specs.