97 XP 787 - Will a bad DESS post cause backfiring and resulting engine damage?

[670hoth]

If you don't want to read my long winded story below then please just answer the following question: Can a bad DESS post cause the engine to backfire while riding in the upper RPM range, push out and bend the rotary valve, scrape up the RV cover, and then cause the engine to quit?

If you want to read on below is my long story below!


I bought a 97 XP last year cheap and as expected (with cheap) it has been a project with numerous problems. When I bought it, it would start fine on the trailer but was almost impossible to start in the water. I changed fuel lines, cleaned out the tank and put in new gas and took the carbs apart and cleaned them but it didn't make a difference. I got lucky and it started once in the water and to run pretty good but I only ran it around the boat landing for 5 minutes and then put it away for the winter. The previous owner told me about the hard starting on water and also said he would get it out on the lake and it would quit and not start again for the rest of the day but would then start again the next day. It didn't quite on me but I only road it for a brief time.

Fast forward to this year, I took the carbs apart again and this time replaced the diaphragms, filters, and some O-rings, checked the pop off, and put the carbs back to together. I also took the RV cover off and found out it had been gouged up before and then cleaned up and put back together. It was in really bad shape with lots of clearance where the cleaned up the gouges. After a new RV and cover I put it together and the first time out on the lake it started up like a champ! Unfortunately, about 1/2 a mile down the lake it started cutting out and then quit. I never could get it to start or even fire that day no matter what I tried including dumping a bit of gas in the cylinders. The next time I worked on it I found some pieces of rubber in the bottom of the fuel bowl which I had cleaned before running it. I figured the shut off valve may have come apart insider and possibly plugged itself up since I know it would only turn with pliers. I bypassed it w/o ever fully removing and testing it and it started up (but probably would have anyway). I got it running and took it to the lake and it would not start in the water. I put some new plugs in and then it started and ran fair but didn't see 100%. I started running it down the lake and it sounded like it may have backfired out the carbs a couple times. I thought it was gonna 'clear up and run better but instead...it quit and wouldn't start. I also noticed that sometimes it would turn over when I hit the starter button and other times it would 'catch' and not spin. I took the carbs and RV cover off and found a bend in the leading edge of the RV and another gouged up cover. I then took the head off and found a banged up piston top and head but it does not appear to have seized like I expected. Is it possible the backfire bent the RV and the RV would then catch sometimes when I tried to start it? Also, if the backfire could not bend the RV then did something get spit out of the engine? I would like to know why it quit the first day even though it started and ran awesome briefly. Also, what caused the back fires? I only question the DESS because it seems like sometimes I would here the two beeps when I put un the DESS cap and other times I would not. Here are the pics. I think the cylinders will be fine with a hone and the RV will probably be Ok if I address the small bend spot. I may also flip it so that is the trailing edge. I just don't know if I can get away with cleaning up the case where the RV rides and the RV cover or if it will not run good with them. I also don't know if the head was clean or dinged up some before since it had been removed before and the RV, case, and cover had been damaged and cleaned up before. I do plan to take the bottom end apart to see how much wear the crank, balance, and RV shafts have on them. I think this ski has a lot of hours on it and it has probably had a top end rebuild before but not a bottom end. Compression was 120 in both cylinders before I ran it this year and 115 in one side and 60ish in the other after the 2nd outing when it backfired and quit. What do you all think and what would you do. I will add another post with pics. Thanks

 

[670hoth]

Here are the pics. Please note that what looks like damage on the cylinders is actually a reflection.

 

[LouDoo]

From the looks of the inside of your engine, you are running lean, which usually means and air leak somewhere. When you overhauled the carbs., did you use Mukini rebuild kits? Check the "O" ring in the fuel filter/water separator. And yes it's time for a rebuild, I'm sorta surprised it runs at all with the compression you have, and with the RV valve I would consider a crate engine.

Lou

 

[mikidymac]

As Lou said that engine is toast. At some point the cases have been apart too because that is not the factory sealant on them. Something went through that engine and your piston to cylinder clearances look excessive from the pictures. To do it correctly you are going to need a new crank, new cases, rebuilt balancer, rebuilt rotary shaft, rotary valve and rotary cover as well as having the head cleaned up or replaced and new oversized pistons and bored cylinders. It might be cheaper just to send your core to SBT or SES.

 

[1983]

no DESS cant do that

 

[670hoth]

From the looks of the inside of your engine, you are running lean, which usually means and air leak somewhere. When you overhauled the carbs., did you use Mukini rebuild kits? Check the "O" ring in the fuel filter/water separator. And yes it's time for a rebuild, I'm sorta surprised it runs at all with the compression you have, and with the RV valve I would consider a crate engine.

Lou

Yes, I did use Mikuni parts. I did not change the needle and seats since they were perfect and the pop-off test was good. One thing that bothered me is that none of the pop-off test instructions and videos I saw address that anything must be done differently to test the carb with the accelerator pump. I had to play around and do something different to be able to pressurize that carb to check pop-off. I don't remember exactly what but it bothered me that this was not mentioned anywhere. I had to do to be able to get a good pop-off test on that carb but I did something different. Is this normal? Even the factory manual does not address doing anything different when you check the accel pump carb. I also used new carb mount gaskets and put gasket sealer on them just to be sure. Also, the o-ring on the fuel filter/water separator is new but I must admit that I don't really like the way the bowl threads on. It does tighten up but give me the feeling that I don't really dare to tighten it as much as I would like. I think if I get it running again I will bypass it and put in a large inline filter. I must admit the air leak does make sense. The first time I ran it this year it seemed like it was fine until I crank on the throttle a bit and then it cut out. I tried yanking the choke and it did not seem to matter. I should go with a reman engine but I am kind of broke right now. I have several money sucking projects going at once. Since someone else said they think the bottom end has been apart it may be worth my effort to tear it apart to see if the crank and balance shaft have possibly been replaced before and may not be too worn to do a top end. I just would like to know if I can get away with the RV case surface being rough. Honestly, it did not look any better before this happened and it started and idled perfect until something happened. At that time the rotary valve and cover I put on it were like new. Now they are damaged.

 

[670hoth]

As Lou said that engine is toast. At some point the cases have been apart too because that is not the factory sealant on them. Something went through that engine and your piston to cylinder clearances look excessive from the pictures. To do it correctly you are going to need a new crank, new cases, rebuilt balancer, rebuilt rotary shaft, rotary valve and rotary cover as well as having the head cleaned up or replaced and new oversized pistons and bored cylinders. It might be cheaper just to send your core to SBT or SES.

Since the bottom end has been apart before I wonder if it is worth my effort to tear it down to see what kind of shape the crank and balance shaft are in. Since the pistons and domes have no carbon I suspect they were replaced shortly before I bought it. Maybe the crank and balance shaft were as well and may not have bad wear. I cant explain why it only had 120/120 psi compression when I bought it since the piston domes look like they don't have much time on them but maybe piston to cylinder clearance and end gap were very loose when it was put together. If the crank and balance shaft look good do you think I could get away with re-using them and the cases? As I mentioned above, the cases looked just as bad before this happened and it started and idled like a top even in the water the first time I put it in the lake after the carb rebuild and new RV and cover. I personally probably have a grand total of 20 minutes of run time in the water in this machine. It probably has 30 minutes total running on the hose. I don't really have the money for a new engine right now. SBT is $1035 ($845 plus $190 round trip shipping) and SES is less on cost and shipping but they have a two month back log.

 

[670hoth]

no DESS cant do that

Thanks, I think I am believing it ran lean now. Why, I am not quite sure. Fuel filter housing, crank seals,??? I don't quite know why it would never restart the same day it quit but would start the next day.