RXP Baackfiring

[Laurence]

Hi all

I have just recently rebuilt a 2004 Seadoo RXP with a SC 1503 Rotax engine.
Today I attempted to start up, on the first couple of tries she would not fire, left it for a while and tried again.
This time the engine would crank over and backfire or misfire with a loud bang coming through the exhaust, I referred back to the troubleshooting manual, and checked the timing, the spark plugs and caps, I put new fuel in the tank and have basically done everything the manual says, but still the engine continues to backfire or misfire with a loud bang.
I have been working on this Jet Ski for over 4 weeks and just want to get on the water.
Anybody have any ideas?

 

[BMANN06]

Might sound like a broken record ... but it definately sounds like timing. With just the N/A 130s or 155s you can get away with slight out of time. But on the supercharged blocks timing is critical and tolerance is very narrow. Timing chain 1 tooth out can cause your symptoms. You must have the crank locked and the cylinder head locked with pins in order to get proper timing. Get piston 3 (or the one closest to handle bars) at TDC, lock the crank. Then lock the head, make sure the lines on timing gear are level with cylinder head.

If your positive your timing is dead nuts on, you might have to pull the head and examine your valves, what was the cause to rebuild?

 

[KK40384]

Sounds a whole lot like a timing issue to me. Double check that and if that's good then your guess is as good as mine. I'm sure someone with more experience will chime in with better advice.

 

[Dennis]

What exactly did you replace when you rebuilt the engine?

 

[BMANN06]

Yah im with dennis...what happened to cause revuild? If its not timing its likely the valves (exhaust)

 

[Laurence]

I got the jetski at a steal, from guy who's business went bust.
I'm not exactly sure what happened, but according to the previous owner the timing chain broke, bent the valves and bent the connecting rod on piston one.
The motor was disassembled and then left in boxes, until I bought it.
What I have noticed is the pin on the cam gear is not exactly in line with the cam sensor, its about one tooth out from being in the centre of the cam sensor. Is this right or should it be central to the sensor?

 

[BMANN06]

Dont worry about cam sensor position, agreed though i scratched my head on that too. Just worry about timing gear lines %100 parralell to cylinder head case. I use a ruler and rest it on cylinder head to make sure lines are dead on.

 

[Laurence]

The timing gear lines are perfectly in line with the top of the cylinder head when #3 cylinder is at TDC and when the crank and cam are locked.

The only things I haven't checked are the injectors and coils, could this cause the backfiring?

 

[BMANN06]

Not likely the coils but possible, i could see this problem being the coils if the harness tape was cut and you had the wrong firing order. Im saying the tape because the stock harness wont allow u to connect coil 1 to 2 or vice-versa. If the harness electrical tape has been removed use the manual and confirm wire colours are going to the proper jug. If the harness hasnt been changed its time to pull the head and check tolerances on the valves. Did u rebuild the head with new valves? They exhaust valvles will always bend when timing chain snaps. Pull the head examine valves and valve springs.

 

[Laurence]

Thanks BMANN06 for the advise.
I'll pull the head and check the valves. Unfortunately I commisioned an engineering company to replace the valves so maybe they stuffed up!
Will let you know what I find on the valves when the head is off.

 

[BMANN06]

Cool let us know, sorry for horrid spelling too been drinkin' at cottage all weekend and punchin replies in with an iphone. Once u go through head you'll find the source of the problem.

 

[Dennis]

You could check compression to verify a possible valve issue. I'd do that rather than just pull the head first. It should be around 160 psi. Check the harness like BMANN said.

 

[BMANN06]

Yep good point dennis, and it could save the cost of a head gasket if problem is not valves. Thanx man did kinda jump the gun a bit there. Just thinking with backfire its always been timing or something in head in my experience. Laurence if u got a gauge post comp test results on each jug.

 

[Laurence]

Sorry for the long wait, decided to take a bit of a break.
I’ve done a compression test on all three cylinders; I only have a KPA compression tester so Iv managed to convert the readings via the web. They are as follows:
Cylinder 1: 700 KPA (101 psi)
cylinder 2: 750 KPA (108psi)
cylinder 3: 750 KPA (108 psi)

They seem to be low, or is this right?

 

[red2blue]

Sorry for the long wait, decided to take a bit of a break.
I’ve done a compression test on all three cylinders; I only have a KPA compression tester so Iv managed to convert the readings via the web. They are as follows:
Cylinder 1: 700 KPA (101 psi)
cylinder 2: 750 KPA (108psi)
cylinder 3: 750 KPA (108 psi)

They seem to be low, or is this right?

I was under the impression 125-135 was in norm range....so...your timing may be a tooth or two retarded.

 

[BMANN06]

Compression is low for a 215hp, get a hold of the company that rebuilt your head and find out what they did and didn't replace.