Seadoo HX 720 factory pipe backfire through carbs

[ALLLCAPSRAGE]

I just bought a 95 hx
Parts:
West coast head bep 7200
Factory pipe
Aftermarket intake grate
40mm carbs
The ski does not have the micro touch limiter or ecwi.






Bought it with a scored pto cylinder and piston, put new piston and cylinder in







I suspect that the guy that had it before was putting 87 in it thus causing detonation

Took the 40mm carbs off and the low speeds were out 4 turns, highs were closed, set them to oem spec.


Went out on the lake yesterday ran okay but fouled the pto plug, put new plug in and it ran fine but back fired periodically through the carbs.

pulled plugs after chopping at wot, looked chocolate brown

My dad had his completely stock hx out too, raced from idle, identical acceleration then he started to pull away from me.

Can’t seem to find a micro touch with ecwi for sale anywhere, do I need it? Looks like there is an unrestricted water line going where the solenoid would be on the pipe.

What could be causing the periodic backfire I hear when riding? Is it detonation? Running half 100LL with 93 pump gas premix tc-w3 40:1 .

Checked the stinger on the factory pipe and the inner pipe was too far inside the outer pipe to get the calipers to touch the inner pipe

Just want a recreational ski not trying to squeeze every last bit out of it but it should be faster than a stock hx right?

 

[mikidymac]

IF you want a rec ski then find a stock pipe and head.

First off you can't run TCW-3 oil, you need a synthetic API-TC mixed at 32:1. You sill need to have oil feeding the rotary gear.

You can't run stock jetting and screw settings.

You have to measure the stinger to see if you have the early pipe that will eat pistons no matter what you do.

The last bad piston was lean, not detonation, deto will make a hole in the top of the piston and erode the dome on the exhaust side.

Finally it won't run without the rev limiter because with that pipe you are going to bouncing off the rev limiter all the time and that could be your backfire at high rpm.

 

[ALLLCAPSRAGE]

Thanks for the info!

I should have clarified a few things, I get the backfire about 1/2 to 3/4 through the rev range and haven't hit rev limiter on flat water.

I Have 40mm carbs I believe off of a 787, I read on a different forum that 787 carbs work well with the FPP

I did not know I couldn't run tc-w3, I have a gallon of brp mineral oil I could run, and the stock oil tank is still feeding the rv gear.

If anyone has a micro touch laying around id gladly buy it.

like I said not trying to squeeze every last bit out of it, but do want to beat my dad on his.

 

[ALLLCAPSRAGE]

This is what the stinger looks like on the inside


The inner pipe ends about 40mm less than the outer pipe does



I have the calipers set to 38mm in the picture above

EDIT: I put a socket with 38mm od into the stinger and it fit! So I have a 38mm stinger!

 

[ALLLCAPSRAGE]

Update for anyone that’s interested, took it out yesterday.

Put new spark plugs in and it ran great, I had the pto carb adjusted way too rich and was getting fouling on pto plug and that was causing the backfire I assume.

Installed a flow control valve where the solenoid would be to try and dry out the pipe a bit


Acceleration is faster than a stock hx, but not hitting limiter on wot, I will continue to try and dry out the pipe until I hit limiter on flat water

Once I hit limiter on flat water I’m planning on buying this CDI Box for Sea-Doo 580 / 720 (1995-2002) Adjustable Rev Limiter - VintageJetSki.com still won’t have ecwi but I hope it’ll run good with hard jetting the pipe

 

[Schneppershoff]

Lass mich wissen, ob der Drehzahlbegrenzer of Vintage-Jetski gut funktioniert

 

[ALLLCAPSRAGE]

Lass mich wissen, ob der Drehzahlbegrenzer of Vintage-Jetski gut funktioniert

Haven’t bought it yet, but will post if I do!

 

[ALLLCAPSRAGE]

Lass mich wissen, ob der Drehzahlbegrenzer of Vintage-Jetski gut funktioniert

Update on the build Micro Touch with ECWI Replacement!