GT 40 bacfires a bit getting skier up then is fine

[eseybold]

[/b] I have a 2000 ski nautique with gt40. Runs great except for the few seconds it takes to pull skier out of wire. There is some backfiring. Then it is fine. Runs WOT no sweat. I have checked base timing, changed both fuel pumps,(fuel pressure at idle 34 and drops to 30psi with throttle) new fuel filter, rotor cap, plugs and wires. Could it be ethanol in the gas? Where do I go from here? I even unplugged my fuel pick up line from the tank and blew compressed ait back into the tank to make sure my pick up lines was not blocked. Should I be looking at my MAP sensors etc. My air intake and flame arrestor are very clean. This started last year and has not changed. S :?: hould I even worry anymore about it? Thanks

[nms1991]

RE: GT 40 bacfires a bit getting skier up then is fine

you should have 42 psi at wot not 30 psi. you could have a fuel restriction in the fuel pickup in the tank, by blowing back thru the fuel line to the tank you are fighting against the anti siphon valve.

[Yabo]

I dealt with this problem for over a year on a 92 GT40. I solved it by running 93 octane. I even changed the carburator in this oprocess and it did not help, but I do not have the problem with high octane gas.

[eseybold]

thank you for the reply - I have always used 93 octane but I will drain the fuel tank and clean the fuel lines and start with some fresh gas. This problem is driving me nuts because I pride myself on keeping my nautique absoutley perfectly looking and perfectly running ! Maybe being a perfectionist is not a good thing.

[01bigair]

Reading your post as I have a GT40 in my 01 air. I read an intersting article regarding fuel octane and how people are damaging engines by using higher octane than the engine is designed for. I have an 04 Yukon Denali and the manual says to use 87 or 89(cant remember) but nothing higher as it may damage the engine and void warranties as well. The article I read was written by an engine expert and he recommended that you not use a higher octane to increase fuel economy or to get better performance, it does not help unless the engine is made to run it. I dont know what the GT40 calls for but I just thought I would throw that out there for you guys. Alot of people pay more for gas because the can $$, but may actually be hurting their engines. I will see if I can find the article and post it.

[eseybold]

update from owner

well I took the fuel line out of the tank today and the screen was clean as a whistle. I measured my fuel pressure again and it was 34 at idle then it would bump up to almost 40 when i gave it throttle in neutral and then drop down to 34 again. Boat idles just fine and reacheds top speed no sweat. I have both new fuel pumps. Ran the boat a lot today and basically on the hole shot it hesistates if I push it hard such as pulling a slalom skier. If I do a gentle acceleration it is fine. Where do I go from here ? Do I look for a new knock sensor? I am still burning off some STABIL in the tank(from last winter) but I have since dumped in fuel injector cleaner as well. I have new cap rotor, fuel filter plugs ,wires and check base timing all good. Could this all be from bad gas ? Would a bad injector give similar problems? Thanks

[Andrew]

RE: update from owner

I didn't read all of the posts, but I've had backfiring issues that were caused by a weak battery...

[eseybold]

RE: update from owner

does anyone have any experience with symptoms from a faulty knock sensor ? I am thinking of disconnecting mine to see what happens when I try to accelerate quickly.Maybe it it\s picking up a false signal and advancing my timing to much ? I am open to all ideas. Thanks

[nms1991]

RE: update from owner

Do not disconnect the knock sensor as the engine uses it to judge how far it can advance the timing of the engine. But with the symptoms I would suspect after you recomfirmed your fuel pressure readings then I would look at the tps or the map sensor for the problems. But you need to run a engine running test as said in the gt-40 service manual.

[eseybold]

Turns out it was just bad gas. I emptied the tank put some fresh 89 octane with fuel injector cleaner and it runs like a champ. I hear now that the shelf life of gas is about 30 days - so I am going to run STP or whatever fuel additive that I can at every fill up. In fact, I will not fill up - just put in small amounts at a time.
There is never a good time for bad gas !