Low idle and intermittent backfire (2003 Viper)

ski

ski

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Joined
Apr 6, 2005
Messages
152
Age
53
Location
Steinbach,Manitoba
I have a 2003 Viper (5000 miles) I bought recently and I have a strange condition at idle. It seems to want to idle around 1000 rpm and sounds a bit like it is only running on 2 cylinders. If I try and turn up the idle screw, it starts backfiring or popping intermittently and the idle doesn't come up much even though I turned the screw a fair bit.

It will pick up and run great for a bit if I give it a bit of throttle though. I also noticed if I flash half choke for a second it picked up the 3rd cylinder and the idle came way up even though the machine was fully warmed up.

I thought immediately that I just had a plugged pilot circuit, so I removed the carbs and opened up the bowls. They are totally clean! I checked all the jets, made sure they were correct sizes and everything else seems perfect EXCEPT... I found the air bleed in the center carb had been partially crushed (more like smooshed) and was probably restricting air flow by 75% or more.... but wouldn't that make it run rich on that cylinder?

The machine runs great otherwise and once up on the throttle it pulls very hard all the way through... which daddy likes BTW :)

I fully cleaned all internal circuits with carb cleaner and compressed air, checked the pilot screw adjustment, it was around 2 turns +/- 1/8, I set them to 2 exactly after cleaning. I also fixed the air bleed so it is the same as the other 2, I just want a second opinion before putting the carbs back on to test. Thanks!

*edit* Not sure how accurate my gauge is (its just the cheap one you hold against the plug hole) but compression cold is 105, 105, 110.
 
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Sounds like you hot it with the carbs being cleaned. As for compression I would get another test and check if numbers are still low I would do a new top end or just new ring as it does have 5000 miles.
 
look at the tors switch on the carb rack it may have come undone from the idle screw. also you can unplug the switch from the harness and loop the sled harness back into each other and by pass it, if the idle is smooth and it doesn't stutter that's where you need to look.
 
This is all good advice,

I also have a 2003 Viper, and I am experiencing the same issue. Your backfiring is most likely caused by the TORS system, which is picking up that your carbs are having an issue. I solved this issue last year by cleaning the carbs really well and replacing the pilot jets. The pilots in this sled are so tiny that you can barely see daylight through them. If the pilots aren't clean, your sled will not idle. I am just getting mine out now and I need to pull the carbs and give them a good cleaning.

My question is, has anyone experimented with going with a larger pilot? It seems like a pilot with a larger hole would stay cleaner and have less issue with plugging. My Viper has the standard #45 pilot. I see that Yamaha lists 47.5, 50, 52.5, and 55 pilots for these carbs. I would be willing to try a larger one if anyone has experience doing this.

I would love to start the season and not have to pull and clean my carbs. I would love to hear from others who have had this issue.

Stevepod
 
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larger pilots means more fuel witch makes it run rich at low end. if it is lean might be ok but usually not nessasary.
 
I seem to have resolved the issue with repairing the air bleed that was partially crushed. The TORS has been disabled by a previous owner. Thanks for the feedback, I think I may replace pistons and rings as a preventative measure.
 
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