Propane and vacuum leak testing

[Denver_FJR]

I'm chasing some driveability issues with my '07 FJR; stumbles on startup, throttle is very on/off at lower RPM. Thinking it could be a vacuum leak and before removing tank I put a propane nozzle directly into intake at air filter. Sure enough, idle smooths out and RPM increases. My question, is this a conclusive indicator of a vacuum leak or a red herring? If conclusive, my next step will be to lift tank and start more propane probing.

 

[FJRay]

I think you need to dig a little more. The Propane isn't a bad idea but it is just showing you that a richer mixture makes it happy. All modern stuff is very lean to pass emissions. I would start with a good sync on the throttle bodies and check for vacuum leaks while you're in there. The throttle is a known issue and some do the barbarian mod or get a power comander to handle it.

 

[Chuck35]

If the propane is making its way to the intake opening (air filter, throttle plates) you will get a false indication of leakage.

 

[lnewlf]

Run the propane nozzle everywhere around the intake system except the air cleaner opening-if idle speed increases you found the leak.. I wouldn't get carried away with the propane tho-could get exciting..

 

[JR'SFJR]

If no vacuum leaks are found from the outside with propane and you decide to remove the air box, you could inject a little propane into each throttle body to see if all cylinders are a little lean or just one bad one.

 

[wmadoty]

I quit using flamable substances to check for leaks years ago, several bad experiences in the shop. Now I use a spray bottle filled with water on mist. Surprisingly, if the leak is small you will still see an idle increase when you hit the spot with the water. Most of the time when you find the leak you will experience the opposite, the engine will slow or even die.

If the leak is only on one cylinder, you should be able to see a difference on spark plug deposits.

Spray the coil packs and wires with some watter as well and listen for arcing. I have seen a weak spark condition look a like like a vac leak.

 

[dcarver]

I'm not a tech, but using propane to find leaks on a running engine sounds like Fun!

Then again, I've used starting fluid and wd40, so guilty as charged.

 

[Panman]

The 07 had some real bad off throttle issues! Mine was kind of scarey to ride, it was really hard to get around a slow corner, you just couldn't feather it enough it would lurch more than slowly take off. If I rode the snot out of it, it wasn't so bad not prefect by any means.

In 08 Mama Yamaha did some work and really smoothed out how the Gen II ran so haven't saw the following mentioned hear as it is old news so disregard if this is old news to you.

I didn't see if you had a G-2 throttle tube or not, it helped some on mine. I put a PC III on mine and it is now acceptable.

 

[JR'SFJR]

"Propane enrichment" was a standard test procedure starting on Chrysler Corp. vehicles when the carburetor idle mixture screws were required to be totally tamper proof, surrounded in metal. Add a little propane while watching a tachometer to see if the idle increased the correct amount for the specified "lean roll". If it was suppose to be a 150 rpm lean roll drop and the engine rpm increased 300 rpm when propane was added, the carb was coming off for drilling and hammering to gain access to the idle mixture screws. Many times, I have placed the propane hose down an individual carb venturi to see exactly which barrel was running lean. Take any propane torch, remove the tip and orifice, maybe add a hose and you're good to go.