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recent gas conversion - flat spot around 900RPM 

 

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 Post subject: recent gas conversion - flat spot around 900RPM
Posted: Fri Nov 10, 2006 10:59 am 
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g'day
just recently had my NC fairlane (4.0L) converted to gas 2weeks ago.
since it was completed on acceleration, it has a noticeable flat spot that almost makes it stall, however once it recovers (around 2 secs later) it goes fine. also when taking my foot off the accelerator around 1/4-1/2 throtle at low speeds, it will still try and stall, and once or twice it actually has. when running on petrol its fine, so it must be a gas problem.

im getting the s**t driving 60km each way to the workshop where it was installed.

ohhh and i replaced the plugs with the Xr options as that is the gas plug, in the hope that it would fix the problem. my next point of call may be the o2 sensor. after that ill just set light to it
haha
thanks in advance
jono
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Posted: Fri Nov 10, 2006 12:38 pm 
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Sounds to me like it's out of tune, take it back mate.
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Posted: Fri Nov 10, 2006 12:41 pm 
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yeah ive been back3 times...been an absolute s**t fight, first there was a tiny gas leak. does anyone here live in brisbane, and know somewhere reputable....this guy is giving me the s**t!
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Posted: Fri Nov 10, 2006 12:41 pm 
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Either the bypass is too low (not enough gas at idle) or the sensitivity is too tight (takes a bigger throttle increase to allow gas to flow).

If it is OMVL I can suggest some things that might help.

cheers
Scott

 

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Posted: Fri Nov 10, 2006 12:51 pm 
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are they the two sdjusting screws on the gas module?
also there is a small black box with a red and a green LED - it is usualy on red, but flicks between red and greeeen. is thsi normal?
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Posted: Fri Nov 10, 2006 6:07 pm 
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i had exactly the same problem with my gas system. i remember something about the screw that is in the gas line to the intake, although im pretty sure ive been told that is a high speed change soooo i dont really know bout fixing it, or how i fixed mine:P soz.

also aslong as those lights on the box are rich and lean your mixture should be good. when tuning gas ur supposed to get them to stay flicking from one to the other because that means your staying between rich and lean.

Good luck, i know how frustrating it is.

 

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Posted: Sat Nov 11, 2006 12:18 pm 
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yeah it is frustrating....i think were having trouble with the self adjusting - ajuster that runs inline with the gas if that makes sense. as the revs increase, the pin moves into the device alowing more gas through....so frustrating. can anyone recomend a good gas place in brisbane?
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Posted: Sat Nov 11, 2006 1:26 pm 
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I had this on my EL, it never got fixed and I suspect it was the gas computer.

 

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Posted: Sat Nov 11, 2006 5:12 pm 
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Yeah, so there are three adjustments on a basic LPG system. The solenoid that is in line with the LPG feed to the intake is controlled by the LPG computer. It should adjust 12 times a second, I think, for optimum efficiency. This is not adjustable on new systems. On old systems that were not computer controlled it was opened to whatever amount gave best speed at 3000rpm.

Then you have 2 screws on the convertor. On an OMVL r90 you have a small one and a big one. The small one is the bypass screw, the big one is sensitivity. The bypass allows a certain amount of LPG to flow all the time. Ideally this should be set so that you car idles off this and nothing else. The sensitivity is a spring loaded diaphram that controls the flow at everything above idle. It is controlled by the vaccum line that is tapped into your airbox. The more vaccum the more the diaphram opens and the more LPG that flows.

So ideally the car should now draw any LPG from the diaphram until the car is off idle. Before that the bypass runs everything.

To tune you need to disconnect the ISC and insert a 0.75mm feeler between the idle stops so the car will idle (a piece of cardboard folded 2 or 3 times works as well). You need to do this so that the computer wont keep opening up the throttle some while you are trying to see the mixtures.

The turn the big screw (sensitivity) out heaps. Now turn the bypass all the way in. The car should still run. If not turn out the sesitivity some more. Now start to turn the sensitivity in untill the car almost stalls. You want the car just running.

Once that is done start to turn the bypass screw out. Do this slowly as it is around 8 times more sesitive than the other screw. Keep turning out and therefore richening the mixture until you get a nice smooth idle. This should be around 3 turns.

What you have done now is tune the car to run on the bypass at idle but the second you increase the throttle the sensitivity diaphram opens and allows more LPG to flow.

You have just home tuned your LPG system. I get around 16l city and 10l highway on LPG in an EF Futura wagon.

I accept no responsibility for people following the above and causing damage to their vehicles. Always make sure stop and seek advice if your vehicle has symptoms of detonation or pinging.

cheers
Scott

 

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Posted: Sun Nov 12, 2006 9:54 pm 
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champion - atlast someone with some good info
out of curiosity, how do you know this, experience or do you have a gas fitting trade?
thanks for that, now to start problem solving why it trys to stall on throtle application from idle
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Posted: Sun Nov 12, 2006 10:09 pm 
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Read and then read some more. Also testing on my car. Also, also I'm studying Mechatronic engineering so designing engines and their control systems is part and parcel of my career path.

If it is stalling on part throttle application you either have a major air leak or the air line that controls the sesitivity diaphram is cut or not installed correctly. Could also be that the diaphram has a hole in it.

Have you installed a pod and removed the airbox lid or anything like that. On most the line is fitted to the mouth of the airbox on top of the radiator.

 

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Posted: Mon Nov 13, 2006 10:26 am 
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there is a line running from the air box inline to the side of the regulator which i am guessing is for the diaphram. no pod, all the factory gear is in place except i have a K&N. there is no hose clamps on the line from the air inlet, so i will throw some on there and see how i go. could the flow properties of the K&N be effecting it?
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Posted: Mon Nov 13, 2006 10:50 am 
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Unless the K&N is sucking air from somewhere else, like holes in the airbox lid/sides, etc then I would say no. Your engine still sucks the same amount of air in and it all comes from the same place.

I am running a pod filter inside my airbox and I have no problems.

 

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Posted: Mon Nov 13, 2006 11:23 am 
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its_jono wrote:
there is a line running from the air box inline to the side of the regulator which i am guessing is for the diaphram. no pod, all the factory gear is in place except i have a K&N. there is no hose clamps on the line from the air inlet, so i will throw some on there and see how i go. could the flow properties of the K&N be effecting it?

That would be your balance pipe... Goes from airbox to the back of the convertor.
If that's off then it'll make the car stall when you pull up at lights and can also cause it to stall when you're driving and back off the accelerator.
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