Gas Research off idle stumble help

Propane, Butane, LPG, GPL, C3H8, C4H10
Post Reply
b73
Posts: 4
Joined: Sat Oct 12, 2024 12:08 am

Gas Research off idle stumble help

Post by b73 »

Hi All,
I know not many here have hands on experience with the GRA gear but I'm looking for a little advice. @storm, I'm look at you :)
I played with and raced LPG powered vehicles using Impco equipment many years ago and always wanted to experiment with a turbo setup.
It took 30 years but I'm finally doing it!
I've just bought a car fitted with a 202ci Holden inline 6, GT3082R and a GRA s350 mixer.
There's not a lot of info available as the heyday for similar setups was 20 years ago or more. I've only found a few posts on o-l-d forums regarding an off idle stumble and none has been of much help.

In my case the motor starts and idles well, but if I crack the throttle slightly there's a dead spot where the motor will stall.
The only way I have found to eliminate it is to use a progression hose with no jet fitted. (results in an idle of 0.9-0.95 lambda). So I know it wants more fuel just above idle and the wbo2 confirms this as it goes way lean when i touch the throttle.
Of course the open progression hose screws up the AFR's elsewhere else.
The documentation says the idle fuel orifice is factory set at 1.2mm. Mines at 1.35mm, an area increase of 26%, so I know someone has been here before.


My first question refers to the position of the progression slot vs the throttle blade at idle. My p.slot is exposed more than I would have expected, so it appears the motor is needing a bit of throttle opening to idle. This throttle opening is pulling fuel from both the idle jet and a good percentage of the p.slot.
I'm wondering if I should be drilling the idle gas port, closing the throttle to cover more of the progression slot (to make more slot length available for cracking the throttle), but I'm not sure if there's enough air available via the adjustable idle air bypass to idle and close off the throttle? I'm thinking this will make the progression slot more active when cracking the throttle?
Am I on the right track? don't want to be drilling out the idle jet unnecessarily.
Thanks in advance!

b73
Posts: 4
Joined: Sat Oct 12, 2024 12:08 am

Re: Gas Research off idle stumble help

Post by b73 »

A minor update if anyone is interested.
I had it apart this morning so I drilled the idle jet from 1.35mm to 1.45mm, and increase of 15% and now 46% larger than the OE 1.2mm idle jet.
I also put the 2.2mm progression jet back in place.
It's much better. I'm certain before I had to wind the throttle open to make it pull fuel from the progression slot in order to idle and now it's getting it from the idle jet.
I adjusted the idle mixture for highest rpm which enabled me to close the throttle some more, which I believe will make the p.slot actually do what it is intended to do.
It still goes lean just above idle but is much snappier and doesn't stall.
I haven't driven it yet so I don't know what the running lambda looks like.
So it still needs a bit more fuel just off idle and I think this *should* come via the p.slot, but I'll wait until I have an opportunity to drive it before deciding whether to use a bigger idle or progression jet to achieve this. If the cruising mixture is good, I'll add fuel via the idle jet. If it's lean I'll add it via the progression jet as I believe this has a bigger influence of the cruising mixture than the idle jet will.
I think I'm heading in the right direction. Any comments welcome.

b73
Posts: 4
Joined: Sat Oct 12, 2024 12:08 am

Re: Gas Research off idle stumble help

Post by b73 »

I acknowledge that not many here have any experience with Gas Research equipment, but I now have a general question regarding boosted propane applications.

I 've got this running REALLY nicely now. There's not much info out there, but I did read that the balance line (converter/reg boost reference line) should be connected pre- intercooler using a 5/8" ID hose for best effect.
Mine had been connected using a 1/2" line post intercooler. ( A lower pressure area due to the pressure drop across the intercooler)
I went looking for this info because I have a big lean spike and slow response from the converter upon flooring the pedal.
Now the 5/8" balance hose is connected pre intercooler (and using a slash-cut quill in the pipe) the lambda response is instant and the lean spike is gone! This also cured the off idle lean stumble, which surprised me.
This change has given the best improvement in driveability of everything I have tinkered with.

However my WOT lambda numbers under boost are now 0.7-0.75, much richer than the recommended 0.85-0.9... I was lean before, hovering around 1.1 under boost.
I was worried about EGT's now that it's rich, but it's not as hot as when it was lean.
Prior to this change I could get it up to 1560F with a pull through 1-2-3rd gears. (1.1 lambda)
Now it's richer I only recorded a peak of 1420F. (0.7-0.75 lambda)

So my question is, should I be trying to lean it out to 0.85-0.9 under boost OR, is it okay as long as the EGT remains safe?

Any input welcome :)

C3H8
Posts: 1144
Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: Gas Research off idle stumble help

Post by C3H8 »

I'm not a Gas Research experienced individual but I will add this. EGT's of 1400 and higher are too high. 1450 is close to valve melting territory depending on the valve material. These temperatures would indicate you still have fuel burning when the exhaust valve opens indicating the timing is still too far retarded. try advancing your mechanical timing a couple of degrees at a time until the EGT temps drop. What are your timing numbers and at what RPM?

b73
Posts: 4
Joined: Sat Oct 12, 2024 12:08 am

Re: Gas Research off idle stumble help

Post by b73 »

Thanks for your input.
I have cured the stumble by relocating the boost ref. hose to after the intercooler and increasing it's diameter.
This affected the lambda values across the board so I made a new metering rod and after a few iterations, it's now as perfect as I imagine possible.

Regarding the timing and EGT, I also suspect it needs more timing on boost. I have recently collected information on other turbocharged engines of this type and when comparing CR, boost pressure, timing, charge cooling and fuel type, I can see I have less timing than most others - and they are all on lowly pump fuel, not lovely high octane propane :).
I've just fitted a megasquirt so I can now control the timing easily and create any curve i want.
I started by mapping what the distributor did, and at max boost of 17psi, that's 21degs btdc. Comp is only 7.25:1, the motor was built back when that's what people believed was required for turbo builds. A naturally aspirated engine of this type typically makes best power at 30-32 degrees.
I ran it on my dyno on the weekend and it made just over 300hp at the tyres. Not too bad for an early 1960's engine designed for boring passenger commuter cars :)
I really wanted to put more timing into it, but currently my knock detection gear is unhappy on this motor. It has a straight cut timing gear and I suspect it's noise is flooding the knock freq range (6.16kHz) with it's whine.
I'll run it again next weekend and sneak a bit more timing into it regardless. Hopefully I can hear it knock in the headphones, even if the knock box can't determine true knock from gear noise.....

Post Reply