Cant get enough gas to the LPG converter

**andrew_mx83** · 30-05-2012, 11:43 PM

so i am having problems with the dedicated gas setup in my new mx83. its basically the same system ive had on 2 other cressy's but this time only 1 tank, its had twin tanks before and had no issues like i do now.

ive just rebuilt the 7m and given it a bit of a tickle, 12:1 and built to rev with some wild cam timing to help it breathe up top.
gas system is IMPCO 425(FB) mixer and Model L converter with APA tank and newish lockoffs at each end (1/8NPT). all new except tank in '09
stock ecu with no AFM running ignition and auto. no rev limit.

prior to the rebuild this car was fine unless revved over 7000rpm, at which point it would slowly die in the ass until it stalled, and wouldnt start again for a few minutes.
now its worse, i basically cant have it over 5000rpm for more than a couple of seconds or it will die. longer you stay on the throttle, longer it will take to fire back up.
in the last car with twin tanks and a slapper rebuild it never had any issues, turned regularly to 7200rpm. car before that had single tank again but was never revved hard.

the converter isnt freezing up, the tank is cold to touch but the weather bloody cold too!
the tune isnt perfect yet but its not running mega rich under load.

ive heard of issues with excess flow valves or lockoff solenoids running out of puff at odd 220rwkw on a single line/tank setup but i dont think the ole 7m-ge is quite at that level!

any ideas or help from the gas guru's much appreciated!

**dumbass** · 31-05-2012, 05:43 AM

Maybe fit a fully sick Holley blue or Bosch 044 onto it mate.
Hehehehehehe

Did the twin tanks run thru the same fuel line
Single fuel line?
Or did u have twin fuel lines?

Try make it run richer see if that helps
I doubt u have flow issues as its all pressurised

Is it a hard rpm stop right on 5000
Or slowly starts to slow rpm as if it's running out of fuel

Could it be with bigger cam your getting more air flow and now
engine ecu is maxing out somewhere since u have no air flow meter
Check for codes on engine ecu

**andrew_mx83** · 31-05-2012, 10:04 AM

Codes galore as heaps of efi shit is missing, but never had a drama like that before. Ecu can't "max out" as its just controlling spark really.

Dies slowly, at what rpm depends how hard I've been pushing it. Throttle goes doughy and the revs slowly drop, enough i can coast to a slow stop somewhere.

Twin tanks had the same single line, the service line was joined by a hydrostatic valve at the back of the car.

**boxh34d** · 31-05-2012, 07:32 PM

Andrew,
What size is the line from the tank, and is there any chance that it has developed a bit of a blockage? I've seen a few solenoids (and converters for that matter) wax up causing similar issues.
Dedicated gas falcooooons are pretty famous for it.

**andrew_mx83** · 31-05-2012, 07:59 PM

3/8 from memory -whatever "standard" copper lpg service line is (was supplied by impco), im hoping to get a chance over the weekend to disconnect the line at both ends and blow it out with the compressor. check/clean solenoids and filter as well.

it does feel like a blockage or something but i religiously run 100% propane from supagas, one of the reasons is its a lot cleaner and not supposed to wax up the converter.

**Redmohawk** · 04-06-2012, 06:34 PM

More likely to be a solenoid than a line if its a blockage, over 150 kw most single unmolested (legal) solenoids will frost up and reduce flow, Due to the venturi effect. I'd be more concerned with your cam , big cams generally dont work with LPG due to the overlap increase causing backfires.

Checking soleniods and filter is a good start , just crack line at conveter and power up solenoid. If you get a good rush of gas you know theres no prob there. Just be careful for f**k sake lol

I'd be checking your ignitor and coils for overheating , higher compression will make ign work alot harder.

**andrew_mx83** · 10-06-2012, 10:05 AM

havent had any time to really play with this.

last weekend i disconnected the service line at both ends, blew it out with compressed air and also LPG from the tank. the line seemed to be clear, i also cleaned out the filter and slightly enlarged the hole in the converter lockoff solenoid....... no effect, still having issues.

i have noticed that while im on power its fine. afr stays about right (just into the "rich" part of the narrowband) under noise. if i go WOT and redline 1st then 2nd, its fine -until i lift.
so this leads me to believe it isnt really a fuel supply issue to the motor.

ive spoken to a couple of local gas places, the more switched on one suggested pulling either the converter or mixer apart. he reckons check the low pressure diaphragm in the converter, or the needle & seat setup on the mixer diaphragm. im thinking more likely the second, apparently they burr up the die cast alloy and get sticky. this makes more sense to me, the diaphragm will be getting pulled right back under lots of airflow and then slam shut when i lift, possibly getting stuck shut and now allowing any air or fuel into the manifold.

i do just wanna confirm ive got spark before i pull the mixer apart, but havent had a monkey free to come out for a drive and crank it while i diagnose yet.