LPG Discussion Forum

Hi all, <BR>newbie here but not an idiot, understand all the principles of the gas system but i just cant get good MPG and reasonable drive / performance. <BR> <BR>running open loop system supplied by wtv-uk who has been really helpful and even took back the VPM in exchange for a pancake type mixer. <BR> <BR>the problem im having is a flat spot just off of idle, if i tune the balance on the mixer in this troubled area then set the power / mixture on the gas hose at 3000rpm (all on a rolling road) it will drive great, as though on petrol. however it runs VERY rich and somtimes stalls at idle but when cold is running so lean i have to purge 3-4 times before i crank to even hope it will fire. mpg is 16. <BR> <BR>so i turn in balance a little (leaner) have a very small but livable flat spot just off idle (which is still rich but not mega) also wind in the power / mixture a couple of turns and lose power but now i can get 18mpg. which is still rubbish! <BR> <BR>petrol i can get 30 mpg so even if i loose 20% thats still 24 mpg which im miles away from. I really suspect ive got a duff vaporisor but how can i test / prove it?? <BR> <BR>cold starting has never been acceptable regardless of settings on either vpm or pancake mixer. <BR> <BR>definatly havent got any air leaks, went extreem the other weekend and carb adaptot and mixer are all siliconed together now! <BR> <BR>any ideas or professionals locally please help. run out of ideas now <BR> <BR><img src="http://www.my90.co.uk/lpgforum/messages/898/2201.jpg" alt="picture">

You have not said what vapouriser you are running. The omvl reducer is the ONLY one to use for good mpg. I have noted a difference of at £2 per 100 miles going from many other good makes to the omvl unit. <BR>Also what car is it? <BR>The flat spot could be resolved by a smaller choke. <BR> <BR> <BR>Mike

Thanks for the reply, that £2 difference would do it, that calculates to 22.2 MPG instead of 18MPG. <BR> <BR>My vaporisor is an autronic (italian i think?) can see it in my not so clear picture (top right). the car is a 2.0 cortina. <BR> <BR>smaller choke to create greater vacum signal on gas delivery hose, and hence diaphram, right?? Unfortunatly its not that practical to change chokes in my application the carb is fixed venturi a 28/36 DGAV webber. The 28mm primary opens first 3/4 throttle travel the 36mm opens in the last 1/4. <BR> <BR>The pancake mixer has a plate which lifts up out the way on petrol and has adjustable studs to set height when dropped down running on lpg. In order to create as much vac demand, this is set to its lowest position. Suppose i could remove studs and grind down heads to get it even lower but that would proberbly be too much restriction? <BR> <BR>Are you local with rolling road and gas analyser, happy to pay for some time, just trying to avoid a trip up north as im sure wtv-uk would be happy to dive in. Im in essex. <BR> <BR>Could it just be a poor open loop system then? I approched a local rolling road last night for some time but as yet they havent come back to me. Once ive satisfied myself fuelling is best im going to get it, albieit with a rich idle should i settle for a 25-30% saving not 40% and live with the fact it wont start and idle cold? the wtv-uk chap wont accept that cold starting is a problem if set-up correctly. <BR> <BR>how often do you find faulty vaporisors? <BR> <BR>thanks to anyone with input. cheers.

my autronic also has idle bypass but because im already running excessivly rich at idle i have no need to use it. My vaporisor definatly has a main bypass, whereby gas will purge constantly (ign on not cranking) with no vacume, is this right? This is basically what it's idling on until just off idle when the diaphram moves and then bang it leans off causing a flatspot, once out the otherside of the flatspot its ok. <BR> <BR>my engine is running rich, slightly lean just off idle and then correct for the remainder. I can adjust balance screw to very rich (almost stalling and lumpy), correct, correct, or correct, v.lean ,correct. (obviously to get the upper revs correct i need to also adjust power / mixture screw) <BR>the last two senario's make the car very unpleasnt to drive so im currently set-up as fisrt described. <BR> <BR>can you see the patern? i have a dip / flatspot just off idle. If you imagime a graph x as revs and y as a/f ratio i can move the entire graph up and down and also make the dip more extreem or less. but i cant seem to eliminate it. <BR> <BR>Im thinking i need a sensative vacume gauge on the gas feed to see how the vaporiser is responding to vac signal created by airflow through the mixer vs engine revs and a/f ratio.

Ok I see you have a lifting plate mixer. As you described you will need to grind the bolt heads down to create more vacume- dont worry you can always adjust it back up.(this is the same as changine mixer choke size). What is ocouring with your set up is the reducer is not sensing the air flow change probable because the mixer is for a 5ltr engine. Adjusting the mixer should sort out the drivability but I doubt the consuption will improve greatly. <BR>I am on Anglesey north wales a long way but I do get many ppl coming here from as far as the top of Scotland and the far reaches of cornwll. <BR>Let us know how you get on. <BR> <BR>Mike

ok thanks, <BR> <BR>your spot on re: my mixer type, supplied with the mixer was a slip of paper that said minimum setting (what im using) is suitable for engines upto 3.5L, then listing other settings lifting the plate for bigger capacity's. <BR> <BR>I'm going to change the three plate height adjust bolts to nylon anyway as ive noticed the ally plate is being eaten by the bolt heads due to vibrations / resonances. Bad design! <BR> <BR>ok so my 2.0L engine is roughly moving 15 CFM at idle and 125 CFM at full throttle. That currently equates to an air velocity of 10.13 mph through the mixer orafice (and therfore across gas holes, creating vacume) at idle and 84.5mph at full throttle. (incidently full throttle air velocity through the carb calculates to 80.77mph, so currently mixer is extreemly small restriction) <BR> <BR>Assuming the mixer paperwork stating "upto 3.5L" is really the setting for a 3.5L and no smaller. <BR>A 3.5L engine would typically be moving 28 CFM at idle and 220cfm at full throttle. So if that was breathing through my mixer as set now (min = 3.5L) that equates to air velocity of 18.93mph for idle and 149.75mph full throttle. <BR> <BR>mmm thats a fair bit of a difference, think we may be getting to the bottom of this. <BR> <BR>ok, a couple more calculations and i rekon i need to reduce the orifice area by 46% (2.85mm to 1.54mm) to give the same air velocity on idle as a 3.5L engine, interestingly it works out to have a higher velocity at full throttle than a 3.5L (157mph) <BR> <BR>As the full throttle velocity is now so high will it not lift the plate a little? or is that the idea? <BR> <BR>Really excited that there is a way forward. Just hadnt thought about this area in enough detail. Thanks, will let you know how i get on.

Ok now im really confused, <BR> <BR>Done exactly what is described and got my mixer plate down to 1.5mm clearence. Started the engine and it would idle but wouldnt pick up off idle. (power valve all the way out, balance screw aparently having no effect on the running of the engine). so I lifted the plate 1/4 turn on the height screws at a time. The result was slowly increasing the point to which it would rev to, and the balance screw begining to work/do somthing. 1/4 turn was moving the rev ceiling by 1500rpm each time. Kept adjusting plate height untill the engine would rev properly without hitting a ceiling, guess where i ended up??? back where i started!! (2.85mm) <BR>With nothing left to try I carried on and started raising the plate, and the flatspot moved from just off idle to further up the rev range, so i adjusted until it went off the top, and ive now got a plate clearence of 3.75mm an increase of 30% area. <BR> <BR>each time I raised the plate the balance screw was comming out, im proberbly 3/4 out now. <BR> <BR>So what have i got now? no flat spot and an engine that is so happy it now idles 1-150rpm faster than before, hardly any lumpyness at idle. This morning when cold i pulled it out the garage, (still had to purge on timer 4 times to get it to fire) it was idleing on the drive while i shut the garage door, but then after 3-4 seconds started to bog down and would have stalled had i not got back and caught it, but its a great improvement. At the bottom of the road when its getting warm enough for the auto choke to come off it stalled but re-started instantly, also it will now pull well from low revs, real noticable bottom end torque improved when hot and cold. <BR> <BR>So its not quite there yet but is greatly improved. <BR> <BR>However im now really confused as i dont understand why its better???, it would appear that was undersized but this is spec'ed for engines upto 3.5L? <BR>any ideas?? <BR> <BR>thanks in advance

ok thanks bob, i'll try that tonight / over the weekend. Unfortunatly i wont be able to give feedback on the results until monday as im using the PC at work. <BR> <BR>Cortinas are great, im fortunate enough to have a good one with only 45K on the clock, so ive got good compression etc. im running opto luminition electronic ignition which cured all starting evils assosiated with a weeker points ign system. Timing is already set as much as I dare push it, not sure exactly what it is but on 99 octane (tesco super) petrol it just pinks if loaded in a high gear going up a hill. Think it ended up around the 12 BTDC mark. <BR> <BR>I think the closed loop is what i need to improve my MPG and good for fine control but at the moment, or just before recent developments, I was doubting if I was even in the right ball park. <BR> <BR>Will definatly add lamda and closed loop later but don't want to introduce another variable until it is running near to correct open loop. Im not being a tight-wod but I refuce to use it as a fix, happy to use as it is intended i.e it only trims / makes fine adjustments opposed what would happen right now, it would be making cource adjustments fighting against an a/f ratio that is jumping from one extreem to the other. <BR> <BR>I'm also thinking about an electronic ignition advance / retard box that comes in / out with lpg / petrol change over. <BR> <BR>Does everyone think my mixer plate is in the right position now or should i try a little more lift? <BR> <BR>Still dont understand why lifting it has made it so much better, would love to hear any theorys no matter how wild. Im thinking that it was a restriction and has bad flow characteristics, more to it than just velocity over a hole???? got more volume now but moving slower so less effected by poor flow???

I might have to go down this road of modifying a spare carb as an experiment, although im already on my second mixer which is the sucky out the way on petrol, plate type, with cooker ring round the top of the carb. The first mixer (a VPM) was £70 and a custom sheet fabricated side to downdraft adaptor which took 3 days to make and this one was £150 and again needed a custom one off adaptor machining from a billet.(see pic in first post on this thread) <BR> <BR>This is why i was initially questioning the vaporisor, although with this second mixer I can eliminate my flat spot just off idle and only have a slightly rich idle. The settings are Idle bypass off, plate height set to 3.5 - 4.0L engine setting, balance about 5 turns out and power about half way. I have a 4 gas analyser so im setting the best each time for idle and 3000rpm. <BR> <BR>This works very well when hot but wont run cold, tried using idle bypass with balance in a bit but flat spot comes back although cold running was improved in as much as it would idle some times opposed to never. <BR> <BR>During tuning i noticed idle mixture is all over the place and would never settle in the same place twice after reved. I checked each time thats throttle was hard against idle speed screw stop. To prove the mixture point further I left the engine idling at 5% do nothing but turn off keys and restart, no touching throttle or anything, only keys and now its idleing at 2%. Just alowing the diaphram to relax and then restarting the engine it returns to a different position, 3% different sometimes!. Is this normal or a sign of an over sized choke? I cant run slightly smaller due to flat spot problems any smaller still and the engine wont accelerate on full throttle and bogs down losing acceleration. <BR> <BR>Normal or a mystery??

There is another way to remove the flat spot. I you fit a closed loop system such as the omvl millennium you could resolve the problem. If you fit a tps switch you can program the stepper to lift alowwing more fuel on acceleration.

No this is a mega flat spot, even if i open the power valve fully it has no effect on flat spot, only richer once cleared. I'm no longer worried about the flat spot as its gone when plate is set at correct height. (Although don't understand why it needs a 4.0L engine setting, or why its so rich at idle (idle bypass is off)) <BR> <BR>The closed loop system has a solenoid / valve that is an electronically adjustable power / mixture valve doesn't it? to go down that road I'd need something that was controlling the balance on the diaphragm, not the flow rate in the gas delivery hose. <BR> <BR>Besides as i said before, technically I shouldn't have to. Something else is wrong, I'm not looking to mask the fault rather fix the cause. <BR> <BR>why does it run perfect (really really sweet, better than petrol) when hot and not cold? are the choke butterflies getting in the way because they will be closed when cold??? I'm sure it's not freezing as its only idle problems, will drive perfect cold once past lean idle. <BR> <BR>Fitting a pipe in the side of the carb venturi (not using a mixer) between throttle and choke butterfly's would create more vac demand when cold as choke flaps closed, right? also what angle does the pipe go in at? slash cut in at 30deg or dead 90?? can see an argument for both. <BR> <BR>Any wild theories as this is obviously not straight forward? <BR> <BR>Wonder if anyone would tune on the basis, "if you fix it you get paid", just don't trust garages, none have the real time to fix anything properly. Much more success learning and understanding then fix myself.

I would be happy to look at it for if distance is not a problem. And I would not expect to get paid unless the problem was resolved. <BR> <BR>Mike

excellent, have you got a phone number? <BR> <BR>might have to take you up on that, what are your rates? obvioiusly id be looking to exchange any faulty parts (ie vaporisor) with WTV. So if you was to diagnose a faulty part without actually fixing it what would you charge then? <BR> <BR>Going to see if i can get WTV to agree to "no fix or faulty part no fee". <BR> <BR>Borrowing some data logging equipment soon so can monitor revs, lambda, ambient air temp and water temp. Dont know at the moment how to log gas delivery hose flow and absoloute pressure, could proberbly use an ordinary MAP sensor? <BR> <BR>Will post up results in the new year.

Richard I've sent you a PM <BR> <BR>Thanks <BR> <BR>Mike