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LPG related high idle

10K views 27 replies 6 participants last post by  Pilch 
#1 ·
over the last day and a half the revs have been sticking at 1500-2000rpm after i give it any kind of a boot. it does this on lpg only. i am currently running a new generation setup on a thermoquad carb.

i havn't had a chance to look at it yet, but it seems like something is sticking in the carby and reducing air flow. typically if i punch the throttle it goes back to standard idle!

Has anyone experienced similar issues before?
 
#3 ·
nah....it will hold revs and just cruise along until i punch the throttle....it doesn't bother me to much though as i'm doing a lot of highway driving, so i dont even realise it. its a bit dangerous though.
 
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#4 ·
had the same thing happen to me ALOT, would nail it, then when i backed off, well i was still going, i was on the highway one time it happened and it was at about 100-110km/h stuck on, so i had cheap mans cruise control for the 40km trip home, LOL

never really tracked it down properly because i changed to straight LPG, and put in new regulator and mixer...

but.... i suspect it was the regulator sticking on, and i had a few guys agree with me, so if i was you i would be grabbing another reg and testing that out first.
 
#6 ·
if its what i think it is it will get worse and possibly cause flooding, i think it will probably be the converter needing a clean, they get a wax build up on the pivot foint that the diaphram opens the impco systems dont suffer this problem due to much heavier springs(no you can't fit a heavier spring!)
i clean my own cos i know lots about gas stuff without blowing myself up, the best stuff for cleaning the wax stuff off is thinners.
it costs about $80 to have your converter cleaned by a professional something you shouldn;t attempt unless you are qualified
usually this problem comes with harder than normal staring when cold
 
#8 ·
A dirty converter will not cause sticky throttle issues. Make sure your throttle return spring is OK and that your butterflies are not sticking open.

Even a half stuck open diaphram will not cause your car throttle to stick as the butterflies should stop the gas flow anyway..... the car would actually flood and you'd stall.

You might want to check the idle RPM adjustments as well.
 
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#9 ·
dunno about that xdc351 because when it was happening to me i got out of the car to check, and butterflys were fully closed, and my return spring was strong anyway....

i gave a quick flick of the throttle on the carb, and it went back to normal operation
 
#10 ·
ZK 351 said:
dunno about that xdc351 because when it was happening to me i got out of the car to check, and butterflys were fully closed, and my return spring was strong anyway....

i gave a quick flick of the throttle on the carb, and it went back to normal operation
How could you tell if the butterflies were fully closed? If they are sticking the spring will still be quite tight, but on the inside of the carb the butterflies are open 1-2mm more than they should be. A quick flick can snap them back to where they should be, but that means its time to have the carb overhauled.
 
#12 ·
Hijack: I was told it was illegal to run LPG through a thermoquad (although this might depend on the date of installation). Apparently people were not letting the petol in the carby run out before switching to LPG and they were getting explosions and fires. I used to have his setup myself and this was what I was told. Anyone heard of it?
 
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#13 ·
i actually asked about that when i got my system installed, because i had heard the same thing
and the guy said they actually prefer the thermoquads because of the solid plastic floats from standard, so they dont colapse like the brass/steel things in the holleys, (this was in NSW)
but in saying that, my grandpa blew the side out of a thermoquad because of backfire on LPG, LOL
 
#14 ·
ZK 351: Yep, thats exactly how sticking butterflies act. All the external linkages are as far back as they can go, but the butterflies stay open. Its dangerous and could lead to a really nasty accident. Best to make switching your car off a reflex action.

xcgxl: All carby dual fuel setups do that. Its because the petrol lock off valve is between the carby and the petrol pump, so when you have to switch to LPG you must put the fuel switch into the 'off' position (which locks the petrol off and the LPG off) until you use all the fuel in the bowles and lines. Failure to do so will result in a massive 'dual fueling'.... which means both fuels are running at the same time. Common result of dual fueling is putting out an engine bay fire.

EFI is, of course, immune to this problem.
 
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#15 ·
xdc351 said:
ZK 351: Yep, thats exactly how sticking butterflies act. All the external linkages are as far back as they can go, but the butterflies stay open. Its dangerous and could lead to a really nasty accident. Best to make switching your car off a reflex action.
ok then, seems to make sense. why did it never do it on petrol though??
 
#16 ·
I couldn't say. Could be the petrol was lubricating something that was drying out with the LPG on, could be LPG was gumming it up. The 465 holley on my 6cyl does it every now and then, but I know its time for a rebuild of that carb!
 
#17 ·
I agree with xdc351 it can only be your butterflys sticking open, I had the same set up as you and occasionally got the same problem only not so bad mine use to stick around 1100 - 1200 RPM. If you are running an imco system I would recomend you go to straight gas and mount your mixer on a square bore holley base. I am running a 800 holley base with an impco 425. Impco make their mixers to fit holley bases. Good luck

joblue xcgxl351 302 heads crow cam lukey headers straight gas T5
 
#18 ·
hey guys.....plenty of good advice in here now...thanks alot. i havn't yet had a chance to check out my car over the w/end.

sticking throttle hasn't happened again, and i havn't run it on petrol so the butterflies havn't been lubricated in any way. throttle return spring is a big sucker, so it always! returns. i'm leaning towards a convertor issue, and as i don't know stuff all about the gas side, it may have to visit a mechanic.

i hadn't heard of thermoquad/duel fuel being illegal, but i'm happy with it as a precisely tuned thermoquad offers excellent economy and streetable performance, so there will be no changes.

the convertor is the same as when I had a 250 X-flow. Could sticking be caused by the convertor being to small?

If the convertor is dirty, hence causing flooding as deankdx mentioned, what would this be bring about? poor economy, stalling?
 
#19 ·
Check that you havent got a split in your GANN hose. Thats the 1 inch diameter wire wound hose between the convertor and the carb. If air gets in it could cause the fuel to run lean and increase the idle. The hole may open and seal itself intermittently so that it doesnt happen all the time.
 
#20 ·
xcgxl said:
Check that you havent got a split in your GANN hose. Thats the 1 inch diameter wire wound hose between the convertor and the carb. If air gets in it could cause the fuel to run lean and increase the idle. The hole may open and seal itself intermittently so that it doesnt happen all the time.
the new generatin mixer(looks like a standard air cleaner) does not have the big wire bound hose it uses a heater hose size(probably IS heater hose) and has a power valve inline, and most likely has i POS landi converter(was this car converted from a six?)
i havent had a clevo for about 7 years(i only miss the sound) so i forgot about this problem i had also with the thermoquad, that someone mentioned earlier,
next time it idles fast(lift off the mixer to see exactly where the accel cable pulls so you can find it with mixer fitted) try to push throttle closed at carb, as this could well be the case, i had this trouble with a 650 holly also.
if you can justify a change the impco set up is far superior than what you have (duel fuel type looks ugly) impco 425 carb on holley base is a GREAT thing and looks horn with a decent (coventional)air filter
ps if you get a backfire on gas dont switch over to petrol unless you are certain it didn't blw a chunk out of the thermoquad allowing petrol to leak over everything and have a BBQ under bonnet like i did.
holden quadrajet is a great pollution carb for clevos and can be tuned easily also good with duel fuel
 
#21 ·
yeah, the new gen does indeed use a heavy duty heater style hose, with a tap in the line between the convertor and carbie (i wouldn't know the technical name)

yes, this car was converted from a six about 14 months/28,000km's ago with a ground up rebuilt 351C.

i'm not gonna pass up the new gen setup as i am currently pulling excellent economy!!! i just can't justify the change.

if it backfires i'm more likely to pull over than swap fuel types, and i havn't yet heard a backfire from the car even since the V8 conversion.
 
#22 ·
Pilch said:
yeah, the new gen does indeed use a heavy duty heater style hose, with a tap in the line between the convertor and carbie (i wouldn't know the technical name)

yes, this car was converted from a six about 14 months/28,000km's ago with a ground up rebuilt 351C.

i'm not gonna pass up the new gen setup as i am currently pulling excellent economy!!! i just can't justify the change.

if it backfires i'm more likely to pull over than swap fuel types, and i havn't yet heard a backfire from the car even since the V8 conversion.
being a 6 cyl converter as i suspected, you could well be running very lean hence great economy(good chance of burnt valve,detonation,blown head gasket etc), performance would suffer with this converter on a 351 be ok if it was a 302, if the converter is a LANDI brand they are common to have wax build up as i mentioned earlier(mine needs cleaning at the moment) they work ok when they are working OK. spose you could squirt some CRC or somethong on linkages anyway, but i personally dont like your combo on a 351
 
#23 ·
it is a good point you make deankdx about the potential lean mixture, but I would have expected signs by now, and the mechanic I use would have swapped if it was remotely necessary as he likes charging big bucks.

i will try and remember to check the brand of the convertor and post tomorrow. that should help out a bit.

as a matter of interest, what economy would a holley/impco setup produce?
 
#24 ·
Pilch said:
it is a good point you make deankdx about the potential lean mixture, but I would have expected signs by now, and the mechanic I use would have swapped if it was remotely necessary as he likes charging big bucks.

i will try and remember to check the brand of the convertor and post tomorrow. that should help out a bit.

as a matter of interest, what economy would a holley/impco setup produce?
with a gas cam almost stock 351 recuved dizzy(for gas) 4 speed sinle rail 2.92 diff mostly(90%) highway driving i was getting 350km per 70ltrs lpg
and everyone said i was getting GOOD economy from it even though i was complaining(i get 500 km fom a 250 crossflow std with impco system 70ltrs)
. these forum things are a disscussion so don't take everything you read as fact as some is BS and some people don't remember correctly. this is my opinion and i'm speaking from MY experience
 
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#25 ·
hmm well with my 351, fmx auto, and 2.77 diff, i have gotten a most of 400km out of a tank, and 86L is the most i have sqeezed in the 2 times i have ran out..
driving to and from work, i get around 250km a tank, out of 86ltrs, sometimes when i fill up after 250ish km there is only 5-10L left in the tank
haha good old economy, NOT, (and no not whinging, dont really care, if i cared i would get a smaller motor, and or drive more economically)
 
#26 ·
ok, checked the convertor for a brand, but still not sure what it is. It says:
O.M.V.L.R 90/E G.P.L.
Is this a brand? or is just some model number? I have come to belive the make is O.M.V.L.

My best economy I reached in the last week was 300km off 47L mainly hwy. i've got a T5 and 3.23 diff, and it has been dyno tuned. more city style driving normally returns about 260km off 50L.

i agree with the comment about BS!
 
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