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V8 running rough at low revs on LPG


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My V8 110 is running rough on gas at low revs and after ticking over ay the lights.

She will slow nearly to a stall and be running on what sounds like one bank and only clears after reving to about 3000rpm.

Seems OK on petrol and ticks over at 700rpm smooth as you like but wont have it on gas.

It is down on power on gas as well and dosen't feel as crisp, I'm thinking that the timing isn't as it should be and need educating as to advance figures for gas / petrol, how different is the timing requirement ?

Dont need figures just to know if it needs a little or a lot more or less advance on petrol/gas post-10430-1216849736.gif

Runs well on petrol but definate/sudden increase in power after 3500 which I'm thinking is timing ?

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Gas is quite demaning on the ignition, make sure your coil, leads and plugs are in good shape.

As for the timing, for gas it needs a bit more advance (about 5° I believe), someone more experienced will explain this further I'm sure.

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Is this problem related to the other problem you posted?

If so, I'd be looking at what is common to both petrol and lpg systems

  • Ignition - already been mentioned about a fat spark and checking the timing.
  • Breather pipes - check for split or perished pipes causing loss of vacuum.
  • Carbs - check for dirt, sticking pistons (not lifting as demanded by air flow), split diaphragms (leaks / loss of vacuum).
  • Fuel changeover - do the two fuel systems switch on/off as commanded?

HTH

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Is this problem related to the other problem you posted?

No, the missfiring is seperate.

The engine is not as crisp on gas and dosen't pull the same. It is not as responsive and has less power in general than on petrol.

Also, when running on gas the power is the same all the way through the rev range whereas on petrol there is a definate increase in pulling power after about 3000/3500 which leads me to believe that timing may be the culprit.

Should it pull at least as well on gas as on petrol, thats the question really :D

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No, the missfiring is seperate.

The engine is not as crisp on gas and dosen't pull the same. It is not as responsive and has less power in general than on petrol.

Also, when running on gas the power is the same all the way through the rev range whereas on petrol there is a definate increase in pulling power after about 3000/3500 which leads me to believe that timing may be the culprit.

Should it pull at least as well on gas as on petrol, thats the question really :D

No, it won't pull as well on LPG as it does on petrol. Generally, about 15-20% less oomph, also about 15-20% less mpg than on petrol too.

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Thanks for that,

Armed with that information I can set about altering the timing, I will set it up for power on gas and see how that affects petrol running, it may pink when back on petrol if my info is correct.

Someone must have done this before ? :unsure:

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Right, advanced the timing today and she starts better on gas and goes better on gas but pinks on petrol so I've knocked it back a bit and will try again tomorrow.

Tickover on petrol has increased to from 700rpm to 1000+rpm and is rock steady

Tickover on gas has increased from 500rpm to 800rpm but drops away slowley to a stall.

Diognosis....

One of the gas feed pipes to the inlet is slightly kinked when the bonnet (hood) is shut, this could be restricting gasflow to the carbs and causing uneven tickover, stalling and fuel starvation after ticking over.

The gas feed pipes are unequal lengths (maybe not a problem).

The air intake is breathing in very hot air from the top of the engine when stationary and could be affecting the mixture.

And my coil is hot, 77 centigrade after a run :o too hot to touch..

And the temp gauge is showing overheating (just below red) but top hose is 71 centigrade and rad is 70 to 45 centigrade depending on where I point the gauge. The header tank is 40 centigrade and the thermostat housing is 80 centigrade and it is hot under there but not I think overheating.

Chew on that lot my friends and lets spit out some solutions :P:lol:

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Thank you Escape, DaveS11A, GrahamThomson , Wouldn't run on eight this morning on petrol so whipped the dizzy cap off to find the usual plastic type film on the cap pick up points. Cracked up loverly after cleaning..

Why I no rissen :ph34r:

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Thank you Escape, DaveS11A, GrahamThomson , Wouldn't run on eight this morning on petrol so whipped the dizzy cap off to find the usual plastic type film on the cap pick up points. Cracked up loverly after cleaning..

Why I no rissen :ph34r:

Glad you found the problem!

I should have mentionned the dizzy right away, but I'm used to working on P38a's lately... :ph34r:

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