LPG + Timing

[Mean Green]

Help all you LPG-heads!

During my previous problems with my rough running 3.9 V8, i had the dizzy changed and the garage that did it set the timing at 14 degrees BTDC.

I have been running it like this for a few weeks and feel that it is under powered both on gas and petrol. I wanted to check what timing set up others have used. I have been advised that it should be more like 21 BTDC for gas.

Can anyone shed light on the correct setting, or is it just a csae of trial and error?

[keithjh]

8 on petrol, it will run okay on lpg at that setting but you will lose a bit of power on lpg also the tickover will be lumpy, best bet is to fit a RPI ignition module about £125 to buy easy to fit will then automatically change timing for lpg, thats what i have on my 4.6 classic, timing module made a big difference.

regards

keith

[Niall_CSK]

set the timing at 14 degrees BTDC.

I have been running it like this for a few weeks and feel that it is under powered both on gas and petrol.

Don't run it at that on petrol for any length of time or there is a good chance of burning piston crowns or valves. If you can't hear it pinking, not so easy to hear with the V8, then there could well be something wrong with the pointer or pulley marks, although the sluggish comment suggests that it is too far advanced.

Try to establish if the TDC mark is correct, the pointer is quite easy to bend and pulley have been known to slip. You can do it with the heads on using a small screw driver down the plug hole but it takes some practice. You can get tools that screw into the plug hole that whistle at TDC but you can make something yourself from an old plug and some plastic pipe that will do the job even better. Break the insulator off an old plug (easier siad than done and cover your face while doing it) and either weld a short bit of pipe with suitable diameter onto the outside or glue your plastic pipe into the hole. Plastic pipe needs to be long enough to make a double U like an S on it's side with each U being about 50/60 mm radius, fix the loops so they can't move. Wth the appropriate cylinder about 20 deg before TDC put enough water in the outer U to fill it half way then start turning the engine slowly, don't think I need to explain further!

Estabished knowledge says a petrol engine generally performs best with as much advance as it can handle without pinking (preignition). Every engine is a bit different so the best way is to try it by slowly advancing a couple of degrees till it does pink (under the same conditions each time) then turn back two degree's.

If you find it difficult to hear then try using a small mike fixed near the outer head into a small amp with headphones, some have used a walkman on record while monitoring with good results.

As I said every engine is different but from my own experience with rebuilt 3.9's I have found that you generally end up with around 6-8 deg BTDC on petrol at 650 rpm with the vac hose off (TDC marks verified during the rebuild with a dial gauge-being an engineer I like to get as basic as possible).

I have used the RPI "advance to retard" unit, Dennis has it now, and didn't note any difference in power but a bit of improvement in consumption, it is very difficult to be objective & I wasn't able to do tests at the time. Be aware that the RPI unit works in reverse i.e. you set the advance on lpg and it will retard the timing by a fixed amount when switched to petrol but only when the revs are above about 1500 rpm so there is still the possibility to have the engine running on petrol at too much advance and heavy load if you are slogging it, not so easy to do with the auto though. With the RPI unit you must remove the dizzy mounted amp and connect directly into the trigger head, the unit acts as an amp but will not function if you leave the original amp in series with it, this means that if something goes wrong with it you have to strip the dizzy & reverse the process.

The Iwema unit does basically the same function but I am told that it works right accross the rev range and simply plugs in series with the existing amp, so if you want to go back you just unplug it. I believe it is a little cheaper also. I have never used one so I can't offer any further advice than that.

[Mean Green]

Don't run it at that on petrol for any length of time or there is a good chance of burning piston crowns or valves. If you can't hear it pinking, not so easy to hear with the V8, then there could well be something wrong with the pointer or pulley marks, although the sluggish comment suggests that it is too far advanced.

Try to establish if the TDC mark is correct, the pointer is quite easy to bend and pulley have been known to slip. You can do it with the heads on using a small screw driver down the plug hole but it takes some practice. You can get tools that screw into the plug hole that whistle at TDC but you can make something yourself from an old plug and some plastic pipe that will do the job even better. Break the insulator off an old plug (easier siad than done and cover your face while doing it) and either weld a short bit of pipe with suitable diameter onto the outside or glue your plastic pipe into the hole. Plastic pipe needs to be long enough to make a double U like an S on it's side with each U being about 50/60 mm radius, fix the loops so they can't move. Wth the appropriate cylinder about 20 deg before TDC put enough water in the outer U to fill it half way then start turning the engine slowly, don't think I need to explain further!

Estabished knowledge says a petrol engine generally performs best with as much advance as it can handle without pinking (preignition). Every engine is a bit different so the best way is to try it by slowly advancing a couple of degrees till it does pink (under the same conditions each time) then turn back two degree's.

If you find it difficult to hear then try using a small mike fixed near the outer head into a small amp with headphones, some have used a walkman on record while monitoring with good results.

As I said every engine is different but from my own experience with rebuilt 3.9's I have found that you generally end up with around 6-8 deg BTDC on petrol at 650 rpm with the vac hose off (TDC marks verified during the rebuild with a dial gauge-being an engineer I like to get as basic as possible).

I have used the RPI "advance to retard" unit, Dennis has it now, and didn't note any difference in power but a bit of improvement in consumption, it is very difficult to be objective & I wasn't able to do tests at the time. Be aware that the RPI unit works in reverse i.e. you set the advance on lpg and it will retard the timing by a fixed amount when switched to petrol but only when the revs are above about 1500 rpm so there is still the possibility to have the engine running on petrol at too much advance and heavy load if you are slogging it, not so easy to do with the auto though. With the RPI unit you must remove the dizzy mounted amp and connect directly into the trigger head, the unit acts as an amp but will not function if you leave the original amp in series with it, this means that if something goes wrong with it you have to strip the dizzy & reverse the process.

The Iwema unit does basically the same function but I am told that it works right accross the rev range and simply plugs in series with the existing amp, so if you want to go back you just unplug it. I believe it is a little cheaper also. I have never used one so I can't offer any further advice than that.

Thanks Niall

We managed to get it running a lot better at the weekend - I threw away the K&N, went back to a genuine LR Air Filter item and everything seems to have fallen into place.

Goes very well and even manages to go above 65!!

Then the lousy handling takes over and does its best to throw you into the nearest hedgerow!!!!

[Niall_CSK]

Thanks Niall

We managed to get it running a lot better at the weekend - I threw away the K&N, went back to a genuine LR Air Filter item and everything seems to have fallen into place.

Goes very well and even manages to go above 65!!

Then the lousy handling takes over and does its best to throw you into the nearest hedgerow!!!!

I used the K & N direct replacement for the original LR item inside the trumpet & can on the CSK & it worked fine. I have found that increased air pressure / draft from either the fan or forward motion can upset LPG by creating false pressures on the intake side of the mixer. One solution is to put a shield to disrupt external dynamic air flow hitting the trumpet end or around the filter.

Above 65, oh good, you be able to get past Nicks 90 then

I think I have said enough about handling already.

[Mean Green]

Above 65, oh good, you be able to get past Nicks 90 then

I think I have said enough about handling already.

Hmm, I seem to remember you mention something about bushes