Nasty bangs

[aRRon]

Set up is a late '89 RRC 3.9l with an IMPCO 200 LPG system installed in 1999.
After any ideas on what might be causing loud explosions in the exhaust under high throttle opening/heavy load on gas? Car runs perfectly on 95 RON petrol.
Fully recoed the motor about 80K ago and fitted 9.35:1 pistons, timing is max advanced and have the Bosch 024 ign amp conversion.
Has run beautifully, including idle for many years. 5km/l consumption. No backfiring.
Recently had the twin LPG tanks retested and all was good - otherwise hasn't been touched.
On heavy load such as running up to 100ks on freeway on-ramp or towing a heavy trailer up hill, get a sudden loud bang in exhaust, then runs on fine.
I always went by rule of thumb that backfiring is ignition and banging/popping in exhaust is fuel. No indication of increased consumption prior to onset of detonation.
Have had some loose exhaust manifold bolts and tightened them up to no avail - can't detect any obvious exhaust leaks there though or at flanges.
Could it be excessive fuelling and are there any basic adjustments I can check?

[bee utey]

Could be that the plugs are running too hot. What heat range are you running?

[aRRon]

Could be that the plugs are running too hot. What heat range are you running?
Hi Bee Utey
Have always fitted NGK BP6ES and last replaced about 10k ago. Engine itself runs pretty cool.

[bee utey]

Hi Bee Utey
Have always fitted NGK BP6ES and last replaced about 10k ago. Engine itself runs pretty cool.

The engine temperature will have little to do with plug operating temp at full load, that's a local issue at the insulator tip. If a plug misfires due to tracking down the insulator you are pushing unburnt fuel into the exhaust system. I'd be closely inspecting the plugs for anomalous insulator colour, looking for an odd one with a brownish tinge. Plug gap too should be checked and be no more than 0.8mm.

[aRRon]

The engine temperature will have little to do with plug operating temp at full load, that's a local issue at the insulator tip. If a plug misfires due to tracking down the insulator you are pushing unburnt fuel into the exhaust system. I'd be closely inspecting the plugs for anomalous insulator colour, looking for an odd one with a brownish tinge. Plug gap too should be checked and be no more than 0.8mm.

Thanks for that advice - I'll go over the plugs and check condition.

[aRRon]

Further progress...
Plugs appear ok but discovered I have a perforated vacuum advance diaphragm. So reasoning is insufficient burn time and leaning out = lots of unburnt gas and extra oxygen in the exhaust = bang? Blocked off the line and no more detonations but no advance either and sluggish and burbly in the exhaust. Have ordered a new distributor as it's done 300k anyway. Will do leads while I'm at it. Funny how it so often ends up being something minor.

[DAMINK]

You could have picked up a new vac advance unit pretty cheap. And there a piece of cake to install.
Although a new Dizzy is always a good option although expensive.

[aRRon]

You could have picked up a new vac advance unit pretty cheap. And there a piece of cake to install.
Although a new Dizzy is always a good option although expensive.

True, I ummed and ahhed over $121 for vac unit + $25 for cap into a 30 yr old 300k dizzy and went for a $195 new dizzy on ebay with the lot. Hope it's not too much of a Chinese toy.

What is the opinion on best HT leads for LPG?

[bee utey]

True, I ummed and ahhed over $121 for vac unit + $25 for cap into a 30 yr old 300k dizzy and went for a $195 new dizzy on ebay with the lot. Hope it's not too much of a Chinese toy.

What is the opinion on best HT leads for LPG?

I buy my vac advances from the UK, now with GST added they're a bit over $60 landed.

Land Rover Discovery Defender V8 replacement vacuum unit Powerspark RTC3201 | eBay (https://www.ebay.com.au/itm/Land-Rover-Discovery-Defender-V8-replacement-vacuum-unit-Powerspark-RTC3201/110890024552'ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649)

Bosch 8mm ignition leads have served me well for the last 15 years or so, you used to have some places where you could buy the leads individually to make up a set on the spot. Bursons still stock them although some sizes have to be ordered in when I ask for them. Kits are dearer:

Bosch Super Sport Spark Plug Lead B4757I fits Land Rover Discovery 3.5 4x4 (L... 9311831058742 | eBay (https://www.ebay.com.au/itm/Bosch-Super-Sport-Spark-Plug-Lead-B4757I-fits-Land-Rover-Discovery-3-5-4x4-L/152737080836?epid=1838169426&hash=item238fd6e604)

OEM style leads work well too, the ones to avoid are low resistance types like NGK or Eagle. Example:

Spark Plug Leads HT x 8 Land Rover Discovery 1 Range Rover Classic V8 | eBay (https://www.ebay.com.au/itm/Spark-Plug-Leads-HT-x-8-Land-Rover-Discovery-1-Range-Rover-Classic-V8/201643566046?hash=item2ef2e4a3de:g:KmwAAOSwc~5a57e j)

[rick130]

I buy my vac advances from the UK, now with GST added they're a bit over $60 landed.

Land Rover Discovery Defender V8 replacement vacuum unit Powerspark RTC3201 | eBay (https://www.ebay.com.au/itm/Land-Rover-Discovery-Defender-V8-replacement-vacuum-unit-Powerspark-RTC3201/110890024552'ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649)

Bosch 8mm ignition leads have served me well for the last 15 years or so, you used to have some places where you could buy the leads individually to make up a set on the spot. Bursons still stock them although some sizes have to be ordered in when I ask for them. Kits are dearer:

Bosch Super Sport Spark Plug Lead B4757I fits Land Rover Discovery 3.5 4x4 (L... 9311831058742 | eBay (https://www.ebay.com.au/itm/Bosch-Super-Sport-Spark-Plug-Lead-B4757I-fits-Land-Rover-Discovery-3-5-4x4-L/152737080836?epid=1838169426&hash=item238fd6e604)

OEM style leads work well too, the ones to avoid are low resistance types like NGK or Eagle. Example:

Spark Plug Leads HT x 8 Land Rover Discovery 1 Range Rover Classic V8 | eBay (https://www.ebay.com.au/itm/Spark-Plug-Leads-HT-x-8-Land-Rover-Discovery-1-Range-Rover-Classic-V8/201643566046?hash=item2ef2e4a3de:g:KmwAAOSwc~5a57e j)Curious why you don't like the low resistance ones?

Too much RFI?

[bee utey]

Curious why you don't like the low resistance ones?

Too much RFI?

Low resistance allows the leads to have moments of resonance, resulting in erratic performance at particular RPM and load. Resistive leads have better damping, and inductive leads although in theory better are often too "stiff" to work well especially with LPG. RFI is simply the external aspect of this problem.

20 years ago I had a particularly nasty little hot weather stumble trying to accelerate off at the lights in my old P76 on gas, and all the usual fixes were applied. It wasn't until I swapped out the relatively new Top Gun leads for the Bosch leads that the problem disappeared forever, leading me to look for a rational explanation. I've since met the problem across a range of vehicles (including D2's and P38's) and high resistance leads generally fixed their problems. Thanks to Mario (Roverlord) for supplying a few sets of OEM leads.

[rick130]

Cheers for that.

My last spark ignition/LPG car was an F100 18 years ago.
I used Eagle leads as they were local to me and supplied me with leads for the race cars.
351C running Bosch/Holden HEI and two ranges colder NGK plugs as the CR was stupidly high.
AFAIK i never had that issue, probably just dumb luck.
Low resistance allows the leads to have moments of resonance, resulting in erratic performance at particular RPM and load. Resistive leads have better damping, and inductive leads although in theory better are often too "stiff" to work well especially with LPG. RFI is simply the external aspect of this problem.

20 years ago I had a particularly nasty little hot weather stumble trying to accelerate off at the lights in my old P76 on gas, and all the usual fixes were applied. It wasn't until I swapped out the relatively new Top Gun leads for the Bosch leads that the problem disappeared forever, leading me to look for a rational explanation. I've since met the problem across a range of vehicles (including D2's and P38's) and high resistance leads generally fixed their problems. Thanks to Mario (Roverlord) for supplying a few sets of OEM leads.