Missfiring, occasional backfireing & hesitation on LPG

Hey Guys,

My RB30 doesn't run on LPG well since the replacement of the cylinder head.
It runs really well on petrol.(I haven't checked the AFR or tune the carby yet though)
I cleaned and renewed all the earth wires and added more earth connections and also cleaned and renewed
connections in LPG wiring. Ignition leads are fine. AFR gauge shows good AFR(14-15).
Basically it sounds like it's starving of LPG, but not sure 'cos LPG economy is pretty bad.
I put 30 litres and got about 100k out of it, so it's horrible economy for LPG.
It idles ok on LPG with a very slight misfire, but not all that bad.
Problem starts when under load. It gets worse as the engine gets hotter.

Cylinder head I put on it came with spark plugs NGK BCPR6ES-11, so I didn't change it.
The factory recommended one is NGK BKR6E.
Is this the problem?!!
Checking the plugs on NGK website there isn't a whole lot of difference between them other than the 2.5mm height difference on top side.
https://www.ngk.com/product.aspx?zpid=9292
https://www.ngk.com/product.aspx?zpid=9292

BCPR6ES-11 has JIS height while the BKR6E has ISO height, so I reckon it's basically the same plug with different height on the top.

is the afr reading at idle, what range does it go thru when driving, eg; 12 under load to 16 after backing off. if your afr reading is only at idle it does not mean that it is not rich when under load. i have no personal experiance with lpg myself but i have seen friends cars in the past that have had problems with the lpg especially if the lpg has not been used for a while. they had the system serviced and some parts were replaced as they could not be cleaned/repaired, they went well after though.

Quote:

---

Originally Posted by nipagu7

is the afr reading at idle, what range does it go thru when driving, eg; 12 under load to 16 after backing off. if your afr reading is only at idle it does not mean that it is not rich when under load. i have no personal experiance with lpg myself but i have seen friends cars in the past that have had problems with the lpg especially if the lpg has not been used for a while. they had the system serviced and some parts were replaced as they could not be cleaned/repaired, they went well after though.

---

Thanx for the post mate. I adjusted the idle AFR and "POWER"(load) AFR both to be around 14.5-15(Impco 300A old skool mixer with stock RB30 Nikki carby). Mixer, converter, tank, filter+solenoid near the converter, etc etc are all old. Only new thing is the cold start solenoid. I replaced the Impco model L converter with a good used one last year.
Yes, the LPG plumbing+gear can do with an upgrade.
I'm ruling out any electrical/ignition issues first before I get into LPG gear.
Most LPG problems aren't LPG gear related but electrical connection & ignition issues(which itself is electrical anyway).

I reduced the spark plug gap from 0.8mm to 0.5mm using a feeler gauge properly this time & the problem appears to have fixed.
It appears the stock gap is no good for LPG as LPG burns slower(higher octane) and apparently needed stronger spark, therefore the narrower gap seems to be fixing the issue. I tried retarding the ignition time to 15 to even 20 BTDC, but that didn't fix it either.
Only the shortening the spark plug gap works.
I am thinking of experimenting with a colder spark plugs for LPG(NGK BKR5E, instead of BKR6E) and see how it goes.

But, in the meantime, can you gents please suggest a suitable LPG spark plugs & leads to improve performance in my RB30 Patrol?
Below are the gear it has.

Standard RB30 Nikki carby stock NA setup
Impco 300A mixer
Impco model L converter
No snorkel(yet, but coming soon).

IMO its either an air leak or running lean they are the onlt 2 things
I can think of atm as to why a motor would back fire Ill add timing as well
so theres 3 things to look at.
Get the timing just of pinging and you'll be close to the ball park .
are you blowing the air cleaner box off yet??
Have you Y pieces instaed of Tees that will aid in water flow and less cavitation

Quote:

---

Originally Posted by threedogs

IMO its either an air leak or running lean they are the onlt 2 things
I can think of atm as to why a motor would back fire Ill add timing as well
so theres 3 things to look at.
Get the timing just of pinging and you'll be close to the ball park .
are you blowing the air cleaner box off yet??
Have you Y pieces instaed of Tees that will aid in water flow and less cavitation

---

It turns out the poor ignition due to spark plug gap being too high(or I think that's what it is 'cos narrowing the gap appeared to have fixed it). I was so sure there's an air leak somewhere, so kept looking for it, but couldn't find any vacuum leaks.
Besides that, the spark plugs in it aren't the factory recommended ones even though the specs are the same except the outside height of the plug and the gap. 0.5mm gap adjustment appears to be working for LPG.
I didn't bother to try it on petrol, but I'm pretty sure it won't be happy without adjusting the timing to around 10btdc or less.
It's not possible to fine tune for both fuels unless I employ a dual curve unit.
I've never invested on the dual curve unit, 'cos I thought it's less complicated to carry a second set of spark plugs & adjust the timing manually when it is necessary to switch to the other fuel. Of course that won't address the timing curve, but it's less complicated.
Adding more electronics on top of having an electronic dizzy in RB30 Patrol complicates things more I thought.

I'll clock the LPG economy again and see how it goes. If the economy is still bad, then I still have a problem.

do you have an O2 sensor on the exhaust...........?????

Quote:

---

Originally Posted by Bigcol

do you have an O2 sensor on the exhaust...........?????

---

Not per say, but there is an aftermarket AFR gauge sensor in the exhaust, about a foot down from the exhaust flange.

Something to consider.
LPG spark gap to be 0.1-0.2mm less than petrol spark gap. Run LPG in a DX2.4 old Nissan tray farm ute, use to adjust gap to 0.7mm/0.028", now don’t bother and use stock gap of 1.1mm.

LPG increases the required voltage to form a spark therefore reducing gap to .020" will assist in getting better spark but cooler (0ne heat range lower) plug is a better option. I use a NGK 4435 ZFR5E-11 Nickel Spark Plug and have the same reach/thread dia/temp range/gap and materials build spec as your suggested NGK 6953 BKR5E-11.

Need good leads but if you are still using your old ones that worked before they are probably OK.

I run old build IMPCO set up (100,000 + km on gas now o/a 190,000ks) never back fired or missed. Some times a bit hard to start on gas when really hot, switch to petrol it floods easily, shut all fuel / crank and evacuate fuel, and then start on petrol and flick over to gas - all good (rare though). Rebuilt mixer 6 mths back agricultural basic build but reliable.

Had one other veh on LPG that loved to blow the resonator box lid off and smash up the airflow honeycomb segments in the air box. Not suited at all glad to get rid of it. Rather Diesel any day but the ute does real well..

Iridium plugs are a more suited for gas but significant more $$$s. Don’t think there needed in your case.

Quote:

---

Originally Posted by Bidja

Something to consider.
LPG spark gap to be 0.1-0.2mm less than petrol spark gap. Run LPG in a DX2.4 old Nissan tray farm ute, use to adjust gap to 0.7mm/0.028", now don’t bother and use stock gap of 1.1mm.

LPG increases the required voltage to form a spark therefore reducing gap to .020" will assist in getting better spark but cooler (0ne heat range lower) plug is a better option. I use a NGK 4435 ZFR5E-11 Nickel Spark Plug and have the same reach/thread dia/temp range/gap and materials build spec as your suggested NGK 6953 BKR5E-11.

Need good leads but if you are still using your old ones that worked before they are probably OK.

I run old build IMPCO set up (100,000 + km on gas now o/a 190,000ks) never back fired or missed. Some times a bit hard to start on gas when really hot, switch to petrol it floods easily, shut all fuel / crank and evacuate fuel, and then start on petrol and flick over to gas - all good (rare though). Rebuilt mixer 6 mths back agricultural basic build but reliable.

Had one other veh on LPG that loved to blow the resonator box lid off and smash up the airflow honeycomb segments in the air box. Not suited at all glad to get rid of it. Rather Diesel any day but the ute does real well..

Iridium plugs are a more suited for gas but significant more $$$s. Don’t think there needed in your case.

---

Thanx for the post mate.

It's running good after shortening the plug gap.
Seems to have developed bit more power as well.
Now I need to clock it and find out whether the LPG economy has improved at all.
Next project is to get the extractor set in and fit a snorkel. Both items are ready(used ones), just
haven't had any time to do that, but it's pure excuse to keep if for a year claiming to have no time. :)

I looked at Impco 300A mixer and model L converter rebuild kits, but the price make is pointless 'cos a brand new one is not too far
from the asking price for a rebuild kit. It's ok to do that just to gain some experience, but I've had enough of that by fiddling with the Nikki carby of this RB30. :)

I keep a used one just it case, but will need to consider better LPG gear sometime soon if I'm to keep this troll going good on LPG with good performance and economy.

Yeah I'm ordering the colder plugs(NGK 6953 BKR5E-11) for LPG and see how it goes.
One experienced mechanic indicated here @Alitis007 that it can improve performance but might burn more fuel.
I'll have to see how it goes.

And yeah, I need to get hold of a lead set that is more suitable for LPG, while keeping the current set for petrol, and do the same with spark plugs as well.
It's not like I need to switch the two fuels on the fly. :), so it's no bother to get out and do it manually.