V8 Ignition

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V8 Ignition

Post by Lazarus » 28 Nov 2022, 8:09 am

I’ve been slowly renovating my 89 Range Rover, I’ve done a new 3" exhaust, new bushes, rebuilt the LPG converter and mixer, but I’ve still got a little firing problem on the gas side.

It runs under low load OK but gets a misfire happening at full noise pedal, but only on gas, goes like a stabbed rat on 98 octane dinosaur juice.

The leads, plugs and dizzy cap are less than 12 months old, the ignition module is brand new, but, the coil is ~15ish years old.

The only gas mechanic withing cooee (actually there’s 2 but I want to stab the other vigorously with a sharp stick) says it sounds like ignition related as gas needs more spark to fire.

I’m thinking, rather than just replacing the coil, it might be worth replacing the old vacuum advance dizzy and the oil filled coil with a Davis Unified Ignition dizzy with integral 50KV coil and mechanical advance.

Anyone here got the knowledge/experience to offer any constructive advice?
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Re: V8 Ignition

Post by Baronvonrort » 28 Nov 2022, 9:15 am

The fact it runs good on petrol leads me to think ignition side isn't the problem. If ignition was the problem it wouldn't run good up top on either fuel.

Jaycar make these kits they're pretty accurate i have compared them with my $3k digitial air fuel ratio meter . I sold my AF meter while i could still get good money for it when people realise how good these Jaycar products are they would never spend heaps on a professional meter. The LED light kits are even cheaper the ones they were selling 30 years ago were really good.
https://www.jaycar.com.au/fuel-air-mixture-display-kit/p/KC5485

I would look at A/F ratios on Gas to see what is happening. Not sure what type of LPG setup you have i know the Gas Research LPG carby was really good worked much better up top than other systems.

The fact you have vacum advance tells me this isn't a fuel injected vehicle. Mechanical /vacum advance systems belong in a museum.

Back in the early 90s i converted my 265 Hemi powered UC Torana hatch to fuel injection. Had to weld up mec advance piss off the vac advance used a Lumenition optical pickup to replace points and stuck a MSD 6A on it running through Haltech ECU. The MSD is under brake booster.

The gains on ignition side were massive at part throttle being able to map it through entire rev range at all load points. It always had a flat spot around 2% throttle 2000 rpm with triple webers. Cranked ignition advance up to 38 deg at that load point flat spot was gone it purred like a kitten.

It's the old saying if you want it done right sometimes you have to do it yourself.

Gas will allow more ignition advance so you will never tune it right to run on both fuels with mech/vac advance. To get best performance on gas ingnition timing will be wrong for petrol. One fuel with have to be compromised for performance with ancient advance controls. If you have a switchable ignition map it can be tuned to run great on both fuels.
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Re: V8 Ignition

Post by Lazarus » 28 Nov 2022, 9:28 am

Thanks for that Baron.

Yeah, it's injected, with MOTEC running that side and a standard IMPCO 200 mixer.

It started as a standard 3.9, in 98 it was breathed on heavily by Canberra Motor Works who turned it into a 4.6 stroker with cold air induction, all manner of expensive internal jiggery-pokery, 3" exhaust etc, etc bringing it from the standard 175hp to 310.
They also upgraded the suspension with King Springs and Bilstien gear and replaced the OEM brakes with big ventilated discs etc.

A few of those ponies have escaped or died in the 360,000km since but he still gets along.

Since posting I spoke to a Land Rover performance mob in Sydney who said the same, get a sniffer up it before doing anything.

I'll get on to Jaycar
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Re: V8 Ignition

Post by Baronvonrort » 28 Nov 2022, 9:40 am

The cheaper kits that use LED lights work good as well. They measure O2 sensor voltage so you know where it's at for AF ratio. I had one of those kits for a warning to let me know if it leaned out.

Since you have Motec piss off the ancient advance controls and do ignition timing with that.
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Re: V8 Ignition

Post by on_one_wheel » 28 Nov 2022, 5:03 pm

+ 1 with everything baron said, he's spot on

I'll take his advice on the jaycar o2 set-up and get one myself.
Cheers baron :thumbsup:
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Re: V8 Ignition

Post by northdude » 28 Nov 2022, 6:54 pm

if you do all that and still have the miss Id look at the valve guides. It used to be a common thing over here but most of them are in the tip now
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Re: V8 Ignition

Post by on_one_wheel » 28 Nov 2022, 7:23 pm

Range Rover hydroscillators are also notoriously sketchy, could be worth checking that after looking at the valve guides.
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Re: V8 Ignition

Post by Lazarus » 28 Nov 2022, 8:18 pm

on_one_wheel wrote:Range Rover hydroscillators are also notoriously sketchy, could be worth checking that after looking at the valve guides.


With close to 500,000 on the clock, the list of possibilities stacking up are giving me the squirting sh!ts.
I recently replaced the water pump and the cavitation erosion in the back of the housing was extremely disturbing.

If I had my druthers, I'd relace the original with an LS3, or in my fantasies, an LSA.

But then the realities of my financial situation bitchslap me back to normality and I'm reaching for the Sorbent again.
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Re: V8 Ignition

Post by northdude » 29 Nov 2022, 4:58 am

Over here moreys did an upper cyl lube kit. They worked well on gas vehicles
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Re: V8 Ignition

Post by Latitude37 » 08 Dec 2022, 5:41 pm

It could be as simple as changing to colder plugs. In NGK, the standard plugs are listed for the Rangie as BP5ES. On gas, I'd go a heat range colder (lpg burns hotter, and "colder" plugs conduct heat out of the combustion chamber more effectively) and that would be a BP6ES, or BP6E - the only difference between those two is that the "S" is standard, the BP6E has a v notch in the positive electrode for better spark placement. Anyway, could be a simple thing to try. Good luck.
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Re: V8 Ignition

Post by Lazarus » 08 Dec 2022, 6:20 pm

Latitude37 wrote:It could be as simple as changing to colder plugs. In NGK, the standard plugs are listed for the Rangie as BP5ES. On gas, I'd go a heat range colder (lpg burns hotter, and "colder" plugs conduct heat out of the combustion chamber more effectively) and that would be a BP6ES, or BP6E - the only difference between those two is that the "S" is standard, the BP6E has a v notch in the positive electrode for better spark placement. Anyway, could be a simple thing to try. Good luck.


""Over here moreys did an upper cyl lube kit. They worked well on gas vehicles""


Thanks guys I'll look into both.
It's booked in for an exhaust analyser tune and the mechanic mentioned something about putting an oscilloscope on it as well :drinks:
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