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V8 110 Misfire under load


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More and more issues on the Land Rover :-)


The wheel bearing in my other thread is a nice job in the rain for tomorrow morning so hopefully that will sort out the helicopter that appears to follow me everywhere.



However I now have had another problem develop over the week....something for the V8 guru's.



I've had what seems like a misfire on a 3.5 V8 110 (Twin SU Carbs, standard dizzy) under load. Started poking around tonight and found the Vac Advance unit was very loose. So have tightened it up and hopefully sorted it, will be able to road test tomorrow. However, to set my mind at rest tonight do you think I have found root cause? :wacko::wacko:


My thinking is, loose vac unit allows distributor base plate to move, therefore throwing the timing out which will be far more apparent under load then any other time...




Or does anyone have a MS kit for sale??? :hysterical: (Nige - I'd love to buy one of your kits but apparently spending the wife's redundancy money on it is frowned upon..)


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So a 15 mile test trip says the problem is no better :mellow: towards the end of the drive it was getting worse if anything

I have a new set of plug leads on order, there is a spare ignition amp in the garage from an old distributor (little box on side of distributor right?). Plugs have been recently replaced....

Anything else I should be looking at?

Or should I just buy a MS kit??

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Oh, the other thing I had was a load of slack in the counterweight springs and that seemed to give the occasional odd misfire + I blame them on eating rotor arms nearly as quick as they arrived (regardless of brand). Its just a PIA from memory to disassemble to distributor to get to the springs as I think you've got to take the pin out of the camshaft gear and a circlip somewhere and the assembly rises upwards (must be 10-12 years since I last took one apart so excuse my memory please).

Rotor arm failure did not seem to be intermittent, it just happened and all went south and got progressively worse but did not improve at any point.

Never experienced it, but was once told capacitor failure could be problematic, on my 3.5/90 it had a capacitor located across one of the ignition coil leads (despite the distributor being a hall effect one with an amplifier (mounted on the side of the dizzy, not as I've seen on the disco which was next to the HT coil), when I was having my ignition problems I disconnected it but is seemed to make no difference.

Other things to try.... run it at night with the bonnet up and see if you can see anything (you can normally see a slight hue on the plug leads from experience when looking closely, especially near any of the earthed metal that the plug leads go past such as the engine lifting eyes), try to run it to the point of it misfiring, pull over and again bonnet up and see if anything is untoward (intermittent spark to earth). [i suspect this won't work, but its worth a try..... for the sake of a few minutes]

I don't think it will be the coil (no basis of this statement, but I've not had one fail personally before) For the coil I'd suggest testing it for spark (old spark plug, increase the gap a bit, think the standard gap was about 0.8mm, so say about 1.0-1.2mm), plug it into one of the plug leads, and ground it, then holding the plug lead shroud with a pair of insulated pliers, get someone to turn it over (being a one man band, you could use a fly lead onto the starter motor solenoid too...) .... if you've got spark, great, now run it a while till you get it missing and try the same thing again, and see if the quality of the spark has changed.

Pulling a lead and grounding it, or leaving it a fair distance away from earth I would not recommend, as voltage will always seek out the shortest gap to jump, and it could cause something to track to earth.

Usual disclaimers apply with HT, it does bite (had my fair share) and I'm told it can kill too... [everyone responsible for there own H&S and know their own competency]

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Just been out to swap the ignition modules over and found my spare distributor that was in the 'massive box of spares' that seems to accompany most Land Rover sales is on older 2 pin type where I am now running a 35DLM8 distributor with the 3 pin module. This looks to be a right PITA to replace without removing the distributor. Has anyone seen a guide anywhere on how to remove and replace the distributor without upsetting the timing (I don't have a timing light and precious little time)

Slightly concerning as we are meant to be taking the Defender away next weekend and it is getting quite undriveable at the moment...

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My advice for swapping the distributor would be as follows (note I'm not sure what landrover support garage of tools you have amassed some of us can rival the trade garages....).

remove the distributor cap, and locate off the distributor cap where plug lead #1 is located roughly.

inspect the cranshaft damper for the timing marks, and rub a bit of chalk to the damper so that the markings can be more easily seen against the marker on the timing cover.

For accurate repositioning I would suggest getting a 12v light bulb (lamp), and 12v lamp, indicator or festoon etc, and solder a length of wire onto one of the contacts / terminals., then connect the lamp fly lead to the switched side of the coil (one side is 12v live, the other switches via the amplifier pack to ground), and ground the other side of the lamp. So that when you rotate the cranshaft (next step), you can see exactly where the amplifier pack breaks the live feed to the coil (which should align with the timing mark on the pulley damper [note these can be out a bit so are not relied upon by everyone, but are a good starting point]).

Next I would rotate the crankshaft using the crank pulley bolt (24mm socket required) slowly in a clockwise direction up to the point that you can begin to see the timing marks on the crank damper, at this point make sure that the rotor arm is point toward the position of where plug lead number 1 is (if it's 180 degrees out, just keep going)

If you're using the lamp method, I'd move the crank as slowly as possible until the lamp goes out, (you'll need the ignition on in the run position so that the amplifier has a live feed).

If not using the lamp method I'd get the damper marks to align with the pointer, and transpose a line (scribe a line) to indicate where the rotor arm exactly is pointing .... I would do this by looking directly downwards from above the dissy and mark two lines either side of the rotor arm (note these must be on the engine and not the disy you're removing..... as the lines won't be there when you need them (sorry if it sounds daft, but take your time to align everything up and double check your alignment marks ( measure twice, mark once).

The next thing I would do is look closely at the two distributor bodies and see if there is something that could be used as an alignment mark on both bodies.... I'd suggest using something like the casting lines as they will more than likley be exactly the same... what you want to do next is transpose another line onto the engine where this line is exactly (you could also use the vacc advance if they align when you study the two of them closely).

If using the lamp method, you don't need the last step (but you can never have too many alignment marks if you know what they are for...).

You are now at the point of no return, and can remove the distributor (suggest that all of your marks are there and align and can be clearly seen), think from memory it's a 14mm spanner (although it may actually be an imperial nut but 14AF has always worked for me), it can be a right PIA to remove and get the spanner in there.

Then I'd swap out the distributors, note that when you lift it out the rotor arm will twist because of the teeth on the camshaft gear, now position the rotor arm in the same plane on the new one and drop the distributor into position making sure that the rotor arm aligns with your marks when it's home, if not you will be maybe a camshaft tooth out either way, just lift it out and try again, rotating the rotor arm a bit toward the way you were out.

Once the rotor arm is aligned with the mark, then rotate the distributor body to align with the casting (or vac advance mark). (if not using the lap method).

If using the lamp method, rotate the distributor (again with the ignition on) until the lamp just goes out, then lock the distributor in position and tighten everything back up, cap on and it should start fine.

Again check all of your marks align before turning the engine over....

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Sorry, keep getting dragged away... (I'll have to drip feed this sorry ... )

If you've done the above, then it will start (either method.... best method with lamp though), you may need to give it a bit of throttle to start if the ignition is a bit out but it should run and idle (but the revs may be a bit high or low)

If it doesn't start of run then you will need to do the lamp method. (probably get a 12 lamp in halfords with a lead on it with a crocodile clip or you could make one up from bits at maplin too), note a lead lamp (led or florescent won't work, you need an incandescent lamp (indicator or interior lamp, interior lamp is probably best given one end will be live and the other end grounded).

Next I would suggest loosening the distributor locking nut and whilst it's idling and up to temperature rotate the distributor slightly to get it at the idling rpm and more importantly a smooth idle or the smoothest you can obtain .

Don't fiddle with the carbs at all as it all goes to pot and you're trying to adjust too many things at once (seen people try that if done a distributor change and it's not idling right afterward).

You could also use a smear of paint or tipex on the crankshaft damper marks, then wipe it off with a cloth so that only the grooves show up as lines and numbers with the residual paint left in them if you don't have chalk. White paint always best as it will reflect the light the most.

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Andy just to clarify you get the misfire when you drive it and Rev it hard yes? Would you say it after mid revs 3-4k.

Have you set the correct timing on the distributor with a timing light? And is your vacuum hose in good condition and is the one way valve fitted and fitted the right way round??

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Hi Robert, Thanks for the incredibly informative reply. That will be very very useful if/when I swap out the distributor and ignition module. I'll let you know how I get on over the next couple of days.

Chris - Misfire occurs when I attempt to drive at low rev's e.g. 30mph in 4th or when I try to accelerate hard. It runs fine at higher revs e.g. you can feel it misfiring at 50 in 5th but in 4th it is ok.

I've not touched the timing since I got the vehicle about 2000 miles ago and it has run sweetly until now. Ignition wise the only thing I have done is put in new spark plugs (NGK BP6ES), they were done one at a time so I didn't get the leads mixed up.

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If you did not have it before the plug change I'd suggest taking the plugs out again and inspecting the insulators just to check that one is not cracked

Also check the leads are pressed fully home.

Changing a distributor although not a major job should be a last resort.

Where are you in the uk. There may be someone close to you who can give you a second look or a timing light loan. Timing light although helpful is not the end all or be all but is useful at checking advance and for inspecting miss fire sometimes

Other things you can do is pulling a plug lead when it's running and misfiring (if it contines once it's started under load and is still doing it at idle) no change in the misfire indicates it's probably that plug lead.

You're not running it on lpg are you? LPG is a very good insulator and actually needs a good quality spark so any weak links you have in the ignition system will show up on LPG (LPG has a higher octane rating than petrol in either of its octane forms. Contrary to popular belief octane by its self does not mean more power but the fuels resistance to knock (self ignition) which it turn means it can be harder to ignite via spark. So using a higher octane fuel means you can use it in a higher compression engine and run it on the limits of knock with advance (using a knock sensor with a fuel injected motor!). to get more power out of the fuel

To eliminate the vaccume advance you could disconnect it and plug off the hose (it will provide excess air to the carb) using a small screw or bolt then give it a run and see if it's less or gone

Sorry just a load of options to consider

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Hi Chris,

Based in South Warwickshire just down the road from the Gaydon Motor Museum.

Before the plug change it had been off the road for a couple of months whilst I did some welding on the bulkhead (batt and alt disconnected). Not running it on LPG till I add MS and EFI.

Will try disconnecting the vacuum tomorrow and check the routing of 5 and 7. I think they might be crossing each other at some point. I've got to run up to Coventry with it tomorrow so will be able to test those out in the morning. Unfortunately it doesn't misfire at idle so it makes the diagnosis quite difficult..

Will report back tomorrow, thanks for the help everyone. Really appreciate it.

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Check the routing of the HT-leads: I have memories of a RV8 fitted in a Rover P5B where HT-leads clipped together to 'look nice' caused crossfiring [a cylinder beside a cylinder-being-fired getting an induced spark while on the intake stroke] which caused major backfires through the carbs and blew all the oil out of the Stromberg dashpot.

This is more of a problem under heavy loads - the more-open the throttle is the more fuel/air mixture the cylinders swallow - and the higher the compression-pressures the higher the voltage needed to fire a plug so the greater stress is placed on the HT-lead insulation.

When I was first called on to diagnose the above problem I just couldn't get it to happen for me - because I was instinctively downshifting-and-using-loads-more-revs at every opportunity [a P5B will happily do 80MPH in 2nd]

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Re-routed a few HT leads today with no luck, and the V8 did not like the Vacuum being disconnected at idle!

Currently awaiting the parts delivery then will crack on with the investigation. Also may need to replace the centre box of the exhaust. It seems that a backfire has blown some of the baffles loose in it!

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