RRC 3.5 Carbies - Another Misfire Thread

[cazbot]

Hi all,

Newish owner of a 1984 Rangie 3.5 on Strombergs. Had a few td5 discos and a triumph with the p76 rover v8 but this is my first foray into the Classic.
I purchased the car with a misfire. Previous owner was a Land Rover mechanic and had recently rebuilt the head and was using the car as a daily when it suddenly developed a misfire. He had multiple landies so parked it up and then sold it to me as he was moving out of town. He did do a compression test though and it was all solid.

I replaced Vacuum lines, Cap, rotor, leads, plugs, coil and thought i'd fixed it however when driving it to get the blueslip it started running like ****e again, got so bad it would barely run after about 20 minutes and was backfiring frequently.

When I hook up my timing light to lead #1 to set the timing its a very intermittent spark. This is 'leading' me to believe its ignition not carbs. Can this happen if the timing is significantly advanced or retarded? I'm finding it nearly impossible to set the timing, it looks to be around 9 BTDC with vac advance attached. I don't have a rev meter so am doing it by ear.
When I hook my timing light to the coil lead it looks to be a steady strong strobe.
I've manually cranked the engine to TDC mark and the rotor is pointing at Lead #1 so I don't think the distributor has skipped a tooth.
I checked the pickup gap in the dizzy and it looks ok, may be a bit tight but i've struggled to gap it much further.

Vacuum advance moves but when I disconnect it at idle the idle drops. According to the manual it should rise.

It has the Bosch Bim024 Ignition Module. I've cleaned all the earths in the engine bay including the modules ground.

When I pull the plugs they're all sooty except for 4 + 5 they're white. See attached photos.

It has what I think is a fuel cut-off valve that is connected to the coil to come on with ignition. I think this is correct.

Things i'm yet to check.
- wiring to and from ignition module looks a bit thin and could do with more insulation
- ignition module could be fried but not sure if there is a way to test
- distributor pickup is damaged
- alternator is toast and interfering with coil

I have balanced the carbs and set the mixture about half a turn downwards from flush.

I will post more pictures and videos when i'm at the car.

I've scoured the internet for months now troubleshooting from other forums but am at a loss and don't want to keep throwing parts at it if there is something obvious to one of the gurus here.

Any input welcome,

Cheers

[PhilipA]

Has it got a Bosch alternator?
if so you have to fit a shielded wire to the coil from the dizzy or alternatively or both run it back away from the alternator.
Bosch alternators produce hash which causes RF interference.
Regards PhilipA

[Old Farang]

Hmm, a long time since I worked on this model. 9 / 10 degrees BTDC is good. The colour of your spark plugs is not!

Black is showing too rich, but those two white plugs are concerning. If they were on the same bank a head gasket would be suspect, but as you have posted #4 and #5 cyclinders, unlikely but still possible.

Cannot remember if the coil has a ballast resister, I seem to recall that it does, so your new coil needs to be the correct one.

I have known those carbs to rupture the rubber diaphragms. The engine will start and idle ok but not rev, but not usually mis-fire.

Those Bosch amps are damn near bullet proof, but could fail I suppose. They are nearly impossble to find new now, but I think some early Ford Falcons had them, so you may find one at a wreckers. Don't know how to test them.

The same with the pick-up sensor; if failing as the engine warms up they will cause mis-firing.

The extra wire on the dizzy is for the rev counter, if I recall correctly.

[cazbot]

It does have a Bosch alternator, thanks for the advice on that i've now routed all my wires well away from it.
Lesson learned to not replace multiple parts at once. After putting all the new ignition parts on it still misfired. Eventually i managed to track the original issue was down to the original coil however it took some trial and error switching between the new and old parts because the new distributor I got was faulty. With the new coil and old cap she's running pretty nicely. Now I can finally get on to fine tuning the carbs.

[DoubleChevron]

Does it run ok until its hot? if so, the electronic ignition, its probably heat sensitive. try spraying it with cold spray (or throwing some points into it).

Also, back to basics, get the motor at TDC... but do it manually, find the point the piston is at TDC by putting something down the plug hole, then check the timing marker to make sure its accurate. If it's not, remark the timing mark and make sure the firing order is right....

[cazbot]

It is running better now however after about 20 minutes of driving it started to stall out. Reminded me of fuel vaporisation so I checked the coolant flow out of the manifold nipple and found it was blocked. Unblocked that and hopefully it will run for longer again.
During that investigation I found when I blow smoke into the carbs it pours straight out around the throttle shaft on both sides of the carb. Haven't investigated yet but I have an old pair of restricted strombergs that maybe I can pinch the throttle shaft from if it isn't as worn as the current one.

[DoubleChevron]

It is running better now however after about 20 minutes of driving it started to stall out. Reminded me of fuel vaporisation so I checked the coolant flow out of the manifold nipple and found it was blocked. Unblocked that and hopefully it will run for longer again.
During that investigation I found when I blow smoke into the carbs it pours straight out around the throttle shaft on both sides of the carb. Haven't investigated yet but I have an old pair of restricted strombergs that maybe I can pinch the throttle shaft from if it isn't as worn as the current one.

throttle shaft leaks will only effect the idle really ... as its unmetered air entering. This is nothing to do with the running issues. You don't have fuel delivery issues? for example something in the tank randomly blocking the pickup ....