Wobbly idle, timing?

[V8david]

(First of all, the sub topic is supposed to say what's yours set to! Sorry hit enter and now can't change it... anyway...)

Hi all. Small problem since the rebuild (3.5 V8 Efi, 3.9 camshaft fitted). When I put the car in D at idle, or slow up to a junction, as the car reaches idle it develops an annoying wobble, sort of jerking to move. This stops if I put my foot firmly on the break, or move the auto selector into N.

I'm thinking it may be the timing is slightly out. On initial start up I set the timing by ear (rotated the dizzy until it was smooth) and the wobble was worse than it is now. I then used a timing light to set it to 8 degrees BTDC (recommended from data at local garage). The wobble improved but it's still slightly there. Truck runs on LPG.

So I guess what I'm asking is could it be the timing needs a tweak?! And if so, what have you all got yours set to? The settings I've heard so far vary massively. All help MUCH appreciated. I have developed a serious paranoia about the timing being out and damaging my beloved RV8!

Edited to sort subtitle - DEANO

[Bowie69]

For LPG you should be running around 12 DBTDC, as it burns more slowly than petrol, I know some do run even more advanced, but 12 is pretty much accepted as 'right' for a Rover V8.

I don't think the symptoms you describe are timing related though... but I am sure someone will suggest something

[RangeyRover]

For LPG you should be running around 12 DBTDC, as it burns more slowly than petrol, I know some do run even more advanced, but 12 is pretty much accepted as 'right' for a Rover V8.

I don't think the symptoms you describe are timing related though... but I am sure someone will suggest something

I think it might be timing. Until i switched to megajolt mine used to hunt at idle, sometimes stalling in gear. Doesn't do that now....

[V8david]

Cheers Bowie69. When I was fiddling with the timing, before I obtained the light, the more I advanced it the better it got. The engine ran faster and faster. It was fun but I had to abort as I didn't have a damn clue what I was doing I then sensibly bought the light and set it to 8 degrees BTDC... I will try the 12 degrees tomorrow though as I'm sure the more the timing was advanced the better the symptoms got.

Cheers also RangeyRover. I would love to megajolt the old girl in the future but for now I'd like to get my head around the basic original system...! It's a bloody steep learning curve!

[vougese39]

Cheers Bowie69. When I was fiddling with the timing, before I obtained the light, the more I advanced it the better it got. The engine ran faster and faster. It was fun but I had to abort as I didn't have a damn clue what I was doing I then sensibly bought the light and set it to 8 degrees BTDC... I will try the 12 degrees tomorrow though as I'm sure the more the timing was advanced the better the symptoms got.

Cheers also RangeyRover. I would love to megajolt the old girl in the future but for now I'd like to get my head around the basic original system...! It's a bloody steep learning curve!

you may have the dissy out one tooth is it a hotwire system or flapper?

[V8david]

you may have the dissy out one tooth is it a hotwire system or flapper?

It's a flapper. It was only happy at around 20-25 degrees BTDC when I first put it back together. I moved it round one tooth and was then able to set it to 8 degrees. It's bloody close so I think I'll try the 12 degree theory tomorrow if I have time. I'll let you know how it goes.

Cheers for the thoughts.

[vougese39]

It's a flapper. It was only happy at around 20-25 degrees BTDC when I first put it back together. I moved it round one tooth and was then able to set it to 8 degrees. It's bloody close so I think I'll try the 12 degree theory tomorrow if I have time. I'll let you know how it goes.

Cheers for the thoughts.

sounds like you are out one tooth i would also clean the throttle pot and reset the throttle switch as well

[Highway_Star]

The timing marks on V8s are notoriously innacurate, then factor in timing chain stretch, timing chain being be tooth out etc you'll soon understand why.

I usually give it as much advance as it will take without pinking under load, from memory my 3.9 is set at around 16ºBTDC at idle, with the vac unit disconnected. I'm running LPG.

It's possible that your dizzy is worn and that gives rise to scatter timing, tricky to see with a simple timing light, but any Crypton type tuning unit with an operator worth his/her salt would soon spot it.

[V8david]

I fitted new timing chain and gears with the camshaft, spent ages lining them up! The engine's only done 52k miles so I assume the distributor should be ok still... I'll have another fiddle tomorrow. Cheers guys.

[Bowie69]

from memory my 3.9 is set at around 16ºBTDC at idle, with the vac unit disconnected.

Just curious... why have you diconnected the vacuum advance?

[Highway_Star]

Standard practice is to set timing at idle with the vac unit disconnected, otherwise wee fluctuations in vacuum will cause it to pull the timing slightly.

It's also the correct way to check mechanical advance, raise the revs with the timing light connect and the vacuum unit off, see how much the timing shifts.

Plug or clamp the hose so it doesn't allow air in and raise the idle speed artificially.

Once it's set, re-connect it.

[Bowie69]

Ah sorry highway star, sounded like you were running without it!

But yes, you are correct in disconencting the vacuum line when setting the timing....

[Highway_Star]

It's worth checking the vac capsule by sooking on the pipe, most of them are burst.

They aren't cheap to replace either.

[V8david]

Thanks to whoever corrected the sub topic on this! Anyway...

Sorted! Advanced the timing and now no wobble. Not only that I've got more power. Was obviously in the wrong place. According to the dial on my timing light it's settled for 20 degrees BTDC. That is of course in relation to the TDC mark on the crankshaft pulley... I advanced it until I went too far, then back a bit until she was/is happy. I may well be one tooth out on the distributor but hey. It works. May not be either... who knows... or cares! Got enough things to fix that are broken without fixing things that aren't!!!

Now that's sorted I'm going to treat the old girl to a nice new rotor arm & distributor cap... Genuine of course...

Cheers for the help and advice guys

David

[Highway_Star]

Nice one!

I learned a lot of tricks like this from my dad, he used to set engines up using his ears, his nose and a vacuum gauge. I think it was when we were working on a MkIII Escort I had when I was a lad, trying to set it up for the then new fangled un-leaded petrol, that I gleaned the 'advance until it pinks, then back it off a bit' trick.

Carb balancing and mixture setting by human senses is something of a black art, not one I ever mastered. Like so much of my dad's engineering knowledge never got a chance to pass on before he died

[V8david]

Nice one!

I learned a lot of tricks like this from my dad, he used to set engines up using his ears, his nose and a vacuum gauge. I think it was when we were working on a MkIII Escort I had when I was a lad, trying to set it up for the then new fangled un-leaded petrol, that I gleaned the 'advance until it pinks, then back it off a bit' trick.

Carb balancing and mixture setting by human senses is something of a black art, not one I ever mastered. Like so much of my dad's engineering knowledge never got a chance to pass on before he died

Cheers Highway_Star. Sorry to hear about your dad mate. Mine died last year. That's how I ended up on the forums I guess. When the first person you call with a problem is gone you need to find answers elsewhere...

Anyway. Car's good. Cheers.