Hi,

I've got an intermittent misfire on my RV8 (3500 1977 SD1 engine with Lucas 35DE8 'opus' ignition and SU carbs) and would appreciate help in identifying what might be wrong based on my description of the misfire.

1. First time it happened was towards the end of a spirited short (5m or so) shakedown run near my home. Fuel levels were around 3 inches in the tank. There was no sign of the misfire after the car had cooled and I went to take it for a short test run.

2. Second time was also towards the end of a 130 mile run where again it started to miss after I threw it round some roundabouts when almost home. Again probably 3 inches of fuel in the tank. The tank is split into 1/3rds with baffles and has a little sump in the middle (only an inch deep) with 3 20mm holes between the tank and sump to let the fuel in but not get out easily. The pump is a big V8 sized carter electric pump with no return pipe and no regulator (it shouldn't need one according to the supplier).

3. Third time was after a cold start following the last ^^^ run. Note that I had expected it to be fine on the cold start as it was the previous time I had this problem. I took it to the petrol station and it was misfiring but seemed better after I had put 25 litres of fuel in although I wouldn't say it was perfect by any means straight after refuelling but it seemed fine shortly afterwards. I then drove it for another 130 miles and it performed fine, even in the rain which tends to suggest this is not a high-voltage arcing problem. It has a relatively new dizzy cap and rotor arm by the way and the leads are magnecor solid leads which are only a couple of years old so should be fine. Spark plugs have done 600 miles or so over 2 years so again should be okay and this is not a regular misfire so unlikely to be plugs or leads anyway.

The misfire itself is quite irregular but seems fine at idle. When you accelerate, it seems worse at first and then seems to clear, a bit like a 2 stroke engine coming "on song" as you lean out the mixture or get into the power band. The engine sounds quite "throaty" with a sort of hollow induction sound and then when the misfire goes away it returns to normal V8 sound. There is significant power loss which comes back in bursts as the problem comes and goes.

My thoughts:

1. My first thought was that irregular misfire usually means fuel issues but could mean a problem with the notorious Opus ignition module. My vacuum advance is locked off (ie. no vacuum advance movement) and so the wiring on the opus amplifier should not be moving back and forwards which was a common problem. Heat is also a problem for the amplifier as I understand it so this could still be the issue.

2. Why does it happen when cornering hard (on two occasions at least)? Could it be fuel surge introducing bubbles into the fuel lines? ( I would have hoped my baffles and sump would stop that but it's possible) How would the SU carbs cope with that in the absence of a fuel return line? Would it perhaps cause changes in float level?

Anyone had any similar issues? Any opinions on whether a return line or new dizzy electronics is the most likely? Neither are trivial fixes. The dizzy is "easy" I suppose but not cheap and the return line is fairly cheap but not trivial.

Any suggestions are most welcome.
Cheers,
Craig.

I'd would vote fueling either air getting into the fuel lines once it is in the only way out is through the SU's.

though I would add a further possible " crud trying to being pulled from the bowels under had cornering" have the Su been stripped or checked.

though 'hopeless' 'Opus' would be a close second.

regards

Agriv8

It seems like you might have a couple of things going on here - fuel starvation as a result of lots of hard cornering (maybe fit a swirl pot)

and as you have the vacuum advance disconnected are you sure your ignition advance curve is correct. The vacuum advance has most effect at lower revs so this could be your low revs issue. Mind you I don't know about the Opus ignition thing so the vacuum advance thing might be irrelevant.

Cheers

Mark

quote:

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Originally posted by Agriv8
I'd would vote fueling either air getting into the fuel lines once it is in the only way out is through the SU's.

though I would add a further possible " crud trying to being pulled from the bowels under had cornering" have the Su been stripped or checked.

though 'hopeless' 'Opus' would be a close second.

regards

Agriv8

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The Aldon Igniter is a reasonably priced, easy to fit and reliable solution.

Bin the opus dizzy. As you say the later ones with ignition module on the side are 1000 times better and more reliable. There's only 2 wires to connect to the coil and everything you need (power) will already be at your coil any way.

Ian

i originally ran this


Rover V8 Dizzi amp and coil

buy there is a condenser hidden in the grey box that died so pulled it apart and used the

inside there is an ign controll modual very similar to this one on ebay Item number: 350476876583

low voltage trigger from the 35DLM8 go in one side and the coil id driven off the other side can supply more info but cant lay my hands on it at the mo.

though I am on Mapped ign now from my VEMS ECU

Regards

Agriv8

[Edited on 13/7/11 by Agriv8]