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running on 3 pots

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Dear all,

I have been scratching my head over why my S3 N/A is running on only 3 cylinders (number 4 bad), more notable during idle and initial acceleration.

Checks done so far;

Compression - 160psi

Spark - OK

Carbs, here's where I think I have a problem, when I screw in the idle screw on carb no. 4, nothing happens to the engine note, whereas if I screw in any of the other 3 screws it nearly kills the engine. I've checked out the jets, blown carb cleaner through and fed a wire down the passageways to see if they were blocked. All fuelways seem to be clear, but when I take the progression (brass) screw off, there doesn't seem to be any flow, so I checked to see if I have a suction to create a venturi and I have.

Any ideas where I should go from here?

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Try the tried and tested method, put your hand over the inlet venturi (don't do it completely, perhaps 95% covered - that way you can get your hand free easily again) and see if that pushes any debris free of the fuel tracts in the carb.

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Right done that little test, things are getting more stranger.

When I immediately cover over the inlet, there's a sudden shift in engine note which seems to be for the better. However this only lasts for a second or so and then it reverts back to the uneven idling. ??????

Did another test by spraying a little easy start down the venturi and that makes a difference as well, so it has to be fuel right?

But I know I'm getting fuel there now because my hand is wet when I remove it?

Also took it out for a blast and it gave the biggest backfire, which is unspent fuel igniting in the exhaust isn't it?

Can all this be caused by unbalanced carbs? I have buggered about with the settings so they may be way out now? But wouldn't this effect both chambers on the carb equally?

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But wouldn't this effect both chambers on the carb equally?

No because both chambers have separate idle mixture screws separate air bleed screws etc.

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If you cover the inlet for that venturi then it will draw more fuel due to the extra vacuum.

It could be fuel but having not touched my carbs yet I'd rather not give too much uneducated advice!

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Yup - I'd start with Steve's suggestion.

You say 'spark OK' - maybe that needs qualifying?

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Spark was the first thing I looked at, have already swapped the lead and plug, which made no difference.

Incidently I have new plugs, leads, dizzy cap and rotor arm.

I did notice the coil arcing from the HT to the LT side, but I bent the LT connection further away and this seemed to fix the problem.

PS. Plug is not really wet, but is a bit sooty.

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Tony - maybe bring the carbs back to a basic setting - ensure linkage is synchronised correctly, air screws all the way in and mixture screws at 6.5 turns out each. That should give a basic balance. It may run rich at this setting, so turn them in progressively, say, 2 turns each. If things aren't right after that, then you may need to take a look inside the carbs.

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Thanks sparky, I've re-set the carbs to the book (how to tune dellorto & webber carbs), which states 3 1/2 turns, I've also synched the pair as it also stated, however I don't have a carb balancer which is my next step (borrowing one tomorrow). I've also had the carbs off and fitted a dellorto refurb kit

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Although odd symtoms, I still think you may be barking up the wrong tree with the carb. If the idle jet is not phsically blocked and you can see fuel when you take the jet cover off the carb, Does it run on 4 cylinders from cold using choke circuit? Does it run normally if you spray easy start constantly into choke 4?

Just re-read your post - Coil arcing from HT to low tension side. Do you mean from the HT lead back to one of the terminals? If so, I think you need to change the coil. NB Are you using Lucas Constant Energy or Lumenition (or something else)

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I'd certainly have another look at the relevant idle jet...as I've mentioned before in these hallowed halls, the jet at the bottom of the idle assembly can get blocked or varnished up. Try swapping the jet assembly with another cylinder; see if the problem moves with it.

Had another thought...I have known the progression holes to get blocked, too...wouldn't hurt to poke a strand of copper wire through them to see if they are clear.

Edited by molemot

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Right, I've found the cause of the problem, fixed it and she's purring as sweet as a kitten.

It was simply the vacuum hose for the brake servo which I must have helped detach itself during my fiddling. Re-attached it and hey presto.

Started thinking today if pot 4 had any differences in its 'system' to that of the other 3 and the brake servo tapping off the manifold struck me as the only thing which was different from the rest.

Loving driving it again...

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Just LOVE those simple, free fixes!! Only thing is finding the problem in the first place...well done!!

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