OT Sebring Sprite Misfiring

[AdamQ]

I need to plunder the wisdom of Blatchat over a problem that's got me somewhat foxed.

I have a Sprite with an A series engine.  It was going well about 8 months ago.  I resurrected it recently (following the changes in the MoT regs meaning that it didn't need one).  I now have a misfire on 1 and 4 at anything below about 3000 RPM.  Above that, all cylinders fire.  This has been diagnosed by removing plug leads with the engine running: <3000 RPM - removing 1 or 4 HT lead makes no difference, removing 2 or 3 does; above 3000 RPM - removing any of the HT leads makes a difference.

The battery appears to be healthy (Powervamp Clubsport) - it starts the engine fine and there's 12.5V there with the engine running and the alternator disconnected, ~14.2V with the alternator connected.  I've also tried running it with it connected to another car with jump leads with that car held at about 3000 RPM - same problems.  At tickover, there's a healthy spark from 1 and 4 with a plug earthed on the block.

Today I replaced the entire ignition system - new distributor, cap, rotor arm, coil, HT leads and I swapped the plugs from 2 and 3 with those from 1 and 4 (all plugs new about 2 hours running ago).  Still the same problem.

I've removed the fuel line at the carbs (twin DCOEs) and have drained off about a gallon of (possibly stale) fuel, having filled up with 5 gallons of fresh fuel.

The timing hasn't been touched since she was running fine last year.  I've checked and double-checked that I haven't mixed up the HT leads in my messing about.  The ignition was a Lumenition electronic system and as of today is a Powerspark electronic system.

One possibly relevant observation is that the non-firing plugs appear oily rather than petrolly when removed - could this be a problem related to idle jets and Siamese ports - or am I getting a little too elaborate in my analysis here?

Compression on all cylinders is 125 - 130 psi.  I've also had the rocker cover off to check that side of things and all appears to be in order - valves all opening and closing fully (i.e. when cold there's the few thou of clearance required (as borne out by the compression figures)).

I'm at something of a loss.  I appear to have compression, a spark and some timing that's not too far off - that only leaves fuel to my simplistic way of thinking, but 2 and 3 are firing beautifully ...

Thought, ideas and suggestions welcome ...  And apologies for the exuberance of my verbosity ...

[Jonathan Kay]

That's a pretty *complete work-up.

I'd get hold of some HT test equipment and see what's actually happening in the circuits of the individual cylinders when the misfire occurs.

Jonathan

* Traditional single coil set-up? Have you checked its earth wire and connection?

[Wrightpayne]

I dont think its fuel as cyls 1&2 and 3&4 share the same siamese port and 2&3 are running fine.

whilst you have checked the firing order and plug leads have you tried swapping the leads on 1 and 4?

Ian

[AdamQ]

Thank you both.  Ian, I'll give that a go tomorrow (though I think it might be something I've tried).  My thinking about fuel is related to a dim recollection of something in Crombac's Chapman biography where, because of the firing order (1-3-4-2) and the shared ports, 2 and 3 could 'rob' fuel from 1 and 4 resp. - Chapman went to the lengths of rigging up something to de-Siamese the ports as far as I remember.  But I'm sure I'm being way to elaborate in my diagnosis - like 99% of things, it'll be something daft and operator-caused ...

[Piers300]

So it has been sitting unrun for eight months with twin DCOE Webber’s. Maybe the jets have gummed up with stale fuel. Have you tried cleaning all jets ?

 

[elie boone]

Are the carbs properly balanced ?

[AdamQ]

Thanks Piers, Elie.  Regarding the jets, I've had them out and inspected them cursorily - they look OK to me (though not entirely sure what I'd be looking for).  The float chambers also looked OK to me.  And it is of course running fine on 2 and 3.

Re. balance, I've listened down a tube and they seem fairly well balanced and the tickover on 2 cylinders is surprisingly smooth.

[Wrightpayne]

another check with the rocker cover off is to get No1 cylinder (radiator end) on its compression stroke (rockers both loose & TDC marker on pulley aligned) and check which lead the rotor arm is pointing at - should be No1.

It is possible to install the dizzy drive 180 degrees out.

As stated, I dont think its fuel related as 2 cylinders that share the same ports with 1 and 4 are running OK.

I learnt most of my spannering skills on mini's and A series engines / gearboxes - favorite was my 1380 Clubman using a megadyne 296 cam, maniflow manifold / RC40 exhaust and stage 2 head. David Vizard was my essential bedtime reading!

Ian

[pikeyboy]

I used to a race a frogeye some years ago.  I once had a spurious miss-fire on a couple of cylinders, after much head scratching it turned out to be an air leak at the combined inlet exhaust gasket.  I found this by spraying WD40 on it and allowing the gasket to swell and seal.

[pikeyboy]

Also I've just tread that its running webers. is this a single or two DCOE's.  If its two it'll only be running one side of each in a split configuration.  Either way that compression seems quiet low for something in that state of tune ie needing a weber to fuel it, my race engine had 190 plus even after a seasons use.  

[AdamQ]

Thanks one and all for all the helpful suggestions and ideas - greatly appreciated ...

I've just had the rocker cover off and have turned the engine to TDC with both No. 1 valves showing a bit of slack and the rotor arm is very definitely pointing at No. 1.

I've sprayed some Plusgas around the manifold side of the head.  As the Plusgas evaporates as the engine warms up, there's no sign of the 'smoke' being sucked into the engine via an air leak (though I'm not sure how noticeable/subtle or otherwise this would be so could quite easily have missed it).  Later or tomorrow, I'll take the inlet manifold off and see what state the gasket is in.

It's running twin DCOEs.  I have to say, I was slightly surprised by how low the compression numbers were, but took some comfort from the fact that they were at least consistent (ISTR reading on here somewhere that consistency was almost as important as the figures themselves (within reason)).  I've just checked them again - hot engine, all plugs out, throttle fully open: 125 - 130 psi across the board again.

Thanks again to all.

[Jim 123]

Are there any signs of gum / deposits on the valve stems / heads that might be inhibiting the function of a couple of cylinders.  A long shot I know but you seem to have covered everything else.
 

[pikeyboy]

Having re read the post my money is on a air leak or more likely a clogged carb.  If it was me I'd be getting it up and running then spraying carb cleaner around all the joints mishab plates etc.  If it sucks in some carb cleaner it'll start to die.  More  likely to be a blocked jet though i think, but its easier to rule out simple stuff first before dismantling your webers.  On that note you're running a 'split' weber configuration, if your engine is in good fettle this would seem over kill really considering the compression readings. Split webers are normally the preserve of full race 1380 plus engines looking for the last ounce of hp, which would have a comp test reading of closer to 200psi.

[AdamQ]

Thanks again for the further suggestions and ideas - I'm working through them ...!

I've ferreted out some more details of the set-up (the car was left to me by an uncle and I've yet to become as knowledgeable as I'd like to be about it (though it's all familiar as I grew up with Midgets)).

It's a 1380 A+ block with a Stage 4 head and a MD286 cam with the 1.5:1 roller rocker gear.

One thing that's always surprised me is that it appears to have servo-assisted brakes with the vacuum being taken from the inlet manifold for 3 and 4.  I'll be blocking that off to eliminate that as a potential cause of problems (though it obviously wouldn't affect 1 and 2).

[Jonathan Kay]

I don't know much about induction... is that particular patter of misfire across cylinders and with engine speed suggestive of those problems or just a possible explanation for any loss of power?

I'd still want to know whether the affected cylinders are getting sparks at the right time when the misfires occur. That's not a suggestion that an electrical problem is more likely than a fuel/air problem, only that it would tell you where to look.

Jonathan

[elie boone]

If you do a leak test ( you can modify a spark plug for a simple test ) then you would know if it all seals correct.

[seriouspete]

I would bet on it being the Weber’s, as it’s running cleanly at higher rpm, loads of carb cleaner and a run with new good fuel and all will be well. Those carbs don’t like sitting unused!!!

[AdamQ]

Thanks again one and all.  I've admitted defeat and the car's at the garage down the road being looked at.  The chap there did notice that the Webers were bolted up tightly to the manifold when they should in fact just be nipped up, leaving some vibration-absorbing capability in the special washers (he gave them a name, which I can't remember).  Apparently the vibration can whip up (sort of aerate) the fuel in the chambers a bit, leading to problems ...

Pete, your suggestion makes good sense - and I'll do my best to make sure lack of use doesn't cause problems again!

Anyway, we shall see and report back when the problem has been diagnosed and fixed ...

[Piers300]

I think they are called Thackeray washers and there is a specific setting/gap needed, otherwise it airiates the fuel.

Piers