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Twincam Spark Plugs

PostPosted: Sun Nov 30, 2003 3:21 pm
by david.g.chapman
Hello there! I have a +2 with an I prefix engine (ex federal converted back to UK spec) and a stromberg head.
I have recently noticed that cylinders 1 and 4 have much darker spark plugs (dark brown center insulator) than cylinders 2 and 3 (mostly white). This seems strange to me, as each carb serves cylinders 1/2 and 3/4 respectively!!
I am using NGK BPR 6ES plugs. I had the engine compression tested 3 years and 15000 miles ago, and all the cylinders were at about 160 psi. The engine seems to be otherwise OK.

Dave Chapman.

Re: Twincam Spark Plugs

PostPosted: Mon Dec 01, 2003 10:03 am
by rgh0

Lots of other things can also influence individual cylinder mixture and combustion temperature even if rest of engine and carburation is in good condition.

1.Exhaust manifolding and how well it scavenges each cylinder
2.Vacuum connections for headlights and brake booster especially if any leaks in these systems
3.Balance pipe between the carbs and how it feeds the cylinders
4.How the cylinders feed from the carb i.e. which leads and which lags if they are not evenly spaced in their timing as in the stromberg twin cam. 2 and 3 suck first from their carb and then 1 and 4 follow on.( this would be my bet on the cause of most of the variation you see)
5.How the flow splits between the cylinders, I understand lotus deliberately blunted off the divider between the 2 siamesed cylinders to minimise the variationin the flow split to the cylinders they experienced with a sharp edged divider.
6. Variations in the valve port area to individual cylinders


Re: Twincam Spark Plugs

PostPosted: Mon Dec 01, 2003 12:39 pm
by david.g.chapman

Thanks for that.

My brake servo(booster) line looks OK, and the headlights stay down for 2-3 hours. It sounds like your suggestion about cylinders 2 and 3 having the first bite at the cherry could be the reason.

I suppose I will just have to live with it - or change the head!

Anybody else with this condition?

Dave Chapman.

Re: Twincam Spark Plugs

PostPosted: Tue Dec 02, 2003 9:05 pm
by dougweall
Hi Guys,

Just to butt in, I have been reading the Bible (Lotus twin cam engine by Miles Wilkins)

Have you got a copy ? :huh:

It may give you some clues / advice on your problem.

Just a thought !

By the way got my copy of this rare book (on advice from this board) from Amazon ?14.95

Good luck


P.s See you guys out on the road on those sunny days when I get my S4 dhc completed. B)

Re: Twincam Spark Plugs

PostPosted: Wed Dec 03, 2003 8:44 am
by rgh0
Yes I have had a copy since it was first published, its where I got the comment on blunting off the divider to balance the flo between the siamesed ports. unfortunately it does not say much else about this mixture variation problem in adjacent ports.


Re: Twincam Spark Plugs

PostPosted: Thu Dec 04, 2003 8:54 pm
by david.g.chapman

Dispite having the engine compression tested only 15000 miles ago, I though it worthwhile to do it again. Here are the results (Oh dear!)

Cylinder 1
Cylinder 2
Cylinder 3
Cylinder 4

Re: Twincam Spark Plugs

PostPosted: Thu Dec 04, 2003 9:04 pm
by david.g.chapman

Please ignore my previous posting - finger trouble!

Despite having the engine compression tested 15000 miles ago, I thought it worthwhile to do it again. Here are the results (Oh dear!):

Cylinder 1 100 psi dry 110 wet
Cylinder 2 120 dry 140 wet
Cylinder 3 140 dry 150 wet
Cylinder 4 90 dry 100 wet

Not a pretty sight! However, I think it might just be valves (ever the optimist). At least it explains the difference in spark plug colour on 1 and 4! The engine does not smoke, or use more than 1 pint every 600 miles plus.

Dave Chapman.