View single post by Mark Rosenbaum
 Posted: 06-03-2005 12:34 am
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Mark Rosenbaum

Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
First, from the symptoms you list, it sounds likely that your carbs do need to be rebuilt.  The mixture setting on Strombergs really works properly only when the rest of the carb is in good shape.

Next, many (probably most) JH engines smoke.  Those that have been sitting for a while smoke a lot more.  And it's even worse if the crankcase oil isn't about half a quart below the 'full' mark.  Fortunately, driving a JH on a daily basis usually reduces the smoke significantly, often to the point that it isn't really noticeable.

Other than annual engine rebuilds, about the only solution is to suck the fumes into the cylinders where they will be burned.  The conventional way of doing this is to use 'positive crankcase ventilation' (PCV) which is nothing more than allowing manifold vacuum to suck the fumes out of the crankcase, generally with a one-way valve so that backfires won't ignite any crankcase fumes.  Despite the relatively low manifold vacuum in a 907 engine at idle, this might actually have worked fairly well.  But Lotus, for some reason, chose not to do so, and my understanding of US emissions laws says that because the manufacturer didn't, neither may the owner, unless he wants to formally certify his system.  But IANAL and could be wrong.

What Lotus did do was to vent the crankcase into the air box, later adding a second vent from the intake cam cover.  Notionally this provides an equal amounts of fumes to each carburetor.  In fact, due to the locations chosen for the hoses, the rear carb seems to get about 2/3 of the fumes.  From an emissions standpoint, this may not matter much, but from a carburetor lifespan and function standpoint, it matters a LOT.

Unless K&N has changed their setup recently, their arrangement dumps all the fumes into one of the carburetors (usually the rear one).  I don't think this arrangement is anywhere near as good as the stock setup.   

To do the job right, IMHO, one needs to deliver fumes to both carbs equally.  There are many ways of doing this.  The method I chose is shown in the attached photo, though the photo does not show that the original fume hole provided by K&N has been capped off, nor the coupling connecting the central hose to the hose from the crankcase vent.  Any fluids in the system, regardless of source or nature, should drain out through the bottom of the filter rather than being ingested into the engine.

As I recall, the hoses and fittings together cost about $10-12, and this could be reduced considerably if desired.  Perhaps more importantly for those folks in California, the car even passed an honest smog test with this system in place.

Attachment: breathers.jpg (Downloaded 182 times)

Last edited on 06-03-2005 12:37 am by Mark Rosenbaum