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Stromberg Zenith CD175 needles and ignition timing

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This topic is mostly relevant to US and Canadian lotus owners with the Fed spec cars, and also Jensen Healey owners.

 

I've been running twin Strombergs in my Eclat (UK) for the past couple of years and have been tinkering with the needle sizes and ignition timing to get the best results (see spec below). 

I'm keen to know if anyone else has tackled the problem of the original carb and ignition settings and is their a recognised updated set up. 

I'd really appreciate some info from other CD175 users for comparison and hopefully round on the ideal set up.

 

The settings that i'm using have been honed over the past couple of years and gives good economy at cruise and part throttle and from 1/2 throttle onwards it goes like a rocket. There's always room for improvement though. Maybe some one has had a bespoke needle profile made up?

 

I look forward to any contributions.

 

 

 

My current settings

 

 

Graph showing the needle profile comparison for the standard Lotus B1DK and the current (in use) Saab B1DS

 

B1DK - B1DS.tiff

 

Screen grab from the ignition software (123Ignition Tune)

It may look a bit odd, but the salient figures are approx 11 degrees of mechanical advance plus 15 degrees of vacuum advance.

 

Stromberg BIDS ignition timing.tiff

 

Other info.

 

blue springs - standard (lightest force)

Damper oil - ATF  or SAE 15 motorbike damper oil

 

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Hi Max,

 

I've got Strombergs on my Esprit at the moment. Pertronix electrical ignition, with the fun vacuum retard installed. As far as I know it has stock needles, not sure what the static advance is.

 

Do you have difficulty starting yours when its sat for an extended period of time. Mine always take 3-4 tries of a few seconds each to get going if I haven't run it in a while. Can't seem to figure out why, but my guess is the fuel evaporates from the bowls then takes a while for the vacuum to suck back up the metering hole.

 

Brian M.

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Hi Brian 

Yes, you're correct about the evaporated fuel. It takes mine several seconds to fill the bowls if left for a few weeks, otherwise it's hard to start.

I've always been curious about the vacuum retard unit and how it functions with the ignition system.  Would I be correct in saying that it works at idle to advance the ignition and also on over run (closed throttle) to advance the ignition. 

I've never seen this system in the uk and it seems a bit mysterious.

 

Max

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Hi Max,

 

I meant mine is hard to start even after I've let the fuel pump fill the bowls. I usually wait until the tick tick of the pump stops then start cranking and it still seems to take a few tries.

 

As for the vacuum retard, from my understanding it retards the ignition 10deg at idle for better emissions. Then as soon as your off idle the retard drops to 0 and you start getting centripetal advance. This is the stock configuration, I've been told by members of the community that know much more than I do that I should disconnect the retard and cap the manifold and re-time my engine without it. 

 

Brian 

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Hi Brian

I had a look at and investigated the vacuum retard and ignition timing set up/specifications this morning. It took me a while to understand the system as it looks counter productive, though it seems a clever system for achieving what they needed to at the time and with purely mechanical means.

I think though with modern fuels the game has changed a fair bit and you could make some substantial improvements to how the car starts and runs. 

From personal experience, an electronic ignition system and different needles really makes so much difference to the starting and running and performance and negates replacing the carburettors.

Regarding the standard set up. I wouldn't try advancing the ignition more than the standard mechanical settings as the total advance of 36 degrees is already very advanced and could lead to engine knock at full throttle. Disconnecting the vac retard unit and leaning the mixture or fitting B1CM ( leaner at idle) needles from the Jensen Healey would help. 

Hope this is helpfull.

Regards

Max

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To update the previous message. I've discovered what I think is an incorrect and misleading piece of information in the Lotus manual (Ignition at idle 2° BTDC - See image below) that could cause all sorts of problems when setting up the ignition timing with Zenith/Strombergs. 

 

I think that the 2 should read as 12. The reason being, If the vacuum pipe from the distributor is connected to the carburettor vacuum pipe (ported vacuum, above butterfly) then there should be little or no vacuum showing at idle. The ported vacuum connection produces a venturi type vacuum signal at acceleration only. In operation, the ignition retards in direct response to the fuel mixture being enrichened. 

 

Another option for the mistake is that Lotus mistakenly used the manifold vacuum as a signal for the distributor retard unit. Which is quite possible, as the Lotus illustrations of the vacuum schematic that I've seen seem to show this.   Maybe someone has an updated and correct schematic.

 

Here is what I think the vacuum connections should be.

 

Ported vacuum (above butterfly on rear carb) >  distributor vacuum retard unit.

 

Manifold vacuum port > air valve > 2 x bypass valves 

 

CV chamber ports (rhs of carbs) > anti run on switch > float chamber vents  (the evaporation control / charcoal canister is also somehow connected to this system)

 

 

 

post-12166-0-54426100-1411367856.png

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Slight correction - the last bit should read as follows

 

Carburettor vacuum port ( underside of rear carb ) > anti run on switch > float chamber vents  (the evaporation control / charcoal canister is also somehow connected to this system)

 

Different versions of the CD175 have varying vacuum ports in different locations.

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Hi Max,

 

You need to be careful reading that diagram of the vacuum ports. The vacuum to the distributor comes of the manifold not the carburettors. Theres a T fitting and it goes to the anti-run on switch.

 

Look at this diagram: http://rdent.com/manuals/esprit/fuel_system/lb.htm

 

You can see hose 1 runs to the anti-run on valve from the left side of a T connection. The downward connection from the T goes to the distributor, while the one on the right turns down 90 degrees and connects to the center vacuum port of the intake manifold (not pictured). Its very confusing and looks like it connects to tube 3. But it doesn't, if you look at every other hose junction in the diagram there is a connector drawn. Hose 3 connects the two ported vacuums of the carb's together and then continues to the charcoal cannister, so that its only purging under acceleration not idle. I've looked a lot of pictures of Federal Esprit's to try and figure that out.

 

My conclusion being that the vacuum to the distributor is highest when at idle and the butterflies are closed and drops as they open, which would make those figures in the manual correct I believe. 10deg static - 8deg retard (under full idle vac) = 2 deg total idle. Which makes sense with the 24-26deg advance at 3000RPM (vac disconnected) when you take 10deg static + ~15deg at 3000. Based on a centrifugal curve, 2000RPM 8deg and 4000RPM 18deg, wouldn't it be nice if they'd given the centrifugal advance at 3000RPM so the math worked perfectly. 

 

I could be wrong about this as my Esprit came with Dellorto's and I put the Strombergs back on, its a long story. But it makes sense in my head = )

 

Brian M.

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Hi Brian 

It seems that the crux of the issue is the ambiguous connection illustration in the hose schematic (the 90° connector to the rhs of pipe/hose 1).

I have a theory on this ambiguous connection. To start, You have to ask yourself why the draughtsman responsible for the technical schematics at Lotus allowed this illustration to leave the office.  

My thoughts are that the illustration is deliberately ambiguous in order to hint at a pipe connection whilst not officially sanctioning the modification. 

As is widely understood, Lotus and other manufacturers, in order to achieve sales in the US in the 70's, had to adhere to strict emission rules regarding the engines of their cars. These emission rules had the effect of reducing the power output of the engines and making the engine inefficient, prone to overheating and run on. 

To negate these issues Lotus very cleverly designed the vacuum control system to have two modes of function. 

 

Function 1. Factory setting to achieve emission requirements. (connection made to manifold vacuum)

 

Function 2 Alternative setting for greater power output and engine efficiency. (connection made to pipe 3, ported vacuum)

 

Function 1 Specifics 

Ignition retarded by 8° up to approximately 4000RPM, where the engine vacuum diminishes to 10 inHg and less. Ignition timing at idle 2° btdc.

Vacuum applied to the control valve which opens the bypass circuits at 20 inHg. (On rapid deceleration only)

 

Function 2 Specifics

Ignition timing at idle  10° btdc.  ( Near normal ignition timing)

Ignition retarded on acceleration/fuel enrichment only.  (Ideal function)

Bypass circuits made redundant. ( not enough vacuum to operate the vacuum valve) (bypass circuit not necessary for normal function)

 

So, you have two settings available by simply moving a vacuum hose connection from the manifold to the carburettor.

ie, One setting for the testing station and one setting for better engine performance. 

The genius part of it is that, from Lotus cars point of view, The inference is deniable and can be attributed to poor drawing communication. 

 

Below (pdf) is a graph to illustrate 907 ignition settings, you'll see how unusual the 2° at idle ignition  set up is. Whereas the mechanical advance is spot on, though could be a bit more advanced.

 

The dotted lines are the extent of the vacuum retard or advance. 

 

My only unknown in the setup is. How much vacuum is lost to the vapour control system? My guess is very little, but its a guess. 

 

Hope this helps.

 

Max

 

 

Zenith:Stromberg 907 ignition maps.pdf

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Hi Max,

 

Interesting graphs, I've been recommended to simply disconnect the vacuum retard system completely. At least thats been the advice I've always received from Tim Engal, as well as to change the cam timing from 100deg (federal) intake to 110deg (euro). I'm not an expect on the subject but I'm planning on moving to that setup eventually.

 

Is the "Lotus 907 Spec 7,8 ported vac retard" named correctly or is that manifold vac? I would have thought that ported vac would have the same curve as the centrifugal until off idle.

 

Brian M.

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Hi Brian 

I think it would be beneficial to retain the vac retard and connect it to the ported vacuum. It will give you ignition retard on acceleration and advance on deceleration. The ideal situation.  Until I can program the specific details into my distributor and do a test I couldn't advise on the static timing (my guess would be around 14 -16° btdc). When I'm fit enough to work on the car (i've been suffering from a slipped disc) I'll do the tests and let you know. Although, if you follow the ignition details in my first post you'll find that the engine is transformed.  Smoother running, more economical for town driving and way more power from 3500rpm onwards. At least equal to the standard dellorto set up. 

The metering needles that I used are from a saab 99 and it's important to use Automatic transmission fluid in the dash pots.

The advice on the cams sounds good. I guess that you have the lower compression engine, so your static ignition timing would be slightly different to my settings.

Max


You're correct about the manifold vacuum. The detail on the graph is incorrect.

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Hi Max,

 

I actually have a set of dellorto's that I've been thinking of putting back on the engine but my mechanic had some trouble with them (but admitted not to have the time or knowledge to get them going) so I stayed with the strombergs. I suspect I'll keep things the way they are for another season until I have a bit more confidence in the car. I put it back on the road a year ago after being in storage for 20 and suffered an electrical engine fire on the starter motor a few weeks later. All rebuilt now but I'd like to drive it some before I start possibly breaking things again.

 

I do have the lower compression federal engine. 

 

Sorry to hear about your back, those injuries are always crippling. Hope you have a speedy recovery, do post your results when you get around to trying the static timing. I imagine having programmable timing makes tweaking it easy. What electronic ignition are you using again?

 

Brian M.

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Hi Brian 

Sorry to hear about the engine fire. I had a similar experience. I restored my Eclat after it being in storage for many years and on the way back from the testing station it burst into flames. The problem turned out to be a higher pressure (7psi) fuel pump from the previous owner, the fuel flooded the carbs and poured onto the starter motor, where it ignited. All sorted now, though I remain fearful and maybe a bit over cautious. 

The electronic ignition system I use is made by a Dutch company called 123Ignition. It's the more expensive Tune version. It really is a technological marvel and a simple replacement for the standard Lucas distributor. The other changes I made were to replace the standard 6v coil with a 12v Lucas Sport coil and by pass the resistor wire to the coil ( the wire with the fabric sleeving). You also need a laptop/computer with Windows and a usb cable to program the settings in. 

 

I'm hoping to be able to sort out a few jobs on the car this weekend and If I can I'll try out the carb settings and let you know the results.

Max

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