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Lean condition with Stromberg head Twin Cam


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This post could easily live in Off Topic since it's not se7en-specific, but I'm sticking it here since a lot of people have crossflows or twin cams and I do (eventually) have a technical question.   

 

Today the Elan visited Vintage Racing Motors to have the advance curve tweaked on the 123 Ignition distributor.  VRM has been a staple in vintage race circles for over 30 years, and works on cars that don't just cost as much as a nice house, but sometimes as much as a nice neighborhood (think 250 GTO).  It's a great shop to have nearby and is run by real car enthusiasts.  Although they keep the 7 figure cars in the back, even the log jam of race cars next to the dyno was full of good stuff. 

 

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Now for some reason, mixed in with all the proper race cars was this Espada in an…interesting color, with an even more interesting interior.  I suppose the owner lost a bet at some point.  The engine was pretty spectacular though.

 

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Anyway, back to the Elan.  The engine runs great.  The only reason it was on the dyno was to take advantage of the 123 Ignition's programmable advance curve and dial it in for this specific engine and local fuel.  Consequently we were surprised to see very lean AFR numbers after the first run.  After brief troubleshooting and needle adjustment, we did two more runs keeping rpms below 5500rpm, but decided to stop further dyno runs to avoid an Oh Sh&t moment.

 

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This engine is from the Stromberg era, and has been converted to the UK engine specs, including the carb needles.  Those needles should have been fine on a stock motor, so something is awry.  Fuel flow is the obvious culprit, but based on the AFR graph and the fact power was still building at redline, that does seem a little odd to me.  Does anyone have any experience here?   Could it be as simple as a failing mechanical pump or clogged fuel filter?   

 

 

Thanks,

John

 

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The engine runs very well.  Is it possible that an air leak sufficiently large to drop AFR by that much wouldn't affect idle or low rpm drivability?  Someone else mentioned that WBO2 shoved up the tail pipe can create lean readings due to exhaust pulses pulling outside air back into the exhaust.  Given my attraction to Murphy's Law, perhaps there are multiple factors at play.  I think I'll play around with things this weekend.  

 

-John

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Clearly, the VRM guys had common sense as they did not let you into the back room where the 8 figure cars are kept.  :classic_tongue:

 

Was that Byron in the drivers seat on the dyno?  

 

 

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After hearing I also had a Westfield, they said something about me not being worthy of viewing the big boys' room. 

 

Yes, that's Byron.  He mentioned your dash is made from crumbly bits.  My condolences.  

 

-John

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Runs well and plugs look ok?  If nothing else is telling the lean tale I'd be leaning towards to tailpipe cause.

 

If plugs are confirming the lean state, you might check the jet height.  I know they aren't technically "adjustable", but if not set at the right height you can never get them dialed in correctly.  But I'd think you'd notice that is running state.

 

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My carbs were converted to the adjustable needles and we did adjust height today between runs.  That richened the low end, but made no change to the top half of the rpm scale.  Based on what I've read, this is expected behavior for Strombergs.  The needle height affects the low end, but higher rpm is governed by the needle profile.  In hindsight, I wish we had pulled the plugs after a run to confirm lean condition, but at the time, the easy assumption was that the needles were too lean and the readings were correct.  I'll try to do a plug check after a hard run this weekend.  

 

Thanks,

John

 

 

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Jet height changes should effect the entire range, I'd think, as relative relation of the needle and jet define initial mix - needle movement with the piston then adjusts with air flow.  Lower jets should mean richer conditions throughout the range, higher jets would mean leaner.  Though I certainly suspect the relative contribution of jet height is less up the range.

 

The temp compensators and idle mix screws can contribute to lean-ness - I'd make sure the idle mix screws are seated fully, and I'd probably fully seat the temp compensators if I had any doubt about their working condition just to take that variable out of the equation.

 

Do you know whether the fuel needle valves were replaced on rebuild?  I can't remember if there was a size change when I rebuilt mine to non-emissions spec.

 

Last, if you haven't seen the LSBs for this modification, I've attached pics - may be helpful.

LSB-StrombergAdjNeedle1.jpg

LSB-StrombergAdjNeedle2.jpg

LSB-StrombergAdjNeedle3.jpg

LSB-StrombergAdjNeedle4.jpg

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My Zetec SVT went very lean in high RPM's and eventually burned a valve or 2. Solution ended up being bigger fuel pumps and a conversion from a returnless fuel system to a return system. Counting the engine rebuild, the new pumps, separate off/on switch for the main fuel pump,adding an AFR gauge to the dash, multiple visits to the dyno, not to mention my trusty mechanic, it was a 5 figure fix.

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Thanks guys.  I found an article where someone experimented with needle height and an AFR gauge.  The results supported that the effect at higher rpm was effectively zero.  The AFR results on the dyno supported this.  I'll research this more.  The PO had a shop rebuild the carbs less than a year before I bought it and the thermal actuator is disabled.  Of course this doesn't mean they rebuilt them correctly.  I'll look through the receipts to see if it lists the replacement parts.

 

The reason I'm not sold on a fuel flow issue is the shape of the AFR curve. Maybe I'm not thinking this through clearly, but if fuel flow was impeded, wouldn't that issue grow larger as fuel demands increased, rather than remain pretty steady?  Other than a few random dips, the AFR vacillated from 15:1 to 16:1 from 3400rpm to 6500rpm (graph above was from a run that stopped before 5500rpm, but the initial run showed AFR remained steady to redline.)  I would expect it to grow increasingly lean as the revs nearly double if fuel flow was a problem.

 

Thanks,

John

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The needles are having the impact you'd expect, given their profile.  Move them relative to the jets (by either adjusting the needle height or by adjusting the jet height) and you'll get a change in initial richness.  Once those are set, but profile does the rest as the piston rises and falls - and, in theory, the right profile allows more fuel at the same time the opening pistons allow more air, and you have relatively stable AFR across the range (assuming slow, smooth acceleration/cruise type conditions, not WOT).  I think I have all that right!

 

The challenge with these adjustable needles is that there is VERY limited adjustment range - just over 3 turns in total as I recall - so the most important bit is getting jet height right.  Only then does the needle adjustment allow you to tweak within the right range.  Also inspect the jet orifice - it should be round, but if the old jets were retained when the carbs rebuilt it could be that they are worn (a proper setup has the needle "leaning" to one side of the orifice, so over many thousands of rises and falls can wear a groove into the jet orifice).

 

Not knowing the PO's shop, I'd double check to make sure piston springs were actually installed - and maybe inspect them to see if they look old and tired (they're light springs, so not sure anything other than missing, broken, or different one to the other would be visually notable).  There are 3 different versions, it may be that you need a stronger spring.

 

A footnote - converting fixed needle 175s to adjustable needle 175s means replacing the pistons.  The Burlens/Zenith technical manuals will tell you these are matched components and should never be changed.  Plenty of us have done so with positive results, but I guess there is always a chance of something being just out of tolerance enough to cause operating challenges.

 

How is your timing set?  You might note in the LSB attached earlier that Lotus suggest a static timing for Stromberg that (as I recall) is lower than standard along with change to adjustable needles.

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Looking through receipts, I found a 3.5 hour line item to rebuild the carbs, but I can't find a receipt for a rebuild kit or any parts.  Also, there was indication from the records that the carbs were converted to adjustable needles.  Discovering that after buying the car was a surprise.  I have no idea when that was done.  Was it performed as part of the rebuild, or was it done at another time in the car's life?   

 

I didn't get a chance to do a plug cut yesterday, but will do that next week as the next step.  I would like some confirmation that the AFR readings were true before I start tearing things apart.

 

Thanks,

John

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as I think someone said of your fuel pump was under peforming your run would lean out during the run,  your afr stays pretty stable,  so I think your just running lean. A plug chop with old already colored old plugs may not tell you what you want to know. 

 

Graham

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All of the discussion has been on needles and the needle jet, but from my experience with motorcycles you have the pilot, needle and main jets, for idle->mid, mid->full and full throttle.  So if this is a fueling issue rather than air, I would have gone to the full-throttle circuit governed by the main jet.

 

Do these carburetors just have a needle jet?

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No, these constant depression carbs have only 1 circuit (unless you count the cold start device) with metering of fuel delivery accomplished by the needle in relation to the jet.  They are ingenious in their simplicity, but as a result adjustability is limited, and all the parts need to work together as designed or they can be very frustrating.

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There is a short description of how they work starting at the bottom of page 4 of this PDF with a diagram on page 5.  

 

@fastg, that was me.  Other than the header, this is supposed to be a stock non-Federal '68 Twin Cam, and the needle is the stock size for that configuration.  It seems there are a few possibilities:

  • The AFR data was wrong -- it's possible that reversion was creating artificially lean reading with the WBO2 probe up the tailpipe.  Other than a plug cut, I don't know a simple way to confirm this, although I will speak to VRM tomorrow to ask the question.  I suspect most of their tuning is done with a sensor screwed into an exhaust port rather than a tailpipe probe.
  • The engine isn't stock -- the hp rating was consistent with what I'd expect from a stock engine, as were the rpms for peak hp and torque, so this is doubtful.
  • There is something else affecting the AFR that mimic what you'd see if the needle was too lean.  If this is the answer, I think it's more likely an issue with how the carbs were rebuilt.

-John

 

 

 

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