Lean condition with Stromberg head Twin Cam

[fastg]

Is the air filter setup stock? For that matter is the exhaust stock?

 

Graham

[Pokey]

John, I just read you original post again and see the AFR is lean from mid onward.  If the AFR is real and it is a fuel issue than doesn't it seem like jetting given you were still building power throughout?

 

 

 

[Pokey]

2 minutes ago, fastg said:

Is the air filter setup stock? For that matter is the exhaust stock?

 

Graham

 

That's where my head was originally Graham, but I was thinking an air leak between the carbs and the manifold.

 

[JohnCh]

It has an RD Enterprises header and a cone filter replacing the stock filter at the end of the factory cold air setup.  I can't see either of these making much difference.  @Pokeythe reason this is confusing is that absent of other information, a needle change seems like the answer.  However, other than the header, it's a stock UK-spec engine running the stock UK-spec needle.  Although that may not be perfect, I wouldn't expect it to be in the 15:1-16:1 range.  It might turn out to be a lot of little things are out of whack and adding up to the lean reading.  

 

-John 

[Pokey]

I've always been fascinated with the engineering that goes into carburetors.  This Constant Depression design looks like it uses an air valve to deal with changing atmospheric pressure.  Any problem with that valve would seem more likely to result in a rich condition though, so probably not the culprit.  But what about the float level?  Is it possible the fuel level is too low in the float chamber?

[fastg]

Spray brake cleaner around the carbs/manifold, if the engine revs change you have vacuum leak

 

Graham

[JohnCh]

I confirmed there is no intake leak.  I also spoke with Byron at VRM.  He had thought about it over the weekend and had come to the same conclusion as someone else regarding exhaust pulses pulling in fresh air, artificially leaning the readings.  As he put it, the engine was just running to well for those numbers to make sense.  I'll find time over the next couple of weeks to bring the car back so they can install a WBO2 bung in the exhaust in preparation for another try.  If the results are still really lean, then I know I have a bigger problem, likely with the carbs, to diagnose.  

 

Thanks,

John

[JohnCh]

Thought I'd follow up with the outcome.  I left the car at VRM Monday so they could weld in an O2 bung in the exhaust for temporary use on the dyno and try again.  With the O2 in that location, reading changed dramatically and matched what we expected.  After optimizing the ignition curve and playing with needle height, the torque curve increased about 5-8 ft-lbs across the entire curve.  Peak HP went from 88.2 at 5720 rpm to 95.34 at 6120 rpm, and peak torque from 86.09 at 4990 rpm to 93.22 at 4520 rpm.  The numbers not only went up, but the distance between the peaks increased.  Well worth the effort.  Thanks again to all who chimed in earlier when I was trying to figure this out.

 

-John