Project: 1939 Jaguar SS100 Replica Classic Roadsters, LTD. "Duke"

[Texas Jim]

I think coincident to our discussion, but worth mentioning -- the choke butterflies were opening on wide open throttle, but were not properly

engaged with their close spring. I adjusted the lid again so that the linkage is properly moving on the close spring and opens at WOT.

I re-adjusted the idle mix. During the test drive the car drove fine, but is uneven at 3500+ rpm -- pulls when you accelerate, but

spits/coughs a bit too just running along at 3500-4000 rpm. So I agree the mixture is perhaps off at that point, or this is another symptom

of why it isn't pulling well from 3500-6000 rpm.

Whatever is causing the roughness at 4000 is why it isn't pulling good at higher RPM... I would be willing to make a small wager on it...

[Cadillac Jim]

It looks like maybe the centrifugal spark advance isn't working.

Another thing that can make the torque curve sag slowly, but not enough to keep the horsepower curve from rising some, is a flat cam.

A wholly unlikely thing is cam timing. The very low torque curve and no top end can be a symptom of cam timing that is late a cog or two on the chain/gear.

[Bruce Nunnally]

The timing is tricky to read directly on the car with a light due to the configuration, but I'll see if I can find an angle/position

that will let me test the total timing to check centrifugal advance. The cam is steel in these (which makes them good turbo

candidates).

Texas Jim is betting the carb is running lean at WOT. I put in smaller jets and that didn't improve it, so he may have a point

(if you go smaller and no change, go bigger). This could also mean a fuel delivery problem since even with the bigger

140/140 jets it was still lean.

I am guessing the carb is still rich at WOT somehow, even with the smaller jets. That might mean it is not getting

enough air.

The car does backfire on overrun, which would argue for too lean at low rpm. I ordered a exhaust mount for the

wideband until I can get a bung installed.

[Cadillac Jim]

There are so many things that could cause the ham-can torque curve, particularly in a car that just came out after sitting for years. You have eliminated most of them, for example your ignition isn't sagging because of your new MSD ignition.

Trucks get a for-them-beneficial ham-can torque curve by late cam timing. But we have no reason to believe that this engine has ever been apart, and such an assembly error is rare. Intentional, for tractor-pulling contests and such, sure; Summit Racing has cam gear mounts for people who want to advance - or retard - their cam timing for the particular tuning that they want to achieve. But accidentally missing just one cog or some such is not something I can remember hearing about. But the ham-can torque curve did remind me that this is what you get if you retard the cam timing.

Since the distributor rotates at half crank speed, you might be able to pull it out, remove the cap and points plate so you can see the centrifugal weights, pivots, and springs, and spin the distributor with a high-speed drill or some other means and observe whether the centrifugal mechanism moves freely.

There is one disturbing possibility: the hard 2,000 RPM shift point may mean that this engine is designed for the driveabilty of a bigger engine and the gas mileage of a smaller engine by use of the ham-can torque curve and shift points designed to keep the engine in it's low-RPM sweet spot for most normal driving. If so, you will need a cam and head to get a more conventional torque curve.

The "nailhead" Buick V8 of 1953-1981 was tuned that way. It had torque all out of proportion for V8s of its size, which they combined with a wide-range toque converter to get even more toque with little or no shifting for great driveability and smoothness. That transmission, which was well underway with the preceding OHV straight eight, got legendarily poor gas mileage - but I knew a fellow whose grandmother had a 1955 Buick with a standard shift and it got great gas mileage, about three times what the Dynaflow models got.

If all is well with the distributor, timing, and carburetor, I would look at the specs for the sizes of the intake and exhaust valves, and specs on the valve timing.

[Bruce Nunnally]

The 3TC is not characterized as having a flat hp curve -- recall it is rated 75hp @ 5000rpm and 95 ft lb of torque @ 2600rpm with a C/R of 9.0:1 stock.

It is basically a miniature hemi, has low tumble, high flow, and likes to be turbocharged. Unfortunately it is hard to find much info on the stock performance,

and I can write books about what I don't know about the engine in my Roadster. This argument is making me think Texas Jim may have a point that the

engine was actually running lean -- although we may see that it is lean at 1/2 throttle (keeping speed at 3500 rpm say) and rich at WOT with both

main and aux open.

[Texas Jim]

The 3TC is not characterized as having a flat hp curve -- recall it is rated 75hp @ 5000rpm and 95 ft lb of torque @ 2600rpm with a C/R of 9.0:1 stock.

It is basically a miniature hemi, has low tumble, high flow, and likes to be turbocharged. Unfortunately it is hard to find much info on the stock performance,

and I can write books about what I don't know about the engine in my Roadster.

This argument is making me think Texas Jim may have a point that the

engine was actually running lean -- although we may see that it is lean at 1/2 throttle (keeping speed at 3500 rpm say) and rich at WOT with both

main and aux open.

If it is... I buy lunch next time we meet....

I told you I was wiling to make a small wager on that... LOL

[Cadillac Jim]

Running lean gets me to a point that fell out of my post when I lost the screen. The jets and power valve in the secondary barrel are the ones that will determine mixture at top end.

Uh, about that oxygen sensor...

[Bruce Nunnally]

Pronto Muffler installed the bung for the wideband O2 sensor.

Did a quick and dirty install so I could see where we are -- at idle first test 10.4:1 Air fuel ratio (too rich), and 14.x:1 under acceleration (too lean).

I was able to choke down the idle mixture screw and get it up to 11.7:1, but needs a smaller jet.

Conclusion: The idle jet is too big, and the main/aux jets are too small. Texas Jim called the main jet problem.

[Bruce Nunnally]

Update: changed the idle jet from 60 to 50. Perhaps could go smaller but right now it is idling at 13-13.5:1 AFR which is ideal.

The previous cowl shake has completely disappeared and now it idles like butter.

Put back in the 140/140 main/aux jets. Under WOT through 4000 rpm AFR came down to 13-13.5 from 14.4 previous.

Probably the engine wants 150/150 or 155 main jet with current air correction jets, or even a 160 main with a bigger

air correction jet.

Larger air correction jets lean out the mix, but as a rule of thumb air correction should be main + 50-60.

So the 150 main 'wants' an air correction of 200-210, but going up in the air correction from current 170 to 210 is like

going down 1/2 that much in main jet, or 20. So a 150 jet / 210 air is like a 130 jet / 170 air. This allows for smaller

adjustments & corrections with the air correction jets.

I ordered several larger jets when Jim guessed it was too lean (just in case he was right, which does happen sometimes).

They should be in next day or two.

[Texas Jim]

Update: changed the idle jet from 60 to 50. Perhaps could go smaller but right now it is idling at 13-13.5:1 AFR which is ideal.

The previous cowl shake has completely disappeared and now it idles like butter.

Put back in the 140/140 main/aux jets. Under WOT through 4000 rpm AFR came down to 13-13.5 from 14.4 previous.

Probably the engine wants 150/150 or 155 main jet with current air correction jets, or even a 160 main with a bigger

air correction jet.

Larger air correction jets lean out the mix, but as a rule of thumb air correction should be main + 50-60.

So the 150 main 'wants' an air correction of 200-210, but going up in the air correction from current 170 to 210 is like

going down 1/2 that much in main jet, or 20. So a 150 jet / 210 air is like a 130 jet / 170 air. This allows for smaller

adjustments & corrections with the air correction jets.

I ordered several larger jets when Jim guessed it was too lean (just in case he was right, which does happen sometimes).

They should be in next day or two.

[Bruce Nunnally]

Also with larger main jet may have to go smaller idle. I am surprised based on that

Reference 3tc reference site that the engine wants more fuel.

Goiing 21 miles to spring shop tomorrow morning; hopefully they can help.

[Texas Jim]

Also with larger main jet may have to go smaller idle. I am surprised based on that

Reference 3tc reference site that the engine wants more fuel.

Goiing 21 miles to spring shop tomorrow morning; hopefully they can help.

having to change the idle jet because you change the main jet may not be necessary.

It depends on how they are interconnected inside the carb.

[Bruce Nunnally]

Maybe; the great thing is with the wbo2 afr gauge we'll see. I wondered if I would even want

that perm installed or just use as a tuning tool, but it is fun to keep an eye on too,

and will make it easy to rejet later if needed -- or detect an emergent issue.

I am thrilled the cowl shake is cured. I am keen to see how it runs once it is tuned in.

When correctly jetted there apparently should be prominent induction noise as well.

Could be the engine was previously modified?

Anyway, if the rain stays away we'll see what they say about the suspension tomorrow.

[Bruce Nunnally]

Made it down to Truett Worrall spring shop and back.

They do a lot of big hardware -- fire engines, dump trucks, buses -- but also do projects like mine.

They will call in a week or two to schedule the Roadster for a return visit and refresh.

Notionally the rear leaf springs look okay and might just need new bushings and hardware, not new springs,

so that would be good news.

Trip down was straight-forward -- down Preston, across Mockingbird, then around the corner to Truett Worall.

Trip back started as simple, but then I was lost in down-town Dallas morning traffic. I ended up streaming back

up Highway 75 at 50-65 mph, buffeted by semi's and other trucks. The Roadster is not aerodynamic --

say a CD no better than 0.55 and perhaps 0.6-0.7 depending. Still a fun trip and I am optimistic we are

one step closer on the rear suspension fix/refresh.

[Bruce Nunnally]

Final AFR installation / location -- in easy view of either occupant, in easy reach for logging to laptop,

but looks clean

[Cadillac Jim]

Sure looks like a great permanent installation to me. I'ts a perfect place to put the boost gauge after you have DFI and need to see the psi.

[Bruce Nunnally]

The main/aux jets and the air correction jets arrived. I got a 150/160/170/180 main/aux, and a variety of higher air correction jets.

Recall the story so far -- AC jets are 170/160.

Main 130/125 14.4:1 on WOT

Main 140/140 13.5:1 on WOT, too lean on steady state throttle higher RPM

Main 150/160 12.2:1 on WOT, so better at WOT, but is still a bit lean at steady state (today's update)

Next I am thinking swap the 160 and 150, to try to get more rich at steady state, then

add some air correction to the aux to try to get slightly more lean at WOT.

So next I will try

Main 160/150 AC 170/180

Which should richen in the main for constant-speed RPM and lean up the aux when both

sides of the progressive carb are open? Other thoughts or suggestions?

The idle was richer with the bigger jets, or a different day/different temps but was 12.7ish

after it settled down. I can dial it in with the idle mix from there, but will order a smaller

idle jet if needed.

[Bruce Nunnally]

Okay, I reversed the main/aux jets from 150/160 to 160/150. So now we have

Weber 32/36 Part #: 22680.033B
Venturis: 26/27 (compare to Optimum Carburetor Choke Size)
Auxiliary venturis: 3.5/3.5
Main jets: 160/150 (OEM setup was 140/140; predicted stock 3TC was 130/125)
Emulsion tubes: F50/F50
Air correction jets: 170/160 (OEM)
Idle jets: 50/50 (OEM 60/50)
Jump jet: 50
Needle valve: 2.00

This resulted in 12.2:1 at WOT still as expected since same amount of air/fuel coming in from both main + aux,

but the main circuit when operating alone at higher RPM seems happier, at about 12.8:1 when on the gas.

Overall the car seems to be in much better tune. I will keep this set and run tests tomorrow, then

perhaps go to a larger aux air correction jet to try to dial it in from 12.2 to 12.5. The curve

is pretty flat in this range so I think we're in the range already. Keep in mind curve shifts left

with our E10 fuel.

[Bruce Nunnally]

Retested with the new jetting this morning. I was expecting to see the car improve somewhat from the previous times.

What I saw instead is that it got slightly worse. It is certainly running better and is better behaved, but not making any

more power.

Today's runs: main/aux 160/150 WOT AFR ~12.4

Run num Speed(mph) Time(s) Dist(f) Graph run Description
2 0.00 0 0.00 Speed Start
2 10.00 1.21 8.91 Speed Line
2 20.00 2.81 44.98 Speed Line
2 30.00 4.85 119.78 Speed Line
2 40.00 7.04 232.31 Speed Line
2 50.00 10.06 433.23 Speed Line
2 60.00 14.44 787.74 Speed End

3 0.00 0 0.00 Speed Start
3 10.00 1.23 8.98 Speed Line
3 20.00 2.85 45.50 Speed Line
3 30.00 4.88 119.97 Speed Line
3 40.00 7.07 232.82 Speed Line
3 50.00 9.98 425.35 Speed Line
3 60.00 14.38 781.56 Speed End

Previous with 130/125 jets: WOT AFR ~14.4

Run num Speed(mph) Time(s) Dist(f) Graph run Description

1 0.00 0 0.00 Speed Start
1 10.00 1.09 8.46 Speed Line
1 20.00 2.63 42.99 Speed Line
1 30.00 4.55 113.70 Speed Line
1 40.00 6.68 223.35 Speed Line
1 50.00 9.50 410.64 Speed Line
1 60.00 13.69 748.99 Speed End

3 0.00 0 0.00 Speed Start
3 10.00 1.14 8.64 Speed Line
3 20.00 2.69 43.54 Speed Line
3 30.00 4.64 115.19 Speed Line
3 40.00 6.77 225.22 Speed Line
3 50.00 9.61 413.44 Speed Line
3 60.00 13.84 755.57 Speed End

Previous with 140/140 jets: WOT AFR ~13.5

Run num Speed(mph) Time(s) Dist(f) Graph run Description

2 0.00 0 0.00 Speed Start
2 10.00 1.22 8.55 Speed Line
2 20.00 2.71 42.07 Speed Line
2 30.00 4.64 42.07 Speed Line
2 40.00 6.77 222.98 Speed Line
2 50.00 9.67 415.39 Speed Line
2 60.00 14.02 769.02 Speed End

This will need some additional thought. I get that it is safer for the car to run slightly rich, and it runs smoother.

I am surprised that doesn't equate to making more power as well.

[Texas Jim]

Does the distributor have both vacuum and centrifugal advance ??

[Bruce Nunnally]

In theory, yes. As Cadillac Jim previously pointed out it would be a good idea to validate exactly what the timing is doing.

Perhaps what we are seeing is timing is all in by 3500 rpm but engine wants more timing?

I'll see if I can sort out an angle or mirror setup to check the timing at various rpm.

[Texas Jim]

Per the numbers,,, After 30mph it is just falling on it's face and not pulling hard...

The bigger jets and enriching it actually hurt it after 30mph.

Something besides fuel is keeping it from revving freely.

The engine only needs 3 things... fuel and air at the proper mixture for combustion and spark delivered at the right time.

You have worked with the fuel and air... now it's time to look at the spark delivery.

[Bruce Nunnally]

Tonight I made a change to the main/aux fueling: I unreversed the main/aux jets to 150/160, reversed

the air correction jets (170/160->160/170) then put a larger air correction on the aux at 185 instead

of 170.

Weber 32/36 Part #: 22680.033B
Venturis: 26/27 (compare to Optimum Carburetor Choke Size)
Auxiliary venturis: 3.5/3.5
Main jets: 150/160 (OEM setup was 140/140; predicted stock 3TC was 130/125; previous 160/150)
Emulsion tubes: F50/F50
Air correction jets: 160/185 (OEM 170/160)
Idle jets: 50/50 (OEM 60/50)
Jump jet: 50
Needle valve: 2.00

This is the equivalent of a 155 jet in main, and leans out the aux by 1.5 steps.

This put WOT back around 13.x:1 but kept the main circuit in a good place for part throttle.

I will try to check the timing, and consider the argument that the fuel pump is not putting out enough

capacity at high rpm. I suspect there is some 'macro' (and obvious) problem with the Roadster

but don't see it yet.

[Bruce Nunnally]

The low pressure inline fuel filter on the Roadster looked like this one

it did have visible sentiment trapped in it, so I put a new inline filter on. I don't think that could be enough to have a pronounced effect

on acceleration, but it would be a nice easy fix.

[Bruce Nunnally]

New clean fuel filter, afternoon testing with Main 150, Air correction 160 and Aux 160, Air correction 185. Loss of 3 whp. Not the correction I was hoping for lol.

So, the 3TC makes more power when it is lean, but it still is laying down after 4000 rpm regardless. After it cools a bit I'll see about checking the timing.

The current jetting is very good for driving around town. etc. I just need to figure out WOT and high RPM behavior -- what is holding it back there.