[TR6 Ignition Timing with Triple Webers]

17 hours ago, Tom Fremont said:

Hi Tom,

I have 40s on both my engines, and concur with the perception that 45s are for larger displacement TR5/6 engines, e.g. 2.7 liter etc.

If your settings give satisfaction across the range ( no low speed - mid range gasp )  you ought to stay with them. Keep a record should you venture into the wilderness so you can find your way back. They say there are over 4 billion permutations.

I have used F2 and now F7 emulsion tubes and have tried F11, F15 and F16 without success - each failed in the low-mid range because the main system doesn't come into play until 2500-3000 rpm with them on my engines. The F16 was the worst, and F11 in 3rd place behind the F7 and F2. Main jets are 125/175 air in the driver and 130/190 air in the concours / higher mod engine. Exhaust manifolds matter a lot, and a true extractor like the GOODPARTS makes for smaller jets and/or  larger air correctors. I have one of these and it was a must for the latter with its really extreme cam in order to get the mid-range up to snuff. Fitting it led to 1 size smaller main and (7) sizes larger air corrector to restore the top end. Chokes are 30mm in the driver and 32 in the concours one. Pump jets are 45 in the driver, 40 in the other with closed bypass valves. A critical factor is float level. I have 40DCOE18 on both engines but the driver's are very old with brass throttle shafts and the same setting in both cases results in a fuel height discrepancy of 3.5mm! The pertinent measurement therefore is 28mm below the top of the well. Below this level ( higher number ) will make it difficult to bring the mid range in low enough; in my case 2000 rpm but others hold out for 1500. Some pundits urge setting the level to 25mm, leaving a scant 1mm below the pipe into the venturi.

I've never had my engines on dynos and I had a brief fling with a a/f meter system when I was tearing my hair out with the wild one; it was a piece of !@#$ and didn't tell me anything I didn't already guess before it stopped working altogether.

So it's been seat of the pants pretty much all the way, with helpful advice here and there, and a very fortuitous set of F7 emulsion tubes in a used set of DCOEs I threw in 20 years ago. The driver's engine now has 135K miles on its last rebuild, runs terrific with scant oil consumption and excellent oil pressure - thanks in no small measure to the wonderful Webers metering fuel as well as can be for a carburetor.

Cheers,

Tom

Hi Tom,

Thanks for the advice and detailed response all of which is very useful. I am tempted to experiment by increasing my main jet from 115 to 120 (and maybe even 125 as you have) as many sources seem to suggest 120 as a starting point (like the SAH data copied above - thanks Bruce). I might try the F7 emulsion tubes too. I can always return to my current settings.

Cheers, Tom

[TR6 Ignition Timing with Triple Webers]

On 6/30/2020 at 8:21 PM, Tom Fremont said:

The 41219 distributor is a 7 degree nominal centrifugal advance type. With 11 degrees static time this gives ~ 25 degrees crank advance maximum and reaches this value at 2600 rpm crankshaft.

I have these on both of my Webered TR250s. The driver has ~ 9.5:1 c/r and CP cam. It will ping if I dial in as much as 30 degrees total advance so I reckon TRIUMPH set them below this value for good reason. The other one has very high lift, high duration cam and 10.7:1 c/r and it can take somewhat more advance before it pings.

Frankly, I haven't found much power difference at all within a range of +/- 5 degrees advance; only pinging and fuel economy. For the latter it would be nice to have vacuum advance for highway cruising but I haven't found a way to get it.

 

Tom

Thanks Tom - really useful data.

I've just measured my head thickness (my engine not my brain!) and get around 3.39" which combined with 0.020" bores gives 10.1 compression ratio - so my engine may need a timing in between your two engines. I've also measured my advance at around 8 degrees at 500rpm and 22 degrees above 2500rpm which seems low but does fit with your 7 degree distributor advance comment. So it seems my timing needs to be advanced a little and I may need to increase the centrifugal advance. I will experiment and see if I can advance to the point of pinging then back off a couple of degrees.

I'd be interested to know what jet sizes you run in your webers - do both cars use DCOE40s or is your higher CR engine on 45s? My DCOE40s currently run well with the following setup: Venturi=30, main jet=115, air corrector=180, emulsion=F11, idle jet=50F6, pump=40.

Tom

[TR6 Ignition Timing with Triple Webers]

Thanks very much for all the comments. The distributor looks original (Lucas 41219) and has weights and springs that have been checked out fairly recently. I am about to try to remeasure the advance but I think it is about 10 degrees in the distributor so 20 degrees at the crank. The static is currently set to 10 degrees so that would make 30 total advance at high rpm. Thank you NTC for suggesting 35 - I may try increasing the static advance to see if I can get some pinking (I can't hear any pinking at the moment but then the car is pretty noisy!).

The 10:1 compression ratio comment came from a garage that tuned the car last year for the previous owner. It was based on cylinder compression test results of around 180psi. So this may not be accurate. Perhaps I should try to measure the thickness of the head and use Bruce's numbers to check - can this be done with the engine in the car?

Tom

[TR6 Ignition Timing with Triple Webers]

I've just bought a TR6 (CP) with triple webers (40s), fast road cam, phoenix sport manifold+exhaust and lumenition optronic performance ignition. I think the compression ratio is around 10:1. It seems to go well (the webers appear to be well tuned) but I am not sure if the timing is optimised for maximum power from the modified engine. I would appreciate your thoughts and recommendations for a distributor timing curve - i.e. the static timing, the maximum timing degrees at high rpm and roughly what rpm this maximum should be reached?