Ignition timing on a 120FHC

Please can I have thoughts on ignition timing on my 120FHC.
Engine 3400cc, 2x2”HD8’s, UN needles, Rob Beere camshafts designed for SU’s with very slightly higher lift at 0.410 instead of 0.375 but usual duration, 9/1 compression pistons.

I have the initial static set at 10 BTDC which is where it is at about 700-800 RPM idle speed. There is a vacuum connection.

What I would like to know is at 1,000, 2,000, 3,000, 4,000 and 5,000 what degrees of TOTAL advance I should see on my strobe light.

Am I right in saying that with higher compression, I should have not have too high advance ? No more than 32 at 3,000 ? Also how early should the ignition advance ramp up at low revs ?

Hi…as you know…you have static advance that you set with the distributor BTDC…which you say you now have at 10 D…That would be with engine not running at all…and should not change at idle as you say…since mechanical and vacuum advance do not (should not) come in at idle. Vac advance depended on your vac unit…the OE 400904 ) (about 1955 the 7 18 12 unit part number was changed to 419066) GC48 .early XK120 vacuum unit, was original 7 18 12. : other distributor or unit replacements that may have occurred over the years may have various GC numbers,.so each GC # has different vac unit specs…) Hope others can provide what the mechanical/centrifugal advance specs are…there were a number of distributor number changes…and each had different mechanical weights to curve the advance…so one has to consider static, mechanical, and vac advance. With your engine set up differing from stock…I’d suggest you also measure your compression, and send all specs to a specialist distributor man…to determine the optimal set up…static, curve, and all in numbers at what RPM. There are various vac mods, and distrib mechanical weight mods that can be done. For now. check that the vac unit is functional, that the mech advance moves, set the points, then.the old shade tree method of advance and test drive until a ping under load…then retard until no ping . Start should be immediate with no “kick back”. Be sure all else in the spark plug ( gap and heat range) , primary wires (no leaks and low or no suppression) , and carb tune is optimal first. 10 to 12 D BTDC should be about correct and a fine place to start. Your engine mods, today’s fuels and spark plus, all come into the formula. A nice winter task is send the dizzy and vac unit out for a rebuild. I did…with very fine results…to
Rob at
British Vacuum Unit
112 Briar Bush Road
Canterbury, NH 03224 USA
603-731-1788
If you are in UK…no doubt there are shops to do it.
Cheers Nick

The Distributor Doctor here in the UK might be able to help, but really the only way to determine what curve you need is a detailed session on a rolling road with an experienced operator. If your engine is modified in any respect, you’re not going to get best results by copying someone else’s figures. Someone can probably give you some ballpark settings, though.
If you set timing at idle, you should have the vacuum tube disconnected and blocked off. Timing a distributor in a modified engine is a fiddly process as carburation and timing interact, responding to any change in the other.

John,

I recall from earlier this year that your engine has a number of modifications including a modern variable advance curve electronic distributor, and that you took your XK120 to a specialist to have it tuned on a dyno.

My XK120 has the 3.4 engine with 8:1 pistons, and the only major modification is an XK140 C-Type head. The distributor is the Lucas 40199 model, which was supplied to both the XK120 and XK140 with 8:1 compression engines. The XK120 Service Manual, page P.5, shows the advance and rpm specs for this distributor.

Note that the maximum advance shown in the Service Manual is 13-15 distributor degrees at 1600 distributor rpm. The distributor turns at half the crankshaft speed, so double those numbers for timing light purposes. Thus 26-30 degrees at 3200 rpm.

Below is a photo of the Shaft and Action Plate found in the Lucas 40199 distributor. Note how maximum mechanical advance is controlled. The two weights mount on the pegs. On the bottom of the weights are small tangs that drop into the two holes in the action plate. The diameter of the two holes in the action plate limits how far the weights can move outwards under centrifugal force. The action plate is stamped “14” which is 14 distributor degrees or 28 crankshaft degrees. I have several of these action plates, each with different diameter advance holes and different degree stampings. The point is that Jaguar supplied distributors providing fairly conservative maximum mechanical advances.

Back to my XK120. I completely rebuilt my 40199 distributor and then tested the results on a Sun Distributor Machine, which showed the distributor was advancing nearly identically to the specs on page P.5.

When I installed the rebuilt distributor in the car, I performed a number of low-speed hill climbs in 3rd and 4th gear to determine the optimum static advance. I settled on 10 degrees BTDC. At this static advance, with 91 octane fuel, the engine gives maximum power with no pinging. So to answer your question, my engine advances a maximum of 38 degrees at 3200 rpm under load, (10 degrees static plus 28 degrees centrifugal, and disregarding the vacuum advance which provides additional advance under low load conditions.)

Last year after my C-Type head was professionally rebuilt, I wanted to make sure the carbs were properly tuned and set-up. So the final tuning was done at a shop that specializes in dyno-tuning British cars. Below is a graph of the results. Maximum HP at the rear wheels is approximately 150.5 at 4580 rpm. Maximum Torque at the rear wheels is 196 at 2269 rpm. And for most of the loaded rpm range, the air/fuel ratio is between 12 and 13.

Thank you to all the replies. It is quite right that I had the car on a Dyno earlier this year and it gave a max of 204 torque with a very flat curve from 1,900 rpm upwards and a maximum at the wheels of 180hp at about 4,500 rpm. But then I had a disaster ! I took the distributor out to tidy up the lose wires with some heat shrink covering.
However I fell over in the garage as tripped over a box and threw the distributor at the wall. It broken the electronic circuit board. So sent it back to be repaired and then it got lost in the post, even though recorded delivery. Luckily I have my old 123 system but that is over 10 years old and a very early simple version. So, the reason I was asking the questions, is that I have bought the new 123 Tune + with Bluetooth that you can make the curve exactly as you like. I must say I find the instructions quite complicated. I have now had an email from the manufacturers in Holland to say set the static at 8 although the instructions say 10. But would we agree that the total, advance should never go more than about 34 for a 9/1 compression. Also to stop advancing at about 3,200 ? In fact what I think is just as important is the advance numbers for lower down in the rpm range from 1,000 to 2,000.

There are various books on “tuning XK engines”. Don’t know whether you’ve seen the one from Des Hammill, but he gives the following answers to your questions.

For tuned engines, total advance (so static + centrifugal advance in the pre-electronic era) should be around 38 degree BTDC. This should be present from 3000 rpm onwards (alternatively a bit higher from 3300 rpm). So this 38 degree total advance is more than the 32 you refer to. He even mentions cases with up to 40 degree total advance (depending on “pinking”) but advises never to exceed 44 degrees.

Hammill recommends idling at 1000 to 1200 rpm for tuned engines, depending on whether “smooth running” can be obtained. He refers to an advance of 12 degrees at 1200 rpm.

Hope this helps.

Bob K.

Cannot even begin to look at the ignition tuning, as the starter motor appears to have broken. It is spinning with a strange noise, but not engaging the flywheel ? It is not very old less than two years, bought from SNG. One of the lightweight versions. I need hands the size of a five year old to undo the nuts and bolts, the access is so tight against body, engine and other pipes !!!

Worth checking that your battery’s fully charged.

Yes, you are quite right. I put the two batteries on charge the other night to check. I will check their state of charge tomorrow. I should have good power ??? as in fact it has two 36 amp hour 12 volt batteries joined together. If I doubt the state of the batteries, I will try jump starting it. Is it often the case that what appears to be a fully charged battery, is not enough to fling the bendix out on the starter motor ?

did this starter work prior? it is possible with replacement starters that the depth of the starter gear teeth does not match/make contact with the flywheel teeth. There are methods to measure and shim to set up…avail on line and You Tube. Nick