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MG Midget and Sprite Technical - Ignition timing for new cam.

I recently fitted a Kent 266 camshaft, set to manufacture's spec (106°ATDC), to my 1098 Sprite, but was a bit disappointed with it, as it didn't seem any better than the BMC 731 that I'd taken out. This was with the ignition set at 7° BTDC. I started to advance the timing and it improved quite substantially.
So, what do people think would be the optimum ignition advance for this cam?
Thanks as always.
Bernie.
b higginson

Bernie,

The Kent 266 has slightly more exhaust valve lift than the 731 (the inlet lift is the same), with 260° vs 268° inlet duration and vs 268° 270° exhaust duration. Do you know what the 731 timing was set to?

Advancing the cam timing by 4° to 110° will increase the lower speed torque, at the slight expense of higher speed power, but with a 1098 engine that could be an advantage.

We had a Kent 266 in our 1380 originally, and with an Aldon Yellow distributor (?24° total crank mechanical advance) we found that the idle was better with 10° of static advance, but that gave too much total advance, so it ran with ~8° BTDC static advance. The CR is 10.5:1.

What distributor do you have fitted? What is the maximum total mechanical advance at 4000 - 5000rpm?

We now have a Piper BP270 (276° duration) fitted and have a direct comparison between the initial timing of 4° advanced, i.e. 111°, and now 'straight up' at 107°. The engine has noticeably less low speed (<2500 rpm) 'grunt', but makes very easy use of the 7000 rpm rev limit set into the Nodiz ignition system!

The total advance, with no vacuum, is 30°, and at idle it runs smoothly with ~20° BTDC advance including vacuum. As soon as the throttle is opened, and the vacuum changes, the advance is reduced to a more normal level of 8-10°!

Unleaded fuel, and high(ish) overlap cams, need more idle advance than was the norm a few years ago.

Richard

Richard Wale

Richard. The 731 was set 4° advanced from dot to dot. The Dissy is a standard 1098 one rebuilt by H&H with modified curve to suit the 731.

I asked the original question because I didn't want to advance the ignition too far and damage the engine.

I'll check total advance and post it here.

As soon as I get chance, I will be taking the car to Peter Burgess to get it on his RR to let him play with it and sort out the correct carb needle. (HIF44)
b higginson

'with 260° vs 268° inlet duration and vs 268° 270° exhaust duration'

This should be 'with 260° vs 268° inlet duration and 270° vs 268° exhaust duration'

Wish you could 'Edit' properly on this site!

'taking the car to Peter Burgess' - good move!!

Richard
Richard Wale

Vacuum disconnected, 850rpm, strobe light shows 11°BTDC.
Vacuum connected, 4000rpm, using the adjuster wheel on the light to get pointer on zero, display showed advance as 45°! Is that correct or do I need a new timing light? LOL.
Rpm on timing light concurs exactly with car's tacho, so that side of it is OK.But I thought that 45°advanced at 4000rpm was a bit excessive.
b higginson

Bernie,

A total advance of 45° BTDC at 4000 rpm with high vacuum is fine, but what is it without vacuum? I would expect the vacuum to be giving something like a maximum additional advance of 12-15°, so looking for ~30° from the distributor mechanical advance alone.

Depending on the compression ratio, I would get a bit nervous if the total mechanical advance, including static, is more than 30/32° BTDC.

From the information I have, an original Aldon Yellow gave 24° mechanical advance and the Aldon Red gave 20°. They seem reasonable, but I cannot validate either of them.

At 850 rpm idle, the 11° BTDC will be more or less the same as the initial static ignition setting.

Do you get any pinking at lowish revs and full throttle?

A session with Peter B should sort out the best settings, but I would suggest that the cam is advanced by the same 4° that you had with the 731, and that the distributor gives no more than 20° mechanical advance, so that you can use a little more static advance to improve the idle.

Richard



Richard Wale

I won't be in the garage today, but I'll check the total advance without vacuum tomorrow.
Resetting the cam will have to wait until nearer the Peter B visit.
BTW. The cam is set at 107°, so it's 1° advanced from spec due to me only having a 4° offset key to hand after I had done the caculations with a dial gauge etc, so I thought it was near enough.

Are you going to Shelsley Walsh this weekend? If so, my paddock number is 130, so perhaps we could have a chat then.
In the meantime, thanks for your expertise and advice.

Bernie.
b higginson

Bernie,
Its a long time since I set my cam but I remember managing to get more advance with a 4° offset key by moving the chain one link on the sprocket and using the key to retard rather than advance. I can't remember the sums but it came out near enough spot on for the recommended timing. I may have flipped the cam sprocket as well - is that possible? - I cannot remember.
GuyW

Just a heads up Bernie, I am trying to set up an audience with the great Peter B and there is about an eight week wait.

Malc
Malcolm Le Chevalier

'there is about an eight week wait'

It's well worth it!!

Richard
Richard Wale

Guy. I don't think the cam sprocket is flippable because AFAIK it has a different profile on either side,but I like the idea of jumping the chain and using the key to retard the timing.

Malc. Thanks for that. I'd better get my skates on.

Bernie.
b higginson

This thread was discussed between 15/05/2018 and 17/05/2018

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