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TR6 timing How far do I go?


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I have a U.K. car with a US spec (lower compression) engine running with twin H6 SU carbs and electronic ignition. 

How far can I go setting the ignition advance? I’ve taken it to 8 degrees and it holds back a little when hot.I can overcome it by giving a tiny bit of choke. I’m reluctant to adjust the carbs further until I’ve got the timing spot on as I have a nice idle quality and a decent exhaust note. 

Thanks in ‘advance’ for any advice

Adi 

 

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Is that 8 degrees static or 8 degrees with vacuum, when idling with a strobe light? Mine runs best with about 18 degrees advance measured at idle with a strobe. That's a UK spec PI engine. It seems to make little difference between 15 and 22 degrees idle crank advance but if I went down to 8 things get a little rough. Full advance with no vacuum (ie wide open throttle) is all in at about 32 degrees crank advance from 3000 rpm upwards. This is a mappable system though so if your isn't mappable you have to compromise a little at one end or the other as best idle might be too much advance at higher throttle. I have always found that seat of the pants and listen for detonation is the best guide without spending time and money on a rolling road unless you are really after that last 2%.

If you are having to give it choke at idle when warm it suggest you re maybe a touch lean at idle or a little retarded on timing (or both). The richer mixture burns a little faster which makes up for a lack of advance so one problem can mask another.

 

 

 

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With most engines fitted with a distributor I always start with the factory setting then advance it until I get light pinging at WOT low RPM.If a vaccuum advance is fitted I disconnect it.  I then retard the timing until the pinging just stops. My TR6 is a PI but I  use a similar methodology with carburetted engines.

 

Edited by Mike C
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2 hours ago, Mike C said:

With most engines fitted with a distributor I always start with the factory setting then advance it until I get light pinging at WOT low RPM.If a vaccuum advance is fitted I disconnect it.  I then retard the timing until the pinging just stops. My TR6 is a PI but I  use a similar methodology with carburetted engines.

 

+1.

But before you get too involved with the timing there are prior things to do if not already done.

If it hasn't been done a compression check is in order.

Assuming that is OK start the tune with making sure the tappets are set correctly. If the engine isn't breathing properly everything else will be out.

Then check plugs gaps and condition. If the plugs, leads, distributor cap and rotor button are of unknown condition fit new ones.

Then balance the carbies. Once balanced set the mixtures, I set mine a tad rich at idle to avoid being too lean at speed.

After all that the engine is in tune and you can play with the timing as suggested by Mike to get the best result. 

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Thank you for the advice, I’ll have a play and advance it a bit at a time until it pinks and back it off. The carbs are pretty well balanced I think as I’ve gone back to scratch and been very careful opening them up. Tappets adjusted and plugs etc. all good. Vacuum advance working ok. 
The 8 degrees was with a strobe and vacuum disconnected. Doesn’t sound like it’s anywhere near enough! 
Thanks again.

 

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Which distributor do you have fitted?
  If the original US spec item, then the vacuum capsule retards the ignition, not advances it.

Check that the dash pots of the carbs have oil in them.

Peter W

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On 9/12/2021 at 3:51 PM, BlueTR3A-5EKT said:

Which distributor do you have fitted?
  If the original US spec item, then the vacuum capsule retards the ignition, not advances it.

Check that the dash pots of the carbs have oil in them.

Peter W

Hi Peter, the vac advance is there and working. The retard at idle vac can has been removed.

SU’s all topped up and ok. 
 

Thanks

Adi

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