Yeah, poor running in traffic is a common prob. I think it's exacerbated by posh exhaust manifolds.
I think the prob is mainly the intake air density, but fuel vaporisation may have a part.
For me it was solved with the pmac/mrf airbox and taking air from in front of the rad.
Also a heatshield between exhaust mani and carbs helps with the fuel boiling.
I do have a Janspeed manifold so that plays to that logic. Also I do have a cold air intake from the front of the rad and off the side but on the standard 240z airbox so undoubtedly nowhere near as effective as a PMAC/MRF set up. Defo need to look into the heat shield thing as the carbs are too warm to the touch for my liking and I worry rubber bits are being over stressed.
If you can't get the required advance from the distributor then I would suggest a "back to basics" check on cam / crank / oil pump installation. The "How to Rebuild..." book has (IMO) the best account with real pictures of what you should see when distributor out and engine at TDC on ~1 cylinder.
The problem of "that'll do" installations and "swap the leads around by one position" is much more common that you may imagine...
If not to the book, then timing will never be right, although I appreciate that changes are often made to accommodate high lift cams (particularly regrinds).
Now that is an interesting trick that hadn't occurred to me (re. Swapping leads). This is great advice Mike, and I do have that book by the bed!
I will try the dizzy again first before going back to basics.
This is just my opinion of how I see the importance of ignition timing. It is all from memory so I'm happy for people to correct me as my memory has never been that good.
If your ignition timing is too advanced. Firstly you are trying to push the piston back down the cylinder before its near the top so not only are you wasting that energy, you are also putting all that energy/heat through the crank and into the engine. Too advanced and you will hear knock.
Too retarded and the energy/heat from the burn will occur too late in the cycle and come out of the exhaust. This is why an exhaust gas temperature gauge can really help to get the timing right.
You are trying to place the spark in just the right place in time to make use of as much of the combustion energy as possible. People generally talk about all in advance of 34deg being about right for an l-series performance engine and I would tend to agree with this but there are other factors to consider that will effect the ideal timing for your engine. For example: chamber design, compression ratio, fuel octane rating, and many more. A lot of these will influence the speed of burn. With a slower burn you will need to ignite the spark earlier in order for most of the energy to be used to turn the crank. Create a faster burn and you need to retard the ignition to achieve the same.
That's about all I have time to write about the subject at the moment and although it may not have answered your question, I hope it has given you a bit of information to allow you to make a more educated decision. I have tried to very much simplify the explanations above in order to not complicate the subject and confuse people even more. It is a complicated subject but I'm a great believer in trying to explain things in simple terms to give people an understanding of it rather than just try to confuse.
Excellent explanation, thanks for taking the time to type all that Johny!! I followed a similar logic in suspecting the timing being the heat culprit as I figured I'm getting half the burn in the exhaust rather than the combustion part of the cycle. Especially with my lowish compression readings.
Liking the exhaust temp idea. At my last MOT, the guys finished the test and said: right, let's put that sensor back into the exhaust and set up your carbs properly. They spent a good 30 mins with me tinkering with the car and giving me readings until we got it bob on! (She was running rich to start with). It was the emissions sensor thing they put up the exhaust pipe.
As you know Im a timing virgin, so can't help there, but, a few other things maybe
1. Could the insulation on one or more HT leads be breaking down as under bonnet temps rise in traffic
2. Is there sufficient fuel return through the restrictor on the end of the fuel rail
3. Viscous fan, is it definitely working 100% at idle
1. I have the Magnecore KV85 racing leads. The car did the same with the old leads (first thing I replaced after overheating at the Blackwall tunnel and causing a 2 mile tail back on my second week of Z ownership!
so I think they are OK
2. Good question, not too sure. But interestingly at the weekend I noticed that my see through fuel filter was only two thirds full at idle and bubbles were travelling towards my carbs!!! So I do wonder if the mech pump has seen too much action and what feels like a misfire is in fact fuel starvation!
3. Great point - had similar doubts but difficult to know as it has a fair bit of resistance and I have nothing to compare it with.
