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First tuneup


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So I've had my Z for about 9 months now and in that time I've concentrated on fixing the major problems (radiator, bent wheel, oil leaks, crappy interior...) but last week it was time for the engine to get some attention. I took it to my local auto-shop and asked them to check the carbs, ignition and compression...

Well it looks like I got lucky. The owner of the workshop loves older cars and is a fan of SU carbs! He balanced the carbs (they were way out) and topped up the dashpot oil (empty on both). He adjusted the dwell and advance of the distributor (again way out). He checked the compression (185, 195, 190 190 190,195psi) for my L24 with E31 head which is excellent. The car starts and idles very easily but has a slight hesitation on acceleration before pulling very smoothly and strongly. He noted the hesitation when he handed it back and asked me to return the car if I cannot live with it...the car is going back on Wednesday!

All in all I am extremely happy - the engine was basically an unknown quantity and I was bracing myself for a big bill and a rebuild in the near future...but no longer! When the tune is sorted, I might consider replacing the muffler to get a better exhaust note...but that is probably all.

(PS. all for $89.00!)

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I have had a simple valve adjustment cure that hesitation. The car then ran like a new motor! Also as simple as a dirty air filter.

A good point. Adjusting the valve clearances may also even up the relative cylinder compressions...but I'm not too worried about them :)

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The slight differences in the compression don't amount to anything. I doubt that an ajustment will change anything , unless you have a clicking , loose rockers. The slight flat spot on first start up from a stop could be caused by a couple of things. Three actually 1. if the mixture adjustments are set a little lean . 2. if the oil in the dashpots is too thin. Recomend 20wt 3. vacume advance is failing or has failed . Just a couple of thoughts to look at. One way to check the lean mixture is to warm the engine to normal running temp. Like when you have driven the car for 15min. or more. With the cover off the air cleaner and the engine at idle, ''blip'' the throttle to full open . That is, open , shut quickly as you can. If you get a pop out of the carbs , one or both , this is a lean pop. Just reach under the carb and turn the mixture adjustment ''knob'' 1/8 of a turn counter clockwise . Repeate the test , if a pop continues repeat the adjustment as before. If you can find out what oil was put in the carbs, ATF or Mystery oil can cause the flat spot also. Thicker oil will richen the first speed up of the engine and may cure the problem. My 2cts

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  • 3 weeks later...

Well here is part 2 in the tuneup can of worms... actually its not as bad as all that. Remember the hesitation I was left with after my z's first ever tuneup? Gone! :love: My over-zealous local mechanic managed to reduce the CO percentage in the exhaust at idle to 0.5% when it should be around 1-1.5%... so it was too lean at idle. Next pearl of wisdom is that the Hitachi SU-style carbs run progressively leaner at wider throttle openings. So, the faster I went the leaner it would get and this accounts for the hesitation at cruise speeds and the constant need for choke. Also accounts for the rapid burning-up of the sparkplugs :rolleyes:

It is running great now - very strong! (cost $AU150)

However, although the exhaust gas readings are now perfect at cruise, the idle readings are around 7-8% CO (remember they should be 1-1.5%). The carb specialist then asked me a peculiar question: "when was the oil last changed?" Err, I had not changed it since owning the car :nervous: Aha! says he, contaminated oil will not absorb any fuel/exhaust that leaks past the rings which means that these elements will be recirculated into the intake manifold. Hey presto - this accounts for the rich idle mixture and high emissions reading.

So, I've changed the oil and filter and I plan to change them again after about 500kms. Even after an oil change it seems to run better!

Oh, they also discovered the coil was underperforming (18Kv max output when it should be 25-26Kv)..so I've got a new one of those too!

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