VAPOR LOCK: SUDDEN RPM DROP FROM 950 to 500 ON 240z - Page 3 - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #21 of 33 (permalink) Old 02-21-2017, 11:08 PM
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Have you solved your problem with RPM dropping after idling for a long time?

Scott
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9/74 260Z VIN RLS30 060061
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post #22 of 33 (permalink) Old 02-22-2017, 12:12 PM
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I still find it hard to believe the the carbs are so well matched that they would require the same number of turns out on both jets. That means the floats, jets and needles are exactly the same.

It might be possible but I still have doubts about it.

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post #23 of 33 (permalink) Old 02-22-2017, 01:26 PM
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There is a tool called color tune. What it does is it allows you to look into the combustion chamber. You screw the tool into a spark plug hole connect a little wire that comes with to your spark plug wire. You can watch the combustion in the cylinders and you adjust your carburetor mixture until the combustion is a blue color. I use number two cylinder for the front carburetor and number five cylinder for the rear carburetor. Very simple and very accurate. They are available on Amazon. If you Google color tune there are several demonstration videos they work.
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post #24 of 33 (permalink) Old 02-23-2017, 12:02 PM Thread Starter
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There is a tool called color tune. What it does is it allows you to look into the combustion chamber. You screw the tool into a spark plug hole connect a little wire that comes with to your spark plug wire. You can watch the combustion in the cylinders and you adjust your carburetor mixture until the combustion is a blue color. I use number two cylinder for the front carburetor and number five cylinder for the rear carburetor. Very simple and very accurate. They are available on Amazon. If you Google color tune there are several demonstration videos they work.

Well i have one of these and have been using it on cylinder 3 for the front carb and cylinder 4 for the rear carb. Maybe i should try as you advise, cylinder 2 for the front and cylinder 4 for the rear carb.


I have set the timing on my distributor which has a petronix ignitor device and flamethrower coil to 12 btdc at idle. I would still like a faster response when revving from stops at 1st gear. Both carbs have been set to 2.65 turns clockwise.


Raced the car 20 mins and came back to the garage, open the Hood and took both spark plugs 3 and 4 out. they look like brownish to black. I have also noticed that when the car is cold, i need to drive it with the choke on for 5 to 7 mins. Is this normal?? I have removed that manifold inline thermostat and have only a 160F (71C) thermostat installed, nothing else. The coolant line that goes on the rear of both carbs was shut down, to avoid percolation at the float valves.


Questions:


1. How can i warm up the car faster when cold? Leaving the choke on for 5 to 7 mins will do any harm?


2. How can i get a faster response from stops? Should i move the distributor timing to 14 btdc?
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post #25 of 33 (permalink) Old 03-07-2017, 11:48 AM
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Open the heater to full hot, do not turn the fan on.

By plugging the bypass through the carbs (which was shut off by the thermostat at the rear of the engine at 170F ANYWAY) was delete your quick warmup bypass. Now the only way to get it hot fast is to open the heater valve to full hot, and not turn on the fan. This will let hot water off #6 route straight back to the pump inlet aiding warmup. The two circuits in the 73 had a very small line continually flowing to prevent icing on the throttle plates, and a larger circuit through the manifolds proper to prevent mixture fall out and aid in cold running by heating the manfolds. Once at operating temperature this circuit was closed off by the thermostatic valve at the back of the engine in the hard line that looped around the back of the head.

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post #26 of 33 (permalink) Old 03-14-2017, 10:37 AM
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Early SU's are only a single circuit carb so you have to decide where you want to optimize: idle, cruise or WOT.

I find it beneficial to prioritize tuning at 3000rpm's rather than at idle as the car spends more time at speed.

FYI: http://www.classiczcars.com/topic/39...rty-su-tuning/
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post #27 of 33 (permalink) Old 03-14-2017, 11:10 AM
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You can slightly improve throttle response when going to WOT by using a thicker damping fluid in the carb pistons. I always liked the results of using transmission fluid.

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post #28 of 33 (permalink) Old 04-03-2017, 05:57 PM Thread Starter
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You can slightly improve throttle response when going to WOT by using a thicker damping fluid in the carb pistons. I always liked the results of using transmission fluid.
Well i have left both carbs at 2.65 turns clockwise and balanced them. It does run fine but sometimes during long idling time at traffic the idle would drop from 950 to 600. However the car will not stall. Is this normal??? I think that the car behaves better with the bp5es plugs than with the bp6es as it has a Schneider 274F cam and not the stock cam. Any opinions?
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post #29 of 33 (permalink) Old 04-04-2017, 11:06 AM
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Well i have left both carbs at 2.65 turns clockwise ...
I find it fascinating that your carbs are so perfectly matched that they end up being set at exactly the same amount of turns.

How do you measure the turns down to hundredths?

Scott
Tucson, Arizona

9/74 260Z VIN RLS30 060061
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post #30 of 33 (permalink) Old 04-12-2017, 10:47 AM Thread Starter
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Open the heater to full hot, do not turn the fan on.

By plugging the bypass through the carbs (which was shut off by the thermostat at the rear of the engine at 170F ANYWAY) was delete your quick warmup bypass. Now the only way to get it hot fast is to open the heater valve to full hot, and not turn on the fan. This will let hot water off #6 route straight back to the pump inlet aiding warmup. The two circuits in the 73 had a very small line continually flowing to prevent icing on the throttle plates, and a larger circuit through the manifolds proper to prevent mixture fall out and aid in cold running by heating the manfolds. Once at operating temperature this circuit was closed off by the thermostatic valve at the back of the engine in the hard line that looped around the back of the head.
Tony,


this means that having removed the in line heat control manifold thermostat from the system and plugging the rear of both carburetors where coolant used to flow will now prevent the car from warming up quicker? I did this because i thought that if i plugged the lines i will have less heat warming the bowls of both carburetors and thus avoiding the gas to evaporate (fuel percolation). I also deleted the heater as i needed space for the ac evaporator and ac lines.
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