SU dampener dry. What are the symtoms?

[SypherSlayer]

I have a stock 72 240z. My dampener pots are dry and I plan to fill them soon but I was wondering what the symtoms are when they are dry. Reason why I ask is because my car is running extreamly rich. It's so bad I'm lucky to get 16mpg. (The pots have been dry since I bought the car) Any help is apreciated. Thanks

[Dragonfly]

If you were to reach inside the carb(s) with your finger and lift the piston you would find no more resistance than the weight of the piston, and if you let the piston drop it will fall instantly with a clunk (That Is When The Dashpot Is Empty). When you put damping fluid (oil, ATF or whatever) into the dashpot and try lifting the piston with your finger you will find it has a lot of resistance to being lifted but when you drop it it will fall quickly with a solid clunk.

 

Connected to the bottom of the piston is a needle that sits inside a jet, as the piston is raised the needle is lifted out of the jet, the higher the needle comes out of the jet the more fuel is pulled into the engine. When the engine is running the vacuum created by the engine is trying to lift the piston/needle in the carb(s), with the engine at idle the butterfly at the back of the carb is almost fully closed creating very little vacuum in the carb to lift the piston/needle but as the accelerator peddle is pushed the butterflies open and a strong vacuum is pulled on the piston/needle, with no damping fluid on the dashpots the piston/needle will slam to the top of the carb creating a wide open throttle condition even though the butterflies are only partially open, this will cause a very rich running condition and poor fuel economy along with accelerated engine wear and other bad things.

 

If you add damping fluid to the dashpots that create the correct amount of resistance to the vacuum applied to the piston/needle for your needle profile you will find that your fuel economy will go up considerably, the black stinky exhaust will go away, your power will increase and you will feel like you are driving a car that performs the way it looks. One of the better ways to find out about the needle profiles and damping fluids to match a certain engine build (stock .... to ..... wild) is to look up then contact ZTherapy they are the true gurus of SU carbs.

 

Dragonfly

[SypherSlayer]

thanks for the extreamly informative post! Now Ive just got to find a store that sells 20wt oil

[WEBEZEEed]

thanks for the extreamly informative post! Now Ive just got to find a store that sells 20wt oil

 

Motorcycle front shock oil is what I use and you have different weights to choose from.

[Konish]

Hate to resurrect an old thread, but I've been dealing with some wonky issues with my SUs. In my research I've found conflicting info on whether or not the engine runs lean or rich under sudden acceleration with no oil in the dashpot.

 

My experience falls directly in line with the excerpt you see below lifted from this site:

http://www.jetlink.net/~okayfine/su/damper.html

 

With empty dashpots, WOT or near WOT my car ran like a bat out of hell and was SUPER responsive to throttle input but it rattled (detonated) like crazy.

 

After I screwed with the floats and filled the dashpots with 20W, the rattle went away immediately, but the throttle just felt soft...like it was running really rich. It just did not hit as hard as I would expect from a leaner mix.

 

Intuitively, I think I buy the "lean condition on throttle" theory more than the one above suggesting "rich condition on throttle" without oil in the dashpots (the high vacuum in the dashpot lifting the piston higher but relatively low pressure in the bridge to draw the requisite fuel causing a lena condition)

 

Thoughts?

R/

D

 

Advanced Techniques: Damper oil can be used to adjust or tune the lifting of the pistons to suit a modified engine. Since I installed my Air/Fuel gauge, I have been able to see that the ATF fluid I'd been using was far too light a weight oil for my application.

For example, under cruise conditions I was getting a fairly rich reading on the air/fuel gauge. Any increase in throttle gave me a lean condition immediately. This was because the oil in the dampers was not heavy enough to keep the piston from opening rapidly. The sudden increase in vacuum from the motor draws up the SU piston (which allows more air into the motor). With a bigger hole from which to draw air through, vacuum over the jet bridge does not rise as quickly to pull more fuel into the stream (despite more air getting into the engine = lean air/fuel ratio). The low vacuum doesn't allow enough fuel to be sucked from the jet to compliment the increased air flow.

3/15/99

I've purchased some 15wt motorcycle shock oil. Replacing the ATF with the 15wt oil has done wonders for the air/fuel readings. Now even with heavy throttle application the fuel mixture does not go lean. Obviously the heavier oil is not allowing the piston to rise as fast as it was before. The seat-of-the-pants differences are not dramatic, but at speed (say, on the freeway, and at higher revs) you can feel a little more urgency out of the car. Whatever the other benefits, the mixture is not going off-the-gauge lean as it was before, and this is good for my peace of mind. Without the air/fuel gauge, however, I wouldn't have known I had a problem as it drives about the same.

4/27/99

The 20wt oil (again, using a bottle of motorcycle shock oil) has gone into the SUs. I haven't noticed much change at all. What change I have noticed has come courtesy of the air/fuel gauge. I was getting a bit of a lean-mix spike with throttle application on the freeway. This has lessened, but is actually still there to some extent (it occurs more often before the motor has fully come up to temp). Once the motor has come up to temp (long after the temp gauge stabilizes), the 20wt oil seems better suited to prevent the lean condition mentioned above. However, with summer coming, I may have to step it up again. We'll see.

11/21/99

Summer came and went. As it is November, and getting slightly chilly 2 out of 7 days here in SoCal, I went back to the 15WT oil. However, on my drive to the SoCal UFO swap meet during the weekend I noticed my air/fuel gauge going lean under acceleration/load. This was most noticable as I went up a few grades on my way. It wasn't terribly warm that morning, so it would seem that 15WT is too light for a SoCal car (at least with my motor). Back to 20WT, where I'll stay.

6/15/01

I've gotten into the routine of going with 15wt oil in the SoCal "winter" months, and 20wt oil in the summer months. When I change my coolant thermostat from a 195' winter to the 160' summer, I also change damper oils. In my climate that's all that's needed to be messed with.