SU Carb red spring 4.5 oz. How to confirm?

[Opie]

My TR3A runs a bit rich even after many adjustments in past couple of days. I ordered a SL (lean) needle pair and will try those  soon. While investigating every SU aspect, I wonder how to test the SU damper piston spring?

Per SU, it should be red, 4.5 oz.

I measured the spring just now at about 6 inches, compressible to about 3 inches with a total force of 6.25 oz to compress it.

Can anyone spec out a good spring as to these compression measurements?

If spring was a bit too strong, then that could explain why I am fine at idle but rich at speed.

Thanks.

Opie

[Lebro]

Yes, I think that would make it richer at higher throttle openings, but I do wonder how a wrong spring would have got into your carbs ?

Bob

[Opie]

Bob,

Yes, good question....

My two suppliers just list a part number for the spring, no spec or color or any choices.  I called them both and they both replied to my spring color question with "no idea" , they said they only provide the "right one."

My memory serves me poorly, so I don't remember if the SU springs are original or I replaced them at one point or another in last 51 years....

 

[Opie]

sorry for not being exact. I am under the bonnet again now.  The SU spring is exactly 5.5 inches and 1.5 inches as it just meets maximum compression.

The effort to compress it to maximum is 6.5 ounces.

 

Edited June 1, 2020 by Opie

[Opie]

My SL (lean) needles came and I installed them. Checked all and cleaned plugs and took a spirited spin for 17 km.  Came back after great run. No lagging, strong acceleration in all gears, no ping.

In other words, I should be totally happy. But, not satisfied until I pulled plugs and feathery black carbon was back as always.

Something is happening at speed.  Correct mixture at idle in garage just won't do the job.

Suspect:

Dashpot oil --- (I tried 3 oils, no change)

Spring too strong -- ?

Piston has encumbered clearance -- (I cleaned it thoroughly and can hear no touching)

Per all theory I understand, rich at speed means not enough air. In other words, piston not rising enough. (It could mean too much fuel, but my float level is fine, needle is centered, idle mixture fine.)

I will continue to investigate.

Today I spent an hour with the pistons on the bench and put tape over the two piston holes and did the piston drop test.  4.5 seconds from up fully to down out of contact with housing while holding carefully in hand.

Then I put piston on bench and raised the chamber all the way to expose piston and let it fall to bottom position. 22 seconds each chamber.

Then I did the same test but without internal spring, 17 seconds.

Do you think SU has specs on this info?

Opie

 

[BlueTR3A-5EKT]

Coming to this from another angle.

Is the engine completely standard?   ie cam spec, compression ratio, cylinder capacity, exhaust type.  If any of these have been changed it will affect the fuelling requirements.

If standard the base line of SM needles in H6 carbs with red springs is a great place to start.

Start with checking the valve clearances.  If you have not read this, perhaps it might help - plus other items in their 'tech' section.  https://www.macysgarage.com/valve-adj.htm

Do not forget modern fuels do not burn the same way as the fuel the car was originally set up for in 1950's

 

Could the carb springs have been fitted by a previous owner with whatever springs they could find? - Does anyone have a spec for Stromberg springs?  They look the same and may have been  the only ones available to the PO.

 

The springs equate to being too heavy/strong.  This means the entire fuel operating range that the SU runs through will be rich, after you have set the tickover mixture correctly, as the piston is not rising fully in the chamber. 

I would try to borrow or buy a pair of Red  SU springs so you have a base line to work from.

 

Cheers

Peter W

 

[Opie]

I want to comment for the record that the SU needles marked SL (lean) are almost no different than the normal SM needles. Maybe 0.005". I found different holes in a kitchen colander and gently placed the SL and SM needles in the different size holes. The SL needle stood proud of the SM needle by 1 mm on average.

So, while the SL needle is indeed larger, it is a very small change.  I found a comment that said the SL needle is offered by SU for high altitude driving.  That may be, but I wanted to warn TR owners that buying a SL needle set for the purpose to solve a overly-rich situation is false hope. I wasted $30 for two of them.

[Opie]

Thank you for your comments, Peter. I will consider each point you made carefully. And comparison to a spring set of yours would be appreciated.

 

[Opie]

1. For accuracy, my carburetor model is AUC878, H6, 1 3/4". Car is TR3, 1960.

2. I just read the long technical explanation on the SU website. link here

Interesting was the part that said SU design is to restrict opening of carb piston by damping so as to effect rich conditions on acceleration!

As my 17 km run was around surface streets with run ups to 80kph for a minute and then a stop light and then another run up to 80kph, it is very likely that my rich condition is the expected result of proper SU design to allow for maximum acceleration. It may be likely that if I try prolonged open road driving at constant throttle for long periods, then the mixture will "lean" out and my plugs will burn off carbon and become clear.

That is my plan.  More news after I try it.

Again, the above linked article is packed with good SU info.

Happy Driving

and stay safe....

Edited June 3, 2020 by Opie

[Opie]

And, walaa!

I can now measure my spring when I get to my TR today.

Here is SU method from SU website:

"

Piston Spring rate measurement

Our SU reference Catalogue gives details on the load ratings of each spring.

The easiest way to measure this is to cut a piece of card to fit inside the spring which is the correct length for the load to be added.

Then place the spring (and card) onto a suitable measuring scale and compress until the top of the spring is level with the card, then read the load rating."

On another page they list the red spring at 4.5 oz at 2.6" compression.  

[BlueTR3A-5EKT]

For added confusion did you look at the Mintylamb SU needle comparison page?

Link that works with windows 7 and firefox  http://www.mintylamb.co.uk/suneedle/

 

Cheers

Peter W

Edited June 3, 2020 by BlueTR3A-5EKT

[Opie]

Peter,

again thanks... The needle link was interesting

[Opie]

I just tested my SU springs as per SU website specs. Link to SU Springs

and I used the measured cardboard insert procedure that SU specifies.

Result is my spring is exactly 4.5 oz at 2.6". Therefore, my spring is correct and is the "red" one.

That ends my spring question, thanks for your help and listening and reading.

My next check is onto the subject of "will extended driving at constant speed overcome the temporary richness due to acceleration and produce correct mixture as determined by examination of plug condition?"

Opie

 

Edited June 3, 2020 by Opie

[stillp]

5 hours ago, Opie said:

 

My next check is onto the subject of "will extended driving at constant speed overcome the temporary richness due to acceleration and produce correct mixture as determined by examination of plug condition?"

 

 

Yes. Plug colour doesn't mean much unless you've been running at a constant speed and do a 'plug cut', i.e. run at a constant speed for a while then quickly dip the clutch and simultaneously turn the ignition off, then stop and check the plugs.

Pete

[Opie]

Pete,

You are right!

So, driving habits affect plug appearance due to carb fuel mixture.

Who knew that?

Pete, for one, but I sure did not know it.  I have always thought that my mixture was unavoidably rich due to poor car set up.

My driving habits were the cause, just like Pete described. I proved this to myself this week. Over many years, 

I thought I was properly "airing out" my occasionally driven TR by fast acceleration and throaty roars.  I never took it out to the open highways to give it long constant rpms.

Today I cleaned the plugs and tailpipe and drove 20 kms keeping the tach at 2200 rpm by using 2nd and 3rd gear on neighborhood long streets.  When I returned home I pulled the plugs and they were tan and clean. The tailpipe was cleaner as well.

So, lesson learned and a change of driving style for me in the future.

Thanks,

Opie