Surging when on the flat

[pgp001]

This is just a heads up to how I solved a problem on my car.

When driving on the level with just enough throttle to maintain speed, it felt to be surging, ie slightly increasing and decreasing in power as you went along.
The only clue was that my vacuum gauge needle was moving ever so slightly in time with the surging, so if the throttle was held constant the only thing that was likely to be moving was the slide and needle asssembly.
I recently topped up the dashpot damper with 20/50 by the way, so last night I removed and cleaned all traces of the 20/50, and filled the dashpot with 10 weight motorcycle fork oil which is a good bit thinner.

All surging sensations have now vanished

Phil

[Murrayminor]

What a coincidence, I happened to be in the garage last night looking at a HIF44 I found under the bench, I noticed the piston was moving without any resistance, The dashpot was empty so I topped it up with my bench oil can containing 20/50 and the piston almost became solid.

I then cleaned the oil out and replaced it with 3 in 1 and the piston regained some resistance but still allowed it to move freely.

I have always run 3 in 1 oil in the dashpots on my Mini's but I suppose any thin oil will do the job.
SU actually sell some "damper oil" http://sucarb.co.uk/damper-oil-sae-grade-20.html

I will stick with my 3 in 1.

[philthehill]

Never had or experienced any problems with 20/50 engine oil in the SU dash pot. It was all we ever used when I worked in the BMC garage.
I have tried various grades of oil in the SU dash pot to control the rise of the piston including EP 90.
The rise of the damper piston is also controlled by the spring which comes in various strengths.
It is not just down to the grade of oil in the damper. The right damper spring must be fitted.
The correct damper spring for both 948cc & 1098cc Minor engines is the RED spring Pt No: AUC4387.

[liammonty]

Is it possible that the dashpot had run low prior to you changing the oil? The thicker grade of oil causes the piston to rise more slowly, as PTH has suggested, but ultimately shouldn’t prevent it reaching the same position it would do with a lighter oil when running at a constant speed, though using a strong spring would, Thicker oil equates to a longer period of richer mixture under acceleration rather than impacting constant running. I wonder if you’ve got a very strong spring in the carb, which in combination with the thicker oil, caused an overly rich mixture. Or whether the piston was sticking before you changed the oil in it, without you realising?

[pgp001]

I have the red spring in place, everything about the carb is standard and it was fully reconditioned in 2018 with new jet, needle & spring etc.
The throttle butterfly bushes are not leaking air either.

The change of oil made a very noticeable difference, I did a test run on the same road before and after within an hour of each other just to make sure.

I know the theory of operation suggests otherwise, but the results of testing worked for me.

Phil

[liammonty]

If it worked, that’s what matters ultimately. I just can’t figure out why!!

[pgp001]

Maybe your comment about the slide sticking is relevant, it certainly felt free enough before I removed the whole dashpot to get rid of the thicker oil.
But I did give everything a really good clean prior to re-fitting it and putting the new oil in.
So I suppose it could have had a bit of friction which was just enough to cause the problem, I used to periodically need to strip the dashpot on my Austin 7when it started to stick, but that was caused by not having room for a filter and it was getting dust particles in it.

Phil

[philthehill]

When the dash pot is held vertical with the piston at its highest the piston inside - the dashpot should fall of its own accord without the assistance of the spring. Make sure that you catch the piston as it drops.
Cleaning the dashpot with cellulose thinners removes all dirt and staining and leaves the inside of the dashpot like new.

[ampwhu]

I've only ever used 3 in 1 oil for the last 30 years and never had a problem. That's on all my SU carbs.

[kevin s]

You have got me thinking now, I have hardly used my TR7V8 this year, when I used it last week it was surging as you describe, can't remember when I last checked the dash pots and have no idea whats in there, something to have a look at tomorrow.

Our minor's got 5w30 in it, (didn't have any 3 in 1) which seems to work fine.

[Murrayminor]

Never had or experienced any problems with 20/50 engine oil in the SU dash pot. It was all we ever used when I worked in the BMC garage.
I have tried various grades of oil in the SU dash pot to control the rise of the piston including EP 90.
The rise of the damper piston is also controlled by the spring which comes in various strengths.
It is not just down to the grade of oil in the damper. The right damper spring must be fitted.
The correct damper spring for both 948cc & 1098cc Minor engines is the RED spring Pt No: AUC4387.

Phil

What spring would you recommend for a HIF44 on a 1330 midget engine with mild modifications to the cylinder head?

Regards

Dermot.

[philthehill]

I would start with the RED spring http://sucarb.co.uk/piston-spring-red-10952.html as fitted as standard on the 1275cc Metro when fitted with the HIF44 SU carb.
You could try the Metro turbo spring YELLOW Pt No: AUD4398.http://sucarb.co.uk/piston-spring-yellow-10967.html

Here is the list of damper springs with their ounce resistance figures
http://sucarb.co.uk/catalogsearch/resul ... per+spring
I would not be looking to use a spring above 8 ounces resistance.
The GREEN spring http://sucarb.co.uk/piston-spring-green-12401.html at 12 ounce resistance is too strong.

Even though the official SU damper oil is a straight 20 grade oil I would still prefer to use engine oil (20/50) in the dash pot.
As I have said above - 20/50 has never given me any problem with any of the SU carbs fitted to my or others engines.

[Murrayminor]

I would start with the RED spring http://sucarb.co.uk/piston-spring-red-10952.html as fitted as standard on the 1275cc Metro when fitted with the HIF44 SU carb.
You could try the Metro turbo spring YELLOW Pt No: AUD4398.http://sucarb.co.uk/piston-spring-yellow-10967.html

Here is the list of damper springs with their ounce resistance figures
http://sucarb.co.uk/catalogsearch/resul ... per+spring
I would not be looking to use a spring above 8 ounces resistance.
The GREEN spring http://sucarb.co.uk/piston-spring-green-12401.html at 12 ounce resistance is too strong.

Even though the official SU damper oil is a straight 20 grade oil I would still prefer to use engine oil (20/50) in the dash pot.
As I have said above - 20/50 has never given me any problem with any of the SU carbs fitted to my or others engines.

I think there is a red spring already in the carb so I may just try that and see how it goes, it will be a while before I'm at that stage but thanks again for the information.

Regards

Dermot.