13 replies to this topic

[Traut]

1) The quick question: There is an amount of free play in the carb damper before the oil slows the piston rise. Why?

 

2) The longer version.: I have got twin HS4s on a 1310cc A+, AAA needles, Red spring. SW5 cam. When I got the car, it was running very rich, you could smell it, and see it on the plugs. However, it had previously been set up on a rolling rd, but it's remit had been max power (98bhp) rather than anything sensible I believe.  Rather than take it to be rolling roaded again, I decided to spend the money instead on an AFR meter & put it on the exhaust near the gear stick recess. Works a treat. Sure enough, the AFR was reading 10-12 at idle, and still rich through to 30-50mph generally. 50-70, too lean at 16ish. I changed the needles to what I thought woukld be a better profie, AAB, and adjusted the idle to be a bit better at around 13. But was still too lean over 55mph. The worst thing though, was the lean spikes when just touching the throttle (at any speed, but worse at low revs and before warmed up) The lean spikes were causing terrible engine stutter, especially cruising at 20-40 & in traffic. Though generally the AFR reading was OK at that speed (14-15) I figured the lean spikes might be being caused by the free play in the damping, before the oil resistance is met. So I removed the circlip in both dampers, and inserted a small washer. It helped. I put in another washer. The lean spikes are almost gone, drives really well  (overall a tiny bit richer too).  I could probably tighten things up a bit more with a 3rd washer, and still leave plenty of room for the oil to pass on when the piston comes down.  So why is there so much free play before damping occurs in the stock damper ? There must be a reason,.

 

I put a data port in the ashtray for logging, so I have graphs of before and after if anyone is interested.

[Ethel]

It's also a crude valve so it resists raising more than it does falling. To achieve that does require a little bit of movement.

[Traut]

Yes, you are right, it is a valve, but there is so much  (about 3mm) extra above what it needs for that to operate. Anyway. 2 washers to take up the play, for the moment, seem to be working a treat.

[nicklouse]

But it is meant to have that free play.

[StefanMini]

Maybe AC Dodd can answer this question..

[Spider]

There are quite a few different dampers available and it's likely you need a different one. As Ethel and Nick have mentioned, they all have some play.

 

In regards to your AFR readings and the needle you found in there, - this is the type of issue I see here with rolling road testers, in that they seldom (read that as none) will change needles and properly set the AFR, but just turn the mixture screw until it's better than when the car drove in. They do the same with the distributor.

[Traut]

I'd be interested to learn more about different dashpot dampers, the differences between them and the reasons for the differences. Burlen's site is not very helpfuil in this respect.

[MiNiKiN]

I'd be interested to learn more about different dashpot dampers, the differences between them and the reasons for the differences. Burlen's site is not very helpfuil in this respect.

You need to watch a few AC Dood videos to get wiser in this matter.

 

IIRC he said that some HIFs had dampers with increased free play. I cannot recall if he gave a reason. Anyway, there are many different standard dampers out there, let alone modifications and wear.

by the way: I second your interest in different damper characteristics

[ACDodd]

The free play is a calibrateable length that is adjusted to give the optimum enrichment or lean out on initial throttle movement. Precise adjustments can be made to give the optimum fueling for your requirements. This is different for every application, and also does not need adjustment on some applications. This is not an adjustment that you can just ‘file’ the use of a lathe enables small and precise adjustments to get the fueling spot on. This adjustment can only be completed when using a Wideband gauge.



Ac

[timmy850]

If you look on Burlen's site there's 5 pages of different dampers. Although some of the photos might be the same they are all made up to different specs depending on what the manufacturer designed the carb to do new:

Different lids, lengths, free play on the damper, tolerances to the piston tube, different oil passage size

 

https://sucarb.co.uk...assemblies.html

 

The oil of course can be used to slow the rise of the damper, but it will also cause the drop to be slower too. If the rise and fall rate can't be both set for your engine with a particular grade of oil, that will be the time to get a different damper or modify your own. 

 

 

As you have an AFR gauge you're in the perfect position to do make some changes and observe the difference to both the AFR reading and how well it drives. Try changing the damper oil to a thicker grade, then a thinner one and see what happens. As you have twin carbs you'll also need to ensure that they both rise and fall at the same rate to give decent performance

[Ethel]

If it's twins it'd be worth doing a head to head drop test. Hold the dash pots & pistons side by side & let the pistons "race" to drop out (but not to the floor!). Start without springs or oil (but damper fitted) and with the holes that feed air to above the pistons blocked. You can swap bits between the carbs to try & get the closest race.

[Traut]

If it's twins it'd be worth doing a head to head drop test. Hold the dash pots & pistons side by side & let the pistons "race" to drop out (but not to the floor!). Start without springs or oil (but damper fitted) and with the holes that feed air to above the pistons blocked. You can swap bits between the carbs to try & get the closest race.

 

I did't think to do the drop test as the carbs have only done about 8k miles, but good idea to see if they are equal.

[Traut]

If you look on Burlen's site there's 5 pages of different dampers. Although some of the photos might be the same they are all made up to different specs depending on what the manufacturer designed the carb to do new:

Different lids, lengths, free play on the damper, tolerances to the piston tube, different oil passage size

 

https://sucarb.co.uk...assemblies.html

 

The oil of course can be used to slow the rise of the damper, but it will also cause the drop to be slower too. If the rise and fall rate can't be both set for your engine with a particular grade of oil, that will be the time to get a different damper or modify your own. 

 

 

As you have an AFR gauge you're in the perfect position to do make some changes and observe the difference to both the AFR reading and how well it drives. Try changing the damper oil to a thicker grade, then a thinner one and see what happens. As you have twin carbs you'll also need to ensure that they both rise and fall at the same rate to give decent performance

I have seen these 5 pages of different dampers with the same pictures and descriptions, it is not much help, neither is the spec catalogue alas.

 

The free play is a calibrateable length that is adjusted to give the optimum enrichment or lean out on initial throttle movement. Precise adjustments can be made to give the optimum fueling for your requirements. This is different for every application, and also does not need adjustment on some applications. This is not an adjustment that you can just ‘file’ the use of a lathe enables small and precise adjustments to get the fueling spot on. This adjustment can only be completed when using a Wideband gauge.



Ac

Thx Ac, as I am using an AFR wideband meter I can see the initial throttle movement is causing too lean a mixture causing stumble. Since I can't find any specs on different dampers, I have modified mine using a couple of carefully drilled and filed washers tro reduce the free play. This has helped enormously. I'm tempted to get a spare set of dampers & thread the end  where the circlip usually goes, put a nut on it so it is adjustable. Might try that in the fullness of time.

[ACDodd]

Perfect!

Ac