Runing SU carb without damper - how does it work?

[BoogWar01]

Fuel eco should fall off a cliff. Sure it will work, but the mix probably isn't any where close to stoich. I would look into the size needle your using, and whether or not the carb / jet is sized right for the engine.

[DeiMuddaSeiMudda]

@BoogWar01 It'S not that bad. Surprisingly good, actually. I average at 35MPG (8l/100km). That's very acceptable for a 'hot'-ish 1300cc engine :-) And I'm not easy on the throttle.

A 1.75" carburetter is percfect for this engine and there's only one jet size available (0.100"). Needle is a modified BBW.

As for the mix being stoich etc. - Most engines will be on the rich resp. maximum performance side. And I bet you, even a 100% sound std. engine with a 100% sound carburetter isn't stoich ;-)

[BoogWar01]

@DeiMuddaSeiMudda Could be that you are running too much carb - the engine vacuum could be not strong enough to snap the piston off the jet when in anger. Experiment with weaker dashpot spring tensions until it starts to splutter, then go to the next firmer spring (at a drag strip). Seat-of-the-pants feel should indicate a better midrange pull. Just my two cents. As to the stoich, I was referring to part throttle operation. I think having such a light car is really confusing your situation.

[DeiMuddaSeiMudda]

@BoogWar01 A 1.75" carb is the right size for a 1300cc unit with ~70HP (though only a estimate from top-speed & 0-100km figures). It's all but too large.

The fiddling with spring tensions made this work - that's allready been done ;)

My english isn't very good - read Des Hammils book "The SU carburetter high-performance manual". The modifications are explained there. Hammil even goes as far as removing the spring!

All in all - it works as it was intended to work, it's not accidentaly working

[zrunner240Z]

I have been running my 240Z with Dual Su's like this for 14 years now. I picked up a tenth in the 1/8 mile after removal and never installed them again. I did have some sputtering on the road but cured that by dropping some long heavy screws in the oil resevoir. My 240Z's Best run to date was a 12.801 @ 107.26 mph in the 1/4 mile on cheap street tires. I also run a LD28 crank with L28 rods and pistons with 2 headgaskets which everyone said wouldn't work but is still running strong after 10 years

[abstractduk]

good for you man-thant was well interesting! so what else did you do other than remove the damper and oil?

[morrisminormods]

Forgot to mention

One of my neighbours coverted a Leyland Mini 1000cc 4 stroke engine to 2 stroke with a different crank etc. He won every single race in two seasons until they banned his car. 12,000 rpm sounded like an aircraft engine.

Mick

[morrisminormods]

Hi

Well if it all works well for you, then all the better. I was told that you couldnt fit a 1275cc head to a 1098cc. Well I did.

Also its impossible to fit a Eaton M45 supercharger to a 1098cc still using SU carbs. I have done.

I have also run an Hilman Imp on camping gas. I found the best way of learning mechanical things is to just have a go. I just wish more people would do the same.

All the best Mick

[DeiMuddaSeiMudda]

Hello Mick! Exactelly what I'm thinking! ;-)

@All: Menawhile I found an educated author who did exactely the same thing; "The SU carburettor high-performance manual" by Des Hammil.

I found one problem, though: cold starting got a bit worse. But it's acceptable for the improvements this mod brings!

[1RadNomad]

Part1: Removing the damping function is bad advice and can only come from someone who does not understand the operating principle of an SU carb. The damper's most important function is to provide temporary mixture enrichment during acceleration phase. It achieves this by slowing the sudden upward movement of the piston when the driver suddenly demands power, this in turn causes an even lower air pressure (or partial vacuum) in the venturi so extra fuel is briefly sucked into the inlet tract.

[1RadNomad]

Part2: Removing the damper will cause lean mixture during acceleration which leads to excessively high combustion temperatures and can damage valves, v. seats, plugs and pistons. Bad idea! If your car seems to run better without a damper it's probably running too rich all the time anyway (a typical result of worn out carbs with worn needles and jets) so the lean-out is less noticable. The designers of the SU carb knew what they were doing to achieve precise fuel metering under all conditions.

[1RadNomad]

Part3: This video shows a Mini using a Metro (or similar) airbox. This is also a bad idea because the inlet trumpet is on the wrong side so the engine is always ingesting warm air from above the radiator. The engine will produce more power if it collects air from the clutch end of the engine bay where the air is less warm and more dense (higher air density = more oxygen = more power). Excessively warm air also interferes with the air/fuel ratio and carb adjustments...

[DeiMuddaSeiMudda]

I respect your position - however; it proofed to be working this way. And if you read all comments; we didn't just remove the damper as single measure. Re: airbox: it's from a Rover Cooper. I rather have constant warm air from a rad than uncosntant temps from nature. Think it this way. And once moving there's not much warm air anyway. Many things don't work in theory - but work great in praxis. Or vice versa. Only downside so far: cold starting got worse...

[DeiMuddaSeiMudda]

The Cooper, Cooper 'S' and Innocentis (all with 2xHS2) actually had the inlet directly above the centre exhaust branch. Warm air mixes better with fuel... '91 on engines even got a heated manifold which actually seems to have positive effects on MPG (1-2 more). Probably not the best thing to do if you'r after the last BHP - on a raod going car; I actuallydon't care... The carb modification was a experiment which is of course unneccesary too - but fun and turned out quite well. :-)

[DeiMuddaSeiMudda]

Oh, yeah, in the video I wrote MG Metro airbox... It's the same. Metro got a long hose to the grille, Rover Cooper got a 110° bend snorkel (much worse for airflow than temps can ever be!).

[phibin777]

interesting

may get better throttle responce as the needle will lift faster un-damped..( no piston/ damping fluid)

if you tuned the carb to your engine and it ran fine( no bogging down in low revs, no flat spots throught the rev range) and it still runs better with a hole in the carb im out of ideas.. haha

if standard needle in carb, mix will be wrong somewhere in the rev range.

low revs=rich

high revs=ok on the profile of the needle?

so if there is a trick im missing please tell.. lol

[phibin777]

hmm?

the damper is to stop 'flutter'

when you put your foot down the engine creates a vacume that lifts the needle up.

no damper = the needle float up/down trying to adjust to the amount of vacume/throttle applyed = juttering like your bouncing the throttle sort of.. lol

if your engine runs faster without the pot cap meens it is running leaner and getting more air as it is leaking in the top of the carb !

is your setting too rich?

replace damper piston (yours missing)

check mixture!

[DeiMuddaSeiMudda]

Thats without a doubt true. But why does it work so good? The mixture is ok. It's tuned to work with that setup! It's not like I removed it because it didn't run. And there's a little trick involved.

It's a well controversial topic. All I can say is that I'm not the only one runing an SU without damper with success. There are however engines that don't run without a damper. On those engines you can hear the piston shooting up & down and hitting the carb on low revs & sudden throttle opening.

[inline6er]

How about a video showing what else is involved to make it work?

[DeiMuddaSeiMudda]

I'll make one soon :-)