Mixture Screw DAT/DATR Carburettors

[capriblu]

Which one is correct?

[Nigel]

My s/h carb, which I bought late last year, has the one on the left.
I don't think i've seen the other one before.

N.

[capriblu]

Thanks Nigel,

Would you happen to know what spec the Carb was,  250/1 DAT,  250 DATR or  ....

I've seen references to both types for various DAT & DATR variants - highly confusing.

The item on the right is what I have removed from the carb on my car but I'm questioning whether this is correct - I will remove carb from car at weekend and see what variant it is - I think it is a 250 DATR of some sort but could be a 250 DAT.   I have another spare 250 DAT carb currently without any mixture screw fitted and both of the types above "fit" and seem to travel same distance to stopping point.

[Nigel]

Capriblu:

yes, it's a 34 DATR 2/250

It also has 9C stamped on it, but i've no clue what that means.

Hope this helps
Nigel

[capriblu]

Capriblu:

yes, it's a 34 DATR 2/250

It also has 9C stamped on it, but i've no clue what that means.

Hope this helps
Nigel

Many thanks Nigel, will report back when I take carb currently on the car off for a closer look.

Kind regards,

Andrew

[capriblu]

So both my carbs including the one on the car are DAT 2/250, i.e.not DATR.

I have seen reference to both types for DAT carbs so very uncertain now.

I have a new 64750.051 - the plain cone shape one and a new staggered profile type one 64750.052 coming in the post.

Will try both and see which works best!

I have a simple CO meter so should be able to check slow running.

Any further clues anyone?

Thanks,

Andrew

[Neil-yaj396]

Andrew

Only going by the drawings in the Haynes Weber Carb manual, but that seems to suggest that the DAT has a shorter and less pointed mixture screw when compared to the DATR. So seems to suggest that the screw you took out of your DAT is correct?

Neil

[capriblu]

Thanks Neil,

Ironically I've been setting the car up this afternoon with the "wrong" screw and it seems fine!

I'll install the new correct item when it arrives.

I have also purchased a selection of new main and idle jets and air corrector caps and will explore how the car performs with various modest increases.

Will report how I get on.

Andrew

 

[capriblu]

So DAT the screw on the right and DATR the one on the left.  New DAT type screw arrived, the old one still seems fine and have tried both screw types in two different DAT 2/250 carbs and no discernible difference in running.

[Neil-yaj396]

Hi Andrew/all

Just re-visiting this post to query the actual procedure for setting the idle once the engine is warm. Are the idle screws above the very small screw in the very base of the carburetor or something else?

My refurbished carb started the car straight out of the box when primed and ran smoothly until the choke cut out. Once the auto choke fully disengaged I had no idle without lightly feathering the pedal. Haynes suggests adjusting the (very inaccessible) screw in the base to enrich the mixture, but I can't avoid thinking that I am missing something. The fast idle screw seems to dictate the throttle opening at rest when the engine is warm as well as with the choke engaged?

Neil

[capriblu]

Hi Neil,

Firstly in all my time with Betas and fiddling with numerous DAT/DATR carbs over the years I've never been able to set so that idle remains perfectly steady through transition from cold start to fully warm. Best is a setting that soon races when fully cold if throttle pedal is left untouched which then transitions after a couple of minutes to something that just about ticks over when switching from choke idle setting screw to main idle screw. Car will settle much quicker when driven (although first mile or so can be a bit lumpy).  Just idling stationary then I find it will take at least 10/15 minutes to warm enough to settle.

Need to deal with one thing at a time.  Firstly the smaller screw that controls throttle position during time when choke is engaged should definitely NOT be in play once choke flap is fully open.  A lightly spring loaded notched plate within the choke body housing moves, i.e. pivots driven by the thermomechanical mechanism within the body as it warms.  Is this working properly?   I think there are either three or four positions from fully cold to fully warm.  The end of the adjuster screw slots into one of these notches and effectively locks the throttle opening position until the throttle pedal is pressed allowing for the plate to move and the adjuster screw to then lock into the next slot.  Once fully warm the plate rotates to a position where there should be no contact with the adjuster screw and instead throttle opening is then governed by the main, larger throttle opening screw.

I assume that you do have both adjusting screws, i.e. smaller adjuster operating off choke mechanism and main larger screw governing idle when car warm?

I'd make sure that the choke mechanism is moving and working as it should - something doesn't seem right?  Are the choke body halves lined up correctly?  Should be small marker lines to indicate.

In terms of idle mixture adjustment then assuming timing is where it needs to be then my simple technique is once car is fully warm (after a short drive) then adjust idle to 1000/1100 rpm - this should of course be using main throttle opening adjuster!  Then very slowly turn idle mixture adjuster screw inwards until you can just start to hear idle speed drop slightly, become rougher etc.   Unscrew slowly just until idle recovers/smooths.  Repeat to find this position and as final setting unscrew a further full turn.  Adjust tickover  down to 900 rpm.

The idle mixture screw runs in a small threaded port at the base of the carb in the back corner on the opposite side to the choke mechanism.  Yes the two items in the pic above are variants of this.

Sorry if all this is obvious ....  

  

[Nigel]

That's a great example of a well written how-to.

Thanks Andrew, it'll help me a lot.

Regards
Nigel

[Neil-yaj396]

Thank Andrew for your detailed description which I will follow. I haven't checked the small throttle adjustment screw, but when warm the choke flap is at full vertical, so the chamber appears to be adjusted correctly.

Haynes reference to the large/long adjustment screw as the 'fast idle screw' made me think that this dictated idle under choke, when in fact it dictates the idle when warm, so I definitely need to adjust that first to achieve some tickover until the engine warms by the sound of things.

Both my non injected Betas have had 'issues' with idling, with the carburetors never quite adjusted correctly for this. My facelift Idled OK after the choke, but the idle became very fast when fully warmed up (1800 rpm+). My pre-facelift car was as my car is now, no idle post choke for several minutes until it attained 900 rpm when warm. I used the fitted hand throttle to control this in traffic etc.....

[HFStuart]

Re-opening this for a second, as some of you know I've got Tim's spider at the moment and I've been swapping carbs around as it's suffering with what I strongly suspect to the crud in the carb syndrome.

Using a colourtune I couldn't get a good idle AND mixture on any carb with the stepped screw. It pretty much made no difference to mixture or idle speed unless it was screwed right in when the engine would die.

I then bought the tapered version of the screw and I now have fine adjustment of the mixture on both the DATR and the DMTR I'm trying - I can make it go rich or lean. I wonder if the stepped screw was for emissions purposes to limit the amount of adjustment that could be made? That might have been good when the carb was new but less so after 45 years of use and wear.

[JohnFol]

Just for comparison, this is mine
(and it's a 34 DAT 11 K8 251)

[WestonTB]

Re-opening this for a second, as some of you know I've got Tim's spider at the moment and I've been swapping carbs around as it's suffering with what I strongly suspect to the crud in the carb syndrome.

Using a colourtune I couldn't get a good idle AND mixture on any carb with the stepped screw. It pretty much made no difference to mixture or idle speed unless it was screwed right in when the engine would die.

I then bought the tapered version of the screw and I now have fine adjustment of the mixture on both the DATR and the DMTR I'm trying - I can make it go rich or lean. I wonder if the stepped screw was for emissions purposes to limit the amount of adjustment that could be made? That might have been good when the carb was new but less so after 45 years of use and wear.

Sadly carb all good, no crud, just shut valves, pretty much impossible to get a feeler gauge under any lobes, valves recessed deeply, carb good though!

Don't always blame the fuel system

[HFStuart]

Tim,

I'm glad you solved it  - I'd not thought to check the valve clearances. She should be a bit happier now!