Twin carbs

[alfadave]

Got the engine sorted now, so back on the carbs.

The colourtune seemed a bit short, so I've got the long throw extension, which gives the same thread length as the plugs.

And the SK type synchrometer sits on (not in) the Dellorto IDF, so I've ordered the adaptor for it.
I don't have air horns.....will fit K&N's when all done.
And the 60's idle jets are here ready to fit.

Nearly there now.

[Crank]

Some very useful information in this thread. So thanks to those who have taken the time to post.

[LooLooSud33Spider]

Dave try to run trumpets inside the K&Ns if you can. As without them you get very messy turbulent airflow through the carbs as a result they never quite go or Sound quite right .

[alfadave]

Gulp!
I hadn't thought of trumpets.
The K&N's are 85mm high.

Will look at Eurocarb/ebay for trumpets......then I suppose the AFM will fit right into them?

[alfadave]

Set of 36/45 trumpets and bolts ordered from Eurocarb.
The only ones they have to suit DRLA 36 carbs it seems.
When they arrive, will fit them before attempting to set the carbs up.

[alfadave]

Amazingly, the trumpets came overnight.
Looks like one of the fixings will need to be drilled a tapped?

[Kegsti66]

I had to do that on mine Dave, both opposite diagonals and remove the factory middle mount.

[alfadave]

Any one on here fitted trumpets to Dellorto DRLA 36's.

Just wanted some info before I start drilling holes in the covers!

[rsfruitbat]

I have don e it a while ago. As keys to says carb flange needs drilling band tapping. From what I can remember there are some dimples in the casting to mark the spots. Think it was M6 10m nut.

Rsfruitbat

[alfadave]

Yes a couple of dimples.
One is directly above the support arm for the float.....so plenty of meat to drill into.
The other is above a thinner section of the cover.....not much to tap into
Did you drill through into the carb body there?

[sudboy1490]

Your carbs are in bits right?

[alfadave]

No,fitted on the car.
Will remove the covers to drill and tap.

[rsfruitbat]

Yes a couple of dimples.
One is directly above the support arm for the float.....so plenty of meat to drill into.
The other is above a thinner section of the cover.....not much to tap into
Did you drill through into the carb body there?

Any chance of a picture to confirm what you are describing

Rsfruitbat

[alfadave]

No chance of that I'm sorry.....if you remove a cover
and look directly under the 2 dimples on the cover, you will see what I mean.

One is directly above one of the float pedestals, and there is plenty of metal to drill and tap into.
The other dimple is above a thinner part of the cover.
Did you manage to drill and tap into this.....
or did you through drill into the main carb body?

Hope that makes sense.

[rsfruitbat]

I definitely drilled through the dimples. If metal is thin is there room to put a bolt into the tapped hole from underneath?
I didn't have any problems when I did it and I didn't do anything fancy.

Rsfruitbat

[alfadave]

Drilled and tapped the covers ok today.
The one over the float pedestal, I drilled a blind hole approx 8mm deep.
The other I drilled through the cover, and both tapped ok.

Got the K&N bases, and the trumpets fitted ok, and fitted 60's idle jets.

Next job is to set up the carbs.....New Years Day maybe?

[alfadave]

Got diverted in January!

But have now got back on the sud.

The Air flow meter is a great little item, and easy to use.

The colourtune was a bit difficult to see in bright sunlight, but got a blue flame ok.

Seems to be running fine.....so its MoT time soon.

As the car was first registered on 01.02.1980 this is its last official MoT required. But lead us not into temptation ! I think I'll continue to have MoT tests......or at least what they used to call pre MoT's

Thanks sprint88qv......your twin carb set up check list is spot on.

[Alfastrut]

Best of luck with the test dave

[Sprint88qv]

Great news Dave.

Fingers crossed for the mot....
...reminds me, I better start thinking about a few spring jobs to get mine ready for its mot.

[Alfastrut]

Got diverted in January!

But have now got back on the sud.

The Air flow meter is a great little item, and easy to use.

The colourtune was a bit difficult to see in bright sunlight, but got a blue flame ok.

Seems to be running fine.....so its MoT time soon.

As the car was first registered on 01.02.1980 this is its last official MoT required. But lead us not into temptation ! I think I'll continue to have MoT tests......or at least what they used to call pre MoT's

Thanks sprint88qv......your twin carb set up check list is spot on.

Dave out of interest what reading was you getting with air flow meter at idle?

[Sprint88qv]

I think mine were between 2.75 and 2.9 kg/h, which seemed to align with the workshop manual tolerance.

[Alfastrut]

I think mine were between 2.75 and 2.9 kg/h, which seemed to align with the workshop manual tolerance.

Would that figure be the same for webber and dellorto 36 and 40?

[Sprint88qv]

I would have thought they’d be there or there abouts for each engine configuration, as the cams, valves, inlet/exhaust flow, piston compression etc., would have been a common design for a set carb. Perhaps 36’s would draw slightly less based on the Venturi being slightly smaller, so a tiny bit less volume of air can be pulled in the same time period?!

Mine are dellorto 40’s on a standard 1.7 8v engine.

[Alfastrut]

Thanks sprint88qv

[Sprint88qv]

My simple brain tries to think of it as the bigger carbs (Venturi/jets) allow you to get more air/fuel mixture into the engine to make a bigger bang and therefore more power. However, it depends on the engine set up attached to those carbs that determine how effective those bigger carbs are. A piston engine is really like a pump, so if the pump system (piston/valve/inlet port/camshaft profile) attached to that carb doesn’t have the right amount of suck for that carb set up, then it won’t give the optimum performance.

Everyone remembers those days of the old Escort boy racers that would fit huge Weber 45 dcoe’s to their 1.1 engine and then be surprised when it would run like a dog and have no power.

At the end of the day you need to have the right engine and carb configuration to get the right amount of suck and air/fuel combination to give the optimum power.

That’s my simple thinking though, I’m sure others have done good thoughts.

[Simonsud]

Air speed is more important than volume which is why over sized carbs run rough . Small carbs will have a high air speed over jets so pull more fuel and mix better where a large carb will have a slower air flow do pull less fuel .large carbs for top end power small for bottom end grunt so it’s slways a compromise. I have a old two stoke dirt bike i use for enduros and after trying three different size carbs ended up with a smaller one than standard that makes jus as much top end but lower end is so much smoother it pulls up most hills on tick over .

[Sprint88qv]

You’re absolutely right, the Venturi effect.

Good to experiment and above all, learn.

[alfadave]

I got approx 3.5-4.0 kg/h on all 4.

The scale is quite small, and difficult to read accurately.

Can't see anything in Haynes re air flow?

[Alfastrut]

I would have thought that for engines of the the same size the air flow would be the same at the same engine speed?

[Sprint88qv]

I’d agree. Although mine is 1.7 with 40’s and I think Dave’s is 1.5 with 36’s.

I’ll double check my numbers though.