weber twin 40 downdraft ?s

[datsunfish]

Took off the twin mikunis today and swapped on the edelbrock mani with the 40dfav1 carb. This oddball carb is essentially the same as the 38 38 dges but bigger. The problem is that I cant get it to idle below 1500 rpms without turning in the side mixture/speed screws or turning the distributor. It had small holes drilled in the throttle plates which I closed off. Its got a thick 1 piece adapter. Basically I have only had this problem before with vacuum leaks but it seems to be pretty tight. It runs great and doesnt bog or miss but I wish I could get it around 900-1000 rpms at idle. Any ideas? As big as it is should I treat it like a sidedraft and advance the timing andtry to run it around 1100-1200 rpm at idle?

[Phlebmaster]

Took off the twin mikunis today and swapped on the edelbrock mani with the 40dfav1 carb. This oddball carb is essentially the same as the 38 38 dges but bigger. The problem is that I cant get it to idle below 1500 rpms without turning in the side mixture/speed screws or turning the distributor. It had small holes drilled in the throttle plates which I closed off. Its got a thick 1 piece adapter. Basically I have only had this problem before with vacuum leaks but it seems to be pretty tight. It runs great and doesnt bog or miss but I wish I could get it around 900-1000 rpms at idle. Any ideas? As big as it is should I treat it like a sidedraft and advance the timing andtry to run it around 1100-1200 rpm at idle?

 

GIVE IT TOOO MEEE. :lol:

 

Where did you find the 40dfav1?

[ggzilla]

Why did you remove the twin mikunis? These are the carbs Datsun Competition sold, they are excellent, I have them on my pickup.

 

 

I cant get it to idle below 1500 rpms

Solution: put the mikunis back on?

[AtomChurch]

spray starting fluid everywhere until you find the leak!

[datsunfish]

I believe the mikunis are over jetted for the L16. Like I say they are saturating the plugs and one float stuck and leeched fuel everywhere so I would like to rebuild them and try them on a L20b or re jet them. I definitely plan to use them again.

Got the 40dfav from ebay.... And I am aware of how to find vacuum leaks. Not finding any.

[Phlebmaster]

spray starting fluid everywhere until you find the leak!

 

Be sure to concentrate the spray around the distributor, that way if you did have a vacuum leak it could be detected by the flame being sucked in. Better visual that way. :lol:

[datsunfish]

Be sure to concentrate the spray around the distributor, that way if you did have a vacuum leak it could be detected by the flame being sucked in. Better visual that way. :lol:

haha. It really is probably a vacuum issue somewhere. Just not in the usual spots. Maybe a clogged fuel port pulling only air? Same sorta thing kinda.

I do dig them mikunis but they need some work or more displacement. This carb rips too so I would love to have it functional so I can pass it on to the 510 when i drop in a L20b.

[Phlebmaster]

haha. It really is probably a vacuum issue somewhere. Just not in the usual spots. Maybe a clogged fuel port pulling only air? Same sorta thing kinda.

I do dig them mikunis but they need some work or more displacement. This carb rips too so I would love to have it functional so I can pass it on to the 510 when i drop in a L20b.

 

Would you keep me in mind if you decide to get rid of the 40/40? I mean if you don't it put into your 510.

[banzai510(hainz)]

make sure the PHH carbs the jet blocks are screwd down tight otherwise they will flood out sometimes.

 

If I remember correctly the 40mm mikunis is like 135 main fuel and 160 air correction as a starting point. And that what MINE is set at .

On stock motor it ran OK and a semi built L16 it ran better.

 

I have the Mikuni book at home that gives the standard specs to start out if you need the 44mm jet starting points. PM me so I can remember.

 

I think SU equip L motors have a timming set at 14BTDC

[Guest jaimesix]

Took off the twin mikunis today and swapped on the edelbrock mani with the 40dfav1 carb. This oddball carb is essentially the same as the 38 38 dges but bigger. The problem is that I cant get it to idle below 1500 rpms without turning in the side mixture/speed screws or turning the distributor. It had small holes drilled in the throttle plates which I closed off. Its got a thick 1 piece adapter. Basically I have only had this problem before with vacuum leaks but it seems to be pretty tight. It runs great and doesnt bog or miss but I wish I could get it around 900-1000 rpms at idle. Any ideas? As big as it is should I treat it like a sidedraft and advance the timing andtry to run it around 1100-1200 rpm at idle?

 

Hi.

 

Are you going to use twin Downdraught Webers? I have been looking into that for a while, a manifold has to be build. I like the idea of twin downdraughts, there are plenty of Datsuns with sidedraughts, but I have not seen any ( except one picture of a suposed 1200 set up in San Diego ) twin downdraught set ups. Twin 38 DGES or the 40s would be great, one runner per cylinder.

 

Jaime.________________________________________________________

[Cuts metal like mad]

Hi.

 

Are you going to use twin Downdraught Webers? I have been looking into that for a while, a manifold has to be build. I like the idea of twin downdraughts, there are plenty of Datsuns with sidedraughts, but I have not seen any ( except one picture of a suposed 1200 set up in San Diego ) twin downdraught set ups. Twin 38 DGES or the 40s would be great, one runner per cylinder.

 

Jaime.________________________________________________________

 

Wouldn't be as good as sidedrafts cause you can't choke them down, nor any cheaper since you need to make a custom manifold and custom throttle linkage... There is a kit for Z cars though.

[HRH]

Yes, my 20b with dual SUs is around 14-16 degrees BTDC at idle. Constant vacuum needs a lot more.

 

As far as the dual downdrafts, they will never flow as well as a pair of sidedrafts, seeing as you have to make the air bend 90 degrees to enter the manifold.

[ggzilla]

timing at idle depends on how you hook up the vacuum advance. Stock is about 5 degrees with vacuum at the carb. If you put vacuum at the manifold, you'll get a false reading it will be much higher. It's OK if you know about this.

 

The engine can only take so much advance. Advancing it at idle also advances it at max revolutions. Beware of damaging the engine.

[ggzilla]

[Dual downdrafts] Wouldn't be as good as sidedrafts cause you can't choke them down, nor any cheaper since

What do mean about "choke them down"?

 

And much cheaper than twin sidedrafts... if you can weld aluminum yourself, just cut up a two throwaway stock manifolds...

Datsun Dual Downdrafts - Webers

[hang_510]

What do mean about "choke them down"?

 

strangle them :D

as in reduced ID of the venturi, side drafts have variable sizes avail to tune to your specs.

[HRH]

timing at idle depends on how you hook up the vacuum advance. Stock is about 5 degrees with vacuum at the carb. If you put vacuum at the manifold, you'll get a false reading it will be much higher. It's OK if you know about this.

 

The engine can only take so much advance. Advancing it at idle also advances it at max revolutions. Beware of damaging the engine.

 

Yep, though SUs like a lot more advance, and I also don't run a vacuum advance. Though often you have to dick around with the distributor springs to get the idle mechanical advance. The stock spring weights on the '69 single point (Pertronix conv.) small body distributors work pretty well for me.

 

With a Weber 32/36, I usually run 8-10 degrees advance at idle.

[ggzilla]

Thanks HRH. So the message is: if you advance the idle timing very much, you should reduce mechanical advance timing to compensate.

 

Yes the beauty of Weber/Mikuni/Dellorto sidedrafts is being able to swap chokes (venturis) sizes. One reason why a 40mm carb can run on small to medium size engines. Most people i think don't tune their carbs, if it runs "OK" that's good enough. Like all those Hunduhs you see pouring out black smoke going down the freeway.