I'm having Weber 44 IDF's installed on my 1.7. The shop says the 28s will restrict enough for it to run well. Any thoughts? Am I getting myself into a can of worms with these big carbs?

Thanks!

Unless you plan to run the engine at high RPMs all the time, I think you'll be disappointed with the drivability. The carbs need a strong intake pulse to pull the fuel, and even necked down to 28mm, you may have too large a throttle bore. I would think that 40s would be a better choice if you need to go to carbs.

"Run well" is relative. I wouldn't say that.

Too big. Go with 40IDFs.

The shop owner is quite confident that 44s with 28 vents will work as stated.
This is not a stock 1.7L engine.
There is so much reversion from valve overlap that it will hardly run with the 34 ICTs that it arrived with installed.
The 44s were installed with 36mm vents initially because I had them on hand and we needed to figure out what was wrong.
The engine started and idled perfectly on the first try.


After the holiday we'll have time to do some tuning.
If for some reason the result isn't satisfactory I'm not too proud to admit I was wrong.


The can of worms was filled before the car arrived at the shop.
I'm trying to remove as many of them as I can within the budget.

Well shit, if Chris says it runs well, then that's all you need to know.

I had a stock 1.8 with 44s and 28 Venturi's that ran well. I was told that either 40s or 44s would work well. But neither of them came right out of the box with the perfect set up.

Based on my understanding of the weber tech manual it would seem that the 28 to 32 would be the best range. But I may have not grasped it completely.

I am taking my 44s and upping the Venturi's to 32s for my new 2056. I have not done the math but I have a hard time buying into the 28s restricting even a modified 1.7 al that much on the top end.

But I did not stay at a Holiday Inn last night.

You're right, it won't restrict a 1,7. Carbs need to do two things, flow enough air for combustion and then also create enough vacuum to both move and atomize the fuel. Venturi's restict total flow, but 28's are fine even for most 2 liter motors. And that much flow past a 28mm, venturi will create plenty of vacuum, more than enough for the carb to work. On the other hand flowing a 1.7's air volume through 44's or 45's past a 36mm venturi, for example, the engine will get all the air it needs, but there won't be enough vacuum under the venturi to pull out the fuel and atomize it well for good combustion. Those are the basics, then the jets for mixture metering and so on, and ALL those things need to be in balance for the whole thing to work.... almost as well as fuel injection.

That was a joke, you've seen people tearing out their hair about getting their fuel injection to work. College graduates, too. That's why all this is fun, it's a challenge.



Oops, forgot to complete that thought. Because of the vacuum created you an use a small venturi with a larger throated carb (28's in a 44, for example) but the opposite doesn't work, putting a large venturi into a small throat, say a 36 venturi into a 40. That 4mm size difference is only 2mm on each side, not nearly enough difference to create a vacuum below the venturi to make the carb function. I actually made 30mm venturi's for some 48IDF's and it worked pretty well. I lost some flow at the top and I don't know if it was the restriction of the small venturi or just TOO much vacuum too soon, but it had the torque of a John Deere down low and ran really well pretty high up the cam.

The car decinately was a can of worms when the shop got a hold of it. No complaints. Just getting the pulse on others experiences and thoughts. I'm going to have to insure this car with Alstate since its coming from "Good Hands"! lol

Thanks Chris! I'm looking forward to the shakedown cruise it never had. New England isn't Mt Hood but there's a fair share of twisties calling out! And if by some chance it doesn't work we can pull the switch off the 356 and I will get new Solexes.

See you soon.
Lou

chris' post told it all. overlap, 28s not the other, hehe. lou, you going with the cable pulley setup?

k

We just learned that the camshaft is a Web 86.
Enough of a performance modification that a pair of 34 ICTs with short intake manifolds is useless on the engine.

Further upgrades will probably happen if Lou decides to keep the car longterm.

Now I'm wondering if I should have gone with larger venturis.

The engine is running really well with 51 idles, 110 mains and 145 air corr. jets.
Idle A/F is around 14:1
On the progression circuit (very light throttle) the A.F is about 16:1
Main circuit A/F is about 12.8:1 at least up to 5000 rpm in 4th gear.

However, in the transition range it gets a little rich, in the high tens and elevens.
This is a huge improvement from being so rich (low 9's) it coughed when it had the 175 A/C jets that were already in the carbs when I started.
I was expecting to deal with a lean transition, not rich, so I had to double check that there were unmodified F11 emulsion tubes.
The rich transition is why I think larger vents might work.
I'll try another size smaller a/c jets before calling it finished.

I've done all this tuning with the pump jets disabled.
If I hit the throttle hard it leans out to 13.6:1 briefly but recovers quickly.
There's a barely perceptible stutter on tip-in that disappears right away.
It won't take much of a boost from the pump jets to compensate.

Just curious - how do you even do that?
On my 40IDFs, I would have to.....plug the accel pump intake valve ??

I honestly dont know how I would do that.
My accel pumps are not the adjustable kind, so to make them not pump would mean remove the pump cam on the shaft.....

Stu

Chris, do you have f7 emulsion tubes?

Might help with the transition....

I proabably do have some in my IDF parts box.

Actually, I think F7 emulsion tubes would add enrichment to the midrange - not what this setup needs.

I did a quick calculation of main venturi crossectional area.
30mm vents have about 15% more area than the 28s. 32mm vents have 30% more area.
I think a 15% increase would be just about right to lessen the pull at lower rpms without having as much effect on upper rpm WOT.
The Weber Main Venturi graph indicates that 30s are a good choice for a 1.7L revving to 5500 rpm.

I only have one size smaller air correction jets (140).
Having just installed them and gone for a ride I can say that the midrange mixture improved slightly and the upper end is now a bit rich.
Of course, testing at 88 deg. F and 90%RH it is expected to be rich.

The engine makes good power right up to 5500 rpm. NO flat spots anywhere.
Now I need to take it on the highway for a high speed test.

Thanks for taking the time to detail this project. This info helps to counteract the obviously widely held belief that 44s are too big for a modified 1.7 (myself included).

How do you ??

Stu

Oh yeah, I meant to answer this earlier.
With the older 40s it is a bit tougher because of the mechanism, but I think you could pull the pin at the bottom of the diaphragm to prevent it from functioning.
Later Webers have a threaded rod, and the nut can be backed off so the diaphragm doesn't get depressed.