[Ben Reynolds]

I have a 1968 1750 GTV with original 40DCOE32s. It was imported new to Japan, probably Australian spec. RHD with km Speedo.

There is a flow mismatch between barrels on one carb. The butterflies align as viewed through the progression holes, spindle bearings and seals are good, and no leaks on mounts or manifold joints. I’m going to try replacing the butterflies as a next step...🙄

I have read that original spec was 79.5 degs, but later cars went to 78 degs. Which is correct? And which is best?!

All advice gratefully received!

 

[yvesmontreal]

79.5 they are. Which cylinder is the offender... number 4? Vacuum leak to (through) the brake booster.

I'm not a Weber expert, but obviously the plate angle has to be matched to progression hole locations.

 

[Ben Reynolds]

Thanks, good call on the vacuum hose leak, but the discrepancy is on the front carb, with 2 flowing higher than 1.

 

[yvesmontreal]

More flow from 2 seems to say it it's 1 that's picking up air elsewhere. Just to make sure you've covered everything, some manifolds have a small vacuum fitting on number 1. Nothing there?

Replacing butterflies is fairly involved and seldom required. Unless one has a visible nick on the edge, or else a visible distortion, there has to be something else.

 

[Ben Reynolds]

All valid but the signs are no1 is getting less air, as idle mixture adjustment on that cylinder doesn’t improve the lumpy idle, and pulling number 1 plug lead off has little effect either. Off idle it’s going well, and compressions are even on the newly rebuilt engine. Someone has done strange things to this carb in the past, and maybe they messed with the butterflies.

 

[zedsn]

Can you looses the 2 screws and adjust the throttle plate a little bit with the throttle shaft closed if that is indeed your problem?

 

[Ben Reynolds]

Good thought...actually I loosened them both, fully closed them then re-tightened, but still have the issue. I’m really quick at carb changing these days but getting a bit sick of it 😆

 

[zedsn]

How much light can you see though the barrel with the carb fully closed? Maybe someone ground down the plate a little bit? There could be a number on the plate and a guy from Italy sells carb parts on ebay and you might be able to get one from him.

 

[Ben Reynolds]

Quite likely, as there were some other mods I have discovered and had to correct! So I intend to replace the butterfly plates for a matching new set. But who can advise on which angle I should go for? If the angle was changed after my car was built, it was for a good reason, and I might get a better result by going that way

 

[patonycam]

If one plate vs the other plate is slightly open in the closed position the throttle shaft may be twisted. There are ways to address this. Check engine compression to make sure the cylinders are behaving the same. Tony

 

[Ben Reynolds]

Good point Tony, but the edges of the plates are perfectly synchronized as they pass the progression holes, so I think they are both at the same angle. I think someone has modified or damaged one of the plates.

 

[vintagemilano]

Sounds more like you have a plugged number one idle circuit or a bad spark plug or wire.

 

[Ben Reynolds]

New plugs, leads, and idle circuits clear. If I crank the engine with closed throttle, it sounds like one cylinder has low compression. But with the throttle open it’s fine. So definitely one throttle valve is more closed than the rest. So next I’m going to change them!

 

[stphers]

Why not try swapping them from 1 & 2 with each other and see if if the problem follows the throttle plate?

 

[Ben Reynolds]

Hmm, haven’t tried that, Thanks, worth a shot...