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Just bought a 521


mainer311

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1 hour ago, Crashtd420 said:

Is this you carburetor or an online picture?

 

Oh, that's one from a Shelby cobra forum, but my 3-hole layout is identical.

 

I was trying to think in my head that maybe I just have some settings so far messed up that the throttle plates aren't opened enough, but I don't know how that could hold true. I'm at 1/4 turn on the throttle stop screws, 10 BTDC, and my idle is 900rpm. Idle jets are 55F9's, which are fairly common. F9 is even a little on the rich side at the mid to high idle circuit. It just seems like things should just fall into place from here, but that throttle tip-in location is the only issue.

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1 hour ago, mainer311 said:

 

Oh, that's one from a Shelby cobra forum, but my 3-hole layout is identical.

 

I was trying to think in my head that maybe I just have some settings so far messed up that the throttle plates aren't opened enough, but I don't know how that could hold true. I'm at 1/4 turn on the throttle stop screws, 10 BTDC, and my idle is 900rpm. Idle jets are 55F9's, which are fairly common. F9 is even a little on the rich side at the mid to high idle circuit. It just seems like things should just fall into place from here, but that throttle tip-in location is the only issue.

 

Just for comparison I'm about 1/4 to 1/2 on my throttle.... and about 750/800 rpm...

 

I think I have a 50f9 in on mine....

 

And I believe my air/fuel screws are out 2 turn .. 

 

So I think your around where you need to be.... 

 

I also checked my throttle plate and I covers the first hole perfectly and is exposed as soon as I touch the throttle.... so I'm not sure what to say, I'm still confused where yours is.... if its past the first hole then try lowering the idles slightly, if its before the hole and you need more throttle to cover the hole then I dont know what to say.....

 

The tip in your talking about is this with a quick throttle opening or slowly ?

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17 minutes ago, Crashtd420 said:

if its before the hole and you need more throttle to cover the hole then I dont know what to say.....

 

The tip in your talking about is this with a quick throttle opening or slowly ?


It’s this. To “fix” the plate location, I would need to turn my idle WAY up. Like a whole turn.

 

Doesn’t matter. Fast or slow. I have a dead spot as soon as the plates start to open. If I’m driving any less than 2800rpm or so, steady speed, my throttle is just barely pressed right in the problematic area.

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1 hour ago, mainer311 said:


It’s this. To “fix” the plate location, I would need to turn my idle WAY up. Like a whole turn.

 

Doesn’t matter. Fast or slow. I have a dead spot as soon as the plates start to open. If I’m driving any less than 2800rpm or so, steady speed, my throttle is just barely pressed right in the problematic area.

Got it..... 

Sorry but this does seem like something other than your basic tuning issues....

 

I wish I had more to offer.... 

In my mind I'm wondering about how they were assembled, could the throttle plate even be installed incorrectly?

Is that even possible?

 

I'm sure our idle screw settings are the same yet our throttle plates are clearly in different locations relative to the first progressive hole? 

Have you checked this on all 4 throttle plates? 

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33 minutes ago, Crashtd420 said:

I'm sure our idle screw settings are the same yet our throttle plates are clearly in different locations relative to the first progressive hole? 

Have you checked this on all 4 throttle plates?


Yes, with a flashlight last night. They’re not identical but very very close.

 

I just tinkerer with it some more. After adjusting the air bypasses last night, I forgot to reset best lean idle. I did each tube separately, then went back and counted the turns. 1.5 for all 4! It idles like a dream. Carbs synced at both idle and up through the RPM.


Took it for a test drive and it’s still there. Even shifting between gears there’s a split second of hesitation right after I let the clutch go and press the gas. It’s just enough to get my head to bob slightly and then it takes right back off. I think it’s hole drilling time. Probably save that for daylight hours when I get a chance. Other than that it drives really nice now. It’s livable, but it will bother me until I get it sorted.

 

Final tune for now is:

34mm venturis

4.5 secondaries

55F9 idle

135 mains

F15 emulsions

175 air correctors

...and whatever accelerator stuff they came with. 

 

Edited by mainer311
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Just seems wierd to have to drill that extra hole....

But maybe its something with the newer carburetors....

I thought I read before they were adding the 3rd hole to older carburetors that only had 2.... 

 

You would thing Weber would think about that if they changed something... 

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9 minutes ago, Crashtd420 said:

Well they eventually figure it out because they started adding the 3rd hole.... wouldnt surprise me to see a 4th hole added soon by weber

 

They already have, well, on the 45's anyway. The model is "152G" and they drilled an extra hole. As far as I know, they never did that for the 40's (151).

 

 

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20 minutes ago, mainer311 said:

 

They already have, well, on the 45's anyway. The model is "152G" and they drilled an extra hole. As far as I know, they never did that for the 40's (151).

 

 

That's interesting,  the holes are in different locations too....

What do your 3 holes look like?

This is a picture of mine... top of picture is engine side.... 

You can see the brass covering the first hole, ignore the shadow on the back edge....

20210511_171230.jpg

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Mine looks like that, but my brass is closer to the engine side. Basically more of my 1st hole is towards the velocity stacks. My holes look the same as yours. (Is this getting weird?) 

 

I also just read on a Datsun Z site some other guy was having the EXACT same problem. One of the replies was that the newer 151's require 2-1/4 to 3 turns out on the idle screws. I'm at 1-1/2. I did try a smaller jet and the popping out from #1 got way worse, so I went back to 55. I guess I could try it again. 

 

Thinking about it, I'm going to start the screws 2-1/2 turns out tonight, and open the throttle plates another 1/4 turn and see what happens. Get it nice and rich and stumbling and then crack the butterflies open slightly more. I know what I'm at now, so I can always revert. These carbs sometimes. What a pain in the ass.

Edited by mainer311
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22 minutes ago, mainer311 said:

Mine looks like that, but my brass is closer to the engine side. Basically more of my 1st hole is towards the velocity stacks. My holes look the same as yours. (Is this getting weird?) 

 

I also just read on a Datsun Z site some other guy was having the EXACT same problem. One of the replies was that the newer 151's require 2-1/4 to 3 turns out on the idle screws. I'm at 1-1/2. I did try a smaller jet and the popping out from #1 got way worse, so I went back to 55. I guess I could try it again. 

 

Thinking about it, I'm going to start the screws 2-1/2 turns out tonight, and open the throttle plates another 1/4 turn and see what happens. Get it nice and rich and stumbling and then crack the butterflies open slightly more. I know what I'm at now, so I can always revert. These carbs sometimes. What a pain in the ass.

 

The beauty of the weber....

Anyone can run one but when you wanna actually tune it properly it takes a minute....

Without a full selection of tubes and jets on hand it's sometimes hard to guess what it actually needs... 

And I gets costly quick....

 

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1 hour ago, Crashtd420 said:

 

The beauty of the weber....

Anyone can run one but when you wanna actually tune it properly it takes a minute....

Without a full selection of tubes and jets on hand it's sometimes hard to guess what it actually needs... 

And I gets costly quick....

 


Yeah, I’m at least a few hundred deep in jets right now. E-tubes are $20/each.

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Just now, mainer311 said:


Yeah, I’m at least a few hundred deep in jets right now. E-tubes are $20/each.

One reason I'm glad I only have 1 carburetor... 

 

Probably late to say this but I actually bought smaller jets and was drilling them out to the next size up as a trial.... 

 

I have toyed with the idea of customizing my other emulsion tubes at some point just to see what I could create. . ..

 

The internet info only gets you so far its definitely the fine tuning that gets annoying....

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30 minutes ago, Crashtd420 said:

Probably late to say this but I actually bought smaller jets and was drilling them out to the next size up as a trial.... 

 

Yeah, I drilled my 45 idle jets to 60's to try, but I'm still convinced that 50, 55, or 60 is where I need to be, so I'm not really at a loss. Can't wait to pop smoke and run home to try my experiment.

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Well, that didn’t work at all. Basically with the butterflies set to where they should be WRT the profession holes, so much air is getting by that idle basically runs away no matter what I do with the mixture screws. I can get it to come down a little bit in RPM if I screw the idles way out to make it very rich, but then the thing is just completely out of tune. 
 

I even tried the Pierce Manifold method and it just does not work with these 151’s.

 

Getting the drills out...

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2 hours ago, Crashtd420 said:

It's good you experimented as much as possible first,  and I'm Glad that worked in the end..

 

Yeah, I wanted to exhaust my options before drilling anything. I'm actually wondering if the 34mm venturis have something to do with the incorrect position of the butterflies, but based on the charts, I have the correct venturis. I arbitrarily picked 6krpm as my max, but theoretically I could go a lot less than that since I'm typically just raising hell around town. 

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