Am I sporting a vac leak?

[siteunseen]

Does it have the little rubber nub on the dome bottom?  I think it's for alignment but the ztherapy dvd says cut it off, I did on my set of SUs.

BTW, damn fine looking car! 

[AK260]

Thank you sir, you are too kind.

The ZT carbs I have don’t have the thing you mentioned above.

I forgot to mention that the idle appears to have dropped by just over a 100rpm too - a classic sign of air leaks. This problem has cost me a lot of £££ revisiting the ignition system as it had all the symptoms of a broken lead/plug/coil/cap/rotor etc, but was simply an air leak.

She’s flying like a scolded cat now and I can’t be more pleased - so what will I do? Tinker even more and break it again. Why break the habit now? I do want to experiment with different needles (currently KD, soon to be MC) to see if I can better the reversion part of the curve. My excel modelling tells me I will get a better all round mix with MCs. More of that on the link below if you’re COVID bored.

https://zclub.net/community/index.php?threads/su-carbs-will-work-perfectly-on-my-modified-l28-and-give-dcoes-a-run-for-their-money.25839/

BTW I really did buy bread ...

[AK260]

 

Oh and the whistle / hissing is even louder now - here’s a video of it!! Normal operation without air cleaner on or another leak now being sucked on even harder!? Place your bets!!!

 

 

 

 

 

 

 

[siteunseen]

I bet your air pull is much higher now. Do you use a "snail" air meter? I bought the Empi off amazon.com

https://www.amazon.com/43-5712-Universal-Pro-Sync-Adapters-Volkswagen/dp/B00BVQGFZG

 

 

[Captain Obvious]

I've had a situation or two when I wasn't confident in the seal between the dome and the base. I just used a small bead of silicone sealant at the base of the dome. Worked great without changing much else about the mounting.

So with the thickness of that foam, I'm wondering if it was a seal issue, or did the thickness of the gasket change your needle station positioning  (On edit, forget this. This is not the case.) ? In other words, I'm not sure I know what it was that caused the improvement.

 

Edited April 24, 2020 by Captain Obvious
I type faster than my brain works. It's a neat trick, but can cause issues sometimes.

[AK260]

I bet your air pull is much higher now. Do you use a "snail" air meter? I bought the Empi off amazon.comhttps://www.amazon.com/43-5712-Universal-Pro-Sync-Adapters-Volkswagen/dp/B00BVQGFZG

 

 

 Yes sir, that is the one I use. That, the AFR sensor and digital callipers I have found to be my best investments in carb tuning tools! In fact, the AFR is better than a Rolling Road IMHO as I get to see what happens in real world conditions.

 

 

I've had a situation or two when I wasn't confident in the seal between the dome and the base. I just used a small bead of silicone sealant at the base of the dome. Worked great without changing much else about the mounting. So with the thickness of that foam, I'm wondering if it was a seal issue, or did the thickness of the gasket change your needle station positioning?  

 

 The foam probably squashed down to about half the size but that is actually irrelevant.

 

Now, the height of the dome does not change the height of the piston as the piston sits dead flat in the middle of the gasket on the cab itself and freely slides up the dome. The only things affecting needle stations should be their mounting within the pistons and the jet heights.

 

The silicone bead is a great idea! I used that on the carb insulators. I did consider it but given that my carb domes are off and on “more often than a tart’s knickers”, I wanted to have something constantly reusable.

 

 

 

In other words, I'm not sure I know what it was that caused the improvement.

 That makes two of us!!!!! ;)

 

My theory is that the suck from engine side of the pistons was being vented and reducing the vac available to raise pistons, causing faster airflow velocity across the bridge and therefore a rich condition. But that is just theory. In an ideal world with more time I should have blocked the balance tube and measured vac before and after on either carb.

 

I will keep an eye on this in case it was just a freak event and report back with any new findings. But I am curious as to why the carbs whistle so loudly now.

 

 

 

[Captain Obvious]

9 hours ago, AK260 said:

the height of the dome does not change the height of the piston as the piston sits dead flat in the middle

Right. Sorry. I got hung up on your alignment issue (see what I did there?) and incorrectly translated that to a potential altitude change as well. Thanks for the nudge back to reality.    I'll go back and cross that out.

I've messed with flat tops more than the round tops, and from what I've seen, the machining quality got better. The precision machined sealing surfaces on the flat tops are pretty good. The bead of silicone I used was on an early set of four screw round tops. I've not had to do that on the flats.

So how did you determine the surfaces were off? Lay it on a flat-ish surface and you can actually see gaps?

As for the whistle, I would suggest it's perfectly normal. I believe that's air rushing past your venture and you should learn to love it. I traveled to Zcon last year in a flat top powered 260 with small aftermarket air filters, and when you punched it, that whistle was very pronounced. Sounded like a turbo spooling up. And the car felt like it too!

Just think of it as your round top turbo!   

[AK260]

Captain, you never fail to make me smile!!

I agree, on the move, when you get on the power you get the “spooling” whistle in the transition from steady to WOT! ;)

The way I saw the imperfections was just with a straight edge (steel ruler), eyes and some sunlight. Enough came through to show me it wasn’t a perfect seal.

I’m still not sure how or why it fixed it but I’m wondering if it’s a case of “managed” airflow v “unmanaged” at that part of the carb.

Someone else suggested it may have been to do with me misaligning pistons before the gaskets went on - but frankly, those domes come on and off enough times to show such an error + I always do the full drop / clunk test with both at the same time to ensure 1. Each is clunking and dropping smoothly and 2. They both clunk at the same time - FBTT (Fast Brain Time Transform) analysis!

[Captain Obvious]

Haha!! Excellent! Then I've accomplished my mission!

So you could try a bead of silicone once you're sure you've got everything else dialed in and hopefully won't have to be taking the domes off again every other day. More hidden than the foam. Just as long as there isn't some other second order effect from the foam thickness that makes a problem come back. In any event, it's just great to hear it's running so well and I'm happy for you!

I think that turbo-esque whistle is the air blowing across the holes in the bottoms of the pistons. Like blowing across the opening at the top of a bottle, or a flute. I had never noticed it until the original air cleaner was replaced with smaller individual ones on each carb. I think the large mass of the original air cleaner housing and the fact that the stock input snout pointed forward towards the radiator muffled the whistle.

And good work with the FBTT!

[heyitsrama]

Ak260, your car looks great, love the colour.

[AK260]

Haha!! Excellent! Then I've accomplished my mission! So you could try a bead of silicone once you're sure you've got everything else dialed in and hopefully won't have to be taking the domes off again every other day. More hidden than the foam. Just as long as there isn't some other second order effect from the foam thickness that makes a problem come back. In any event, it's just great to hear it's running so well and I'm happy for you!

I think that turbo-esque whistle is the air blowing across the holes in the bottoms of the pistons. Like blowing across the opening at the top of a bottle, or a flute. I had never noticed it until the original air cleaner was replaced with smaller individual ones on each carb. I think the large mass of the original air cleaner housing and the fact that the stock input snout pointed forward towards the radiator muffled the whistle.

And good work with the FBTT!

 

 

Once my needle work is complete (in the year 2031) I will defo hit the silicone!

 

I’m quite please too and itching for the next shopping run and tinker - which these days is quite often with four of us eating at home and more than doubling the grocery throughput!

 

I absolutely love the look of those round filters - they are phenomenally expensive to get this side of The Pond. If I buy from Zcardepot they end up at $239USD delivered!! As a matter of interest do you know the K&N part number (or dimensions) of the actual filter element? I am tempted to see if I can source the elements and fabricate my own metalwork.

 

I know people say they will cost a few small ponies due to sucking warmer air but the sound is quite a way closer to DCOEs!

 

These are several options but I think the E2410 is the closest!

 

 

Also to be controversial, I have heard those flat tops can be great if you know how to setup / tune properly + maintain them.

 

 

Ak260, your car looks great, love the colour.

 

Thank you sir - very kind of you to say so.

 

 

[Captain Obvious]

10 hours ago, AK260 said:

Also to be controversial, I have heard those flat tops can be great if you know how to setup / tune properly + maintain them.

That's not controversial to me at all. I completely agree 100%!!

I'll check on those round air cleaner filter gizmos for you... Tagging @GGRIII . That's his car and he bought the air cleaners. I don't know where he got them, but I remember something about "cheap clones of the expensive ones". Hopefully he come in and shed some light. And hopefully they might be cheaper on your end of the pond as well.