Dual IDF Tuning - 36HPVW

[txoval]

Dan:

Distributor/advance is working correctly, like you said I can see it with the timing light.

I think the bottom line is everything you guys have said about compression/velocity. It runs like a beast above 2800 rpms.

It's not bogging at lower rpms, it's simply lazy. It's amazing how you can hear the difference as the velocity increases.

This is why I'm hoping Jack at Jaycee will help with fabricating a set of 24mm vents

[txoval]

Jaycee will fabricate the 24mm venturies for me. I understand this will not "fix" all my issues, but it should allow the use of a smaller idle jet correct?

[[email protected]]

no, the idle jet is completely independent of the main jet sizing WRT to the venturis. Re-read the wideband tuning thread, it's covered there several times and you missed it.

However, SOME guys run a bigger than needed idle jet in order to cover up the lean hole that is present from a late main circuit. Not the proper way to tune the carburetor(s), because it makes the entire lower end richer than needed, to fix one small spot at the end of progression.
_________________
It's just advice, do whatever you want with it!

Please do NOT send me Private Messages through the Samba PM System (I will not see them). Send me an e-mail to john at aircooled dot net

"Like" our Facebook page at
http://www.facebook.com/vwpartsaircoolednet
and get a 5% off code for use on one order for VW Parts ON OUR PARTS STORE WEBSITE, vwparts.aircooled.net

[txoval]

John:

I'm referring to these earlier comments below:


"Running 27 vents at 1200cc is like using 32 on 1600cc
32 vents on a 1600 is not going to work with common 40 idfs, but it could be made to work with 44idfs or 40 dells. you need more transfer ports and idle jet to carry the engine until you have enough flow to kick in those "big" Wink 27mm vents "

"Morning. I just realized that I goofed up on the math last night. I woke up and had that notion that something was wrong. So I checked, and sure enough, I was too fast on the trigger. - The formula for venturi size is rpm x displacement, take the square root and divide by 50. That brings some totally different numbers. This also brings Modoks thumb close back to normal size. Rolling Eyes Very Happy
Sorry about that. Embarassed
But you should still up the idle jet."

If I install properly sized (per the formula) 24mm vents, this should increase the vacuum (or velocity) through the carb, which should allow smaller idle jets. Unless I am reading both threads wrong.

If I am wrong, then what will properly sized vents correct?

[[email protected]]

When the venturis are the correct size, the main circuit will come in at the proper time. If they are too big, the airflow is insufficient to get the mains started early enough, and there's a lean hole between when the progression ends, and the mains begin.

If the venturis are too small you have the opposite issue, where there is an OVERLAP, which creates an excessively rich contion, with both metering fuel at the same time.

The "larger idle jets" are a crutch used to cover up the lean hole, but as I stated it's done at the expense of being too rich at low throttle settings.
_________________
It's just advice, do whatever you want with it!

Please do NOT send me Private Messages through the Samba PM System (I will not see them). Send me an e-mail to john at aircooled dot net

"Like" our Facebook page at
http://www.facebook.com/vwpartsaircoolednet
and get a 5% off code for use on one order for VW Parts ON OUR PARTS STORE WEBSITE, vwparts.aircooled.net

[txoval]

John:

That's exactly what I'm asking/saying. It has been stated that my venturis are too large, creating the lean hole...which has led to larger idles being used as a crutch.

By going to correct size venturi, I shouldn't have to use the larger idles as a crutch. This is what I'm trying to confirm, and I believe you just did.

Thank you for the help again.

Obviously the flip side is that the 24mm venturis will be too small causing another issue, but I guess that's just part of it

[[email protected]]

the difficulty is the idle and progression circuits operate off manifold vacuum, while the main circuit operates off of airflow.

In difficult tuning jobs, the issue is synronizing the 2 circuits, so the main circuit turns on when the progression circuit turns off, with "just enough" overlap. This can be very challenging, especially when carburetors are not properly sized to the engine.

It's also totally different in an engine with a lot of intake vacuum (small cam), compared to one with low intake vacuum (large cam ie: large overlap).

This is why a set of carbs which operate perfectly on one engine, can be completely wrong on a different engine (even the same size with a different cam, or different size with the same cam).

Heck, even the "same" engine with the same cam will need different jetting if the compression ratios are different on the 2!

Or imagine an engine which runs perfect, and then as the ring seal goes away (compression and vacuum drop), all of a sudden it needs more idle jet, since intake vacuum is lower.
_________________
It's just advice, do whatever you want with it!

Please do NOT send me Private Messages through the Samba PM System (I will not see them). Send me an e-mail to john at aircooled dot net

"Like" our Facebook page at
http://www.facebook.com/vwpartsaircoolednet
and get a 5% off code for use on one order for VW Parts ON OUR PARTS STORE WEBSITE, vwparts.aircooled.net

[modok]

Smaller vents is going the right direction, yes. I do not know if 24 is absolutely the right size, but certainly it is easier to bore them larger than it is smaller, so sure try 24mm for now.

The venturi should not be the smallest point in the system, so if it is, that means to me the rest of your system would be improved if it was smaller, OR you could enlarge the engine to match the size of the ports and manifolds.
It's up to you how FAR you chase ideal performance.

[txoval]

Glen, did you mean to say the Venturi should be the smallest or should not be?

Even at 27mm, the Venturi is the smallest diameter in the system. Head and manifold intake ports are match/oval shaped, skinny dimension is ~30mm, Intake valves are 33mm

I came up with 24mm based on Alstrup's formula...Jaycee's price to fabricate them was very reasonable. I may be wasting money, but at this point I'm interested in how it effects the performance

I should probably just go with 55 idles and 115 mains and call it good, but that would be too easy

[[email protected]]

the best venturi sizing chart I've seen was in the Solex book back in the 70s and 80s. Black cover with white lettering.

I still have it, but it's not handy ATM, but I believe this is it

so 28mm is for a 1200 turning 6k RPM. so 24-26mm should be very close to what you want.
_________________
It's just advice, do whatever you want with it!

Please do NOT send me Private Messages through the Samba PM System (I will not see them). Send me an e-mail to john at aircooled dot net

"Like" our Facebook page at
http://www.facebook.com/vwpartsaircoolednet
and get a 5% off code for use on one order for VW Parts ON OUR PARTS STORE WEBSITE, vwparts.aircooled.net

[modok]

[txoval]

I would have to measure the port size in the head(1.5" behind valve), but I bet it's close to 86% of 33mm or ~28mm. The head was designed for 1200cc engines, but with single 32mm carbs

I would be curious to see where 4500-5000rpms falls on that chart John provided.

In the end, I may be stuck with sluggish performance down low. When Ray with Speedwell designed the manifolds to utilize DRLA's or IDF's on Okrasa heads, I'm sure he intended them to be used on stroked/bored 36hp style engines. Some guys have turned 36hp engines into almost 1600cc, but the most common change is to utilize a 69.5mm crank or even 72mm.

I'll keep plugging away at it

[txoval]

What would be a lot of advance? Currently 8 BTDC at idle and will have to recheck full advance...trying to remember what the advance is on an 019, but I think it is 15-17 degrees, so total would be 23-25

[modok]

lets see.... small bore engines usually use less advance, and higher CR uses less too, so maybe 10 idle, 32 all-in would be "a lot", for the tiny thing.

[txoval]

To get 30 or 32* all in, my idle advance would need to be 13*BTDC

The VJ4 BR8 (019) only provides 17* advance

[Boosten Ya(Josh)]

If you go that route you could start at 10* and see what happens. Then work you way up and if the motor doesn't like it go back down
_________________
07' Ford Ranger - New DD.

71' Super beetle - prostreet trans 4.10 gear, 1641cc with dual 40 IDFs, W110, and a 1-1/2qt deep oil pan.
Needs: cage, seats, harnesses, better suspension, new wheels, and rear disc brakes.

Almost there....

[txoval]

A couple of views into the Okrasa heads:

Hard to tell how the intake port necks down

[txoval]

Jack at Jaycee finished my 24 mm vents today, so hopefully in 2-3 days I'll be able to let you all know how they work, or what needs to be adjusted next.

I will be curious to see how much I can decrease my idles...if any

[txoval]

Installed the 24 mm vent this afternoon, re-sync'd, checked/adjusted timing to 10*BTDC, reset mixture screws to LBI...and then the floods came.

Sounds much better at lower rpms, but I'm afraid it may be choking it at high rpms...hard to tell without driving it after tuning it.

Will see tomorrow how it goes

[mark tucker]

personaly I think the 27's were not to big. but the cr is too low and makes low power&tq at lower rpm more noticeable. and also needs to be richer. if it were mine the deck/piston to head clearance would be .035" then mill the heads to get the cr you need . kinda late to be doing that, but it will make for a happyer motor and cleaner running eazer tuning, happyer driving.
didnt alsurp say 30 vents would be right for that motor? I would think that would be about right too .I also recomend adding gaskets to the booster vents like webber should of done.