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Discussion Starter #1
I've been reading like a g

I've been reading like a grad student, learning about sync procedures for our 308 carbs. I've read many different procedures, and understand the basic principles. I've identified the common points across everyone's carb sync procedure.

I'm currently struggling with one thing, though, and was looking for some help.

1. I know that the progression holes in the carbs are critical, since they help smoothen things out when transitioning from idle to off-idle.

2. I know that the throttle plates should just cover the first progression hole, so that the moment the plates are opened the progression holes become active.

3. I just had my carbs rebuilt by Pierce Manifolds. I checked, and I noticed the plates were configured to ALWAYS stop right where they were supposed to--at the first progression hole. A call to Mike Pierce at Pierce Manifolds confirmed this, this was done on purpose.

4. What I'm struggling with is, there's all this talk about setting the "Idle Speed Screw," which basically translates to moving the throttle plates and MODIFYING their relationship to the PROGRESSION HOLES (bad bad) in order to set your idle. I don't want to set my idle this way. My forming opinion, based on things that I've read, is that there is only ONE good position for the throttle plates, and that position is covering the first progression hole.

Point #4 was heavily influenced by an old thread I came across with Ferrari-Talk's own JRV as well as another knowledgeable carb person, Dr. Mike Adams.

Thanks to Hans Hansen for pointing out the merits of this logic as well as a few other more subtle points.

5. So it seems to me, therefore, that because my throttle plates are ALREADY set in their one perfect position in relation to the progression holes, I don't need to adjust the Idle Speed Screw aka Throttle Stop Screw on ANY of the carbs. For my purposes, they can be backed out or otherwise just touching the two carb throttle levers. Note that I'm also sensitive to the left/right linkage across the carbs, and have studied the mechanics of this and have found what I believe to be settings that leave all carbs throttle plates in their fully closed, "natural" positions.

Point #5 is the big one. Workshop manual and lots of folks say to adjust idle speed here. This is going to affect the fundamental carb response, because I'm messing with the throttle plates in relation to the progression holes.

Dr. Mike suggests that with this set, I can now leave the Idle Speed Screws ALONE and adjust the idle with the air balance / air bleed screws (which I'm not really proficient with at this point).

So before I go too crazy and sync the carbs with some fundamentally false assumptions, what are your thoughts on this logic?

If we can concur that this makes sense, I think we might be able to improve the carb response in many folks' 308s by syncing things differently than they have been synced in the past...

And finally, I defer any success this method may have to Hans Hansen, who had these new sync methods in mind well before I laid a hand on a carburettor.

Would be interested to hear comments on this, as well as ideas for how to set the idle speed with the air balance screws, and then also how the idle mix screws will play a role.

--Mike
 
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Discussion Starter #2
Hi Mike,
I recently taught my


Hi Mike,
I recently taught myself to rebuild the three Weber DCOE's on my 240Z and was reminded by your point in item #5 of what my Weber book said in reference to initial set up BEFORE starting the engine for the first time.

Not verbatim; In essence, the book walked me through a procedure to mechanically sychronize the linkage to all three carbs so all three would open at exactly the same time.

The book then told me to turn the idle speed screw in enough to crack open the carbs slightly for the express purpose of getting the engine running ONLY.

It then told me to back the idle mixture screws out a nominal amount from the seated position for the purpose of getting the engine started with the intention of making fine adjustments later.

The next step was to run the engine to full tempurature and proceed with the adjustment of the idle mixture screws while at the same time BACKING the idle speed screw OUT so there was no mechanical interference with the throttle plates.

The eventual, intended result is the engine idling on the adjustments of the idle mixture screws only.

This is how my car is set up. The idle speed screw does not touch the linkage cam at all.

I am thinking that the basic theory should be pretty much the same on your DCN's.

As usual I am always open to correction. Hope this helps.

DJ
 
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Discussion Starter #3
Good thoughts, DJ and Bob. Tha

Good thoughts, DJ and Bob. That's good info on the Nissan carb setup. What you and they are saying is, really, seriously, don't sync these things using the throttle plates. They should be in their neutral position. Sync the carbs with air bypass and idle mixture screws.

Bob, since the carb sync is dependent on timing, I don't think I would use timing to set the idle. Rather, I think using the Idle Mix screws and the Air Bypass screws, I can sync the carbs to one another (relative value of barrel flow with the air bypass screws), and achieve the idle speed via the absolute value of the air bypass screws, all the while maintaining mixture with the Idle Mix screws...
 
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Discussion Starter #4
Mike,

What book are you rea


Mike,

What book are you reading? The Haynes book?

Thanks,

Anthony
 
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Discussion Starter #5
Anthony,

I am currently &#3


Anthony,

I am currently "following" no book to the T, though I have ordered the Haynes book (and it arrives today). I also have read the procedure in the GT4 workshop manual. I also have the Pierce Manifolds tuning manual.

Most of my information comes from reading these books, and from reading tens upon tens of informative posts (everybody ELSE's opinion on how to sync the carbs).

I have heard that the Pierce Manifolds book basically suggests this new method, but have not yet found where.
 
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Discussion Starter #6
Mike,

agree the progression


Mike,

agree the progression ports are important. Have to say though that tuning is soooooo filled with variables that what works for one may not work well across the board.

In that old thread with Dr. Mike, who I hold in high regard, you may notice that our procedures vary a little but the results are clearly the same. The truth is that discussion with Dr. Mike promted me to write one of the first F-Talk Tech Articles on Tuning Daytona Carbs.

Tuning carbs is more an art than a science imo, because so many factors can influence each firing of each cylinder..

Just a general read of this new procedure leaves me wondering if it's really possible to set idle speed and balance without touching any throttle stop screws, seems unlikely to me but I'm all for learning new tricks.

Regards, JRV
 
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Discussion Starter #7
I forgot to add that to set th

I forgot to add that to set the idle mixture screws on each carb I used a 'Unisync' on each venturi to measure flow for balance.

DJ
 
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Discussion Starter #8
No offense Spasso but a Unisyn

No offense Spasso but a Unisync is only slightly better that listening to the hiss in a hose. I've used them all. A BK or SK Synchrometer is much better but a bank of manometers is about as good as you can get for synchronizing carbs.

Progression hole relationship to the throttle valve is important geometrically but not as good as evaluating its real effects on the actual running of the engine IMO. It's what happens that is important not what it looks like. If the engine doesn't idle properly, no matter how perfect the progression hole to throttle valve relationship looks, it just isn't right. Fluid or more specifically air flow is a funny thing and it's effects aren't always obvious. It is air flow past the progression holes that lowers the local pressure to the point that an emulsified air flow mixure is sucked out of the progression holes to mix with the air flow past the throttle valve and support running of the engine. Air entering the engine through the balance channels, will bypass the progression holes and will not suck the emulsified air/fuel mixture into the engine to support running. With no fuel, the engine will not run.
 
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Discussion Starter #9
>>It's what happen

>>It's what happens that is important not what it looks like<<

exactly!

>>If the engine doesn't idle properly, no matter how perfect the progression hole to throttle valve relationship looks, it just isn't right.<<

exactly again( and where Dr. Mike and I's procedures differed
), and the variables I mentioned MUST be isolated, analyzed and corrected, and timing is one of those variables..

Regards, JRV
 
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Discussion Starter #10
Mike,

To clarify a few poi


Mike,

To clarify a few points that have been made, thus far...some correct and some, well not exactly on the money.

-Timing does not affect carb synch, only the resulting idle speed...and possibly a minute mixture tweek.

-Idle speed, timing and mixture tweaks are all going to be necessary, more than once to dial your engine in

-Throttle stops are "stops", and not the end all for setting the idle speed. They are merely a mechanical stop that prevents the throttle plate from going "over-centre". I look at them as strickly a point of reference.

-What is important..is that once the timing, mixture and idle speed are set correctly, the first progression hole (ideally in all carbs) is covered/uncovered by the exactly same degree of throttle plate.

-Drew, JRV and myself spent alot of time going through and hashing out the details of setting up the carbs on a BB. Drew has posted this, tried and proven, technique in the tech archives. It is worth a read. (Note: BB carbs do not use a throttle stop.)

-Any possible vacuum leaks from ancillary hoses, or the intake manifolds themselves, will make carb set-up a total waste of time. If the air bypass screws are loose in your fresh carbs, these orifices will leak false air...and cause a rise in idle speed (if the mixture is on the lean side) when carb cleaner spray is detected.

-As good as Pierce may be, it's a dangerous presumption to make that everything is A-OK with your rebuilt carbs. They may be fine...but all of us who have been working with cars and vendors through the years have been burned by making such assumptions.

With all the talent available here on F-Talk, there's no doubt in my mind that you'll get your car running perfect...!

Regards,
David
 
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Discussion Starter #11
"No offense Spasso but a U

"No offense Spasso but a Unisync is only slightly better that listening to the hiss in a hose. I've used them all. A BK or SK Synchrometer is much better but a bank of manometers is about as good as you can get for synchronizing carbs."

I agree 100% Bill. It's funny you should mention the hiss in a hose. I saw a guy at the SCCA races with a stethescope on his 240, listening to the venturies.

The rest of your paragraph I agree with also. I would dearly love to have a bank of manometers to set my carbs with. I looked around on these DCOE's for vacuum ports in hopes of hooking up better instruments but to no avail.

Could you direct me to information on the set up and use of manometers? I will be the first to admit I don't know much about them.

DJ
 
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Discussion Starter #12
I just found my answer by Bill

I just found my answer by Bill in another thread.

http://www.ferrari-talk.com/discus/messages/5/971.html?1075511744

"I removed the hard plumbing for evacuating the charcoal canister and tapped the resulting holes in the intake manifolds for 1/8 inch pipe taps, right angle barbs. Flexible hoses were connected between the barbs on each intake manifold and 2-carb tuner manometers, like the motor cycle guys use. Balancing becomes really easy with this instrumentation. It measures the intake manifold vacuum in all cylinders simultaneously and is exquisitively sensitive. One merely equalizes the vacuum between cylinders at a given rpm and you are done.

Thanks, what hot set up!
DJ
 
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