Tuning Vacuum Operated Secondary

[edwjmcgrath]

Hopefully Pinhead or Jim or one of the other carb gurus will see this.
I rebuilt my carb a few months ago and it's been running great. When I did the rebuild, I verified that the vacuum diaphragm was holding and just remounted it. However, recently I did the paperclip test to see if my secondary was opening and determined that it was not.
I checked the following items:
I had operate the throttle and I verified that the secondary does kick open a bit when the throttle is fully depressed. I had checked this when I reassembled the carb but I never measured the 28*.
I verified that the throttle opens fully and freely when I pull in on the diaphragm rod by hand.
I disconnected the rod from the secondary linkage and did the paperclip test again and the rod still does not move.
I did the suck test again on the vacuum secondary unit and the rod does pull in and holds vacuum (although it takes a pretty good suck to get it in.)
I replaced the diaphragm just for grins and got the same result.
I sprayed carb cleaner into the vacuum port for the vacuum operated secondary and I can see it coming out down onto the secondary throttle plate-so the passages are clear. The restrictor jet on the vacuum secondary is clear and the little gasket that goes around that port is in place.

So based on all of this, I think that I've determined that I do not have enough vacuum to get the secondary to open. I'm basically at sea level and my vacuum runs around 18 at idle. I can think of three things that would help get me get the secondary to open:
Adjust the tang to make the secondary plate kick up more at full throttle. (I'm not sure how sensitive this setting is.)
Put a weaker spring in the Vacuum unit. (This would be easily reversible.) I checked my existing spring and it looks like it would take about 200g to compress it about half way.
Open the restrictor jet some. (I'd rather not be drilling this out unless I need to)
Advice?

 

[SteveH]

I have exactly the same problem (after a rebuild) on my '78 carb and would love to hear some ideas.

 

[kurtnkegger]

Subzali did a great write-up on his carb...Helped me get mine better tuned in, but still need to do some work.

http://risingsun4x4club.org/forum2/showthread.php?t=16462

Hope this helps you too.

 

[Pin_Head]

The secondary vacuum signal comes from high up in the venturi above the restriction. There is a hole drilled between the primary and secondary bores which feeds vacuum to the actuator. How this works is a bit of a mystery to me, but it seems like it needs to have vacuum in both barrels to send a vacuum signal to the actuator. If vacuum is high in just one bore, it merely sucks air from the other bore. At WOT, vacuum will be high in the primary, but with the secondary plate closed, it will be zero in the secondary. I think the purpose of the manual secondary opening is to pull some vacuum in the secondary to start the actuator opening the secondary. If it doesn't kick open far enough, there won't be enough vacuum to move the actuator. Try making it open a little more by bending the tab on the mechanical linkage. The stock opening is about 2 mm. Try 3 and then more until it takes off.

 

[edwjmcgrath]

Thanks Pinhead. Since the carb is installed, I can't measure how much the secondary opens without taking the carb off.
I think I'll just try bumping it open a bit more-once or twice. If that doesn't work, I'll take it off and see where I stand.

What is the best way to test for the secondary opening? High speed plus WOT?

 

[Pin_Head]

It opens in response to high load on the engine, like going up hill in 4th with the pedal to the metal.

More about vacuum secondaries:
http://www.youtube.com/watch?v=AgLwehs8-EM&list=UUjEngWGChKu-OlRPOYrb4bg&index=1&feature=plcp

 

[edwjmcgrath]

Thanks. Actually, it was that video that inspired me to check if my secondary was opening.

I decided to be scientific about it and remove the carb and do the measurements right.
When I removed the carb, I found that the secondary kicked up 29* (the FSM calls for 28* and 25* for CA).
I did the measurement two ways-with an iphone level and with an old school angle gauge.
This tells me that the opening theoretically is already a little bit more open than the FSM calls for and the vacuum secondary is not engaging. (I can't imagine it opening with a CA spec carb-that's probably what they had in mind. ) Then I measured the opening and it was smaller than my smallest drill bit (1.6mm). I measured the angle with a 2mm opening and it was 32* and at 4mm it is 36*.

I'm definitely confused about the kick-up gap vs angle. It seems like I have to open it way more than the spec. I'm going to try the opening at somewhere around 3mm (34*) and see how that works.

 

[edwjmcgrath]

Progress!

I decided to adjust the secondary kick up to 2.5mm or about 33* and try it.
The secondary is opening now. I did a longer trip out to the highway and back and it felt like it was working, but when I opened the hood, the paper clip was gone. I did a short trip and the paperclip moved around 9mm. I really think that I can feel it opening slowly like it's taking several seconds for it to open all of the way. This makes sense since there's no accelerator pump in the secondary, it shouldn't open quickly. The place where I really noticed the difference is on the entrance ramp to the highway. Getting from 50-65.

 

[SteveH]

Thanks for inspiring me to do the same - I will pull my carb this weekend and see if I cannot make this happen as you did.

 

[Hudson High]

Progress!

I decided to adjust the secondary kick up to 2.5mm or about 33* and try it.
The secondary is opening now. I did a longer trip out to the highway and back and it felt like it was working, but when I opened the hood, the paper clip was gone. I did a short trip and the paperclip moved around 9mm. I really think that I can feel it opening slowly like it's taking several seconds for it to open all of the way. This makes sense since there's no accelerator pump in the secondary, it shouldn't open quickly. The place where I really noticed the difference is on the entrance ramp to the highway. Getting from 50-65.
View attachment 671459

Can you post pics of which tabs you bent to open the secondary more?

 

[edwjmcgrath]

Here's a pic showing the tab and the drill bit that I used to measure the opening.
It should be pretty obvious with the carb off. Operate the mechanical linkage until the throttle is almost fully open and you will see the tab hit and cause the secondary butterfly to kick open a little bit.

 

[Icantdrive55]

I too am working on the same issues in my '86 FJ60, de-smogged/recurved, 2F with Aisan carb. When operating the throttle linkage in my driveway, revving the engine to see the secondary kick-in only as the engine nearly redlines, seems way too high RPM's--nearly 4.5K. Is there a proper RPM range where one should see the vacuum secondary begin to engage? Or, am I wasting my time trying to set vacuum secondary as a function of RPM's? It would seem that all RPM's are the same whether chugging up a hill or sitting in the driveway.

Linkage is set to kick open the secondary to about 2mm.

Paperclip test shows the secondary working, though on a long incline I can't particularly feel any difference. (Not expecting warp drive, but at least little boost.)

Recently refurbished Aisan 2A and found that a PO, or their mechanic, installed a primary venturi in the the place of the factory venturi of the second barrel. Does the primary venturi have the same (correct) vacuum characteristics such that it can be used in the secondary?

Thanks for this this thread. It's been a big help.

 

[Pin_Head]

The secondary only opens in response to high engine load; not RPM. There is no load on your engine revving it up in your driveway. You will throw a rod before the secondary will kick in. Drive with it in 4th gear up hill with the trottle floored at 2,000 RPM and it should open. As others have noted above it takes several seconds for the secondary to kick in so you might not notice it.

On some model years the primary and secondary main nozzles are the same; on other years they are different. It probably won't matter to use a primary nozzle as a secondary, because the primary always has two booster rings.

Here is a description of how it works:

 

[Icantdrive55]

Pin_Head,

Thanks for the clarification of the vacuum secondary responding to engine load, not RPM's. And, for the perspective of 2K RPM's up a hill in fourth gear. My test runs have been at 75MPH up a 9-miles long, 200-feet change in elevation on a stretch of Interstate near the house, so I'll have to go across town to the foothills to put it to the 2KRPM/4th gear test.

Thank you for the Aisin carb rebuild videos and the vacuum secondary video posted to youtube. Over the last 2 months I've probably viewed them a dozen times.

 

[Pin_Head]

I don't think the RPM is that important. It needs sustained periods of wide open throttle to kick in the secondary and it is easier to do in 4th gear.