Question for the auto tranny gurus

[Drummerboy4as]

I recently put R1 carbs on my 73 620. It had the stock mani with a weber 32/36, L16 with 3 speed auto.

The manifold I had made has 1 vacuum port for the brake booster and 1 for the vacuum advance.

There are 2 vacuum lines coming off the transmission. 1 I know has something to do with smoothing out the shifting, the other I have no idea. In the stock configuration the brake booster port tee'd off to one vac line to the tranny. There was another port below the carb that had a vac line going to a distribution block or something, that had a line going to about halfway down the tranny on the drivers side and a line going to the crankcase pcv.

 

With my new setup at first I had the brake booster coming off one port then tee'd off of that and ran the two lines to the tranny with the vac advance getting its own port. It was shifting really hard and only at high rpms.

So I ran the vac line that goes to the back of the tranny off of the brake booster and tee'd the other off of the vac advance. Still shifting hard, no difference :/

Do I need to use that vac distributor block? Should I run the line to the back of the tranny off the vac advance instead of the brake booster?

What do these lines even do?

 

If clarification is need on the vac line routing let me know, little hard to convey via text.

 

I didn't change anything else and it was shifting fine before the new carbs.

[Drummerboy4as]

Bump

[ggzilla]

bump within 2 hours is futile. This isn't a telephone center, you know.

 

Teeing into one of the manifold vacuum lines is the right way to go. Do not tee into the booster itself as that is constant vaccum. You can use the booster line before the booster check valve. The transmission vacuum line wants manifold vacuum.

 

The vacuum line/hose goes to the transmission modulator. With high vacuum (light throttle) shift are smooth, with no vacuum (as in full throttle) shifts are hard. And inbetween varies too.

 

You might measure the vacuum where you Tee it. It should be around 20":HG at idle.

[Drummerboy4as]

It wasn't in the "new content" so I assumed it wouldn't be seen :/

 

Anyway, that's how I have it, tee'd right off the manifold before the booster check valve.

I'll have to check for vacuum leaks I suppose. I used all new vacuum lines and new tee's so it should be ok, but I'll check

 

I'll also check the vacuum pressure.

 

What's happening is that it won't shift at all unless I punch it. So it'll stay in first way too long and it won't shift into third unless I put my foot to the floor.

It's also down shifting hard when I'm coming to a stop.

[ggzilla]

Gurus check all content, not simply what appears on the first page.

[Drummerboy4as]

Did some tinkering after work and found that I'm only getting about 10hg and also that there aren't 2 vacuum lines going to the tranny, there's only 1. The other goes to the fuel tank.

 

So I assume that line goes to the modulator and the 10hg of vacuum isn't enough for it to do its job.

 

So my new question is, is there a way to adjust the modulator to shift better with less vacuum? Also, where the hell is the modulator??

[ggzilla]

10hg of vacuum is not normal.

[datzenmike]

Just got in. Yes there is only one vacuum line to the 3N71B. The vent to the gas tank is connected to the PCV system. If you have it installed.

 

10Hg of vacuum is way too low so you may have a leak somewhere in that line. Pinch off the vacuum line to the manifold .... does the idle change or the vacuum reading go up?? If yes, then you definitely have a leak somewhere on that line. If it remains at 10 inches of vacuum perhaps the manifold is leaking at the gasket or at one of the carb connections?

 

The modulator is located on the left side of the tranny just above the pan. Usually there is a small metal pipe going to it and a small rubber hose connecting the modulator to the metal line.

 

 

The electrical thing is the kick down solenoid. To the left of it is the modulator valve. A rubber hose normally goes to the center of it. Inside is a small rod and a spring. Changing the length of the rod or stiffening/softening the spring might change the shift points but I wouldn't. It was working until the carbs and manifold were added. I would try and find out why the low vacuum.

 

 

This modulator valve is for the 280zx turbo automatic. They are white colored because they have different quality of shift and different shift points.

 

[Drummerboy4as]

The vacuum is running off of only 1 cylinder because of the R1 carbs/manifold. I get 10hg off of 1 cylinder (#4). When I connect both I still get about 10hg (#1).

I've checked both ends of the manifold for leaks and there are none.

All the vacuum lines and fittings I'm using are brand new. That doesn't mean they won't leak, I just assumed they wouldn't.

 

Could it be that because of the way the manifold is setup that it could only pull max 10hg at idle?

 

I'd prefer to not tinker with the trans shift points as they were good before. My other option is to find an electric vacuum pump or something to run as an auxiliary.

[Drummerboy4as]

Just brainstorming here because 12v vacuum pumps are stupid expensive....could my mechanical fuel pump be use as a vacuum pump?? I removed it because I opted for an electric fuel pump. I know air is different than fuel, but it's an idea

[datzenmike]

Vacuum pump wouldn't work. It has to vary with load or how hard you step on the gas. The tranny needs to know this.

[ggzilla]

10hg is not because of R1 carbs. It is 'normal' if you have a very long duration cam. Or it is a vacuum leak.

 

With multiple carburetors, normally they put a small line on each intake runner, and feed all four into a little canister to accumulate/average/spread out the vacuum signal.

[Drummerboy4as]

Alright, I'll start checking for leaks :/

[datzenmike]

Most large cams would still be around 15 inches of Hg.

 

Low vacuum causes...

 

Tight valve(s)

Retarded ignition timing

Valve timing retarded (unlikely because chain wear and stretch advances a cam)

Intake manifold/carb gasket leak

Crushed exhaust pipe clogged muffler

[Drummerboy4as]

Cool thank you. Ignition timing and smashed exhaust are 2 things I'll definitely check along with vacuum leaks.