Motorcraft 2150 ported vacuum

[JimsCJ5]

Hey Gang, Installed the Gronk 2150 on our CJ and it runs noticeably smoother and quieter but the timing is all jacked. Checking the ported vacuum and there is full vacuum at idle. There should be close to none right? I don't see any other vacuum ports that would work. What am I missing?
Here is where the port I am using is located:

 

[mriplaybass]

You should have the greatest vacuum at idle. The harder that you accelerate, the lower the vacuum will be. Think of it this way, the engine is trying to suck in fuel and air all the time. When the throttle plates are close to fully closed, the engine is trying to pull fuel and air through an obstruction, thereby creating strong vacuum in the intake manifold below the carb. Once the plates open, the fuel and air are able to rush in, but there is lower vacuum because of less obstruction due to the open throttle plates.

 

[JimsCJ5]

Thanks Bass. I get it but I thought ported vacuum (for the dizzy advance) was low at idle and increased as RPMs increased? I am referring to Gronk's instructions for ported vs. manifold vacuum.


Ported Vacuum -Ported vacuum is drawn in when the throttle is opened up. This draw is increased as more air is sucked into your air through cleaner and down through the venturis. Ported vacuum is highest at W.O.T. and lowest (or non-existent) at idle. You will have almost no vacuum pull at idle with ported vacuum unless you have moderately high idle speeds..

Manifold Vacuum -Manifold vacuum is drawn when your throttle is closed or at idle to very low driving speeds/RPM’s. This is everything under the carb. It is highest at idle and lowest at W.O.T.


I've read the ported vs. manifold vacuum arguments but am still concerned about having full manifold vacuum where I should only have ported vacuum.

 

[Posi]

You should have ported vacuum going to the distributor if your running a carb

 

[JimsCJ5]

You should have ported vacuum going to the distributor if your running a carb

Yes, but the source for ported vacuum has slightly less than full manifold vacuum (14 Hg) at idle and drops to 10Hg as RPMs increase.
I sent an email to Gronk to see where I am going wrong. Might be a bad carb, wrong gaskets, etc.

 

[Posi]

You should read zero at idle.

 

[JimsCJ5]

You should read zero at idle.

Exactly right. Not sure why though... There are no other vacuum fittings for ported vacuum I can see.

 

[Posi]

Yes, but the source for ported vacuum has slightly less than full manifold vacuum (14 Hg) at idle and drops to 10Hg as RPMs increase.

If 14" is slightly less then your idle vacuum there is an issue somewhere. I'm assuming your close to sea level in Ga should be around 19"-20" for base idle in a good running engine. Spec is 17"-21"

 

[JimsCJ5]

Yes, Full manifold vacuum is 21Hg and rock steady.

 

[Posi]

I see what looks like a boss for a vacuum port on the right side of the accelerator pump. I'm not able to see if there is a port or not. Its hard to see, may be nothing. The ported vacuum should be in the front on the right side.

 

[JimsCJ5]

I see what looks like a boss for a vacuum port on the right side of the accelerator pump. I'm not able to see if there is a port or not. Its hard to see, may be nothing. The ported vacuum should be in the front on the right side.

No that is not a port. I think you are seeing a part of the manifold.

 

[mtnwhlr]

where I should only have ported vacuum.

That would be above the throttle plate

 

[JimsCJ5]

Right. I am taking the vacuum from where I should but it seems to not be ported vacuum but closer to full manifold vacuum.
Here's a question. Since it appears that some are running full manifold vacuum on the dizzy should I still set timing with the hose disconnected or connected?
Disconnected and plugged it runs like (set to 8 degrees BTDC) . It doesn't smooth out until the vacuum is reconnected.

 

[Posi]

I’m thinking the problem may be the carb
If your taking ported vacuum where you should and have full vacuum there is a problem with the carb.
Where you running ported vacuum with your previous carb?
What was the reason for the new carb?
Disconnect and plug vacuum at the distributor, take a vacuum reading and post then advance timing to see if it smooths out.
It may run better at 10 degrees
I’m running full vacuum, I’m also running gm fuel injection with a V8 which Howell recommend, ported for 6 cyl.

 

[Manic Mechanic]

any chance you have a leak at the carb gasket? also what are you idle air screws set to? these carbs are very forgiving but 1-1 1/4 turns is a great place to start.

 

[JimsCJ5]

Yes, I was using ported vacuum with the previous Carter. We changed the carb because we wanted an electric choke and we were only getting 9 MPG.
Vacuum with the port plugged is 21HG but it runs very rough. It smooths out when I reattach the vacuum to the carb. Timing is at 8 degrees. It starts easily but does tend to turn over very slowly like the timing is too far advanced.
Thanks,
Jim

 

[JimsCJ5]

any chance you have a leak at the carb gasket? also what are you idle air screws set to? these carbs are very forgiving but 1-1 1/4 turns is a great place to start.

We tested for leaks with propane but didn't come across any. Not sure how that would affect the ported vacuum. Idle screws were set with the vacuum gauge.
I have an email out to Gronk.
Thanks,
Jim

 

[Posi]

Yes, I was using ported vacuum with the previous Carter. We changed the carb because we wanted an electric choke and we were only getting 9 MPG.
Vacuum with the port plugged is 21HG but it runs very rough. It smooths out when I reattach the vacuum to the carb. Timing is at 8 degrees. It starts easily but does tend to turn over very slowly like the timing is too far advanced.
Thanks,
Jim

So we’re on the same page. When you had your previous carb mounted you run 8 degrees timing and the engine ran good with 21” of vacuum? The only thing you did was removed the old carb and installed a new one, nothing else was touched?

With the new carb installed and distributor vacuum disconnected and port plugged the engine runs very rough and you still get 21”?

Unless I’m missing something I’ve never seen an engine that ran very rough read 21” of vacuum with a steady needle.

 

[JimsCJ5]

That's weird. I could have sworn I responded to this last night when I had the Jeep in the garage...
I can't tell you what the timing was before because we never checked it. The timing marks were obscured under a thick layer of dirt and grease. Since it ran fine we just left it. It wasn't until I had the radiator out that I could get to the timing marks and clean them. So I broke a cardinal rule and changed two things at the same time. Anyway, with the port plugged I am running 8 degrees timing. The starts and runs (although still doing the slow starting/too far advanced thing) but tends to pop and stumble at full throttle. At low RPMs it runs OK but seems down on power.
With the dizzy unplugged it runs rough but still pulls 19 Hg of vacuum. When I plug the vacuum in it cleans up and the timing goes to 10 degrees.

 

[Posi]

When you ran ported vacuum to the distributor with the carter the engine ran good.
Disconnecting the distributor and plugging the carb vacuum port should give the same results. However, the engine runs poorly.
The only difference from your good running engine and your poorly running engine is the carb and/or carb installation. This is where I believe you will find your problem.