Low Vacuum / Runs Fine....

[Rocky_LC]

So I determined pretty conclusively that I am measuring low vacuum on my '85 2F Motor. I bought the motor back in 2010, and swapped into my '74 FJ-40. No problems, has run fine and passed smog ever since. I will say the engine has a lot of valve train noise.

The motor starts and runs fine, I wouldn't have known if I wasn't just "experimenting" with the vacuum and timing for fun.

I am measuring the vacuum at a spare port on the "gas filter", coming off manifold vacuum.

At idle, I am getting ~6" Hg with ~9 Deg BTDC timing (BB close to edge of window).

I drove the truck up and down Mt. Lemmon. On the downhill slopes, the most vacuum I saw was 12" Hg, with my foot off the gas, and coasting downhill. I thought in that case I should be above 20".

If I really have such low vacuum, wouldn't I have other issues with the truck (hard to keep running, poor idle, etc?). If it was a bad idle screw adjustment, wouldn't I have to be running so rich that I wouldn't pass smog?

Should I be measuring vacuum somewhere else?



The truck has been desmogged (I thought it was reasonably well done) and has the HAC valve installed.

Reading the info available, the suggestions are:

1. Vacuum Leak (Brake Booster?) - I have good brakes

2. Vacuum Leak at Cracked Manifold

3. Vacuum Leak at Carb Base

4. Bad Valve Adjustment

5. Bad adjustment of Carb Air Idle screws?

Any other ideas of something to check?

It seems like I should check them in order Starting at 5 - 4 - 3 - 1 - 2

BUT.... I am hoping for a miracle cure or suggestion that makes this all moot - like: Measure vacuum at "such and such port at the base of the carb".

Thanks -

Rocky

 

[Pighead]

I'm not familiar with that gas filter. Does it have some sort of restrictor inside of it?
I plumbed my vacuum gauge line directly into the intake manifold.
A bad valve adjustment will certainly decrease vacuum readings, as will poor timing.
How about adjusting your valve clearance just a tad tighter than factory specs, then adjust timing (advance will usually increase vacuum) watching the vacuum gauge. I usually advance timing to maximum vacuum, then retard just a bit.

 

[Rocky_LC]

Thanks - I don't know about the gas filter either - I though it just had some filter paper or foam in it...

I will look and see if I can find another port on the manifold.

I plan to do the valve adjustment, but wanted to start with the easy stuff first.

My timing is good, checked that yesterday.

 

[Pighead]

My timing is good.

Timing to the BB is just a starting point...the further away from the factory these engines get, the less the factory settings apply. adjust your valves first, set timing for best vacuum, back off a tad, back off more if it pings under load. You're looking for the best settings for your particular engine at your altitude, etc.

 

[77mustard40]

Take your vacuum reading where you know you have unobstructed manifold vacuum. I measured mine at the line going to the break booster. I think you will find a better reading.

 

[Splangy]

Every 2f came with a gas filter in the manifold. They shouldn't have any restriction. Did you try another vacuum gauge?

 

[Rocky_LC]

17" at idle.

I set up a new vacuum port into the manifold at a hole in the manifold that was previously capped off - by the brake booster.

My low vacuum nubers must be something going on in / with the "gas filter".

I will still mess around with the valve adjustment, timing and idle screws, but my original symptom (low vacuum) seems to not be a problem. Thanks for all the advice.

Rocky