Hello all,

I’m a 25 year old automotive engineer from Europe and I moved to Mexico for my job.
I’ve always loved American trucks so a while ago I bought a 1979 F150 2WD Supercab with a later model 302 engine and automatic transmission (early 80s).
I bought it because I'm planning a road trip to the USA soon. For that reason I’m trying to fix it up as good as I can.
This is my first time using this forum and the first time working on a ford truck, so please be gentle with me ;)

When I bought the truck, it was running quite rough, low idle and it would even die in idle (especially when the steering wheel all the way to one side).
There was no idle screw in the carburetor, there wasn’t even a idle screw hole for it. I was welded shut and they replaced the bolt with a spring pulling on the lever…
A lot of vacuum lines are plugged, not working, modified, …

I already tore down the complete carburetor and rebuilt it, but now I’m having some issues with a vacuum leak. I’m measuring around 10-12 “ Hg, which would point to a vacuum leak.
I do think I used the wrong gasket during the rebuild of the carburetor because the hot-idle compensator valve hole doesn’t seem like it’s been sealed by the gasket.
I plugged it temporarily with some electric take. There was another inlet manifold hole which wasn’t plugged, I also plugged that whole.
I before plugging these hole, spraying brake cleaner near the carb resulted in an increase in idle speed. With these holes plugged I cannot get the idle to increase anymore by spraying anywhere around the inlet manifold or carb.
But the vacuum is still only 10-12 “ Hg and I still hear the hissing sound. It seems to me that the hissing sound is coming from near the EGR valve.
Nothing is currently connected to the EGR valve (no vacuum tubes) and by spraying around the EGR valve I can’t get any increase in RPM.
Any ideas where my vacuum leak could still be and how I could fix it? Could it be internal?

Additional information:

• The idle screw doesn’t seem to have any impact on the RPM or vacuum. I read that this is a sign of vacuum leak, is this correct?
• The transmission is only shifting to second gear at around 4000 RPM, at the point where my steering pump is screaming (I installed a new pump, radiator and hoses). Is the 4000 RPM shifting due to the low vacuum? Or is this another problem?
• Keep in mind it’s a later engine/gearbox. According to the guy I bought it from, it’s early 80s.

Any help would he appreciated.

Kind regards,
Simon Vancoillie

 

Modulator valve on the transmission is controlled by vacuum from a port on the carb. for example, a new edelbrock actually has a port labeled for that purpose. I know if you use the wrong one it can give you trouble. I believe it is ported vacuum.

if that doesn't do it, use a smoke machine. I built my own with an ammo can. you will blow smoke into the vacuum system and it will leak out from wherever your leak it.

 

If you have an EGR plate under the carb they are known to burn out and leak.

 

Would seem to me that given all the issues with the current carb, replacing it would solve a lot of issues or at least give you a better starting point. You can look at the EGR after that. JMHO.

 

Low vacuum doesn't always mean a leak. if you slowly cover the air horn with a rag does it speed up or die ?

Have you checked your timing ? advance it a little and see if your vacuum picks up noticeably.

 

Late ignition timing can contribute to low manifold vacuum. Verify the TDC mark on the balancer is accurately located, and has not moved off index due to a defective elastomeric bearing using a "piston stop tool." The TDC "0" mark is important because all ignition and valve timing is based on this point.

There is generally a correction factor applied for altitude when performing a vacuum test. Depending on the elevation at your location, 16" or 17" may be considered excellent, where these same figures would indicate a serious defect when at sea level. So some judgment and experience is required when interpreting the results. Late valve timing - often due to a stretched or worn timing chain set - is another characteristic of low manifold vacuum. Have you performed a cylinder compression check?

 

Build a diy smoke machine like this and the smoke will point directly to the leak

 

Dang potheads!

 

We use to trace them down with an automotive stethoscope, if you can hear it this will help you pinpoint it.

 

I second the late ignition timing scenario. Also get a mity vac vacuum tool you can use it to check vacuum devices to see if they hold vacuum like break booster, distributor advance, cruise control, egr valve, etc. Also if you have an intake gasket leak, spraying carb clenaer or other fuel like propen while the engine is running will affect engine rpm and you can find it that way.