I have a 78 ford with a 300 i6 with a single bbl Carter YFA carb. when i got the truck the vac. lines were a jungle and it barely run. it has 150,000 miles and the carb has been rebuilt and all the lines replaced. The engine idles good but will bog a little bit at mid throttle and sometimes alot. i have a va. gauge nd i have adjusted it properly but it still does it. the dist. advance is connected to the port on the side of the carb on the throttle blades and gets about 7hg of vac at idle. I tried to put the dist advance on the other port higher up so it gets no advance at idle, but it died so i readjusted the carb for that setup and it dieseled when shut off. it ran great with no bog ping or loss of power. but still dieseled and idled rough so i switched it back. the truck still has a small loss of power at mid throttle. i am unsure what to do because i cant find a clear answer if the engine should have that advance to the timing at idle. can anyone help?!?!?!?!?!

 

For now, you want the "ported" vacuum connection to the distributor. No vacuum signal at idle. Be sure that the idle RPM is at factory spec (or lower) when making ignition timing or carburetor adjustments.

A healthy stock engine should draw 18" to 20" of steady manifold vacuum at factory idle RPM at sea level. Have you performed a cylinder compression test? Can also test compression using a vacuum gauge.

Make sure ignition timing is set correctly before carb tuning. 150k is a lot of miles btw, the damper ring that timing marks are on may have slipped, this makes the marks innacurate. The distributor itself may not be advancing and retarding smoothly, vacuum advance diaphragm leaky, etc. If you're only pulling 7" of manifold vacuum that's a problem.

Ignore the timing marks for purposes of this test, and whatever the current timing setting is when using a light, put that away in a drawer or something and forget about it for a while. Try advancing the distributor timing for maximum manifold vacuum achieved with engine at factory idle RPM, then, back off about 1" as indicated on the dial, this should be around 18" or 19" of vacuum indicated at sea level. This will be very close to optimum timing for pump gas.

Then readjust carb for best idle and idle mixture. Make sure idle transition slot is not overexposed, we want only the idle circuit to be in play at idle, don't want the transition circuit cutting in at idle. Basically when tuned properly the throttle butterfly should be almost completely closed at idle, this is why tuning instructions start with idle mixture screws backed out so far 1.5 turns say - so it will start and idle. Then tuning can actually be performed from that point. Start turning in the idle mixture screw (leaning it out), and as the idle RPM starts to rise, turn the RPM screw out to lower RPM back down to spec. Then go back and turn the idle mixture screw in some more - the carb will start to hiss a little maybe - good, though it's best to have the air cleaner installed while adjusting. The idea generally is to lean out the idle mixture as far as practical while still maintaining a smooth idle, where it just starts to drop off the RPM and start to stumble. Might want to back the mixture screw out slightly from that point.

Don't mess with idle mixture again unless outside temperature swings wild, might want to fatten it up some in the wintertime. If there's still a problem it lays elsewhere, with the accelerator pump circuit, or maybe a leaky power valve, vacuum leak etc. In the latter case it would need to be retuned once fixed.

 

The dizzy should use the no-idle port. You want to set your static timing with the vac advance disconnected, and then connect the line. Then adjust your carb according to that, and see what you've got.

Mine bogs and the jerks when I get on the throttle, and I'm certain that I need to adjust the accelerator pump.

 

Its usually best to get ignition dialed in first before carb adjustments. Set timing with vacuum advance removed & plugged.

Sounds like a stock setup so stock specs for timing. Then use "ported" vacuum for the advance as mentioned.

Inadequate timing could cause low idle vacuum and other issues.

Also, as mentioned there is a decent chance the timing marks are not accurate. Here is how to check with the "positive stop" method before checking timing.

 

Hopefully the attached will help. There is another vacuum port just above and to the rear of the S port (the one at the base of the carb) - check and make sure if it is capped that the cap is in good shape...Mine was originally capped and I had a vacuum leak from a bad cap. Sounds like you have a vacuum leak somewhere.

Story of the say - Saturday I got in my truck, started it... engine was racing, it died when put into gear, and my brakes were rock hard. Opened the hood and sure enough - the line had worked its way off the vacuum valve on the booster and was just hanging there. Instant fix and it was fine...what was a little spooky was that the last one to drive it was my grandson, who is not an experienced driver - can only imagine the combination of non-existent brakes/racing engine/rookie driver. Glad the hose decided to fall off when it did!