I just hooked up a vacuum gauge and the results dont mean a whole lot to me so I thought I would share and ask for feedback. Below is a link and if you click on scenario 2 it pretty much describes my exact ready so I figure thats my issue, but not sure what I need to fix the issue. I am getting a pretty steady reading at idle of 12-14ish, which seems low but I am not sure. Thanks for any advice, sorry just to ask questions hope I can contribute more as I learn.

How to Use and Interpret a Vacuum Gauge

 

Well maybe a combination of scenario 11 and 2, I thought my timing was advanced not retarded, but I will check again tomorrow. Also, my plugs are gapped at .40 and I believe they need to be at .35, so I guess I will remove and re gap them. Any other ideas or do you think my ideas are off base.

 

I guess I could mess with the idle mixture as well, ok done brain storming for now, thanks for you guys thoughts and help

 

Scenario #2 is normal; vacuum drops as the throttle opens and manifold pressure equalizes to atmospheric pressure.

As for #11, don't use a vacuum gauge to adjust timing. Use a timing light, and nothing else. Once you set the timing with a timing light, check the vacuum gauge. If it still reads low, try adjusting the idle mixture.

 

Ok, I was going to use a timing light to adjust timing, sorry I didnt indicate that, but thanks for looking out. I will work on the timing and then re check it. Thanks again

 

Alright, the timing was set at*8 and I advanced it to *12, the vacuum gauge is holding steady right at 14, which my understanding is it should be closer to to 20. It has a hesitation when I hit the throttle hard and it has started backfiring. My thoughts, on where to go, please correct me if my thinking is wrong. I need to adjust the idle mixture, possibly regap my plugs to a smaller gap, thats all I got, which doesnt seem like a lot.

Anyone have better advice on what to do next to try and improve my readings and fix my back fire and hesitation?

Thanks

 

You're on the right track - now use the vacuum gauge to adjust the idle mixture. Setting the timing wasn't supposed to increase your vacuum reading - it was to eliminate it as a problem and establish a baseline so you can move on.

With the truck shut off, gently turn both screws in until they seat, then back them both out 1.5 turns. Start the truck with the gauge connected, and move both screws in or out (most likely out) until the needle pegs.

 

Pegs isn't the proper thing to look for since the vacuum gauge may have readings all the way up to 100 and you will never get it to peg that high.

Adjust the idle mixture screws until the vacuum readings don't go up any more.

 

You are absolutely correct. I meant to say that it holds steady and is at a maximum - thank you for the clarification.

 

Ok, thanks for the responses thus far. Here is what I tried to today and the readings I am getting. I hooked up the vacuum gauge and it holds pretty steady at 14. I adjusted the idle mixture screws in and out and it had no effect at all on the reading, starting at 1.5 turns out from seating and turning in pretty quickly roughens the idle, turning them out I can just about remove the screws with very little to no effect on the vacuum or the idle.

In doing some reading on here last night I checked the vacuum advance and it appears that it is not functioning properly as I did as someone recommended and detached the hose from the carb and sucked on it and it was like sucking air through a straw no vacuum created. So I anticipate this being an issue. By the way I have a Mallory distributor.

I havent yet regapped the plugs I didnt have time to do this morning so I need to let the engine cool a bit before pulling them.

The time is set *12, but as far as I know this could be a little off since I am probably not idling at the right rpms anyway. Just guessing.

Oh, and it still hesitates bad if I gun the throttle and then backfires through the carb.

Anyway, as frustrating as this is I am bound and determined to figure it out. So I still greatly appreciate any suggestions.

 

One last thing, when I gun the throttle the vacuum gauge drops to 0 and when I let off the throttle it rises to about 23

 

This is a textbook vacuum leak - idle mixture screws have no effect, bogging down, and backfiring. Check all hoses for leaks and make sure no vacuum fittings are left open. Try idling the motor and spraying a can of carb cleaner around areas that may have a vacuum leak - if a leak exists, the idle will jump as the motor takes in the carb cleaner through the leak. Common areas to look for a leak include the brake booster and base of the carburetor.

You do need to replace the vacuum advance if it won't hold vacuum; however it's not related to your other problems. Good job so far though; it's good that you're using the proper tools and taking a step-by-step approach to the problem.

 

Thanks for the encouragement, I sure need it. I have a friend that basically said the same thing in regards to finding and solving any and all vacuum leaks. I have a 1" spacer for my carb that doesn't seem true and has a little wobble in it. This is the second spacer I bought from summit in the past 2 weeks one aluminum and one phenolic and they seem to have the same problem. I think there is a slight leak because of that because it does rev up a bit when I spray around the base. I guess its no spacer for me and go back to just a gasket. I have checked the hoses and vacuum tree and no leaks there. I will check the brake booster too.

Thanks again for the advice, I am grateful.