mat in tn Posted February 12, 2023 Share Posted February 12, 2023 David/1986F150Six had a similar problem and just could NOT find a vacuum leak. He tried squirting brake cleaner everywhere, to no avail. Finally he tried the smoke test and immediately found the problem - a vacuum cap had apparently split - in all senses of the word. What he did was to get a cheap cigar, block off the inlet to the carb, and puff smoke into a large vacuum line - either the PCV line or the brake booster hose. That put smoke throughout the engine and magically a wisp of smoke came out from under the intake manifold on that 300. Upon close inspection he found a port was open way back in the depths where he couldn't see nor get brake cleaner to. Ditto. Happened to me twice on my 300... and both times I caught it with a party fogger converted to a cheap smoke machine (I don't smoke, and have no intention to start!). The whole top of the cap was gone in both cases, resulting in a very rough/unstable idle. If the caps are old, I'd replace all of them as insurance. go the extra inch and get a pkg. of vinyl vac caps. they do not degrade the way rubber does. these are the pretty bright color coded ones. Link to comment Share on other sites More sharing options...
delco1946 Posted February 12, 2023 Author Share Posted February 12, 2023 go the extra inch and get a pkg. of vinyl vac caps. they do not degrade the way rubber does. these are the pretty bright color coded ones. Tried smoking out my engine and couldn’t find any leaks :/. Not sure what to do next. Link to comment Share on other sites More sharing options...
FuzzFace2 Posted February 12, 2023 Share Posted February 12, 2023 Ditto. Happened to me twice on my 300... and both times I caught it with a party fogger converted to a cheap smoke machine (I don't smoke, and have no intention to start!). The whole top of the cap was gone in both cases, resulting in a very rough/unstable idle. Could you post up in the garage / tool area how you did the party fogger to smoke machine. I have a car club member that has a vacuum leak that I think is between the intake & heads (v8) as we have not found it in the quick checking 2 of us have done so far. I want to fill the crank case with smoke and see if it will come out the carb. It started with the idle speed going to about 1500. Told he changed carbs and it is still doing it. I need to get some time to get with him to really look it over because it was only a min or 2 before going to a show the last time I looked it over. Dave ---- Link to comment Share on other sites More sharing options...
Gary Lewis Posted February 12, 2023 Share Posted February 12, 2023 Tried smoking out my engine and couldn’t find any leaks :/. Not sure what to do next. So the problem you are now having is that the idle isn't smooth but it is idling best with the screws 1 1/2 turns out? Right? Link to comment Share on other sites More sharing options...
85lebaront2 Posted February 12, 2023 Share Posted February 12, 2023 Tried smoking out my engine and couldn’t find any leaks :/. Not sure what to do next. So the problem you are now having is that the idle isn't smooth but it is idling best with the screws 1 1/2 turns out? Right? I found years ago that the 351M and 400 engines were a problem, Ford actually got fined over them by the EPA. In order to idle smoothly the mixture has to be quite rich. For those of us who did not have emission inspections, opening up the idle mixture tubes in the cluster worked wonders. Link to comment Share on other sites More sharing options...
Gary Lewis Posted February 12, 2023 Share Posted February 12, 2023 I found years ago that the 351M and 400 engines were a problem, Ford actually got fined over them by the EPA. In order to idle smoothly the mixture has to be quite rich. For those of us who did not have emission inspections, opening up the idle mixture tubes in the cluster worked wonders. Bill - You'd recommend opening up the idle screws more to smooth out the idle? Link to comment Share on other sites More sharing options...
delco1946 Posted February 13, 2023 Author Share Posted February 13, 2023 Tried smoking out my engine and couldn’t find any leaks :/. Not sure what to do next. So the problem you are now having is that the idle isn't smooth but it is idling best with the screws 1 1/2 turns out? Right? They’re currently about 2.75 turns out. However if I place a rag/ gloved hand over the carb it clearly idles better and smoother. I think the goal would be to pass less air thru the carb rather than simply increase the gas thru the carb as that’s why I’m failing emission and getting poor mileage. Also, I have noticed that the hissing/ sucking noise from the top of the carb/from inside the carb is quite loud. Is that normal? Link to comment Share on other sites More sharing options...
delco1946 Posted February 13, 2023 Author Share Posted February 13, 2023 Ditto. Happened to me twice on my 300... and both times I caught it with a party fogger converted to a cheap smoke machine (I don't smoke, and have no intention to start!). The whole top of the cap was gone in both cases, resulting in a very rough/unstable idle. Could you post up in the garage / tool area how you did the party fogger to smoke machine. I have a car club member that has a vacuum leak that I think is between the intake & heads (v8) as we have not found it in the quick checking 2 of us have done so far. I want to fill the crank case with smoke and see if it will come out the carb. It started with the idle speed going to about 1500. Told he changed carbs and it is still doing it. I need to get some time to get with him to really look it over because it was only a min or 2 before going to a show the last time I looked it over. Dave ---- I held 5 small cigars to one end of my shop vac and held the blower side hose up to and taped to the brake booster line. The cigars burn fast but it def blow a lot of smoke out the top of the carb at the rag! Link to comment Share on other sites More sharing options...
85lebaront2 Posted February 13, 2023 Share Posted February 13, 2023 Bill - You'd recommend opening up the idle screws more to smooth out the idle? He's between a rock and a hard place with that engine and emission testing. I would rather try to get a hot cammed eye burning exhaust small block Chevy through. If he opens the screws he will be richer. One thought, fresh plugs gapped a bit wide, oil change, new PCV valve and set it to factory mixture. This is what some of my friends in California do before their tests. A wider plug gap will fire a leaner mixture reliably. Link to comment Share on other sites More sharing options...
FuzzFace2 Posted February 13, 2023 Share Posted February 13, 2023 I held 5 small cigars to one end of my shop vac and held the blower side hose up to and taped to the brake booster line. The cigars burn fast but it def blow a lot of smoke out the top of the carb at the rag! Thanks for that I Googled on how to make one also. Dont know if I want to take up smoking (again) so may make one. Thanks again Dave ---- Link to comment Share on other sites More sharing options...
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