I have an 86 f250 460 7.5l holley 4bbl. 4180. It keeps stalling out at red lights, but not untill it has been driving a few miles. I rebuilt the carb to see if this might help, of course not. What is the best way to find a vacuum leak. If anyone has any ideas of what other problems it could be and the order I check I would appreciate your help.

 

Get a can of Carb cleaner or small propane bottle without lighting it >

Open the valve and point it toward connections.

if the engine speeds up when you do this ,you found your leak

 

Dennis is correct, but if you aren’t familiar with the test let me go a step further in explaining.

I like to use carb cleaner because you can point it at a hard to get to spot and shoot, but other things can be used. The areas you want to test are anywhere that air can be pulled into the intake system. Some of those areas are; the union of the intake manifold to the cylinder heads, the EGR system, and the carb mounting point. Also, any vacuum hose (and yours should have plenty if the emissions equipment is still all there) is a potential source of a leak. They will dry rot and break.

Then there are the vacuum diaphragms that many vacuum hoses are supposed to activate. They too will dry rot. The main ones are the brake booster, EGR valve, and the distributor vacuum advance. You may also have diaphragms in the HVAC system and the carb secondary. The best way to test for a bad or leaking diaphragm is to crimp off the vacuum hose – with vice grips or similar tool - and see if the engine speed smoothes out. If the brake booster is leaking, it is so large that you may well hear the hissing. If your hoses are rather brittle crimping them will probably create leaks so you may end up replacing a lot of hose as a matter of principal. On the smaller diaphragms such as the distributor vacuum advance you can pull the hose on the non-distributor end and simply suck on it. If the diaphragm is bad you will be able to draw air through the hose, but if it is still good you will feel the resistance of the diaphragm. Make sure you don’t pull the hose with some kind of switch remaining between the device you are testing and the end you are sucking on. It may give you a false reading of a good diaphragm.

Hope that helps,

Mark

 

also be shure all of your hoses are flexable and like new I had this problem with my 460 all it was that the idle needed turned up because of the cold weather. it would do the same thing as you describe, also check to make shure you choke is working properly

 

I appreciate all your help, I need it desperately. fI have had no time to put into looking as of yet due to 16 hr days at work, which adds to the problem cause I have to drive 40 minutes each way and try to keep it from stalling the whole time. the engine was replaced by previous owner by the factory, is a vacuum leak the most likely thing to be or is there something else I could be in for?

 

There are a number of things that can cause it to stall.

As part of your investigation of a vacuum leak, if it has not had a major tune up in a while I would do that. New plugs, cap, rotor, ignition wires, PCV valve, air filter, crankcase filter, set ignition timing, and an absolute adjustment of the carb. To do this properly you will need both a tach and a vacuum gauge. Plus, confirm that the intake manifold bolts are torqued properly.

As part of the carb adjustment and after all else has been done then you should look for vacuum leaks.

Mark

 

thank you mchild, I did a quick tune up when I bought it in sept. but did only new plugs and wires, cap, and air filter. I was going to do a thorough job if I can find a few hours off. Is there any reason it acts up only after it has been driving at a higher rpm and its warm?

 

There are a bunch of things that change as a motor warms up and will affect how it runs. This may include the ignition system and certainly any possible vacuum leaks. Electrical systems are greatly impacted by heat so if you have a failing component then it will probably show up when the environment is the worse. A vacuum leak may not appear until the heat has expanded and opened up the source of the leak.

As and example, EGR valves are not supposed to be open at idle, but after the motor has warmed up and the temp time switch (on top of the t-stat housing) allows the EGR to get a vacuum source it then may not close at idle or if the EGR diaphragm is bad it will then cause a vacuum leak. But, only after the motor is warm.

Many things can cause the problem you are having, which is why trouble shooting is like detective work. Pulling all the evidence pieces together. That’s why a complete is important and as part of that confirmation that all components are functioning properly.

Mark