Perplexing Vacuum Leak

Matthew K

Matthew K

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Broke Engineer
Hi All-

Have what seems to be a perplexing issue. I replaced the intake and exhaust manifolds on my f100's ford 300 and upgraded to a four barrel carb (single barrel before :skeptical:). I seem to be having a never ending vacuum leak. I started with installing a 1 inch spacer, gasket on each side, noticeable vacuum leak. After checking everything, I replace the gaskets in case the spacer's included gaskets were junk. After replacing, the leak seemed to go away for about ~30 seconds, and then resurfaced, albeit its not as bad as before as I have more idle screw control now. Is it possible I'm doing something wrong with the gaskets?? Seems like a trivial task. The perplexing thing to me is the vacuum leak seemed to be gone at first after replacing.

Admittedly this is my first time swapping carbs on something that's not a lawn mower/atv, so it's possible I've missed something.

Thanks all
 
Do you know for sure that you have a vacuum leak and where it is coming from? There are various ways to figure that out and it needs to be the starting point.

Aside from that, the 240 and 300-6 use shared fasteners to retain both the intake manifold and the exhaust manifold. If the manifold flanges aren't similar in thickness there's a potential for one or the other to leak at the cylinder head.
 
Use some carb spray with the long thin straw and shoot a few burst around the carb, adapter, intake, and vacuum lines. A change in RPM occurs when you find the leak. WD-40 will do the same trick.

4 barrel carb on a 300 (straight six) is a lotta carb. I once owned a Mustang (1967) that had a two barrel on the 200 straight six replacing the single barrel. Made a big difference.
 
mondtster said:
Do you know for sure that you have a vacuum leak and where it is coming from? There are various ways to figure that out and it needs to be the starting point.

Aside from that, the 240 and 300-6 use shared fasteners to retain both the intake manifold and the exhaust manifold. If the manifold flanges aren't similar in thickness there's a potential for one or the other to leak at the cylinder head.
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There’s a high pitched vacuum type noise, so pretty sure it’s a vacuum leak. Not sure on location, need to go ahead and spray around with some starter fluid or something to find as you and @Daleandee suggest. Definitely agree on potential for the manifolds, they were a PIA to install together so it’s worth another look at how the flanges are being held.
Daleandee said:
Use some carb spray with the long thin straw and shoot a few burst around the carb, adapter, intake, and vacuum lines. A change in RPM occurs when you find the leak. WD-40 will do the same trick.

4 barrel carb on a 300 (straight six) is a lotta carb. I once owned a Mustang (1967) that had a two barrel on the 200 straight six replacing the single barrel. Made a big difference.
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Will do. And it’s definitely a big carb but it’s a street demon 625 and has been used in a number of builds successfully that I’ve seen, it has a small primary and then goes to bigger secondary.
 
Matthew K said:
There’s a high pitched vacuum type noise, so pretty sure it’s a vacuum leak. Not sure on location, need to go ahead and spray around with some starter fluid or something to find as you and @Daleandee suggest. Definitely agree on potential for the manifolds, they were a PIA to install together so it’s worth another look at how the flanges are being held.

Will do. And it’s definitely a big carb but it’s a street demon 625 and has been used in a number of builds successfully that I’ve seen, it has a small primary and then goes to bigger secondary.
Click to expand...

You can use a propane torch to find a vacuum leak too. And depending on the intake you have, it may or may not be suitable for use with an exhaust manifold. I know the Clifford manifolds specifically call out for using a header (due to flange thickness differences).

A 625 cfm carburetor is likely more carburetor than that engine will ever be able to use but I wouldn't expect it to be undrivable.
 
Simple things first. Check vacuum lines for cracks, splits or plain off the fitting. Check power brake line from intake to booster.

I had a '76 Ford 250 Van with the 300-6. I put on a 4 bbl intake with a 600 holley, hedman hedder and a camshaft recommended and ground by Ed Iskenderian along with a 4 speed out of a mustang, plus some custom head and valve work. It pulled my race car trailer just fine. Empty it would outrun a stock Chevy 350 pickup. It was a blast to drive, but cornering was a little hairy...
 
Matthew K said:
There’s a high pitched vacuum type noise, so pretty sure it’s a vacuum leak.
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That's indicative of a leaking gasket. The loose gasket will act like a reed and vibrate and howl. Are all the surfaces clean and level? Any old bits of gasket on the head will interfere with sealing.
Zeldman said:
Simple things first. Check vacuum lines for cracks, splits or plain off the fitting. Check power brake line from intake to booster.
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Or open, unused ports on the new carb. I seem to remember two ports on some carbs for the distributor advance, one above the throttle plate and the other just under it, depending on what the engine manufacturer specified for ignition advance at warm idle. Then there's the typical vacuum manifold screwed into the intake manifold, with several nipples on it. All unused ports need to be capped.
 
Dan Thomas said:
That's indicative of a leaking gasket.
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You hit the nail on the head. The spray method helped me track down part of the intake manifold gasket letting air in due to one of the hard to reach bolts being a little looser than I remember. Vacuum leak fixed.

In other news-- I drove the truck a few miles and back to check over things since its been sitting since last summer, and I found that oil is coming out of the dipstick, and no its not overfilled on oil lol. It didn't do that before. I'm thinking either the extra power went ahead and finished off one of the piston rings or perhaps this was already near happening and the engine was so suffocated from the old one barrel that it didn't have enough air to show itself until now.

I've got a compression test kit to figure out which cylinder the blow by is likely coming from...Assuming that is in fact it, it is a tired, 1983 motor with ~140k miles, only thing I'm aware that's ever been done to it is a head gasket 25 years ago. I've always wanted to do an engine rebuild and would be cool to upgrade the internals of this motor, so this might be my opportunity.
 
I have an 86 F150 with 300 six, Holly 4160 with vacuum secondaries, Offy manifold, Cliffordheader and a bunch of other stuff. New or used carb? What make and model? What manifold?

I chased a vacuum leak for a long time. Finally found the bushings for the throttle shaft in the carb were worn, but in a Holley it could be the vacuum diaphragm.

Plenty of places other than a spacer plate for a leak, including the intake/exhaust manifold bolts not being correct. Need more details on your setup.
 
Matthew K said:
You hit the nail on the head. The spray method helped me track down part of the intake manifold gasket letting air in due to one of the hard to reach bolts being a little looser than I remember. Vacuum leak fixed.

In other news-- I drove the truck a few miles and back to check over things since its been sitting since last summer, and I found that oil is coming out of the dipstick, and no its not overfilled on oil lol. It didn't do that before. I'm thinking either the extra power went ahead and finished off one of the piston rings or perhaps this was already near happening and the engine was so suffocated from the old one barrel that it didn't have enough air to show itself until now.

I've got a compression test kit to figure out which cylinder the blow by is likely coming from...Assuming that is in fact it, it is a tired, 1983 motor with ~140k miles, only thing I'm aware that's ever been done to it is a head gasket 25 years ago. I've always wanted to do an engine rebuild and would be cool to upgrade the internals of this motor, so this might be my opportunity.
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Aha, didn't see this. Glad you found your leak.

140K on a 300 six is nothing. It's probably the previous head gasket job.
My six is bored .030 over, decked with stock pistons, RV cam, the Holley 450 cfm 4bbl with Offy dual plane intake with heat plate, the Clifford header and 21/2 inch custom exhaust. Ignition is a Ford Duradpark II with a GM module. Runs like a scalded cat, and with Rev to 5000 no problem, but all your making there is noise. Torque tops at around 3k.

Couple things you'll want to watch out for. Without a heated manifold you'll have a cold stumble that's impossible to tune out. Ford TFI ignitions suck, and will leave you on the side of the road. And a port and polish is very important, unshrouding the valves is critical to getting this engine to flow.

In addition to being a strong motor, it's fun when someone says " " I've never seen a built six like that before".
 
Chip Sylverne said:
Aha, didn't see this. Glad you found your leak.

140K on a 300 six is nothing. It's probably the previous head gasket job.
My six is bored .030 over, decked with stock pistons, RV cam, the Holley 450 cfm 4bbl with Offy dual plane intake with heat plate, the Clifford header and 21/2 inch custom exhaust. Ignition is a Ford Duradpark II with a GM module. Runs like a scalded cat, and with Rev to 5000 no problem, but all your making there is noise. Torque tops at around 3k.

Couple things you'll want to watch out for. Without a heated manifold you'll have a cold stumble that's impossible to tune out. Ford TFI ignitions suck, and will leave you on the side of the road. And a port and polish is very important, unshrouding the valves is critical to getting this engine to flow.

In addition to being a strong motor, it's fun when someone says " " I've never seen a built six like that before".
Click to expand...
What makes you think its head gasket related? Only times I've heard of blow by showing symptoms like mine are from worn piston rings.
 
Matthew K said:
In other news-- I drove the truck a few miles and back to check over things since its been sitting since last summer, and I found that oil is coming out of the dipstick, and no its not overfilled on oil lol. It didn't do that before.
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You have mussed up the PCV system. That system draws fresh air from the air cleaner through the crankcase and out the rocker cover via the PCV valve, and into the intake manifold. The suction end of things isn't working, and maybe the fresh air supply, too. The crankcase builds pressure and blows the dipstick out. It will soon blow a crankshaft seal and you will be taking a lot more stuff apart.

So what did you do with that PVC plumbing?

1704558612899.png
 
Mr. Thomas nailed it.
Bad rings won't blow oil out the dipstick unless the crankcase vents are plugged.
 
Dan Thomas said:
You have mussed up the PCV system. That system draws fresh air from the air cleaner through the crankcase and out the rocker cover via the PCV valve, and into the intake manifold. The suction end of things isn't working, and maybe the fresh air supply, too. The crankcase builds pressure and blows the dipstick out. It will soon blow a crankshaft seal and you will be taking a lot more stuff apart.

So what did you do with that PVC plumbing?

View attachment 124050
Click to expand...
So I may have a bit of a misunderstanding with this PCV system, and maybe some info that collaborates a potential rings issue.

IMG_9012.JPG
This is a picture from when things were still being put back together. My prior thinking was that both the front and rear hoses acted as breather tubes. The front one piped into the air filter intake before, and up until about 30 seconds ago I had been thinking the rear tubes just breathed to outside air. I just found an old photo that definitely shows the rear tubes connected to something on the carb or intake manifold.

So what I had done with this prior thinking, was cap off the front tube as it had been throwing some oil into the old carb, although seemingly a very manageable amount, and left the back breather tubes to vent to air.

Now with last nights situation, the rear tubes were unquestionably throwing oil out, so they are breathing.

So how off base am I here? Given that the rear tubes can still breath, and that the intake on the prior carb had oil in it, is there definitely still rings on their way out?
 
Zeldman said:
There is always that possibility, but the first sign of weak rings is usually bluish gray smoke out the tail pipe.
Click to expand...

Reminds me of a very funny story. Years ago a friend had a '63 Chevy with a worn out 283 that made a bunch of smoke but still ran pretty good. He got into a red light race one night. He did beat the other guy but the police stopped him a wrote him a ticket ... for excessive smoke. He was so embarassed!
 
Matthew K said:
So I may have a bit of a misunderstanding with this PCV system
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The PCV valve in the rear has to be hooked to a vacuum source (intake manifold or carb). The PCV valve is normally closed and vacuum opens it. I think you just need to hook that back up to vacuum to properly vent the crankcase.
 
Yes, the PCV system is an "emission control", but one of the side effects is a very significant increase in engine longevity due to the fact that you don't have moisture and combustion byproducts building up in the oil. I would hook it back up.
Now, are the rings bad?
Matthew K said:
I've got a compression test kit to figure out which cylinder the blow by is likely coming from...Assuming that is in fact it, it is a tired, 1983 motor with ~140k miles, only thing I'm aware that's ever been done to it is a head gasket 25 years ago. I've always wanted to do an engine rebuild and would be cool to upgrade the internals of this motor, so this might be my opportunity.
Click to expand...
Could be that it has set for a long time somewhere and one or more bore accumulated a lot of rust. Or, it has 240k on it (I don't remember when they added the extra digit to the odometer). Or...

How much oil does it burn? 1000 miles per quart would be on the high side for my tastes. Blue smoke (as others have mentioned) is another indication. Multiple bad rings look like this: :)

1704569826939.png
 
Matthew K said:
What makes you think its head gasket related? Only times I've heard of blow by showing symptoms like mine are from worn piston rings.
Click to expand...
Because even with a modicum of maintenance and the occasional oil change, a 300 six should go well beyond 145K. Redneck compressiontest: warm it up, let it idle, remove the oil filler cap and hold your hand over the hole. Little gentle puffs are what you want out of an engine with that mileage.
 
Another easy tip about checking for bad compression rings: Your engine oil will be much hotter than normal. That hot combustion gets past the rings to heat oil in the sump.
 
Daleandee said:
Reminds me of a very funny story. Years ago a friend had a '63 Chevy with a worn out 283 that made a bunch of smoke but still ran pretty good. He got into a red light race one night. He did beat the other guy but the police stopped him a wrote him a ticket ... for excessive smoke. He was so embarassed!
Click to expand...

I had a 283 in a '64 Impala that leaked and smoked so bad that I just changed the oil filter every 6 months. It was in a constant state of oil change...
 
Zeldman said:
I had a 283 in a '64 Impala that leaked and smoked so bad that I just changed the oil filter every 6 months. It was in a constant state of oil change...
Click to expand...
I never had to worry about what to do with my used engine oil. A guy in the back woods had a truck that used so much oil. He would take all my old oil to keep feeding the engine.
 
Alright so on a compression note, (I ran this cold, so it would just increase from here), 5 of my cylinders sat at 120psi, and one at 150psi. While I'm intrigued by the one cylinder at 150, all of the cylinders seem to be alive.

So I'll have to figure out how best to rerun the hoses for the PCV system and report back.
 
Matthew K said:
This is a picture from when things were still being put back together. My prior thinking was that both the front and rear hoses acted as breather tubes. The front one piped into the air filter intake before, and up until about 30 seconds ago I had been thinking the rear tubes just breathed to outside air. I just found an old photo that definitely shows the rear tubes connected to something on the carb or intake manifold.
Click to expand...
The old air cleaner had a "moustache" filter in it to clean the air being drawn into the crankcase. It did not take air from inside the engine's air filter, as a backfire (from the carb, not the tailpipe) could conceivably set off the flammable vapors in the crankcase. That tends to blow oil pans off. The engine's filter acted as a flame arrestor. That mustache filter did get oily, due to blowby at idle when the airflow though the engine is low. No big deal.

The PVC valve has a check-valve function if the engine backfires. The valve seats and prevents flame setting off the fumes in the case. During idle, the manifold vacuum is high, and pulls the check off its seat a long ways and up against a stop and close to another seat so that only small amounts of air are fed to the manifold. Too much would lean the mixture and the engine would quit. With the throttle open, the vacuum falls and the valve falls so that more air is drawn through the valve. That valve is adjusting itself all the time. It gets coked up with varnish and stuff and can start sticking. Take it out and look inside. Shake it to see that the valve is rattling freely. The hose should go to a vacuum manifold or nipple on the intake manifold. The airflow is too high to find a nipple that big on the carb itself.

You have replaced the original air cleaner with an open filter. That's asking for carb ice and rotten performance in dampish weather at almost any temperature. The old air cleaner had a thermostatic valve in it to work a flapper that directed warm air off the muff on the exhaust manifold into the air cleaner to keep the temp high enough to prevent icing. The engineers that designed that engine knew what they were doing, and the stuff some people take off to "improve" things can make things much worse.
 
Matthew, work on the stuff you changed, don't start looking at rings, pistons, head gaskets whatever. I was going to suggest the pcv system. I would unplug it from the block and seal it off to see if your problems disappear or at least get better. Those valves are relatively cheap if it hasn't been replaced.

Also make sure you have the carb configured correctly and that it isn't defective. If all else fails put the old carb back on to see if that solves the issue.
 
Matthew K said:
You hit the nail on the head. The spray method helped me track down part of the intake manifold gasket letting air in due to one of the hard to reach bolts being a little looser than I remember. Vacuum leak fixed.

In other news-- I drove the truck a few miles and back to check over things since its been sitting since last summer, and I found that oil is coming out of the dipstick, and no its not overfilled on oil lol. It didn't do that before. I'm thinking either the extra power went ahead and finished off one of the piston rings or perhaps this was already near happening and the engine was so suffocated from the old one barrel that it didn't have enough air to show itself until now.

I've got a compression test kit to figure out which cylinder the blow by is likely coming from...Assuming that is in fact it, it is a tired, 1983 motor with ~140k miles, only thing I'm aware that's ever been done to it is a head gasket 25 years ago. I've always wanted to do an engine rebuild and would be cool to upgrade the internals of this motor, so this might be my opportunity.
Click to expand...

Check your crankcase vent system before assuming something is wrong with the engine. You would have messed with it during the intake and carburetor install.
 
Daleandee said:
Agreed. I was thinking back to the days when guys would come into the store and buy a 850cfm Holley and put it on a stock 350 and wonder why it it would billow out black smoke and stumble all over itself ...
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A high school buddy installed an 850 dual feed double pumper on his 1969 Mustang running a stock 351. He couldn't understand why it became undrivable.
 
Anyone know where to get a replacement cap for the hose that goes to the air filter? My new air filter has a hole for the pcv hose but I need a new cap. Having a hard time finding a replacement with a proper hole in it.
 
Dan Thomas said:
The old air cleaner had a "moustache" filter in it to clean the air being drawn into the crankcase. It did not take air from inside the engine's air filter, as a backfire (from the carb, not the tailpipe) could conceivably set off the flammable vapors in the crankcase. That tends to blow oil pans off. The engine's filter acted as a flame arrestor. That mustache filter did get oily, due to blowby at idle when the airflow though the engine is low. No big deal.

The PVC valve has a check-valve function if the engine backfires. The valve seats and prevents flame setting off the fumes in the case. During idle, the manifold vacuum is high, and pulls the check off its seat a long ways and up against a stop and close to another seat so that only small amounts of air are fed to the manifold. Too much would lean the mixture and the engine would quit. With the throttle open, the vacuum falls and the valve falls so that more air is drawn through the valve. That valve is adjusting itself all the time. It gets coked up with varnish and stuff and can start sticking. Take it out and look inside. Shake it to see that the valve is rattling freely. The hose should go to a vacuum manifold or nipple on the intake manifold. The airflow is too high to find a nipple that big on the carb itself.

You have replaced the original air cleaner with an open filter. That's asking for carb ice and rotten performance in dampish weather at almost any temperature. The old air cleaner had a thermostatic valve in it to work a flapper that directed warm air off the muff on the exhaust manifold into the air cleaner to keep the temp high enough to prevent icing. The engineers that designed that engine knew what they were doing, and the stuff some people take off to "improve" things can make things much worse.
Click to expand...
I'm not disagreeing with you, but why would carb ice be an issue here when the setup is exactly like most old carb systems? Or did old carb/muscle cars experience this and this was aimed at fixing it?

Understood on the PCV system. I have learned a lot about this system the past couple days that I did not know about before. I went ahead and bought a new PCV valve for a few bucks since a bunch of oils been forced through the old one.
 
Matthew K said:
I'm not disagreeing with you, but why would carb ice be an issue here when the setup is exactly like most old carb systems? Or did old carb/muscle cars experience this and this was aimed at fixing it?
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It was a factory system. Some explanation of carb heat in both cars and airplanes:
https://en.wikipedia.org/wiki/Carburetor_heat

1704773557220.png

You might get away without it. And you might not. The car will bog down and have almost no power, and all you can do is pull over, shut it off, and wait awhile.

Been there, done that when I was young and knew no better.
 
Matthew K said:
There’s a high pitched vacuum type noise, so pretty sure it’s a vacuum leak. Not sure on location, need to go ahead and spray around with some starter fluid or something to find as you and @Daleandee suggest. Definitely agree on potential for the manifolds, they were a PIA to install together so it’s worth another look at how the flanges are being held.

Will do. And it’s definitely a big carb but it’s a street demon 625 and has been used in a number of builds successfully that I’ve seen, it has a small primary and then goes to bigger secondary.
Click to expand...

You can put fuel hose near your ear and use the other end as a stethoscope too


Sent from my iPhone using Tapatalk
 
MIFlyer said:
You can put fuel hose near your ear and use the other end as a stethoscope too
Click to expand...
I have a cheap mechanic's stethoscope that I modified. I took off the probe and diaphragm assembly and stuck a length of 1/4" aluminum tubing into the stethoscope tube. It will find the tiniest leak as air blows across the end of that tube.

BUT BE CAREFUL! That affair could blow you eardrums out if it got near any significant leak, or a vacuum source. Magman's idea of a hose near the ear is safer, but not as sensitive.
 
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