Vacuum leak ?

[Robert]

Bought a fresh can of carb cleaner and started spraying individual hoses around top of engine, not idle increase - - till I sprayed in between the 2 four barrels, then I got definite idle increase.

Also, rubber bushing where PCV valve mounts into valve cover a bit loose, got to source one.

Auto parts manager told me to take PCV and soak it inside a small glass bottle with carb cleaner overnight, then spray out again. He said rarely PCV's need replacement, mostly cleaning if caught early enough.

Guess I'll replace all the little plastic caps over brass carb vacuum tubes first inside, see if that works.

Otherwise, gonna have to remove both carbs to inspect manifold, carb bases, and replace both carb gaskets (after checking all mating surfaces with straight edge).

If plastic cap replacement does not work - - - if I can get my hand inside between the 2 carbs might try smearing silicone glue along the carb bases one at a time, dry overnight, then re-test.

Anything I am overlooking ?

 

[Mike]

Auto parts manager told me to take PCV and soak it inside a small glass bottle with carb cleaner overnight, then spray out again. He said rarely PCV's need replacement, mostly cleaning if caught early enough.

I keep threatening to write a book on the stupidest things I've ever heard. If the book gets written, this will appear in the prologue.

Imagine a parts store manager turning down the opportunity to sell both a can of carb cleaner and a new PCV valve. You know, that reminds me of the people who used to say that if you shook a PCV valve and it rattled, it was OK. My question was if the plunger in the valve was broken, which would cause the valve to rattle twice as much, would that make the valve twice as good?

Seriously, just buy the flipping valve and replace it. Save your carb cleaner to, uhhhh, let me see, well, clean carbs with?

If plastic cap replacement does not work - - - if I can get my hand inside between the 2 carbs might try smearing silicone glue along the carb bases one at a time, dry overnight, then re-test.

If air can get in, raw fuel can get out. And that will eat up the silicone. Buy a couple of good, cold-flow gaskets, replace them and be done with it.

Two quality carb base gaskets and a PCV will run you under a double-sawbuck. Just do it.

 

[railroad]

You may have stumbled upon the source of many related issues. I would just block off the pcv valve and hoses, probably the cause of running lean, overheating, high idle, flat spots, etc. It's a hot rod, stick a couple of breathers in each valve cover and call it a day. If you still detect a vacuum leak, it will need correcting. PCV valves are not one size fits all. The manufactures designed them to operate with the vacuum levels of their engines. For this they served their purpose. That is not to say, there is a specific one that will work with your engine and vacuum range.
Good luck,

 

[PotvinGuy]

Ah, PCV valves. You can buy hot rod PCV valves or breathers with PCV's built in, but as RAILROAD points out, one size can not fit all. I tried the usual arrangement of an intake breather on one valve cover and a PCV valve on the other cover. Noticed that the intake breather was letting fumes come out and oil down the firewall. Then used a trick from my old Karmann Ghia. Intake breather on one cover, and a hose from the other cover to the air cleaner. The intake is constantly sucking fumes from the crankcase, and no fumes from the intake breather. No PCV needed.

 

[Screaming Metal]

Thats my favorite hose routing for non-pontiac motors. My Old 421 Drag Catalina had the 2x4 intake on it, the valley pan had the grommet and pcv in it (Came from the factory like that) and both the covers had breathers till I added collector hoses to keep ring flutter down. After a hard run, it'd mist oil slightly and the collector hoses cured it.
As PotvinGuy said, it works....vents the whole motor.

 

[Mike]

With the exception of what happens when the motor starts pulling oil vapor (and we all know it will) into the air filter, and from there into the carb. if you've rebuilt more than two carbs, you know which ones have been eating oil and which ones haven't. Get a bit of oil moving through a Holley and then the air bleeds start attracting dirt like magnets. Suddenly, the carb is getting blamed for all the evil in the world, even though it was victimized by the oil vapor.

I don't even like the idea of sticking breather hoses onto the base plates of carbs. Carburetors are meant to meter air and gasoline, not oil.

And @Robert, please understand I was laughing at the advice your parts guy gave and not laughing at you.

 

[Keeper]

Glad you started tracking those down. All your symptoms from your posts pointed to a huge vacuum leak.

Hopefully fixing those up will get you up and running smoothly.

 

[Screaming Metal]

You may have stumbled upon the source of many related issues. I would just block off the pcv valve and hoses, probably the cause of running lean, overheating, high idle, flat spots, etc. It's a hot rod, stick a couple of breathers in each valve cover and call it a day. If you still detect a vacuum leak, it will need correcting. PCV valves are not one size fits all. The manufactures designed them to operate with the vacuum levels of their engines. For this they served their purpose. That is not to say, there is a specific one that will work with your engine and vacuum range.
Good luck,

Yeah, Keep, we kinda hinted around on that on the other post, on the 1st page, about vaccum leaks. Thought a vaccum gauge would be hooked up, [sigh]...oh well...anyway.
It'd be worth yanking the carbs off and getting new gaskets on there, doing it right, since all else is unknown at this point about the motor. But thats just me.
I'm with Mike, the Oil could cause a mess with the carbs. NEVER EVER RUN A OIL BREATHER LINE UP INTO THE AIRCLEANER, I DON"T GIVE A DAMN IF THE DIRECTIONS SAY SO OR THE MFG'ers SET IT UP LIKE THAT. Said oil will FOUL a carb, as per Mike....
He said there were no places tap in on the intake for vaccum, which is very strange. The only intakes I have seen with no NPT fittings on the manifolds were/are race manifolds....
Usually on the 2x4 intakes, the rear of the front carb has a hose line that most folks block off. Rear carb has one, too. I've seen many times when folks first start a motor, it'll sometimes backfire while setting the timing, blowing a vaccum plug off.
I would check all those vaccum plugs and lines before ripping things loose, it just might be that simple....
It has happened before....

 

[Golly]

If you have carb spacers under the carbs, check to see it you haven't bent the mounting ears down a bit on the baseplates of the carbs due to the extra gaskets compressing. They leak under the middle of the baseplates sometimes with cheap paper gaskets.

 

[bobs66440]

This is a temporary fix I used on my '55 Chevy's 402. When I first got it, it was using 1 qt of oil in 100 miles! On a fresh rebuild! To see if the PCV was the cause, I installed a makeshift catch can (peaces jar, lol) and bingo! That jar would fill up in about 5 miles! I have the hose routed to the carb base. Turned out the valve cover had no baffle...and the PCV was located very low on the valve cover so it was sucking all the oil out and into the intake. I moved the PCV location farther up on the cover (shown), added a rubber grommet baffle, Mopar style oil separator and viola! oil consumption went to almost zero (well, not quite zero, but you get the point). BUT, if I remove it, crankcase pressure pushes oil out of the rear main seal and even the dipstick tube. So, this motor at least needs the vacuum to ventilate the engine. I've had many engines and this is the first that acts this way. Normally I would say that it has excessive blowby, but it runs great and doesn't use oil so....I've read that some BB Chevys are like this.

 

[Screaming Metal]

This is a temporary fix I used on my '55 Chevy's 402. When I first got it, it was using 1 qt of oil in 100 miles! On a fresh rebuild! To see if the PCV was the cause, I installed a makeshift catch can (peaces jar, lol) and bingo! That jar would fill up in about 5 miles! I have the hose routed to the carb base. Turned out the valve cover had no baffle...and the PCV was located very low on the valve cover so it was sucking all the oil out and into the intake. I moved the PCV location farther up on the cover (shown), added a rubber grommet baffle, Mopar style oil separator and viola! oil consumption went to almost zero (well, not quite zero, but you get the point). BUT, if I remove it, crankcase pressure pushes oil out of the rear main seal and even the dipstick tube. So, this motor at least needs the vacuum to ventilate the engine. I've had many engines and this is the first that acts this way. Normally I would say that it has excessive blowby, but it runs great and doesn't use oil so....I've read that some BB Chevys are like this.

Alot of diff. motors respond in diff. ways to diff. PCV setups.... especially long stroke motors. I'd say 95% of the time, the stock baffles in the right valvecovers usually cure oil pickup into the PCV system. On a strong long stroke motor, I've seen a misplaced PCV Hose/valve in the valvecovers wreak havok with other tuners and builders, causing a major mess.
On one particular Mountain Motor, I had to make a baffle can similar to the setup Bob made. Motor was strong, and healthy, but because of the long stroke/big bore it had a mess going on in the crankcase. And it leaked at the seals, also, but it won alot of races at Major Meets. I really had to work to keep it from oiling down a track.
I've also seen what Golly has described happen also. I've also seen porous castings causing vaccum leaks.
Now, we know Roberts motor has a leak at the bottoms there somewhere by the carbs bottoms....so I'd just dive in and fix it. God only knows what OTHER MESS he might find in there. Maybe a chunk busted out and JBWelded into place....hope not but you never know....
Close visual inspection with a flashlight, and a inspection mirror and some quality time with his motor might turn up a clue to the vaccum leak....might even be a simple, cheap fix....
If not a open/missing Vaccum cap on a line, probably a leaking base gasket. Maybe even loose base to throttle body screws....
Are the carbs Barry Grants/Demons by any chance?

 

[Swampdog]

Is the vacuum leaking out or is the atmosphere leaking into the engine?

Swampdog

 

[Screaming Metal]

Hey bobs66440, cutdown a beercan cooler and put around that glass jar....

 

[railroad]

Is the vacuum leaking out or is the atmosphere leaking into the engine?

Swampdog

The intake plenums are usually in a vacuum, which is high at idle, so the engine is pulling in air without fuel in it.
PCV valves do this, but have a check valve and spring or weight to not let it happen when it will make the engine run poorly. When you sprayed carb cleaner or starting fluid around the intake and it was pulled into the vacuum leak, it improved the air fuel ratio and your engine ran better/ faster. By stopping the vacuum leak the carb can be adjusted for all the air that passes through it.

 

[PotvinGuy]

Is the vacuum leaking out or is the atmosphere leaking into the engine?

Swampdog

Both, sorta. "Vacuum" is just what we call it when manifold pressure is less than ambient atmospheric pressure, which is always the case in a normally aspirated engine. If there is a leak, air flows into the manifold, from higher pressure to lower pressure.

 

[Swampdog]

Side note: back in 1968 I was stationed in Japan. I got to tour a Honda Racing Shop. They had motor (v-8/V-12) that had a bundle of snakes exhaust and some ram tubes intake. They claimed that they were slightly over atmosphere pressure. They must have had some kind of mechanical fuel injection, but I don't remember. They had monometers all over the dyno. They also had pictures of a Mopar 413 long tube Ram Engines posted on the wall.

 

[Screaming Metal]

Side note: back in 1968 I was stationed in Japan. I got to tour a Honda Racing Shop. They had motor (v-8/V-12) that had a bundle of snakes exhaust and some ram tubes intake. They claimed that they were slightly over atmosphere pressure. They must have had some kind of mechanical fuel injection, but I don't remember. They had monometers all over the dyno. They also had pictures of a Mopar 413 long tube Ram Engines posted on the wall.

Hahaha... Pops Yoshimura had a 125cc, 2-cycle bike that was either a 5 or a 6 cylinder road racer, it put out 25 or 28 horses @ 22,500 RPMs. Yep, read that right, 22,500 rpms. Extremely small bores and extremely short stroke.... I heard this bike run around a track 2 times, and the sound was unbelievable....This was back in 70'....in Japan. We taught them well. I saw the motor you spoke of....it was a work of art....
Just think, theres a Toyota Pickup truck coming down the road with a Cummins V8 Diesel....

 

[Screaming Metal]

Sorry, my memory was a little rusty, I believe it was indeed back in 68' instead of 70'....That was actually the very first input that actually made me become a EngineBuilder all those years ago....
heres a article....
http://www.examiner.com/article/eighth-liter-wonders-the-honda-125s-singles-twins-fours-and-fives

 

[Swampdog]

Screaming Metal

I was a crew chief on a Marine H-34 helicopter. It had a Radial R-1820 Wright Engine. These Japanese guys came on our base during an Air Show. There were 6-8 of them. A couple could talk some English. They ask a lot of technical detailed questions about the beast. This engine had a low tension magneto system, where there is a coil on each cylinder and I had to draw pictures to explain it to them. Now all engines are coil on plug. They called the base CO and invited me to there shop (along with Him)

 

[Robert]

You may have stumbled upon the source of many related issues. I would just block off the pcv valve and hoses, probably the cause of running lean, overheating, high idle, flat spots, etc. It's a hot rod, stick a couple of breathers in each valve cover and call it a day. If you still detect a vacuum leak, it will need correcting. PCV valves are not one size fits all. The manufactures designed them to operate with the vacuum levels of their engines. For this they served their purpose. That is not to say, there is a specific one that will work with your engine and vacuum range.
Good luck,

I like that idea.