Help! Engine troubles

[Sloan66]

Checked manifold with starting fluid, no leaks.

[71K20chevy]

It could also be leaking from the lifter valley into the intake, and that won't show up with starting fluid. I wouldn't pull the intake just yet..

[Legolas894]

I'm still thinking vacuum. Did it run any different with the vacuum advance off? Can't remember if an HEI has a vacuum advance...I'm still a points man. What about the choke? Does the same thing happen after it's warmed up?

There's a reason people went fuel injected...No the least was the fiddling with carb mixtures, timing, choke etc

[knomadd]

Originally Posted by Sloan66

Checked manifold with starting fluid, no leaks.

Did you try the carb to intake? I've had two vehicles where the gasket either developed a vacuum leak or had one from the beginning and caused all sorts of headaches before it was discovered.

[BMERDOC]

Did you get around to verifying the condition of the plug wires? I think East Tx was pointing you in a good direction. Sounds like an ignition misfire to me. Bad wires will "leak" under load and cause a misfire.

[BMERDOC]

I pointed you out to this guy so I'll point him out to you too. You guys are both having similar problems.
http://67-72chevytrucks.com/vboard/s...d.php?t=622906

[70SBC10BOB]

What is the manifold vacuum at idle? If it is north of 17" of vacuum, I don't think you would have a manifold leak. If it is less, it could be internal leak.

If the vacuum is less, put the vacuum guage on the dipstick tube and plug all other vacuum sources (ie pcv, brake booster etc.). If you have vacuum at the dipstick, chances are you have an internal leak.

Good luck.

[litew8]

Reiterating my previous comments - if you removed the carb and reassembled with an old base gasket, I'd replace with new. You'll find the source, keep looking.

[Sloan66]

Thanks men, it rained all day yesterday so I was un able to do anything. I will check vacuum, and other ideas today. Internal vacuum leak? where would that come from? Just never heard of such before.

[70SBC10BOB]

Internal vacuum leak would be caused by a bad gasket seal on the intake. Instead of leaking on the outside where you could locate it with carb spray along the intake/head joint, it would leak inside or at the bottom of the same joint. The leak would be into the engine valley creating vacuum in the crankcase. Sometimes this type of leak will cause blue exhaust smoke because oil is sucked in.

Keep swingin' at it.

[Sloan66]

Worked on it yesterday.
Moe info: Vacuum was 20, plugged all the ports did not get an vacuum at dip stick.
I did not get to replace carb gasket yet as no one had one, will pick one up today. Researched further still think I might have a small vacuum leak internally. When you first start motor in morning it sound slike it has a mild cam, then as it warms up it smooths out. Thoughts? If I end up pulling the manifold how can I assure that I it get it to seal. I have been using felpro gaskets, fyi.

[engineer_gregh]

Hi Sloan66. I live in Smyrna and have a 71 with a 307 two barrel. You're more than welcome to stop by and compare the two. I have some extra plug wires, and a freshly built 2 barrel carb. Let me know.

[70SBC10BOB]

With 20" of vacuum, I don't think you have any leaks. That is a great vacuum reading.

Don't we just love working on trucks?!?

[70SBC10BOB]

Before you pull the manifold. Have you checked the float level of your carb?

With engine [U]not running[U] can you see if the accelerator jets are spraying out of both squirters when you operate the throttle?

The spray should pretty much start and stop with the throttle movement. There should be no delay at the beginning and no stopping until movement stops. (You would only work the throttle a couple of times before you have to start the truck to refill the accelerator bowl.)

We are havin' fun now!

[Sloan66]

Greg, Thanks for the offer, may take you up on it will PM you.

[Sloan66]

Thanks for everyones help. Wanted to update, figured out the issue. After replacing intake gaskets, checking carb, fuel pump, it was a bad plug in #5!!!
I swear I thought I checked them all earluy on, but apparently I missed #5. It was fouled pretty good. I checked compression and for an old motor all cylinders were good, averaged 130, so that plug must have been bad out of the box.

Again, thanks for all the help! Forum is awesome.

[litew8]

How does a bad plug act like a vacuum leak?

[70SBC10BOB]

Glad you killed the dragon!!!!
Just remember to do ALL of the basics first. You can not short cut the basics or you get bit in the butt!!

Good job at not giving up.