Need diagnostic help

[Doug&Deb]

Had a rough running problem with my Coronet. One cylinder had half the compression of the other 5. There are 500 miles on the rebuild. Turns out the intake valve wasn’t sealing properly. I fixed the problem but another issue I was having was that after being at operating temperature for a while the engine doesn’t want to idle. The two issues showed up at the same time but I’m not sure if they’re related. It’s always tricky for me to differentiate between fuel and ignition problems. I definitely fixed the low compression issue and the valves are all adjusted to spec. Any thoughts?

[DJ194950]

Sounds like a vacuum leak at the intake/carb the most likely culprit?

 

DJ

[keithb7]

A vacuum gauge is a great diagnostic tool to ensure if you do, or don’t have a vacuum leak. 
 

What is the history on your carb? Last cleaning?  If you carb is not reacting adjustments of the idle air fuel mixture screw, you likely have an air leak leak somewhere. Leaky carb base gaskets? Shaft wear?

[FarmerJon]

I am a big believer in having a vacuum gauge. Having it in the driving area allows you to see what is normal and spot what condition it changes.

Have you tried resetting the idle mixture and idle speed once fully hot?

[Doug&Deb]

I’m going to check the carb today. Keith the carb is recently rebuilt. Would a vacuum leak cause problems after the car is fully warmed up? The problem starts after I’ve been driving for a while. I’m going to check settings and see if I can locate any leaks. I also have another rebuilt carb. If all else fails I will swap and see if that helps.

[greg g]

When you say won't idle, wha does that mean? Is it misfiring, over reving, needs choke to idle smoothly, you need to keep your foot on the gas to keep it running?  If it's a miss fire, do the old pull a spark plug wire one at a time till you isolate the dead plug , did you check to see if the choke is fully open?  What is the status of your heat riser flap?  Is the carb percolating and I've flowing fuel?  Checked the float  and or needle and seat?  Is the coil over heating?  Is the carb linkage fast idle cam dropping out as supposed to?

[Doug&Deb]

Greg it sounds like I have a crack in the manifold somewhere. The car gets louder after it heats up like it’s expanding. I’m confused because I was careful with the installation during the rebuild. I didn’t tighten the 4 bolts holding the intake and exhaust together until the whole assembly was fastened to the block. I also retorqued after a few heat cycles. It wasn’t as bad today but I did install my spare carb and I spent time tuning everything. I should have been more specific about the symptoms. I had to keep giving it throttle for it not to stall and with the M6 trans that’s tons of fun lol. Heat riser and choke are operating properly. 

[greg g]

Well in order for the trans to work right your idle speed needs to be in the stezdy 450 to 500 rpm neighborhood.   You probably need to get hold of a vacuum gauge and check for leaks from the carb to the block.

[Doug&Deb]

As the engine warms up the vacuum drops slightly. I’ve already removed the manifold from my old engine and I have the necessary gaskets so that’s my next project. Learning how to rebuild my engine was both rewarding and frustrating. I’m seeing light at the end of the tunnel so hopefully this is the last project. My idle speed is about 600 until the engine gets hot enough then it drops to 450 or so. Once I get the leak fixed I’ll set everything back to normal. 

[D35 Torpedo]

Use non chlorinated brake clean to find vacuum leaks. Spray it around the manifold while it's running and listen for rpm change. 

[Doug&Deb]

As an update I found that the gasket between the manifolds has failed. That explains the vacuum leak and the louder exhaust sound. I’m puzzled by the failure. I was careful about the installation remembering to only finger tighten the 4 bolts until the manifolds were fastened to the block. I’ll do some checking before re-installing. Hopefully that’s the last of the gremlins (except for the electric fuel pump no longer works). 

[DJ194950]

Did you check the manifolds for straightness of the mounting surface.

 

Hard to believe but cast iron does warp when uneven force is used in the mount bolts.

People over the years always try tightening the nuts some more even after the gasket has blown out causing the uneven mount pressure and warp the manifold as it heats in use.

 

Since you will need to remove them for gasket repair , check Both manifolds for flat mounts.

 

DJ

[Sniper]

7 hours ago, DJ194950 said:

Did you check the manifolds for straightness of the mounting surface.

 

So what do you do if the surface is not flat?

 

In the link I go thru a new water pump and I check the gasket surface for flatness, it wasn't so I fixed it.  Same repair would apply here

 

http://www.yourolddad.com/cooling-system

[Sam Buchanan]

13 hours ago, Doug&Deb said:

As an update I found that the gasket between the manifolds has failed. That explains the vacuum leak and the louder exhaust sound. I’m puzzled by the failure. I was careful about the installation remembering to only finger tighten the 4 bolts until the manifolds were fastened to the block. I’ll do some checking before re-installing. Hopefully that’s the last of the gremlins (except for the electric fuel pump no longer works). 

Failure of the gasket between the manifolds will not result in a vacuum leak but can certainly cause a lot of noise.

[Curt Lee]

I've flattened warped mounting surfaces by getting a large pane of glass, aluminum oxide paper, and sticking the paper down with WD-40. Carefully set the item on the paper, soaked with WD-40, and move it around in an oscillating pattern, then X-pattern, circles, and back to an oscillating pattern, checking frequently to see the surfaces all the same appearance. You're not looking for grinding away, you're looking for uniformity. Once you get that, clean the WD-40 off and reinstall it. Doesn't take much except time.

[Dave72dt]

You may need to take your assembled manifolds to a machine shop and have them trued as a unit.  Intakes don't erode or warp like the exhaust does so you may still have some standout, even if the bolts were loose.