Any ideas?

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swinger 74

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Ok so ive been trying to figure this car out for a while now and ive gotten some improvements the more I work on it. Anyways ive gotten the car to run fairly well now but still having some issues. The engine I cant get to idle smoothly and it shakes at idle but when rived is pretty smooth. Car has new motor mounts, spark plugs, cap & rotor, plug wires, new refurbished distributor. It has a holley 550 & Edelbrock performer 318/360 intake manifold. There are no vacuum lines hooked up and are blocked. Carb came off my truck and ran smooth. Any ideas what could cause the engine to shake at idle? Engine is a 1974 360 out of a motor home.
 
Double check your plug wire routing first. It's easy to mix 5 and 7 up if you aren't paying attention.

A little more description of what you put together would help, engine (360). What trans? What flex plate did you use etc etc.
 
My money is on timing or a vacuum leak.
Have you tried 10-12 degrees initial advance, or going with the old zero like some of the engine stickers say?

There ya go Bruce, I forgot about the 5/7 wires.
They are REALLY easy to get switched.
 
I'm using the stock 904 transmission with a B&M flexplate made to mate the stock 318 torque converter to the 360. Also I will check the plug wires again but I'm positive that they are right because ive checked them before. Also if the plug wires were crossed id think it would backfire and stuff. Another thing is where would a vacuum leak be coming from? all ports are blocked off and no lines going to anything. Carb and intake aren't leaking. I checked while running with propane torch.
 
You can do a "backyard" cylinder balance test by getting yourself a grounded probe, AKA a screwdriver and an alligator clip lead

Pull the distributor boots up and loosen the wires in the cap so you can pull them up and out easily. This is best done with insulated pliers AKA "fuse pullers.........."

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With the engine warm, or at least somewhat warmed, and at as smooth an idle as you can manage, pull one wire at a time, and as you pull the wire up and out, work the grounding probe down into the distributor. This is to prevent flashover to other cylinders, and to protect the ignition

Listen for each cylinder to drop. A dead hole will show up easily.

Suspect bad / open plug wires or even a bad /broken plug. Check plug wires with your ohmeter

As Rusty said, run a compression / leakdown test.

Have you done ANYTHING to the engine mechanically that might bring this on?

Also, with the engine warm, see that the carb idle screws have "equal" effect. This is a clue as to problems in the carb, or vacuum leak problems.
 
I haven't done a compression test on the motor. Engine is all stock accept for the intake and carb. If I turn the mixture screws all the way in the motor shuts off. My truck I took the carb off ran smoothly with no shakes. I went and purchased a brand new Holley and now my truck is doing the same thing uggh lol. But that's another problem. Trying to figure out the car
 
I would bet on the 5-7 swap as i did it on my last rebuild
 
Put a vacuum gauge on it. This is classic vacuum leak territory. Also, what's a Holley 550? Never heard of one of those? You got a pic? Is the engine stock? Modified? If so, how much? What cam, etc.
 
holley 550 is a 550 cfm holley from the 60s. believe they came on ford trucks. carb is good and works properly because it came of my daily driver. The 360 I have in my car is stock. im just gonna pull it out and have it rebuilt because I didn't like the compression test numbers I was getting. Also I don't have any vacuum leaks anywhere. all the ports and everything are blocked. ive checked with carb cleaner and propane. Dad is thinking its the valves. did wet and dry on the cylinders tested and compression was the same both wet and dry. also the gauge would leak down too.
 
swinger 74 said:
holley 550 is a 550 cfm holley from the 60s. believe they came on ford trucks. carb is good and works properly because it came of my daily driver. The 360 I have in my car is stock. im just gonna pull it out and have it rebuilt because I didn't like the compression test numbers I was getting.
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Yeah I have seen a few Ford specific Holleys that were oddball CFM sizes. I had a 450 vacuum secondary carb from a Rustang that looked similar to a 600.
 
So the numbers I came up with when I did 5 cylinders. 2 cylinders were at about 100psi 2 at about 80psi and 1 cylinder was about 40psi. could the gauge be bad or wrong or what? the engine runs but shakes at idle and doesn't backfire through the carb. also when given gas it revs up nice. The cylinders I tested were 1,2,4,6,8.
Or did I do the test right? I pulled the plug and connected the tester and cranked it over about 4 or 5 turns.
 
Do them again in a different order and see if you get the same numbers for the same cylinders.
That'll answer the gauge question real quick.
 
swinger 74 said:
So the numbers I came up with when I did 5 cylinders. 2 cylinders were at about 100psi 2 at about 80psi and 1 cylinder was about 40psi. could the gauge be bad or wrong or what? the engine runs but shakes at idle and doesn't backfire through the carb. also when given gas it revs up nice. The cylinders I tested were 1,2,4,6,8.
Or did I do the test right? I pulled the plug and connected the tester and cranked it over about 4 or 5 turns.
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Throttle plates should be blocked WOT though that is just for factory specs and should not effect differences in cylinder balance.
 
I'd suggest doing the compression test right. Only doing some of the cylinders won't tell you anything. You have to do them all. You have to do them right.

Do the same amount of cranks per cylinder. An extra crank can throw a reading off. Do all the cylinders. Write down your results on each cylinder. They should all be within 10% of each other. The cylinders that aren't. Put a teaspoon of oil in them through the plug hole. Run the test again. If pressure goes up thing rings/piston. If it doesn't. Think valves/heads.

Don't half *** it. It's easy enough to do it.
 
ok ill rent a gauge from another part store and see if theres any difference. also I didn't have it blocked WOT when I did the test. What should the cylinders be reading if they have good compression?
 
Why rent a gauge? What makes you think yours is bad? The 40psi reading? The two 80 psi readings? If the engine is running like crap I wouldn't be surprised if that's what they are. Use your gauge. Do it right. Twice. See if the numbers line up.

As for what compression. 100psi is ok. 140-150 is good. 170 is a strong street motor. 200+ is serious territory. Depending on the cam of course. You should care more about how close the numbers are to each other. Worry about how high they are second.
 
Lustle said:
I'd suggest doing the compression test right. Only doing some of the cylinders won't tell you anything. You have to do them all. You have to do them right.

Do the same amount of cranks per cylinder. An extra crank can throw a reading off. Do all the cylinders. Write down your results on each cylinder. They should all be within 10% of each other. The cylinders that aren't. Put a teaspoon of oil in them through the plug hole. Run the test again. If pressure goes up thing rings/piston. If it doesn't. Think valves/heads.

Don't half *** it. It's easy enough to do it.
Click to expand...
Im not trying to half *** it. the driver side is a ***** to get the hose screwed into because the shitty design Chrysler did with the exhaust manifold. also ive never done a compression test before in my life. Never had to. I did the passenger side of the motor because it was easier. shouldn't make a difference if I only did on side of the motor. the readings on that side are still gonna be the same. Right?
 
Lustle said:
Why rent a gauge? What makes you think yours is bad? The 40psi reading? The two 80 psi readings? If the engine is running like crap I wouldn't be surprised if that's what they are. Use your gauge. Do it right. Twice. See if the numbers line up.

As for what compression. 100psi is ok. 140-150 is good. 170 is a strong street motor. 200+ is serious territory. Depending on the cam of course. You should care more about how close the numbers are to each other. Worry about how high they are second.
Click to expand...

I don't own a gauge. the one I used I rented from oreilly auto parts. That's why I was saying I should rent another one but from another parts store.
 
The WOT thing is not necessary. All that is necessary is that you do the compression test the same way on all cylinders. Either open or closed will tell the story. Opening the throttle does not make that much of a difference. What you are looking for is to make sure the cylinders are fairly even with regards to compression numbers. That will show up whether the throttle is open or closed.
 
swinger 74 said:
Im not trying to half *** it. the driver side is a ***** to get the hose screwed into because the shitty design Chrysler did with the exhaust manifold. also ive never done a compression test before in my life. Never had to. I did the passenger side of the motor because it was easier. shouldn't make a difference if I only did on side of the motor. the readings on that side are still gonna be the same. Right?
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Not to be blunt. But you are half assing it. Why do a compression test on only some of the cylinders? It's half the work. It makes a difference if you want to find which cylinder or cylinders is bad. Each cylinder is its own. The passenger side won't tell you anything about the drivers side. Just like one cylinder won't tell you anything about the cylinder next to it. If you do one side and not the other. Your diagnosing half the engine. For all you know there are bad cylinders on each side. You say you've never done one before, then listen to the guys who have. Test all the cylinders. Otherwise it's useless.
 
I understand that I need to do both sides. I was trying to get an idea of how bad the compression is on one side to see if I need to bother with the other side. If one side is bad then I need the motor redone anyways right. If they are reading low. I'll do it again and do both side.
 
But you don't know what low is until you do the engine? 90 sounds low until you get it on all cylinders. It's the overall that matters. And do the oil test. Maybe you don't need the motor redone. Maybe all you need is valve work or something easy. Or maybe it isn't the cylinders at all. The test will point you in the right direction.
 
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