umfan92

Hey everyone. I'm new to this forum. I have a 1966 Mustang Coupe and I just finished swapping the engine for a rebuilt 1987 302 HO. It has an Edelbrock 500 cfm carburetor, an Edelbrock Performer 289 intake manifold, and pertronix coil so no points.

The problem is, it has a cyclical shake at idle. It's not a big deal but I don't think it's supposed to run that way. It's an HO, but I still don't think it's supposed to shake at idle. It was rebuilt to stock specifications so the cam should be stock and I think the cylinders were bored .030 over. What could be causing this shake?

I had an old 1985 302 in it before and it would do the same shake but worse. The only things I reused on this engine were the distributor, the carburetor, and the coil. I don't think it could be the carburetor, and I doubt the coil is causing this. Or can it? I have tried messing with the timing but I can't get it to run smoothly.

Does anyone know what might be causing this? I'm leaning toward the distributor, but someone told me a cyclical shake doesn't sound like a distributor. Thanks.

jojobanks

generally when an engine shakes like that it's becuase it's idle is too low. Do a vacuum test and make sure you don't have any vac leaks. Check and make sure all the adjustments on your carb are where they need to be.

Otherwise it could be a missfire. Check all your plugs. Make sure they're firing and getting fuel. A fast way to check for misfires is to get the engine up to operating temp and take a temp gun to each cyl and see if one or more is firing hotter/colder than the rest.

umfan92

Thanks jojobanks. I don't think it's a big enough shake for a misfire though. And my vacuum is good. Here is a link to a video of what my vacuum does. http://www.youtube.com/watch?v=OoWX27024OY
And this is the acceleration: http://www.youtube.com/watch?v=SxwjjmDOs4U

I think the gauge is off. The needle is less than 0 when not connected to anything so I think my vacuum is a little higher than that but you can still see what the needle does.

I'll still try to check the temp of each cylinder. Should I take the spark plug off? Or can I just point it around the spark plug or the header pipe?

Thanks.

jojobanks

I usually just point at the header pipe. look for a difference of about 150* or greater between cyls.

boogerschnot

Check that your pcv is working and not causing a vaccum leak. Could just ne low idle in gear..

boogerschnot

You may just reset your idle and check timing. Block the front tires and make sure in gear when tuning. It will be higher rpm in park but will level you out in gear and warm. make sure your tune the carb for highest vac. With the car warm. Check for vaccum leaks. And you say you took the points out.. did you shim the magneto with the plastic feeler gauge when you put it in?

Xring01

De ja Vu.
My engine shakes also at idle...on my build...have about 500 miles
302 roller cam 30 over built to 1985 gt specs.
Early 70s 4 barrel intake. Holley 600cfm elec choke
Msd pro billet dizzy..
Stock 65 mustang exhaust manifolds.
I get about 15" vacuum.
running a auto c4 tranny.

I have no idea how to get this engine to idle smoother...

umfan92

I bought it not too long ago but I wasn't sure if it was working correctly. How can I check if it's working right? The PCV valve is a controlled vacuum leak, so plugging it will make the engine run better even if the PCV valve is running well in the first place right?

I'm sorry I didn't take the points off. It's a 1987 302 and I'm not sure which distributor it is but do I still have to do that magneto thing? I'm not familiar with that. Thanks

I will try this later. Thanks for the tip. I don't think it's a big enough shake though. If a cylinder wasn't firing, shouldn't it be a bigger shake? Thanks.

jojobanks

not really. Some misfires are actually worse than others. If you're having problems with your ignition system somewhere you could just be getting a weak spark.

Personally i think your vac should be higher than that if you have a stock cam. it's a steady reading but it's borderlining on the bad side. The lowest your vac should ever steady is about 16. Highest I've seen is 21-22.

In fact, if your previous engine did this as well, I think it's very likely that the problem got carried over in one of the parts that you swapped over.

umfan92

Yeah the vacuum is higher than that. The gauge is broken. It functions but the needle sits at below zero when there is nothing connected to the gauge. So I'm pretty sure the gauge is giving a lower reading than it really is. I'm pretty sure it's around 18 or 19. I'm going to buy a new vacuum gauge and see what reading I get.

Hey I didn't even see your post. Yeah I know how you feel. If you figure it out, let me know! I have no idea what's going on.