Wrong size 2100 on my 1966 390? Low compression on a cylinder also
 

My 390 has a 2100 but its 1.21 bore instead of the 1.23.
It also has 53F jets which sounds small to me.
The engine runs a bit hot being lean and it has dead spots in the throttle.
Should I just buy an =listing-final-manufacturer AUTOLINE =listing-final-partnumber as-link-if-js buyers-guide-colorC834A which is the right carb or should I try to get this working?
=listing-final-partnumber as-link-if-js buyers-guide-colorI opened it up and its clean as a new carb should be but its not the correct one.

thanks

Are you certain that it's running lean, or, maybe the ignition advance isn't operating correctly? That will cause hot running and poor performance as well. The rule of tune-ups is always ignition first, the. carburetion. You might be right, but I'd think a 1.21" should run just fine on that.

Believe it or not, carburetors that are too big tend to run lean. Post a clear close up pic of a spark plug or two. When engine tuning it's important to use the stock heat range plug starting out. Post a clear, closeup pic of a couple spark plugs.

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I pulled these out, drove it 30-40 miles when I got it

Thanks for the pics. Those look real bad, and they are fairly new looks like. Have you done a compression test? Don't worry about the carb or jetting just yet. A couple of those are so fouled they have barely been firing, if at all.

Yea, it's not running lean. The cfm rating between a 1.21 and a 1.23 is miniscule. Sitting on a 390, it'll virtually flow the same as a 1.23. Looks like you have at least half the valve seals shot. Or the guides under them bad.

Half of those plugs are running on oil rather than gas.

Three oil fouled. Three look Ok. One looks slightly hot on the Ok side and one looks slightly lean and maybe questionable. What is the timing running at? That strap mark on the lean one, which is visible looks too far down.

I got a compression tester and a vacuum gauge coming I'll post the results.
Pretty sure I'm gonna be pulling the engine out

Time to start testing!

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I just did one tonight

Cylinder:
1 130
2 145
3 165 cleanest pug
4 170 second cleanest
5 75 (and with oil added: 75) plug oil fouled black (only 50 miles on them)
6 130
7 155
8 155

#5 is hurtin.
I hope its a pushrod and a lifter is half out maybe that would explain low oil pressure but not sure.

What's the history on this truck, 75 psi on #5 is kind of a deal breaker, but overall the rest of the numbers aren't too bad,

This is cold engine too mind you but its a 66 parklane.
History unknown.
I guess next is pull the drivers side valve cover and have a look.

I think a leakdown test might be next. I guess maybe it could be a stuck or bent valve on #5? Seems to me, if the #5 unpleasantness can be straightened out without too much drama it could be a good runner.

Could be burned valve. Our '66 390 2V managed that at 41000.

I'm defiantly worried about the seats.
If its not hardened seats the heads gotta come off.
This year engine needed leaded gas or hardened seats at least my 289 did in a past vehicle.
If the valve doesn't close tight it will get hot too.

I got some more parts today for the carb.

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I had to plug the vacuum port on the choke so I made a solder blob, dropped it in and tightened the housing back on.
It squished in great.
The edelbrock choke fit but the choke fork was a bit too long and had to be shortened a couple 16th's.
Now I'm gonna blow it all out and assemble it shortly.

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https://i.imgur.com/nIx5J
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