Thread: 383 Stroker low vacuum and sluggish

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Good afternoon,

I recently got a project car. I was told it is a Chevy 350 crate with the 383 Stroker kit. It has been very sluggish when I floor it. I checked the vacuum and it’s very low, bouncing between 8-10 inches. I have tried all three ports on the edelbrock 1406 carb and same vacuum. However, when revved to about 2k rpm, it goes to a steady 20ish. I sprayed carb cleaner to try and find a leak but nothing. I also tried adjusting the timing by finding the highest vacuum but that didn’t change anything at idle. What should I check?

Thanks!

Do you know anything about the engine, especially the cam specs?

I don’t know too much about the internals. It’s a mild cam. Comp cams. Not sure if the exact specs though. I just put a timing light on at idle and it’s at nearly 50 degrees. If I attempt to adjust the distributor to lower it, engine sounds awful.

Originally Posted by Devon32

I just put a timing light on at idle and it’s at nearly 50 degrees. If I attempt to adjust the distributor to lower it, engine sounds awful.

Disconnect your vacuum advance when checking/adjusting timing.

If it still shows high timing with vacuum advance disconnected, verify your timing marks are correct by finding true TDC.

Norfolk, Virginia is 7' elevation, so you should have more vacuum at idle (unless it's a huge cam). Check compression, and check and verify your valves are adjusted correctly.

Verify you do not have a vacuum leak from the underside of the intake manifold by plugging both valve cover breathers (remove PCV and plug) and attaching vacuum gauge to the oil dip stick tube, should be very little to no vacuum (slight bouncing only).

What is your fuel pressure? Have you adjusted the carb?

Are you certain that when you checked the timing you were on the #1 cylinder?

Yeah I’m using number 1 spark plug wire. I did your method to try and find a leak from the underside and everything seems good. I’m thinking these marks are off because engine sounds the best when it’s at about 50 degrees timing. Any lower and it starts to stutter. What’s the best method to find TDC with everything installed? Is it possible it just runs a very low vacuum?
Fuel pressure is about 5 pounds. Adjusting the carb doesn’t seem to do very much either way I turn the adjustment screws?
Also I noticed I don’t have a PCV valve hooked up on either side of the carb. Both ports are plugged.

One way to accurately find TDC and check the 0 on your balancer is by using a piston stop that screws into the #1 spark plug hole. Pull your #1 plug and with your thumb over the plug hole have someone rotate the engine clockwise using a socket & breaker bar on the balancer bolt to find the compression stroke. When your thumb is pushed away stop rotating. install the stop and slowly rotate the engine clockwise until it resists (piston hits the stop. On your balancer, mark the exact point that aligns with the 0 on the pointer. Now rotate the engine the opposite direction, CCW slowly until it resists. Again mark the point on the balancer that aligns with 0 on the pointer. Using a dial caliper, measure the distance between the two marks. TDC is exactly 1/2 way between the two marks.
If you want, you can back your piston stop out 1/2 turn and repeat, which will move your marks closer together, and continue that process until the distance between your two marks on the balancer is very small, increasing the accuracy of your TDC mark.

Originally Posted by Devon32

What is the best method to find TDC with everything installed?

There are numerous ways to check TDC.
However, I DO NOT recommend using a spark plug piston stop method on an assembled SBC unless you really know how to do the method, as the piston stop if set wrong can bend valves.
On assembled/installed motors I recommend a non intrusive method (I’ve included a Verify TDC PDF), this is the method I use (modified slightly from other non intrusive methods) and it works well in tight places.

Originally Posted by Devon32

Is it possible it just runs a very low vacuum?

It is possible, if you have a big cam along with low compression, though I suspect other things are also adding to the low vacuum and poor performance, i.e. tight valve lash and carburetor adjustments comes to mind (I've included a valve adjustment PDF).
Is the motor real rummpity at idle (idles rough, has a lot of cam lope)?
What is the compression?
What do the plugs look like?

Originally Posted by Devon32

Fuel pressure is about 5 pounds.

No matter what else you hear, it is important with Edelbrock carbs to have no more than 5 psi. Higher pressure can cause an overly rich idle and very poor performance. Check your pressure, be sure what the pressure is.

Originally Posted by Devon32

Adjusting the carb doesn’t seem to do very much either way I turn the adjustment screws?

Most likely, you have the throttle opened up too far overly exposing the transition slot trying to get it to idle. This can be caused by a large cam or other issues. However, if you use a lot of throttle stop (throttle speed adjustment) to keep it running, it throws the carb out of its idle/transition circuits and screws the low speed performance up and can also add to low vacuum. There are other ways to get the idle speed up without messing up the carb’s idle/transition circuits.

Originally Posted by Devon32

Also I noticed I don’t have a PCV valve hooked up on either side of the carb. Both ports are plugged.

I run PCV on all my motors. Contrary to what some will tell you, the PCV system helps the motor's performance by lowering crankcase pressures (motor spins easier) and it keeps the crankcase cleaner.

VERIFY T.D.C..pdf
Adjusting hydraulic valves .pdf

36 sedan, thanks for posting that TDC method! I was trying to find the old post where I'd explained that method but having no luck I described the hard stop method which I've never liked either! I downloaded your PDF to save, along with your valve adjusting explanation. That cutout is great!

Is it a hi rise intake manifold? They are notorious for only being happy at WOT.

or possibly wrong timing with the gears/chain----

Originally Posted by Devon32

I don’t know too much about the internals. It’s a mild cam. Comp cams. Not sure if the exact specs though. I just put a timing light on at idle and it’s at nearly 50 degrees. If I attempt to adjust the distributor to lower it, engine sounds awful.

By chance does this have multi-spark ignition (MSD) and an un-compatible timing light? If it has MSD move the timing lights inductor as close to the distributor as possible, this sometimes eliminates the ghost reading (second strobe).

36sedan, Thanks for the procedure for finding TDC. Just bought the piston stop tool but think I might try your way instead since I’ve never done it.
Just did a compression test and every cylinder came back at around 120. Lowest was cylinder 8 at 118 and highest was cylinder 7 at 122. Should I be worried that they are very low but are all consistent?
Cylinder 5 and 6 have extremely tight clearances for the spark plug boot and they were slightly melted so I replaced all the spark plugs with NGK UR5 shorty plugs and gapped them to .035 and have new wires on order. The plugs didn’t look bad, slightly fouled.
Before pulling everything I played around with the carb and timing some more and got vacuum to sit at a steady 11.5-12 inches, timing was still around 55 though. The idle isn’t rough and don’t hear any pinging or knocking.
And no I don’t have a high rise intake manifold. Just a basic edelbrock performer. Distributor is a pertronix flame thrower.

Originally Posted by DennyW

____________________________oops

That's me, lol!!

Sorry it’s taking a little while to get back. I work a 12 hour shift so only have so much time to do what you guys recommend and get back to you.