383 Stroker low vacuum and sluggish

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.

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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.

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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.

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Originally Posted by Devon32

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

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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.

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Originally Posted by Devon32

Is it possible it just runs a very low vacuum?

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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?

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Originally Posted by Devon32

Fuel pressure is about 5 pounds.

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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.

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Originally Posted by Devon32

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

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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.

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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.

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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.

Attachment 72844
Attachment 72845

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----

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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.

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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.

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Originally Posted by DennyW

____________________________oops

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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.

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Originally Posted by Devon32

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.

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You’re welcome, I can’t take the credit for it though, it is just a modified version from others, I just re-did it to suit me.

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Originally Posted by Devon32

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?

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Compression is equal that’s good. However, the numbers point towards a lower compression motor (below 9-1) and this surprises me for a stroker kit, they’re usually 9.5-1 > 10.5-1 and occasionally higher depending on the heads and deck heights.

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Originally Posted by Devon32

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.

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This is not unusual with tight motor spaces. I have to use ceramic boots on mine or they arc out, and this may be part of your problem. If these cylinders are arcing out then they are not firing and performance will be way down. Picture of the plugs would help.

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Originally Posted by Devon32

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.

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What is the idle rpm?

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Originally Posted by Devon32

Distributor is a pertronix flame thrower.

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Does it have a Pertronix ignition box between the distributer and the coil?

36sedan,

Idle rpm is about 900. And I don’t have an ignition box between the distributor and coil.
New wires and heat shield for them should be in tomorrow and I’ll hook everything up and see if that makes a difference and get back to you.
Is the PDF you attached to check the condition of the timing chain? Attachment 72855Attachment 72856

Idle speed is a little high, try dropping the idle speed down to 700rpm and see if the carb adjusts better. You may find once the carb adjusts you won't need the higher RPM. If the carb adjusts at the lower RPM but when you drop in gear the idle is too low, we'll work on that next.

Sorry, I didn't explain. The drawing shows you a way of checking the balancer on the front of the motor for slippage, which was mentioned as a possible timing error problem. Check your balancer to make sure it has not slipped throwing the timing marks off. And please verify the timing marks line up once you find true TDC.

That plug looks good aside from the oil leaking on to it (probably valve cover gasket), also a picture of the face of the plug would tell us more. And would help to see all the plugs as they are the windows into the cylinders. I have spark plug racks that I use to keep the plugs in order to the cylinder they came out of. You can use a piece of cardboard with a hole for each insulator tip and marked per cylinder. It is a useful tool to keep track of your tuning.
Attachment 72857

Your boots are definitely burned, watch it at night to see if it's arcing (even with new boots), this may be a big part of your troubles. As I said, I have to run ceramic boots on mine.

Does your Pertronix distributor have vacuum advance?

Devon32,


You should really appreciate the time it takes to post great comments like 36 Sedan has done for you! It takes time to find the graphics, create the text comments and pull it all together. He is really on point with his comments, covering all the possible variations.
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Ill drop the idle down and try to readjust the carb and see if that helps. Last time I tried lowering rpm to about 700, vacuum dropped very low to about 5 and was a rough idle but maybe with the new plugs and wires, it will be better. When brought up to around 2k rpm, the vacuum is around 20 and steady. If this site allows me, I can try to post a video. My distributor does have vacuum advance. When checking the timing, I disconnect the vacuum hose and hooked the vacuum gauge to one of the ports on the front of the carb.
I can upload pictures of all the plugs tomorrow as well. I never thought about keeping track of them, that’s really smart. I appreciate all the help and will try to do everything you’ve mentioned as soon as possible to get back to you, thanks!

Ill drop the idle down and try to readjust the carb and see if that helps. Last time I tried lowering rpm to about 700, vacuum dropped very low to about 5 and was a rough idle but maybe with the new plugs and wires, it will be better. When brought up to around 2k rpm, the vacuum is around 20 and steady. If this site allows me, I can try to post a video. My distributor does have vacuum advance. When checking the timing, I disconnect the vacuum hose and hooked the vacuum gauge to one of the ports on the front of the carb.
I can upload pictures of all the plugs tomorrow as well. I never thought about keeping track of them, that’s really smart. I appreciate all the help and will try to do everything you’ve mentioned as soon as possible to get back to you, thanks!

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Originally Posted by Devon32

Ill drop the idle down and try to readjust the carb and see if that helps. Last time I tried lowering rpm to about 700, vacuum dropped very low to about 5 and was a rough idle but maybe with the new plugs and wires, it will be better. When brought up to around 2k rpm, the vacuum is around 20 and steady.

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This sounds like your gauge may be connected to the timed port with the throttle stop adjusted to high and the initial timing low.

Drop the throttle stop so you can adjust the carb, then give the motor a little more advance to bring the RPM up. If this helps, you'll need to confirm the timing marks are accurate so you can adjust the mechanical advance in the distributor.

36sedan, I have the gauge hooked up to the right manifold port one. With throttle stop backed all the way out, adjusting the carb still has no effect. I can shut both mixture screws and it’ll still run, but makes some squeaking noise. With idle screw backed out and timing advanced to keep rpm around 700, vacuum was at around 5. Bring it to about 1000 rpm and it’s at 10 inches vacuum. Also, looks like I don’t have a choke at all. Starting to think I just have a lot of problems. I checked the balancer and for slippage and it was good. I’ll find TDC this weekend but even if the marks are incorrect, that won’t help because I’ve tried finding the best vacuum with a wide range of timing adjustments and can’t seem to get over 10ish. I really appreciate your time and help, but I might have to either take it somewhere or just work on it over time. Thank you so much!

First off - Welcome to CHR!

You are correct in thinking that a 383 stroker should run like a scalded hog. It looks like you're already getting a lot of great help/advice and after reading through this thread, here’s my 2 cents.

When all things are correct with this engine you should be looking at 8-10 degrees initial and 32-36 degrees total. That said, at sometime this engine was set up and ran correctly (or so I would assume) and now that you’re seeing a 50 degree reading something has changed and I suspect there’s a mismatch between the timing tab and the balancer (one or the other has been swapped after initial build?). Could you get us a picture of the front of the engine showing the timing tab and balancer?

Your plugs look to be okay but definitely show some wear. For the cost of a set of plugs, I'd recommend replacing them. The Autolite APP25 is a good plug and come pre-gapped at .044 (I still check em). You mentioned .035 – that’s too tight, they should be .044-.045.

As mentioned, the boots look like they may be compromised and a new set of wires would be my next step. I like ceramic boots as well.

What is the condition of the roter and the cap? Replacing them would not be a bad idea.

As to the low vacuum, I have found it is almost always caused by either compromised intake/carburetor gaskets or late timing. Worn valves or rings can also cause poor vacuum. You mentioned a compression test and as stated above, the numbers seem a bit low. As you’ve acquired this engine used, I’d do a leak-down test to see if you have a tired set of rings. You do not need to spend a ton of money. You’ll need a source of compressed air and a kit like this one from Amazon:
https://www.amazon.com/gp/product/B0...d-b31d6b1bfbee

We’ll keep asking questions and making suggestions – hopefully you will find the solution and experience the power of a well running 383.

Glenn

Thank you!
I honestly don’t know the condition of the engine prior to this, I got it from an unfortunate circumstance. I can check the distributor cap tomorrow and see how it is.
I do want to mention I did the compression test on a cold engine, no way I could do it after warming it without being burnt. I already swapped the plugs with NGK UR5s and have some heat shield for them on the way. If that still melts, I’ll get the ceramic ones. I can pull them and go ahead and increase the gap like you suggested though.
What’s a typical width of the carburetor gasket? Mine is quite thick. Any method to test leakage past them? I tried the spraying carb cleaner on them and listening for rpm increase.
Thanks for that link, I’ll go ahead and order it and give it a shot.

if possible swap a known carburetor onto it . first thing i do in eliminating problems .

Shine,

Unfortunately I don’t have an extra laying around. I don’t mind ordering a new gasket if that’s a probable cause.

Great pictures - really helps to know what we're looking at. It appears that there is a thin and a thick gasket between carb and manifold. The thick gasket approach helps to isolate the carb from intake manifold heat. Let's get a new gasket and make sure both mating surfaces are really clean. Don't use any gasket sealer here. Tighten the carb and re-tighten after a few hours run time. Picture below is more what I'd like to see.

In addition - note that I'm using the manifold port for my vacuum advance and have plugged the timed port. In most all cases, I have found this to be the best fit with the Edelbrock carbs.

Glenn

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Originally Posted by Devon32

I really appreciate your time and help, but I might have to either take it somewhere or just work on it over time. Thank you so much!

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WHOA!, don’t give up yet, you’ve made great progress. This is an unknown motor that the previous owner may not be telling you the truth about, so CONGRATULATE yourself, you’ve already found a lot about it. Lets move forward, relax, take your time and enjoy the journey. You’re not offending or putting any of us out, lets getter done.

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Originally Posted by Devon32

I have the gauge hooked up to the right manifold port one.

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Perfect.

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Originally Posted by Devon32

With throttle stop backed all the way out, adjusting the carb still has no effect. I can shut both mixture screws and it’ll still run, but makes some squeaking noise. With idle screw backed out and timing advanced to keep rpm around 700, vacuum was at around 5. Bring it to about 1000 rpm and it’s at 10 inches vacuum.

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OK, this tells us you have a sizable vacuum leak (still running with mixture screws shut and whistle noise gives it away).
Not all carb cleaners raise the rpm, some cause it to lower, lets try it again as follows;
Seal all the vacuum ports (front and back) on your carburetor and spray a mist of water around everything, the water mist usually lowers the rpm and is quite noticable.
Start at idle and mist everything, then raise the rpm to around 1500 and mist again.
With the oils signs on your plug’s housing, you may not have a good enough valve cover seal to use the dip stick vacuum gauge trick for underside of intake leaks, try tightening your covers and checking again.
With the carburetor ports sealed only your intake manifold and carburetor base gaskets are left as a leak source, so this should show up quick.
Your plugs show signs of some oil consumption on a couple plugs, this could be the cylinders that were not firing and why I keep plugs in order.

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Originally Posted by Devon32

Also, looks like I don’t have a choke at all.

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You can add the choke back later to help cold starts, but it’s not the issue right now.

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Originally Posted by Devon32

Starting to think I just have a lot of problems. I checked the balancer and for slippage and it was good. I’ll find TDC this weekend but even if the marks are incorrect, that won’t help because I’ve tried finding the best vacuum with a wide range of timing adjustments and can’t seem to get over 10ish.

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If you have a vacuum leak, usually you’re not going to get the timing to raise the vacuum enough to make a difference on the vacuum gauge, especially with a larger leak.
And beware, setting timing to the highest will usually be too much timing for the motor. Usually this method requires backing back off a little after highest vacuum is achieved.

The pictures of your motor show the tab at a higher placement than usually is used, there are some motor balancer combinations that use it, but it warrants checking to be sure. True TDC never lies.

I’m not a big fan of opening plug gaps up, but as Glen noted with your possible oil consumption, opening the plug gap usually helps to run the plug a little hotter and burn the oil off.

Hang in there, you’ll get this, we’re rooting for you, there are a lot of great people here helping and we all enjoy helping each other.

Low vacume but even compression figures----------oily sparp plugs-ummmmmmmm????? Valve stem seals and/or guide condition on the intake valves---------

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Originally Posted by jerry clayton

Low vacume but even compression figures----------oily sparp plugs-ummmmmmmm????? Valve stem seals and/or guide condition on the intake valves---------

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Strong possibility Jerry, and it should show with a crankcase vacuum test.

36sedan,

Alright just did the vacuum test on the oil dipstick again. Valve covers were tightened down and breathers were plugged. I plugged all ports in the carburetor and no vacuum. Actually saw maybe half a pound of pressure. Steady .5psi all the way to 2k rpm.

I just tried spraying water on all the gaskets and hoses and got no change. I’m going to order a new carburetor gasket anyways since they’re relatively cheap. Leak down test kit should be here this weekend and I’ll also find TDC then as well. I really appreciate everyone’s help on this.