Thread: 383 Stroker low vacuum and sluggish

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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.

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.

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?

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.

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.

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.

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.

What is the idle rpm?

Originally Posted by Devon32

Distributor is a pertronix flame thrower.

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? 3F27FBBD-5562-423A-9392-9643C1359F28.jpg6513E97A-9E7B-4846-9BCB-55C782F4AA19.jpg

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.
plug rack.png

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.
.

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!

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.

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