383 4bbl running with an intermitent shake...

[StockMan]

I have a 383 4bbl that I had running out of the car for testing purposes, following a full gasket replacement.  The only major thing I did was pull a couple of caps to check the bearings and added a new cam, lifters, timing chain and sprockets.  The motor ran very nicely out of the car but now that I've installed it, it just doesn't sound as smooth and crisp as it did.  It seems to have a slight shake, that seems to occur intermittently.  I did recently add a carb kit due to the carb flooding the motor periodically but am still experiencing the same thing.

Could it be the carb?  Again, just after the motor starts up it runs very smooth, a few seconds later it begins a bit of a slight rough idle.  The throttle response seems just a slight bit sluggish as compared to how it used to be.

Any ideas?

Thanks

[Just 6T9 CHGR]

You said you did a reseal of the engine.....did you check for vacuum leaks at the intake manifold?

[StockMan]

Thanks for your reply.  I did reseal the entire engine.  It did run perfectly when I had it running outside of the car during testing/tuning.  Its only after I installed it where now it has this issue.  The only differences are.
1. New plug wires.  While it was outside of the car I had a non correct set of yellow 8mil ones.
   - note that I triple checked to ensure they were installed correctly.
2. Different transmission.  I'm in the process now of filling it, almost there.
3. Carb kit.  But it wasn't running properly even before the carb kit.
4. I've moved the timing around a bit.
5. A thicker gasket from the carb to the intake.

Some things I tried.
- Adjusted the air screw on the front of the carb.  Note that it appears that it runs the best when this is screwed all the way in.
- Plugged all vacuum lines to see if that was the problem.

When it starts up initially it runs very smooth, a few seconds after that it suddenly begins with a very slight shake like someone pulled a vacuum line or an arc started from a plug wire.  The motor is still running ok, just not a smooth as it was initially.  Due to the air screw needed to be turned all the way in concerns me.

Thanks

[firefighter3931]

Thanks for your reply.  I did reseal the entire engine.  It did run perfectly when I had it running outside of the car during testing/tuning.  Its only after I installed it where now it has this issue.  The only differences are.
1. New plug wires.  While it was outside of the car I had a non correct set of yellow 8mil ones.
   - note that I triple checked to ensure they were installed correctly.
2. Different transmission.  I'm in the process now of filling it, almost there.
3. Carb kit.  But it wasn't running properly even before the carb kit.
4. I've moved the timing around a bit.
5. A thicker gasket from the carb to the intake.

Some things I tried.
- Adjusted the air screw on the front of the carb.  Note that it appears that it runs the best when this is screwed all the way in.
- Plugged all vacuum lines to see if that was the problem.

When it starts up initially it runs very smooth, a few seconds after that it suddenly begins with a very slight shake like someone pulled a vacuum line or an arc started from a plug wire.  The motor is still running ok, just not a smooth as it was initially.  Due to the air screw needed to be turned all the way in concerns me.

Thanks

It could be many things but start with a check for vacuum leaks. The idle circuit doesn't seem to be functioning correctly from your description (idle mix screws turned all the way in). As the engine warms up and things start to expand the leak could be leaning the mixture out...that's one posibility.

Get a vacuum guage on it and ee how much vacuum you've got cold and if it change after the engine warms up to operating temp.



Ron

[StockMan]

Thanks, I'll do that.

I just went out and checked the plugs and they are all very black.  I'm sure this occurred before I installed the new carb kit, sometimes a float would stick and dump fuel into the motor.  The plugs on the pass side of the motor appear to have been running a little richer.  Now that I've been running the motor following the carb kit, the #7 plug (rear passenger side) appears to have burnt off the black soot and running good to maybe a little lean.  Why would this plug be doing this?  Is this where the vacuum line for the booster feeds from?

[firefighter3931]

.  Now that I've been running the motor following the carb kit, the #7 plug (rear passenger side) appears to have burnt off the black soot and running good to maybe a little lean.  Why would this plug be doing this?  Is this where the vacuum line for the booster feeds from?

That cylinder appears to be running leaner than the others. On some intake manifolds the booster vac fitting does indeed fit on the #7 intake runner. Try closing off that fitting and see if the same conditions persist. If the engine runs better then you've got a leaking PB booster.

I would throw a set of fresh plugs in the motor and see where you stand....your's are fouled and should be changed, inmo. I like NGK XR5's in an iron head bigblock street build. 



Ron

[StockMan]

I'll grab a new set of plugs.  I've got a set of champions in there now, I'll try the NGKs.  Is it true that after a plug is fowled that they are at risk of shorting internally, even if they are cleaned?  The other thing I have hooked upto that vacuum fitting is the pod that control heating/AC under the dash.  This is one line I didn't plug off.

Thanks again.

[firefighter3931]

  The other thing I have hooked upto that vacuum fitting is the pod that control heating/AC under the dash.  This is one line I didn't plug off.

That is definately a problem....all vacuum lines need to be connected or the fitting ha to be capped. 

That's  good place to start.

Is it true that after a plug is fowled that they are at risk of shorting internally, even if they are cleaned?

I haven't experienced that but it is possible...

Once a plug fouls i just replace it, they're cheap enough to not take any chances. 



Ron

[Purple440]

StockMan,

I have that intermittent shake on my 440 also, and I can't figure it out.  My #7 plug was also lean and I mostly fixed that by clamping the hose ending on the brake booster, as it was very loose (thx Chryco).  This did not fix the intermittent shaking though.  When you plug in a vacuum gauge is the needle shaking or steady? Mine shakes like a belly dancer...

I'm very curious to see if you find the cause of this shaking as it's driving me nutz and I can't figure it out. 

- Doug

[StockMan]

Has this shake always been there in your motor?  This 383 of mine was running excellent and very smooth until I installed it into the car.  I'm thinking its because the plugs have been running rich due to the bad carb.  I recently added a carb kit, I now will replace the plugs and see where I'm at.  The motor starts us and runs smooth then about 8-10 seconds later it begins to move between a smooth idle and a bit of a slight shake.  It does sound like a bit of an intermittent miss on a couple of cylinders.  I don't know why turning the air screw all the way in makes it run better?  This would almost mean a vacuum leak somewhere.

I'll keep you posted.  Thanks..

[Purple440]

No my engine didn't always do this, but I can't remember when it started happening.  I'm on my third carb, and it didn't happen when I still had my first I know that much...it ran great on the engine stand also.  Had a Carter, now on my second Demon. 

yep, starts and runs fine, but then after a few seconds (20?) it starts shaking, then smooth, then shacking, ect ect ect.  ugh.  I'm getting a Holley this spring so I'll see how that works.

hey you don't happen to get shocked when grabbing the distro while running do you?  I have that happening also and am wondering if that's related.

[StockMan]

I cleaned up the plugs and reset the air needed to 1.5 turns out and its running nice again.  I'm suprised that black residue on the plugs would make that kind of a difference.  I ensured that all the plugs were gapped properly as well.  Thanks you all your advise.

[firefighter3931]

I cleaned up the plugs and reset the air needed to 1.5 turns out and its running nice again.  I'm suprised that black residue on the plugs would make that kind of a difference.  I ensured that all the plugs were gapped properly as well.  Thanks you all your advise.

Good work ! 

Keep an eye on those plugs 



Ron

[Purple440]

Glad you got it figured out StockMan!  I wish that would have worked for me, oh well.  Summer is coming and it's almost time to bring the beast out of storage and give it another try. 

I'll check my plugs and make sure they're uber clean!

- Doug

[StockMan]

Thanks.  I'm going to review some of the threads that get into carb tuning.  I picked up a vacuum gauge, it doesn't look to hard.  Do you still have the stock distributor on yours?  Do any of your plugs appear to be running lean?  I wonder if it could have anything to do with pressure building up in the block, then releasing every few seconds when it gets high enough??  Do you have a steel shim head gasket as opposed to a composite?  I'm wondering if you might have a leak between cylinders, but I guess if this was the case the problem would be consistent.