Welcome to the Corvette Forums at the Corvette Action Center!

Question: Vacuum leaks

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Well this weekend has been fun.
Pulled the carb off of the car just to be sure about the manifold layout.
It seems like the is a 5/8"(?) hole in the manifold between the two sides, I'm assuming this was for the EGR.
Adjusted the idle fuel while I had it off as well.
I noticed the two spare base gaskets I have for the car were different to the one I took off. 20160507_135221.jpg
The one on the left is what came off. and the one on the right is the replacement I fitted. (still have one spare)
I can't see it making much of a difference.

The next day (today) I took it for a short drive to let things warm up before doing some checks. about half way through the idle speed ended up at 3500RPM. found the secondaries had become stuck on small amount of sealant and wouldn't close, removed that and all was good, idled at 800rpm.
I have now been able to get it to idle at 700RPM so that's a step in the right direction.
I also placed my hand over the primaries while it was idling. not enough to block off all the air just to restrict it. That made an obvious rise in engine speed. I would have expected the opposite to happen which makes me think it's still running lean, as the dieseling still occurs as well.
Connected the vacuum gauge and it still reads a steady 15"hg, if you quickly open the close the throttle it drops to almost 0, before rising up to 22 and the back down to 15.

I'm still very tempted to just pull the carb and do the rebuild on it since I have all the bits anyway, at least that way it can be eliminated.
 

dougelam

Well-known member
Joined
Jul 19, 2011
Messages
453
Location
Michigan
Corvette
2002 Roadster
I assume you were adjusting the air/fuel mixture screws on the throttle body while you had it off.
Those should both be a 3 3/4 turns, final adjustment of the idle air is by the idle air bleed on top.
The beauty of this design is, for example if you run the idle air bleed all the way down and still need more fuel you can open up the air/fuel mixture screws a 1/4 turn and adjust as needed.
The 5/8 hole creates a single plane plenum so their is equal flow from both venturies. So both air/fuel mixture screws need to be the at the same number of turns.

I did just notice you don't have the 1.304 gage, it is VERY important when setting up a E4ME


Sent from my SM-T530NU using Tapatalk
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
I assume you were adjusting the air/fuel mixture screws on the throttle body while you had it off.
Those should both be a 3 3/4 turns, final adjustment of the idle air is by the idle air bleed on top.
The beauty of this design is, for example if you run the idle air bleed all the way down and still need more fuel you can open up the air/fuel mixture screws a 1/4 turn and adjust as needed.
The 5/8 hole creates a single plane plenum so their is equal flow from both venturies. So both air/fuel mixture screws need to be the at the same number of turns.

I did just notice you don't have the 1.304 gage, it is VERY important when setting up a E4ME


Sent from my SM-T530NU using Tapatalk

Never seen anywhere about needing a 1.304 gauge. What's it used for?
 

dougelam

Well-known member
Joined
Jul 19, 2011
Messages
453
Location
Michigan
Corvette
2002 Roadster
It's for the lean mixture adjustment
Should be fig. 2 in your instructions

Sent from my SM-N920P using Tapatalk
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
I assume you were adjusting the air/fuel mixture screws on the throttle body while you had it off.
Those should both be a 3 3/4 turns, final adjustment of the idle air is by the idle air bleed on top.

3 3/4 turns! This may be the solution to the problem. Thats a 1/4 turn more thatn I've seen anywhere else. (I've seen 3 1/3 to 3 1/2 turns)
I haven't adjusted anything yet but will when i get home from work if there's time.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
:happyanim:
Adjusted to 3 3/4 turns. Took it for a drive, had to drop the idle down again from 850 to 700. I noticed it was taking longer for the oil temp to go up than it had been. Haven't set the idle air bleed yet, but did check it and it was still low 30's. Checked the vacuum still down at 15, but again not sure if my gauge reads correctly.
But the best part was when I got home and shut it off it just stopped. No dieseling.
 

dougelam

Well-known member
Joined
Jul 19, 2011
Messages
453
Location
Michigan
Corvette
2002 Roadster
That's great
Now you need to stream clean your combustion chamber.
Get the engine up to operating temperature, it needs to be hot to be effective.
With the engine at 2000RPM, poor cold water down one side of the carb then the other side.
You will use at least a quart if not more.
Poor to quickly the engine will stall, to slow is OK just takes longer.
You will know is clean when the burnt smell is gone

Sent from my SM-T530NU using Tapatalk
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Haven't driven it since then. The altenator warning and choke warning lights had started to come on and off. I'm guessing it'll just be a ground. Right now I'm replacing a few wiring harnesses as well and have the speedo out. (Replacing lamps with leds at the same time). I'll post an update once it's running again.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
I think it's going to be a bit rainy for a little while might be a while before I get a chance to give her a proper run and test.
Speedo is back in with the new LEDs. Wow what an improvement:D. I also painted the reflective surfaces red to match the interior. Still have to do the small gauges but will soon.

Won't let me upload a photo for some reason.;shrug
Dropbox - 2:love:521_173938.jpg
 

Peer81

Well-known member
Joined
May 21, 2003
Messages
2,497
Location
Netherlands
Corvette
'81 Black
Looks great!

Btw. On the steam clean... Looks nice if it's only carbon in the intake to take care of. But with the crank vacuum also into the intake it's an oily mess inside so I think some steamed up water wouldn't make any difference.

Greetings Peter
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Looks great!

Btw. On the steam clean... Looks nice if it's only carbon in the intake to take care of. But with the crank vacuum also into the intake it's an oily mess inside so I think some steamed up water wouldn't make any difference.

Greetings Peter

When I had the carb off it appeared to be more carbon than oil. Still not sure how much of an effect the water will have, but it's not going to hurt to try it.

Its been very wet here recently but may give it a try this weekend.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
OK finally I was able to get it out for a drive. I haven't changed the plugs yet as there was some damage to one of the plugs in the first set so I'm waiting for a replacement. Seemed to run fine, idle got a little rough when I was waiting for a few minutes at some roadworks. Oil temperature is definitely slower to rise and falls quicker when idling. It's still dieseling however. But I am still to do the steam clean and change the plugs so that might be all it is.
Hopefully I'll get the steam clean done later this weekend.

Alternator and choke warning don't come on anymore. Not sure exactly what the problem was as I replaced the forward lamp harness, alternator to battery harness and one of the ground straps, as well as cleaning most of the others. But i did find that the ground connection on the alternator had stripped so that could've been it too. I've also fitted a new circuit board to the small gauges and change all of the lamp in there out for LED also. Which led to one interesting observation; the the LED lamp in the fuel sender will dim like the normal lamp did. I was expecting it to just be a straight on or off.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Tried the steam clean today. Does seem to have made a difference in the idle quality. Checked the manifold vacuum and that's up to 17. Still dieseling. I'll pull a few plugs tomorrow and check the insides out again. But could still just be the old plugs too.

Have noticed one strange problem now though. The oil pressure gauge never drops below 40 and is at times pegged well past 80. Could just be a poor connection though I guess. Hopefully that's all it is and it's not correctly reporting those pressures.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Have noticed one strange problem now though. The oil pressure gauge never drops below 40 and is at times pegged well past 80. Could just be a poor connection though I guess. Hopefully that's all it is and it's not correctly reporting those pressures.

Poor connection it was. at the plug that goes through the firewall. The bolt that holds the two together seemed a bit loose, but gave the connectors a bit of a clean too.

I had it out for a short run this weekend, after fitting the AC delco plugs. I would have had it out longer but it started to intermittently miss fire; and it seemed to be on the primary side of the coil not the secondary. This is what led me to check the plug at the firewall, and I found the hot wire was loose in the connector and was pushing out the back of the plug instead of going into the terminal. Hopefully I'll get it out for a good run soon. :)
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Dieseling issue seems to be sorted. I've made a few small changes since it last happened so not sure which one it was. I suspect it was changing to the AC Delco plugs; but I did find a couple of small issues in the distributor as well.
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
Been A while since I updated this. (took a little bit of a break from it for winter and holidays + just doing other things instead)
I've been doing a few small changes over this weekend trying to get the idle issue finally sorted. (It would get rough if left idling for more than about 30 seconds often less) I was thinking about when this would happen and it occurred to me that it seemed to be temperature related. This led me to replace the O2 sensor with a heated sensor. No more rough idle now. even when idling for a few minutes. Vacuum is still around 15-16 (I have checked my gauge on my brothers C10 when helping him with the timing and it read around 19) but I'm not sure why. I'm wondering if it's not a valve timing issue or a aftermarket cam.
 

LLC5

Well-known member
Joined
Sep 28, 2004
Messages
2,299
Location
Wa.
Corvette
98 black 6spd convert.
Been A while since I updated this. (took a little bit of a break from it for winter and holidays + just doing other things instead)
I've been doing a few small changes over this weekend trying to get the idle issue finally sorted. (It would get rough if left idling for more than about 30 seconds often less) I was thinking about when this would happen and it occurred to me that it seemed to be temperature related. This led me to replace the O2 sensor with a heated sensor. No more rough idle now. even when idling for a few minutes. Vacuum is still around 15-16 (I have checked my gauge on my brothers C10 when helping him with the timing and it read around 19) but I'm not sure why. I'm wondering if it's not a valve timing issue or a aftermarket cam.


Sorry, I don't have time to read the whole post but 15-16" vacuum at idle is too low for a mild or stock cam. If you have a quadrajet, then sunk floats or leaking well plugs come to mind but that should also cause sooty plugs. For vacuum leaks I like to use propane, with a small hose attached to the end of the bottle. With a small enough hose you can really pinpoint the leak, plus it is safe to use, easy to modulate how much to dispense, and propane is heavier than air and goes down not up. Any time you spray an area with a propellant you never know for sure if the residual just got sucked up and into the throttle opening or that there actually is a leak, plus most propellants are flammable.
 

Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Supporting Vendors

Dealers:

MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!

Parts/Accessories:

Vetteskins

Advertise with the Corvette Action Center!

Double Your Chances!

Partners

Top Bottom