vaccum leaks

[eric 87]

got a question. If it sounds like an existing let me know where to find and I will go. I have looked though and didn't see.

Problem. My 87 GMC shortbed 305tbi th400 stock JUST got a carb swap. Its done and we can discuss which is better till the cows come home.....I like both.
The problem is idle it dies. If I go to lightly give throttle it kills it if it hasn't died. IF I get the revs up first and take off down the road it runs stronger than ever with the TBI. Again that may have been the TBI issue. However it will roast the tire/s easily now when it wouldn't think of it before even with a 2.73 gear. It is a light truck though so there is that.
I took the can of starting fluid to the carb gasket and hoses and t fittings etc. At first nothing. Then I got one of those straws from another can and pin pointed the stream. FUNNY THING is...
The two center bolts on the intake (drivers side) when sprayed actually made the engine REV pretty high. Could there be a vacuum leak at those bolts? But then I was able to get the stream behind the throttle linkage a bit better and it too also rev'd the engine a bit.
But the biggest rev came at those bolts repeatedly. SO I got a universal 9/16 socket and lo and behold I got about a half to 3/4 turn on the front bolt of the two. I got about a full turn from the rear of the two. These are the two bolts that are at the different angle than the front and back bolts. Now if there is a vacuum leak on a fresh install of a new intake and new gaskets do I need to pull it completely back off?
One other question. I noticed I didn't put washers under the carb mounting nuts. Do they need washers? I know it would give maybe a little better spread of pressure but.....
I have never had an intake bolt leak vacuum....is that a thing? Of course I can't remember having this bad an issue with idle
IT has had two locally known HOLLEY carb guys who own their shops make adjustments. I bring this up because one had my accelerator pump so tight I think the lever/arm was pushed down constantly. So not sure of his HOLLEY GUY status.
I have myself looked at every holley carb adjustment video from Holley and others. I have set things to base and then made minor 1/8 turns of idle mixture evenly.
It runs good. I have a slight clatter and wonder if I am still a bit lean or if the leak is still existing since I tightened the bolts on the intake.
Timing I have at 8 BTDC. One of the shops had it at 3-4.
Shame you pay people to fine tune things and you end up doing a better job yourself.
What does the folks out here say. IS it possible to have a vacuum leak at those two bolts or is the spray close enough that the gasket above it behind the throttle is pulling it in.

 

[WP29P4A]

From my past experience with intake gaskets, here are my thoughts. The bolts are not the leaking issue, the fact that the bolts were not tight and not compressing the gasket enough to seal I believe is the issue. If it's all new there is a chance it can be torqued down to spec and still seal. If you tighten it down and do the spray test again and the RPM's pick up, that would confirm the need to pull it back off and seal it again properly. I spent a reasonable amount of time listening to engine builders on how and when to use sealers with gaskets. Lots of people think the more crap you pollute the gasket with the better it will seal, the engine builders disagree with this fantasy.

Washers under nuts are good at spreading out the contact area especially on aluminum and pot metal, they also allow the nut to tighten without digging in and binding on the surface so you can get consistent tightness on all the nuts.

 

[Bextreme04]

got a question. If it sounds like an existing let me know where to find and I will go. I have looked though and didn't see.

Problem. My 87 GMC shortbed 305tbi th400 stock JUST got a carb swap. Its done and we can discuss which is better till the cows come home.....I like both.
The problem is idle it dies. If I go to lightly give throttle it kills it if it hasn't died. IF I get the revs up first and take off down the road it runs stronger than ever with the TBI. Again that may have been the TBI issue. However it will roast the tire/s easily now when it wouldn't think of it before even with a 2.73 gear. It is a light truck though so there is that.
I took the can of starting fluid to the carb gasket and hoses and t fittings etc. At first nothing. Then I got one of those straws from another can and pin pointed the stream. FUNNY THING is...
The two center bolts on the intake (drivers side) when sprayed actually made the engine REV pretty high. Could there be a vacuum leak at those bolts? But then I was able to get the stream behind the throttle linkage a bit better and it too also rev'd the engine a bit.
But the biggest rev came at those bolts repeatedly. SO I got a universal 9/16 socket and lo and behold I got about a half to 3/4 turn on the front bolt of the two. I got about a full turn from the rear of the two. These are the two bolts that are at the different angle than the front and back bolts. Now if there is a vacuum leak on a fresh install of a new intake and new gaskets do I need to pull it completely back off?
One other question. I noticed I didn't put washers under the carb mounting nuts. Do they need washers? I know it would give maybe a little better spread of pressure but.....
I have never had an intake bolt leak vacuum....is that a thing? Of course I can't remember having this bad an issue with idle
IT has had two locally known HOLLEY carb guys who own their shops make adjustments. I bring this up because one had my accelerator pump so tight I think the lever/arm was pushed down constantly. So not sure of his HOLLEY GUY status.
I have myself looked at every holley carb adjustment video from Holley and others. I have set things to base and then made minor 1/8 turns of idle mixture evenly.
It runs good. I have a slight clatter and wonder if I am still a bit lean or if the leak is still existing since I tightened the bolts on the intake.
Timing I have at 8 BTDC. One of the shops had it at 3-4.
Shame you pay people to fine tune things and you end up doing a better job yourself.
What does the folks out here say. IS it possible to have a vacuum leak at those two bolts or is the spray close enough that the gasket above it behind the throttle is pulling it in.

Those bolt holes go all the way through into the valley and the bolts should have thread sealer on them as well. If the bolts are not tight, you will get an intake leak, possibly an exhaust leak from the crossover, and possibly oil into both the top of the intake and into the intake/exhaust runners. All intake bolts should have thread sealer on them, and should be properly torqued down or you will have multiple issues.

 

[eric 87]

Those bolt holes go all the way through into the valley and the bolts should have thread sealer on them as well. If the bolts are not tight, you will get an intake leak, possibly an exhaust leak from the crossover, and possibly oil into both the top of the intake and into the intake/exhaust runners. All intake bolts should have thread sealer on them, and should be properly torqued down or you will have multiple issues.

I do appreciate that. I did notice on the drivers side there was about half teaspoon of oil below that back center bolt. THEN noticed I had an open hole for some bracket mount or something next to that bolt. I put a bolt in it. I have to pull it back out and put sealant on it. I went back around all the bolts to retorque them and all needed a good half to full turn to tighten even before torque. I still get a rev when near the gasket on the carb but also a little rev still in that valley area near those two bolts at the center. I tried to look for any crack and do not see any. This is a new Holley intake but that doesn't mean it doesn't have a crack somewhere. My torque I have at 25 I think on all of them. I did put a red gel sealant on all of them. I think it actually might have been the red gel thread locker now that I think about it. I do have a white thread sealant to stop any leaks. But is it advisable to take one bolt out at a time and put that on them and retorque each as I go or will that cause a break in gasket and sealant I used around each port? Risk reward on that?

 

[gmbellew]

you can remove one bolt at a time and add sealant without disrupting the gasket. if you used red thread locker, I'd try and remove them ASAP. red is the permanent thread locker. I have used a permatex thread sealant with good success on those bolts.

sounds like your carb gasket is leaking for sure. I'd correct that, properly seal the intake manifold bolts, and then see what you have.

 

[eric 87]

From my past experience with intake gaskets, here are my thoughts. The bolts are not the leaking issue, the fact that the bolts were not tight and not compressing the gasket enough to seal I believe is the issue. If it's all new there is a chance it can be torqued down to spec and still seal. If you tighten it down and do the spray test again and the RPM's pick up, that would confirm the need to pull it back off and seal it again properly. I spent a reasonable amount of time listening to engine builders on how and when to use sealers with gaskets. Lots of people think the more crap you pollute the gasket with the better it will seal, the engine builders disagree with this fantasy.

Washers under nuts are good at spreading out the contact area especially on aluminum and pot metal, they also allow the nut to tighten without digging in and binding on the surface so you can get consistent tightness on all the nuts.

appreciate. I did go around and retighten all bolts. All the intake bolts have washers however my carb nuts do not. I was thinking maybe I might switch those to the bolts that came with it as well. They have that odd head is why I didn't used them. Not the typical bolt head. I did tighten and got a slight rise in the same area of the intake valley where the center bolts are. I did note that there was an open bolt hole next to the back center bolt. I didn't know it was all the way through but it is. I put a bolt in it. I will take back out and put sealant on it. I used red gel thread locker sealant on all intake bolts. Now thinking maybe I should have used the white gas resistant thread sealant instead. Not sure if I can take each bolt out one at a time and reseal and retorque. I don't want to take off the intake for a 2nd time since it was installed. The first time was due to valley sealant not holding. Bought new gaskets with the mega grey sealant around each port. A thin coat smeared evenly and thin. Especially where the valley meets the walls and around the water ports. I had used the wrong sealant in the valley walls and is why it blew the first time.
Man I do not want to pull that thing again. I will say there is no rev at the top edge of the intake just below the valve cover area. And none on the passenger side or hoses. Just the drivers side of the carb and the two bolts. Which honestly I think all the retightening helps tremendously.
I DO note that I don't have washers under the nuts holding the carb down and may pull it....put new gasket and use bolts this time....still washers should be used on Carb I take it.

 

[eric 87]

you can remove one bolt at a time and add sealant without disrupting the gasket. if you used red thread locker, I'd try and remove them ASAP. red is the permanent thread locker. I have used a permatex thread sealant with good success on those bolts.

sounds like your carb gasket is leaking for sure. I'd correct that, properly seal the intake manifold bolts, and then see what you have.

Thanks for the vote of one bolt at a time. I noted a small amount of oil before tightening them down again. Now nothing. I have a white sealant to stop leaks on bolts I think is permatex brand. It worked well on the intake plug I put in the coolant runner that was there for a temp sensor. But it does seal and make it tough to take apart. I used it on a fuel regulator fittings and took it back apart and It was TOUGH.

 

[topper]

I am not a mechanic so I'm just throwing this out there but with a better carb would you need to change out the fuel pump to feed it ?

 

[Old60Driver]

The stock mechanical fuel pump should work just fine for that, IMHO. Unless it's not working right. Then you'd need to change out the fuel pump.

 

[eric 87]

Okay. On my truck the block doesn't have the provision for a mechanical pump. Man do I wish it did.
BUT guys let me tell you this journey only to share the oddest things. Turns out my TBI system had two problems. I had replaced everything related to include the computer. All sensors and some of those twice. Ended up being the pickup coil in the ignition. That was one issue. But it still wasn't right after me and the shop I ended up using worked on it. Fed up I did the swap. Many said I would regret.
While dismantling the sensors and prepping to take the intake off I was taking out the ignition module plug. Let me go back. When fixing the pickup coil I just replaced the entire distributor. New module in it. So when it still would not run correctly as mentioned I took out the cluster to see why my codes....oh a burnt bulb in the SES socket. Put a new one in and code 42. Fuel relay circuit short of ignition module. I BACK pinned to module and the circuit was good. I replaced both the fuel relay and the ignition module.
So fast forward I am dismantling the intake and pull the plug out of the ignition module to see one of the four pins was gone. No copper pin. This was throwing the code. Now at this point I have nearly a grand in parts new intake carb and HEI distributor along with the needed extras. Do I stop and buy a new four pin pigtail , see if that works and keep the TBI system only to have something else go bad MAYBE. Or use the parts I just bought and continue marching. This is what I Did.
Now myself doing the work but never getting the carb exactly right even though Holley wet tests them it wasn't right. I had to reputable shops I THOUGHT adjust on it. Still not right. I decide to watch every holley youtube on every adjustment and follow word for word.
Still nope.
Decide I will do what I must to get it running well enough to sell. Be honest that I can't get the carb exactly right. Break even is all I hoped for. Actually stopped and test drove 3 nearly new vehicles at a dealer. I was driving my 69 firebird and one of the older shop mechanics at the dealership came out to check it out. Discussed my truck with him. Told me there was a retired guy that lived within blocks of me that liked old school stuff and did work in his own shop. Maybe check him out. I didn't even go home I went to his house from there. Glad I did. I told him I had found massive vacuum leaks all the reputable guys missed when adjusting the carb. BUT that I can't get rid of the leak along the base of the carb behind the throttle linkage. I had replaced the gasket twice.
Took it over to him. He did the same. Used his own gasket he had a little thicker. NOPE.
He took his Holley 650 off his boat and put it on my intake and....PERFECT RUNNING TRUCK. No vacuum leak even using that used gasket. At first he thought the base of my carbs throttle plate wasn't true. Even being a new Holley which he loves. It bugged him so that night he took the throttle plate off. NO GASKET between the body and the plate. Just metal on metal. He took one of his carbs apart and found one real close and then put mine back on and he nearly had it sealed.
I called Summit and told them not knowing he had done this and knew what it was. They thought the base wasnt true either. They sent me a new one, THEY PAID for express overnight shipping. I got it the next day. Printed me off labels to send it back.
Two good things came about. Found a competent guy to have when I get into a pickle and found in this case Summit was amazing. I have to send the other back within 30 days. He brought the truck to my house about 2 hours after I took him the new carb and it runs better than it ever did. Who knows. It might actually get to stick around now and not be replaced by a new vehicle!
Also I know to question everything as he did and search for that answer instead of accepting that it is a new part so therefore it is fine. But for him to think about that leak maybe being between the throttle plate and carb body good catch. Odd thing is it didn't leak all the way around without that gasket. Only on the throttle side of the plate.
Vacuum leaks are amazing at how bad it will make an engine run.