15 inches of vacuum

[Trippin3030]

Howdy folks sorry to bug you I'm sure there's already a thread on this but I've searched and searched and cant find one with solutions I haven't tried. First off I'm a Carpenter by trade with a love for turning wrenche's so my skill level may not be up to par.

I just rebuilt an edelbrock 1406 to replace my q-jet and like a rookie I didn't check how much vacuum I was getting with the Rochester. So I install the edelbrock set the choke as best I could and turns out the whole "screw them all in and then twist them out a turn and a half" for the idle air screws us where it draws the most vacuum but it's only drawing 15". I read somewhere that it should be about 18-21 inches. I can't find any leaks and when I palm the top of the carb it dies fast the only things I can think of is that the 1" spacer I have on the intake which I'm gonna pry off tomorrow or the timing is playing havoc. Any thoughts would be appreciated.

Engine specs:
355 sbc
Edelbrock aluminum intake
Edelbrock 1406 with a fresh rebuild and the floats are 7/16 at the top and 15/16 at drop (I followed the build sheet to a T)
Short tube headers
Stock cam

Sorry for the longwinded post just wanted to make sure you had as much as I could provide. Thanks for taking the time.

 

[Handy Andy]

What is your timing set at? Try advancing it until you stop gaining in vacuum and then back it off till your vacuum drops 1".
Not 100% sure on this but I don't think messing with idle mix screws will increase/decrease vacuum. Mainly I think it will just get it to steady out. Advancing timing will increase vacuum. 15"isn't horrible especially if your motor has a few miles on it. My 400 smallblock with dart shp heads only makes about 19" of vacuum... and it has only about a thousand miles on it

 

[1987 GMC Jimmy]

You may have 2 or 3 small vacuum leaks, too. I've found that small vacuum leaks on mine tend to lose about an inch of vacuum. I'm assuming your new intake doesn't have EGR, but did you keep the evap canister or the PCV valve?

 

[HotRodPC]

What do you know about your cam that's in it? Cam will certainly affect Vac at idle. If you have the least bit of valve overlap you're not going to be in the optimal readings.

 

[Camar068]

After you adjusted the idle mixture, did you have to go back and adjust the idle? I bounce back and forth on both to get it just right. I can only get 17.5 on mine but I blame that on plenty of miles on the engine. The carb is fairly new.

 

[Trippin3030]

Whew boy I did not expect this much help this fast I'm not complaining either I really appreciate you all taking time out to help me.

Handy andy
I have not yet dealt with the timing i had a shop do it because I had no idea about retarded or advanced timing but I'll take a shot at it today just for clarification though Turning it clockwise is advancing and counter clockwise is retrading right? And I should still set my gauge up on the manifold vacuum of the carb while I do it?

87 Jimmy
When I rebuilt the truck I took everything like that off but I might be thinking of a different part then your talking about. Are you talking about the coffee can sized emissions cleaner(I swear I'm not asure dumb as I sound please bear with me) as far as it goes the intake is barren of any ports.

Hotrod
When I say I rebuilt the engine I mean I tore apart the top end and cleaned and replaced everything but I didn't trust myself to take it any further. I had a buddy set the valves (again I really hope I have the correct idea of what a valve is) but this buddy also did some other things I had to fix later so I don't quite know what I'm doing there but if there's a link or if you could swing me to educate me on what to do properly under the valve cover I'd be very grateful

Camaro
I installed a tach for just that reason. It was pretty tedious running back an forth to check rpm but my kid though reading the tach would take away from the fun of pretending to run me over. It definitely gives me piece of mind that your telling me I did it properly thank you.

I won't lie to you folks I'm green as grass with all this and I know it probably frustrates the mechanical veterans on here but I don't like to many things as much I like puttering on this truck and I promise I'm doing my best again I truly appreciate your help....... and I'll try to shorten my posts

 

[1987 GMC Jimmy]

It's clockwise to retard and counterclockwise to advance. What you're thinking of is the EVAP canister. The EGR valve is a little saucer that mounts to stock and certain aftermarket intakes. The PCV valve is the little elbow shaped rattle that goes into the valve cover. Anything that uses vacuum can be a cause of a leak, though. Such as your brake booster. Vacuum can leak around the carb base gasket, too. A line comes off the factory setup at the back of the intake for cruise and HVAC setting control. I would do a good visual inspection of anything vacuum coming of the carb or intake. You can also spray starting fluid around the base gasket or any vacuum connections. If the idle changes, there's likely a leak. The only good way I know to test a brake booster is with a vacuum pump to see if it holds.

 

[Camar068]

you could always disconnect the brake booster hose at the carb, put your finger over it or plug it and see if your vacuum is higher.

This might be the test Jimmy's talking about:

http://ssbrakes.com/attachment/84298-Vacuum%20Brake%20Booster%20Testing%20and%20Diagnosis.pdf

Below is a cut and dry test and makes sense. Maybe 5 minutes to complete these tests. Guys please correct if this is BS, as I'm not positive.

http://www.roadkillcustoms.com/hot-rods-rat-rods/hot-rod-power-brake-booster-test.asp#axzz4gM3wTry7

 

[1987 GMC Jimmy]

Yeah, you're right. Or clamp the hose down with something that's not gonna cut the vacuum line. Those tests make sense to me. I've capped my finger over the vacuum line to test it before, too. I should have thought of that. It's finals, though, and I got up first thing Friday morning with just under three hours of sleep, and my brain is acting accordingly.

 

[Camar068]

your minds in the right place......finals first

 

[Trippin3030]

I took off the 1" spreadbore adapter plate put a new gasket down reinstalled the carb and I gained an inch of vacuum I'm up to 16" so a small victory. I did what camaro said and sprayed starter fluid around all the vacuum connections making sure to isolate the spray so it didn't suck down into the carb. None of the connections bogged the engine (I shot a bit into the carb just so I knew what I was looking for). Pulled the pcv and put my thumb over the hole the vacuum didn't change. I'm really leaning on the timing or maybe a leak on the "t" connector going to the tranny. It runs fine am I just being picky about how mucheap vacuum I need? Thanks for the input

 

[Trippin3030]

I really can't Thank you all enough for helping me out I got maybe 2 guys that I trust for info on trucks. The rest of the boys are into tuners and anytime we talk shop they won't stop trying to convince me to put a turbo on it.

 

[1987 GMC Jimmy]

It's just a hair low, I think. Seventeen is good, eighteen or more is perfect. You may could advance the timing a little, or you may still have one leak towards the trans modulator like you said. It's hardly anything, though. You're welcome!

 

[Rusty Nail]

I thought 15 inches sonded pretty good and don't consider 18-21 a realistic expectation for an engine with balls.
Whatever.

 

[1987 GMC Jimmy]

I'm operating under the assumption that this is just a slightly modified engine with a mild cam. If it has a meaner than mild cam, then a 15" reading with kind of a wiggly needle is great.