350 Tuning Issue

[cebra]

I am having a hard time getting my 350 to run correctly. I recently tore the motor down and replaced the head gaskets and everything that goes along with that. It didn't idle very well before requiring constant peddling to keep it from stalling. Now, in neutral or park it idles very high (1,700) and as soon as you drop it into gear it stalls out. I have hosed around the intake manifold and carb with starting fluid and replaced the intake manifold gasket/carb gasket a 2nd time and it didn't fix the issue. I have 17PSI on a vacuum gauge, timing is 6 degrees advanced. Any thoughts on things to check? Thanks.

[azle]

what carb?

[C-ries]

I've been having a similar issue with my pickup when I changed carburetors...a friend that has been helping me said it is a vacuum leak. We sprayed around the base of the carburetor and all vacuum lines with carb/choke cleaner until we found where it was leaking. When the carb/choke cleaner is sprayed into a leak the engine would race. One other thing he did was block the top of the carburetor with his hands, instead of killing the motor it would rev up uncontrollably. Haven't had a chance this week to get the leaks (in my case the manifold to carburetor gasket) fixed and vacuum hose replaced and to try it again. I'm very interested to see what others have for input as well.

[cebra]

I have the stock Holley 4150. I did notice if I unhook the throttle cable it stays at 1,700 rpm but if I pull back on it (not throttle it) the rpm will reduce. I wondered if the throttle plate was worn and letting vacuum in/not letting it fully out? Covering the top of the carb with my hands is something I have never heard of.

[clay68c10]

Are you running a return spring? Sounds like the high idle part of the problem is your throttle not closing completely.

[68 P.O.S.]

Is the engine stock other than the carb? You definitely want to correct the vac leak if you do indeed have one. Since you said “It didn't idle very well before”, it may be time to rebuild the carb as well. You should pull your carb off and re-square the transition slots too. This will ensure it’s idling on the correct circuit. This thread (http://67-72chevytrucks.com/vboard/s....php?p=7670124) has 2 good pics of the slots on post #37. That whole page really. Your timing is a bit low as well. Bump it up to 12 or 14 initial. Also, hooking up your vac advance correctly affects idle as well. It all works together. As clay68c10 said, make sure the return springs are doing their job.

[dagnabbitt]

Is your vacuum advance working?

[Coley]

"....your timing is a bit low as well. Bump it up to 12 or 14 initial." by 68 POS

I definitely agree with this......job #1
I wouldn't start to touch the carb until the timing was advanced and set.
As a result of this, you should see you vacuum rise at least a couple of points...maybe more and up to 20 or 21 if things are good inside the motor.

Then check your vacuum needle to see how steady it is. If it bouncing a lot, there are a few things that need to be checked.
If it is steady or just wavering 1/2point up/down........then so far so good no big problems yet.

Next, go over every single rubber vacuum line attached to either the carb or the intake manifold to check for leaks.
There are lots of ways to do this, including removing them temporarily and plugging the port with your finger......if things check out and there are no changes in the way it runrs, this will eliminate the vacuum lines from the equation.
As well don't forget to check the power brake rubber vacuum line, this is a big one and should always be checked....plus it will indicate if your power brake booster has failed and is leaking. I had this happen once when I couldn't find a vacuum leak....only to find my power brake booster was the culprit.


Next....check the base of the carb to make sure you have the right base/insulator gasket and it is secure and not leaking vacuum.
Also, check to make sure you have NO open vacuum ports on the carb...investigate.

Keep us posted.
All good
Coley

[Fbks_Cruiser]

You didn’t say whether you have an automatic transmission. I so, another culprit is the vacuum line to the modulator. There is a piece of rubber hose down between the steel line and the modulator on the transmission that gets brittle with age and can leak.

[cebra]

I have a TH350 trans. The motor has a mild cam (I don’t know thespecs), Edelbrock intake manifold, HEI distributor, and headers. Should I advance the timing more with this setup? Thanks for all the input, I will check all these this weekend and reset the timing. One other note is my idle screw is not making contact at all (as low as it will go) and 1,700 is as low as it idles in park.

[phat69]

Setting your initial without knowing your total mech advance is not the correct way.

[Fbks_Cruiser]

Okay. If idle speed screw is not contacting, but idle is still at 1700 rpm. Check throttle plates in carb. Are they partly open? If so there is binding in the throttle linkage not allowing them to close even though idle speed screw is backed out.

[cebra]

I know the advance is 6 but I guess that is wrong so I will move to 12-14. I had a few manuals I was cross referencing which is where I got the 6. I should note, I have worked on cars a long time, done some motor swaps and rebuilds etc but none that I couldn’t sort out with a AF gauge, laptop, or a light tester (i.e. first foray into carbureted engines) but I am eager to stumble through it and learn.

[68 P.O.S.]

As Fbks said, check your throttle plates and throttle linkage to make sure they're not being cracked open or hung up. Also, you need to pull the carb and square your transition slots. Once squared and it's idling, attach the vac advance to manifold vac and the idle will jump up a bit more. The squared slots and vac advance will have your idle close to dead on. You'll only have to fine tune the idle to 750. Now that it's running you can time it. Again, you want 12 to 14 initial just to start. Your HEI should have 20 to 22 degrees mech advance in it. You should shoot for 36 degrees total timing. I personally set total by removing the advance springs in the HEI and reving it to 2000 rpm. This ensures the advance is all in and you don't have to rev it very high. Reinstall springs and check timing at idle again to see where your initial falls, probably 12 or 14. Reattach vac advance to manifold vac and you're good to go. Now that its timed well, recheck your idle and adjust if neccessary. Also a good time to adjust idle mixture screws.

[Boog]

Not knowing what your engine is in we're guessing to some extent here. As far as the carb not idling down and the idle screw is not setting on the base, remove the throttle linkage/cable at the carb and see if the throttle will then back down. It could be misadjusted or binding.
The engine needs to idle down to check timing anyway.

[Coley]

I'm not sure about or familiar with the Holley carbs but on the quadrajets if you dial in the (passenger side) choke circuit in enough you override the regular idle and it can end up too high.
Does the Holley have a choke circuit and its own adjustment?

C

[HO455]

Here is a simple thing to check and to overlook. Is the throttle return spring still under tension at idle or is it bottomed out and not pulling the throttle completely closed?

[tdangle]

You have a carb issue. Don't worry about the timing right now, Factory settings were anywhere between 0 and 6 degrees BTDC and they idled just fine. A small vacuum leak will raise the idle slightly but not to 1700 RPM. A large vac leak will cause the engine to stall normally.

I suspect the float got stuck while you had the carb off and you may have fuel drizzling from the primary boosters. This happened to me once. The reason the RPM drops when you force the throttle closed even more is that fully closed they have a very slight gap in the bore.

Your original idle problem may very well have been a vac leak or mis-adjusted carb. Increasing the initial timing only helps throttle response and will with all things equal will slightly increase idle speed

I was always taught KISS (keep it simple stupid)

[cebra]

Thanks for all the feedback, I have a list of things to check. I have forgot some of the things I have done and I did reset the float levels watching a Holley video. The back one was letting fuel pour out of the bowl so I got that dialed in but will check again.

I set the timing to 12 and every degree past 6 made the engine open up, it ended up at 2,600 at 12...so I moved it back for now. I think the carb/vac issue will need to be sorted out before I can properly set the timing.

[demian5]

You never mentioned if you had a return spring. I have a new demon carb that has a pretty stout spring on the carb itself, but to get it to close fully, I still needed a return spring.

[Coley]

If you work on and adjust the carb before redoing your timing....you will be doing all your carb adjustments over again.
This will be due to the increased idle speed, vaccuum, etc.
Hence the general tune-up rule or 'circle', timing before carb

Regardless, 6° is ok and the truck will run there, but most of these trucks started at an 8° advance and most guys bump them up to get better performance, response, etc...highly recommended as a first step.

my two bits.
Coley

[68 P.O.S.]

Yes, as Coley said, timing is job #1 and always the first thing to set correctly before anything else. If your foundation isn't right, you're gonna be chasing your tail and redoing your work because you set it up on a crappy running engine. Since it still runs but doesn't "idle", you can set total timing to 36 instead of initial timing and move on to the carb from there. Once you set the transition slots, hook up vac advance, and set the idle, it should be running like a champ.

[cebra]

It is at 6 now, I have tried to set to more advance but the motor runs uncontrollably and heats up. My truck is fairly loud as well to be running at 2,500 rpm at 12 degrees. I will reset to 12 and adjust idle and af once I fix the issue. tried plugging each vacuum port and it didn’t change. I can unhook the throttle cable, with a double and tight return spring on and when I pull back on the throttle get it to idle fine. As soon as I let go it runs back up to 1,400. Could this be a worn throttle plate? The idle screw is all the way out or not touching so it should just die.

[geezer#99]

Check the high idle screw isn’t causing the rpm or maybe the secondaries are too far open.

[firedemon]

check the shaft for the throttle plates where it goes through the base of the carb . these carbs are getting old and can wear that hole out and cause a fairly significant vacume leak and is often overlooked . it dosent take very much either