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Old 05-11-2018, 06:41 AM   #1
justin_jj_Taylor
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454 Throttle Hesitation/Vacuum Leak??

My truck will hardly get out of it's own way, and it's getting worse. It acts like it's cold all the time, choking and falling on it's face until about 2500RPM or so, then it takes off. Currently hoping it'll take me home after work...

I bought an 80's burb last year with a rebuilt 454 that had been said to have sat for 5ish years. I put it in my 73, and replaced the timing chain and gear, oil pump and pan gasket, rear main seal, etc just to be safe. I installed a 74 454 stock intake (for divorced choke, trying to keep the truck era-correct) that went to the machine shop to be planed. I also bought a rebuilt Quadrajet (Jet Stage 2) and a Mallory HEI distributor.

Here's what I've done so far.
-Sprayed all around the intake with carb cleaner, doesn't seem to be leaking.
-Checked timing and advance, all works properly
-Checked voltage at distributor, only 10 volts, ran wire direct from fuse panel, now has over 12 volts.
-Replaced carb gasket, as it was leaking but no longer is.
-It appears to be getting plenty of fuel, you can smell it all time, it's definitely not burning what it's being given.

My next step is to see what springs they put in the distributor for the advance, the advance works instantly, so my thought is possibly different springs will make a difference? They sent a few different ones with it. I was also going to throw a new set of plugs in it, since when it was first put in the truck, it was started and moved several times with open manifolds so they never had a real chance to get hot.

Also, choke is currently wired open, haven't purchased a new divorced choke thermostat yet.

Does anyone have any other ideas to try before I start digging deeper?

Last edited by justin_jj_Taylor; 05-11-2018 at 06:49 AM.
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Old 05-11-2018, 08:55 AM   #2
geezer#99
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Re: 454 Throttle Hesitation/Vacuum Leak??

Couple things!
You had the intake ‘planed’. Why?
And if you smell gas (you shouldn’t) your carb might be flooding a bit. Look inside the carb for any fuel while it’s idling. Shouldn’t be any.
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Old 05-11-2018, 09:33 AM   #3
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Re: 454 Throttle Hesitation/Vacuum Leak??

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Couple things!
You had the intake ‘planed’. Why?
And if you smell gas (you shouldn’t) your carb might be flooding a bit. Look inside the carb for any fuel while it’s idling. Shouldn’t be any.
The intake had been sitting awhile, I just wanted to assure it wasn't warped. As for the fuel issue, it doesn't have standing fuel in the carb while idling if that's what you mean, and gives it fuel as soon as you give it throttle, so I'm not sure how a quadrajet could over-fuel the engine, but I suppose it's possible.
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Old 05-11-2018, 09:46 AM   #4
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Re: 454 Throttle Hesitation/Vacuum Leak??

Float can be a little high.
All it takes is a little drip from the boosters to flood.
Your shaving the intake could create a vacuum leak on the bottom side of the intake where it connects to the head.
Shaving it changes the mating surfaces angle.

What are you running for initial timing?
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Old 05-11-2018, 10:02 AM   #5
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Re: 454 Throttle Hesitation/Vacuum Leak??

I'm fearing that the intake is leaking from underneath but planing the surfaces would cause it though? We've had intakes planed before numerous times at the machine shop and we've never had an issue with them. 10 degrees on timing
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Old 05-11-2018, 10:20 AM   #6
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Re: 454 Throttle Hesitation/Vacuum Leak??

You could bump the timing up a bit.
Can you adjust either mix screw all the way in without the engine wanting to quit?
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Old 05-11-2018, 10:38 AM   #7
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Re: 454 Throttle Hesitation/Vacuum Leak??

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Originally Posted by geezer#99 View Post
You could bump the timing up a bit.
Can you adjust either mix screw all the way in without the engine wanting to quit?
I haven't adjusted the carb much yet. Jet claims you don't have to adjust at all, but I believe it's close enough. I can try that though.
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Old 05-11-2018, 10:59 AM   #8
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Re: 454 Throttle Hesitation/Vacuum Leak??

Is there a good way to test if the intake is leaking underneath before I take it off?
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Old 05-11-2018, 11:08 AM   #9
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Re: 454 Throttle Hesitation/Vacuum Leak??

Check your plugs.
Underside leaks suck up a little oil that shows on the plugs.
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Old 05-11-2018, 11:10 AM   #10
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Re: 454 Throttle Hesitation/Vacuum Leak??

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Originally Posted by geezer#99 View Post
Check your plugs.
Underside leaks suck up a little oil that shows on the plugs.
Awesome, planned on changing those anyways. Thanks man!
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Old 05-11-2018, 12:19 PM   #11
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Re: 454 Throttle Hesitation/Vacuum Leak??

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I haven't adjusted the carb much yet. Jet claims you don't have to adjust at all, but I believe it's close enough. I can try that though.
Adjusting a mix screw like that and the motor not quitting tells you that you have the primary throttle too far open, the transfer slots are exposed totally and your motor is idling on the power circuit.
You don’t want that. Makes for boggy bottom end.
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Old 05-13-2018, 06:52 PM   #12
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Re: 454 Throttle Hesitation/Vacuum Leak??

I always suspect the carbonator. Are you sure that your choke is open all the way when the engine is warm (which won't take more than one minute in the summer?) I would remove the air cleaner when the engine is warm and make sure that the choke valve is in the full open position at idle, and yank the throttle to make sure that it stays open under moderate acceleration.
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Old 05-15-2018, 06:37 AM   #13
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Re: 454 Throttle Hesitation/Vacuum Leak??

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I always suspect the carbonator. Are you sure that your choke is open all the way when the engine is warm (which won't take more than one minute in the summer?) I would remove the air cleaner when the engine is warm and make sure that the choke valve is in the full open position at idle, and yank the throttle to make sure that it stays open under moderate acceleration.

The choke is currently wired open.
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Old 05-25-2018, 08:43 AM   #14
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Re: 454 Throttle Hesitation/Vacuum Leak??

Well I got it to run better but the hesitation is still there. So far I've done the following:

-Put on a quadrajet that is in use in another truck of mine, no fix
-Changed plugs, old ones looked good just smelled of fuel
-Poured Sea Foam down the carb, that seemed to help a little

I'm starting to think the valves are sticking. I plan on doing a running compression test to rule that out.
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Old 05-25-2018, 09:08 AM   #15
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Re: 454 Throttle Hesitation/Vacuum Leak??

Did you try bumping your timing up?

What’s your timing at now?
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Old 05-25-2018, 10:03 AM   #16
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Re: 454 Throttle Hesitation/Vacuum Leak??

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Originally Posted by geezer#99 View Post
Did you try bumping your timing up?

What’s your timing at now?
I haven't tried that yet, I can do that as well. Only reason I haven't yet is I recall it having it's hesitation before putting a timing gun on it, no matter where I had the distributor at.
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Old 05-25-2018, 08:41 PM   #17
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Re: 454 Throttle Hesitation/Vacuum Leak??

My 73 454 had a hot air, not divorced choke. I have found the divorced chokes to be the worst. Not relevant to your problem, but just sayin.
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Old 05-29-2018, 08:28 AM   #18
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Re: 454 Throttle Hesitation/Vacuum Leak??

I've always had good luck with them, I think it's really just a matter of personal opinion.
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Old 05-30-2018, 07:10 PM   #19
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Re: 454 Throttle Hesitation/Vacuum Leak??

I can adjust an automatic choke of any type to close fully when the engine is cold, but then it won't open fully when the engine is warm. Or, I can adjust the choke to open fully when the engine is warm, but then it won't close fully when then engine is cold. I have owned about 20 different carbureted vehicles - same story with each one, regardless of the type of automatic choke. So, I now use only manual chokes.

I would call this experience, not personal opinion, but each to his or her own.

Not sure why you have the choke wired open if it is functioning properly.
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Old 05-31-2018, 08:16 AM   #20
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Re: 454 Throttle Hesitation/Vacuum Leak??

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Originally Posted by tucsonjwt View Post
I can adjust an automatic choke of any type to close fully when the engine is cold, but then it won't open fully when the engine is warm. Or, I can adjust the choke to open fully when the engine is warm, but then it won't close fully when then engine is cold. I have owned about 20 different carbureted vehicles - same story with each one, regardless of the type of automatic choke. So, I now use only manual chokes.

I would call this experience, not personal opinion, but each to his or her own.

Not sure why you have the choke wired open if it is functioning properly.
Well technically, divorced choke is manual choke, but I get ya. I can always get a divorced choke to close and open all the way. Maybe that is just the "experience" as you mentioned.

It's currently wired open because I'm waiting for a new thermostat in the mail.
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