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Engine rough at part throttle. EGR delete?

r3bartl

r3bartl

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canada
Hey guys,

I have a sbc 400 in my 77 blazer. When cold the engine runs good. Once it's up to running temp part throttle around 1500 rpm gets rough. Idle is fine and lots of throttle is fine as well. This was a smog motor and got all the smog stuff deleted. I'm assuming it has the egr quadrajet still on it. Could this be the root of my issue? I'm not against buying a new carb as this is a project. I do have a 700r4 and got the TVcable set up and don't really want to play around with that again tho lol. If anyone got any insite on this it would be great. Should i get a new carb? Play around with timing? New distributor.

Just to add to i bought this truck with the engine and trans already swapped on it. So I know nothing about what's been done or how used anything is. The distributor and carb i would suspect are as old as the engine. Which I believe to be a 79. So things might not be in perfect working order. I do know the vacuum advance works and the mechanical advance works as well. Haven't check to see if it's fully advanced tho. but this is a part throttle issue.

Thanks.
 
Does this happen in neutral/park as well? If so what does a vacuum guage say while symptoms are present.
Not sure any egr qjet is different from non egr qjet.
Having cruise circuit issue could be a plugged passage, dropped air bleed tubes.
Ethanol fuel relay messes with metering after a few years, a good cleaning can do wonders.
Be sure to get ethanol tolerant parts to put back after cleaning
 
I removed EGR on my '83, with no other changes I had a *tiny* bit of lean surge at part throttle cruise.

With that age of components I wouldn't blame the carb being "emissions". As Wes said, carbs were not really that different emissions or not.
 
Wes Harden said:
Does this happen in neutral/park as well? If so what does a vacuum guage say while symptoms are present.
Not sure any egr qjet is different from non egr qjet.
Having cruise circuit issue could be a plugged passage, dropped air bleed tubes.
Ethanol fuel relay messes with metering after a few years, a good cleaning can do wonders.
Be sure to get ethanol tolerant parts to put back after cleaning
Click to expand...
Hey wes

She idles perfect. No issues there. Not sure what the vacuum reads at part throttle. There's definitely some data I gotta collect to get the whole picture. Also I don't believe my truck has a ethanol fuel replay. I don't think she has any relays lol. I read in a forum that the metering rods on a EGR quadrajet are different from not EGR one. Causing the engine to run lean at cruising speeds. Which seems to line up pretty good with my issue. Now some people have zero problem deleting there EGR accessories and other seem to have this issue I'm having.

Now a fresh rebuild on the quadrajet and a new distributor wouldn't be a bad investing anyway. But the engine runs good otherwise and my sparkplug look correct.
 
dyeager535 said:
I removed EGR on my '83, with no other changes I had a *tiny* bit of lean surge at part throttle cruise.

With that age of components I wouldn't blame the carb being "emissions". As Wes said, carbs were not really that different emissions or not.
Click to expand...
That lean surge is what I'm thinking is my issue. We have a lot of 50km/h roads here and I'm doing about 1500 rpm in 3rd. And the engine just shutters when ur maintaining that speed up small inclines or hills.
 
Not hard to swap metering rods, or main jets.
Carburetor unlimited has a nice kit that proves typical needed parts to fine tune a qjet.
As for a new dist. If yours is in good shape I'd leave it. Very difficult to find quality replacements today. Maybe just a new gm or non counterfeit ac Delco, ignition control module
 
Last edited:
Wes Harden said:
Not hard to swap metering rods, or main jets.
Carburetor unlimited has a nice kit that proves typical needed parts to fine tune a qjet.
As for a new dist. If yours is in good shape I'd leave it. Very difficult to find quality replacements today. Maybe just a new gm or non counterfeit ac Delco, ignition control module
Click to expand...
Ah here we go. That's the info I needed. And you're right about the distributor. Good parts are hard to find. I'll toss a test light on it and make sure I'm getting full advance before making that call. I just wanted to see if this egr delete infact has this issue associated with it and it seems to be. He didn't do the 700r4 right so I'm assuming he didn't do this right either lol. I may have a cracked flex plate too. But that's for another day lol
 
dyeager535 said:
Yep, I threw in a slightly richer set of rods and my surging was gone. But that was back when the wrecking yards were packed with vehicles with q-jets. :(
Click to expand...
Yeah that's the kinda info I was wanting to hear. Just a small change needed to get the drive ability back. Thanks.
 
dyeager535 said:
Yep, I threw in a slightly richer set of rods and my surging was gone. But that was back when the wrecking yards were packed with vehicles with q-jets. :(
Click to expand...
Did you use 4m rods or m4 rods do you know? I know one set is longer than the other but can be made to work.
 
Wes Harden said:
Not hard to swap metering rods, or main jets.
Carburetor unlimited has a nice kit that proves typical needed parts to fine tune a qjet.
As for a new dist. If yours is in good shape I'd leave it. Very difficult to find quality replacements today. Maybe just a new gm or non counterfeit ac Delco, ignition control module
Click to expand...
Also I just decoded my qjets serial number. 17059229. It was on a 79 manual transmission. It's now on an automatic. Could this be a potential issue. I've read earlier models some would use ported vacuum or manifold vacuum for the vacuum advance.
 
Quadrajets are actually extremely complex when it comes to the amount of tuning available. Almost guaranteed rods are different manual to automatic. But enough to actually notice? Probably not.

Honestly can't recall which rod sets I used, I was always partial to the later q-jets vs. earlier, so whichever are correct for that series. Early I call the squared off front ones, the later the casting was a bit triangular at the front, I believe for the PCV line? Haven't messed with a q-jet in decades. Used to re-gasket them in the living room lol. Carbs unlimited was right down the hill from me and would bush the always worn throttle shafts for like $25.
 
dyeager535 said:
Quadrajets are actually extremely complex when it comes to the amount of tuning available. Almost guaranteed rods are different manual to automatic. But enough to actually notice? Probably not.

Honestly can't recall which rod sets I used, I was always partial to the later q-jets vs. earlier, so whichever are correct for that series. Early I call the squared off front ones, the later the casting was a bit triangular at the front, I believe for the PCV line? Haven't messed with a q-jet in decades. Used to re-gasket them in the living room lol. Carbs unlimited was right down the hill from me and would bush the always worn throttle shafts for like $25.
Click to expand...
I spent all night shift at work researching this. It's wild how much tunablity there is on these. So I found i have a late model qjet which is very tunable. Has the apt screw. There's a carb company that makes full tappered rods that u adjust with the jet and the apt screw. U can even knock out the aluminum plug above it so u can adjust it without taking it apart. 10/10. I'll give it a rebuild along with it and should be golden after that.

I plan on doing it at the kitchen table as well. Don't tell the wife hahaha
 
If you have deleted the EGR be sure that the vacuum port is properly capped off on the carb. Don't forget that vacuum port way in the back of the carb towards the firewall that the air cleaner vacuum hose plugs into. It's really hard to see and hard to get to, so if you have deleted that air cleaner snorkel vacuum flap be sure that the related carb cap in back is properly plugged. This does sound like it could be a vacuum leak issue.
 
mpg7_400 said:
If you have deleted the EGR be sure that the vacuum port is properly capped off on the carb. Don't forget that vacuum port way in the back of the carb towards the firewall that the air cleaner vacuum hose plugs into. It's really hard to see and hard to get to, so if you have deleted that air cleaner snorkel vacuum flap be sure that the related carb cap in back is properly plugged. This does sound like it could be a vacuum leak issue.
Click to expand...
I'll add that to me list of things to run through. I also have to check the bushings on the throttle linkage. Apparently that's a common place to have vacuum leaks on these qjets.
 
Wes Harden said:
Not hard to swap metering rods, or main jets.
Carburetor unlimited has a nice kit that proves typical needed parts to fine tune a qjet.
As for a new dist. If yours is in good shape I'd leave it. Very difficult to find quality replacements today. Maybe just a new gm or non counterfeit ac Delco, ignition control module
Click to expand...
Hey wes,

Have you heard of the company called quadrajet power. I reached out to them and they said they recommend a 73 jet and a custom full tampered rod.
 
I have used them for some parts, 73 sounds big to me but I have no experience with the custom primary rods.
 
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