Engine stumble at idle

gsgtx · Apr 20, 2012

LARRY70GS said: ↑

Looking back at the posts in your thread, you seem to be describing two separate problems, and I don't believe that they are related. One problem relates to the title of this thread, engine stumble at idle. I take this to mean a miss at idle, like the engine stutters every so often at idle. I think that might just be the plugs or the gap, or both.

The other problem you describe is when you mash the gas, the engine bogs and then goes. That is the carburetor. You are experiencing a bog because the secondaries are not coming in cleanly. But here's the problem with the Edelbrock carburetors that are modeled after the Carter AFB. They use a counter weighted secondary air valve. There is no way to adjust that air valve without taking it apart, and grinding on the counter weight, or adding more material to it. The Edelbrock and Carter AVS have an adjustable air valve. The tension is adjustable by increasing the spring tension, much like a Q-jet, but the Q-jet also has the primary vacuum break to slow the air valve down, and even that is adjustable. If a Q-jet is adjusted correctly, and calibrated correctly for the application, the transition from primary to secondary operation is seamless, and smooth. The primary side of the Q-jet is also much more efficient than any Edelbrock or Carter carburetor of this type. That isn't to say that some guys can't get this type of carburetor to work reasonably well on their engine. Sure TA Performance wouldn't sell you the carburetor if it wouldn't work with their intake, but I think if you asked them which carburetor they thought would be better for you, they would tell you that a properly calibrated and set up Q-jet was the better choice. In any case, you may be able to improve upon what you have by tuning it. You can adjust the metering, and the accelerator pump volume and rate, and that may get you improvement, but Like I said, there is no easy way to adjust the air valve, and that is a large part of your second problem IMHO.
Click to expand...

Larry, do you think the secondary air valve is opening up to soon or to late ?

LARRY70GS · Apr 20, 2012

gsgtx said: ↑

Larry, do you think the secondary air valve is opening up to soon or to late ?
Click to expand...

Well, with a Q-jet, bog happens when it opens too fast, so I'm thinking, too fast. To slow the Edelbrock air valve down, you would have to add weight to the counter weight. That's why the Edelbrock AVS is an improvement. You can adjust spring tension on the air valve.

http://www.popularhotrodding.com/tech/0403phr_edelbrock_avs_carburetor_dyno_tune_specs/index.html

SteeveeDee · Apr 20, 2012

LARRY70GS said: ↑

Looking back at the posts in your thread, you seem to be describing two separate problems, and I don't believe that they are related. One problem relates to the title of this thread, engine stumble at idle. I take this to mean a miss at idle, like the engine stutters every so often at idle. I think that might just be the plugs or the gap, or both.

The other problem you describe is when you mash the gas, the engine bogs and then goes. That is the carburetor. You are experiencing a bog because the secondaries are not coming in cleanly. But here's the problem with the Edelbrock carburetors that are modeled after the Carter AFB. They use a counter weighted secondary air valve. There is no way to adjust that air valve without taking it apart, and grinding on the counter weight, or adding more material to it. The Edelbrock and Carter AVS have an adjustable air valve. The tension is adjustable by increasing the spring tension, much like a Q-jet, but the Q-jet also has the primary vacuum break to slow the air valve down, and even that is adjustable. If a Q-jet is adjusted correctly, and calibrated correctly for the application, the transition from primary to secondary operation is seamless, and smooth. The primary side of the Q-jet is also much more efficient than any Edelbrock or Carter carburetor of this type. That isn't to say that some guys can't get this type of carburetor to work reasonably well on their engine. Sure TA Performance wouldn't sell you the carburetor if it wouldn't work with their intake, but I think if you asked them which carburetor they thought would be better for you, they would tell you that a properly calibrated and set up Q-jet was the better choice. In any case, you may be able to improve upon what you have by tuning it. You can adjust the metering, and the accelerator pump volume and rate, and that may get you improvement, but Like I said, there is no easy way to adjust the air valve, and that is a large part of your second problem IMHO.
Click to expand...

Now that there is sage advice. If I do it again, I won't be buying an Edelbrock ever again. I've been through these problems (plus heat soak issues with zippo response from technical support regarding their vacuum-leaking chinesium base gasket), and can attest that the Q-Jet on my 3/4 Ton Chevy truck can be/has been rebuilt and adjusted just like it needs to be for best performance.

chrisc1541 · Apr 23, 2012

Should I still gap the new plugs at .045 even though the info I find for a 75 350 says the gap is .060?

LARRY70GS · Apr 23, 2012

chrisc1541 said: ↑

Should I still gap the new plugs at .045 even though the info I find for a 75 350 says the gap is .060?
Click to expand...

.045 is a good safe gap to use. .060 is pushing it with any ignition system. That might have been OK in 1975 with a GM HEI, low compression and lean fuel mixtures. Bigger is not always better. It won't get you any more performance.

chrisc1541 · Apr 25, 2012

Got new delco plugs gapped them at .045
also got the new metering rods and a sd card for my phone so i can take video
hopefully tonight i will swap the plugs see how that goes if no improvement i will swap the out the rods
I will take before and after videos of everything and link them here so you can see what the outcome is and offer suggestions based on what you see and hear
thank you everyone for your help

Also what number q-jet would i need to get or where could i look up that info
1975 buick 350 block # 1241748 head ID #1248452 f/30

LARRY70GS · Apr 25, 2012

chrisc1541 said: ↑

Got new delco plugs gapped them at .045
also got the new metering rods and a sd card for my phone so i can take video
hopefully tonight i will swap the plugs see how that goes if no improvement i will swap the out the rods
I will take before and after videos of everything and link them here so you can see what the outcome is and offer suggestions based on what you see and hear
thank you everyone for your help

Also what number q-jet would i need to get or where could i look up that info
1975 buick 350 block # 1241748 head ID #1248452 f/30
Click to expand...

Just contact Ken at everyday performance and ask him to build a Q-jet for you. Doesn't get any simpler than that.

chrisc1541 · Apr 26, 2012

[video]http://www.facebook.com/profile.php?id=1102777428[/video]

this is what i am starting with no plug changes or anything else done
let me know if there is something else you'd like to see

chrisc1541 · Apr 27, 2012

new easier video location

[video=vimeo;41142609]https://vimeo.com/41142609[/video]

chrisc1541 · Apr 30, 2012

new plugs, wires, cap and rotor made no difference swapped in different metering rods and lowered the idle seems a lot better will be doing the timing and idle in gear later this week

[video=vimeo;41292839]https://vimeo.com/41292839[/video]

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Engine stumble at idle

gsgtx Silver Level contributor

LARRY70GS a.k.a. "THE WIZARD" Staff Member

SteeveeDee Orange Acres

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LARRY70GS a.k.a. "THE WIZARD" Staff Member

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LARRY70GS a.k.a. "THE WIZARD" Staff Member

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