75 qjet choke pull off adjustment
Thread Starter
Racer
Joined: Jun 2009
Posts: 374
Likes: 18
From: Merrimack NH
75 qjet choke pull off adjustment
Hey CF,
I recently got a 75 that has some trouble cold starting. What i've found is that after setting the choke, It fires up but dies down a few seconds later. The vacuum break is opening the choke too much when the engine starts. When it first runs, i can hold the choke plate by hand where it's supposed to be (~1/4" open) and it will stay running fine in high idle. the only way i can see to adjust this is by adjusting the choke spring to more tension to counter the pull-off actuator but it does not seem to be enough. What am i missing??
(This is my first carbed car so i have a lot to learn
)
Thanks!!
I recently got a 75 that has some trouble cold starting. What i've found is that after setting the choke, It fires up but dies down a few seconds later. The vacuum break is opening the choke too much when the engine starts. When it first runs, i can hold the choke plate by hand where it's supposed to be (~1/4" open) and it will stay running fine in high idle. the only way i can see to adjust this is by adjusting the choke spring to more tension to counter the pull-off actuator but it does not seem to be enough. What am i missing??
(This is my first carbed car so i have a lot to learn
)Thanks!!
Safety Car
Joined: Oct 2007
Posts: 4,356
Likes: 1,723
From: Seattle Area Washington
This link will get you to the off vehicle instruction sheet for Rochester Models M4MC, M4MCA, M4MEA carburetors. On page 3, fig 9 shows the steps.
Hope this helps.
LINK:https://www.carburetor-parts.com/***...s/50-505-3.pdf
It's good information to save.
Hope this helps.
LINK:https://www.carburetor-parts.com/***...s/50-505-3.pdf
It's good information to save.
Last edited by bmotojoe; Dec 7, 2019 at 03:23 PM.
Thread Starter
Racer
Joined: Jun 2009
Posts: 374
Likes: 18
From: Merrimack NH
This link will get you to the off vehicle instruction sheet for Rochester Models M4MC, M4MCA, M4MEA carburetors. On page 3, fig 9 shows the steps.
Hope this helps.
LINK:https://www.carburetor-parts.com/***...s/50-505-3.pdf
It's good information to save.
Hope this helps.
LINK:https://www.carburetor-parts.com/***...s/50-505-3.pdf
It's good information to save.
Thanks again!
Advanced
Joined: Jul 2009
Posts: 94
Likes: 1
Does your carburetor have a vacuum choke pull off with an adjustable screw? My 78 does (though I'm not sure if it's the original qjet), and that is the screw that adjusts the choke opening when the engine starts (vacuum pulls the choke plate open). I'm not certain, but I think if you bend that rod to solve this, you might prevent the choke plate from fully opening when the engine warms up and the carb drops down to the slow idle setting.
Thanks to Lars and his excellent write ups for teaching me this.
Last edited by wgrea3; Dec 21, 2019 at 10:24 AM. Reason: Fixed typo
Thread Starter
Racer
Joined: Jun 2009
Posts: 374
Likes: 18
From: Merrimack NH
I was just browsing posts, and saw this... thought I'd reply because I have a question, and maybe an answer for you.
Does your carburetor have a vacuum choke pull of with an adjustable screw? My 78 does (though I'm not sure if it's the original qjet), and that is the screw that adjusts the choke opening when the engine starts (vacuum pulls the choke plate open). I'm not certain, but I think if you bend that rod to solve this, you might prevent the choke plate from fully opening when the engine warms up and the carb drops down to the slow idle setting.
Thanks to Lars and his excellent write ups for teaching me this.
Does your carburetor have a vacuum choke pull of with an adjustable screw? My 78 does (though I'm not sure if it's the original qjet), and that is the screw that adjusts the choke opening when the engine starts (vacuum pulls the choke plate open). I'm not certain, but I think if you bend that rod to solve this, you might prevent the choke plate from fully opening when the engine warms up and the carb drops down to the slow idle setting.
Thanks to Lars and his excellent write ups for teaching me this.
Advanced
Joined: Jul 2009
Posts: 94
Likes: 1
Advanced
Joined: Jul 2009
Posts: 94
Likes: 1
https://www.ecklers.com/1975-1977-co...MaAnY6EALw_wcB
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Safety Car
Joined: Oct 2007
Posts: 4,356
Likes: 1,723
From: Seattle Area Washington
Here are my thoughts why the rear pull-off and linkage was used on the 75. 1976 and later Corvettes used a vacuum thermo sensor inside of the air cleaner to keep the fresh air inlet closed drawing in only Warm-Hot air from the exhaust stove pipe, once warm the sensor changed state shutting off vacuum and exhausting opening the cold air intake diaphragm on the air cleaner. That sensor uses the same vacuum source from the throttle plate as the rear pull-off did on the 75. I think as far as Corvettes, 1975 was the only year the rear pull-off was used.
Yes, I believe the same front choke style pull-off was used on the 75 but it just served a different purpose. If you removed the rear pull-off and linkage and capped the vacuum source from the throttle plate could you adjust the choke plate opening distance on a 75 with that screw, sure. Right, Wrong???
Yes, I believe the same front choke style pull-off was used on the 75 but it just served a different purpose. If you removed the rear pull-off and linkage and capped the vacuum source from the throttle plate could you adjust the choke plate opening distance on a 75 with that screw, sure. Right, Wrong???
Last edited by bmotojoe; Dec 21, 2019 at 12:14 PM.
Tech Contributor
Joined: Aug 1999
Posts: 14,365
Likes: 6,345
From: At my Bar drinking and wrenching in Lafayette Colorado
For a complete step-by-step procedure for adjusting the choke on the stock carb, just send me an e-mail request for the paper.
.
Lars
V8FastCars@msn.com