Edelbrock 1405 mixture screw backed out

[berserk]

Has anyone experienced an idle mixture screw backing out presumably from vibration over time? The carb is 10 years old with around 15k miles and I haven't touched it once since the initial setup and it has been solid.

I recently had to move my K10 into daily duty and with the 50-90 % humidity in Oklahoma I have been having some drivability issues. Once the heat index reaches 90 or above I'm getting a bog/stumble/hesitation. I definitely thought it was carb related but until yesterday I hadn't had time to look.


I got off work and it was starting to act up as it was hot and I decided to readjust my idle mix screws. And..... One of the screws was so loose I could adjust it with my fingers!! It was backed out so far I could see at a glance how much more it was turned out than the other. I zeroed them out and noticed the passenger side was difficult to turn while the driver side was very easy and smooth.... Anyways, backed them out 2 full turns and adjusted by ear and it's and entirely different truck!!!

I have way better throttle response, idle isn't as choppy, smooth acceleration and what feels like 20+ more HP!

Any ideas on my issue with the idle mix screws?? Why is one difficult to turn the other way to easy? Anyone had the screw back out on it's own? I looked at a parts list and didn't see a seal for the mix screws which surprised me.

[VileZambonie]

Overtightening can damage the threads. Is there a spring on them? You should be able to turn them with your fingers.

[berserk]

Yes there is a spring. I just got off work and decided to mess with it again. I can in fact now turn both screws by fingers. Originally I could not and I'm wondering if there was some Grim or grit as there seems to be some vapor leakage. I'm not sure if this is a sign of something else or not, I'm going to attach a picture if I can get it resized correctly.

[Mr Diesel]

Quite a bit of grime. Wouldn't hurt to clean that up. The idle mixture screws only affect air at idle so won't make the engine run any different above idle or have any additional power.

Not sure what your fuel supply looks like but you need a fuel return system in place to help keep the fuel circulating and cool. If problem returns you might try diagnosing that since it seems to have been a heat related issue.

[berserk]

Thank you! Yes, I was just driving it in 100 degrees and high humidity and I'm having a very slight bog during cruising and it's loading up at the stop light. This morning before it was really hot it would rev crisp and clean.

I will say I was having a fuel pressure issue at one point a few years ago(been sitting in my shop getting upgrades since), the fuel filter wouldn't even fill up but the truck ran pretty good still. I had an FPR because Edelbrock, and removed it and it seemed to run better. I thought there may have been an issue with it.

After I tore apart the entire truck last year I dropped the tank and found the rubber gas lines rotted allowing air in. Makes sense now, remove the FPR and get more fuel pressure. Well I replaced the lines and the filter fills right up and I have plenty of fuel. But... I still don't have an FPR, is fuel pressure affected by the heat?

I ask because yes I need an FPR with an Edelbrock and will get one. But my drivability issue happens only when it's hot and humid out. And like I said it's a new issue, the decade this set ups been in my truck I can drive 3 hours and 100° weather and not have any issue. So something has changed...

Thanks for any input!!

[Mr Diesel]

And like I said it's a new issue, the decade this set ups been in my truck I can drive 3 hours and 100° weather and not have any issue. So something has changed...

^^^^^
This is a good observation. Your current setup might be functioning fine, but something has changed like a line leak, fuel pump crapping out, etc. Don't worry about changing anything yet since it was working great for years, focus on what has changed unintentionally.

I have found the Edelbrock doesn't necessarily need a FPR. I currently run an Edelbrock 1406 on a 305 in a 1985 Camaro for 14 years now. The current fuel routing is basically factory, using a 3 port mechanical fuel pump. 1 inlet from tank, 1 outlet to carb, and 1 outlet for excess fuel return to tank. This keeps fuel circulating through the pump constantly, even at idle when fuel demand is low. I also experimented with a 3 port fuel filter at the carb body years ago. It accomplished the same thing, but allowed fuel to circulate constantly right up to the carb. At first I wondered if fuel pressure would get too low but it was never an issue. I experimented with a FPR on it once and found it to just be an unnecessary extra component for maintenance.

[berserk]

My truck does have the return style fuel pump and it's all hooked up. I think I'll run compressed air through the hard line to make sure it is clear. I also was made aware of the fuel filter you described recently. It is inexpensive so I was going to add it to my set up and route the return inline with the return line from the pump.

I went to the mechanic I use when I don't feel like wrenching and he is dragging his heels on my daily driver as usual. When I get my Silverado back I'll put the K10 in my garage and I'm going to try what we discussed to start with.

[78 Chevyrado]

Maybe a weak spring behind the screw?  the spring is there solely to stop it turning.  get some new springs and maybe screws (if they been overtightened) and try again.  don;t oil the threads.