I have been having some problems getting my v8 s10 to idle correctly, now I do have a very aggressive cam, but I was wondering if I might be able to get it to idle better, the idle mix screws don't do anything when turned, so this leads me to believe there might be a vacuum leak, I haven't had the chance to test for leaks yet, but I was wondering on the side of the carb where the vacuum break is and choke linkage there is a long tube that sticks out the side, is this a vacuum port? The carb body is cracked where the tube is pressed into the carb, I will add a picture later on, the PCV is hooked up with a breather on the other valve cover, this goes to the large center port at the front of the carb, vacuum advance port is plugged, just was wondering about the other possible port on the side with the vacuum break, its a tube about 2 inches long that sticks out from where the choke and break linkage is
The only piece cracked is just the boss that sticks out where that tube is, if its not needed I can pull the tube and seal the hole with epoxy, and the cam, duration: 303/313, lift .487/.508, LSA 114 degrees, rpm range: 3000 - 6,500, I have a feeling the cam might have a lot to do with it, but wasn't sure if a vacuum leak could be the problem too, compression should be around 10:1
Sounds like the summit 1107 cam. There is a vacuum leak out your exhaust valve. That cam is huge and it's not gunna idle good under 1200 RPM. Watch the crack....they can grow over time.
Here's what you can do...hook up a vacuum gauge...Safety first. Chock the wheels, tie it down etc. In Neutral see at what RPM it make the most vacuum. Bump the dist a few degrees in either direction and try to find the lowest RPM where is makes the most vacuum. Set the idle 50 to 200 RPM below that. Do it all again making small adjustments to the idle mixture around the RPM where you get the most vac. There you go...best idle is found.
I will give that a try, I think the reason the idle mix screws aren't doing anything is cause I have the throttle screw turned up to the point where the idle adjustment screws aren't working
That's a good point. Post what carb you have perhaps some one knows what the idle circuit limit is on that carb. I'm no carb expert but I've read some people have drilled a small hole in to the butterfly plate to get enough air flow for the car to stay on the idle circuit. Perhaps some one with experience with this can chime in.
Motor needs more ignition advance at the crank. Put in 20 degrees at the crank, 14 in the weights if using fast burn chambers, 16 in the weights if using slower chambers. To prevent kick-back when cranking to start, interrupt hot wire to the coil and wire in a momentary-off, normally-on pushbutton switch so that you can push the momentary switch while cranking the motor, then let go of the momentary switch and the starter switch and the motor will be running, no muss, no fuss.
Use this tutorial to find true TDC so that you can time the motor properly..... http://www.crankshaftcoalition.com/wiki/Determining_top_dead_center
Im not a big fan of locked out timimg,but,You might consider using locked out timing because of the Rochester.Is your compression ratio over 10:1 or were you guessing it might be?
If it was a solid lifter cam it would probably be a decent road racing cam.,
I was just guessing at the compression, factory dished pistons with a thinner head gasket and maybe an 1/8th shaved off the heads, how would I lock out the ignition timing? And the cam uses hydraulic lifters
Go with what tech said.
20 initial, 14 in the dizzy and turn down your idle and get your mix screws working. You're idling on the power circuit.
If you still can't get the mix screws to work after that then hook your vac advance up to manifold vacuum (limit it to 10 degrees). Then you'll get your mix screws working.
And yes that tube on the side is manifold vacuum and needs to be fixed.
you can buy a lock plate from summit/jegs etc for most applications. as for the cam being hydraulic flat tappet,its a little large for a good idle with hydraulic flat tappets.I meant if it was a solid you would have better tune ability. make do for now,,,lock timing at 34º to start and back down the idle so you are using the idle circuit in carb
I got the timing adjusted and got the idle down, adjusted the mix screws and got it idling good, shut the engine off and when I restarted it wouldn't idle just kept stalling out, I guess the next thing to try would be locking out the advance?
Messed around some more with the timing and mix screws, got the idle down a good bit, but when I put it in gear it wants to bog out and stall, I can keep it runnning in gear by tapping the gas pedal and if I let off the brake and floor it, it bogs out and takes a few seconds to rev back up and make power
If you're tapping the gas pedal to get it to run you're activating the accelerator pump enrichment. This suggests you're lean...check yout jetting or perhaps air is getting around the card plate.
Checking for vacuum leaks was the next thing I was planning on trying. With the quadrajet, do I back out the idle mix screws to richen it up? I heard some carbs are different and just want to be sure
I'm no carb expert...but when I played with Q jets years ago my understanding was that there is a idle circuit mixture adjustment that is for the idle circuit only. Once off idle fuel metering is done by the primary metering jets/rods.
If the Qjet is stock and the engine now has a larger cam you will need different metering rods and jets to get the fuel mixture right.
The mix screws are the normal set up. Lean in, richer out.
Can you screw one or both mix screw all the way in at idle and it not quit running?
If so you need less idle speed or more initial timing.
The lean stumble can be caused by poor acc pump shot or lean set up on your metering rods. THe metering rods can be adjusted externally if you modify your carb.
We'd need a pic of it to tell if you can access the apt easily.
Did you hook vac pot to manifold vac?
Accelerator pump is working strong, it was replaced when i rebuilt the carb, i think the bogging may have been due to the secondary plates not opening, i havent had a chance to test it yet, but the secondary plates dont open until the engine is fully warmed up, could this be the problem?
Secondary won't open due to the choke lockout. That's there to protect the cold motor until it's warmed up.
Do you have an electric choke or a divorced choke.
It has the choke where there is coil mounted to the intake that explands when it heats up, is that the divorced choke? and i got kinda ahead of myself with the testing and the engine was never fully warmed up, still need to tie up some loose ends before it is safe to fully warm up, so that is probably the problem with the bogging, kinda feel like an idiot now
That's a divorced choke. Not as easy to lean out.
THe linkage rod that runs from the coil on the intake to the carb needs to be made longer by straightening the bend or making a new one that's longer. I've made them out of old wire coat hangers.
The secondaries open fine when fully warmed up, I just got excited and forgot an important rule, don't start testing and tuning until the engine is fully warmed up, I havent been able to let it warm up since the engine was put in the truck
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