250 engine drops 600 rpms from park to drive

[jfw432]

I recently purchased a 79 C-10 with a 250 straight 6 in it and the idle speed drops pretty significantly when I put it in gear. This truck has a Varajet II carb in it and I've been making some progress getting things working better. It idles around 1100rpms when fully warmed up and drops down to about 500rpms with my foot on the brake and the truck in reverse or drive.

1. First thing I did was fix the timing. Kind of a pain since the timing marks broke off but I've set the timing to about 8° at idle with the vacuum advance removed. (helped a lot but engine had to be semi warm before I could put it in gear and not have it die)

2. Next I inspected the wires, cap, and rotor. I only changed the plugs though. (no change)

3. Changed the dirty air filter. (no change)

4. Searched for vacuum leaks around the carb. (I found one leak coming from a dry rotted hose coming from a large bottle(?) near the drivers headlight. This made a large difference and the truck wouldn't die when cold and in gear but the rpms still drop a lot)

5. I've been searching for other vacuum leaks but it seems like there are about 30 vacuum lines going in and around the carb on this engine but haven't found anything else.


Additional information:
Vacuum reading on the engine is a little jittery. It only bounces +/- 1/4 in-hg. However, the rpms waiver from time to time which causes the vacuum to vary between 18-21in-hg. It may take a couple minutes for it to go from 18 to 21 though.

Compression numbers are as follows:
1 - 150
2 - 150
3 - 170
4 - 160
5 - 150
6 - 155

I'm still thinking vacuum leak but I can't find any others. Any advice would be appreciated.

[geezer#99]

Bump your initial timing up to 14 and turn your idle down to 650 rpm.

[dwcsr]

I would run the vales to see if you can stop the flutter. run them loose and see if it doesn't help with the vacuum. if it does you may have some lifter that bleed down when shut off.

I'm not a Automatic trans guy but I think I'd be looking at the Converter next to solve that rpm drop

[jfw432]

Originally Posted by geezer#99

Bump your initial timing up to 14 and turn your idle down to 650 rpm.

Thanks. The idle is as low as I can get it and if I advance the timing any more, the rpms go up even higher when I reconnect the vacuum hose.

Originally Posted by dwcsr

I would run the vales to see if you can stop the flutter. run them loose and see if it doesn't help with the vacuum. if it does you may have some lifter that bleed down when shut off.

I'm not a Automatic trans guy but I think I'd be looking at the Converter next to solve that rpm drop

Do you mean check the valves to see if they are adjusted properly? If so, do you know off hand what the clearances are supposed to be?

[geezer#99]

Originally Posted by jfw432

Thanks. The idle is as low as I can get it

Find the source of the high idle!
Either curb idle or high idle screw or a throttle plate hung up.

[dwcsr]

Originally Posted by jfw432

Thanks. The idle is as low as I can get it and if I advance the timing any more, the rpms go up even higher when I reconnect the vacuum hose.



Do you mean check the valves to see if they are adjusted properly? If so, do you know off hand what the clearances are supposed to be?

Yes , if they are hydraulic then back off til it clacks and tighten til it just stops and tighten half to 3/4 turn 1/4 at a time. I would adjust all until they just stop making noise and see what the vacuum gauge says and how it acts. check for vacuum leaks around the manifold. use carb cleaner and spray the manifold gasket on top and under side see if it makes any rpm changes.

here is a video on use of the vacuum gauge and what it can tell you. With that and reading the spark plugs you'll find the answer

http://4.bp.blogspot.com/-j0zzs4Ml8N...ug+Reading.jpg

https://www.youtube.com/watch?v=SdlNwm8OHco

http://www.classictruckshop.com/club...ts/vac/uum.htm

[jfw432]

So this is going to sound really stupid but apparently there are two idle speed screws on this carb and they have to be adjusted together... I've never heard of this but I was able to bring the idle down to about 700 and then it only drops to 500 when in gear. I'll get a picture of these screws and maybe someone can help explain what's going on. There are sooo many arms and linkages to this carb.

[jfw432]

So here at the pictures. I was adjusting the silver screw and it affects idle to a certain point. I just noticed the rusty vertical screw. I played with it a little and it didn't do anything until I started messing with the silver screw again. Figuring maybe the silver screw was just in the way, I tried playing with the rusty screw again but that didn't do anything.

I've done a lot of engine and carb work but the complexity of this thing is blowing my mind.

[geezer#99]

Being that's a progressive two barrel, the rusty one could be a secondary throttle adjustment similar to the set screw on a holley.
FWIW I wouldn't bother adjusting the valves. That slight deflection in your vac reading is likely timing chain stretch.

['68OrangeSunshine]

Originally Posted by geezer#99

Being that's a progressive two barrel, the rusty one could be a secondary throttle adjustment similar to the set screw on a holley.
FWIW I wouldn't bother adjusting the valves. That slight deflection in your vac reading is likely timing chain stretch.

Good advice, [well-intentioned -- anyway] except Chevy L6s have Timing Gears, not chains. No stretch.
Compression readings are not bad.
18" Hg Manifold Vacuum is a good sign.
Does it bump to 21'' every so often, like one-in-six? Or does it warm up and settle at 21", or come to 21" when decellerating? If you have the time, get a rebuild kit and overhaul the Carb. I'm unfamiliar with the Varajet II, but wikipedia says it's like half a Quadrajet. Should not be too rare to find a kit for it at the autoparts store.
Is your '79 L6 250 the integrated-manifold type?

[Wrenchbender Ret]

Those are ornery carbs to work on. How are the choke pull offs? Maybe the choke is staying on too much & running a high fast idle. I wouldn't go into the carb unless I really had to. If you do take lots of photos of the linkage. They are a night mare to get back on right.

['68OrangeSunshine]

OK. I'd never heard of a Varajet II. I've had Quadrajets and Monojets, and there's a reason I run a 4Bbl Edelbrock on my 292 L6. The OP's V-Jet looked a little corroded on the surface, and I thought it would benefit from cleaning overall. Given the degree of difficulty, then a good blasting inside and out with carb spray may be safer.

[400/400]

Originally Posted by jfw432

Thanks. The idle is as low as I can get it and if I advance the timing any more, the rpms go up even higher when I reconnect the vacuum hose.



Do you mean check the valves to see if they are adjusted properly? If so, do you know off hand what the clearances are supposed to be?

Sounds like your vacuum advance is on hooked to a manifold source if it's increasing your rpm when its hooked up. Something to try would be hook it to ported or set you idle rpm with it disconnected and plugged and with it disconnected and plugged put the truck in gear and see how much of a drop there is.

If it doesn't drop as drastically w/ it disconnected your set up may work better off ported vacuum.

Also verify that your vacuum advance canister doesn't leak. If it does that could also increase your idle speed.

[jfw432]

The vacuum moves between 18 and 21 as the idle speed changes. Although the idle speed changes less with some work last night but I haven't hooked up the vacuum gauge since.

The vacuum advance on the distributor is connected to the carb. It was connected to some kind of coolant plug at front of the engine. My guess is when the coolant warms up, the vacuum lines opens up allowing the vacuum to advance the timing. Those pieces are plastic and broken so I've currently got them bypassed for now. No idea what they're called.

I fixed a lot of leaky vacuum lines around the carb last night and got the idle lowered. Now, it doesn't like to idle when it's cold.....

[D13]

GM did make a 2bbl version of the QuadraJet where the secondaries are blank but that was used only on small v-8's in the late 70's, when the old 2bbl was phased out for emissions reasons. This is not it. This is a staged 2bbl carb unique to inlines.
Contrary to popular opinion you can rebuild this carb, they are a bit finicky but when properly set up they work fine.
Given that carb you must have an integral head 250 (head and intake are 1 casting). These are notorious for cracking and creating vacuum leaks, causing high idle and eventually running lean and burning valves. Spraying carb cleaner on the manifold may let you find a vacuum leak.

Do you have a service manual? Seriously recommend you get one and read the sections on vacuum advance and carb setup. These motors were set up for emissions compliance and removing / rerouting vacuum hoses can have some serious side effects.
Set idle speed (the screw that moves the primary throttle linkage) per the timing decal. Then adjust the mixture screw on the primary to get the highest idle. Turn a little at a time, let it stabilize, then a little more. Then rest the idle.

The plastic thing was indeed a thermal vacuum switch (TVS). You can run without it but it was altering your timing changing when vacuum is being applied to the advance.

[jfw432]

Here's a video of the vacuum readings. Sorry about the middle...was trying to figure out how to operate the throttle, hold the gauge, and hold the camera.

https://www.youtube.com/watch?v=O1Jh...EWeYbBwaudLyXQ

I bumped the idle back up to 900ish (warm) because it's a pain to start when cold otherwise. I think the choke may need to be adjusted as it seems to open way too fast...or maybe it opens because I'm pumping the throttle trying to coax it into life. I shot carb cleaner all around the intake and carb and have fixed any leaks I could find. I can't seem to find anymore vacuum leaks so I may have gotten them all.

edit: I have the Haynes manual but I don't see very much information on this carb in there or on vacuum advance.