Poor idle, Wanna be mechanic

[Tommyups]

Looking for some help troubleshooting low idle...
Test drove my truck a few times after installing brake booster and everything was great. Came home from picking up my pizza and truck started running bad, real low idle. Had not put an air filter on it yet, had to find a different size, maybe sucked some thing into the carburetor?
A list of troubleshooting options in order would be appreciated. 292

[Sheepdip]

Get a can of carb cleaner and spray around your vacuum hoses carb base and booster connections including where the master cylinder bolts to it....sounds like a vacuum leak. If it's leaking your idle will jump up, you can then pin point it.

[Tommyups]

Thx sheepdip, will do

[yuccales]

Also grab that carb and give it a shake. Maybe the nuts came loose?

[Tommyups]

I’ve tried using the carb cleaner, cannot pinpoint any vacuum leaks. It did increase the idle a little bit enough to keep it from stalling, but it’s still running pretty rough, low idle

[AussieinNC]

Clamp off the vac line to both the PCV and the power booster and see if the idle improves.

These engines have a habit of leaking between the head and the manifold...check the nuts are tight

Also check the nuts that hold the carb to the manifold

Check all screws on the carb body and make sure they are all tight...

Let us know how you go...

[bry593]

Yup, plug the booster line with a bolt and see if it improves. That Chinese booster probably already crapped out.

[Tommyups]

Yeah I’m still having no luck locating any type of vacuum leak.
As previously mentioned, I ran this a half a dozen times on short trips without the air filter, is there any chance that I suckd something in the carburetor that’s causing the very low idle.
Any other suggestions to look into if it’s not a vacuum leak?

[RichardJ]

>>It did increase the idle a little bit enough to keep it from stalling,<<

If the idle changed when you sprayed the cleaner, that is what you were looking for. That's where the vacuum leak is.

[toolboxchev]

Originally Posted by Tommyups

I’ve tried using the carb cleaner, cannot pinpoint any vacuum leaks. It did increase the idle a little bit enough to keep it from stalling, but it’s still running pretty rough, low idle

Carb Clean can be misleading when looking for vacuum leaks. Generally when you spray it directly into the Carb it will lower the idle.

Get a torque wrench and try to tighten all the manifold bolts and carb studs.

[toolboxchev]

Originally Posted by bry593

Yup, plug the booster line with a bolt and see if it improves. That Chinese booster probably already crapped out.

Have someone step on the brakes with the engine running. Have the press the pedal multiple times and spray the entire booster with soapy water. I use Dawn mixed in a Dollar Store spray bottle.

Boosters go out rather frequently.

Leave the vac line attached to the booster.

[Tommyups]

I disconnected the vacuum off of the booster and plugged that hose, I thought that would eliminate the booster being bad ??

[HO455]

I am assuming that plugging the vacuum line to the booster did not help the idle?

[Tommyups]

No it did not

[Tommyups]

This doesn’t look right. The vacuum hose to the booster is completely sucked flat while the engine is running

[HO455]

Since your checks for a vacuum leak have turned up nothing I would check your ignition timing before going further just to rule out that as a cause.

[BigBird05]

I would check fuel pressure.

[tim_mc]

Sometimes unrelated things happen at the same time. After reinstalling an updated PCM in my Z28, it started running rough and backfiring. I thought for sure it was related to the new tune, but checking the fuel pressure at the rail indicated a bad fuel pump. The hose on the fuel pump had hardened and cracked in the tank.

Anyway, start with cheap and easy. Does it run OK when you increase the speed, meaning that it only has a rough idle? If so, then maybe the idle fuel circuit is dirty or plugged. Find the air mixture screw, turn it in gently until it seats while measuring the number of turns. Then remove the air screw, hit the passage with some carb cleaner, and reinstall to the same number of turns. See if it behaves any differently.

Block off any suspect vacuum leaks and check the PCV valve. Make sure the choke isn't hung up and is wired open if it's not used.

If they are old, I'd replace the fuel filter and rubber fuel hoses and as BigBird suggested make sure you have good fuel pressure into the carb. As HO455 mentioned, is it possible that you bumped the distributor or did anything that may have changed the timing?

If easy fixes don't help then the next step might be to take off the carb and clean out float bowl & check the float valve/needle & seat. If you're that far, might as well give it a good cleaning and rebuild.

Question: Did you replace a bad booster, or add a new one for power brakes? That's a lot of vacuum to pull the hose closed, assuming it's a 3/8" fuel line.

[Tommyups]

Yes this was a new booster for power brakes. I’ve had no issue with the brakes, they work great since installing, but I did think it was funny that that vacuum hose was completely sucked flat, I didn’t look right. Can somebody recommend a fuel pressure gauge to purchase, I don’t need the most expensive one out there. I appreciate all the advice and ideas, since I have Mostly ruled out vacuum problems I’ll start getting to know the fuel system

[Getter-Done]

Originally Posted by Tommyups

This doesn’t look right. The vacuum hose to the booster is completely sucked flat while the engine is running

This type of vacuum hose in the link below,
would be better suited for a Booster hose.
Booster hose needs a thicker wall or to be reinforced more than fuel line.
Hope this helps.

Link: https://www.pegasusautoracing.com/pr...20group%20%231











.

[toolboxchev]

Since this is going to be a journey into starting to learn mechanics lets start here now.

It seems you have plenty of vacuum. Get a vacuum gauge with fuel pressure in the same at most likely Amazon.

If your distributor has a vac advance can on it, most likely this can be the source for a leak, I have had those things go bad in 6 months.

https://www.amazon.com/INNOVA-3620-V...s%2C261&sr=8-9

If you have no leaks, Then get a quality dial back timing light. Again shop around on Amazon. Hard to beat their prices. They are thinking of dividing Amazon up because they have a monopoly.

Then move on to timing. As the rule of thumb rule out any ignition issues then move on to carb. Be sure to do a quick check of fuel pressure before this.

Now for timing.

http://www.crankshaftcoalition.com/w...EI_distributor

Timing principles are the same for 6 cylinders, just follow the guide and get the timing checked and dialed in.

If you follow the above and insure every aspect in timing is functioning let everyone know and someone will chime in on the carburation thing. Not my cup of tea!

oh yeah and do what the above poster mentions about the booster line. You can grab that at any FLAPS. (friendly local auto parts store)

[Wrenchbender Ret]

I can't see by the photos but if you have a idle solenoid, the wire may have come off or a blown fuse or the solenoid itself defective. When you turn on the ign. switch the plunger should come out & kick up the throttle after you open the throttle a bit.
Running without the air filter will not likely harm the carb but could harm the engine if it injested something.
George

[Tommyups]

Thanks for all the advice guys, I will keep you updated

[bry593]

Tommy,

The booster hose should be vac rated, else will collapse. Different SAE ratings for different types of hoses (fuel, vac, etc.).

Since you have checked your booster line and no change, and since it ran great and then suddenly did not, I would suspect possible trash in the needle and seat, or maybe a fuel soaked float. Sometimes a float will dry out if it sets awhile and then you get it running and drive, the fuel soaks into a pore or fault in the float and this causes the float to be heavier and not float at the proper level. Net result is the fuel level increases beyond normal and this will make it run rich! Does the exhaust smell rich, see any blue smoke when starting? What about the top gasket on the carb, does it look wet? Both conditions can be fuel seat, bad float or could also be that the gasket for the needle seat is not sealing (maybe has some material left from the previous gasket holding the seat slightly off register). Another possible cause, although not likely, is leaking around the main jets. If the threads are boogered up, can cause some issues.

A way to see if it is running rich is to simply pull off a vacuum hose so un-metered air is going into the carburetor. Did the idle increase? If so, the extra air has helped balance the fuel/air ratio (14:1 ish). If the idle gets worse, then you probably got a vac leak somewhere. Simple test, try it.

Did you recently rebuild this carb?

Carbs, they can be a pain but when you get 'em working, they generally are good for a long, long time. Not to mention, the Rochester is the most advanced carburetor available to this day. Holley nor AFB can compete with a Rochester for efficiency, reliability and power. Rochester is however, more complicated.

[Tommyups]

I have an update on my idling problem. With the help Of my Mopar Motorhead friend. The first thing he noticed was how the engine vibrated so much and ultimately realized that the fast idle solenoid set screw was missing and causing the Solenoid to move around on its own Raising and lowering the idle. I think someone mentioned I have a Rochester carburetor, so I’ll just need to find a setscrew that fits. I really appreciate everybody helping me out.