Surging idle with Q'jet

[CCCC]

Freshly rebuilt Q'jet with manual choke and i took great care in the process - well, for as much as I know. SB 350 starts runs and revs just fine, and will idle at fairly low revs, but there is a slight "surging" action (not bumpy or stumbling) that I cannot eliminate with idle adjust screws or fuel pressure regulator. I have carefully checked all vacuum inlets and believe there is no air sucking in topside.

I have seen mention of too much opening at secondary (or primary?) throttle plates as a cause of idle issues. Not sure how this would have developed because I changed nothing there, but maybe this is an issue. Is there a quick and good test to see if this is a cause? Can anyone point to some info on how to check and correct such an issue with adjustment - preferably with carb as mounted?

Thanks for any help.

[geezer#99]

Before the manual choke conversion, what type of choke was it? Divorced, hot air or electric?
And of course, what’s your timing like?

[Steeveedee]

Do you have EGR? I see a long list of vehicles, so it became a question.

One thing to check is that you got the base gasket on correctly. On some Q-Jets, they are asymmetric in the back and will give a vacuum leak. Spritz the back with WD40 or carb cleaner and see if the idle speed changes or smooths out.

Also look down the air horn and make sure that there is no fuel dripping. Its easy to get the float hanger off a little when installing the air horn. Also, if it is spilling over, it could be a heavy float (saturated with fuel). These newer fuels sometimes eat them up.

[D.PASSMORE]

You can use a vacuum gauge hooked to manifold port to help. Sometimes a simple way to check if its a running rich is to push the brakes and see if the idle increases. The power brake booster pushes air back in manifold. Did you replace your intake/carb gaskets?

[CCCC]

Thanks for these replies - with your help I am certain to get this idling correctly.

Checked timing - was about 8+ deg before TDC (no vac) at time of install. Used new gaskets in carb and at manifold. Installed brand new float and set the level very carefully, maybe a tiny bit low. The engine revs quickly and is very smooth doing so.

Previous auto choke was OEM divorced, but this manual setup started//idled/ran great for years before the ethanol attack and this rebuild. This engine (1970 SB350) does have the vent system (vac port with hose and fitting to valve cover opening - is that EGR?) so will plug that tight and see if that helps.

I will apply the vac booster brake to check for rich mixture. Did look into into horn and could not detect any extra/dripping fuel, and very slight application of choke seems to give better running - which is why I think idle is lean due to air leak. Will try the carb cleaner leak test - forgot about that one.

Thanks again - will be back after I do the above.

[geezer#99]

That hose to the cover is likely the pcv. The wrong one can cause idle fluctuations.
You need way more timing especially at your altitude. Float level a little low is good especially at your elevation too.
Can you screw in either mix screw all the way in without the motor quitting?

[CCCC]

The weather has improved and I plan to re-install and try the Q'jet today. Have checked for plane on the manifold facing and base of carb as best I can and those seem OK, and have a new two-piece manifold to carb gasket (very thin above thick) ready to go on. Used a vac meter to check the vac advance diaphragm on dist and PB booster

I have securely plugged every vac opening in the carb and the power brake vac opening in the intake manifold behind the carb. All new vacuum lines will be used in eventual re-connects. If the problem was a vacuum leak, the thing should idle fine with carb install and progressive hookups should ID any trouble site.

QUESTION: Is there a downside to not using the PVC valve and leaving that carb port securely plugged?

geezer - memory of previous efforts seems to say that running the idle mix screws all the way in did not shut it down, but the overall idle speed may have been set quite high at the time. And, thanks for the timing advance comment. Never had occasion to check this one because this truck ran good when I got it up here and have used it mostly on-site for years - only run the 21 miles to town once, and one of my kids drove it on that run - noted no issues. It has an HEI dist and I have noticed with some of those a bit less advance is needed to get optimum result. Does that make sense to you?

Will report here after install/run.

[Steeveedee]

Your carburetor is calibrated for the PCV "leak". If you disconnect/plug it, the truck will not run well. You will also get a lot of crud built up inside from not venting the blow-by out of the crankcase.

[Coley]

As Geezer mentioned...get that timing pushed up....10, 12-14° or thereabouts first- that is job one and should happen before any further carb adjustments.

Then with the timing moved up you will probably see that your idle is higher.
Next, you need to adjust/turn down the idle on the carb to about 600-700 rpm, that way your low idle circuit will function properly.

Remember this adjustment is the idle adjustment on the drivers side of the carb.
The slightly hidden idle adjustment on the left side (passenger side) of the carb is the choke/fast idle adjustment.

....overall, this sounds like there is a vacuum leak problem within your carb / line system.

Coley

[brown7373]

Don't eliminate the PCV. It does NOT decrease performance! It vents the buildup of gas and pressure in the crankcase. Without the PCV you will force pressure past the rings or out the oil dipstick and probably blow oil all over you engine.

Older engines had a vent cap on one valve cover and a draft tube that extended down toward the ground. This blew air through the crankcase to relieve pressure. The PCV is a better and cleaner way to do it.

[CCCC]

Driving another truck today and got bad sound with braking - checked and had to replace two disc brake caliper bolts that had broken, and it was fairly cold out in the garage today, so will give the carb install a go tomorrow.

Thanks for these replies - I have installed a new hose on the PCV valve - it will go back in with the carb. Easy enough to advance the timing from 8 BTDC to 10 or 12 for this next test. Can't imagine that I have not eliminated any possible previous vac leaks with the steps taken since.

We will see.

[CCCC]

Thanks for the observations and suggestions. I found an odd vac leak, fixed it - all is well and the idle is good. Even though not needed for good idle, I did advance the timing to 12 BTC on general principle. Thanks again to good fellows.