Rough idle

"Hi,
Happy Friday the 13t

"Hi,
Happy Friday the 13th!

My 5.7l with Rochester 4bbl carb is having trouble idling. We just rebuilt the carb and it did not help (made it slightly worse actually) chasing a high end issue. When trying to set the idle mix screws, we cannot get the engine to run at all below about 1000 RPM. It is super lean (covering the air intake with a hand to simulate choke clears up the roughness and the RPM jumps back to 1000-1200). With a little throttle to get the RPM at 1000, it purrs just fine. Any less, and it hesitates and stutters. Choking it helps, and adjusting the mix screws does not seem to have any effect. When it is running right at the threshold, looking down into the carb, it seems that one side is spraying more fuel than the other.

I think I can't adjust the idle screws until we get the RPMs down to the 600-800 range (with no extra throttle), so what should we check to get the engine to run that slow? Looking at the manual, I am guessing to check timing? I don't think there is a vacuum leak, since the vacuum break actuator sucks in just fine, and with just a bit of throttle, it runs great. I am going to get a vacuum gauge and timing light for the next trip to the boat. Maybe check compression? I don't suspect ignition, since it would probably be worse as the RPM goes up.

[The previous problem was that the engine would stumble and stutter when you tried to go above 3000 RPM. That just started to happen on the last two trips. I think that was a clogged carb fuel inlet filter, now replaced.] We have not tried it in the water yet - trying to set the idle mix now.

Thanks for any advice...
Chris."

"Ayuh,.... Have you changed t

"Ayuh,.... Have you changed the Fuel Filters,..??

I'd take that carb,+ give it the "Float Test" at the dock,....
When it Fails,...
Go buy an Edlebrock, bolt it on,+ Go Boatin'...."

Is the old base gasket the sam

Is the old base gasket the same as the new one? Did you use compressed air to blow out the idle circuits when cleaned?

Did you check and blow air or

Did you check and blow air or carb cleaner through every passage and tube in the carb including air bleeds beside venturies. A plugged air bleed or one of the small tubes will cause lean problems.

This has all the symtoms of a

This has all the symtoms of a vacumn leak

"Had a Rochester quadrajet reb

"Had a Rochester quadrajet rebuilt,($450.00)only to find out later that there was a bad vacumn hose. It looked like new on top but was cracked on the underside. $2.00 hose/plug vs $450.00 wasted dollars. Acted the same way you discribed. Cover the air intake -ran smooth and idle had to be high."

"Chris...

Sounds like a vac

"Chris...

Sounds like a vacuum leak to me. As Carl has stated, cracked hoses can hide in corners where you will never see them. There can be plenty of vac inside the carb to operate that choke pull-off, but still have a huge vac leak elsewhere. When the motor is running at the lowest idle it will sustain on its own, open up a propane bottle (small plumbing style torch)(DON'T light it) and move the nozzel around the intake gaskets to see if a vac leak will suck in the propane and improve the idle. Don't have the propane turned on so high that you fill the whole area with raw propane, but just enough so you can hear the torch hissing. Keep it moving so the propane doesn't build up. Be warry of getting too much in around the dist. area becuase of sparks inside the cap. Slowly move the torch closer to the base of the carb to see if there is a vac leak. Old carbs can have vac. leaks at the throttle shafts that can be very difficult to pinpoint. Move the torch around all the vac. lines to check them. Remember that some vac. lines are quite long and the whole length needs to be checked. Vac. leaks will usually have a whistling sound to go with them while the extra air is being sucked in, But... not always. If all else fails, go with Bondo's suggestion.

Wrench"

"Thanks for the suggestions, I

"Thanks for the suggestions, I am packing tools for the day at the boat, will report back what we can find. I did blow out all the passages during the rebuild, and everything looked good when it went back in. I think the only vacuum line is the 1 inch hose from the carb to the vac break actuator, so I will replace that. The fuel pump sight tube fitting at the carb was completely clogged, so I better check that hose too. (I did look at it last time, no fuel).
Thanks,
Chris."

"1 inch hose from the carb

"1 inch hose from the carb to the vac break actuator" Remove it from the actuator and plug the hose; if the idle improves the problem is the diaphragm in the actuator is leaking.

""The fuel pump sight tube

""The fuel pump sight tube fitting at the carb was completely clogged, so I better check that hose too."

If you have a mech. fuel pump then the nipple on the pump is also plugged. The internal diaphragm has been seeping and the slow leak turned to gum varnish. You may need a new pump. Did you change the pump's internal filter?"

"Ok, back from the boat. Took

"Ok, back from the boat. Took the carb back off, and re-cleaned out all the passages, reassembled.

Checked timing, OK.
Checked spark plugs, a little oily, but otherwise OK (checked three out of eight)

Started right up, and idles slightly better (can keep running with no throttle now) and the idle screws have some effect.
Still pulses a little, the RPMs drop to around 500, then a visible squirt of fuel comes out of the starboard venturi and the RPMs surge up to 800, then slowly falls back down. The Port venturi is spraying a small amount of fuel continuously. [I think they should be the same!?!]
Does the idle fuel come out the top of the venturi just like the regular fuel from the primary metering rods, or is there a different outlet port?
Plugged off the sight tube fitting, no effect, plugged in a vacuum gauge, and there is no vacuum on the sight tube fitting. [It is plumbed into the top of the air horn, so probably expected.)
Pulled the vac break tube, and plugged in the gauge, no change in behavior (so the actuator is OK).
Still running lean, covering the intake improves the running, and steadies out at around 800 rpm.
I have not checked anything on the fuel pump (it is mechanical). I did not know it had a filter, I thought the water separation and the carb inlet were the only two. (both replaced).

Since it will stay running with the throttle straight up, I think it is close, but the surging RPMs tell me something is still not right. We plan to take it out on the water tomorrow to see how the high end is working now. (Weather permitting... It was nice here in southern California today!)

Thanks for all the ideas,
Chris."

"Ayuh,...
Those carbs are kno

"Ayuh,...
Those carbs are known to leak air around the throttle butterfly Shafts,...

Spray around the base of the carb with wd-40 or ether,...
If it revs,... see my post above..."

Chris...

Watch how long and

Chris...

Watch how long and how hard you run that motor in a lean condition as this raises combustion chamber temps. It can start burning pistons and valves.

Wrench

"Chris Bell
At 500 rpm the ve

"Chris Bell
At 500 rpm the venturis shouldn't be atomizing any fuel, the nozzles should be dry. At that rpm the air/fuel mixture should be maintaining the engine with the idles circuit. Check for misaligned gaskets, missing gaskets etc, possibly the float is set too high?
Bert"