QJ off idle lean bog.

[rfoll]

I have 2 cars with similar engines and similar off idle problems. Both engines have Vortec heads and I am using a Performer RPM with a modified 4 hole spread bore adapter. Both have a huge flat spot just off idle when the engine is cold and it is really aggravated when the weather gets cold. I can tune some of it out with the choke adjustments, but I don't like drowning my engine in fuel just to drive 3 miles to the store. My assumption is that the nearly stock late 70s vintage carbs are not happy with the increase in plenum volume over a stock QJ manifold. My research tells me I need to increase the area of the Idle channel restrictions near the idle down tubes in the main body. The necessity of needing the idle mixture screws turned out 3 or more turns to achieve a good idles makes me think this is true. I have no idea how much to increase the diameter, and I fear ruining an otherwise good running carb and screwing up the 18 + MPG I have been getting. Any thoughts on this are welcome. Thanks, Rick

[77cruiser]

How big have you made the channel restriction? Might have to make the tubes bigger too. I have drilled & tapped to make mine more adjustable.

[bill jones]

--this link is about a TVS system---"Trapped Vacuum Spark"
viewtopic.php?t=2178&start=30

--is pretty simple really---runs the vacuum advance vacuum thru a thermally controlled brass valve screwed into the the thermostat neck
---it locks the vacuum advance full advanced until the engine warms up and then it works normal

---it was like a miracle on my own 400 SBC
---might need to get an illustrated parts guide catalog of that sort of sensors etc---where they show you the temperatures and various temp sensor applications that different temp ranges--like maybe using a Ford sensor of some sort

[F-BIRD'88]

This is why every OEM for the past 100 years uses a exhaust HEATED plenum
on all carbed engines..
You need to heat the plenum . Heating the carb spacer will help too..
The non air gap. vortec intake manifolds can be plumbed for exhaust heat or hot water heated plenum.
Hot exhaust is better faster.

The AIR GAP manifolds have space under the plenum to stuff with a bladder or a coiled tube heated element you can plumb with hot water or hot exhaust....

[F-BIRD'88]

A electric heating element added under the plenum of AIR GAP type intakes is also possible.
Liquid fuel will not burn.

If every OEM could not get around not using a heated plenum on every carbed engine in the last 100 years you won't either.

[F-BIRD'88]

An example of a add on under plenum heater box.
https://www.aussiespeedshop.com/product ... er-heater/

[rebelrouser]

I have 2 cars with similar engines and similar off idle problems. Both engines have Vortec heads and I am using a Performer RPM with a modified 4 hole spread bore adapter. Both have a huge flat spot just off idle when the engine is cold and it is really aggravated when the weather gets cold. I can tune some of it out with the choke adjustments, but I don't like drowning my engine in fuel just to drive 3 miles to the store. My assumption is that the nearly stock late 70s vintage carbs are not happy with the increase in plenum volume over a stock QJ manifold. My research tells me I need to increase the area of the Idle channel restrictions near the idle down tubes in the main body. The necessity of needing the idle mixture screws turned out 3 or more turns to achieve a good idles makes me think this is true. I have no idea how much to increase the diameter, and I fear ruining an otherwise good running carb and screwing up the 18 + MPG I have been getting. Any thoughts on this are welcome. Thanks, Rick

I worked as a dealer tech in the 70's you should be able to tune the bog out by proper vacuum break, or choke pull off adjustments, provided as some have said your exhaust cross over under the carb is open and working. Will it run fairly rich on a short trip, yep, they always did. I think some people forget how miserable it was to drive a cab in cold weather. I remember when the weather turned cold spending a couple weeks doing nothing but overhauling carbs and replacing vacuum breaks, back in the day.

The other thing is that the fuel the carb was designed for, no longer exists. The gasoline formulations now are for the burning of fuel using high pressure injection systems that can easily atomize cold fuel. I was on a forum a few years back discussing gasoline and driveability of carbs, and a petroleum engineer posted, we don't make gas to run in carbs anymore, we formulate fuels for injection. I guess we should just be happy that they will still run on what is being produced and sold? And on most muscle car engines that I work on which mainly get driven in summer, I block the cross over heat, as on a hot summer day the extra heat will boil the fuel causing all kinds of driveability issues.

[rfoll]

How big have you made the channel restriction? Might have to make the tubes bigger too. I have drilled & tapped to make mine more adjustable.

I have not altered the idle channel restrictions. The purpose of this post is to get opinions as to whether or not this could be a solution to the problem. The necessity of opening the idle mixture screws twice what would be normal lends me to think so. I have a similar setup on the 400 in my truck, and it doesn't have the problem despite not having a choke. These are not high performance engines, and the manifolds are not air-gap.

DSCN1421.JPG

DSCN1421.JPG

I have played with the choke and vacuum pull-off arrangement endlessly, and if I tune the flat spot out by that arrangement the fast idle is still on 2 miles down the road. Most of the electric choke heaters are very lazy and slow to respond. I am very leery of making a no-reversible modification to carburetors producing 18+ mpg.

[dannobee]

First things first. What is the float level? Sometimes the rubber (viton?) tip of the needle valve swells up and causes a really low float level. Check that first. There's a tool to check float level. Takes about 2 seconds to check without pulling anything apart. It looks like a tiny plastic popsicle stick with graduations.

[77cruiser]

Do you have a stock snorkel air cleaner with the stove on the ex. manifold?
3 turns isn't out of the norm for for a Qjet sometimes can be more.

[F-BIRD'88]

If you look at every OEM Qjet intake manifold there is a heat track under the carb to heat the carb base. The cold carb spacer slows the time any intake manifold heat conducts up to help heat the carb base for better fuel vapourization and drivability during cold weather.
If you use the hot water method to add heat to the plenum and carb spacer you speed things up by forcing the whole engine cooling system to heat up quicker during engine warm up. The hot exhaust gas method is quicker at getting heat to the plenum and carb spacer.
Thats why it is prefered over the hot water method.
A electric heater under the plenum can be quick too and easy to shut off and on to regulate plenum temps.
But the energy is not free.
A manual choke control cable and a manual (fast idle) throttle cable control scheme may help some.
But it comes down to the need to address (a lack of) fuel vapourization in the manifold during cold weather warm up. It is caused by a LACK OF HEAT.
The QJet carb is only fuel efficient because it can deliver just the needed amout of fuel while driving and no more IF when the supporting systems that aid engine warmup and manifold plenum heat regulation are operating.
Otherwise a cold manifold requires EXTRA FUEL to account for poor vapourization.
When ever the engine is running fuel is using up HEAT to vapourize. When it runs out of heat vapourization stops.
Pure chemistry and physics.

[F-BIRD'88]

Back in the day the fuel at the pump was adjusted for the season to aid fuel vapourization wigh carburated engines. Don't know if they still do that where you are..
Modern EFI systems can adjust fuel injection on the fly and most use a "dry intake manifold".
Even fuel octane does not now need to be so tightly controlled as the EFI system adjusts spark timing on the fly to avoid any engine knock on cars used primarily for daily commuting. 99% of drivers don't know don't care.

[F-BIRD'88]

Rethinking the spark advance curve control if with a cold intake manifold, cooler weather , during engine warm up can and will help some..
Poor fuel vapourization creates both rich and lean and incomplete fuel mixing, all want more spark advance under that condition when driving.
Again, modern EFI does this on the fly.
A method of dash board spark timing control the driver can work with during engine warm up helps.
A method of driver feedback about the temperature of the intake manifold plenum, carb base and air cleaner air inlet (temp gauges or indicator lights) helps the driver make on the fly engine spark/fuel/temp control choices while driving when cold. A dashboard guage or lighted indicator that shows engine misfire condition while driving would be usefull feedback for manual driver control of these systems.
A (heated) narrow band LED AFR guage can show engine missfire as it jumps around during this condition as well as showing relative rich- lean either side of stochimetric .
You soon learn what is normal vs abnormal without needing to know the exact AFR at the moment.

[rfoll]

First things first. What is the float level? Sometimes the rubber (viton?) tip of the needle valve swells up and causes a really low float level. Check that first. There's a tool to check float level. Takes about 2 seconds to check without pulling anything apart. It looks like a tiny plastic popsicle stick with graduations.

I have been inside both carbs many times and have tried changing the float levels. The Impala has a dual snorkel air cleaner with the heat stoves intact. It doesn't really help When the motor is stone cold. The El Camino has such a low hood clearance that I have to run a drop base air cleaner. I have tried an open element to allow the engine to breathe hotter air, but it doesn't really help either. F-Bird, you are not telling me anything I don't already know. What I want to know is if increasing the size if the idle restriction will help the transition from idle to the main jets.

[F-BIRD'88]

I would play with the primary metering rods.
The APT adjustment is critical. The APT trims the whole fuel circuit including the idle off idle transition.
If you drill the IFR you are stuck with it.
If you set it up for COLD (extra fuel) it won't be right for when warm. too rich.
The height and shape of the primary metering rods is much easier to adjust and MANIPULATE ±.
When stone cold its all about the choke and fast idle system.. Manual override (driver) control helps. Same with cold spark advance..
Quick heat to the plenum is key reguardless.
Carbed engines need time to "warm up".