I have an electric-choke Q-jet on my '75 and I'm changing intake manifold gaskets. I have the choice between the open heat crossover or closed heat crossover models of gaskets. I assumed I'd use the closed heat crossover gasket because I'm using an electric choke, but when I pulled the intake off today I noticed the engine builder (rebuilt, non-stock 383) used open heat crossover gaskets. Should I assume they used them because they were allowing for people with the non-electric type carb, and should I go ahead and install the closed gaskets?

Another thing: if one were to use the open-style intake gaskets and an electric choke, would it cause any drivability issues? (Vapor lock, flooding, etc.?)

I have a '79 Chevy truck that originally had the little non-electric, exhaust-heat style choke (whatever the official name is I'd be curious). The carb needed to be replaced and the mechanic threw on a factory rebuilt unit that used electric choke. I hate the thing and have had real drivability issues with it since day 1, and now that i have a look at my Vette's intake/carb setup it's gotten me to wondering if I've invited problems on the truck by having the stock open-crossover gaskets and an electric choke. (It mainly runs beautifully when absolutely cold-for a couple minutes-then it won't hold an idle (tries to hold 500 rpm) while it's part-way warm for about 20 min. of driving, and then when it's fully warm it'll just purr at 500, no problems. That in-between/partly warm time is a real pain. And when I re-start it warm it races at 1000 rpm and when I go to restart it half-warm or warm it floods very easily. Anyway, just wondering if the old open-crossover gasket and electric choke combo are causing issues.

Thanks, Patricia/'75 Vette

Do you still have an EGR valve? If so, that is why you still need the open heat crossover gaskets. It should not have any effect on the choke. The difference between the heat riser choke and an electric choke is the way that the choke is controled untill it is opened. The heat riser choke has to wait for the intake to transfer heat to it to open the choke were as the electric choke has a electric by-metal spring that opens it after the key has been turned on for a couple of mintues.
If your truck isn't running right you should check to see if the chike is openning properly. If it is then you have something else that needs help.

Good Luck

Redwingvette is right. The EGR is the issue. Also either choke will work well if it's set up properly. Yours is not. A readjustment will fix the problem. It's opening too soon. There needs to be a increase in tension on the choke thermostat coil. This is done by rotating the choke coil housing. You can do this your self and do some trial and error until you get it right. The worse you can do is put it back where you started and be where you are now. Rotate the choke coil in the direction the choke rotates to close. That will make it stay closed longer. You will find marks of some kind on the housing or coil cover in most cases. Some after market housings aren't marked. Go about 2 marks, or 1/4" at a time. That may be too much when you fine tune it, but it's good for rough adjustment, and usually final adjustment is about 1/2 of that, unless you get lucky the first time. If there are rivits holding the coil housing, drill them out and replace them with screws. If the housing is notched and cannot be adjusted, cut the tab that goes into the notch, and adjust it anyway.

If it is not closed long enough you have the problem you have now. If it is closed too long it will stay on fast idle too long, and run rich.

Ken Styer

Hi Brian,

The Vette does NOT have an EGR valve on this motor. So I should use the gasket which closes that crossover area on the intake, correct?

When I ordered my gasket from Summit I wasn't sure which to purchase, and the tech asked me if I had an electric choke or not. I said I did, so he said I probably need the closed-gasket. He said to use the open one would unnecessarily allow heat near the intake air. Perhaps I misunderstood him, but it sounded like the whole thing depended upon what type of choke I have. I then assumed the exhaust crossover in the intake manifold is there to warm up the heat-riser style choke, but I guess that's not the case; the crossover is just for EGR vavle useage, correct?

So with a carb that has the heat-riser style choke, does it simply absorb heat from off the intake, not from the exhaust crossover underneath it?

Also, the intake manifold has a riveted-on sheetmetal "heat shield" on its underside. Does it have anything to do with the choke style or the EGR use, or is it merely to keep engine heat away from the intake air?

If you do not have an EGR valve and you have a electric choke than you do not want the open gasket. Some heat is used to transfer heat to a heat riser choke because it helps heat up the intake however there are some metal plugs that are usualy used to make the hole smaller. The steel plate that is attached to the underside of the intake is there to keep oil from cooking as it splashes up on to the bottom if you have the crossover (Hot exhaust and oil don't mix). The crossovers main purpose is emissions related. If you have a heat riser choke an the crossover is pluged it will take longer for the intake to heat up so the choke will take longer to fully open. Note: all chokes should open about half way as soon as thw motor is started (Vacuum line from the carb base}. Than as the motor warms up, it should be opened fully (about 3-4 minutes).

Brian, thanks for explaining about the crossover. I'll definitely use the blocked intake gaskets (which I have). So in the case of an engine with NO EGR valve but a heat-riser choke, you're saying there's usually a sort of plug in the end of the crossover to make the passage smaller; that way the choke gets some heat but not the volume you'd need to run an EGR?

Out of curiousity, if one were to run an open-gasket with NO EGR valve and an electric choke, would you have any issues?
(thinking of my truck now: they pulled the heat-riser-style carb off and threw on an electric choke carb and, of course, left the original intake gaskets in place.)

Patricia

Ken, thanks for your help with the choke adjustment. Presently, I have the choke set for about 2 min. The guy who sold me the carb said the longer you set the choke for, the quicker it will close again when the motor is shut off, such that when you stop for an errand for 5 or 10 min. the darn choke will be back at idle-speed again quickly and will take longer to open back up (as opposed to having the choke set for a shorter time it will not close on you as quickly during warm-restarts).

So I have been hesitant to adjust it for longer choke time. What I have done, with my brother's help, is wire in a switch (on the dash) that I can interrupt power to the choke when the motor's cold, thus causing the choke to stay closed until I give it power again and it can start warming up. Unfortunately, the situation hasn't improved using this switch idea; for some reason the choke still behaves erratic, sometimes going to warm idle speed even though i've interrupted the power. It's a frustrating problem and makes the truck very annoying to drive until it's warmed up all the way; then it's very happy at 500 rpm. However, it takes about 15 min. of across-town type driving before it's warm, so most of the time I'm running it in its cold condition.

The guy who sold me the carb pretty much chalked up all the issues to the fact that the motor has been stripped of all its exhaust heat shielding and EGR valve, etc. (somebody before me), so there is nothing to help the carb warm up quickly when cold. He thinks i'm getting carb icing in the winter. Funny thing is, with its old heat-riser-style choke the truck never had these idling issues. Wish I could convert back! However, seems to me there's got to be a way to get a BB w/o emissions to idle well with an electric choke.

Typically, you want to set the electric Q-Jet two notches rich from the center index on the choke housing. The factory usually says center index, but it doesn't work as well.

With the old heat riser, the intake maifold was heated with exhaust gas. This caused the fuel to evaporate better during cold operation, and helping to warm up the engine and run better cold. This requires the open manifold gaskets to allow exhaust flow into the manifold. But without the heat riser, there is equal pressure on both sides of the intake and it won't flow anyway. The heat riser partly restricted exhaust flow on one side and caused flow across the manifold.
You could adjust the choke pulloff. This is the vacuum break the opens the choke as soon as the engine starts. It will remain at that spot until the coil warms up enough to further open the choke. By playing with the pulloff, you could set the choke to stay on a bit longer (not too much) and the truck should be more stable while warming up. If the pulloff is too lean (open) it will hesitate during cold operation. If it is too rich it may bog or foul plugs. If you tighten the choke too much it will kick back to fast idle very soon. But this would be way too much for normal driving anyway. Timing adjustment and warm fuel mixture will also affect this. Make sure they are correct first, or you will be wasting your time.

Next, do you have the air preheat on the air filter? This is the tube from the exhaust manifold stove to the air cleaner snorkle. Above the tube in the snorkle is an air flap. The flap is controlled by a thermo sensor in the air cleaner assembly. The sensor is supplied by manifold vacuum. It senses air temp in the air cleaner. When it's too cold it pulls air from the hot manifold. As it warms it moves the flap to allow cold air to mix, but maintains the incoming air at a constant temp. The target temp is about 105 to 125 deg F. as an average for most cars. The sensor changes the vacuum to the flap. This improves cold driveability and helps the engine warm up quicker. If you don't have this, or if it isn't working, getting it straight will help. Many people flipped the air cleaner lid over to allow more air during hard acelleration. While this did indeed work, it created a cold driveability problem.

Another option is install a resistor to the choke coil so it opens a bit slower. This will not affect how quickly it resets. Just make sure the resistor will handle the amperage.

If this is erratic, check and make sure you have a good ground at the choke coil. This includes all the way to the engine, the carb itself may not be grounded very well.

The only time you should need the switch is when the key is on and the engine is not running.

Keep me posted on whats happening. We will get this fixed.

Ken Styer

|UPDATED|3/2/2004 3:50:11 PM (AZT)|/UPDATED|

Ken,

Thanks for all the help; sorry I've been so long in responding. I looked for the air preheat pipe and flapper unit you mentioned. I see a vacuum-type hose running from air cleaner up to snorkel and into a little device on top of the snorkel, but inside the snorkel there is no flapper, and I see only a short metal pipe dropping straight down from underside of snorkel but connected to nothing, so I guess someone completely disabled that system.

No EGR; no nothing. Presently choke set about 1/4-1/2" counter-clockwise past end of indexing lines.

Regarding a ground on the choke...all I see is the power wire that plugs into electric choke. It seems to me if the choke was getting no power it would be closed all the time and motor racing all the time (that happened once when the guy's shoddy wiring job fell off and I took it to him because motor was racing horribly).

Another symptom of choke operation (or lack thereof): if I touch the gas pedal at all, just after start-up, the choke knocks off and the poor motor tries to run without it, at 500 rpm (which doesn't work for long!). We live on a hill which I can coast down (1.25 miles) so if I don't touch the gas pedal all the way down the choke will maintain around 750 rpm until the bottom of the hill when I have to use the gas and it knocks the choke off.

However, when the motor's warm, shut it off 5 or 10 min., come back out, start up, the choke sets the motor on 1000-1250 rpm and WON'T knock down for anything (tapping gas pedal) for a couple minutes. Very erratic behavior!

Here's a question: I still have the little rod and stove and spring thing from the original heat riser choke; can you swap out just the choke portion of the carb so I could return to heat-riser type, or would it require a whole different type of carb; and, can you even buy a heat-riser style carb still? (My carb mechanic said you can't, but presently I don't place much faith in him.)

And last thing I can't help but wonder: Aren't there hotrods and custom cars running around where the owner has removed all emissions-related equipment? Do they all have cold-running issues, or are they somehow getting around it? Also, my 383 crate motor in my '75 Vette (no emissions of any kind) never had cold-running issues at all, with ancient electric-choke carb and everything. Could the difference be explained because of my truck having a BB vs. a SB which would warm up faster? My truck takes 15-20 min. driving town speed before up to temp.

Thanks, Patricia

The way the electric choke works is this:
Inside the plastic disk is a metal spring
when power is on it acts like a resistor that heats up
as it heats up it expands which causes it to open the choke

If the wire came off the choke would not open

When the engine is cold and it is started up the choke should open about half way. aAt that point the idel should be alittle above 1200rpm's after the choke has opened fyully (about 3-5 minutes) the idel should drop back down to the normal run speed (about 800 rpm's).