Carb or Timing adjustment? or both?

neal64ss · May 24, 2007

My 396 has always ran "hot". It is an oval port, edelbrock dual plane intake, hydraulic non-roller cam .515 lift 280 duration, headers, q-jet, crane cams electronic ignition. I replaced the 3 core brass/copper radiator with a 2 core aluminum. It used to run hot on the highway. Now it doesnt run hot at highway speeds but still runs "hot" at idle. By hot I mean 15-20 higher than thermostat.

In an attempt to see if timing has anything to do with it I pulled the vacuum advance line and plugged it, the timing is about 12 advanced. I plugged the vacuum advance back in and the timing did not change. Vacuum advance is connected to the front bottom passenger side port of the q-jet.

Also the q-jet idle screws have almost no affect on idle until they are almost completely closed. I have noticed this issue before, thought it might be worth mentioning here. Also when I back it into the garage I can usually smell gas after shutting it off, as if some fuel is passing thru without being burned????

Also have thought about upgrading to an aluminum high flow water pump to help with the cooling.

Weird thing is, it runs great. Idles smooth and the 4 barrel comes in smooth and rapidly, no bog or pinging. Just seems to me something is wrong.

Anyone have a suggestion on what to check, fix, or adjust.

LevonH · May 24, 2007

Where are you measuring the temp? If it is off of the head it can read 20 deg higher than actual because of heat soak into the sensor. Common problem;simple fix.

onovakind67 · May 24, 2007

What method are you using to attract air through the radiator?

65cayne · May 24, 2007

I run around 200-205 with a 190 degree thermostat. No issues at all. If you are not overheating, I wouldnt worry about it. JMO.

Schurkey · May 24, 2007

neal64ss said:
My 396 has always ran "hot". It is an oval port, edelbrock dual plane intake, hydraulic non-roller cam .515 lift 280 duration, headers, q-jet, crane cams electronic ignition. I replaced the 3 core brass/copper radiator with a 2 core aluminum. It used to run hot on the highway. Now it doesnt run hot at highway speeds but still runs "hot" at idle. By hot I mean 15-20 higher than thermostat.

Overheating at high speed generally means water circulation problems.
Overheating at low speed generally means air circulation problems.

HAVE YOU VERIFIED THAT YOUR GAUGE IS CORRECT?

What are you using to move air through the radiator? Tough to beat a thermostatic clutch fan and decent shroud. If you have a fan clutch, is it thermostatic, or the low-dollar junk? Does it engage?

neal64ss said:
In an attempt to see if timing has anything to do with it I pulled the vacuum advance line and plugged it, the timing is about 12 advanced. I plugged the vacuum advance back in and the timing did not change. Vacuum advance is connected to the front bottom passenger side port of the q-jet.

Is that nipple for ported vacuum, or for manifold vacuum? Do you get suction above idle? Does the vacuum advance leak? Will it advance if the carb is set to "fast idle"?

neal64ss said:
Also the q-jet idle screws have almost no affect on idle until they are almost completely closed. I have noticed this issue before, thought it might be worth mentioning here.

Perhaps the throttle is open too far at idle. I'd move the vacuum advance to manifold--if it isn't there already--make sure the vacuum advance WORKS, and the extra timing may allow you to back off the idle speed screw. Then re-adjust the mixture screws.

neal64ss said:
Also have thought about upgrading to an aluminum high flow water pump to help with the cooling.

Doesn't sound to me like you have a water flow problem now. 'Course, it's good to remove weight from the front of the vehicle...

neal64ss · May 25, 2007

A little more info. I checked and no vacuum at idle to the port on the front passenger side of the Q-jet where the vacuum advance is plumbed. Now this confuses me as I thought all the ports on the bottom plate of the carb were the same as manifold vacuum.:clonk:

I checked and I have a port on the bottom rear of the carb that has good vacuum at idle. I think I should plumb the vacuum advance to here. Then I plan to set the initial timing back to about 4-6 BTDC and the vacuum advance should add about 12-14 more degrees of advance when hooked up.

Please some advice if I am on the wrong track.

Also, does anyone think this will help with the fact that I cannot get the idle screws to do anything unless they are almost completely closed?

Schurkey · May 25, 2007

neal64ss said:
I checked and I have a port on the bottom rear of the carb that has good vacuum at idle. I think I should plumb the vacuum advance to here.

OK. Does the vacuum advance actually work?

neal64ss said:
Then I plan to set the initial timing back to about 4-6 BTDC and the vacuum advance should add about 12-14 more degrees of advance when hooked up.

How many degrees of centrifugal advance do you have? You're going to want around 35 degrees of initial + centrifugal altogether.

neal64ss said:
Also, does anyone think this will help with the fact that I cannot get the idle screws to do anything unless they are almost completely closed?

Might. If you have more advance at idle than before, you can close the throttle blades more via the idle speed screw to maintain the same idle speed. That "might" help make the screws more sensitive. I'd be pulling the screws out and either spraying aerosol carb spray in the holes; or hitting them with compressed air. Either way, verify that the idle bleeds are open by seeing the carb spray squirt out; or feeling for the compressed air.

gnicholson · May 25, 2007

The vacuum port you had your vac. advance hooked up to was a ported source, in other words the throttle has to be opened enough for the port to be uncovered by the throttle blade; about 1500 rpm. I would first set the total timing to 36 deg and see what that produced at idle to decide whether you want ported or manifold vacuum to your advance. Usually you will have 12 to 16 deg without the vac adv. hooked up with the total set to that figure which would work fine for the ported advance. Bigger cams like more initial timing and usually work better with the dist hooked to manifold vacuum. While your at it check your mechanical advance rate and see what rpm it is all in by, usually around 3200 depending on your combo. After you get everything adjusted with the car hot and idling try your mixture screws again then slowly choke the air horn until the rpm increases as much as its going to . If the idle speed goes up more than 100 rpm the idle mixture is to lean. A lot of q jets are lean on the idle circuit and need the idle feed restriction opened up as well as other mods.

SWHEATON · May 25, 2007

You need a lot more initial timing with the cam your runing,thats part of your running hot issue and why your prim throttle plates are adj almost past the idle circut so idle mix screws are not very sensitive/reactive.

Set the initial to 18 deg btd with that cam and your motor will wake up a lot.

When you set the inital make sure the motor is idling low like 600rpms to ensure the mechanical adv in dist is notstarting to activate upsetting the inittial timing. Also make sure you have the vac adv unhooked & plugged when checking initial timing.

Advancing the initial to 18 deg will raise the idle some which will force you tolower it again with in turn will position the primary throttle plates back into the idle circut where they belong when at an idle. Now your idle mix screws should be a little more sensitive/reactive to adj made to them.

But check the total after that and if it goes past 38 deg pull the dist to have a shop recurev the mechanical adv for 20 deg max . The 20 deg mech all in by 3k rpms with 18 initial gets you 38 deg total and your good to go.

This is the right way to dial in your ign timing,i realize pulling the dist to have the mech adv recurved/limited to 20 is a pia but its the right way to do it.
Backing off the initial to get the total to come back down where you want it is the wrong way to go about it. When you do that the initial timing is then retarded which looses power,wastes fuel,and also makes the motor run hotter too.

I see many people that choose to go this easy way out route rof etarding the initial to get the total down where they want it not realizing just how much power & fuel is wasted in doing so not to mention the motor running hotter too.

You also need a vac adv with your motor setup and that will bring your part throttle cruise timing up to approx 50-54 for better responce & fuel economy if there is such a thing with a perf BBC(LOL!!!) Try the vac adv on a ported port 1st that only has vacuum when off idle,it that work goo fine,but if not try a port with vacuum on all the time and see how that works out.

Today in 90 deg heat my 69 chevelle ss with a mild 396(Now 402) with approx 370hp/440 lbs trq ran approx 190-195 deg at cruise & 205 in some traffic but after 8-10 mins in very slow traffic if went to 210 . The temp sensor is mounted in the head by the exhaust port & manifold so thats a known hot spot so i would subtract at least 5 deg from my readings to be more realisitc. My cooling system has a stock type "25 yr old 4 core raidator",shroud,7 blade 772 clutch fan,std duty thermostatic fan clutch,stock type w/pump with the better pumping cast iron impeller which is whats considered the 30% better flowing units sold today with cast iron impellers,a 180 deg hi flow stat,and i run 18 deg initial with a mild hyd flat tappet perf cam along with 20 deg mech for a 38 total. These temps my motor runs are very tyoical for a bbc on a 90 deg day esp with a 25 yr old radiator in the mix. but i would have to say if my motor ran much htter i would feel likemi had to do something and that would be to replace the 25 yr old radiator witn a new 4 core or an Alumitech AL radiator that looks basically stock,it would depend on $ avail at the time i needed it.

If your timing & general motor setup/timing is dialed in correctly along with a stock cooling system in good operating cond there is no need for an aftermarket AL w/pump or AL radiator with your mild setup IMHO.

Let us know how your motor responds to 18 deg initial if you decide to take 5 mins to try it out,you wont be sorry as long as you get the mechanical adv recurved if needed. It should run a little cooler and have more punch esp in the low & mid range power with more initial.

Scott

neal64ss · May 25, 2007

OK, I have checked some stuff as asked for. Heres what I found. These numbers are probably +/- 1 or 2 degrees.

Idle with no vac advance = 17 BTDC
Idle with vac advance = 17 BTDC
high rpm with no vac advance = 22 BTDC
high rpm with vac advance = 38 BTDC

Looks like this is set pretty good, and it runs about the same as before I started. I am a little confused with what seems to be 5 degrees mechanical advance, however.

So. . . I guess I have some carb adjustments to do. How do I spay the idle screw passages out? Do I remove them and spray from the front of the carb in the holes they were in or do I need to "backflush" these passages? After I reinsert them how many turns on each from seated do I need to start with?

Schurkey · May 25, 2007

neal64ss said:
OK, I have checked some stuff as asked for. Heres what I found. These numbers are probably +/- 1 or 2 degrees.

Idle with no vac advance = 17 BTDC
Idle with vac advance = 17 BTDC

So. You have vacuum advance on ported vacuum. I'd consider trying manifold vacuum.

neal64ss said:
high rpm with no vac advance = 22 BTDC
high rpm with vac advance = 38 BTDC

Your vacuum advance works. Looks like you have 16 degrees of vacuum advance. So far, so good.

neal64ss said:
Looks like this is set pretty good, and it runs about the same as before I started. I am a little confused with what seems to be 5 degrees mechanical advance, however.

You should be. 5 degrees is NOWHERE NEAR ENOUGH. Time to pull the distributor apart to clean and lube the centrifugal advance. SOMETIMES you can do this in-the-vehicle, but if it were in my driveway I'd pop the distributor out and do it on a nicely well-lit workbench.

First Guess: You're going to find the advance mechanism is nearly seized from rust or varnish. A little polishing and some lube and you'll be set.

neal64ss said:
So. . . I guess I have some carb adjustments to do. How do I spay the idle screw passages out? Do I remove them and spray from the front of the carb in the holes they were in or do I need to "backflush" these passages? After I reinsert them how many turns on each from seated do I need to start with?

Pull the screws out, (Count the number of turns IN until the screws seat. Record that number for both screws. When you're done, you'll screw them in until they seat again, then back them out the same number of turns that you recorded earlier.) insert red straw from carb spray nozzle into passage. Spray the passage, looking at the top of the carb--you should see some carb spray coming out of tiny air bleeds on the same side of the carb that you're spraying.

Final adjustment on the screws will be to adjust them in or out until you get the highest manifold vacuum. Adjust the idle speed if needed to keep the idle speed where it should be. Re-adjust mixture screws again for highest vacuum. Then turn them clockwise 1/8--1/4 turn to lean the mixture just a bit. Final adjustment of idle speed. Done.

But take care of the centrifugal advance first.

gnicholson · May 25, 2007

As the guy before said you dont have near enough mech advance, you need atleast 20 depending on where you set your initial.What kind of dist do you have? Is it a stock points type converted to a crane electronic? Did I understand correctly. I would remove and disassemble the dist as described earlier and clean and inspect for wear in the advance mechanism. After its back together the best way to check and adjust the advance rate and amount is a Sun dist. machine if theres anyone in your area with one. Failing that I would get a recurve kit from Moroso and do it myself in the car. I would start with a total of 36 deg. all in @ 3200 to start with. You may want to try a little more total or change the rate some if it pings under hard accereration for instance but these figures will be close. Also adjust your idle mixture as prescribed earlier and afterwards try richening the mixture by hand like I described in my last post .If you have an original q jet for a 396 the idle circuit is probably ok but if you have a newer one it is almost certainly to lean and will not allow you enough control over the mixture screws without modification. Qjets are also notorious for debris being lodged in the idle feed restrictions .

neal64ss · May 26, 2007

Thanks guys, I am starting to understand some of the earlier post better now. Will go to work now on that mechanical advance and then check out the idle screw adjustments.

neal64ss · May 27, 2007

Yes the distributor is the stock points type converted to the crane optical trigger electronic ignition.

neal64ss · May 27, 2007

I checked the mechanical advance weights, they are free and moving properly. I found one of the four bolts that hold the carb to the intake is stripped out, I have pulled carb and insereted a helicoil, mabey this was causing a vacuum leak?

Carb or Timing adjustment? or both?

neal64ss

LevonH

onovakind67

65cayne

Schurkey

neal64ss

Schurkey

gnicholson

SWHEATON

neal64ss

Schurkey

gnicholson

neal64ss

neal64ss

neal64ss

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