396 Cold Start.. I Need Some Advice

[James W.]

I'm working on getting my Operations/PV Cold Start checks dialed in on my 1965 Corvette equipped with a 396 engine. Once the engine starts I've gotten the cold start fast idle dialed in at approximately 1,800 to 2,000 RPM and the hot idle curb idle set at 850 to 900 RPM based on the NCRS Performance Verification guidelines.

The problem that I am having is getting the car to start and stay running initially when cold. Per the supplemental 1965 396 chassis service manual I have the initial timing set at approximately 9 degrees; the manuals states that 10 degrees is the nominal setting with a range of 8 t o14 degrees. The choke is set at 3 to 4 notches lean. I've tried depressing the accelerator fully to the floor thinking it needed a bigger shot of fuel and also tried just barely depressing the throttle enough to get the fast idle cam up to the top step per the manual. The car will start but runs rough for about 5 to 10 seconds and then stalls. The only thing that seems to help is to have my helper hold the choke plate open until the engine starts and when it does it runs rough and requires that I feather the throttle until it smooths out at about 1,400 to 1,500 RPM and then escalates up to around 1,800 to 2,000 RPM.

When the car is up to operating temperature and then shut off I have attempted a hot restart without any manipulation of the throttle. The car does not want to start unless I feather the throttle a little until it starts. I want the car to start when hot without touching the throttle. I have not adjusted the carb idle circuits on the metering block for best curb idle based on vacuum yet.


Question: Should I go a notch or two leaner on the choke setting?

Question: Would bumping the initial timing up a few more degrees help both the cold and hot starts?

Question: Should I adjust the hot curb idle circuits now or wait until I get the timing set so the car will start and run when cold?

The carb was recently rebuilt by Bob Kuntz and ran on his engine stand before being sent back to me. The spark plugs in the car are AC Delco R45XLS gapped per the manual and I'm running 110 octane racing fuel.

What should I be looking for to get these issues resolved? Any advice would be greatly appreciated.

Thanks in advance.


Regards,

James West

[Timothy B.]

Re: 396 Cold Start.. I Need Some Advice

James,

From your description I would say the choke circuit on the carburetor is not adjusted correctly. There is lots of info in the archives about the choke adjustment, do you have a service manual because it's very important understanding the adjustment.

At approx 70* the choke coil should close the choke blade completely and set the fast idle cam on the highest step. When the engine starts the vacuum choke piston should pull the choke blade open so the motor does not load up, approx 3/16", at that point if you tap the throttle the fast idle cam should drop to the next to highest step. As the choke blade opens the fast idle cam will drop to lower steps until it's off and the choke blade is full open.

All these adjustments work together so it's best to read and understand them before hand. Keep in mind the choke high step RPM is adjusted with a fully warmed engine so for example if the book say's 2000RPM the engine may only run at 1500 RPM when cold.

Timing sounds OK and the emulsion screw adjustment is made with a hot engine so choke adjustment should not have anything to do with it. I would do that next time you warm the engine. If Bob Kunz rebuilt the carburetor it's probably perfect except for a few adjustments.

[Duke W.]

Re: 396 Cold Start.. I Need Some Advice

A few degrees initial timing one way or another shouldn't have much effect as long as it's in the recommended range, and being at the low end where you are should ensure it won't detonate during the PV, but are you setting it correctly? You need to set initial timing at below where the centrifugal starts, and when you disconnect and plug the VAC to check/set initial timing revs should drop well below normal idle speed of about 900.

The OE VAC is a 201 15, and it should be connected to full time manifold vacuum. Does it work?

Check that you are actually measuring initial by bringing revs up slowly from less than 1000 to determine where the centrifugal starts. The AMA specs say centrifugal starts at 1000, but someone may have installed light springs

You need to get a baseline. First, verify that the timing and entire spark advance map (vacuum and centrifugal advance) is in the ballpark. Then with the engine fully warmed up, go through the idle speed/mixture adjustment procedure and shoot for about 900 hot idle speed.

Then set all the carburetor adjustments IAW the service manual. You ask about where to set the choke housing adjustment. What does the book say?

Be sure the choke vacuum break adjustment is right on.

Once everything is set to OE spec evaluate cold and hot start behavior and go from there.

Back in the day a typical hot start was done with about one-quareter throttle. Nowadays, given the percolation problem, a normal hot start may require full throttle because the engine may be essentially flooded. Review the flooded engine start procedure in the owners' manual.

Duke

[Carl N.]

Re: 396 Cold Start.. I Need Some Advice

James, Be sure valves are lashed correct - I like .024 intake .026 exhaust - set with engine at idle and hot - I know this is messy and not what the book calls for but after trying many different settings this works best for me. Also make sure your connections on your heat tubes to exhaust manifold on choke side are tight and free of any obstructions. You may also need to play with choke settings. Mine will not run right with book settings for idle and choke. When mine has sat for more than 4-5 days cold I have to pump it 3-4 times before it will start. After it reaches 150 or so it will start fine with turn of key. Your timing sounds ok to me. I have idle set at 900 rpm hot, anything lower I have to feather it at traffic lights. I have 3124 (also by Kunz) stock cam, TI ignition and so on. You may also want to be sure the vacuum tube in the meter block is tight, if not you can have a leak. If you have power breaks check line off back of manifold if not check the plug and be sure it is tight. Last of all I feel you are wasting $$$ with racing fuel. Keep playing with it and when you are frustrated leave it for a day or two and come back to it!

[James W.]

Re: 396 Cold Start.. I Need Some Advice

Tim,

If the 3/16" dimension you referenced is opening between the choke butterfly and the air horn (choke unloader setting) the 1965 Chevrolet Interim Shop Manual section 6M states that this dimension should be .375" or 3/8". I am going to take the carb back off the car tonight and check this setting and also the fast idle cam setting reference measurement of .025" on idle transfer slot side of the carb.


Thanks,

James West

James,

From your description I would say the choke circuit on the carburetor is not adjusted correctly. There is lots of info in the archives about the choke adjustment, do you have a service manual because it's very important understanding the adjustment.

At approx 70* the choke coil should close the choke blade completely and set the fast idle cam on the highest step. When the engine starts the vacuum choke piston should pull the choke blade open so the motor does not load up, approx 3/16", at that point if you tap the throttle the fast idle cam should drop to the next to highest step. As the choke blade opens the fast idle cam will drop to lower steps until it's off and the choke blade is full open.

All these adjustments work together so it's best to read and understand them before hand. Keep in mind the choke high step RPM is adjusted with a fully warmed engine so for example if the book say's 2000RPM the engine may only run at 1500 RPM when cold.

Timing sounds OK and the emulsion screw adjustment is made with a hot engine so choke adjustment should not have anything to do with it. I would do that next time you warm the engine. If Bob Kunz rebuilt the carburetor it's probably perfect except for a few adjustments.

[Timothy B.]

Re: 396 Cold Start.. I Need Some Advice

James,

I don't have a 1965 shop manual for reference but follow the procedure. When you start a cold motor the choke blade closes completely so the engine vacuum at cranking speed sucks fuel not air. In your post #1 you stated the choke coil is set 3-4 notches lean which may not be right. If you set the coil too weak the vacuum piston will pull the choke open to far to fast. The vacuum piston balances against the coil which is why these need to be set correctly.

The choke vacuum break opens the choke blade just enough to keep the engine from running rough from to much fuel. The 3/8 sounds wide but that check may be at the front of the air horn, my memory tells me some are made at the rear of the air horn but that may be on the AFB carburetor.

The .025 throttle blade fast idle setting is probably a good starting point, does the book give a fast idle RPM speed as this can be set after full warm up.

One other thing to keep in mind when adjusting a carburetor is the primary throttle blade position, set the primary throttle blade so at curb idle (no choke) there is approx very small squares of the transfer slot exposed below the blades as that's just enough to keep the transfer circuit wet with fuel. If the engine needs more air to idle at the desired speed open the secondary blades a bit with the hidden screw on the right side of the carburetor. Adjust the emulsion screws after full warm up because the hot plenum will vaporize the fuel load better and that's the condition the engine will see.

[James W.]

Re: 396 Cold Start.. I Need Some Advice

Tim,

Per the '65 396 Interim Shop Manual (ST-71) the 3/8" (.375") opening is between the choke butterfly and the air horn (choke unloader setting) is measured at the rear edge of the choke butterfly, this is verbatim from the manual. I took the carb off and checked and/or set everything per the manual and started the car. I still had to pump the throttle 3 to 4 times to get it to start. The choke coil was set at two notches lean but I think it still needs to go leaner since the car would not start and run without the aid of my helper holding the choke butterfly open until the car cleaned up and began to run smooth. From this point on the choke butterfly opened to full open as the car warmed up to operating temperature and the fast idle climbed from an initial 1,700 RPM up to 2,100 RPM which is 1,000 RPM from the high side of the PV acceptable range. I blipped the throttle after reaching the cold start high idle RPM and the car settled back at a steady 900 RPM where I had set at the end of the previous test. The car was shut off and I open the throttle and set it on the highest step of the fast idle cam, hit the switch and the car started and went to 2,100 RPM which is how it is to be tested per the manual once the car is at operating temp. I was able to back the fast idle cam adjustment screw one-half turn and will see if that reduces the fast idle a few hundred RPM when I retest again tonight.

I'm going to set the choke two or three more notches lean and see if that helps with the choke butterfly opening upon initial cold start.


Regards,

James West

James,

I don't have a 1965 shop manual for reference but follow the procedure. When you start a cold motor the choke blade closes completely so the engine vacuum at cranking speed sucks fuel not air. In your post #1 you stated the choke coil is set 3-4 notches lean which may not be right. If you set the coil too weak the vacuum piston will pull the choke open to far to fast. The vacuum piston balances against the coil which is why these need to be set correctly.

The choke vacuum break opens the choke blade just enough to keep the engine from running rough from to much fuel. The 3/8 sounds wide but that check may be at the front of the air horn, my memory tells me some are made at the rear of the air horn but that may be on the AFB carburetor.

The .025 throttle blade fast idle setting is probably a good starting point, does the book give a fast idle RPM speed as this can be set after full warm up.

One other thing to keep in mind when adjusting a carburetor is the primary throttle blade position, set the primary throttle blade so at curb idle (no choke) there is approx very small squares of the transfer slot exposed below the blades as that's just enough to keep the transfer circuit wet with fuel. If the engine needs more air to idle at the desired speed open the secondary blades a bit with the hidden screw on the right side of the carburetor. Adjust the emulsion screws after full warm up because the hot plenum will vaporize the fuel load better and that's the condition the engine will see.

[Stuart F.]

Re: 396 Cold Start.. I Need Some Advice

Just a short word of advice about hot starts: I do a lot of hot starts down here in central Florida, and the best advice I ever got and followed was to re-start by spinning the engine over for a couple seconds with my foot OFF the gas, then apply just a small amount of throttle and it will light right up. Follow with a few blips to finally clear it and it will be running fine. You see, your manifold is full of packed up over rich mixture that needs to be pumped out through the cylinders to make way for a proper new correct fuel/air mixture. This is specially true with today's "DC wonder fuel (ethanol)". Try it. You'll be surprised how well it works. Trouble is we've all been taught to drop the pedal to the metal to clear the engine and we can't help ourselves. This just makes the situation worse.

Stu Fox

[Timothy B.]

Re: 396 Cold Start.. I Need Some Advice

James,

With regard to the choke coil setting, what notch does the book say to set it? The setting for this spring is at approx 68-70* air temperature the spring should COMPLETELY close the choke blade.

If you start the engine with choke blade completely closed and the vacuum choke pull off does not open the blade 3/8" then you need to adjust the vacuum choke pull off. That procedure is in the book, you should not need to hold the blade open to clear the engine. When starting the engine just pump the accelerator pedal two times, you may be flooding the engine. These two adjustments are very important for proper operation of the choke. Keep at it until it's right and make sure there is no air leak around the circumference of the coil cover.

Set the high speed choke RPM on high step of fast idle cam only after full HOT engine run. On my 67, the fast idle is set hot at 2000 RPM and when it's cold start the high RPM is approx 1500RPM (FWIW).

[Duke W.]

Re: 396 Cold Start.. I Need Some Advice

Is the choke vacuum break system functional?

As stated the choke valve should fully close when you pump the throttle for a cold start at room temperature.

Immediately upon startup, the choke valve should pull back to the 3/8" opening.

You can test this by applying vacuum to the vacuum break diaphragm when the choke valve is closed or you can visually check for proper operation while someone else starts the engine.

Duke