I have a problem with my split window that seems to have stumped everyone in my circle of Corvette buddies
My car is a 300 hp/ auto. The car recently under went a complete rotisserie restoration and done to NCRS specs. The engine is original to the car as well as the power glide. The engine was rebuilt to original GM specs but the shop did install a roller cam ( mild specs).
Here is the issue, the car starts and idles fairly well, when I put the transmission into reverse or drive, it stalls . The vacuum at idle is 15 and when I put it in gear it drops to about 10 or less. Chicago Corvette rebuilt the stock Carter AFB carb. We have changed the vacuum modulator on the trans. Any ideas would be appreciated! Our consensus is that the cam could be the problem but the specs show it's an RV type which should give enough vacuum to handle the automatic transmission. Thanks, Jim

 

What is your base timing? It could be that your vacuum advance can is not correct for your engine vacuum. I would guess that when your vacuum drops to 10 you are losing all your advance that the can has added. You will probably have to change the advance to one that is all in about 12-13" of vacuum. Is there a letter and number stamped on the advance you have now?
I believe that 15" of vacuum is low for a 300Hp engine.
The 300HP 327 IMHO is one of the best running engines of the era.
Joe

 

Is it the Correct modulator?

Also, hows the 'cam' in the distributor for the points...is it worn out?

 

Chicago corvette is your problem.

 

Hi Joe, Thanks for your response. We have played with the timing and get our best idle etc. at about 12 degrees. It has a newly rebuilt distributer with a new vacuum can. Not sure what type of can they installed. It certainly looks ok. Are you saying there are different vacuum ranges on these distributor cans?

 

There are MANY..................chose wisely.

Larry

PS: You can use a Mighty-Vac with the engine running to see when the can begins to pull in and when it is "all in". Just slowly apply vacuum to the vac can hose with the MV and watch the timing mark with a light. If you tell us the markings and ID on the can, we can probably provide the design specs. But you can measure them as well.

 

Joe:

The 327/300 HP should pull 19-20 IN HG at normal idle. Even my 327/350 HP engine will pull about 17-18 IN HG at idle, but this idle is at 750-800 RPM. The 327/300 normal idle should be around 500-600 RPM.

Larry

 

they are just the middle man. They send carbs to a few people.

 

Here are some of the numbers and specs:
The number is normally on top of the arm near the chamber.
You will probably use the B-20 or equivalent.


Stamp NAPA Standard Start @ Hg All in @ Hg Total crank degrees
#
B-22 VC1802 VC171 7-9 14-16 16*

B-20 VC1765 VC181 5-7 11-13 16*

B-28 VC1810 VC177 3-5 6-8 16*

 

Thanks, Are you suggesting this is the cam from NAPA that would give me proper vacuum?

 

When the car is at idle speed how far out are the two emulsion screws.

 

1 1/2 turns. We know the carb isn't the issue. Thanks

 

I recently rebuilt my 1963 340 HP with the original rebuilt AFB and had a horrible time getting it to idle properly and setting the timing. It stalled constantly. It ended up being an incorrect PCV valve. I bought the PCV from one of the major parts houses, specified for my application, but it allowed way too much flow. I installed an NOS AC valve and it solved my problems. I was told the correct PCV should be restrictive. This was my experience, sounds similar to your issue. It was like having a huge vacuum leak.

 

What does "split window" have to do with engine idle?

 

Just as much as your question

 

Does that mean if you can blow through it easily, it's not working correctly. And are they all the same, regardless of engine size.

 

My thoughts are the torque converter slippage, it's simply not enough with this camshaft.

When you adjusted the curb idle emulsion screws did you do it in drive with a warm engine. When vacuum drops to 10" there is not enough vacuum to gas (vaporize) the mixture and the engine goes lean. Remember, it's vacuum and heat that changes liquid to vapor, liquid does not burn.

That being said, the 3460 Carter carburetor idle circuit is probably lean for this camshaft along with the above.

 

You might try using a "ported" vacuum port for your vacuum advance. If your vacuum falls off that much then you are most likely losing vacuum operated timing advance and the engine is stalling. Like already mentioned you can pop the distributor cap and observe the vacuum advance with a "Mighty Vac" hand pump connected to the vacuum canister to see where the vacuum starts to move the timing plate and also when it's full in. Now locating a ported vacuum nipple is pretty easy - just take a fine wire and push it into the carb nipple and see if it comes out above the throttle blades in the carb throat. Or use some WD40 spray to see where that comes out. Now the difficult part using a ported vacuum port is you have no vacuum advance now until the throttles start to open where as before with manifold vacuum you had advance added from the canister while the throttle blades were closed. So what I'm saying is you need to adjust the initial timing advance to try this.

Now it maybe the vacuum canister is incorrect for your application but I think the ported vacuum check is gonna be faster and cheaper to try.

Finally there maybe a better metering rod for your combination as Carter AFB's had tapered/1step, 2 step and 3 step metering rods. But I would suggest installing a air/fuel O2 monitor to test with this.

Hope this helps ya.

 

You say it a Automatic 2 speed Glide did you change the torque converter stall , get rid of the roller and put back the stock cam that come with the car ,

 

A few items IMHO............

Even if you changed the vacuum to intake vacuum as opposed to ported, that shouldn't change a thing if the OP has base set to 12 degrees advanced (assuming the balancer has not slipped or the timing cover has been changed and does not correctly reflect timing).

As mentioned before, that engine should produce in the neighborhood of 18-20" of vacuum........there is a sign of a problem. I don't think that the idle circuit in that carb would support that kind of vacuum level. By the characteristics that are described, it sounds like a dead lean fuel mixture at idle. If it's that lean, it won't take and sort of load (placing the car in gear for instance).

What is the voltage going to the pos. side of the coil when running? I believe it should be around 8 volts after the ballast resistor. Make sure you are not dealing with a voltage drop.

Now having mentioned what I did, there are so many variables that can affect the above. Is the cam installed where is should be? Was something done incorrectly in the carb rebuild? The PCV comment is spot on.........I got burned by that some time ago.

I honestly do not know why so many engine builders try to use a roller cam these days. I understand it in an OE and performance application, but not for a restorations. We have the specialized oils at our disposal.

On a different note......if you were going to change the cam, maybe Duke Williams will chime in. He came up with a neat little cam for the 300hp engines. Very stock with it's manners and torque production, but makes some good overall power.

Hope this helps,
Tim