bruizer

Hello,
I've been chasing issues on my 1970 cutlass 4spd, stock 350, HEI conversion, 2bbl Rochester 2GC, stock carb for quite a while. I've been trying to track down what's been causing off-idle hesitation, hesitation tipping in the throttle at cruise, and surging at cruise. I'm pretty certain it's not an ignition/timing problem and I've verified the accelerator pump is working properly. Plugs are reading lean, so I just rebuilt the 2GC and rejetted it increasing from 54's to 56's for the main metering jets. As I just got it running again after the rebuild (setting idle mixture w/ vacuum gauge, etc), the hesitation and surging were even more pronounced on the road test. I sprayed carb cleaner around the carb and I discovered a vacuum leak where the choke heat (stove) tubes press into the manifold tube. When removing the carb for rebuild, these came off very easily and rotated loosely in the female choke tube seat once disconnected from the carb.

So here's my question. As I understand it, these tubes are part of a "controlled" vacuum leak that draws clean air from the air horn through these tubes, down through the manifold (where the air is warmed up) and then up into the integral choke housing on the side of the carb to heat up the bimetal choke spring to pull open the choke as the vehicle warms up. Great. But, if additional air is being introduced from around these choke tube "press-in" connection points,I'm thinking this could be the source of my problems. I'd like to verify my thinking here. Should these tubes be tightly sealed together? If properly sealed, should I be able to spray carb cleaner on them and not see any change in engine rpm? Trying to understand what is "normal" here so I can solve this properly. I'm willing to purchase new choke heat tubes, but don't want to throw money at it unnecessarily.

BTW, please don't suggest an electric choke. I'd like to keep things as original as possible.
Thanks!

Oldsguy

I believe you are understanding everything correctly (at least the same way I do, so if not then I am wrong too). The solution would be a tighter seal where the tubes from the carburetor seat into the heat tube/plate on the manifold. You can buy those things still and maybe the new ones have a smaller diameter and would provide a tighter fit than the old one.

https://www.inlinetube.com/products/...BoCKbwQAvD_BwE

bruizer

Thanks for the reply, this is what I'm thinking. I was considering using a sealant of some kind that might work (i.e., Permatex copper?), but I wonder if that would effectively glue things together making it difficult to service later. I'm also wondering if I could "encapsulate" the exterior of the connection points with heavy grease to temporarily close up the leaks just to ascertain that this is, indeed, my vacuum leak causing a lean condition. If I see improved performance, then I know for certain this is my issue and I'd then go ahead an order the new choke tubes. Will post back here if I do this and what the results are.

joe_padavano

The choke tubes are not your problem. The choke housing draws air through those tubes so it is heated by the exhaust crossover. The amount of air pulled through that system is controlled by an orifice in the choke housing itself, downstream of the tubes. The air normally comes from the tube that runs to the air horn at the back of the carb, and guess what? That's an even bigger "vacuum leak" than what you are seeing. It doesn't matter where that air is coming from (other than the air from the air horn is filtered, but the amount coming through the slip joints is really negligible). Stop worrying about this and find the real problem. This isn't it.

Sugar Bear

Try driving it with the vacuum advance disconnected, if the problem disappears the wires on the distributor pole piece are breaking in which case the pole piece would need to be replaced.

​​​​​​Is full battery voltage being supplied to the HEI, i.e., it is no longer fed via the original pink resistance wire?

bruizer

Makes sense, will keep looking elsewhere.