70 LT1 carb help

[Dennis D.]

Diagnosing what seems to be a percolating/heat soak issue. Want to get back to stock configuration before I start. Previous owner seemed to combat with carb shield/insulators and fuel line/ heater hose wrapping.

I'm thinking this vacuum hose routing isn't optimal for direct advance. Also, Appears to be 2 manifold vacuum sources. Haven't noticed that port below choke diaphram on 70 LT1's. Is this normal?


Here's a diagram I borrowed from an old Dr. catalog I had.

[Joe L.]

Re: 70 LT1 carb help

Diagnosing what seems to be a percolating/heat soak issue. Want to get back to stock configuration before I start. Previous owner seemed to combat with carb shield/insulators and fuel line/ heater hose wrapping.

I'm thinking this vacuum hose routing isn't optimal for direct advance. Also, Appears to be 2 manifold vacuum sources. Haven't noticed that port below choke diaphram on 70 LT1's. Is this normal?


Here's a diagram I borrowed from an old Dr. catalog I had.

Dennis-------


The vacuum to the choke unloader is manifold vacuum. The other vacuum fitting below the choke unloader is ported vacuum. Very easy to confirm this. At idle, you should feel vacuum at the choke unloader fitting. At idle you should not feel vacuum at the fitting below the unloader.

[Dennis D.]

Re: 70 LT1 carb help

Thanks Joe

Let me ask you, if the intent in the photo was to have full vacuum to the distributor, that was not the best way to achieve it, correct?


Even in the Dr diagram, the tcs eventually gets vacuum from the manifold source when energized. If I connect the hoses in the photo to the manifold vacuum, and cap off the ported, I should have full vacuum?

[Joe L.]

Re: 70 LT1 carb help

Thanks Joe

Let me ask you, if the intent in the photo was to have full vacuum to the distributor, that was not the best way to achieve it, correct?


Even in the Dr diagram, the tcs eventually gets vacuum from the manifold source when energized. If I connect the hoses in the photo to the manifold vacuum, and cap off the ported, I should have full vacuum?

Dennis------

I would say yes to both questions. However, keep in mind that with the TCS solenoid in the system, you will not have manifold vacuum to the distributor at all times.

[Mike E.]

[Chris H.]

Re: 70 LT1 carb help

Dennis, here’s a photo from the Dobbins book of a 70 LT1. It appears there is no tap for ported vacuum. The diagram appears to support that. The second photo is of a 71 that clearly shows a tap for ported vacuum.

[Dennis D.]

Re: 70 LT1 carb help

Mike. you just answered my follow up question.

I have seen the 70 version holley without this port. Not even drilled out in the base plate. Also looked at the 71 version and they all seem to have the tube in the area you mention. I assume its for CEC in 71 and if it's there on a 70 would be a California version?

If this is correct, than the carb I have isn't correct, or at the very least the base plate is wrong.
The carb number and date are correct for the car. 3972121, 4555, 054 date. block 6333 and 4519. Early July car

Is the California version the same part number? BTW is that port manifold vacuum?


70 carb





71 carb

[Dennis D.]

Re: 70 LT1 carb help

Thanks Chris. Can't seem to locate my Dobbins book. Appreciate the photos.

[Chris H.]

Re: 70 LT1 carb help

Dennis sure thing. If it were me I'd see if I could scrounge up a '70 base plate or one without that port. Then you would have manifold vacuum to your vacuum advance can (VAC).

Since your motor seems to be fighting hot drivability problems, the correct base plate may help facilitate a potential solution using the 2 Inch Rule and VAC swap out.

[Dennis D.]

Re: 70 LT1 carb help

I'm wondering if I correct the base plate, I may run the vac lines in the stock configuration. Might be a good starting point to figure out the heat issue. Actually ,blocking off that port would simulate replacing the plate. If that has good results I could change out the base plate later.

Was told there's 110 octane in the tank and I wouldn't expect to be having this problem if the setup was correct. Thank 's again Chris

[Joe L.]

Re: 70 LT1 carb help

Dennis------

The 1970 LT-1 carburetor throttle body assemblies (the actual nomenclature of what you refer to as "base plate") were different for California and non-California carbs. The 49 state carb, , Holley R-4555A, used throttle body GM #3979534. The California carb, Holley R-4489A, used throttle body GM #3979497. The 3979534 had no ported vacuum port on the left side of the carb. I do not know, for sure, if the 3979497 had such a port but I am almost certain it did.

The 1971 LT-1 carb, Holley R-4801A, used throttle body assembly GM #3979694. So, it was not the same as either of the 1970 carbs. How it differed, I know not.

As I mentioned previously, the vacuum source for the choke unloader is a full manifold vacuum source and can be tee'd into for any other functionality for which full manifold vacuum is desired (e.g. distributor vacuum control).

[Dennis D.]

Re: 70 LT1 carb help

Thanks Joe. Is there a part number on the throttle body assemblies?

[Joe L.]

Re: 70 LT1 carb help

Thanks Joe. Is there a part number on the throttle body assemblies?

Dennis-------


Unfortunately, no. At least, there is no GM part number. There MAY be a Holley number but usually there is not. If you do find one I may be able to cross-reference it to the GM number assuming it's a number for the complete assembly and not just for some component of the assembly..

[Duke W.]

Re: 70 LT1 carb help

Converting a LT-1 from ported to full time vacuum advance requires replacing the OE VAC (15" 201 15 IIRC) with a current 8" B28 (Airtex 4V1053) in order to pass the Two-Inch Rule... recommend about 900 idle speed, and it should pull about 12" manifold vacuum with near 30 deg. total idle advance.

Duke

[Dennis D.]

Re: 70 LT1 carb help

Yup. Got it Duke. Read your paper a few times.