Engine idles better with vacuum leak?

Allison Winn

Veteran Member
Jan 10, 2021
401
Doesn't make sense to me - when I went to check the timing on 70 LT1 engine, as I disconnected the vac hose from the distributor (and before I plugged it) the idle runs up maybe 500 rpm. The engine seems to really like the huge vacuum leak. Shouldn't the idle actually suffer because of the lean condition I created? I've got the idle mix screws set perfectly
 

Lowend

Administrator. .a car, a man, a maraca.
Staff member
Lifetime Gold Member
Mar 25, 1999
16,775
San Jose, CA, USA
It's not unusual. The engine is drawing in more air so increasing the idle speed.
Once you fix the leak, you'll probably need to lean out the mixture screws
 

1980RS

Veteran Member
Jun 17, 2006
6,257
MN
All that means is you have the throttle blades open to far exposing the transfer slots too much. Drill some holes in the blades and it will idle better for sure.
 

G72Zed

Veteran Member
Sep 8, 2015
3,917
Canada
Doesn't make sense to me - when I went to check the timing on 70 LT1 engine, as I disconnected the vac hose from the distributor (and before I plugged it) the idle runs up maybe 500 rpm. The engine seems to really like the huge vacuum leak. Shouldn't the idle actually suffer because of the lean condition I created? I've got the idle mix screws set perfectly

If the vacuum hose was plugged into the side of the metering block, as in a ported source, I'm with 1980RS, your primary throttle is into the port just above the throttle plate.

If you are on manifold vacuum, and not over activating the primary trans slots, chances are you may in fact be a little on the rich side, and the extra air leans out and brings the idle afr's in check, so, the engine is not labouring as much and picks up rpm naturally, and that in itself picks up some timing centrifugally, and that pick's up more RPM.

In my opinion, get the primary and secondaries adjusted before you go drill some holes in those plates, that's the last thing to do, and more often than not it points to other underlying issues. I get some of these carbs in with holes in the butterfly's, they do not need them 90% of the time, heck, I take out the original ones with holes and put solids in.....and wow, things get better LOL.

To be really straight up forward, getting the idle mix screws "perfect" is not a walk in the park sometimes, 2 corner or 4 corner, and I'm on the pump every weekend, and soon to be at the track. Every one of these carbs I built are different going out the door, if you have a "tuners" secret, I'm all ears !!!! heck I'll pay for it LOL.

This is my "completed" carb waiting shelf.....always looking for tips.

Carb week.jpg
 

2ndGenCrazy

Veteran Member
Aug 18, 2011
888
Upstate New York
If the vacuum hose was plugged into the side of the metering block, as in a ported source, I'm with 1980RS, your primary throttle is into the port just above the throttle plate.

If you are on manifold vacuum, and not over activating the primary trans slots, chances are you may in fact be a little on the rich side, and the extra air leans out and brings the idle afr's in check, so, the engine is not labouring as much and picks up rpm naturally, and that in itself picks up some timing centrifugally, and that pick's up more RPM.

In my opinion, get the primary and secondaries adjusted before you go drill some holes in those plates, that's the last thing to do, and more often than not it points to other underlying issues. I get some of these carbs in with holes in the butterfly's, they do not need them 90% of the time, heck, I take out the original ones with holes and put solids in.....and wow, things get better LOL.

To be really straight up forward, getting the idle mix screws "perfect" is not a walk in the park sometimes, 2 corner or 4 corner, and I'm on the pump every weekend, and soon to be at the track. Every one of these carbs I built are different going out the door, if you have a "tuners" secret, I'm all ears !!!! heck I'll pay for it LOL.

This is my "completed" carb waiting shelf.....always looking for tips.

View attachment 138843
As soon as you move the carb to another engine, you start over with the tuning. The holes will be required for tuning some engines, and not required on others. It should be the final step and only used when required.
 

G72Zed

Veteran Member
Sep 8, 2015
3,917
Canada
As soon as you move the carb to another engine, you start over with the tuning. The holes will be required for tuning some engines, and not required on others. It should be the final step and only used when required.

Yes, I agree with you on that 2ndGenCrazy, every engine is different, even the ones that look the same on paper. I have even disabled the "idle EEZ" on a 1/2 dozen carbs as well, and removed the holes.

I actually build custom carbs "on site" at the dyno for "unknown" combo's, or if the owner does not want EFI, and does not know what carb to buy, and want's one custom built.

I actually have 1-11/16 and 1-3/4 SS butterflies with holes predrilled in various incremental sizes just for that purpose, most times, my predictions are correct if they are needed, does not take long to figure it out, love the dyno for that.
 

G72Zed

Veteran Member
Sep 8, 2015
3,917
Canada
Yep, I can't argue that point ChevyReb LOL, having a hard time keeping up with demand of my modified "EFI delete kits" these days.

Going on a dozen of the new HP dichromate aluminums alone as of late, on the dyno yesterday, and got 2 more out the door, cleaning the warehouses as quick as they come in. Holley has stepped up production so they are in stock now...... The last 3 I had to get them in from Lincoln Nebraska because of "NO Stock"

This Covid has really buggered up production schedules.
 

Allison Winn

Veteran Member
Jan 10, 2021
401
Maybe this is the problem? I had slapped a regular thin gray carb base gasket on it. Looks like the accel pump parts are very close to the intake casting, they hang below the plane of the baseplate. I can't slide a feeler gauge in between the raised firing order numerals and the accel pump screw heads. I have this thicker base gasket I could try, to see if the interference had the front of the carb slightly off the gasket.
 

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