3810 holley

[Timothy B.]

I ran my 67 yesterday for the first time since pulling the intake, carburetor throttle shaft bushings, breakerless ignition etc..

Car runs great but I have a carburetor question again. At idle 500/550 RPM, emulsion screws approx 5/8-3/4 turns out on each side the throttle blades are not exposing any of the transfer slot. Turning the idle screw to increase idle I encounter a lean miss with transition from curb idle to transfer circuit. If I open emulsion screws to 1 1/4 turn this gets covered up but idle is dirty.

I can't set blades to expose transfer slot because idle is to fast. I noticed a small pin hole below the transfer slots on primary side that's below the throttle blades I guess to keep prime, (like the secondary pin holes so the gas never gets stale). The secondary blades are closed, no gas dripping perculation etc that I can see at this time. The car ran for approx two hours yesterday this is the only issue.

I can get the car to idle at 500/550 RPM with 19" vacuum, this is not a accellerator pump problem. Any tuning help is appreciated, could I have to much idle timing?? Idle timing is 26*- (10* initial, vacuum advance 16*)

[John H.]

Re: 3810 holley

I can't set blades to expose transfer slot because idle is to fast. I noticed a small pin hole below the transfer slots on primary side that's below the throttle blades I guess to keep prime, (like the secondary pin holes so the gas never gets stale). The secondary blades are closed, no gas dripping perculation etc that I can see at this time.

Tim -

The hole below the throttle blade isn't a "prime" hole - it's the idle mixture discharge hole - that's where the idle fuel/air emulsion enters the intake. The throttle plate should be set so only about .020" of the transfer slot is visible below the plate - it should appear as a square when the plate is set properly. Photo below shows the various holes with the throttle blade slightly open - the transfer slot, the idle mixture discharge hole, and the manifold vacuum pickup.

Using a vacuum gauge, adjust the idle mixture screws to achieve highest steady vacuum, and the secondary throttle plate stop screw should be set 1/2-turn in from plate-closed position.

Your timing is on the money.

[John D.]

Re: 3810 holley

Tim -

The hole below the throttle blade isn't a "prime" hole - it's the idle mixture discharge hole - that's where the idle fuel/air emulsion enters the intake. The throttle plate should be set so only about .020" of the transfer slot is visible below the plate - it should appear as a square when the plate is set properly. Photo below shows the various holes with the throttle blade slightly open - the transfer slot, the idle mixture discharge hole, and the manifold vacuum pickup.

Using a vacuum gauge, adjust the idle mixture screws to achieve highest steady vacuum, and the secondary throttle plate stop screw should be set 1/2-turn in from plate-closed position.

Your timing is on the money.

I had the same problem you describe with my 3810 and it was the secondary throttle plate stops open too far. After I set them as John describes the problem went away. I also had an off idle flat spot when the throttle pedal was pressed though....

[Timothy B.]

Re: 3810 holley

Thanks for the reply John and John,

I am aware of the curb idle port under the throttle blade. The air/fuel screws control this discharge and there is a channel in the throttle base plate to distribute fuel to secondary curb idle holes supplied from the primary,(poor mans four corner idle, shown in pic).

Just under the primary transfer slots there are small pin holes about the same level as the curb idle holes. These holes are under the throttle blade and if you stick a small wire in them they connect to the drilling for the transfer slots. I don't believe they have been added, I guess they act as primer for the transfer slots. Has anyone ever seen these on there base??

The secondary blades are closed and sealed, I can't seem to open primary throttle blades to expose even a small transfer slot because the engine will idle higher. I should have taken a pic of the transfer slot pin holes, they are like the small holes in the secondary idle circuit that are there to keep fuel from getting stale in the secondary bowls. This base is stamped 3810.

[Clem Z.]

Re: 3810 holley

if the idle transfer slots are not exposed at idle you will have a hesitation because they cover the time it takes for the fuel to get down from the pump shooter to the base of the carb.

[Timothy B.]

Re: 3810 holley

What I am trying to accomplish is run the car at 500 RPM. At this setting the throttle plates are almost completely closed so the problem is that the transfer slots are not exposed hence going lean when turing the idle screw even to 6-7-800 RPM (sputter).

With the old leaded gas could the throttle position have been open more? The car runs well at 500-550, good vacuum etc. It was suggested on another forum that the transfer slots need to be lengthened so the correct position is achieved.

All thoughts and comments are appreciated, maybe I should just increase idle to 600+- and let it be. I would like to know from others if they have ever seen small pin holes under the primary transfer slots (which would be under the throttle blades) drilled into the transfer slot well in the base. These holes are small just like the secondary idle bleed and located the same way and I am wondering if someone could have done this.

Car runs great, throttle shaft bushing job turned out very nice, I think with 600 RPM the car will not go lean.

[Gerard F.]

Re: 3810 holley

Tim,

I have an original 3810 base plate in front of me and I see the little pinhole below the transfer slot on both the primary and secondary sides. It doesn't appear in John's photo.

It is extremely small, and with my eyes, I can't tell if it goes through to the transfer slot hole. But it is located at the base of the transfer slot drilling.

If it is an actual hole it might be the answer for a smoother lower RPM idle on a 67. However, it is so small that I would think it would be subject to blockage over time.

I gave up on trying to get 500 rpm on mine but had it running pretty good a 550 to 600 just after the fast idle kicks off. After a while, you will then see it varying 500 to 600 before it dies. When hot you will never get it down to run smoothly below 600.

For the summer, I'm running 600 at idle, but with the Vintage Air system on. With it off, it kicks up to 650-700 at idle.

[Timothy B.]

Re: 3810 holley

Thanks Jerry,

At least I know the little holes are supposed to be there below the transfer slot. These holes are connected to the transfer slot well in the base so this is fuel feed at idle because the holes are below the blades.

Do you recall the transfer slots exposed when idle is around 5-600 RPM. I am thinking carburetor is fine but it's not going to idle the car at 500 like I wanted.

[Gerard F.]

Re: 3810 holley

Tim,

I have the original 3810 base plate off the carb with a replacement on the original carb. I'll check and get some pictures this weekend for the hole and transfer slot on the car.

I'm still not sure it is hole blow the transfer slot on the original, as it is extremely small. Will check with a mini drill bit set tonight when I get home. If it is a hole any little spec of stuff will block it off.

I have my original (refurbed) distributor and vacuum can, and original cam on my 67 327/300 base engine. The book says 6d initial, and I keep it at 8d initial. Maybe this has something to do with a smooth low idle.

However with the 91 gas I get here in CA, I don't think I'll ever get it to idle smoothly down to 500. That would be only with a semi warm engine just after it gets off the fast idle cam. When the engine gets really hot, the idle seems to go up to 700-800.

[Clem Z.]

Re: 3810 holley

remember all this talk about cam lobe wear and the cam is lubed by spash off from the rotating assy so i would think you would be better off with a higher RPM idle. that is the reason you use 2,000-3,000 RPMs for camshaft break in to keep a lot of oil flying off of the rotating assy onto the camshaft. JMHO

[Brian M.]

Re: 3810 holley

I keep my 327/300 4spd 3810 at 750 RPM's.

[Joe R.]

Re: 3810 holley

Hi Tim:

You mention the possibility that modern gas may be a factor here. A couple years ago I read an interesting article where the author claimed that today's gas is sufficiently different from 1967 gas that some recalibration of the original settings is appropriate.

Starting with a stock 1967 327/300, the author used a wideband oxygen sensor to monitor the fuel/air ratio under different operating conditions. I recall that one of the observed problems was an off-idle lean condition. In the end, I think that changes were made to the jetting, the accelerator pump, and the ignition timing curve. I do not recall any modifications to the transfer slots.

I can try to find the article if you are interested. It was the first time I had read about the benefits of a wide band oxygen sensor for engine tuning. I'm planning to install a bung in my exhaust system so that I can use a wide band oxygen sensor to tune my own car.

[Timothy B.]

Re: 3810 holley

Thanks everyone for the suggestions. If I idle the car at 600-650 I don't think there is even a problem. The engine runs very well with the new advance curve and breakerless ignition along with the other changes I made.

I am curious how the factory spec calls 500RPM but everyone has such a difficult time with this. Like I stated in a earlier post, my car will idle there but the throttle position is not correct hence the stumble during tip in. I should be able to increase the idle speed screw and the motor should progress from one circuit to the next without any lean stumble.

Joe, I even left out the allen plugs you gave me to keep exhaust heat on the carburetor as to aid gasoline vaporazation. The car ran for two hours, not a drop of fuel leak (perculation). Thanks again for your help.

[Clem Z.]

Re: 3810 holley

Thanks everyone for the suggestions. If I idle the car at 600-650 I don't think there is even a problem. The engine runs very well with the new advance curve and breakerless ignition along with the other changes I made.

I am curious how the factory spec calls 500RPM but everyone has such a difficult time with this. Like I stated in a earlier post, my car will idle there but the throttle position is not correct hence the stumble during tip in. I should be able to increase the idle speed screw and the motor should progress from one circuit to the next without any lean stumble.

Joe, I even left out the allen plugs you gave me to keep exhaust heat on the carburetor as to aid gasoline vaporazation. The car ran for two hours, not a drop of fuel leak (perculation). Thanks again for your help.

those number were probably set in a lab under controlled condition. yes the new gasoline does not have the same specific gravity of the old gasoline. if your corvette uses the stainless steel plate under the carb without a gasket i would install a carb to intake gasket there and see what happens.

[Gerard F.]

Re: 3810 holley-Here's the mystery hole

Tim,

Here's a pic of my 3810 base plate showing the mystery hole below the transfer slot on the primary side:



Here's with a #80 (.0135") drill bit stuck in it to the transfer slot passage:



Tried a #75 (.021") bit and it won't fit, so the diameter is somewhere in between. Boy that is a small hole.

Same thing on the secondary side, except the hole on one side was plugged tight with stuff, or never drilled all the way through.

Checked a replacement 3810 sitting on the shelf, same holes in all 4 corners.

Checked a 1966 3139-1 also sitting on the shelf, it had the holes in the primary but none in the secondary.

I wonder what these mystery holes are supposed to do?