3810 holley

[Clem Z.]

Re: 3810 holley

Clem, I haven't had the chance to block the PCV port yet but will tonight.
The engine has a steady vacuum of 18-19", could it still be getting excess air?

what changes the idle speed is the amount of air getting into the engine when you open the butterflies farther so if you have to close the butterflies to get ths idle speed down air is getting in somewhere. do you have power brakes ?

[Timothy B.]

Re: 3810 holley

Clem,

I am in agreement that the transfer slot/throttle blade position is causing a stumble. I also understand your point about excess air causing idle speed to be faster requiring a more closed throttle blade position. The PCV channel is large so restricting air at that point may help some, I will have to check that out.

What I am saying is that I don't believe all these carburetors are pulling air from other places. I believe the primary transfer slot needs to be lengthened and the small .020 bleed hole under that plugged on the primary side. I am also giving some thought to blocking the snake like passage in the base that supplies the rear curb idle ports.

[Gerard F.]

Re: 3810 holley

Tim,

You got me reading my Holley books on the transfer slot and idle feed. Couldn't find anything on that seemingly double feed with the pinhole and the curb idle.

Found a tuning tip in one of the books about drilling a hole in the throttle plates, to restore the adjustment quality of the mixture screws.

Says it leans out the idle, and allows the throttle plates to be more closed. Maybe going in the opposite direction then your off idle stumble, but maybe worth a try, rather than modifying the transfer slot on the throttle body.

Throttle plates are a lot cheaper than the throttle body that you have so much work in.

Wonder if anyone here has tried this hole in the throttle plates?

[Clem Z.]

Re: 3810 holley

Tim,

You got me reading my Holley books on the transfer slot and idle feed. Couldn't find anything on that seemingly double feed with the pinhole and the curb idle.

Found a tuning tip in one of the books about drilling a hole in the throttle plates, to restore the adjustment quality of the mixture screws.

Says it leans out the idle, and allows the throttle plates to be more closed. Maybe going in the opposite direction then your off idle stumble, but maybe worth a try, rather than modifying the transfer slot on the throttle body.

Throttle plates are a lot cheaper than the throttle body that you have so much work in.

Wonder if anyone here has tried this hole in the throttle plates?

you drill the holes in the plates when you have to expose the transfer slots too far and you are too rich at idle,not the condition we are discussing here

[Timothy B.]

Re: 3810 holley

Agree with Clem, Jerry, those throttle blade holes are opposite of the way we want to go. Purpose of that is with big camshaft and high idle where transfer slots are exposed to much. A hole in the throttle blade will allow air to enter and throttle blades closed to proper position and regain idle adjustments.

I am not sure if it's better to restrict PCV air to lower idle speed or lower the transfer slots to the bottom of closed throttle blade position. Before I do anything, I want to run the car again to establish a idle speed that's stable but I think it's about 550.

Anyone with a AFB in good condition will attest to a stable 500-550 idle. Any changes in the holley base are reversable, lowering the slot simply exposes fuel to engine vacuum sooner. I understand lowering too much can cause richer idle and mess things up which is why the small holes under the primary slots need to be blocked as well as the snake channel that feeds the rear curb idle holes.

John McRae, I just re read your post about surge at low speeds and by getting the engine to surge then installing two jet sizes larger is the way many tuners say to tune the main circuit on holley. This is the leanest most efficent setting for the carburetor. If you change anything in the idle circuit you may have to re visit this so it does not go the other way (rich).

Everyone's thoughts are appreciated..

[Joe C.]

Re: 3810 holley

Tim,

You got me reading my Holley books on the transfer slot and idle feed. Couldn't find anything on that seemingly double feed with the pinhole and the curb idle.

Found a tuning tip in one of the books about drilling a hole in the throttle plates, to restore the adjustment quality of the mixture screws.

Says it leans out the idle, and allows the throttle plates to be more closed. Maybe going in the opposite direction then your off idle stumble, but maybe worth a try, rather than modifying the transfer slot on the throttle body.

Throttle plates are a lot cheaper than the throttle body that you have so much work in.

Wonder if anyone here has tried this hole in the throttle plates?

Jerry,

The modification which you describe is a cure for the exact opposite situation Timothy is describing. It is done to carburetors, usually installed on race engines with cams having large overlap, and thus, low idle vacuum. It enables the engine to idle within a reasonable RPM range with the throttle plates in a normal position which exposes the proper amount of the idle transfer slot. Without the holes in the throttle plates, the race carburetor would otherwise have to idle with the throttle plates open far wider than they should be.

This has the same effect as a throttle air bypass, a vacuum leak, an improperly calibrated PCV valve, or the "Idle-Eze" (see Barry Grant Demon series carburetors) bleed screw which I mentioned jokingly in my post above.

The pinhole existing in some carburetors will do nothing to increase idle speed, since it is a Pitot tap for drawing air/fuel emulsion into the intake, and not a source of induction air.

Joe

[John M.]

Re: 3810 holley

I checked my car last night with PCV in the mix and then blocked. The idle speed drops 100 RPM when the PCV port on the carburetor is blocked. I also have power brakes. Blocking that on the intake had no effect although when you hit the brake pedal there is a slight effect.
In my case niether would be enough to get that slot exposed.
John McRae

[Clem Z.]

Re: 3810 holley

looks like the next question is, did these engines idle at 500/600 RPM when new? can anyone answer for sure ?? is this number something that just apears in the GM shop manuals with no proof it happened ??

[Joe C.]

Re: 3810 holley

looks like the next question is, did these engines idle at 500/600 RPM when new? can anyone answer for sure ?? is this number something that just apears in the GM shop manuals with no proof it happened ??

I asked him that question earlier, and he said that it did, but had the same stumble.

[Clem Z.]

Re: 3810 holley

I asked him that question earlier, and he said that it did, but had the same stumble.

my question is did the cars when new ever idle down that low without the stumble caused by the ITS not being exposed ? there is no way there is not going to be a stumble if the ITS are not exposed and i am sure GM would have had a TSB to fix the problem. anyone ever try 100LL or race gas in these cars to see if the gasoline is the problem ??

[Timothy B.]

Re: 3810 holley

Clem, I never tried high octane leaded gas to find out if idle changes, I was never that concerned until now. My neighbor with a 65 Impala SS 327/300 and carter afb idles butter smooth very low RPM's. The modular design of the holleys is supposed to make tuning easier but it sure seems to present lots of problems.

If you think about it the curb idle and transfer slot location on these 3810's seem perfect for a L-79 car. Maybe it's tuned more to this than the 300HP. I have a holley 4555 for a 70 LT1 and there are curb idle holes in the primary only and no pin holes under the transfer slots (primary). There are pin holes in the secondary under the slots as this is to keep fuel from getting stale. It seems that holley changed the base plate design in 1970 or so.

As I said earlier, I am going to tune it more and will post results for everyone to see. No sense calling holley, the answers are right here with all of us.

[Gerard F.]

Re: 3810 holley

Tim,

Wonder if a sluggish heat riser valve has any effect.

Notice with mine that even when it is warm, it sometimes sticks full closed at idle. After I hit it with a stick, it then opens a third like it should and then fully opens when you give it gas.

Wonder if wiring it open whould improve the idle as well as the off idle operation.

[Timothy B.]

Re: 3810 holley

Jerry,

I don't think the heat riser valve has anything to do with this particular problem. The worst case would be if it stuck either way causing poor engine performance (shut) or not allowing the intake manifold to heat properly the vaperize to fuel (open).

I will say that when the intake was off to capture that small freeze plug that fell, I removed the oil splash shield to find a large area approx. 3/8" thick of oiley crud stuck to the bottom of the manifold, it was even hard to scrape with a putty knife. After putting everything back together and running the engine, with shut down it seems the manifold cools much faster than before. I think this helps with fuel perculation so this could be a problem with some of these engines.

I also tapped the hot slot holes for plugs but decided not to install them for now, of course the proper stainless baffle and gasket is new. I just remember the car running better a few years ago when the slot was open. We definately don't want to go there with this post.

[Timothy B.]

Re: 3810 holley

I ran the car yesterday to confirm idle speed and exactly the problems I want to try and fix. The engine is very efficient at idle and will run with the throttle blades almost completely closed, the air screws opened a bit to 3/4-7/8 turn out.

The engine runs completely on the curb idle ports, I tried to unhook PCV thinking a reduction of air may allow for throttle blade position to be changed but the engine died and I soon became frustrated and decided to leave the PCV as is.

FWIW-If the engine is brought to approx 1300-1500 RPM where it's running on the transfer and curb idle circuits it's lean because additional 1/2 turn out with screws will increase RPM. I have come to realize I can't even begin to tune until I establish the correct throttle blade position at idle.

I am going to block the snake channel feeding the rear curb ports and lower the primary transfer slots. I have read to use lead shot or JB weld to block passage, where can I find lead shot?? If I decide to use JBweld will it hold up to gas??

I am starting to think this pin hole under the primary transfer slot is a attempt to cover the problem of not having the slots the correct length for a low idling engine. The throttle blade position on a 350HP car is probably perfect just the way this carburetor is made.

[Clem Z.]

Re: 3810 holley

JB weld will work in 100% gasoline but i am not sure about with 10% ethanol. did you block off the vacuum port in the carb when you removed the PCV ?