3810 holley

[Gerard F.]

Re: 3810 holley-Think I got it figured out

Tim,

Spent a little time with a replacement 3810 which I have the fuel bowls, metering block/plates off. One of those projects I was going to complete three years ago, but some other project took priority.

But I spent some time with compressed air and fluids trying to figure out all the little channels between the fuel bowls and the barrels.

It looks to me, that on a 3810 there are two idle circuits to each barrel.

One is through the mixture screws on the primary block and the snakelike track in the top of the baseplate which is adjustable though the mixture screws, and goes to the larger hole below the throttle plate in the photo in the last post.

The other is through the small pin hole below the transfer slot which is also shown in the photo. This feed is not adjustable except on the primary side. When you adjust the idle screw, it rotates the primary side throttle plate and exposes more of the transfer slot, therefore enriching things on that side.

The secondary feed is adjustable only through the mixture screws, the pinhole feed is fixed as the throttle plates at idle do not expose the transfer slot. The pinhole feed is from the rear bowl, whereas the feed through the mixture screws, on both the front and rear barrels is from the primary bowl.

So to test out my theory, I screwed both mixture screws all the way in, and the idle screw all the way out, when it was just slighlty warmed up. The 67 still idled although a little rough.

Then I adjusted up the mixture and idle, tried to get it down to 500. Best I could do was 550 with an annoying variation from 550 to 600.

Then I took a hot run to test out the new fan clutch. When it was hot, I really couldn't adjust the idle and mixture below 700 rpm.

So, I think it is the nature of the beast, and the gas.

[Timothy B.]

Re: 3810 holley

Thanks for the reply's Jerry and Clem,

The carburetor has the stainless shield and gasket like it's designed. The curd idle holes (larger holes) under the throttle plates on all barrells are metered by the primary air/fuel screws and designed to provide equal wetting of the manifold under each barrell, (poor mans four corner idle).

The small SECONDARY hole under the transfer slot is there to prevent gas from getting stale in the secondary bowl. Some grocery getter four barrell carbs could be used in such a manner, hence the secondary idle circuit.

I don't understand the purpose of this small hole in the primary under the transfer slot. I personally think the transfer slot should be longer extending down closer to the closed position of the throttle plate in a closed position. It was suggested on another forum to extend it slightly and this will prevent my stumble with low (500) idle.

This 3810 is also used with a 350HP engine so it's calibration may be richer for the L79 camshaft.

[John S.]

Re: 3810 holley

My 2 cents here. I tuned my car for the trip to Tahoe and found that the best idle was at 650-700. Seems to be a theme here! Did not try 500. My idle quality even at the 650 is not as smooth as I would like. I am sure the car either would not run or protest badly at 500. I am going to go out now and check my secondary stop as suggested.

John Seeley
67 black/ teal coupe

[Dale S.]

Re: 3810 holley

I am going to the Hot rod reunion next week. I am going to get some fresh C-12 VP gas(fresh drum). When I get home I am going to check the idle on my Corvette. Then I am going to change from some fresh Phillips 91 octane to the C-12. I will measure the reading with a dwell -tach at idle. Then I will have a better idea of what is going on. Anytone have a suggestion on how to do this in a better way? Dale.

[John H.]

Re: 3810 holley

I wouldn't dream of trying to achieve a stable 500 rpm idle on my '67 327/300; maybe Chevy could do it in a lab setting, but it's improbable in the real world. I set mine (with an electronic tach, not by the one in the car) at 650-700, and it's happy there.

[Timothy B.]

Re: 3810 holley

When the car is checked by NCRS don't they check idle speed? If I set the idle at 600-650 there are no problems with tip in. Is the 500-550 idle speed not sufficent for camshaft lubrication, the car still has 20-30 lbs oil pressure.

Thanks everyone for the good help, I still don't understand why those small holes are below the transfer slot on the primary side of the carburetor, I understand they bleed fuel into the manifold.

[Jack H.]

Re: 3810 holley

Various RPM's are 'loosely' checked during the Operations check in Flight Judging. They're more closely monitored during Performance Verification tests...

[John M.]

Re: 3810 holley

Tim and John
I have the same engine with the same problem. I set the idle screw with the carb off the engine so that about .020" of the slot was visible below the plate. This resulted in an idle speed of over 1000RPM. In John's post he says " 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." How do we set the plate properly? Is there enough play in the attaching screw holes to do this?
Thanks for any help, great thread.
John McRae 30025

[Stuart F.]

Re: 3810 holley

I believe John means that when your idle stop screw is set properly, then the plates relationship to the transfer slots will be about .020". Do NOT loosen the plate screws to attempt any adjustment in this regard, or at least you should not have to if they have not been messed with before or diassembled. A quick check would be to close them all the way seated and hold the base up to a strong light to see if the plates seat well in the throttle bores.

I look for a similar relationship of the transfer slots in my Carter AFB's as well, but I don't concern myself with a specific dimension or square appearance. About the only time that comes into play is on engines with extreme racing cams that require a greater than normal throttle adjustment to get them to idle properly - beyond the normal design limits of the carb.

Stu Fox

[John M.]

Re: 3810 holley

Thanks Stuart,
The plates close nice and tight in the bores but there's no way the slot will be exposed in this carburetor at 550 RPM and there is a slight stumble below 1000RPM; after that it is fine. It's the original carb. Just wondering how this condition can exist as compared to what John showed.
John

[Timothy B.]

Re: 3810 holley

Stu and John, If you close the idle speed screw on our AFB's, the transfer slots are exposed approx .010-.020+- but the 3810 holley is different. I bet an AFB will idle a 300HP car to 500 with no problem.

I think the calibration of this 3810 carburetor is set for the 350HP car meaning if the idle is set at 750-800, no problem with transfer slot exposure and that's a good idle with L-79.

If the throttle shaft is not wore and surfaces flat etc, 500 RPM idle is possible. My car will do this with 19" vacuum and be steady but it's to lean on tip in because no transfer slot exposure. The transfer slots need to be made slightly longer to be just at the bottom of the throttle blade at the idle position. Also, the small .013 hole just underneith should be closed because idle will become rich.

I always thought a NCRS PV test would include idle speed because this is a indication of carburetor/engine problems.

Please don't get me wrong, I am not rushing out to modify my carburetor but it's nice to understand certain things after all this is a technical discussion board...

[Clem Z.]

Re: 3810 holley

Thanks Stuart,
The plates close nice and tight in the bores but there's no way the slot will be exposed in this carburetor at 550 RPM and there is a slight stumble below 1000RPM; after that it is fine. It's the original carb. Just wondering how this condition can exist as compared to what John showed.
John

make sure the secondary butterflies are completely closed and you may even have to center them in the bores by loosening the screws. as long as the butterflies do not stick in the bores you can close them down to try and get the carb to idle off of the front barrels so the idle transfers are exposed and not have too fast of a idle speed.

[John M.]

Re: 3810 holley

Tim, Clem,
The secondaries are nearly completely shut and do not stick and the idle can be set at 500 like Tims. It's that off idle stumble that's annoying. I tried a lot of other things only to learn that it's those slots. I din't think I can modify the slots; has anybody tried?
John

[Clem Z.]

Re: 3810 holley

Tim, Clem,
The secondaries are nearly completely shut and do not stick and the idle can be set at 500 like Tims. It's that off idle stumble that's annoying. I tried a lot of other things only to learn that it's those slots. I din't think I can modify the slots; has anybody tried?
John

i wonder if a carb that has all it surfaces remachined flat and true has this same problem because it sounds like air is getting past the carb into the engine but not thru the primary butterflies causing them to have to be shut too far and closing off the idle transfer slots.

[Timothy B.]

Re: 3810 holley

Clem,

I had my 3810 put on a mill at a local machine shop and main body (bottom and sides) trued. The base I did myself on a flat table with some 220 wet paper, I know it's flat.

Also installed throttle shaft bushings using valve guide inserts into the complete length of the bore in primary and secondary and installed a new secondary shaft with new teflon bearings, it's NICE but smooth.

This carburetor gets idle fuel from curb idle ports under all four throttle blades plus secondary idle circuit (bleed hole) located under secondary transfer slot plus the same hole in the primary under the transfer slots.

The only thing I can think of to try outside of lowering the primary transfer slot is to block the secondary curb idle holes and that may let the primary throttle blade be opened more to expose the slot. I am not sure how not wetting under all four barrells would effect idle but the AFB's are like that.

I like this post as it seems others have had the same questions so it's good to learn. Like I said before, AFB's will idle a 300HP car to 500 no problem.