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340 Holley 6 Pack Info needed #2650639
04/30/19 07:38 PM
04/30/19 07:38 PM
Joined: Jul 2007
Posts: 291
St. Louis
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Imrare Offline OP
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Imrare  Offline OP
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St. Louis
I am needing information on the outboard carburetors on my Holley 340 6 bbl set up, list numbers 4789 & 4790. I have been told that the metering plates between the front and rear carburetors are different. However, looking at the 1970 parts book, each metering plate has the same part number listing which to me would indicate they are the same.

Does anyone have the spec information on the front & rear metering plate that will provide the metering jet sizes (PVCR) and the number that is steel stamped into each of the metering plates?

Also, the parts book lists a different part number for the base plate and the diaphragm but they both look identical to me. Does anyone know the difference between the two base plates and diaphragms?

Holley originally delivered the end carburetors with plugs over the idle mixture screws so no one would be tempted to adjust them. Does anyone know how many turns from fully closed those screws were set at from the factory?

Thanks.

Re: 340 Holley 6 Pack Info needed [Re: Imrare] #2650641
04/30/19 07:43 PM
04/30/19 07:43 PM
Joined: Jan 2003
Posts: 9,436
Blair County,PA
62maxwgn Offline
master
62maxwgn  Offline
master

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Posts: 9,436
Blair County,PA
Originally Posted by Imrare
I am needing information on the outboard carburetors on my Holley 340 6 bbl set up, list numbers 4789 & 4790. I have been told that the metering plates between the front and rear carburetors are different. However, looking at the 1970 parts book, each metering plate has the same part number listing which to me would indicate they are the same.

Does anyone have the spec information on the front & rear metering plate that will provide the metering jet sizes (PVCR) and the number that is steel stamped into each of the metering plates?

Also, the parts book lists a different part number for the base plate and the diaphragm but they both look identical to me. Does anyone know the difference between the two base plates and diaphragms?

Holley originally delivered the end carburetors with plugs over the idle mixture screws so no one would be tempted to adjust them. Does anyone know how many turns from fully closed those screws were set at from the factory?

Thanks.



Both use a # 41 metering plate,bases and diaphragms are the same,have the specs but can't do right now.

Re: 340 Holley 6 Pack Info needed [Re: Imrare] #2650664
04/30/19 08:31 PM
04/30/19 08:31 PM
Joined: May 2012
Posts: 1,812
South Bend
John Brown Offline
top fuel
John Brown  Offline
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South Bend


July 19th should be "Drive Like Rockford Day". R.I.P. Jimmie.
Re: 340 Holley 6 Pack Info needed [Re: John Brown] #2650780
05/01/19 09:01 AM
05/01/19 09:01 AM
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Posts: 291
St. Louis
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Imrare Offline OP
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Imrare  Offline OP
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St. Louis
Thanks for the information. The two metering plates in my end carburetors had a stamping of #41 with a casting #34R-6153B. The PVCR (Power Valve Channel Restriction) or jetting (fig. A above) is staggered at .086 (driver side) and .073 (pass side) with the Secondary Idle Speed Restriction being .036 on both plates (front & rear carb), driver and pass side. Based on the parts book and what I am understanding then, the metering plates were the same for both front and rear carbs.

Is anyone familiar with how many turns out the air mixture screws were set from the factory on the end carbs? I know this must have been a very precise setting because all end carbs came with "plugs" over the adjusting needles so they couldn't be messed with.

Thanks

Re: 340 Holley 6 Pack Info needed [Re: Imrare] #2650792
05/01/19 09:35 AM
05/01/19 09:35 AM
Joined: Jan 2003
Posts: 9,436
Blair County,PA
62maxwgn Offline
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Blair County,PA
This may give you some idea .I've done "lot" of sixpack carb restoration,almost all big block that still had plugs were 2 turns or "very" close.I don't think I ever had a small block set that still had the plugs that hadn't been messed with so I really can't answer your question,Scott Smith possibly could. I will tell you if you remove the plugs to adjust,be very careful,those screws are brass,been there almost 50 yrs,the cork seal gets like concrete and you can take half the head off quicker than you can blink.If you can't get the broken screw out,this is the only alternative.

Picture 194.jpgPA310551.jpg
Re: 340 Holley 6 Pack Info needed [Re: 62maxwgn] #2650797
05/01/19 10:03 AM
05/01/19 10:03 AM
Joined: Jul 2007
Posts: 291
St. Louis
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Imrare Offline OP
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Imrare  Offline OP
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St. Louis
Thanks for the info. I am replacing all four idle mixture screws with new screws from Holley. I don't have a starting point for the old screws because they were not adjusted properly to begin with. I don't know if the small block carbs would be that much different than your big block carbs but previously I've been told several different settings for the mixture screws on the end carburetors ( anywhere from 1/8 to 1 1/2 full turns open). I know this setting is very precise and an 1/8 or 1/4 turn could make all the difference between a smooth idle and a terrible one. That is why I was asking for information on an untouched, original carburetor, hopefully to unlock this mystery. The problem is, if I get the setting wrong and the engine doesn't idle properly, I still have access to the front carburetor idle mixture screws and they can be adjusted. However, the only way to adjust the rear carb is to take it off and on, adjusting the needle setting each time until it is determined to be right. It is virtually impossible to adjust the rear carburetor with it installed. Does anyone know if there is a "spec" setting on the idle mixture screws for the 4789 and 4790 carbs?

Thanks

Re: 340 Holley 6 Pack Info needed [Re: Imrare] #2650805
05/01/19 10:42 AM
05/01/19 10:42 AM
Joined: Jan 2003
Posts: 9,436
Blair County,PA
62maxwgn Offline
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62maxwgn  Offline
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Posts: 9,436
Blair County,PA
Originally Posted by Imrare
Thanks for the info. I am replacing all four idle mixture screws with new screws from Holley. I don't have a starting point for the old screws because they were not adjusted properly to begin with. I don't know if the small block carbs would be that much different than your big block carbs but previously I've been told several different settings for the mixture screws on the end carburetors ( anywhere from 1/8 to 1 1/2 full turns open). I know this setting is very precise and an 1/8 or 1/4 turn could make all the difference between a smooth idle and a terrible one. That is why I was asking for information on an untouched, original carburetor, hopefully to unlock this mystery. The problem is, if I get the setting wrong and the engine doesn't idle properly, I still have access to the front carburetor idle mixture screws and they can be adjusted. However, the only way to adjust the rear carb is to take it off and on, adjusting the needle setting each time until it is determined to be right. It is virtually impossible to adjust the rear carburetor with it installed. Does anyone know if there is a "spec" setting on the idle mixture screws for the 4789 and 4790 carbs?

Thanks


Switch the rear carbs back to front,adjust the front then put it back,it will get you real close.There are a few alternatives,someone on here some time ago had mixture screws and a tool to adjust without removing,someone should remember who.The other is replace the rear base with a Promax base with angled screws,a little pricey but it works.

Just got two sets of small block carbs about 20 min. ago,haven't opened yet,I'll check later today and see if they still have plugs,if so I'll find out what you need to know.

Re: 340 Holley 6 Pack Info needed [Re: Imrare] #2651308
05/02/19 06:07 PM
05/02/19 06:07 PM
Joined: Jan 2003
Posts: 6,840
The Swamp
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Sixpak Offline
master
Sixpak  Offline
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Posts: 6,840
The Swamp
Originally Posted by Imrare
Thanks for the info. I am replacing all four idle mixture screws with new screws from Holley. I don't have a starting point for the old screws because they were not adjusted properly to begin with. I don't know if the small block carbs would be that much different than your big block carbs but previously I've been told several different settings for the mixture screws on the end carburetors ( anywhere from 1/8 to 1 1/2 full turns open). I know this setting is very precise and an 1/8 or 1/4 turn could make all the difference between a smooth idle and a terrible one. That is why I was asking for information on an untouched, original carburetor, hopefully to unlock this mystery. The problem is, if I get the setting wrong and the engine doesn't idle properly, I still have access to the front carburetor idle mixture screws and they can be adjusted. However, the only way to adjust the rear carb is to take it off and on, adjusting the needle setting each time until it is determined to be right. It is virtually impossible to adjust the rear carburetor with it installed. Does anyone know if there is a "spec" setting on the idle mixture screws for the 4789 and 4790 carbs?

Thanks


All 3 carbs - start at 1 5/8 turns out, stone stock jetting. This is what I ended up at on a T/A with a Comp 268 XE Hydraulic and stock exhaust. Get the best idle from the center, then the front, using the 'finger over the bleed hole' trick to tell if that bore is lean or rich. Get the front adjusted, then as suggested, swap front to back and tune the new front. Make sure everything is right first - linkage adjusted right, fast idle cam bend tab not hanging up and causing the motor to rev too high (DAMHIK..lol), vacuum lines correctly plumbed between the center and outboard carbs YMMV...don't worry about what the settings are out of the box (most likely 1/2 to 3/4 out on the end carbs) because it won't be close enough. It has to idle on all 3 carbs, not just the center one. I imstakenly thought for a long time I could get it to run well at idle just on the center carb and that was a mistake. Could never get it fat enough on just the center carb. Once I opened up the end carb idle screws it became much nicer and happier at idle, and cleaner too.

Re: 340 Holley 6 Pack Info needed [Re: Sixpak] #2651385
05/02/19 09:54 PM
05/02/19 09:54 PM
Joined: Jan 2003
Posts: 42,714
Spokane Washington
ScottSmith_Harms Offline
Mr Wizzard
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Joined: Jan 2003
Posts: 42,714
Spokane Washington
So far good info that I agree with. Like Bill said, about 2 turns out is where most I've seen that were untouched (including 340 versions). Also, keep in mind that the cork O- rings and lead plugs are what held the screws in adjustment, if you remove or replace the screws you may find that they do not stay in place and can vibrate loose. Replacement cork rings from Holley can be used, or some other method to stake them in place, even a tiny daub of clear silicone will work. Also, a little on the rich side is better than lean, if you go too lean you can hurt the engine.

Last edited by ScottSmith_Harms; 05/02/19 09:55 PM.






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