Idle Fuel

[1970LS5]

I have an original 7040205 EH carb on a completely stock 454. Cliff installed front shaft bushings and repaired all of the fuel bowl plugs and I installed an HP kit during the rebuild. The engine at idle exhibits all of the symptoms of a vacuum leak but I could find no leaks. After re-reading Cliff's book and searching the forum, I concluded that modifying the idle fuel system was in my future.

Since the car is completely stock, I intend to use the specs found in Chapter 6, page 96. Are these preferable to Recipe 1 under the above conditions? Additionally, if the upper idle air bleeds are found to be .090, is there a correction which can be applied to these measurements. The information in Chapter 6 states that all that a .003 to .004 increase should be applied for high performance applications.

Thanks,
Gene

[429bbf]

gene if you read the next 20 sentences right after the .003 to.004 it says you can plug the holes in the main casting with a piece of tig rod and redrill them in the air horn from the bottom side . he also states that this is a lot easier than plugging them off in the main casting and resizing them in the main casting. id start with .050 in the air horn if you choose to go this way. hope i didnt confuse you. dean

[1970LS5]

Thanks for your reply.

I did read the next twenty sentences about resizing, but since a correction was listed, I want to see if that applies to all applications. I was trying to avoid the resizing process, if possible, in the interest of time and expense since I don't have access to a single TIG rod to fill the hole. Has anyone used JB weld or Marine Tex for this purpose?
Thanks,
Gene

[Cliff Ruggles]

Pretty common to find large upper idle airbleeds in Chevy carburetors made in the 1970's.

They also used pretty large idle tubes in many of those applications.  If your engine lacks enough fuel to the mixture screws, it will take larger idle tubes or less idle air to get it done.

I prefer to restrict those large upper idle airbleeds on those units in lieu of using huge idle tubes in them.

They easily tap for 4-40 brass set screws, which I re-size to apprx .059", then use smaller idle tubes in the casting. 

They can also be left blocked off and upper idle airbleeds drilled in the airhorn instead.  They will need to be apprx .050-.055"......Cliff

[1970LS5]

Cliff,
 The original idle tubes that you returned to me after completing the machine work were less than .040". (that is the smallest drill size I have at this time) Just so I understand,  I should leave the current idle tubes in place unmodified and reduce the size of the upper air bleed. Would this action then eliminate the need to modify the remainder of the idle system as per recipe 1 or the specs on page 96?

Thanks again,
 Gene

[Cliff Ruggles]

If you need more idle fuel to the idle system, reducing the size of the airbleeds will do that.  No way to tell if any further modifications would be needed, without testing it to find out.  If every thing is completely "stock", the stock idle system usually has enough idle fuel and doesn't need to be modified anyplace.....Cliff

[1970LS5]

Cliff,
 I finally removed the carb and measured all the relevant components. I have not yet removed the idle tubes to  measure those but they were supplied by you as part of the kit.

Idle down channel: .052
Upper idle air bleed: .070
Lower idle air bleed : .0785
Idle mixture screw holes: .070
No idle bypass air.

"If every thing is completely "stock", the stock idle system usually has enough idle fuel and doesn't need to be modified anyplace."
I agree with this completely which is why this problem is so frustrating.

Thanks again for your help,
Gene

[1970LS5]

I finally overcame my anxiety about removing the idle tubes. They measured .035". using the specs in the book as a guide. I increased, the idle tubes size .003" to 0.038". After adjusting the idle speed to factory spec, the rpm no longer increases when the air horn is partially covered. There is a noticeable unburnt hydrocarbon smell at the exhaust, but this may be normal for this year of car. There is no noticeable black smoke at idle. Now off to adjust the mixture and hopefully this project is complete.

Gene

[Cliff Ruggles]

All of the big block Chevy carburetors I've seen from that period have a LOT of idle bypass air.  I'm not sure why your throttle body wouldn't have been drilled for it?  I don't have one of those carbs in the shop right now to check it, but remember the last 1970 big block carb that I did has around .120" idle bypass air, and pretty large idle tubes in it......Cliff

[1970LS5]

Cliff,
 I looked for idle bypass air in the throttle body with the book open to that photo and it did not appear to be drilled for it. Just for my edification, is .035 a large idle tube?

Thanks,
Gene

[Cliff Ruggles]

They used idle bypass air on all the big block Chevy carbs I've seen in 1970, and later units. 

Idle tube size was determined by engine requirements, along with the size of the idle airbleeds.  Tyically they used larger idle airbleeds on those carbs, so they had large idle tubes in them from the factory....Cliff