Holley question

[sportsroof]

I have a Holley 4 barrel which has the reverse idle system. I want to use a different metering block for some testing but this other block is 'normal' idle system. Will this work as intended if I fit it to the same carb the reverse idle block came from?
Cheers, Martin

[pamotorman]

the reverse idle system controls only the air to the idle system not the fuel mixture like in the standard idle system.

[Tuner]

You can use the standard block with .063" orifice (instead of the reverse idle .106" orifice) at the tip of the mixture screw on the "reverse idle" carb body, but the idle adjustment will be relatively ineffective. Probably won't matter as those carbs are famous for ineffective idle adjustment in the first place.

If it is actually a "reverse idle" carb, it does not have the passage from the mixture screw through the body and base for the curb idle discharge. The reverse idle mixture screw bleeds air from a port in the bore wall above the throttle to the transition slot passage. Idle fuel is entirely from the T-slot and there is no separate curb idle discharge port.

However .... there are some mixed up carbs that by a screw up on Holley's part (click, drag and paste "engineering"??) have the "reverse idle" metering block on an otherwise standard curb idle configured body and base. The problem with this is the orifice in the "reverse idle" metering block for the tip of the mixture screw is .106" instead of the usual .0625" (approx) in the standard block and the RI mixture screw has a very short taper, which combined make the screw so sensitive just turning the width of the screw slot usually moves the mixture out of reasonable A/F range.

If your carb has a horizontal passage straight though into the bore from #3 in the upper image, the standard block will not convert it to standard "forward idle".

If #3 goes in the body and turns 90 degrees down connecting to a passage in the base discharging through a .078" (approx) hole below the throttle, you can use the standard block and that will improve the idle mixture screw adjustment behavior.

[sportsroof]

Tuner: as usual, informed and comprehensive response, thank you. Tonight I attacked it to convert the carb body to accept either type metering block as it was made with that stupid brass accelerator pump transfer tube affair which prevented me using other type blocks. This is a 6709 which utilises the 4165/4175 type metering blocks and odd ball float bowl gaskets. Great fun. At least now I can test conventional 4150/4160 metering blocks easily, whether reverse idle or not. Thanks again.
Cheers, Martin

[Walter R. Malik]

I have a Holley 4 barrel which has the reverse idle system. I want to use a different metering block for some testing but this other block is 'normal' idle system. Will this work as intended if I fit it to the same carb the reverse idle block came from?
Cheers, Martin

Not without other modifications.

[In-Tech]

There are many on this site that have a grasp of things that we have an aspire to learn. None of us know it all... the kindness of people like Tuner, Mark and plenty of others help us find the things that we did not take to time to learn as we were learning other things.

The hard part is to ask the right question. Goodness abounds.

[HS Garage]

If your carb has a horizontal passage straight though into the bore from #3 in the upper image, the standard block will not convert it to standard "forward idle".

If #3 goes in the body and turns 90 degrees down connecting to a passage in the base discharging through a .078" (approx) hole below the throttle, you can use the standard block and that will improve the idle mixture screw adjustment behavior.

I have a list 8679 650 spreadbore and #3 goes to the bore and also down to the base. There is a hole in the base but it does not appear to go anywhere. My question is could you epoxy the through hole to the bore shut up to the point of the 90 degree turn to the base and then drill the base hole below the throttle blade for "forward idle"?

[sportsroof]

But if your existing idle circuit is workable for your application, do you need to change it? I only wanted to change mine as it was way too weak and the idle screws were very ineffective. I have now found how to make it much better and have control of the idle and low speed circuits such that the transition between low speed and high speed is seamless with no weakness-induced 'flat spot' with gentle acceleration at about 2000 rpm so the whole question of changing to the old style idle adjustment is moot. HTH
Cheers, Martin

[Tuner]

If your carb has a horizontal passage straight though into the bore from #3 in the upper image, the standard block will not convert it to standard "forward idle".

If #3 goes in the body and turns 90 degrees down connecting to a passage in the base discharging through a .078" (approx) hole below the throttle, you can use the standard block and that will improve the idle mixture screw adjustment behavior.

I have a list 8679 650 spreadbore and #3 goes to the bore and also down to the base. There is a hole in the base but it does not appear to go anywhere. My question is could you epoxy the through hole to the bore shut up to the point of the 90 degree turn to the base and then drill the base hole below the throttle blade for "forward idle"?

Hypothetically, you could. Practically, not a good idea, unless you can come up with a metering block with .063" orifices at the tapered tip of the idle adjusting screw, or modify your blocks by inserting a bushing where the tip of the adjusting screw seats.

If memory serves me, that carb has the short brass tube with O-rings between the block to the body in the accelerator pump passage, so this limits the metering block availability. You would need an old mid-70's emissions carb block from a R-6619 or R-6919 or similar with the tube.

The "backwards" idle metering blocks have a .106" hole and a blunt larger diameter tip on the screw. When used with "forward idle" the larger hole and blunt tip make the slightest turn of the screw very sensitive, a quarter-turn will go from too rich to too lean, or visa versa.

I think your time and effort would be better spent tapping threads so you can use brass set screws to make the Idle Air Bleeds adjustable.

[HS Garage]

Thanks for the reply Tuner. I appreciate the response.