Author Topic: idle help  (Read 1065 times)

Offline 73ss

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Re: idle help
« Reply #30 on: June 30, 2021, 06:25:09 PM »
I installed .039 idle tubes over the weekend. It made a small improvement. Now it responds to pulling off a small vacuum line to where it didn't before.
 
Question: Did all BB carbs have a .136 bypass or just the smogger units? My original '73 carb is at .136. Do any of you go this big on a BB carb? I'm currently at .125"

To re-cap, I started with 10" vacuum and am now up to 12, 850 rpm, But it still requires lots of timing lead. Its getting there in small steps. I'll have to put my carb tuning skills on hold (Or lack thereof) :D for a while until I get a fuel pressure issue solved. I'll start a new post for that rant. 

Offline Kenth

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Re: idle help
« Reply #31 on: June 30, 2021, 11:52:19 PM »
I have found most if not all 1970-74 Chevy 454 units uses .138" idle bypass air.

Offline 73ss

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Re: idle help
« Reply #32 on: July 11, 2021, 06:28:04 PM »
While trying to get this thing dialed in, I have found that the #2 plug turns black. I had convinced myself that the intake was not sealed properly and causing all of my issues. I pulled it off and took it to the machine shop, They said it was slightly bowed and took a small amount off. While I had it apart, I took the springs off on #2 to check for sticky guides or valves not sealing. I do not have leak down test equipment but the cylinder had what sounded like minimal leakage while filled with compressed air. everything checked out great. I tested the plug wires and they all ohmed out within spec, I decided to replace them anyhow since they are about 7 yrs old. Put it all back together and the same thing. Does not idle well and #2 turns black. Temp readings with a laser temperature gun show that # 2 is running about 250 degrees, the remaining 7 are all high 300's low 400's. I have read on line where the experts say that this is a irrelevant test at idle. The rest of the plugs look ok, some darker than others but #2 is turning black. It doesn't look like oil, the black easily flakes of the plug to where oil will be sticky. The engine hasn't used any oil. Buy comparison, #1 plug is white, looks like it is running lean if anything. Compression test is +- 3 psi across the board.  Plugs actually look good after a good 10~15 mile run, but then #2 gets black with idling and in town driving.
I don't claim to be an expert but I've been doing this 25yrs and am stumped. I've never had a motor that has needed this much timing or fuel at idle. Could my intake be junk and just not sealing ? I would think it would suck oil from the bottom side if that were the case. Why would #2 appear to be getting all the fuel? One side of the carb dumping more fuel than the other and it's all running to #2? I have no nozzle drip at all. Any ideas?

Offline Cliff Ruggles

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Re: idle help
« Reply #33 on: July 13, 2021, 01:19:23 AM »
Did you replace the spark plug or move the one on the troubled cylinder to a different location?

Where is you PCV hooked up?

Offline 73ss

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Re: idle help
« Reply #34 on: July 13, 2021, 04:41:45 AM »
Replaced the plug, PVC is on right valve cover connected to the correct port on front of baseplate. Stock aluminum gen 6 valve covers.

Offline Cliff Ruggles

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Re: idle help
« Reply #35 on: July 13, 2021, 10:01:58 AM »
I was hoping it would be something that simple like the PCV plugged into one of the runners and not into the carb to be evenly distributed to all cylinders.

A few years ago my engine developed a slight miss at idle and low RPM's.  Started idling rough as well.  I isolated the troubled cylinder by removing one spark plug wire at a time from the HEI until I got little to no change.  Coincidentally it was the #2 cylinder. 

At that point I replaced the plug and wire, no improvement.  Then I did a compression test and it was same as all the other cylinders.

Hum?  So I removed the intake and replaced the intake gaskets and it corrected the issue.  Good thing that I did because the gaskets were failing everyplace.  They were the grey paper type gaskets.  I replaced them with the Felpro blue variety with the metal reinforced centers and material bonded to both sides and haven't had any issues since......


Offline 73ss

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Re: idle help
« Reply #36 on: July 17, 2021, 02:23:22 PM »
Thoughts on this? I don't like the way the plug is slightly recessed. I noticed this while assembling the motor and sent some pics to Brodix and they said it would be fine. This is the correct NGK plug that Brodix lists for this head. Could this have something to do with why this motor needs so much lead? I would think that the plug should be at least flush with the head opposite side of the valves.

I finally figured out how to re-size images!!! ;D
« Last Edit: July 17, 2021, 02:27:18 PM by 73ss »

Offline old cars

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Re: idle help
« Reply #37 on: July 21, 2021, 10:00:30 AM »
There called extended tip spark plugs.

Offline old cars

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Re: idle help
« Reply #38 on: July 21, 2021, 11:12:55 AM »
pics

Offline 73ss

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Re: idle help
« Reply #39 on: July 22, 2021, 07:31:47 AM »
There called extended tip spark plugs.
Thank you for the info. I did some research and decided upon an autolite 3924, extended tip. Installed a few and rotated motor over with no interference. Edelbrock & Trick flow both list the 3924 for their BB oval port heads. It didn't make any difference on the amount of timing lead that it needs, but I'll be curious to see if the #2 plug stays clean.