Author Topic: idle help  (Read 9633 times)

Offline 73ss

  • Carb lover
  • ***
  • Posts: 120
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

  • Jet Head
  • ****
  • Posts: 388
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

  • Carb lover
  • ***
  • Posts: 120
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

  • Administrator
  • Qjet Hero
  • *****
  • Posts: 5347
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

  • Carb lover
  • ***
  • Posts: 120
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

  • Administrator
  • Qjet Hero
  • *****
  • Posts: 5347
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

  • Carb lover
  • ***
  • Posts: 120
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

  • Carb lover
  • ***
  • Posts: 131
Re: idle help
« Reply #37 on: July 21, 2021, 10:00:30 AM »
There called extended tip spark plugs.

Offline old cars

  • Carb lover
  • ***
  • Posts: 131
Re: idle help
« Reply #38 on: July 21, 2021, 11:12:55 AM »
pics

Offline 73ss

  • Carb lover
  • ***
  • Posts: 120
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.

Offline 73ss

  • Carb lover
  • ***
  • Posts: 120
Re: idle help
« Reply #40 on: December 09, 2021, 02:17:44 PM »
Wanted to give an update. I my above post I described how the #2 plug was turning black and was stumped as to why. I decided to pull the engine back out because I was never satisfied with the way it idled and wanted to degree and or replace the cam. I did a leak down test and all cylinders are 10~13% on the gauge with the exception on #5, It has a 35% leak past the intake valve, you can hear and feel the air escaping from the intake. #7 is the next to fire followed by #2. Since the #5 is still on it's intake stroke while #7 begins to open I'm thinking that when #5 starts the compression stroke, #2 is starting its intake stroke and receiving what is getting pushed past the #5 valve. # 5 & 2 do both share the same side of the intake, dual plane. I think this is why #2 is rich and turns the plug black. #2 also runs about 100 degrees colder than the other cylinders using a infrared heat gun.

What do you all think about this? That's the only thing I can come up with as to why it turns black. I second guessed myself and thought maybe I installed a ring upside down on that cylinder but the leak down and compression results are good.

I was quite surprised to find a leaky valve, When I got the heads we tested all the valves with a home-made tool to seal the ports and pull a vacuum on each valve to check for leaks. More than half of the valves were leaking. These were new heads, I called Brodix and they said to send them in for repair but I decided to lap all of the valves myself. Afterwards all valves held 20"'s vacuum for 15 mins or more and I called it good.

I also bought the tools to degree the cam and after checking I found it is 2 degrees retarded. I have it installed straight up. I have the specs above, open to cam suggestions, I don't think I made a good choice on the cam.

I did take the car to the track, Ran consistent low 12.30's @ 110, 1.7 60'. OK but was expecting more for a near 500 inch motor. 4100# chevelle, Through a full exhaust, 3.73, 2500 convertor. Q-jet ran flawless.
 

Offline Cliff Ruggles

  • Administrator
  • Qjet Hero
  • *****
  • Posts: 5347
Re: idle help
« Reply #41 on: December 10, 2021, 03:37:40 PM »
Advancing the cam 2 degrees isn't going to do much for it and may even slow it down a bit.  The cam is pretty small right to start with for 489cid with 10 to 1 squeeze and the head flow available.  That's about how much cam I'd use in a 454 build with less compression.

Below is a dyno chart from a 455 cid engine with less than 9 to 1 compression using Pontiac Super Duty round port heads and a 230/236/112lsa HR cam.  Idled with 16" vacuum at 800rpm's and I knew right then we needed a little more cam in it.

Low 12's at 110mph is showing about 460-480hp being used just doing some rough "math" in my head using the vehicle weight.  Certainly some room to grow there for sure......IMHO......Cliff

Offline old cars

  • Carb lover
  • ***
  • Posts: 131
Re: idle help
« Reply #42 on: December 11, 2021, 03:53:39 AM »
You didn't answer what intake manifold you are using, at least not that I could find

Offline 73ss

  • Carb lover
  • ***
  • Posts: 120
Re: idle help
« Reply #43 on: December 16, 2021, 03:36:20 PM »
You didn't answer what intake manifold you are using, at least not that I could find
Edelbrock performer RPM Q-jet. part # 7164

I believe my original poor idle problem was due to #5 intake leaking and also a poor cam choice.
 233.6in 246.6ex @ .050, 110C/L,  114 lobe sep.

Too much exhaust duration for the amount of intake duration? This cam would be better suited for a blown application. Lift, duration & center line were all spot on with the degree wheel, other than everything being 2 degrees late. Cranking compression isn't that great either at 155 psi.

Offline Cliff Ruggles

  • Administrator
  • Qjet Hero
  • *****
  • Posts: 5347
Re: idle help
« Reply #44 on: December 16, 2021, 05:47:10 PM »
I'm not sure exactly what's going on but something isn't quite right with the build.

489cid, 10 to 1 compression, 230/246/114LSA cam and you are only seeing 155psi cranking pressure and it takes 20 degrees initial timing to produce 10" vacuum at idle speed.  It should EASILY make 13-14" vacuum at 700-750rpms with only 10-12 degrees initial timing and cranking pressure is down at least 20psi......IMHO

Hopefully you'll get it figured out at some point......Cliff
« Last Edit: December 16, 2021, 05:53:56 PM by Cliff Ruggles »