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| Rough idle when warm? https://slantsix.org/forum/viewtopic.php?t=48858 |
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| Author: | b5cuda [ Tue May 22, 2012 5:52 pm ] |
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OK, the idle was affected when I removed the vacum line to the distributor. It slowed down the idle noticeably so maybe there is a problem. I'm away from home this week without a camera (sorry - no pics) but my lines are run as follows: - vacum line from distributor to outlet on base of carb - hose from PCV to an outlet on the spacer between carb and intake manifold - hose from valve cover breather to the air cleaner housing Is something routed wrong? |
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| Author: | Mroldfart2u [ Tue May 22, 2012 6:14 pm ] |
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Yes you have the advance plugged into full vac and not ported (timed) vacuum.. I am not familiar with the carb so someone else needs to tell you where it needs to go.. |
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| Author: | THOR [ Tue May 22, 2012 7:12 pm ] |
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What Carb? If its a Holley 1920, the the port will be on the base of the Carb, but there won't be any vacuum on it at idle. With the vacuum line disconnected, what is your base timing? Try setting it at 5 degrees BTDC to start, then adjust the carburetor to lower the idle to around 750-800 in park/neutral. After that, there is a port on the Carb which will not pull vacuum at idle, though if you snap the throttle open, you will feel it with your finger over the port. Try hooking it up to another port which is the same size and see if it does better. ~THOR~ |
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| Author: | b5cuda [ Tue May 22, 2012 9:42 pm ] |
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Carb is a Weber DGEV (2bbl, progressive not synchronous opening, electric choke). I can only find one vacum port - at the base of the carb on the side facing toward the valve cover. |
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| Author: | /'d Box [ Wed May 23, 2012 6:16 am ] |
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Under your current conditions, you most likely have the warm-idle set high enough to uncover the port in the bore which is sending vac signal to dist. I had this same symptom while trying to tune my weber. Once I established the correct mixture with the proper idle jet size, I was able to bring the throttle blade closer to the closed position. The idle mixture screw will not be effective if the jet is not sized properly for the 225. If all other causes are eliminated, valve adjust, initial timing, etc., I would bet getting the proper jet size will solve your issue. Currently, in order to get it running, if your mixture screw is less than 1-1/2 turns out, you need a smaller jet. More that 2-1/2 turns out you need a larger jet. I needed a larger jet. Here's a better link for weber tuning: http://www.redlineweber.com/html/Tech/c ... _best_.htm |
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| Author: | b5cuda [ Thu May 24, 2012 4:22 pm ] |
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Thanks again for your input, and patience! I know it’s hard to diagnose a problem when you can't see it and I don't understand it. I’ve re-read the Weber instructions and that’s pretty much how it was set-up already. My mixture screw was 1.5 turns out. I did dial down the idle speed, but there was still a noticeable drop when I unhooked the vacuum hose between the carb and distributor. It was less, but then again, the idle speed was lower too. I have a few spare primary jets so I guess will play around with those, but I’m still lost as to what timed vacuum is and what’s wrong with it. |
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| Author: | wjajr [ Fri May 25, 2012 5:14 am ] |
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Quote: I’m still lost as to what timed vacuum is
I’ll take a shot at explaining what you called Timed Vacuum. You agree that slant six engine has to idle with no mechanical and vacuum advance, just static or base timing. To accomplish this, the carburetor’s vacuum tap to the vacuum advance is located above or top side of the throttle plates when closed isolated from manifold vacuum. In other words when engine is idling, throttle plates are closed, vacuum advance tap only sees atmospheric pressure, no vacuum. Once throttle plates are cracked opened, and carburetor is no longer exclusively running on idle circuit, that tap is exposed to manifold vacuum, and depending on available vacuum strength, activates the vacuum advance canister between zero advance and full advance on top of preset static and rpm dictated mechanical advance. |
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| Author: | WagonsRcool [ Fri May 25, 2012 12:07 pm ] |
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The internal combustion gas engine speed is controlled by restricting airflow- via the throttle plates in the carb. from the carb to the intake valve is an area of low pressure- known as manifold vacuum. This vacuum is steady- highest at light load (idle- 18-20"/ decel-23+"), lowest at heavy load (wide open throttle accel- almost 0"). The pcv valve & brake booster are connected to man vac., as are the choke pulloff & thermastatic air cleaner control. Timed or ported vacuum is designed to be 0" at slow idle, & increase as the throttle is opened. You should have a vacuum gauge as one of your basic tools to measure this. The idea is (as wjajr stated) that the engine should idle on "base" timing- without any mechanical or vacuum advance. If not, idle speed is "artificially" increased by the additional advance. So now you have to adjust carb stop down to get the idle speed you want. The "base" idle speed (without any advance)may be too low to keep the eng running. What can happen is if some thing causes the engine to hiccup (sudden snap throttle decel?), you can lose the speed or vac that sustained the "good" idle speed- resulting in idle surge or stall. |
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| Author: | b5cuda [ Mon May 28, 2012 10:10 pm ] |
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Hey guys, great explanations and it makes more sense now. Thanks for taking the time to help me learn! I'll start working the issues again this week (was away for the long weekend) and try to make more progress. |
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| Author: | b5cuda [ Mon Jun 04, 2012 10:46 pm ] |
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Updated - see next post. |
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| Author: | b5cuda [ Sun Jun 10, 2012 5:10 pm ] |
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Rest of the story… as a recap, symptoms on my /6 with Weber DGEV and HEI electronics were: - Rough warm idle - Hard to start when warm - Poor idle when slowing from highway speed (engine wanted to die) - Improper vacuum at idle, when I disconnected the distributor vacuum line the engine dropped a lot of RPM An old mechanic who’s good with carb’s looked at it and quickly determined that the float level in my carb was too high (at factory setting). I adjusted that and the first three symptoms got a lot better. Then I could adjust the idle down so there’s virtually no change in idle speed when I pull the vacuum hose from the distributor. I’ve also been playing with slightly larger idle jets, following advice (and weber links) received here. Now the warm start is great, engine doesn’t try to die when decelerating from high speed, and warm idle is better. I’m not 100% where I want to be but think I’m getting pretty close! Thanks everyone for the great help, Bill |
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