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Thread: Mild engine tune-up causing it to run rich.

  1. #1

    Default Mild engine tune-up causing it to run rich.

    Looking for some help with my 400 CI SBC. I recently did a tune-up and it's now running rich and has a good bit of white smoke in the exhaust. I adjusted the timing from about 28° ATDC to 4° BTDC just going off what an old Chilton repair manual I have said the timing should be set at. It suddenly went from not wanting to start or run and trying to diesel back to life after shutting off the key to running great. I've also recently gone from 70 primary & 80 secondary jets to 67 & 77 respectively on the 750 double pumper. But I found that the rubber fuel line going from the frame to the pump was rotted & collapsed, so I'm not sure if that's causing it to run rich or not. The only other thing I did was swap the plugs for another set of R44T plugs just like what came out and what the book called for gapped to .035", but now I'm getting this fairly thick white smoke, and it's running like it has a miss, but all the cylinders are firing. Also the old plugs had white on the face of the plug but the porcelain was completely black. Not sure if this is an issue of too much fuel, or if I should look into valve stem seals.

  2. #2
    Join Date
    Jan 2010
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    Sounds like the engine needs a bit of work. You're running a 750 DP on a stock 400? How about 750 vacuum secondary, and some more timing?
    Regards, Gary

  3. #3

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    I put a vacuum secondary 750 on when I got it to replace the junky Edelbrock that it came with. It’s a stick shift and the secondaries never opened on it, so I put the double pumper on it. I'm halfway through swapping the valve seals though and about half of them are pretty hardened up as well as one being fairly mangled. Only done the odd side of the engine so far. Other issue I’ve discovered is that I only gapped the plugs to .035", however with it being HEI I needed to gap them to .045".

  4. #4
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    So if the secondaries were not opening, either the secondary system is binding up some how, the spring is too heavy, or the engine is not flowing enough air to open them as you think they might need. Once you have have replaced the bad valve stem seals, then hopefully the plugs will stay a little cleaner, so you can sneak on the jetting. .045" gap is a good start. A distributor recurve with less mechanical in the curve, will allow you to run more initial timing like 10-15° at idle. Check timing at 2500-3000 to have around 32 with vacuum advance disconnected.
    Last edited by Gaz64; 11-07-2018 at 10:13 PM.
    Regards, Gary

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