Tim972 Posted October 21, 2020 Share Posted October 21, 2020 IMG_3754.MOV After rebuilding my Qjet for my rebuilt 402, I’m getting an idle surge. I initially was going to send it to Cliff Ruggles, but he’s backed up til next year sometime, so he set me up with the parts. Before the carb rebuild it was rich at idle but it ran smooth. After I rebuilt it and fired it up, it idled at 10-12 hg and 12.5 on a/f gauge, but idled fairly smooth. I had no control with the idle screws, so I advanced the timing to 20° Initial and then backed out the idle speed screw, and also lowered fuel pressure to 5 psi I regained some control. The mixture screws are now 1 turn out and I'm getting 13-15hg, 14.5- 15 or so on a/f gauge but surging at idle, 800-1,000 rpm or so. I cannot find any vacuum leaks on intake or around carb. The vacuum and a/f look decent, but the surge makes me think it’s lean. Any ideas ? I’ve been emailing back and forth with cliff, just waiting on his response. Some engine details: 1972 402, .030” over, 9.0:1 CR, stock oval port heads, Lunati hydraulic roller cam, .535/.545 lift, 220/230 duration, 110 lobe sep, 800 cfm Qjet from 80’s motorhome w/454. IMG_3754.MOV Quote Link to comment Share on other sites More sharing options...
wallaby Posted October 24, 2020 Share Posted October 24, 2020 Are you running manifold or ported vacuum to your distributor? Mine has a similar setup and it always shows a rich AF mixture at idle. Mine idles happy at about 12.5 on the gauge. Don't forget that a cam with a bit of lope will mess with the AF gauge..in other words, the gauge can't be trusted. I would start over and forget what the AF gauge is telling you. Your ear can be a better judge of what the engine wants, and you should adjust for the highest vacuum you can get. The timing sounds reasonable, so hopefully it's just a matter of getting the carb adjustments dialed in. There is a possibility that with the low vacuum at idle your primary rods are lifting and making the mixture rich. There are spring options to correct that, but I would hold off on that for now and see if it can be made to run right without having to take the top off the carb again. If you can get the vacuum to come up, it will pull the rods down where they should be and you will regain mixture control from the screws. Quote Link to comment Share on other sites More sharing options...
Tim972 Posted October 25, 2020 Author Share Posted October 25, 2020 I’ve tried manifold and ported vacuum. I believe the vacuum signal to my vacuum advance is playing a part in my troubles. My vacuum output is right on the edge with the vacuum advance can being all the way open, which I believe is causing it to hunt back and forth rapidly. So I temporarily unhooked my advance and cranked the timing higher, the idle has smoothed out alot. I also started adjusting the vacuum advance with the engine off using my Mityvac pump and I’m going to reset my timing and go from there. I had the a/f gauge from a prior vehicle so I thought I’d throw it on and see what it read. I’ve mostly been using the vacuum gauge. I have a few more things to try, but my carb might need a few more modifications to get more idle bypass air. I’ve only done what Cliff’s suggested so far. Just tinkering with it on the run stand until my car gets back from the body shop, the rest of the adjustments will have to wait until it’s installed. Quote Link to comment Share on other sites More sharing options...
Tim972 Posted October 26, 2020 Author Share Posted October 26, 2020 I decided to pull the carb back off and enlarge the idle air bypass holes in the base. I drilled them out to .095”. After firing it back up, playing with the timing and mixture screws some more, I now have an almost steady 14.5hg of vacuum at idle and it occasionally bumps up to 15hg. With the cam that I have, I’m pretty happy with those readings. We’ll see how it acts when I get it back in the car. Quote Link to comment Share on other sites More sharing options...
wallaby Posted October 28, 2020 Share Posted October 28, 2020 I hate going in there with drill bits. Glad it worked for you. It's going to act a bit different with the load of a transmission hooked up, but you are on the right track. Once you have it all together, try to get the idle speed as low as you can. That way, shifting into gear isn't such a shock and the idle will be manageable. Mine likes manifold vacuum; it picks up the speed and then I can dial it down as needed. Once the speed gets reasonable, fine tuning is pretty easy. Oh...the A/F gauge can be a real help, but not at idle. It's good for cruise information or wide open throttle, but lopey cams confuse it and the info isn't accurate. New cars run at leaner cruise settings than you want to aim for. Wide open throttle is pretty standard at 12-12.5. Quote Link to comment Share on other sites More sharing options...
Tim972 Posted October 28, 2020 Author Share Posted October 28, 2020 I appreciate the help. I’m definitely going to try the a/f gauge going down the road, whenever that happens. Hopefully by spring Quote Link to comment Share on other sites More sharing options...
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