Surge problem on new motor.

Engine, Trans & Driveline

  1. Heimedw

    Heimedw Well-Known Member

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    Hey guys just had the car out for a good test drive today and I'm getting a surge at just tip in throttle at about 30-55mph. The car run great if you get in the throttle and keep the speeds above 60. Im not sure if this is caused by the carb settings or the convertor? I have always had manual transmissions. The motor is a nasty stroker with a Holley 850 xp and the convertor is a 2600 stall. As soon as I get into the throttle is cleans itself out. Any ideas of what direction I should start. Timing is good and it runs at 180f all the time.
     
  2. Jerry Hall

    Jerry Hall Well-Known Member

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    My opinion is that it may be doing what I call riding the convertor. What is your RPMS in the 30-50 mph range? If it's in the 25-27 hundred range the convertor kicking in and out will feel like a surge. On acceleration it goes away because the convertor is locked in.
     
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    • steve340

      steve340 Well-Known Member

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      It could be a bit lean you may need to go up a size or two on the primary main jets.
       
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      • beanhead

        beanhead FBBO Gold Member FBBO Gold Member

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        Lean baby lean! What camshaft did you put in it? Getting your carb dialed in will take time and testing (that's the fun, right?). If you have an A/F meter it eliminates some guesswork but you can get dialed in either way. Like 340steve said, jetting may need to be increased as you should be cruising on the mains at that speed. Tip-in stumble generally means you need a bigger/quicker pump shot when the throttle blades start moving. Try swapping to different pump cams (you can get the assortment kits, I prefer AED's kit, they're all the same but AED's is about $10 cheaper). Experiment with cam and squirter combos, you'll wake that thing right up. Personally I would start with a blue cam up front in the #2 hole, 31 squirter and drive it. (Pump arm preload will have to be re-adjusted with each cam, lots of how-to vids on this...it's quick&easy) Good hunting!
        Disclaimer: Before monkeying with the more detailed stuff timing has to be good first. Then the basic carb settings need to be correct (float levels, idle mixture/vacuum etc.) I'm also a firm believer in getting an 8' piece of tubing and driving with the vacuum gauge inside the car, hooked to manifold vacuum...keep track of gauge readings under all the various rpm's and throttle conditions..
         
      • Suregrip391

        Suregrip391 Well-Known Member

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        Could be a vacuum leak as well , mine was on the intake,
         
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        • Heimedw

          Heimedw Well-Known Member

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          It seems to be just off idle to about 1/4 throttle. Factory had 84 mains front and rear and the engine shop put a 85 in the primary. I just bumped the primary and secondary up to 86's. It's still doing it. I'm starting to think it is the convertor. I really noticed it at about 2000-2500. Also it's with a steady pedal if you very your speed it seems to be better. If you get out of the throttle it idles fine no surge or wants to stall.
           
        • forphorty

          forphorty Well-Known Member

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          I seriously doubt it's the converter unless it's broken. I have had a variety of converters in street cars over the years , ranging from factory 11.75 low stall units , factory 10.75, aftermarket 10.75 , 10.00 and 9.5. Stall speeds from about 1700 to 4500. Never had one feel like it was surging/ kicking in and out. I suspect it is carb/induction related. The first thing I do if a carb is acting weird is to spray the crap out of the air bleeds with carb cleaner.
           
        • beanhead

          beanhead FBBO Gold Member FBBO Gold Member

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          Take a peek at the plugs. Are the basics set? Float levels? Proper T-slot exposure? Try bringing your mixture screws out a little. Transfer slots still have some effect here.

          Also possibly timing curve. How is the distributor curved? Vacuum advance hooked up?

          Suregrip391s post- vacuum leak can also cause this.
           
        • Heimedw

          Heimedw Well-Known Member

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          I pulled a plug from each bank tonight they actually look a little rich now but my road is gravel off the main road so I just take it easy coming in. Timing is all mechanical msd. The float sight windows are just under half. This is after I bump the primary up one and the secondary up two. I wonder if it's not making enough vacuum to draw the proper amount of fuel? I will brake clean the intake tomorrow to see if I have any leaks. These xp carbs have no vacuum ports to hook up a gauge. My cam is 252/258 610 lift on a 110 lop and 108 centreline.
           
          Last edited: Aug 31, 2019
        • Heimedw

          Heimedw Well-Known Member

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          Well I didn't get anywhere today. I put the primary jets back to 85s. Then I turn the idle jets out from 1 1/8 to 11/2 and took the car for a drive. For about five minutes it seem to run well with no issues. Then I come to a stop and was idling ok. As I started to accelerate I was getting a slight pop or backfire through the exhaust and my surge was back at a steady cruise under 60mph.I stop a few times and adjusted the idle screws to 13/4 to back to 11/8 and it made no difference. I took a video not sure if you will be able to hear the surge.
           
        • Heimedw

          Heimedw Well-Known Member

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        • Heimedw

          Heimedw Well-Known Member

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          You can hear it better in this video
           
          Last edited: Sep 1, 2019
        • Curiousyellow71

          Curiousyellow71 Well-Known Member

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          Does it idle ok? Maybe you have a bad power valve or the wrong sized one. A backfire can take them out.
           
        • BSB67

          BSB67 FBBO Gold Member FBBO Gold Member

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          Too much timing in the surge rpm range could cause the condition you describe. If you have vacuum advance, try disconnecting it as a trial. If you don't, but have all of your mechanical advance in early, like 2000 rpm, try slowing it down.
           
        • Heimedw

          Heimedw Well-Known Member

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          I'm starting to think this big cam is not making much vacuum and you are correct curiousyellow that the stock 4.5 power valve is staying open. When I said backfire that might have been a harsh word it was more like a light popping from the exhaust. It idles perfect.

          I will check the timing again tomorrow because I will say I wasn't looking at what rpm total timing was coming in at just seeing what it was set at around 3000-3500
           
          Last edited: Sep 2, 2019
        • Curiousyellow71

          Curiousyellow71 Well-Known Member

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          It sounded like a Holley power valve problem to me. Does that carb have a 50cc accelerator pump on the primary? When I looked the xp specs said 30cc. I have always ran a 50cc...so I was curious if the spec I read was right.
           
        • Heimedw

          Heimedw Well-Known Member

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          32 I believe. It's running like a champ if I can get this surge problem solved.
           
        • Curiousyellow71

          Curiousyellow71 Well-Known Member

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          It doesn't sound like it has a blown power valve then....but maybe it's not opening up until your WOT. I think it works the opposite of what your thinking. I think the power valve would need a higher number so it came on sooner at more vacuum. You might have more vacuum then you think compared to a large roller cammed engine. I think the rule is take your engine vacuum and divide it by 2 for the PV #....a 4.5 is for a engine with 9 in of vacuum. I think I would try a 5.5 or 6.5 and see if it changed it. I haven't tuned a PV in While so I am guessing.
           
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          • beanhead

            beanhead FBBO Gold Member FBBO Gold Member

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            Don't guess, hook up your vacuum gauge so you can watch it while driving! Vacuum line is cheap..and absolutely no disrespect meant to CuriousYellow, but when we're talking modified engines, throw the old "half the idle vacuum" method of PV sizing out the window..
             
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            • Curiousyellow71

              Curiousyellow71 Well-Known Member

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              No offense taken....I agree beanhead. Vacuum gauge and a drive. Take time with the Holley and dial it in.
               
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