72 LS5 Posted September 13, 2011 Share Posted September 13, 2011 I have an 850 Holley double pump 4781-2 that is a fresh rebuild and bone stock with 80s all around and 6.5 power valves front and rear per the book. I was getting this off-cruise stumble....only when I give a very tiny bit more gas while cruising. If I give a decent shot, it would not stumble. I played with every pump cam in every position looking for the most shot as early as possible. Also spent $20 on a set of 37 squirters. Finally sprung for a 50 cc pump kit and using the most aggressive cam, the problem is 95% cleared up. But I'm still getting a VERY TINY lean stumble during the transition from cruise to main, one that you can't really feel, but can hear it ever so slightly if you listen closely. This is only under very light throttle from cruise. I thought an 8.5 pv may clear it up, but I drove with a vacuum gauge and the "stumble" happens at 10+ inches of vacuum. Then I thought of upping the primary jets from 80 to 82, but the plugs are perfect light tan as it is. Plus, it bothers me to have bigger primary jets than secondary (for reasons I can't explain). Air Fuel screws are 2 1/4 turn out for highest vacuum (11.5 in drive, 13.8 in park) The air bleeds are fixed. I betcha adjustable ones may do the trick. I understand that this is not considered a street carb per Holley. and maybe I'm expecting too much. The carb was not really designed for "tip-toeing" around. Any thoughts? Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted September 13, 2011 Share Posted September 13, 2011 my 750 and my 1000 do the same Quote Link to comment Share on other sites More sharing options...
72 LS5 Posted September 13, 2011 Author Share Posted September 13, 2011 Sam - do you have the adjustable air bleed screws? Quote Link to comment Share on other sites More sharing options...
ScottNM Posted September 13, 2011 Share Posted September 13, 2011 I am not as knowledgable as alot of these guys but I think you might be a little over carbed. Consider the 770 street avenger. Quote Link to comment Share on other sites More sharing options...
72 LS5 Posted September 13, 2011 Author Share Posted September 13, 2011 Scott, I ran this carb on my mild 454 for a short time and it was too much carb - it bogged with slicks. But now I have 509 cubes. Everybody I've spoken to says the 502 needs a lot of carb. I have a nice AED 750 HO that ran great with this motor, but I felt I may be leaving something on the table with that carb. I'm going to run them both at the track this winter, one right after the other, and see which is faster. Quote Link to comment Share on other sites More sharing options...
CrazyDavey Posted September 13, 2011 Share Posted September 13, 2011 Mike it seems like a transition issue so I don't think jetting will do anything. Usually that means it's getting too much air or not enough fuel on initial throttle blade change. First how much have you played with the accel pump lever adjustment? Most Holleys are very sensitive to this adjustment some liking no clearance and some around .015. Second I know you're trying to get a lot of vacuum but 2 1/4 turns on the idle screws is way too much, should be from 1 1/2 to 1 3/4. I would play with those 2 items and see if you can clear it up. Quote Link to comment Share on other sites More sharing options...
680HPStroker Posted September 13, 2011 Share Posted September 13, 2011 carbs are tricky and I'm no expert let alone a novice, but I rely on a guy who is a carb guru to tune my 1050 AED Street Dominator. If you have 454 cubes with somewhat of a radical build, or more than 454 with a milder build you can effectively utilize a carb with over 1,000 cfm. It's all in the tune. If you are still unhappy after all your efforts find a guru who specializes in performance carbs. Quote Link to comment Share on other sites More sharing options...
72 LS5 Posted September 13, 2011 Author Share Posted September 13, 2011 Davey, I totally agree with it being a transition issue. I should have mention that the 4781-2 is not a 4 corner idle - its only got air/fuels on the primary block. I got the highest vacuum at 2 turns out, but I made it a little richer to help out with this issue and warm up (no choke). When I put the 50 cc pump on (which cured the stumble 95%), I set the lash of the pump arm at 0, since I needed most of the shot early as possible. I could try slight lash.... At very low speeds, like driving in a neighborhood, if I gently go off idle, I can *hear* a slight stumble but can't feel it. Since I thought it was not involving the jets or power valve, my only other choice (after installing the 50cc pump) is the air/fuels. Like I mentioned before, this carb was not designed to be "streetable" like today's custom carbs. An $800 carb is in my future but not at the moment. Maybe I should quit [you said a naughty!]-footin' around and just drive it harder already! Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted September 14, 2011 Share Posted September 14, 2011 I would try a larger idle feed jet (may not be adjustable with your carb) this will richen your idle/idle transfer circuitin turn should close your idle jets and give more fuel to the idle transfer if you want to just make do...try opening the idle screws a bit more and/or raising your float level a smidge and give it a try Quote Link to comment Share on other sites More sharing options...
Heckeng Posted September 20, 2011 Share Posted September 20, 2011 Sam hit the nail on the head. At cruise, you are usually operating significantly on the idle feed port and the idle transfer slot which are both restricted (or jetted) by the idle feed restrictor. The idle fuel port is the hole in the base plate right below each throttle plate. The idle fuel port adjusts the idle fuel mixture as I'm sure you know. The idle transfer slot starts to flow under part throttle cruise situations before there is enough flow to get the boosters (main jets) to work efficiently. If you can adjust your idle feed restrictors which are usually in the metering block you should put in some larger restrictors. You will need to readjust your idle mixture screws to set the idle, but after that, the larger restrictors will let more fuel go to the transfer slot and get rid of that lean hesitation you are getting. If you wan to try and fix it easier and don't mind a rich idle, open up both of your idle mixture screws an extra 1/4 to 1/2 turn and see if that helps out. Quote Link to comment Share on other sites More sharing options...
72 LS5 Posted September 20, 2011 Author Share Posted September 20, 2011 Thanks Scott. All those restrictors are fixed, so I'm going to try just running richer air/fuels for now. Thanks for the feedback! Quote Link to comment Share on other sites More sharing options...
CrazyDavey Posted September 20, 2011 Share Posted September 20, 2011 you could try opening up the primary power valve sooner...on the dyno mine had a transition issue and we went to a 8.5 and fixed it... Quote Link to comment Share on other sites More sharing options...
72 LS5 Posted September 21, 2011 Author Share Posted September 21, 2011 Davey, I had the vacuum gauge with me and the lean stumble happens at over 10 inches of vacuum. Would a power valve have an effect on that b/c whether its a 5.5 or 8.5, it won't be opening at 10 inches. I was going to try this initially, but didn't once I saw the vacuum readings. It may be worth a try. Quote Link to comment Share on other sites More sharing options...
CrazyDavey Posted September 21, 2011 Share Posted September 21, 2011 umm probably wouldn't hurt to try it, don't know how accurate the gauge is so it won't hurt anything. I would also fatten the primary jet a couple of steps just for grins... Quote Link to comment Share on other sites More sharing options...
72 LS5 Posted October 9, 2011 Author Share Posted October 9, 2011 Update - put the 8.5 pv in and no change. Bumped the primary jets from 80 to 82 - FIXED! The 850 dp now works perfectly all around. Everything on paper pointed to the pv, but the main jet cured the transition problem. Quote Link to comment Share on other sites More sharing options...
downset71 Posted October 16, 2011 Share Posted October 16, 2011 what jets do you have in rear? Quote Link to comment Share on other sites More sharing options...
John Burns Posted October 16, 2011 Share Posted October 16, 2011 C'mon- try the staggered jetting! Quote Link to comment Share on other sites More sharing options...
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