have big block chevy 540 in a willys coupe. drive on street and strip. have two 750 mech secondary double pumpers sideways. getting everything to work pretty good except off idle ( around 1000 rpm) right now stumbling on full throttle attempt off idle. upped my shooters and getting better but still stumbles. up to 52 shooter in primaries and 50 in secondaries. read articles and most say to go to 50cc pumps which i have ordered. my questions are has anybody been down this road and what have they ended up with for shooters and cams on the accelerator pump arm? also what would be close for a starting point for jetting. will i need 50cc pumps front and rear? thanks for any input.
Need more info, tell us about tune up,what dist,what base timing what cam is in the motor, rear end gears, stick or auto what stall speed if auto.Many stumbling problems are not carbs.
has msd about 38 degrees at full advance. has hydralic roller in 650 range lift. 410 gears turbo 400 with switch pitch converter 1200/ 4000 tuning is being done in tight side of converter. when holding brake and taking rpm up it launches like a rochet with no stumble. pretty sure its in accelerator pump area cause getting better as i go up in shooters.
started out with the same jets that were in the carbs when i bought setup on ebay. 76 primary and78 secondary. but upgraded with proform main bodies. decided to stay with jets that were in carbs cause car was driving around town good and not fowling the plugs. proform recommended to start with 78 and 84 but i wanted to sneak up on that.
Like was said by sdluck above.. It ain't the carbs. You answered your own question when you said it doesn't do it on the high side converter. That engine combo needs some rpm. You have the engine pinched down too far under the torque curve at idle with that cam, manifold, carb combination that is has a lean backfire under those conditions. Carbs are nothing more than an organized leak and there isn't enough velociity in the engine at that speed to draw fuel in that it needs just off idle. Your choices are mild down the engine or run the higher stall converter. You could try bigger accel nozzles and the big pumps and it will bandaid it some, but I doubt you will ever cure it completely.
sounds probably right banana. i can drive it around town on tight side. anytime i want to nail it i can switch to hi side of conv and raise starting line rpm and do my thing. will try 50 cc pumps since i have them coming. whats your thoughts on jetting? am i too small on secondaries? thanks a bunch.
I had the same problem with a high horse power 327". My fix was lower rear gearing and higher stall torque converter.
its hard to get a car to perform the way you want it both on the track and street. have to compromise on both accounts. my problem is i dont want to compromise my track side of it.
Been down this road many-many times.. you say 38 degrees total ? Is the timing "locked out?" Should be 38 degrees all the time.. this will more than likely fix your low speed stumble.. been tuning these things for many years, and the timing is the cure for the "carburetors".. almost every time. need to lock the timing advance mechanism .. (MSD distributors have a provision for this.).Also, don't use a vacuum advance, wow, there is a disaster.. on a tunnel ram engine Do yourself a favor, don't try to cover up the stumble with pumps, it'll make it so fat, you'll hate it,and have plenty of time to cuss it, while you're changing the plugs every time you drive it. 36 or so, is about as big as you should go, on squirter size, and be sure they are adjuster correctly. no free play at idle..may have to open up the idle fuel orifice, if anything.. believe it or not, we have made best power(depending on combination,comp. ratio/etc.) at 42 degrees advance on a rat chevy.. on pump gas. tunnel rams,and superchargers really like the timing to be "up there" down at low rpms. just lock it,and have a good starter..
dont know what you mean by locked out. its thirty eight when i speed the motor up and put a timing light on balancer. dont know what it is at idle . never have grasped the timing thing. run an msd no vaccum advance. running pump gas with about 10 or so to 1 compression. this is a street rod first - ha so i say - then a drag car. what do you mean lock out the advance mech? do i even have one with an msd? sure i am showing by ass here but what the heck. thats how you learn right? thanks
reread your reply and seen where you said msd's have a provision for locking out the timing advance mech. will research this on good old google. thanks again.
wow! making sense now! reading the pros and cons of loccking out my advance. need to find out what the duration is on my cam and rethink this thing.
I would look down the carbs with the motor not running ,and slowly work the throttle watching one squirter at a time make sure it sprays all the way through.if it doesn't make sure the needle under the squirter is seating and holding a reserve of fuel.if they are holding fuel you may have to adjust the cams or the levers that work the pumps good luck I would also start it after I did each carb just to get the unused fuel out
yes, be sure the accelerator pumps are squirting,and have no free play at idle. But the reason i bring up the timing issue.. I have fixed about 70% of the "carburetor problems" by tuning in the timing.. we run the timing mechanism "locked out" on about everything. makes for easier starts,better throttle resonse,and fixes most of the "sneezing" on acceleration. just need a good starter. and battery. As for the m.s.d. lock out proceedure, basically, take out the distributor, knock out the roll pin from the drive gear, and remove the shaft,and advance assembly. remove the weights,springs, etc. and pull shaft from the plate the rotor screws on to. then remove the advance limiting bushing,**they usually come with three sizes when new...takes an 11/32" wrench, and then turn the shaft one half turn, and reinstall the nut..and reassemble the distributor shaft. put all the advance stuff in your tool box, for ? . (this will all make sense once you start taking it apart.) **E-Z> tip, set up distributor for reinstallation. add 10 degrees to the total you want,, (38+10= 48 ) set the balancer to 48 degrees before tdc,and line up the rotor with # one in the cap. (mark the distributor housing) and then rotate the distributor, until the reluctor lines up with the center(metal part) of the pick-up coil.. tighten up the hold down.. the thing will be within 2 degrees when ya start it.. yaaay !! if you have an msd box, 6a-etc.. run the thing at 3200 rpms to set timing,as this is when the box will lose the mulitple spark,and make you timing light happier(not jump around as much) really hope this helps.. see ya
thanks guys for your help- hey tricky i thought i'd try the easy stuff you mentioned first. real cold out today car in heated garage so i did get it to start and warm up. put timing light on cause curious to see where timing was at idle on balancer. its on 26 at idle and 40 at full in. turned out idle mix screws half a turn like you said to try. went for ride. engine idled down some like 800 rpm after turning out screws. went up the road and stopped and tryed not quite a full throtle start but almost. cartook off clean and didnt stumble . tryed it twice and was freezing my butt off so came home. going to try it there for a time and see if it is ok. waiting for a warm day to give it a better go. thanks again.
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