off idle engine stumble

1oldtimer

1oldtimer

Well-known member
'47 stock flathead, with a msd pro billet dist, 6al box and blaster coil (12v system). it has a 2 jet on it now but had a rebuilt 94 (had the same problem with both carbs) and it has a steady stream from the accel pump. I have to make a timing set up for it to fully check it. it has a little bit of adjustment and i've messed with the springs and stop (no pinging now). question is would no vacuum advance cause this? should i look for a stock dist and convert to electronic. it acts like a bad accel pump, you give it gas, it starts to rev and dies for a second and picks up. i have to slide the clutch to get past it, once past the flat spot it runs great.....the other thing i noticed is it stumbles/bucks when your cruising in that spot (at part throttle).
 
there's no vacuum leak, runs great after the stumble. it's running a rochester 2 jet right now, the stock 94 had the same problem and had plenty of squirt from the accel pump. i've had a few newer cars (60's and 70's) that had a hesitation off idle when the vacuum advance was not hooked up, so i was wondering if this MIGHT be the same....this is my first time dealing with the crab style dist. when i first got the car the carb was in bad shape and the idle was about 1200 so i couldn't tell if it had a problem. all the msd ignition stuff was already on there and i personally rebuilt the 2 jet so all the parts are in it. also i put the 2 jet on another car and it runs great. i guess when i get some more time i need to check how the advance is working.
 
Check the advance weights ,should have a little tension on them ,The vacuum advance, these we run almost fully screwed out these days due to the fuel .so its unlikely that .does it have choke ??
 
it had the strongest springs on it out of the box, i've tried the weakest and it pings (the hesitation still there), last night i put on the next up (medium) so far no pinging but the hesitation is still there. the choke isn't hooked up, running a little rich so it doesn't need it.
 
Is the linkage positive . activating straight of. I have seen them were the spring is over the ball and the linkage spring compresses first . If its running rich then the problem maybe spark tracking of Idal ,rotor /terminal plates .
 
1oldtimer said:
it had the strongest springs on it out of the box, i've tried the weakest and it pings (the hesitation still there), last night i put on the next up (medium) so far no pinging but the hesitation is still there. the choke isn't hooked up, running a little rich so it doesn't need it.
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Sounds to me like PV are too high a # like 7.5 2x2 setups usually use 4.5 or even 3.5. 7.5 open almost right away flooding the engine momenterily till speed picks up. OMO ken ct.
 
ken ct said:
Sounds to me like PV are too high a # like 7.5 2x2 setups usually use 4.5 or even 3.5. 7.5 open almost right away flooding the engine momenterily till speed picks up. OMO ken ct.
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Youve also made so many changes on it it hardly a stock FH anymore. Maybe try the hot rod guys on the hamb.Your past the stockers already. ken ct.
 
Where is your accelerator link on the throttle arm? You may need to move it. closest to the thottle shaft shortens(lean) the pump travel. You will get stumble on either a lean or rich accelerator stroke. I just went through this on my dual setup. With Ken's help on the jets and me finding the right accel pump setting, all is good, no bog no stumble.
 
ken ct said:
Youve also made so many changes on it it hardly a stock FH anymore. Maybe try the hot rod guys on the hamb.Your past the stockers already. ken ct.
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i thinking of putting the 94 back on it, maybe finding a stock dist (convert to electronic with bubbas) and a 12v coil. i really don't like the look of the spark box and msd coil. the only other non stock thing is the fentons.
 
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