Hesitation while driving at constant speed?? 88 d50

[cnatsu1]

The vacuum line to the brake booster had slipped down a little. I pushed it up and made sure the clamp was tight, but this happens while I'm driving and not braking. Where was your vacuum leak in the booster at??

I also found a couple of caps on my carb that were pretty sad and most likely leaking, but I've still got the stumble. I was driving it tonight and feel like it might be the carb. It only happens after it's warm, a couple of miles, and it seems to be on deceleration. So intermittently on the flat, when I hit a slight downhill, or slowing down for a stop.

The mikuni carb is super clean and I can tell the exposed gaskets are new and soft. Maybe I still have a vacuum leak, float or other carb adjustment?? Wish I could do the Weber carb, but the wife has cut me off and I don't have enough mad money for that right now. Might have to check out the local parts yard again as I still some credit form the tranny debacle.

[BradMph]

The vacuum line to the brake booster had slipped down a little. I pushed it up and made sure the clamp was tight, but this happens while I'm driving and not braking. Where was your vacuum leak in the booster at??

I also found a couple of caps on my carb that were pretty sad and most likely leaking, but I've still got the stumble. I was driving it tonight and feel like it might be the carb. It only happens after it's warm, a couple of miles, and it seems to be on deceleration. So intermittently on the flat, when I hit a slight downhill, or slowing down for a stop.

The mikuni carb is super clean and I can tell the exposed gaskets are new and soft. Maybe I still have a vacuum leak, float or other carb adjustment?? Wish I could do the Weber carb, but the wife has cut me off and I don't have enough mad money for that right now. Might have to check out the local parts yard again as I still some credit form the tranny debacle.

I found this tidbit on hesitation with carb. Camoit's idea seems very likely and it's an easy check. Just throttle the carb and look into barrels for a squirt from pump.
Here is some other info about a lean setting causing the hesitation. I have the entire document which it talks about the Weber, but carb hesitation is pretty universal when it comes to diagnosing the problem.

8. The last thing I do after this is make sure that there is little to no off-idle hesitation which would indicate a lean mixture. The accel pump in the Weber should prevent that, but sometimes, it’s possible that your final mixture setting is on the lean side and so you might get some hesitation. If you do, turn the mixture screw out just a hair, like 1/16th of a turn and then run the engine at 2000rpm for about 10 seconds and then let it idle. Now snap the throttle open quickly. Hesitation? No, great, you're done. Still some hesitation? Make another 1/16th turn out and repeat.
9. Now when you're doing this, it’s important that you are able to tell if you are hesitating due to a lean mixture, or stumbling due to an overly rich mixture. There are several clues that will help you out here. The first is if its leaning out, you may get some backfiring through the carb. The second is if its over-rich, you'll see lots of black smoke when you snap the throttle(s) open. The third is if turning the idle mixture screw out a sixteenth of a turn makes it worse, then its over-rich. Ultimately, what you >want to see when you snap the throttle open is a hint of black smoke out of the tailpipe. No smoke, you're too lean. Just a little smoke is what you want. Now, if you've got a nice, perhaps *slightly* burbly idle around 900rpm, and an engine that snaps enthusiastically off idle, you're done! The main key to remember is that you want your idle screw having to move the throttle as little as possible, since, again, if you have to open the throttle very far, it will uncover the transition ports, and that suggests that your idle jet is too lean.
One other consideration is that if you have the throttle just about closed and the mixture adjusting screw pretty far in, then your idle jet may be too rich.
Another note: I keep saying that you want the throttle as closed as possible, but in actuality, having the idle speed screw turned in about a half to one turn (after making contact with the bellcrank) is about right. Any more than that and your idle jet are too lean, any less than that and your idle jet maybe too rich. Your plugs can also show whats going on inside the chambers. In my experience the trucks run a little rich anyways, but it could go either way.