1949 Olds 88 with the 303 V8, 6 volt, 29,000 original miles and I've got a hesitation whenever I suddenly accelerate. It's so bad it often backfires through the carb and sometimes stalls the engine. I've heard before that typically if it coughs through the carb, it's too lean. I've done a bunch of work to isolate and eliminate the problem but it persists. I'm beginning to wonder if I did the work in the wrong order. Here's what I've done so far and the order in which I did it:
1- replaced spark plugs, points and rotor with Pertronix (I was going to do that anyway).
2- adjusted the distributor timing following the guidance in the 1949 Olds shop manual.
3- changed fuel filters and serviced carburetor following guidance and order in the 1949 Olds shop manual. This included float height, accelerator pump examination, accelerator pump linkage, needles and jets cleaned with solvent.
4- checked for vacuum leaks. At idle I can hold about 18psi from the T fitting I added to the vacuum pump port on the base of the carb.
5- adjusted the air/idle screws in my Rochester AA carb. I found about 4 turns out offered the best vacuum pressure.
6- adjusted the main idle screw on throttle linkage.

It idles amazing, but still hesitates although less than it did once I adjusted the air/idle screws out so far. The carb base mounting gaskets all look good. I tested with starter fluid while the vehicle was running to find leaks. None were found. I'm wondering if I was suppose to disconnect the vacuum before adjusting the distributor? Was I suppose to disconnect vacuum before adjusting carb? If so, the automatic choke vacuum and the vacuum pump? I know a vacuum leak can affect idle, but I think something else is going on. At this point Im throwing money at it for no good reason. I ordered a carb rebuild just to be sure all the gaskets were good and that the accelerator pump was good (even though I know it was pumping). It seems to me the problem is with the accelerator pump. I just don't know how to adjust it. Ideas? I'm open to any other than selling!!

 

Sure sounds like a accelerator pump issue. Plus the backfire on entail acceleration is usually a sigh of a lean contrition caused from a poor shot of gas from the accelerator pump. It could be electrical but to me it sounds like the pump is bad or going bad. ...Tedd

 

That’s sure what I’m thinking. I’ve ordered a replacement and will keep everyone posted. Following the instructions about the linkage is definitely a bit confusing. Any pics or tips for that?

 

There's a lot of events that take place in your carb before fuel is discharged. I've never worked on one but very intrigued by the accelerator pump system.
http://www.oldcarmanualproject.com/m...A_0006_jpg.htm

 

I'm going to agree with Tedd. Usually it's a shortfall in the accelerating pump system. Be certain that it enriches as soon as the throttle begins to open.

 

The accelerator pump carb rebuild sounds like a good starting point.

if that doesnt help try some good points. Ive had pertronix in the past, theres a thread on here called pertronix never again. My experience w pertronix was i bought 3 for 3 different applications at the same time. 1 was bad out of the box, 2nd offered nothing but poor running engine until it was replaced, the third ran fine w no issues until i sold the tractor 3 yrs later. YMMV

 

Sometimes the accelerator cup is still good but the linkage that operates it is either worn or out of adjustment, like Ozzy said above you need gas immediately with any movement of the throttle linkage or there will be a bog till there is air movement over the venture..... Tedd

 

Was the problem there before the listed work was done? Does the engine reach full temp/produce good heat?

 

Yes, the car has had the hesitation since before any of my recent work. The work I’ve done was really to get it out of moth *****. It’s breaking the bank, but when it’s done it will be done right.
I stumbled upon an interesting thing today while installing the carb kit from Mike’s Carburetors. While it’s a 1949 Olds, the carb has a tag on it indicating it is actually a 1950. The accelerator linkage was “correctly” set in the position that a 49 should run. But a 1950 carb actually specs that the accelerator pump linkage use the center mounting hole, not the outer hole as is used in 49. I’m going to reassemble using the settings for a 1950 and see what happens!

 

The Rochester carburetor book says that while both '49 & '50 used model AA carburetors, there were slight differences. The '49 was number 7001570 while the '50 was number 7002570. The book agrees with what you found for the pump lever (outside hole for '49 & center hole for '50). It's worth it to try the other position.

 

Time for a project update! I found the hesitation. It came in multiple forms. As I said earlier, the accelerator pump was "good" before the carb rebuild but the hesitation persisted. So I rebuilt and what do you know, no change. So the carb came apart again, and again, and again. In the end, I found that the accelerator pump was clocked improperly and that the tension spring on the accelerator pump was slightly deformed. Just enough to cause it to hang up every once and a while.
So, if you've got hesitation between idle and full throttle, check that accelerator pump closely. Don't just look for damage, look for smooth operation and be sure to follow the mounting instructions in the book. It's a little different than the instructions that come with the rebuild kit.

 

Top notch troubleshooting!

Now- will that Rocket 88 raise its tail when you hold the brakes and stand on the gas? That's what they did back in the day... then launch like a shot when you let off the brakes! Hey it was muy impressive for its time.