VCCA Home Forums TECHNICAL - 1945 AND NEWER 1949-1954 Serious hesitation/bogging problem

I'm hoping that someone will have some ideas as to what might be causing a serious hesitation/bogging problem.

First, a little background. The vehicle is a '52 Fleetline Deluxe with 216, 3-speed and Rochester B with manual choke. The engine and drive train have racked up approximately 10,000 trouble-free miles since being totally rebuilt in 1996. Then, for a number of reasons, the car sat idle for a little over a year until late this spring. After priming the oil passages a bit, the first attemp at starting failed due to the needle valve being stuck closed. A quick rap above the fuel inlet with a plastic hammer rectified this and she fired right up. After warming up for a bit, I took off down the street, but the throttle seemed to be sticking on a high idle. After getting back to the driveway, I shut down the engine and then proceeded to work the throttle at the carb with the linkage disconnected; it felt as if something was binding and there was no jet of fuel from the accelerator pump. after taking the cover off, I found that the accelerator pump piston had stuck to the sides of the pump well and the piston a shaft had parted company -- carb rebuild time.

Fast forward: with the freshly rebuilt carb adjusted and the function of the accelerator pump and fuel pump checked and verified, I attempted another test drive. This time, the throttle worked properly and so did the accelerator pump. However, when starting off in 1st while attempting to pull out into the main road (low speed and more than half throttle), or when applying half to wide-open throttle immediately after the 1st to 2nd, and 2nd to high shift, there was a serious hesitation or bog; I could only accelerate slowly with light throttle or the engine would all but stall -- no backfire, no sputtering -- just a severe bog. Once the car was up to 35 MPH or above, it ran smoothly and seemed to respond normally to the throttle. After coming to a stop, it was the same story: I couldn't accelerate with more that the lightest throttle, and the engine would almost quit with any serious throttle applied at the bottom RPM in any gear.

Thinking that maybe something inside that carb was FUBAR, or that I screwed up something somehow when rebuilding the carb, I purchased a professionally rebuilt carb from someone I have done business with numerous times in the past, and always with excellent results. No luck. I am still experiencing the serious hesitation/bogging problem. Just as before, the problem occurs both curbside when suddenly opening the throttle, and when underway and applying throttle at the bottom of the RPM range in any gear. I've replaced all of the tune-up parts just to rule out an ignition problem, the distributor advance -- both weights and vacuum -- appear to be working correctly, and the timing mark hold steady as a rock. The only oddities I have noticed are the exhaust sound at the tail pipe at idle. Normally, the exhaust is smooth with the occasional light "poof" which I heard someone described as a symptom of "Rochester nasal drip"; now, it seems to have slight bit of a burbling sound. If the throttle is open gradually, the distributor body tracks the increase in throttle opening perfectly; however, if I snap the throttle wide-open, the distributor body begins to rotate, falls back when the engine bogs, and then snaps right back to where it should be once the engine gets past the bog. (The accelerator pump has been, and is still working perfectly.)

When first started, the car had a quarter tank of gasoline treated with "Sta-bil," and the very first thing I did after getting it running back in the spring was to fill the remainder of the tank with fresh gas and the requisite dose of "Sta-bil." The fuel visible in the pump bowl and the AC filter is light in color and free of water and debris, and has no stale odor. I know that today's fuel can be problematic for many older engines: does anyone think it possible that a short run on year-old modern gasoline could have caused one or more sticky valves that quickly? Any input would be greatly appreciated.

You may have a vacuum leak that is hindering enough vacuum to the distributor to allow advancing on acceleration????
With the engine running spray some starting fluid around the intake manifold ...If the engine speeds up any you have a leak.... Good LUCK Ed

I am also thinking vacuum leak. Also the Rochester B has a power valve that feeds fuel based on vacuum. It could be the problem. I am not that familiar with the Rochesters but there are a few that frequent ChatII that are. Hey Mr. Mack could it be the power valve?

I'm not Mr Mack but use several Rochester B's. The power valve in the bowl base is accuated by a spring loaded plunger held up position by a vacuum signal from under the throttle plate. Vac decrease under hard accel allows plunger to open valve. My experience is that failure to accuate the p.v. does not cause bogging. usually the opposite occurs with plunger stuck down- p.v. open and poor gas mileage.
He didn't say but maybe there is no fuel filter and slugs of residue from tank plugged jets in both carbs ? Long shot but I've had bowl residue cause bogs.--=Dan

I am trying to understand how the power valve actually operates. I had a piece of plastic (I have not idea how it got in there) plug the main jet in the Rochester B on my '51. It would still run but not well. Once I took it apart and removed the plastic it ran well. So my conclusion was the idle circuit was okay and the gas to get the engine to run was from the accelerator pump and power valve. My understanding is that as the vacuum dropped the power valve added more gas so the engine continued to run enough to get where I was going.

So in the case of hesitation (subject of this thread) if the throttle is opened quickly and the accelerator pump is adding a squirt of gas (apparently not enough but maybe too much????) why is the mixture so far off to cause the bog? Can it be too much gas? If the throttle is eased open it continues to run (abet not too well). Which means that the mixture is at least close.

Anyone else want to help us learn?

Hi Dan,

I thought about junk from the gas tank, but the AC filter at the carb is clean as a whistle and so is the screen in the glass-bowl fuel pump. I also put an in-line filter in the fuel line back by the tank and have changed that numerous times -- again, clear. Most importantly, there wasn't a speck of dirt inside either carb. FWIW, I attached my MityVac to the carb end of the distributor vacuum pipe and it held the same vacuum for 10 minutes before I gave up and disconnected it,; the distributor promptly returned back to the normal engine-off position.

I'll have to do the vacuum leak test as suggested above around the carb and manifold. I should probably also pick up a vacuum/pressure test gauge so I can see what's going on at the manifold. I used to have one years ago but a friend "borrowed" it and I never saw it again. (I'm sure every one of us has a tool story that goes something like that!)

I just had a similar problem with my '54 Rochester. Had sat for a long while and had some rust in main bowl. Went through it, cleaned, blew out passages, etc. Put back together but had a serious stumble on acceleration. Checked and had a stream of gas coming out of accelerator pump outlet. Still stumbled. Then I remembered that the last Rochester I rebuilt for a friend had the outlet port from the accelerator pump was completely stopped up. Took my cutting torch cleaning tool and CAREFULLY inserted the smallest one from the side where the spring and T shaped holder goes. Passed very easily. Went to next one, still passed easily, but see crud coming out of the passage. Went to next one, still passed fairly easily, but now LOTS of CRUD coming out. The next size up was tight so I knew I would remove metal if I forced it, so stopped right there. Put the top back on, fired up and runs perfectly. So, my conclusion is that even though the passage is open and you can see gas squirt out, it is corroded from the sides causing a restriction and not allowing enough gas to pass for the instant requiarement. Cleaning the passage to the original size resolved the issue for me. You might want to check and be sure your passage is clean to bare metal.

Ron

Ron's discussion about partial plugging of the accel pump circuit is something many of us have probably encountered and would likely affect off-idle accel performance. But on two separate 'rebuilt' carbs?
The vacuum gauge could tell you alot- that was the first thing Dad would use to troubleshoot. Just remembered an old experience. An old Buick sitting behind the garage I worked at was started up after a long while (years?) The boss spent hours messing with it trying to stop the problem you describe to no avail. We had it on the rack and for reasons I can't remember, disconnected the exhaust pipe at the manifold-- it ran fine. Turned out that the old muffler han corroded with pieces blocking the outlet. Our motors are big air pumps- constrictions preventing air in or out have big impacts.
Just a thought. Good hunting. Dan

I like this comment. I had a very similar issue with my '86 Cavalier Convertible where it sat off & on for about 2 years. When when I fired it up for the following summer...it bogged badly. I also noticed that the exhaust sound changed pitch too just like you described with your 216. I suspected it was a restriction in the exhaust and took it to the local exhaust shop...and sure enough a baffle had come loose inside the muffler. They replaced it and it was as good as new.

I would guess that if your Chevy has sat off & on and there was moisture in the muffler, you could have a baffle or some packing inside the exhaust that's the cause of your hesitation. I also imagine maybe a mouse or other little critter could have made a mess in there and packed it with junk. Here's what our '47 Chevy's looked like when we got it. It was filled with a few pounds of seat material and dead mice.



Now I'm sure your exhaust is in MUCH BETTER shape than that, but its still possible that its plugged or a baffle came loose inside the muffler. If there's a way to pop your muffler off and run it to see if you get the power back, that would be a good way to test it. I think shops may have a backpressure test they can do too.


I think you're on the right track. I would also check fuel pressure, check the point gap again, use your vacuum gauge to make sure that's hunky dorey and then move to exhaust.

I agree with Scott.........sounds better too

Well, you've all certainly given me plenty of stuff to check (and re-check!) and a few things that hadn't occured to me, i.e. the muffler -- still clean and bright on the outside and no indication of leaks or rattles from the interior, but who knows what lurks inside.

I picked up another vacuum gauge on the way home from work last night; hopefully I get the time to use it this weekend, and try out the numerous suggestions you've all given me!

Thanks to all,

I didn't read through the posts but is the ball in the accelerator pump missing or loose. You should hear it rattle when you shake the plunger. Is the accelerator passage check ball the correct one and in place?
Is the power piston free and working and is its spring in place?

Hi Gene,

With the original carb, which I rebuilt myself, I can answer "yes" on both counts, however I can't say for certain about the professionally rebuilt carb that's on the engine now. The rebuilder I got it from both flow bench tests and then engine tests his all his work; you'd think that would all but rule out such internal problems -- particularly the engine test. However, since I'm still having exactly the same problem, I suppose I had better pop the cover and have a look to confirm things for myself. Given the ample stream of fuel expelled at the accelerator pump opening, the pump check ball will probably prove to be in proper order, but as for the power piston.... It's a shame there's no way to visually check a power piston while in actual operation!

Regarding the check ball in the accelerator passage, this is the '52 and later Rochester B in which they eliminated the check ball that goes in the pump well. Originally, I thought this might have been the problem, but I found out about the difference between the '51 and earlier and '52 and later B's from both the shop manual and the Rochester manual online at the old chevy manuals website.

By the way, if worse comes to worst, does any one have a good suggestion for freeing up a sticky valve or valves? I'd rather leave pulling the head as a last resort.

Thanks,

Find the thread about bill Barker's new find. This subject has been covered quite well in that thread.

Somewhere around HERE will get you started.

Ray,

Thank you for the tip on that discussion. As you said, they're covering that subject very well.

Thanks,

You would be surprised how much information is in the history of this Forum. I would be amazed if a subject has not been covered in detail. The secret is finding the information.

Hi all,

I just wanted to post an update. I got myself a vacuum gauge and ran the tests for idle vacuum, back pressure (i.e. restriction in the exhaust) and vacuum drop and recovery in response the throttle. Except for a significant response to the bog, which follows the reaction of the distributor advance exactly, everything seems to be normal. The gauge needle wanders at idle by less than an inch either side of 18 inHg and holds fairly steady at whatever reading I get at various engine speeds from idle all the way up to the highest steady RPM I dared hold it at; this leads me to believe that I probably don't have any sticking valves. I did the carb cleaner test around the intake gasket and carb gaskets -- again, no resonse from the engine one way or the other, so there doesn't appear to be a vacuum leak. I also checked thing with the wiper hose connected and then disconnected and the manifold opening plugged; all checked out fine.

I also checked the fuel pump for pressure and flow -- the pressure was 5 psi and the pump delivered just shy of 15 fluid ounces into a gas can in 30 seconds -- more than enough in both cases.

The one test I did (at the risk of losing my mustache, beard and eyebrows!) that made any sort of difference was spraying a shot of starting fluid down the throat of the carb at the same time I whacked open the throttle. If I got the timing and amount just right, the engine responded the way it should to sudden opening of the throttle. Strangely, the accelerator pump still seems to be working properly, so I'm beginning to wonder about the power piston. I've already sent the one carb back to the guy who rebuilt it, and this weekend, I'm taking apart the one I did myself for a thorough check. If I have any success, I'll let you all know!

Have you verified the vacuum advance is working properly, as tested with a vacuum pump?

Have you verified the distributor weights and springs are not binding / sticking?

Is there a defective / broken dist. weight spring?

After a bit more fiddling around with this carb, it seemed to me that I needed to move the throttle a bit more than I suspected was necessary to get any fuel flow from the accelerator pump outlet. Also, I was still suspicious of the power piston since that's the one part you can't put eyes on while the carb is assembled and running.

At this point, I did LAST what I should have done FIRST; I boxed up the #@*! carb and sent it to Doyle Stokes, our VCCA tech advisor on single barrel carburetors, along with a long note explaining the problems I was having. After receiving the carb and looking it over, Doyle called me and told me the problem was two-fold.

First, the accelerator pump, though brand new and properly assembled, wasn't working at 100% because the cup fit a bit too loose in the bore. He told me that in his experience, with the aftermarket kits we have to work with these days, with the earlier Model B's he often has to test-fit a number of pump assemblies into a carb before he gets one that works the way it should. This has to do both with wear in the pump bore of some carbs, and with the somewhat sloppy tolerances of the non-USA parts found in some of the available kits.

The second problem was a bit of a mystery, since this was a NOS carburetor, still in the original carton, which I purchased in 1995 from a reputable and well-known Chevy parts vendor. As far as Doyle is concerned the "mystery" is why this carburetor ran so well prior to the recent problems I've related in this thread. The piston appears to have been a defective part from the factory that shouldn't have passed inspection. The pin at the bottom, which depresses the power valve ball, was too short to properly open the power valve. Additionally, the shaft was slightly bent, which was causing the piston to bind occasionally, depending upon what position it had rotated to.

I received the carb back from Doyle yesterday evening, put everything back together this morning, and then went for a test drive. The car now accelerates smoothly in any gear at any speed, even after allowing the car to drop almost to an idle in high while going uphill.

Thanks to everyone for their help and suggestions, and especially to Doyle Stokes. If anyone you know is being driven crazy by a problem with a Rochester or Carter single barrel, I highly recommend Doyle.

Thanks for sharing your positive experience with us and following up to your original post. I'm glad your Chevy is purring like a kitten again thanks to your work and the work of a fellow VCCA member who knows his stuff! Happy motoring.