The Stovebolt Site Forums The Shop Area Engine Shop carb hesitation of idle

Hello-

I have a rochester carb of a 1954 Chevrolet, I have rebuilt it and every thing works great except a hesitation or hiccup right off idle. At first I thought it was the vaccum advance changing the timing to quick. I have since disconnected that temporarily to isolate the problem. I have come to the conclusion it is the accelerator pump is not giving any gas right off idle. I have bent the connecting rod linkage to the accelorator pump to try to remove any slop and possibly move the accelorator pump down the bore a little to accomodate some wear in the bore.

No luck, the blip or hiccup is large enough if I don't feather the pedal through it, the engine will die. Idle is great along with the rest of range of throttle.

I have not messed with the spring and ball on the outlet for the accelorator pump yet, is that the cure??

Any suggestions are appreciated.

Thanks,

Brent

If it was me I would do the following. Remove the top of the carburetor and check to see if the accelerator pump check balls are seating for one and not allowing the fuel to return to the fuel bowl instead of coming up to the discharge nozzle. I would also probably have not bent the accelerator pump so the pump was further down in it's well but would have raised the fuel level a little more so the fuel level is higher in the well and thus allowing more of a squirt.

Joe,

I have checked the check balls to see if the inlet ball was sticking and allowing the fuel to return back to the bowl or the outlet ball not opening to allow fuel out. They both seem to be working. It is intresting on your concept of the float level. At one time I had it to high, thought it was to spec but it started weeping between the top gasket. Maybe I set it to low, I thought if I was to low I would start starving on more on wide open throttle.

I will have to double check float level.

Any other ideas out there while I'm digging into this?

Thanks,

Brent

The float level is critical, as it affects how much venturi vacuum is necessary to make the transition from the idle circuit to the midrange and high-speed systems. The gasoline will find its own level in the carb body, filling the passages with liquid gasoline, which is then atomized as it enters the airstream through the venturi. A too-low float level results in a flat spot just off-idle, as the idle system can no longer support the engine's fuel needs, and the high speed system hasn't gotten into full operation yet. Try tweaking thd float up a little at a time, and stop just before the fuel level gets high enough to trickle gasoline out of the venturi at idle speed. Also, try to determine if the carb part number is correct for your engine. Similar-looking carbs often have very different internal calibration, and all the tinkering in the world won't help a carb that's being used for the wrong application!
Jerry

Have you tried leaning the carb Air fuel mix screw in a little?...

Been my experience, a Rochester B is nortorious for off idle hesitation. Does your intake heat valve work properly? a little additional help on fuel vaporization can make a big difference.

All the above _plus_ the venerable " B " series carby is _supposed_ to be damp at the air horn gasket , they were when new , the only way to stop it is to use Permatex # 1 on the gasket , a very bad idea indeed .

Better raise the float level to the spec.

Tight valves will also cause flat spots so check and adjust as needed , engine _hot_ , not warm .

All,
I have removed my exhuast heat riser flap, it was froze solid. I can live with the cold driveability issues with that removed. As for the damp horn gasket, it was more like a fuel leak. I will re-adjust the float level in the next day or so and give it a try. I appreciate all the comments and more is appreciated to help me resolve this. I will give an updated soon.
If the rochester B carb has this problem, has anyone gone to a carter for a 216? I would think the cfm of the carb would be limiting and I'm not sure on bolt spacing.

Thanks,

Brent

I am in the middle of this dilema right now. I'm having problems with my B and I may have found a a Carter YF calibrated for a 216. I'll let you know how it turns out in about a week. Do a search for carbs, carb problems, Rochester B etc here on the bolt and see what you turn up. There is a lot of discussion on this issue.

Well,

I have raised the float level some and it seemed to work better, the hiccup is still there but not as bad. I'm starting to have the gasket between the carb horn and body weep alittle more than I would like. Is there any suggestions on the leaking?

Thanks,

Brent

I amn hopeful that I will recieve my new YF by Friday, so I have to wait to post how it works. On my B I tried to very carefully put a very small bead of Hylomar on the gasket between the halves. Diaster! Despite my best efforts some of the gasket sealer got into the carb. Had to completely disassemble and clean up. I did smear a little on the outside of the carb between the 2 halves, it seems to have stopped the seep on the outside, cosmetic at best, as it doesn't solve the internal problem. I have read somewhere that you can driil and tap corresponding holes in a piece of flat stock, bolt the carb up tight and bake the warp out of the B. Haven't tried that yet!

You guys are amazing! I have had the same hesitation off idle problem for the last two years (even after rebuilding the Rochester carb, checking my timing, replacing my vacuum advance). After reading this post, I rechecked my float level, which was just a hair (maybe 1/32) below specs. I adjusted it to 1/32 above specs, and my hesitation disappeared immediately.
Thanks.

In the tech tips section of inliners.org. there is an article on multiple rochesters that deals a lot with off idle hesitation with some pretty detailed info.

Try setting the float level to specks & change the main jet .002 larger.