Hot Rods carburation help

Chevy 250 single barrel carb runs great at all speeds except when going uphill at speed.

It will start to missfire, if I let off the throttle it clears out until I press again and then it misses again.

Key points
It only does this on one large 7 percent grade. Normal hills and driving it runs out great. I live at 9000 ft elevation.
Carb rebuilt about a year ago.

Any thoughts? Could this be incorrect float adjustment?

 

Carbs are usually picked on the most with all engine performance problems and are in most cases not the problem.Have you checked your distributor shaft for wobble ? This will change your dwell and was a common problem with inline 6 - Vacuum advance - rubber fuel hose collapsing internally

 

Plugs and wires? Can u make it miss while power braking?

 

Maybe the fuel level is a little high? When going up a long grade the bowl is facing forward and the fuel would collect against the mainbody and the weight of the fuel will let more fuel into the engine than what the engine is calling for. The fuel would be dribble out in droplets and the engine would 'miss'.
Try to lower your float about .020 or so and see if there is a difference.
Or, you could just drive downhill wherever you go. Rt 81 is nearby, for some reason it is goes uphill in either direction. We locals have deep discussions on it.

 

^x2. Or it's dribbling out the bowl vent into the venturi.

 

try a new fuel filter first. The steeper grade will require more fuel,if there is a flow restriction,the engine will go lean.

 

My first thought is its a lean issue. If it only happens on a grade (and not on a flat road punching it) I'd surmise it's a low float level... on a grade and under acceleration its allowing the jet to become uncovered.

 

Totally agree but wouldnt the timing issue be evident at other times. This literally only occurs on one steep grade not on any of the other million hills around my commute. Thanks for the help

 

Interesting...I was thinking to high might perhaps your on to something. Thank you

 

LOL Im all for the downhill driving its the everywhere I go thats the problem. Thank you

 

no shes steady as can be but Im going to replace them as its time anyways. Thank you

 

Thanks everyone I will keep you posted once I tear into it....right now stuck turning wrenches on the misses car.

 

It could act this way if the enrichment circuit in the carb is not operating properly. Check to make sure that the power piston and metering rod are able to move freely thru their entire range of travel. Make sure the metering rod tip is not bent.

Does the distributor have a working vacuum advance? Is it working freely and properly? Do you have it connected to a source of full manifold vacuum or to a ported or off-idle vacuum source? Is the mechanical advance working freely so that the timing retards like it's supposed to at lower RPMs?

 

I'd say the float level is too low. I'd raise it a bit and try it

 

I had a similar problem with my o/t F-250, misfiring while pulling a heavy load up a steep incline, turned out problem was needle and seat was too small, was starving for fuel, changed to a larger needle and seat and the problem went away.

Good Luck
Lynn W

 

Does it puff black smoke when going up this hill? If so, it is runnin rich and the float is probably adjusted incorrectly.

 

A miss sometimes will only exhibit itself under a heavy load, I forgot to add if you have a subs***ute coil try hanging that on. Will you post the fix when found?

 

Look for a broken primary wire inside distributor. (inside the insulation)