VCCA Home Forums TECHNICAL - 1945 AND NEWER 1946-1948 My 48 is surging at speed.

I tuned up my 48 and installed a rebuilt Rochestor carb on her this Summer. She's been running great. I took her to a show this morning, and, the trip involved about a 20 mile interstate run. I got on I 64 and ran her up to about 60, and, she's surging noticeably. Below about 55, it goes away. It feels like a high speed plug misfire, but, what else could cause this? When I got back to my town, She runs up through the gears just fine, but, I never got over 35. I'll pull the plugs out this week and inspect them, but, they're new Champions. Appreciate any advice you can give me.

Regards:
Oldengineer

Bad spark plug, plug wire, points (or set too close), condenser or the last thing, the coil.
Weak fuel pump or junk in gas line.

Thanks for the help. I know the gas line and tank are OK, plus, I've got two clear inline gas filters on her, one in front of the pump, and, one ahead of the carb. They're both clean. I'll get busy checking the other stuff you listed. Thought I'd also pull the valve cover and see if everything is OK as well.

Regards:
Oldengineer

Sometimes too many gas filters will cause a restriction at high speeds. Try removing one of the gas filters and see what happens.

Does sound like starvation for gas. While you could blame ethanol, I suspect that the trouble may lie elsewhere. You've had some good suggestions on where to look.

How about floating points? Gene mentioned points but they may look ok while having lost strength in the spring. How about float setting? (Actually, I'm not sure that could explain it.)

Did it run faster coming home than going? Maybe it has a personality and wants to act like an old mule. Try attaching a carrot by a string on a stick and mount it where it will stick out beyond the grille a little ways to where both headlights can see it real good. Should improve performance both ways. For best results Instead of a carrot, substitute a quart of Mobil One. (Sorry, I couldn't help it)

After you have it figured out, please let us know what the problem was.

Charlie

More often I have found surging or loping at speed to be overfueling and not under. Reducing the jet size in the carburetor would be an option for me to look at.

Just did a '31 that I had to replace the metering rod and main jet so it wouldn't pop on high engine speeds. Both were worn just enough to cause a problem.

Charlie:

I'm half Italian, so my 48 responds better to a Pizza hung out in front of her headlights. My prime suspect right now is her fuel pump. When I changed the fuel filter on the suction side of the pump the other day, the gas in the glass bowl on top of the pump tried to drain out through the suction line. Wouldn't that mean one of the check valves in the pump is leaking? She's got a fresh set of Echlin points in her - I haven't had a chance to hook my dwell meter up and check to see if setting is OK yet.

Regards:
Oldengineer

Did you install the arm spring and not just install them using only the copper weak ground spring that is attached to the point set.

Chevy Nut:

I installed both springs when I put the points in. I knew weak copper spring wouldn't get the job done by itself.

Regards:
Oldengineer

After work, I took out the fuel filter on the suction side of the pump, and, checked the dwell to see if it was OK. I then headed off to a cruise-in. She ran perfectly on the back road I took. On the way home, I decided to take the interstate, and, see what she'd do. What she did was march herself right up to 70, and, I backed her off to 60 - ran nice and smooth. She's got a lot more top end power now. Thanks for your help - You gave me excellent advice.

Regards:
Oldengineer

Today I scored a back-up set of points and a proper Fram gas filter for my 48 at Autozone. I replaced the small clear plastic gas filter in front of the carb with the larger Fram metal can unit. Just to make sure I'm not restricting her gas flow.

Regards:
Oldengineer