"Stumble" at 4500 RPM

Stumble is a polite word- the car just flat out hits a brick wall and won't rev any higher. You can roll in and out of the throttle, and it'll rev back up to 4500, but that's it. Once in a while, it'll work through it and keep reving, but that's getting rarer.

It's a '86 Civic with the CVCC 1.5L. It'll do it in corners and on the straights, so I'm not suspecting fuel sloshing. We did a bunch of work to the fuel system over the winter (cell, new lines, carb rebuild etc.), but it feels more like an electrical thing to me- the engine doesn't bog, it just refuses to rev higher. If you upshift to drop the rev's, it's fine until you get back up to 4500. If you downshift to get the revs over 5000, it'll go all the way to redline. It's been getting progressively worse over the past couple schools/races/practices, and today it was flat out a pain to drive- we were doing all sorts of crazy stuff to shift around it. Timing (at idle) seems to be consistent day-to-day, but my guess is that it's the distributor. I'm trying to avoid buying a $160 part on a fishing expedition, but I don't really know what to look for inside there to replace, so all I can think of to do is swap the whole thing.

Any thoughts? Race weekend approacheth...

[This message has been edited by Dom Pirinchinci (edited May 30, 2002).]

Dom--You should suspect what you were tinkering with--FUEL--rather than ignition.

Sounds like that rebuilt carb but could be a fuel pump or clog issue.

Frankly, it is real tough to distinguish between a fuel and ignition issue, especially on a carb car. However, if it ran fine before you made all those changes on the fuel side, start there.

You may be able to hook up a readable fuel pressure gauge to see what its doing as you reach 4500 on the track.

Put it on the cowl, however.

Dom:

I had a similiar problem, but it acted as a studder around 5K. It turned out to be the electrical ground on the back inside near the hatch lock. It seems that ground wire was the ground for the fuel pump system. When I got up to high revs, the vibration would make it miss. I sanded the contact areas, tightened it down and the rest was history.

Thanks to Village Auto Works for that find..
Tom

Are you running the stock Honda 3BBL carb or a Weber? If you are running stock, is the secondary opening up? Do you have all of the old polution control lines plugged? Did you mess with that small screw that sets the position of the prechamber throttle plate? (Don't ever, ever, mess with this screw!) Will it rev past 4500 in neutral (no load?)?

The car had a stumble before, but that was a known fuel problem that the previous owner had warned us about (hence the change to the cell etc, and it cured the left-hand corner problems). The car is carb'd, but we run a Holley pump with about 100x the capacity we need, and regulate it down to 3.5 psi. And we didn't mess with that little screw. Bad things happen when people mess with that little screw. I'll try the rev'ing in neutral test tomorrrow at the track, and will rig up the fuel pressure gage when I do it. I've done a little too much "neighbor testing" lately to try it tonight. And the secondarys open- I've tested them with a MityVac, and made sure there was vacuum on that line.

[This message has been edited by Dom Pirinchinci (edited May 30, 2002).]