The Infernal "Flat Spot"

[highcking]

I've finished going through the engine on my '58 Roadmaster. By that I mean most ignition parts have been replaced, timing checked and rechecked, Rochester carb rebuilt (by a Buick club member, not one of the well-known outfits), new fuel pump, filter, etc. Vacuum checks out at 14 inches (correct according to the shop manual). Compression is down from factory specs but not bad, around 140 - 150. The car now starts well, idles well, and is smooth at high rpms. No overheating. I checked the exhaust and it shows right between rich and lean as it should.

What I haven't been able to solve is the flat spot at partial throttle, just a bit above idle. In Park the engine does not run smoothly at this throttle opening, and under driving load it is even worse, lugging and almost stalling. Higher throttle - it smooths out and roars. I've checked vacuum advance and it is working just as it should. A vacuum leak is still possible at the carb base, but I've run all the usual checks and don't see any evidence of it (or any sound). The exhaust check would have shown a significant leak.

I'm back to suspecting the rebuild job on the carb. On the first round of the rebuild the carb leaked gas all over the place. After a second round, the leak was no longer present, but some other things weren't right (like missing choke gaskets). I don't have another carb known to be good to swap.

Ideas?

Bill Shields

[NTX5467]

Gotta be something in the idle circuit and "transition fuel" circuit.

With the carb removed and emptied, the transition "slot" is the vertical slot just above the idle hole in the carb's throttle bore. At curb idle, there should be about .020" of the transition slot below the throttle plate, visible.

If you have the throttle cracked open farther, it will get more transition fuel and cause driveability issues, including hesitations until enough airflow exists to pull fuel through the carb venturis, at which time the "main system" will be the primary fuel metering circuit. Or if the idle circuit is flowing poorly, then in order to get more fuel, the throttle plates are openned farther (to get some transition fuel) and then have a hesitation as there's not enough air flow to get fuel into the venturis just yet . . . AND a weak accel pump might not be strong enough to cover that particular transition period.

It could well be that there are some residual deposits in those fuel passages which normal carb cleaning solutions won't and can't remove. To effectively remove them will take a deeper knowledge of the carb's various fuel circuits and how to get to them rather than just "kitting" it. Is the idle speed correct AND is there sensitivity in the idle mixture screw adjustments, side to side?

On many carbs of that era, there were "large tubes" and "small tubes" whick hung down into the float bowl from the venturi cluster. The smaller one is the idle fuel with the larger one being for the main system fuel. There can be some hidden orifices in the bottom of the smaller tubes, as an "idle feed restriction". When THAT fuel is restricted, it decreased ALL fuel to each of the idle circuits. I'd suggest you get some twist drills and start probing the bottom areas of those tubes, carefully, starting small and going up in small increments until you "get brass", then stop and spray some carb cleaner through the tubes to help flush them out. Then reassemble and see if things don't work better. I had that very issue on a Carter BBD 2bbl . . . when I got it mechanically cleaned, then the idle circuit worked again.

As for the accel pump, it's been mentioned many times that using a leather-cup accel pump is better than using one of the later "rubber" pump cup accel pumps.

Please keep us posted on your progress.

Just some thoughts,

NTX5467

[highcking]

Many helpful suggestions - thanks for taking so much time. I can certainly follow up on them - but I did spend some $300 on the rebuild that was supposed to be "correct." One curious aspect is that the "flat spot" is not momentary as it might be if the accel pump was faulty. I did have to correct the hole the pump arm was in and also adjust the rod itself to correct specs. It spurts adequate gas into both barrels. But the hesitation continues under steady part throttle, for any length of time you stay at that position. This suggests that the problem is in carb's transition circuits - it certainly feels like the mixture is off at that point. I'd love to swap the carb for another one and see if the problem changes or goes away, but so far no luck on that front. I'll post again when I have something to report.

[Smartin]

Have you made any adjustments to the idle mixture screws? I had this exact problem on the way home from South Bend on Sunday, and I backed out the two idle adjustment screws a full turn and it really woke it up.

[highcking]

I am going to try the idle screws first - easy to do. I have them set for maximum rpms but that may not be right.

Have you made any adjustments to the idle mixture screws? I had this exact problem on the way home from South Bend on Sunday, and I backed out the two idle adjustment screws a full turn and it really woke it up.

[Smartin]

Make sure it is really warmed up well before adjusting.

[highcking]

I tried the mixture screw method but did not get any real change. Here is another data point. If I open the throttle to the bog point with one hand and then cover the carb throat with the other, rpms go way up and the engine roars smoothly. Take my hand away and rpms drop way down. To me this means it's running way too lean at part throttle, more than the idle jets can compensate for.

[old-tank]

When I had a Rochester on one of my 55's, I was advised to lean out the idle mixture screws and it helped. I finally replaced it with a Carter because every time it dried out from disuse, a white fuzz clogged the idle and transitional circuits.

Willie

[Smartin]

Pinch the vacuum line closed that is going to the brake booster and see if there is a change.

[highcking]

That's easy enough to try. I already checked the vacuum advance - it does not leak.

Pinch the vacuum line closed that is going to the brake booster and see if there is a change.

[highcking]

Ok - several folks suggested pulling the "rebuilt" Rochester 4gc and taking a close look at the primary throttles. I did this and found something I can't explain. One of the barrels seems to have no idle orifice. The other primary barrel does and there are idle orifices in the secondaries. I looked very closely to see if the orifice was simply obscured, but I don't think so. It's not there.

Anyone ever see this? Every 4-barrel I have has an idle orifice in each primary barrel. Otherwise why have two idle mixture screws? Could this be a factory defect? It certainly explains why the mixture is too lean until the throttle is open pretty wide.