VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc Getting the Wrong Vacuum Signal???

Here’s a toughie cuz it’s not about a completely stock Stovebolt. I’ve a friend that finished up a stock ’52, 3100 last year. The 216, even though he had gone thru it when he was building it, was giving him some bearing problems. Last fall he decided to put the stock ’55, 235 power glide engine in it that he had rebuilt.
The stock Rochester ”˜B’ was giving him head aches because of the all to common, “Rochester Stumble”. He had switched out the ”˜B’ on the 216 with a Carter W-1 for the same reason and we all know what a good carb the W-1 is and how well the
Stovebolt ran with them. So he went looking for a Carter that would fit the 235. As was suggested by several people in the know, the later Carter YF was supposed to be a good replacement for the ’B’ on a 235. So he found a Carter YF4480S, rebuilt it and gave it a try yesterday.
The problem that he’s running into with this carburetor is with the timing. Normally with a Rochester ”˜B’ at idle the vacuum port is in high pressure or blocked off so the vacuum advance isn’t getting a signal and the vacuum advance spring holds the distributor in it”˜s fully retarded position.
The Carter YF4480S has the vacuum port below the throttle butterfly exposed to the high vacuum and at idle holds the distributor timing advanced. You can block off the vacuum signal and set the ball on the needle when setting the timing, then all is well as long as your running down the highway. However, now at idle the timing is pulled to far advanced with this carburetor. Supposedly or according to the Carter books the YF4480S is the correct aftermarket replacement for the 235 Stovebolt.

Anyone have a comment on this or run into this problem themselves?
Denny Graham
Sandwich, IL

I never experimented with an "old" 6 cylinder but have converted small block 8 cyl. engines to full advance at idle. In fact in the late '50's/early '60's Chevrolet made them that way. They claimed cooler running at idle speed among other advantages. They changed back when the quest for cleaner emissions came along.
I can think of no reason not to have the full advance, in fact I want to experiament with it on one of my cars come Spring.

When I was raceing, I only had mechanical advance. I set it at 36deg. full advance. That is were an internal combustion engine runs the best.

That how my 1957 with dual 4 barrels was. No vac. advance and just a very "advanced" setting of the timing at idle speed.

Well this '55 Stovebolt runs very rough at idle with full advance. He's got the timing set to the ball without the vacuum advance. With it re-connected at idle his new timing light is telling him that it's advanced about 30°. That seems like about two or three times as much vacuum advance than it should be. I can't find any specs but I'm thinking it should be more like 8° or 10° the rest of it coming in with the internal weights as the rpm's increase to give you a total advance of 25° to 30° at speed.
The distributor has an aftermarket vacuum motor from Classic Parts of America and I'm wondering if it's just pulling the clamp way further than it should. He's 50miles away from me so I'm just throwing out ideas his way. I've suggested clamping off the rod to restrict the vacuum advance to about 10° and see how it runs there. He's gonna try that this afternoon.
If anyone has any info on the advance curve for the 50's truck engines I'd sure like to see it.
Denny Graham
Sandwich, IL

Vacuum control - Advance begins 7" to 8.5" with full advance at 18 to 20 deg. at 16" to 18"

Centrifugal advance = begins at 550 to 725 RPM, 32" to 39" at 3450 rpm. and up.

Thanks Gene, where'd you find those specs?? I'm assuming you meant 32° to 39° at full advance?
Denny Graham
Sandwich, IL

Oh, its just something that I always remembered
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
Its in the 1949-1953 shop manual.

Right, page 12-26 "Automatic Spark Control".....1948-51.
I was looking at the 1954 Shop Manual and it doesn't have those specs in it as far as I can see. I thought I'd seen them somewhere, guess I just need to look a bit deeper next time.
The '48-51 reprint doesn't give any vacuum readings where it begins to pulling in at and the 235 specs are a bit different, total advance being 29° to 33° at 3700rpm.
Thanks for jogging my memory, hate to think what it gonna be like when I'm REALY, REALY old.
Denny Graham
Sandwich, IL