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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.

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

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.

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.

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.

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

Thanks for the replies - that tells me what I need to know! Bill Shields

I need to replace the temperature sensor on my '58. A previous owner installed an aftermarket electric unit that I've removed. The original unit is still in place - the sensor is hanging loose near the distributor. The threading on the aftermarket sensor exactly matches the threading on the original sensor. It is clearly pipe thread, but I am puzzled by the fact that both fittings have four flat sides on the threads at 90 degrees from each other. It looks like the threads were ground down by a factory process. The fitting does not leak. Nor does a standard 3/8" NPT fitting screw into the block hole. What am I missing? I can't recall seeing a fitting like this, and I wonder why it doesn't leak. Bill Shields

Thanks to all who replied. It turns out that the critical dimension is the thickness of the drum flange that mates up to the wheel hub. The original drum is about 1/8" thick, the replacement about 1/4" thick. Good for sturdiness but that extra thickness eats up the portion of the wheel hub needed to mount the wheel. Really no way to machine that safely. These drums may well be a generic fit for other GM cars, but they cannot be used on '58 Buicks. Bill Shields

I recently purchased a set of rear drums from Kanter Auto Parts for my '58 Roadmaster. (My originals had been turned paper-thin.) The drums look good and have the right bolt holes, but there is a problem. The drums are about 5/32" deeper than the original measured from inner lip to the outside of the wheel hub opening. What that means is that when the drum is mounted, there is nothing left of the hub on which to mount the wheel. I've emailed Kanter - anyone have any ideas why this could be? It seems odd for a drum to be that far off in this critical dimension. Bill Shields Luray, VA

As many of you know, rear brake drums on 1950s Buicks are held to the hub by three small bolts. One of these bolts has an unusual long head that sticks out through the wheel when it is mounted to the hub. I am missing one of these odd bolts. Questions: 1) What is that special bolt called? 2) Is there a Buick part number for it? 3) Where can I get one? Bill Shields Luray, VA

John - I will email him.

John - I agree with that plan. These units are as rare as an NOS part for a 1910 Buick. Bill Shields

Leif - I certainly will consider that. The '58 unit is mounted on the firewall so you'd have to find a new unit that would bolt up there. So far I haven't found anyone supplying this for a 1958 Buick. Bill Shields

Doug - thanks. Even the power of Google is failing me. I'm assuming '58 must have been a one-year unit. The '57 unit is available though it looks different. I just can't believe the master cylinder doesn't interchange. I think I see a number on mine that I'm going to look at more closely. Bill

I am continuing work to get my '58 Roadmaster ready for safe driving. The previous "mechanics" made every possible mistake, used bad parts, etc. At the moment, the power brake booster works OK as does the master cylinder. I'd like to put a good rebuild kit in the master to be on the safe side. But I can't seem to find either a kit or a rebuilt master (or even an unrebuilt master). Advice? Bill Shields Luray, VA

Adam - that may be the only solution. It's nice to know that someone else has tried it with success. But what a nuisance!

Gents - as you know, plugs on '58s (also '57s and other nailheads) are recessed into the block. No problem - except that on my "new" car, a previous owner sprayed plenty of green paint into the plug cavities. Result is that none of my 13/16" sockets will squeeze between the paint and the plug body. I tried tapping the socket in with a small hammer - with predictable results. It was hard to get back out and hadn't touched the plug. The plugs fire, but I think they may have been in there a long time. Ideas? Bill Luray, VA

Bob - I plan to make a priming tool from a used oil pump shaft and do as you suggest. I'm intending to rebuild the distributor anyway.

Adam - it did look the same minus the vacuum unit. Apparently the 57-58 pumps were problematic, at least that's an often- expressed opinion. I can see how the vacuum part would fail, but to be honest the oil part looks just like the 59 unit.

John - I may have slipped up. It's been many years since I had an oil pump issue. The 1958 Buick Shop Manual's section on checking/replacing an oil pump makes no mention of priming. Now that the oil pan is back on ... would it be worth trying filling up the oil filter and overfilling the pan to submerge the pump? Pull the spark plugs to spin faster? I might be able to use the method of pulling the distributor to spin - but the distributor is so far back I'm not sure I can set my drill directly over it. The previous pump could have had a gasket failure - the gasket was in poor shape when I took it apart. That would explain a sudden loss of pressure.

The sending unit itself is the ground for the circuit. A ground wire goes from the dash gauge to the tank sender and from the tank sender to ground. The tank sender provides a variable resistance that is sensed by the dash gauge.

So, here's the story. I acquired my '58 Roadmaster a couple of months ago. After going over the car pretty thoroughly (and finding lots of things to fix), I decided it was OK to run the car and drive it short distances. The oil pressure gauge worked and showed, as expected, fairly high pressure after a cold start and medium pressure when warmed up. A 6-mile drive to and from a gas station at 50+ mph showed no problems with the engine. After that, I started the car occasionally to move it or charge up the battery. One day a few weeks ago, I backed it out of the garage at idle, and noticed the oil pressure gauge pegged at the low end. I thought about a gauge issue, but I started to hear valve and lifter clatter so I shut it down. So far I have done the following: 1) Dropped oil pan, cleaned the sump, disassembled original oil pump (with vacuum booster). The oil pump looked to be in very good condition - nothing obvious to explain a total failure of flow/pressure. 2) Installed a used '59 style oil pump that seemed to have almost no wear on the gears. The pump bolted in with no problem - the groove the the shaft seemed to seat easily in the fitting under the dstributor. (I assume if it didn't, you can't run the mounting bolts all the way in.) 3) Reinstalled the pan, installed a spin-on filter conversion kit and a new PH-30 filter. 4) Filled with 10w-40 oil and cranked for 10-15 seconds to get some oil pressure before starting. No pressure. 5) Unscrewed the oil filter and found it completey empty. 6) Unscrewed the high-pressure bypass fitting, found the ball and spring in good condition. (This would only kick in to keep pressure from getting too high anyway.) 7) Cranked the engine without re-installing the oil filter - nothing coming out. 8) The car will start if I connect the coil wire. I am stumped for the moment. I think both the original pump and the replacement are good enough to generate full oil flow and good if not perfect pressure. The gauge is fine. There is no oil flowing, yet oil pressure was originally right on specs. What to look for that would yield a total loss of pressure? I am hoping it doesn't have to do with the fitting under the distributor. Obviously the distributor is working fine as the car starts and runs. I thought about a blockage - but it would take quite a blockage to resist 40-50 pounds of pressure. Ideas? Bill Luray, VA

Chris - no AC, so I'm OK there. I appreciate the offer and will probably be taking you up on it. Bill

Chris - thanks! That's how I hooked mine up. But your picture begs another question - I don't think I have a relay in the place pictured. Do you know which relay this is? (I am strugggling to put the car back together from a previous unfinished restoration.) Bill