So I finally get some time to mess with my new project this afternoon. After some carb and timing adjustments I was able to get it to start easily and idle good. Then I noticed that when I gave it some gas and it would come back down to idle, it would have a bad vibration around the same rpm, but would stop when it got back to idle. Also it seemed like the vibration would sneak in and sneak out radomly while at idle. What could it be? Where should I look first?
Give it a tune up and double check the motor mounts, also it wouldn't hurt to look at the fan clutch while your in there.
sounds like a usual engine miss my 425 had one went to a pertronix ignition then had idle and run issues ..finally got ahold of dave's small hei and man ..it runs like a raped ape !!! he takes your stock dist.remans it builds an hei that fits under your stock cap and guarrentees it..it all looks oem..car has never run this good...also timing these things are horrable..if you get lost or need advise on where to get parts i have a few numbers..good luck..ROB
Take a close look at your fan. One or more of the blades might be cracked at the rivets and is loose. You may have to take it off for a thorough inspection. You can take the belts off temporarily and run it for a short while too. Bob
Well, after a closer inspection we determined that the fan is good, and the balancer is good. We did find that the flex plate wasnt lined up correctly with the fly wheel. We fixed that and although not as bad, it still has a vibration. Now we are looking at the torque converter. My mechanic that is helping me with this problem says the only way to prove it is to buy a new one and put it on. Which leads me to my next question. Where can I find a torque converter for this car?
I have no idea what year or model of carb you have, but some older O.E. carbs had a "dashpot" which kept the throttle blades from closing too quickly when you suddenly lifted off the throttle. Yours may need replacement or adjustment if so equipped. Without it, on a quick return to idle, the engine could stall or develop a miss for a few seconds until the idle circuit stabilized and took over. I think there was a tendency for some carbs to be a little slow about stopping fuel flow thru the main nozzles when transitioning from cruise to idle conditions.
I have an aftermarket Edlebrock carb. I can tell you that when driving the car the vibration is just as bad, and never goes away, untill it gets back down to idle
when its running at idle slowly advance your idle and watch your crank pulley ....i had a bad vibration at 55mph that shook the whole car and found my crank pulley is bent got another set from flat top bob in texas ..waiting on a nice day to put it on
One more thing to look at that doesn't cost anything. Put a wrench on the bellhousing bolts and make sure that they are all tight and all there. I've worked on more than one car that had a loose bellhousing. Before pulling it apart I'd do as Saltflats suggested and pull the three converter bolts out and slide the converter back in the pump and start the engine. If the vibration goes away you have a converter problem. If it is still there you have a problem somewhere on the engine. Eliminate the possibilities as much as possible before pulling it all the way apart or spending a bunch of money. I'd look at that flexplate real good too as they do crack quite often.
I changed out many a cracked flex plate, usually for noise on our Buicks. If you have clearance you can put two long 3/8 bolts in the bottom bellhousing bolts, drop the driveshaft, take out the crossmember bolts from the frame rails, disconnect shift linkage, undo converter bolts remaining bellhousing bolts, and slide the trans back. Then use a long combination wrench (11/16 I think) to unbolt the flex plate. Be sure to mark the bottom of the flexplate so you can line up the new one, it only goes on one way.
Sorry, its a 425. The car doesnt have the th400, it has the dynaflow trans. Thanks guys for all the help. I need all I can get.
I had an old carter carb that the metering rods fell out and into the bowl, it would crank and run, but shook and rattled at idle (very bad), like you, I check rotating things, like harmonic bal, clutch and so on, even replace a few things before I found out it was the carb.It shook at 850-1000 rpms then leveled out just fine, and even pulled good under a load. I reinstalled the metering rods, and the problem was gone. Good luck. Godspeed MrC.
I assume this below is what you lined up? On a nailhead, the flexplate will have a hole through which you line up a dimple in the hub of the crank. There is no particular pattern for the bolts to line up to bolt on the flexplate, i.e. they're symmetrical, so all the holes line up in any rotation. If you bolt it up out of balance, it'll shake your fillings loose. When I rebuilt the 425 in the '66 Riviera I owned, I learned this the hard way. I just thought I got lucky the holes lined up on the first try.
If it's a balance problem it should come in multiples of RPM. Example 800-1600-2400 although the amplitude will change. Words from EngineBuilder.com The important thing an engine builder looking at this powerplant should do is find the correct year before starting the rebuild. “Certain engines cannot be used in some Buick cars because of bellhousing, engine mount, block, oil pan and crankshaft changes,” Martin explains. “Plus, since all Nailhead engines were externally balanced, mixing dampers and flywheels with some models is not going to work.” Getting your balance with a the Buick is critical. “Because all Nailheads are externally balanced, the flywheel/flexplate must be installed with the index holes lined up,” Martin explains. “They can bolt on six different ways but only one way will be vibration-free! The 1957-’66 Engines MUST have their dampers tightened to 225 lbs ft. or the crank and damper will be damaged. They are a light press fit so the bolt must be this tight.” Nailing Down The Buick Nailhead
Thats what it does when its in park. The vibration comes in and out at a certain rpm. When I am driving it, it seems like the vibration never goes away. We are pulling the converter off today, and I am taking it in to get rebuilt tomorrow. Hopefully when the rebuilders take it apart they will be able to find something wrong with it.
Well, I got the trans pulled yesterday, and found out the trans mount was completly shot. After checking all the local parts shops, we discovered that the mount is obsolete. After a very stressfull internet search, I found a company in Arizona that rebuilds them. He isnt cheap, but what other option do I have? The name of the company is GM Obsolete. I ordered the mount from them, so it will be headed my way as soon as he recieves my core in the mail. Here is his website. http://www.gmobsolete.com/
Oh yea, I also dropped the converter off to be rebuilt. When they disassembled it, they found that the shaft bushing was worn pretty badly, and also one of the small springs was missing. They found some traces of metal shavings, and figured that was what was left of the missing spring.
A bad transmission mount will easily give you a random vibration, be nice if you could have isolated that first, oh well... the main thing is between the mount and converter, it should do the trick. Bob
The Jalopy Journal
