Buick v6 shaking.

[BobD]

Hi guys, this might be a long post. I have a buick 3.8 in my vega, block casting shows it to be 79-83, wiend single plane intake, edelbrock 500 cfm carb and hei distributor. After putting all new gaskets and new distributor ( previous owner has an oddfire distributor in it). Turned it to tdc on #1 cylinder, lined everything up and installed the distributor. I was suprized when it fired up no problem. With the vacuum advance plugged, I set the timing to 12 degrees btdc at 700 rpms. I only have a simple timing light without the dial on the back. I was all happy, let it run for a few minutes, checking for leaks, no milky oil. The head gaskets worried me the most. Then I started revving it up to about 1200rpms watching the timing go to about 20 degrees btdc. When i revved it up slowly to about 1500rpms it starts shaking til about 2000 rpms, after that it seems to get smoother. I tried advancing and retarding the timing but it still shakes in that rpm range. I did change the flexplate but you would think if it was the flexplate it would shake at all rpms. So I started to try to eliminate thing, I unbolted the converter and push the trans back but it still shake, a little less but it is still there. Then I took the belt off, still shakes. Unbolted the balancer, some of the rubber was cracked but not much, it does look like it wobbles a little at low rpms. I started the engine without the balancer on it , still shakes but not as bad. Next step might be to put the old flexplate on and try it but i can't just push the tranny back far enough to get at the the bolts.
Has anyone ever run into this problem before? I have read that these engines are externally balanced, I was going to buy another rebuild balancer but that just might make it worst.
Wow that was long!!! I have a to get a beer now. Any help or comments would be appreciated! After reading this i might have to buy you guys a drink!

[chevyart]

bob are you sure the newer flexplate was externally balanced(welded weights o it) just a thought. art

[cammerjeff]

From what I understand the only Buick Even Fire V-6's that were externally balanced were the 4.1's. all the even fire 3.8's I have seen have been internally balanced. How are the Motor Mounts? if the car has the T-350 trans the stock rear mount is not a good unit, it is not a fail safe design, and can fail and let the rear of the trans do what ever it wants.

When I had the shaking issue at idle on my 3.8 it ended up being a seized torque converter. Swapeed out the torque converter and cured it. But that does not sound like your issue. The shaking still could be an ignition issue, are all cylinders firing? Start with the simple things 1st.

[BobD]

Hi guys, thanks for the replies. Art, the flexplate i bought said it was externally balanced, no weights welded to it but alot of holes and one partially drilled hole. The picture below is the one i have. Jeff, I just put new stock motor mounts and an Energy suspension trans mount. I don't understand the whole externally balanced thing. Does that mean if I change the flexplate or the balancer, I would have to get everything rebalanced? I know in the crank counterweights there are some holes drilled in them. Thanks again Bob.

[vegastre]

I assume this is an odd fire motor since you have a odd fire distributor in it?

The Buick 231 is 30% externally balanced which is significant. Unlike most GM motors which can take a wide range of flexplates Buicks are limited to about six if memory serves and matched from the factory to a wide selection of dampeners. Mismatching these two items promotes this shaking. Some very noticeable, some not so much. The key is when replacing a flexplate on a Buick you match the holes up and not by weights. The holes are how the flexplate is matched to the dampener. By your picture you will notice a bunch of holes =) I assume the old flexplate was cracked? So, go get your original flexplate and go to a transmission parts jobber and they will have a selection of flexplates to match to. Don't go by part numbers or block casting or some such. Visual only and beware the holes may not match up perfect but should be close. May or may not have some odd small holes drilled here and there.

Caveat: If the holes in your original flex matches the new flex then you have no balance issue.

[BobD]

Hi Phil, no it isn't an oddfire motor, others on here have told me it is an evenfire, the previous owner had an oddfire distributor in it, I switched it over to an evenfire distributor. I still have the old flexplate, the only reason I switched it out was when cranking it on the stand it looked like it wobbled slightly. The new flexplate looks better but still has a little runout. When I was bolting the new flexplate up to the crank, I matched it up how the old one was on. I think the bolt holes would line up only one way. Maybe it is time to take to somebody. A friend of mine recommended Jensen Engine in Nescopeck, PA, only about 30 miles from me. He had engine work done on his vega by Jensen but his vega does 9 seconds in the quarter, mine sits on jack stands

[vegastre]

I think the bolt holes would line up only one way

Hmmm,
I didn't mean the crank bolt holes? Sorry =) I'm talking about the circle cut outs and half moon as well as misc holes drilled here and there. They need to be a match. I would just put the old flexplate back on. A little wobble is ok and even a little vibration is not unusual for a stock Buick V6. The only way to get rid of all the vibration is to have the rotating assembly balanced. Beentheredonethat. Even if you have the rotating assembly balanced you will still need the correct flexplate. Its one of those critical deals with these V6 Buicks.

BTW, you mentioned the balancer having an issue. The dampener is not as critical. You can go by part number on that if you need to replace that. They are a rubber sandwiched design and as they get old they do tend to slip and separate.

[Monza Harry]

I met a guy years ago and his 3.8 had an issue it acted like it was way advanced and retarded at the same time he didn't mention a shaking (he might not noticed it with spark knock and two fouled plugs though) after talking to him for a bit turns out he put the distributor out of his '80 Riviera into a '76 motor (even fire car odd fire motor) so I wouldn't assume it is an even fire motor with out confirmation. There are posts on here detailing the differences (CammerJeff as I recall had quite a bit of detail, I've only had an odd fire and that is a long, long way from smooth) GM ended up with a balance shaft in these motors to smooth them out in the end if I remember right, so smooth as silk probably not going to happen, but should be smoother than you are describing Good luck Harry P.S.and my car is offended by the "Jack Stands" remark. Don't worry about that though as my car is offending me at the moment, and it resembles that remark

[BobD]

I guess the next step is to go back to the old flexplate, after comparing them they are a little different, the new one has 3 complete large holes and 3 crescent shaped holes. The old one has 4 large holes and only 2 crescent shaped things. I can live with a little shaking but when it does shake, the whole dash, steering wheel, and the grill and valance shake like hell. What is strange about it is, that it smooths out after 2000 rpms. Here is a post about if the motor was an oddfire or evenfire,seems like the most people think it is an evenfire. The mystery continues! Thanks Bob.
viewtopic.php?f=1&t=33587

[vegastre]

The mystery continues!

The symptom you are describing is a classic imbalance in the flexplate/dampener. I guess the V6 never ran that smooth with an Odd fire distributor in an even fire motor. Unless you have a perfect matched dampener, crank, and flexplate you will get a ever so slight vibration at about 1500 RPM but will go away instantly as you accelerate. This is the reason they added the balance shaft later on. That said, I had my 3.8 professionally balanced and to this day has no vibration whatsoever. My point here, even putting the correct flexplate back on you might still get a little vibration but of course should not shake the dash and grill.

[OldsStarfire]

I've experienced 5 Buick V6s, 3 even fire and two odd fire and they all shake. Odd fire is worse, but not much. Automatics are less noticeable than the manual cars. I had a dealer mechanic from back in the day come up to me a car show and tell me how people used to bring their cars back all the time complaining the engine wasn't running right, but in reality it was just the nature of the 90° V6.

I have driven one other manual V6 that was reasonable smooth, but not like a V8.