Engine Vibration

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Engine Vibration

Postby doan » Tue Nov 08, 2011 2:02 pm

Now that my car is legal, I've been driving it a lot more. I was noticing what seemed to be like drivetrain vibration that I think is more engine related, mainly in the 2500 to 3500 RPM range. There are times when it seems to resonate more than others. While cruising at say 40, if I push in the clutch and let the engine drop to idle, the vibration goes away. You can also definitely feel the vibration when just sitting still and revving the engine through 2500-3500.

My car has a stock 948. Is this normal for these engines? I don't really know what to expect for a 50 year old british 4 banger. I was raised on Japanse 4's and big american V8's.

Thanks,

David
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Re: Engine Vibration

Postby Nick » Wed Nov 09, 2011 12:52 am

Not the norm....what has been done to the motor?...did you do any clutch work and remove the flywheel?..have you done any investigatory work.?...compression test?..leakdown?..scope? Nick C
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Re: Engine Vibration

Postby Charlie Tolman » Wed Nov 09, 2011 1:20 am

Several years ago, in my '63 Sprite that was modified for road racing, I sometimes would have significant engine vibrations in the high rpm range.
This was caused by broken external straps on the clutch pressure plate, and created an unbalanced condition. Downshifts that were harsh (not smooth) would bend and break the straps. Of course, it was very difficult to engage first gear.

This might not be the same problem you have, but it could be clutch related.

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Re: Engine Vibration

Postby doan » Wed Nov 09, 2011 9:40 am

I've not done any major work to the engine. The engine was rebuilt and clutch replaced by the PO about 9000 miles ago in 1985 (I have the receipt from the shop). It sat in a garage for about the last 10 years. I got it out this summer and rebuilt the brakes, carbs, and ignition. Compression test shows 165,170, 170, 175. I did not do a leak-down test.

The vibration is not really bad, but makes the glass on the gauges buzz. Some days it is worse than others.

Thanks,

David
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Re: Engine Vibration

Postby Nick » Wed Nov 09, 2011 6:01 pm

Those numbers are good. Does it also make the vibration in neutral without the clutch being depressed?..and has it always done it or is it a new problem?...I am trying to determine if it is an engine balance issue or if its fuel/spark related...Nick C.
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Re: Engine Vibration

Postby doan » Wed Nov 09, 2011 6:09 pm

Yes, you can feel the vibration in neutral with or without the clutch pressed. I've only had the car since late July. It's been drivable for about a month a or so. It takes a while to get used to all the noises of this car vs. a modern car. I've been fixing a lot of stuff, now I'm down to the finer stuff like vibration, rattles, etc.

Others have suggested that it might possibly be the water pump.
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Re: Engine Vibration

Postby Nick » Wed Nov 09, 2011 8:11 pm

It could very well be....that would be an easy check by removing the belt and spinning ..the water pump and dynamo....I was also pondering the thought that the person who built the motor did not line up the 1/4 balance points on the flywheel...have heard of the issues you mention when that step was overlooked during a rebuild..Nick C.
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Re: Engine Vibration

Postby doan » Wed Nov 09, 2011 8:58 pm

Thanks, I will check the easy stuff first.

How smooth should it be at say 5500 rpm? In 1st gear above 5000, the gear shift vibrates A LOT, more like a rattle really, like the shifter is about to jump out of the transmission.
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Re: Engine Vibration

Postby Nick » Wed Nov 09, 2011 9:32 pm

While you are at it have you checked all of your mounts?...in the tunnel (3)...motor(2)? and while you are under there check the routing of the exhaust pipe ( make sure its not binding or hitting)....then the spring washer setup on the shift housing ?....no it shouldnt vibrate that bad even at 5k in 1st, when you are cruising you should be able to put your hand on the shifter and just feel the engine's humm....granted at 5k in 1st its screaming however, it shouldnt feel like its ready to come unglued ...Nick C
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Re: Engine Vibration

Postby Nick » Wed Nov 09, 2011 9:44 pm

I run a Supercharged 948 with a close ratio box...and it doesnt feel like its coming unglued at all...and I can wind it up tight and not get that kind os a vibration..EVER...so check out the things I mentioned and it should tell you a lot..plus give you peace of mind knowing what IS sound, so if none of that sorts out the issue we can press on and look elsewhere....Nick C.
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Re: Engine Vibration

Postby doan » Wed Nov 09, 2011 10:09 pm

Thanks again Nick... Hopefully this weekend I will have time to check all the stuff that does not require pulling the engine.
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Re: Engine Vibration

Postby Nick » Wed Nov 09, 2011 10:17 pm

Dave,
Even the marks on the flywheel can be checked with a mirror and a flashlight thru the starter aperature without removing the engine. Just put the engine at TDC remove the plugs and starter....back up the motor by hand while looking at the edge/tooth side of the flywheel..and you should see the 1/4 mark line up in the aperature with less then a quarter of a turn..Nick
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Re: Engine Vibration

Postby Charlie Tolman » Thu Nov 10, 2011 12:45 am

Nick is on the correct path of trying to determine if the vibration is due to an engine balance problem or to a engine misfire condition.

I would be temped to remove a spark plug wire, check the severity of the vibration, and repeat for all four wires, one at a time. Does the vibration increase significantly with the removal of a wire? Does the vibration increase a little, or not at all, with the removal of a wire?

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Re: Engine Vibration

Postby doan » Sun Nov 13, 2011 11:26 pm

Troubleshooting the vibration got sidetracked by having to replace the fuel pump - converted to electric. I did check the water pump and generator, both seem to be OK. While I was in there I replaced the 20 yo belt.

The motor mounts are all snug. I'm starting to think it might be misfire related. While idling, It did pull each plug wire one at a time. At first the back two had less effect than the front two. I tweaked the idle screw on the back carb and now pulling any wire has the same effect. When I have more time, I need to go through the whole carb balance procedure again.

I want to repeat the spark plug wire test per Charlie's advice at speed. While driving around tonight, it seemed like the vibration got worse when the engine was completely warmed up with the water temp at about 180 degrees.

I still need to dig into checking the 1/4 marks - I'll have to get the manual out to see what they are.

This is not a huge problem, the car is very drivable. I'm just trying to learn about what's normal and not normal for a 50's technology engine. I wish I had another sprite to drive for comparison.

Thanks,

David
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