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MG MGA - Australian harmonic crank balancer
|Has anyone bought the Australian made harmonic crank balancer from Moss. It looks to be well made but at close to $300 it's a little pricey.|
I have a 1960 MGA with the 1600 engine which has been bored out to 1622 + 30 and uses the 1622/early MGB rods and completely balanced.
The engine pulls great but there is an annoying vibration between 2400-2800 rpm. I am considering splurging out for the crank balancer but wondered if anyone out there has any experience with it.
|Andy Preston 1|
|Why not consider a used one. And are you also certain that the vibration in that range is not caused by predetonation down to lean running? I would try rich ending up the carbs a couple of flats first and seeing if there is any change, and certainly move to a richer needle than standard with that engine.|
|Dominic thanks for your response. You might be on to something there because the cylinder head is the stock 15 type which has smaller combustion chambers than the 16 type used on the 1622 engine. I calculated the compression ratio to be around 10.5:1. I have to retard the timing by at least 7 degrees or buy 100 octane gas from the local race track or the engine pings.|
|Andy Preston 1|
|There is your problem. With such high compression you need more octane and you need more fuel. If you are running the original needles you definitely have predetonation.|
I would suggest that as a minimum you update the carb needles to at least those for a 1622 engine. You would probably be much better doing both that and fitting an early MGB head (basically the same as a 1622 one)
And stop driving your car until you do something. Even if you can feel the vibration only in one range, it is happening at a lower level across the entire rev range. Predetonation eats pistons, so your nice new engine will not last long if you carry on using it in the current setup.
|I've got a similar setup to Andy with a 1600 engine and a standard head with the engine having been rebored +60thou which makes it around 1652 cc.|
I wondered whether to change the carb needles but in the SU reference catalogue it says that the same needle is used for both Mk.1 and Mk.2 engines so I kept them the same. At the time I didn't realize that the heads were different.
It makes me think now that I should have chosen richer needles. However, I don't get any pinking or vibration at all at any speed above idling and the timing has been checked on more than one occasion. The only thing I'd like to improve is the idling performance which could be a bit smoother.
Question is : are there any articles written about the need for changing needles after a rebore and by how much they should be changed. It would be nice to have a reasonably accurate estimate bearing in mind the hassle of removing the carbs,etc.
I would think there are many MGA owners out there with a rebored engine still using the original needles.......................Mike
|1622 bored out 030 brings the engine to about 1650cc, which is more than 10% over the original 1489. That means more fuel is needed to avoid running lean. High compression, poor quality fuel or running lean are all prime causes of pre detonation. With probably both at work, you are indeed likely to have trouble. (all this is standard stuff to consider with supercharging too!)|
The 1500 uses a GS needle as standard, CC as rich, 4 is lean.
The 1600 and 1622 use a 6 as standard, AO is lean, RO is rich.
As you can see from the picture, the various other MGA carb needles are basically the same in the lower area, but in the higher positions (i.e. the areas used under full throttle) there are quite marked differences. That's enough to get you into trouble with the wrong needle, especially as with high compression you are already in pre detonation territory.
You can make other comparisons by using the excellent http://www.mintylamb.co.uk/suneedle/
The fact that 100 octane helps is also a clear indication that it is a predetonation problem
|And this picture shows the differences from the standard needle very clearly too. You can use mintylamb to find other needles to match the carb too. This is what I did with the selection of a fresh needle for the Judson SU conversion|
SU needles can be changed without removing the carbs, just remove the dashpots and take the SU piston to the bench - really really easy to do!
This thread was discussed between 15/10/2013 and 16/10/2013
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