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T-Series RPM drop when depressing clutch

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Lynn Kirkpatrick

Lynn Kirkpatrick

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Ijust read something in the Moss magazine that has me wondering.

I have a '68 MGBGT with a '74 lump. At idle, when sitting still, if I push in the clutch pedal, the revs drop slightly (say from 1000 to 900). It has been this way 3+ years when I bought it.

In the letters to the fixit guy, in the Moss magazine, a reader mentioned that his Triumph 1500 "bogs" when depressing the clutch. (He didn't say how much the revs dropped, does "bogg" mean stall?) Mr. fixit said that the crankshaft thrust bearings were probably worn, putting side loads on the connecting rods. If the wear is severe enough a connecting rod could break, and "ventilate the crankcase" (doesn't sound good).

Does this explian why my "B" does it too?

They don't say how much endplay is too much.
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I haven't tried to measure the endplay (do you do it stopped or at idle?

Are "B"s different? Can anyone help?

Lynn
 
I believe this is mostly normal. MGBs do not have a throwout "bearing" in the sense of something spinning. The bearing on a B is a round piece of graphite and can actually wear down. It will cause the RPMs to drop a little.
 
I'm not sure about the model year, but I know at some point the Midget and the Spitfire (1500) shared the same engine and supposedly the same dropped-washer problem. Over time the worn (or lost) thrust washers can cause damage to the crank. As I recall you can diagnose the problem by looking at the front pulley on the 1500 engine while someone works the clutch. The crankshaft on the engine will noticably slide fore/aft as the clutch is being cycled.

The lost-washer problem was due to the design of that particular engine, I don't believe any of the MGB engines suffered it as a normal occurance, at least it isn't something I've heard talked a lot about.

[ 06-06-2003: Message edited by: aerog ]</p>
 
The same diagnostic procedure works on the "B", though. Push the front of the crank back with a suitable bar, and have someone depress the clutch. There sould be no noticable movement. I can't remember off hand, but I think the Max. end float on a "B" is somewhere around .005"to .006. If you want a definitive answer, put a dial indicator on the crank pulley and have someone work the clutch, and see if the end play is excessive. If the thrust washers are bad, eventually it will make a very loud, very expensive noise.
Bugeye58
 
I remember the Midget/Spit 1500 thrust washer issue (in fact, I may have read about it in a Moss catalog), and that at something like 60,000 miles, they pretty much had to be replaced as a service item, because they'd be so worn out. I hadn't noticed the RPM drop on the B, but don't pay too close attention to it. that's the exact opposite of my Miata, the RPM's of which will rise when I depress the clutch (especially when it's very cold outside and the engine is cold right after first startup), and they'll drop when I let the clutch back out. In neutral, of course!
-William
 
MY MGB has always done it, and I've driven it since '71 and 16,000 miles. It now has 130,000 miles. My MGA also does it, at 22,000 miles (really!). Depressing the clutch drops the RPMs by a hundred or two.
Don't worry. Be happy.
Safety Fast!
 
All
Thanks for for the input... well, actually I was hoping this would be a good excuse to rebuild the motor (bore it out, mill and polish the head, bigger cam and bigger SU's, elctronic ignition).

I'll just have to find another excuse, or just find another motor that has been tweaked and is too good of a price to pass up. (Let me know if you know of one, not that I'm looking right now.)

Thanks
Lynn
 
Oh, by all means, this is something that should be attended to immediately. My guess is that a complete "performance" rebuild is in order before the summer slips away. That should without a doubt cure the problem.
Is that more along the lines of what you wanted to hear?
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Bugeye58
 
sounds right to me. I wouldn't drive that car another foot without a complete performance rebuild or without locating a hot rod B engine
lol.gif
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