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MG MGB Technical - Lumpy tickover

Trying to sort a lumpy tickover. 18V engine, + supercharger (single HS6 carb). New plugs. usual timing at 11* btc at 800.
At tickover, I removed plug lead 1, and the engine ran like it would on 3 cylinders. Same with lead 4. But with either 2 or 3 removed, the effect was hardly noticeable.
Thinking it might be head gasket related,possibly a small blow between cyls 2&3.
Compression is 170 for 1,2,&4. 150 for 3

Testing cyl 2 with a plug in cyl 3 dropped to 165; testing 3 with 2 plugged dropped to 145. Any ideas?

Judging from the gas analyser output, the carb is running a bit rich, which may be the cause of the lumpy idle but I am a bit concerned about the above! Thanks Michael
Michael Beswick

Hi Michael

It sounds like you are getting typical charge robbing effect of siamesed inlet ports.
Did you do the compression check with wide open throttle?

Peter
peter burgess

Hi Peter, Thanks for your reply. No it was at idle. I can do it at wide open throttle, what differences should I expect based on the above figures. Thanks M
Michael Beswick

Michael,

I'm not sure you understood Peter's question, or else I'm about to learn something new.

You don't mean that you did the compression test with the engine idling do you?

The cranking compression test would typically be done by cranking with the starter motor, while the throttle is wide open, and all the plugs removed.

Charley
C R Huff

Charley-you are right, and I did not make myself clear! No the engine was not running, but the throttle was closed, with all plugs removed. M
Michael Beswick

Hi Michael

You will only get a 'true' reading if you do the test with all plugs out and wideopen throttle.

What cam and CR are you running?

Peter
peter burgess

Peter. I must obviously do the test again!

The engine is standard as far as I know; it was rebuilt prior to my buying the car. The only info is the pistons are 30 thou oversize, but I have no idea whether the head was skimmed. As far as I know it was not rebuilt to any "higher" spec. I also need to check the valve clearances.

This all started trying to cure a lumpy idle; what figures would you expect for CO, CO2 and HC emissions? Current figures are CO 4.04% CO2 6.12% and HC 1013.

(I have access to an MOT analyser!)

Thanks again
Michael Beswick

Michael,

Adjust the vavles before you do the compression test. All else being equal, being 30 over will raise the compression a bit compared to std.

Charley
C R Huff

Thanks again- my week end work now sorted....weather and wife permitting
Michael Beswick

Thanks for your help. Useful day yesterday and learnt a lot! Valves were slightly tight except for 8. Lumpiness now gone tho valve train is noisier. Subjective test -it "feels better" Compression (throttle open) is 1-175, 2-160, 3-170, 4-175. Will put it on the analyser next week.

When adjusting valves using the rule of 9, how do I tell the correct point when the corresponding valve is fully open? Using a socket wrench on the crank nut, the handle can turn thru a noticable arc, during which the valve does not appear to move.

I also came across my clik-adjust which I remember finding rather cumbersome. (I used a stubby spanner and blunt screwdriver) Its boast is that it compensates for any "peaks and troughs" between arm & valve top. But to set it, you use a feeler gauge between arm and valve, thereby presumably carrying any inaccuracies forward. Is there a relationship between the pitch of the adjusting screw and the cliks? Or even a number of turns of the screw (from "tight") to achieve the (or any) required gap?

Thanks again
Michael Beswick

It is not critical the exact cam position for the rule of 9 because the back of the cam has the same radius for a considerable arc. All you need is the "opposite" valve to be "open" and you are OK to adjust.
Art Pearse

That is the theory, but doesn't work on my car, for whatever reason. The back few valves are still changing clearance when their complement is fully down. This made it impossible to get repeatable results. I now adjust at the point of maximum clearance, which *is* quite a few degrees one way or the other from the theoretical point, which gives repeatable results as well as being quieter. I did wonder about recession or burning from them being tighter, but several years later it hasn't happened yet, they don't need readjustment from one year to the next. I've only ever noticed a very small radius of movement before the down valve starts to come up again, BTW, but then I nudge it forward and back in 4th gear using the top of the right front wheel rather than using a socket on the crank pulley.
Paul Hunt 2010

Thanks for the info. Will have another go at valve settings next week end! (Either for better or worse!)
Michael Beswick

This thread was discussed between 24/03/2010 and 28/03/2010

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