Engine Idle

[daren007]

I have both "A" and "B" engines that have been rebuilt. The "A" did not have the lifter bores reamed to oversized lifters nor the cam bore wear repaired. The "B" lifter bores were not repaired and I believe the cam bore either. At 500 RPM they both idle smooth but as you add throttle or spark advance they both start to run rough-not smooth as you would expect I read on MAFCA cam bore wear can cause this. I can see cam (whipping) affecting the valves and distributor as well. This makes sense to me. Can anyone add to this as I am thinking about going for the repairs but do not want to spend the $$ if it does not give a smooth engine at high idle. Also what causes the center cam bore to wear?

[Chuck Sea/Tac]

The model A doesn’t idle smooth with the timing advanced. It idles smooth at retard. Actually no car will. Sounds normal to me. A worn center cam bore will make a knocking sound.

[100IH]

Many folks set timing with the lever is 3 clicks down (advanced) so that if they wish to compete in a contest of who can idle the slowest and smoothest. That's when the levers go all the way up. Normal idle at a stop would be 2-3 notches down, just short of using GAV fuel. When advancing only go down if you can feel the additional power. Never all the way down.

[daren007]

My "T" idles so nice even with spark advance.

[daren007]

The model "B" uses 19 degrees BTDC initial timing, Wouldn't this give a rough idle opposed to TDC for the "A". Somehow this does not make sense. The "A" owners manual says to advance 1/2 after start. This would be approximately 20 degrees. It pretty much states that is where you leave the lever for most driving although we know that is not how it works in the real world. Is there anyone out there that have had their lifter and cam bores repaired to stock tolerances, If so what were the results on engine smoothness. From what I have seen this is something almost if not all engine rebuilders do not address.

[100IH]

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As has been said, the lifter bores normally would not affect idle smoothness, only the click noises and they would mostly be from valve guide and lash. Same for center cam brg. I think that the looseness at the cam would be worth chasing. How much wear is acceptable and how many dollars per thousandth inch is your call. You'll only know if it was worth it after taking the leap. The faster and harder you drive the more important tolerances should be corrected. Few people do these repairs.

[Patrick L.]

I believe the Model B base timing is 9.5º, which is about right. More than that and about any engine could kick back upon starting.

Our camshafts have 3 bushings/bearings and there is quite a bit of stress on the center bushing. I would doubt that a loose bushing would be much of a cause for a rough idle.