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Point Gap vs Dwell Angle

Posted by nrichard

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Point Gap vs Dwell Angle #1
nrichard	Posted: May 18, 2005 12:44AM Registered: 8 years ago Posts: 1

I have a late-model Sprite Mark II with the 1098cc (10CG) engine, which, as most of you know, is basically an early Midget.

I am having trouble tuning my engine. It seems to idle fine, although I can't get it to run smoothly below ~1200 rpm (timing is specified at 600rpm!). The car drives fine in first and second gear, but when the engine is under load in 3rd and 4th, it begins to stutter and loses all power. I need to rev it a few times with the clutch in before it runs normally (especially at high rpm).

I've heard that I must make sure the ignition is perfect before adjusting the carburetor, and setting the ignition is what is giving me the problem. The car specs are for a point gap of 0.014 to 0.016in, with a dwell angle of 60+/-3deg. Well, when I set the gap to 0.016, the dwell angle is between 30 and 35 degrees. I have to increase the gap significantly to get a dwell angle of 60deg. I even installed a freshly rebuilt distributor with new points, and the problem remained. Can anybody explain the discrepancy between my gap setting and dwell angle? And yes, I think I am reading the dwell correctly (doubling from the 8 cylinder reading). In the end, does the gap matter, or should I just set it for the proper dwell angle?

Beyond the gap, I am checking the timing with a timing light, have set the rocker clearances to spec, and have new spark plugs with the correct gap. I replaced the vacuum line from the carb, and verified that there is vacuum when I rev the engine and that the distributor advances under vacuum. I am using an old coil, but I don't see how it's performance would change based on the engine load (shouldn't it be a function solely of RPM?)

Any help would be appreciated.

Thanks,

Nick

Nick

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Re: Point Gap vs Dwell Angle #2
sureshot-!	Posted: May 18, 2005 07:24AM Registered: 8 years ago Posts: 9

I am troubleshooting a similar problem.

About gap vs. dwell:
Less gap yields more dwell - the points are closed longer.

Too much dwell and the coil gets too hot & starts to fail (especially idleing).
Too little dwell and the primary current surge dosen't have enough time to overcome the coil reluctance and spark gets weak at higher revs.

Electronic points overcome this by switching on for a fixed time duration.
This yields a short dwell at idle (so the coil won't overheat) and a long dwell at high RPM's (to give a full current surge to the coil).

Plug gap is also a factor - especially if the coil is getting old an weak.
The higher cylinder pressures of open throttle acceleration increase spark resistance.

Now - if I can just find my problem.. (Maybe a weak ground through the distributor?)

1964 Midget - resurrection project - on the road.
1991 RX-7 - daily driver.

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Re: Point Gap vs Dwell Angle #3
scoot	Posted: May 20, 2005 02:02AM Registered: 8 years ago Posts: 14

I would try decreasing the gap a bit to .014 maybe, and read the dwell. Significant difference between your readings could indicate camshaft wear. Most distributors specify the 60degree +/-3 you mentioned. Also check your fuel pump, and carbs, as they could be sticking or something too.
Good Luck!

Scott A. Huber

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