Shrinking Gap

[Smittie]

OK motorheads. Riddle me this.

The book says the initial gap should be set to .019, proper dwell angle is 31 to 34. Setting the points to .019 yields a dwell angle of about 20, at a dwell of 32 the points are gapped at < .080. My question is what is wearing out that causes the gap to shrink? In all other respects the engine runs smooth and steady.

The engine is a 1976 GM 250 CID inline 6 cylinder with a single barrel Rochester B manual choke carb which now lives in a 1963 Chevy C10.

[PGSigns]

Something sounds wrong. The dwell is the time the points stay closed. If the dwell is longer the points will not be open as far. Almost sounds like you are reading it on the 8 cylinder scale one time and the 6 the next.
Jimmy

[Wrenchbender Ret]

The gap gets closer as the rubbing block on the points wear down as they rub the cam. A small pinch of grease needs to be on it. Sounds like its time for electronic ign. It solves all those problems. A 76 model came with HEI originally.

[jtrichard]

Originally Posted by PGSigns

Something sounds wrong. The dwell is the time the points stay closed. If the dwell is longer the points will not be open as far. Almost sounds like you are reading it on the 8 cylinder scale one time and the 6 the next.
Jimmy

i agree that it could be that he is reading the 8 cly. scale on his dwell meter

[Smittie]

While I will not rule out user error, I am reading the dwell meter correctly.

The points and condenser are new.

The cam is lubricated.

Not interested in going to an electronic ignition just yet.

When the previous owner did the engine swap, he kept the 230 distributor and probably the carb. The one on the truck has a manual choke which, as far as I can find, came only on trucks.

-- Smittie

[luvbowties]

Originally Posted by Smittie

While I will not rule out user error, I am reading the dwell meter correctly.

The points and condenser are new.

The cam is lubricated.

Not interested in going to an electronic ignition just yet.

When the previous owner did the engine swap, he kept the 230 distributor and probably the carb. The one on the truck has a manual choke which, as far as I can find, came only on trucks.

-- Smittie

You said, "In all other respects the engine runs smooth and steady." At which setting?... .019" & 20 degrees? or .080" and 32 degrees?
When you INCREASE the gap by about 4 times as much(.019 to .080), the dwell should DECREASE; and yours INCREASES...doesn't make sense, so something's definitely amiss here

I'd HAFTA GUESS ONE OF 3 THINGS: (1) user error in reading, setting, and/or connecting the dwell meter correctly: NO accusations here, but I've made meter reading errors myself, and still often hafta double-check, so please re-check these steps (2) dwell meter defective(check meter battery) (3) You said, "My question is what is wearing out that causes the gap to shrink?".....Distributor worn out, allowing play in the shaft-bushings, resulting in inconsistent/senseless meter-readings with possible gap-changing.

As I began, GUESS...
HTH,
Sam

[markeb01]

I had the same experience years ago. Turned out my dwell meter was completely inaccurate. A new dwell meter solved my problems.

[Smittie]

I think my meter reading and feeler gauge skills are OK. My proofreading skills are definitely lacking. I wrote .080 but it should have been .008.

The engine runs OK at .019 but quite a bit better at .008. Once set and tightened down, dwell angle and timing seem to remain very consistent. The timing is right at the book indicated 4 before, dwell angle sits very steady at 32 degrees.

-- Smittie

[Smittie]

I have an Actron CP7605 analog dwell/RPM/volt meter, no batteries. Given the readings I was getting, I zeroed the gauge needle to make sure it was set right. That's the only adjustment the meter has, that I know of. If the meter was very far off would that not show up in how the engine ran?

-- Smittie

[luvbowties]

Originally Posted by Smittie

I think my meter reading and feeler gauge skills are OK. My proofreading skills are definitely lacking. I wrote .080 but it should have been .008.

The engine runs OK at .019 but quite a bit better at .008. Once set and tightened down, dwell angle and timing seem to remain very consistent. The timing is right at the book indicated 4 before, dwell angle sits very steady at 32 degrees.

-- Smittie

Smittie, I think I would remove cap and rotor, set points cam-lobe for adjusting gap, insert the 0.019" gauge & set gap there. Then try wiggling the shaft to and fro while simultaneously "feeling" the gap with the feeler gauge. That could determine if the dist. has excessive play. **You may get better feel/feedback by inserting, say, a 0.014" or so, to see if the gap closes when wiggling; whereas with the 0.019", it may better show the gap widening.**

I'm strongly of the opinion that, barring the dwell meter being off-calibration, your dist. shaft has worn bushings and allows it too much room to "play"--and it likes to play at the 0.019" setting.

One last thing to experiment with. I seriously don't know how this would affect dwell on a points setup, but it won't hurt to try. With rotor off shaft, lightly place a non-magnetized screwdriver against the metal dist. shaft & see if the shaft has become magnetized. [When they get magnetized on later ignitions, it will cause them to not start or at least run improperly.]

HTH,
Sam

[jtrichard]

are the points new? or be run for a fair amount of time? pitting on the points will play havoc when setting with feeler gauge ... check the shaft/bushing wear if there is wear then you can get odd point gap... then just use the dwell and if it runs good go with it

[ERASER5]

Originally Posted by Smittie

I think my meter reading and feeler gauge skills are OK. My proofreading skills are definitely lacking. I wrote .080 but it should have been .008.

The engine runs OK at .019 but quite a bit better at .008. Once set and tightened down, dwell angle and timing seem to remain very consistent. The timing is right at the book indicated 4 before, dwell angle sits very steady at 32 degrees.

-- Smittie

Ahha! Now you may get better responses. Dang decimal points! Nearly as aggravating as the points you are dealing with!

[Smittie]

The points and condenser are about a month old, have maybe 1000 miles on them.

A rebuilt distributor is $40. Swapping it out is easy enough. I can't imagine there is a down side to doing the swap. I did want to make sure that the significantly smaller gap was not indicative of some other issue.

-=-=-

As additional background, I am just starting this build. A glutton for punishment, my project vehicles are always my daily drivers. When I bought the truck it got about 10 mpg on the 120 mile drive home. I've replace the valve cover and push rod cover gaskets, points, plugs, condenser, coil and rotor. The cap looked pretty good so I didn't replace it. Changed the oil once, planning to do it again soon as the oil that came out the first time was extremely black. Rebuilt the Rochester B manual choke carb. At this point the truck starts and runs pretty smooth, gets about 12 mpg and has no problem at California freeway speeds to 70 mph.

The plan for now is to keep the 250 CID straight 6 and see what I can do with it. I'm looking to make it a good driver with "reasonable" gas milage (reasonable being relative to its 50 years of age). The only engine modification I currently have planned is headers to split the exhaust and add glass packs to give it a throaty retro sound.

I have some cosmetic plans but those have to wait until the drive train is a lot closer to perfect.

Comments, advice, and teasing welcome.

-- Smittie