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Opinions on distributor cam lobes
Posted by: Vin ()
Date: April 26, 2018 05:57PM

The wear on my six distributor cam lobes vary and I am wondering from those more experienced than me if the variance could cause what feels like slight surging especially at slower steady speeds. A couple of the lobes are visibly rounder and flatter than the others and I have to sort of split the difference when setting the point gap. I can set the point gap such that a couple lobes create a .019 points gap and a couple lobes create a .016 points gap. Both measurements by themselves are within range but I am suspecting the variation between them is causing just enough advance then retard and advance then retard that I am feeling the slight surging and that at highway speeds it is there but not as noticeable because I am skipping across the lobes much faster. So could this difference in cam lobe wear cause what I am feeling? And if so is there anyway to even out the lobes?


Vin
Dayton, OH
1965 Corvair Monza Convertible (4 speed)

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Re: Opinions on distributor cam lobes
Posted by: MattNall ()
Date: April 26, 2018 06:24PM

Sounds like a bent shaft...

Find another distributor to try first!





MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo

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Re: Opinions on distributor cam lobes
Posted by: Vin ()
Date: April 26, 2018 06:28PM

Well what I didn't say is that this is a rebuilt unit from a vendor with a full rebuild. So I am assuming the shaft would be ok? But I can see the cam lobe differences. But maybe the shaft is bad and they didn't catch it?


Vin
Dayton, OH
1965 Corvair Monza Convertible (4 speed)

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Re: Opinions on distributor cam lobes
Posted by: MattNall ()
Date: April 26, 2018 06:36PM

most likely!





MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo

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Re: Opinions on distributor cam lobes
Posted by: Vin ()
Date: April 26, 2018 07:03PM

So besides a possible shaft difference, can cam lobe differences cause the timing to advance and retard back and forth enough to feel it? The difference in gap certainly is changing the timing but I don't know if that would be something enough to feel?


Vin
Dayton, OH
1965 Corvair Monza Convertible (4 speed)

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Re: Opinions on distributor cam lobes
Posted by: MattNall ()
Date: April 26, 2018 07:18PM

Hard to say for sure.... why I suggest borrowing another unit.





MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo

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Re: Opinions on distributor cam lobes
Posted by: RobertC ()
Date: April 26, 2018 07:52PM

Maybe it is the bearing surfaces (dist shaft or inside of the dist cam) that the dist cam rides on.

Is there any play of the dist cam on the dist shaft.

I really find it hard to think that the dist cam lobes were worn - since the points have a plastic or fiber piece that rides on the lobes.

I assume there are a number people hear that have put well over a 100,000 miles on a single dist cam / dist shaft.

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Re: Opinions on distributor cam lobes
Posted by: Seth Emerson ()
Date: April 26, 2018 08:49PM

Any of the electronic units that use a hall effect or magnetic pulse to trigger the ignition will eliminate that issue. Gap between the reluctor and the pickup can vary (somewhat) with no change in trigger point, because the pulse goes from positive to negative (or the opposite) at the exact point that the reluctor tip goes past the pickup and that change from positive to negative is what the electronics uses to fire the plug. The last time I put one of my new distributors on the Sun Machine, all six firing points were within one-half of a degree. That, and maintenance were the reasons points disappeared.

I'd bet you have a run-out issue on your shaft.

Seth Emerson

Check my new Performance Corvair Web site [www.perfvair.com]

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Re: Opinions on distributor cam lobes
Posted by: rowin4 ()
Date: April 26, 2018 08:54PM

As stated above it's hard to believe that the lobes a re worn, but that being said they could be. If you see a difference by eye and a setting of up to .003 difference I would get a different distributor.

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Re: Opinions on distributor cam lobes
Posted by: Vin ()
Date: April 27, 2018 03:11AM

Thanks all. Yes, I also think it is odd a couple lobes could be so worn compared to the others. But they are, enough to see it and of course the .003 difference is measurement. I guess I will only know if I try another unit. And Seth, I appreciate the electronic option


Vin
Dayton, OH
1965 Corvair Monza Convertible (4 speed)

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Re: Opinions on distributor cam lobes
Posted by: dryenko ()
Date: April 27, 2018 04:14AM

If that doesn't fix your issue, suggest you check carbs next.
Any vacuum leaks??

Bob C aka Dryenko
Dobson, NC 27017

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Re: Opinions on distributor cam lobes
Posted by: JimBrandberg ()
Date: April 27, 2018 04:40AM

You might also check the pivot on the point plate. If it has play from side to side it can mess up the Dwell or the even firing of cylinders, especially when the vacuum advance is pulling on it. Look for changes in Dwell with a meter as you rev it up a little.

Going back to your original question, you can just change the part with the lobes. They have a number so you can get the right one. I usually turn the engine so it's at the #1 firing point and remove the distributor to the bench to dig into it. I have aluminum vice jaws with a hole for Aeroquip fitting to hold it. It's just easier for me to mess with the little springs at the bench. The CORSA Tech Guide has a listing of the part numbers in case you want to get a good used one beforehand.
I usually see variance because of a bad bushing rather than the cam lobes but I suppose anything is possible. A new single bushing doesn't always take care of it.

Jim Brandberg
Isanti, MN
CorvairRepair.com

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Re: Opinions on distributor cam lobes
Posted by: cnicol ()
Date: April 27, 2018 06:19AM

Distributor cam lobes do wear out; especially since most distributors no longer have their foam lubricator and many people don't bother with that little dab o'grease on the rubbing block.

Lobe wear is one of the things you look for when you use an ignition 'scope. With a scope you can check the dwell on each cylinder (worn lobe) and in a stacked display you can see timing variation caused by a bent shaft.

The points cam is a replaceable part; At the Chevy dealer where I was a parts counterman in the 70's, we had a few common ones in stock and sold them to our tune-up guys and shops. We also sold lubricator wicks with every internal tune-up kit.

I doubt you could feel it as a surge though, a given cylinder fires four times a second at 60 mph. Most likely symptom would be a lumpy idle and loss of pep.

Craig N. Coeur d'Alene ID.
66 Black Monza 4dr, 4.2L V8 49k
61 Seamist Jade Rampside 140 PG
60 Monza coupe (sold, sniff sniff)
66 Sprint Corsa convt - First car! Re-purchased 43 years later
2+2 gnatsuM 5691

+17 Tons of parts

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Re: Opinions on distributor cam lobes
Posted by: Vin ()
Date: April 27, 2018 08:50AM

Thanks, very interesting. I didn't know the cam lobe part was replaceable. Supposedly the rebuilt unit I got had a rebuilt points plate and two new bushings etc... firing 4 times a second - with that I think you are right, doubtful I could feel that. I rebuilt the carbs to include new shafts and shaft seals and all other hoses are new. I don't think I have a vacuum leak. It runs really strong- another club member drove it and remarked how strong it was (for a 100). It is just this slight surging that is killing me. In 2nd or 3rd going at a steady slower speed is when I can really feel it. The higher speeds it feels more like hitting small bumps in the road.


Vin
Dayton, OH
1965 Corvair Monza Convertible (4 speed)

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Re: Opinions on distributor cam lobes
Posted by: Vin ()
Date: April 27, 2018 08:51AM

*for a 110 not 100


Vin
Dayton, OH
1965 Corvair Monza Convertible (4 speed)

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Re: Opinions on distributor cam lobes
Posted by: American Mel ()
Date: April 27, 2018 10:35AM

dryenko Wrote:
-------------------------------------------------------
> If that doesn't fix your issue, suggest you check
> carbs next.
> Any vacuum leaks??


X2 ! ! !

Not saying there is zero problems with the distributor,
but I have Seth's Distributor, and have noticed the vairy same symptoms in my car.
So I am doubting that it is timing.
I KNOW that I have some carb issues to deal with.
Why I have 4 new rebuild kits waiting to be used.
Just do not want you to be chasing ghosts in the machine, that do not exist.

.
-----------------------------------------------
WA. state, 1 mile south of the Canadian border,
I am not at the end of the world, but you can see it from here.
'66 Monza Coupe - 4spd, 140 Daily driver beater
'67 Monza Vert. - PG, 140 Daily driver beater

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Re: Opinions on distributor cam lobes
Date: April 27, 2018 11:48AM

Other than a problem with the points plate this is not a distributor problem. Slight variations in fuel mixes in the carbs are more likely the culprit. I would also check for consistent output from the coil.
As to wear on the cam lobes, as with all 55+ year old Corvair components, this will become an issue at some point. The only solution is to find a better condition cam.
It's something I'm going to have to watch for in the future.

Lon Wall

Fireman Sumpter Valley Railroad #3 and #19
svry.com
www.pacificmodelloggerscongress.com

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Re: Opinions on distributor cam lobes
Posted by: Vin ()
Date: April 27, 2018 04:05PM

The wisdom of the group is great, thanks. I am.going to leave the distributor alone for now and open the carbs up, checking for things like venturi cluster flatness, enrichment needles sticking etc...


Vin
Dayton, OH
1965 Corvair Monza Convertible (4 speed)

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Re: Opinions on distributor cam lobes
Posted by: Vin ()
Date: May 16, 2018 06:21PM

Finally had some free time to dig into this and thought I would report back. I didn't have a spare distributor so I dug into the carbs first. I just took the tops off and pulled the venturi clusters. I put a straight edge best I could on the base of the clusters and they were nice and flat. None of the venturi holes were plugged but blasted them with carb cleaner. Used a q-tip to push into the top of the enrichment needle, pulled the needles out and blasted carb cleaner through the holes. 5he driver side carb the carb cleaner shot right out into the bowl but the passenger side did not initially spray out into the bowl very good. Shot it a handful of times then the spray came out into the bowl nicely. Then I checked my float levels. I rebuilt these myself a couple of years ago and in my log book I noted I followed Dave Motoheads advice on this forum and set the floats to 1 and 1/8 inch. Well, my driver side was that exactly but my passenger side was at 1 and 1/4. I am not sure how I managed that but the ruler didn't lie. So I adjusted that float to 1 and 1/8 as well. Put it all back together and got a test run in last night. Success!! The little bumping or surging at steady speeds is gone! Nice even power and it is great to have that nagging little bumping sensation gone. I don't know if it was a plugged enrichment needle hole or the float setting inconsistency or a combination but it was the carbs. Thanks again to everyone who offered their thoughts.


Vin
Dayton, OH
1965 Corvair Monza Convertible (4 speed)

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Re: Opinions on distributor cam lobes
Posted by: Seth Emerson ()
Date: May 16, 2018 11:48PM

Glad to hear success!

Seth Emerson

Check my new Performance Corvair Web site [www.perfvair.com]

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