Technical SBC DISTIBUTOR TIMING SYMPTOM QUESTION

Sbc points distributor set up as follows...

Mechanical advance limited to 20 crankshaft degrees, initial timing set between 16-18 degrees, vacuum canister connected to manifold vacuum (when connected...note, it is not connected when timing the engine). My goal is to have 36-38 degrees of total advance at 2800-2900rpm with the vacuum canister not connected and the vacuum line plugged. I have that dialed in. The damper has a timing tape that shows 50 degrees btdc.

The problem is, if I continue reving the engine past 3000rpm...the mechanical advance seems to start backing off again down to 30 degrees or less at around 3500+ and doesn't regain the lost advance as I continue raising the rpm.

Anyone else encounter this? Anyone know the cause? Also, I set my distributor up on my Sun 500 machine and it does not do this on the machine (though I did not have the cap or rotor on when setting it up on the machine...so maybe that plays a roll??). Even, when I took the distributor up past 2500 distributor rpm (5000+ engine equivalent) the advance seemed to react properly as expected.

Thanks in advance.

 

remember it is kind of a vac retard--less vac -less advance

 

Thanks for the response. The vacuum advance is not connected during these timing tests.

 

A worn timing chain will allow the timing to retard at high rpm, which isn't always a bad thing

 

Thanks for the response. The engine was fully rebuilt in the winter of '23-'24...has less than 6000 miles.


But thinking about that idea...it has a retrofit roller cam...perhaps the nylon cam button is wearing and allowing the cam to walk. So that maybe worth looking into further.

 

If you have extra parts , try different distributor just to see to check /verify If Cam walk , This way you know to look fix cam or look deeper into your custom tailored distributor.

 

Timing light issue?

 

I have a pretty decent digital timing light with tach and dial back timing ability. I could try my old basic timing light to see if it shows the same issue.

 

Might be worth running the timing light of a seperate power source, if not already?

Chris

 

New morning, new thought. Is the timing going down steadily or suddenly dropping? Suddenly would indicate spark phasing from the rotor jumped to the next pole on the cap.

 

Good question and thought...can't say I made specific note of that (frankly, I was just surprised it was happening). I'll be heading down to the shop later this morning to continue the investigation/quest. I'll follow this up.

 

If it is erratic ( bouncing around ) that is common spark scatter. I have seen a couple oil pumps come loose and have really bad spark scatter. If you ever put a timing light on a motor with a crank trigger set up you will see why that is the way to go. Rock solid timing mark.

 

Follow up test...different timing light hooked to an isolated power source...same symptom.

As rpm increases past 3000 to around 3500 the timing slowly backs down (not instant). I'm thinking either the timing chain cover is flexing as the cam trys to walk forward (the timing chain cover was supposed to be compatible with a retro-fit roller cam button)...or the nylon cam buttom has worn some.

I haven't tried a back up distributor...yet.

 

It's pretty solid...not real erratic. And is reacting predictably.

 

If the distributor doesn't retard on the Sun machine, it looks like the problem is not the distributor.

 

It doesn't retard on the Sun.

I'm just scratching my head trying to figure how to get at the timing chain cover...it's behind a short water pump so space is really tight/restricted. I thought I might have been able to jam a screwdriver in as a temporary support while I ran another quick timing run, but I can't get one in there...not to mention what it might do if it came out with the engine running.

I may have to drain coolant, remove accessory/drives, remove water pump etc.

 

Is it possible that you may have a little vertical looseness in the distributor? I'm thinking that as you accelerate the motor the looseness may take up the slack and once its at the specific rpm the slack regresses. Just a guess on my part............

 

Good thought...I shimmed the distributor to .020" when I rebuilt/set it up (it originally had almost .090").

 

I just tried installing a back up distributor that was set up with the same advance tune as the primary distributor...same results.

This really seems to point at the cam walking. And I'm debating whether I have the time or energy to properly address this now.

 

Spark scatter is the partner to mechanical/timing chain/natural engine harmonics thoughts. Cams and cranks twist, and they'd bounce when revved with no load. I'd guess those low to mid 3000 RPM ranges are just enough to half-assed "bounce" the crank n cam vs holding a steady twist rate under load. The cam is a billet, not a hardened casting so there's little to no absorption of twisting harmonics. If it's scatter you can paint the rotor up to the edge and insulate it from random spark jumping at speed. It happens and that's one of those have it deals like indexing spark plugs. If you do don't use black paint as carbon is conductive. Unless you can read timing in gear under load it's hard to prove much of this, but I got there because you said it's all good in the distributor machine. In a commercial break sponsored by the FWIW dept, I always drop in and time with all the backlash taken up in the distributor. Once in the hole I hold the rotor gently against the backside of the gear and set the static lead where I want it.

 

Thanks everyone for your suggestions, thoughts and for taking the time to put them in writing...I appreciate it.

@theHIGHLANDER and @TA DAD perhaps this is spark scatter...but I've seen the "spray like" timing light strobes of spark scatter...and this does not appear that way. The timing strobe is clean and linnear up to the 36 degrees at 2800...stays solid at 36 degrees while the revs are raised past 2800. At around 3200 the strobe stays consistent and starts to cleanly back down degree by degree as revs continue to raise until it settles at 28 degrees around 3600rpm...and then it stays solid and clean at 28 degrees as the rpm is brought all the way up to 5000. It returns with similar consistency and pattern.

 

My question is, how does it run at higher rpms? Is this noticable on the road? Could be you are chasing a problem that isn't a problem. The reason I say this is that I have chased non-existant problems a few times.

 

That's kind of where I am and the way I'm thinking at the moment too. The car runs fine and the revs stay below 3200 98% of the time....I'm likely giving up hp and efficiency when my foot is testing the floor board stuctural integrity.

I think, to fully investigate and address this properly would require removing and partially dismantling the engine....which sounds like a winter project (not 2 1/2 weeks before Deuce Days...with one of those weeks being lost to a trip back east to help out my parents).

And, just because this is the HAMB and this thread hasn't had a picture...I don't have any photos of the cam button install, but here are a couple engine assembly shots anyway.

 

Did you measure camshaft end play during assembly? The last roller I installed in a non roller block, I added the star shape support to the cover. Could watch the cover flex a bit when checking end play, so welded an additional support across the outside of cover. This tied the cover into the area where the bolts are. No flex with it now.

 

Indeed, I measured the endplay when I was assembling the engine. But, being my first roller cam install, I wasn't sure how much forward pressure to apply to the cam. My timing cover had the inside re-inforced "tri-bar" plate...but, I'm suspecting the outside may also need re-inforcing.

 

I had to "smash" the cover a bit to get end play set at .005. Then welded the additional support in place. Did not use a gasket, but RTV instead so the end play would not change.

 

Just thinking out side box,
How a bout one of those machining block that has 2 triangles , as you tighten the screw it gets taller like a Jack ,
Or tube nut & bolt , just so you can
Place between water pump & cover
To check for flex , if so temporary fix,

 

I was hoping to be able to do something like that, even temporarily to run further timing tests...but there is barely the thickness of a putty knife between the back of the water pump and the timing chain cover. I tried a timing test with the putty knife in place. It made no difference...but I'm not sure the putty knife was applying pressure to the absolute correct location.

 

More putty knives needed! Or steel rules, stripes of sheetmetal / flatbar?

Chris

 

well THERE'S the problem-you need higher compression!