Shdggsdv

So had an exciting day today. My timing light failed rather spectacularly and left a trail of molten plastic all over my '75s fender and cracked my new phone's screen in the process of trying to keep it from combusting. Anyway, in the process I discovered something that's got me scratching my head.

I've been having performance issues on a newly built engine with an XE274h cam that should make above 400ftlbs just off idle. Feels sluggish and meh. My timing was set as follows: 18* initial, 18* limited mechanical, with 36* total all-in by 3000. As far as I understand, what should be perfect for the cam. I've also been hearing a tin-can rattling noise off and on seemingly coming from underneath the engine I wrote off as some bit rattling against something else. The idle pattern was also super even and sounded disappointingly mild despite this cam's large overlap. Struck me as odd since YT of other SBC cars with that cam absolutely don't have a smooth idle.

Well now all I did was back off the initial to 8 degrees and the idle pattern developed a **** ton of lope and sounds just like the video linked below with a super satisfying lope. The rattling disappeared too which has me wondering if that might've been detonation? I always though detonation should be heard through the valve covers, so that part has me scratching my head. Sounds a little healthier revving as well, but haven't been able to road test. I am deeply confused by this, however. 8 degrees seems way too low for this engine, but it sounds quite a bit healthier.


You all have any idea what's going on?

Rescue Rogers

I have the CS XS274S-10 https://www.summitracing.com/parts/cca-cl12-677-4 And I think I ust took out the solid lifter version of your cam

14* initial, 36* total and have the vacuum advance connected. Maybe just a hair too much advance on yours. MY idled very nicley and has a nice burble to it. It was very peppy and responsive to the throttle. Its got to be your timing and maybe the valve lash is too tight....

lars

Your numbers don't make any sense - it should not run well at all with only 8 initial, and it should run great at the first setting. This is indicative that your balancer timing line may not be in the correct location. I'd do a TDC verification if I were you.

Lars

Shdggsdv

Well.. I verified my TDC and confirmed I am at TDC. Used a borescope in the spark plug hole to see exactly when the piston reaches the highest point on the compression stroke and it's exactly at my 0 mark. Any idea what else could be up with it?

MelWff

was the timing light you were using compatible with whatever type of ignition system you have? As an example certain timing lights do not work well with MSD systems.

Shdggsdv

I've got an MSD Streetfire, which as I understand has no multi-spark system that would cause incompatibilities

MelWff

borrow another light and see if you get the same readings

Shdggsdv

Just used an inductive Bosch light and got the same reading. Was using a Craftsman digital dialback before

cv67

Do you know anyone in the area with a sun machine, never had luck with their bushing kits. Mine used to be the same way, flat, no power no matter what I did. took it to an old timer who got it right, night and day. Definitely sounded like you had some detonation going on, at least you caught it.

Big2Bird

I have found street fires out of the box with the rotor out of phase/gear 180* off.
Someone forgot to tell China there is a difference.

Shdggsdv

Not really sure what a sun machine is, so my guess on that is that I don't know anyone with one.

Saw on YT how to check rotor phase, so I guess I'll try that and see if I come up with anything.

Big2Bird

The dimple on the gear should align with the rotor tip.

cooper9811

I think 18 initial sounds a bit high for a the 274 cam. I would try different setigns till you get the performance you want. In my corvette, I run a 286/286 duration cam and have 20* initial timing, and I think that's close to the max I should run with that setup. Other things besides camshaft duration could be a factor too. My stroker is just over 10 to 1, AFR heads and lots of cubic inches, and seems to like about 20* best. I have 18* less with no real issues, 20* just feels more responsive.

While the following example is not a corvette here is an illustration of what too much timing can cause. I recently worked through this with my 71 Mach 1. Engine is a 351 Cleveland at about 10.25 to 1 with trick flow heads. 272 degree duration can with high lift, set at 12 initial and with vacuum connected, was around 28 to 30* degrees at idle. Car ran good at speed, but at low RPM/cruise conditions, it was a little rough and sluggish. I would occasionally miss too, resulting a soft after fire in the exhaust.

I dialed back the vacuum to about 8 degrees, and the symptoms went away - car responds instantly off idle and screams 0-60, even with very conservative rear gearing (2.73:1). Huge difference just by allowing less timing off idle. I still need to find ideal amount of vacuum advance. I bumped my initial to 14*, and it got even better - but no question I had too much advance with my original setup.

This is all a long-winded way of saying that I just had to keep looking for that ideal setting that my engine "likes".

Shdggsdv

Well new news, guys. Replaced my distributor with MSD's Pro-Billet HEI and the problem disappeared and she runs great with 14 initial and 36 total now! Must have been a faulty dizzy

cv67

Those streetfires dont have the best rep good you got it figured out
Thats a good size cam for a 350 it wont make loads of low end tq needs gears/converter

Rescue Rogers

Awesome news!!

I had a dizzy come apart internally and it was doing some goofy stuff. I realized it when i saw that the cap was eating into the rotor