Odd timing issue, built engine runs best with very little advance..? Comp xe274h - Hot Rod Forum : Hotrodders Bulletin Board
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Old 07-07-2019, 03:42 PM
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Odd timing issue, built engine runs best with very little advance..? Comp xe274h

I'm scratching my head hard at this one. I have a freshly built 350 I've been troubleshooting an issue with lacking power and RPM range. It's a 350 with a CR of about 10:1, aluminum heads, and a Xe274h cam with a pretty decent amount of overlap and the engine combo should be capable of above 400ftlbs right off 2k RPM with a redline of 6k.

Now, the timing is where it gets weird. I'd just set my timing to 18* initial with 18* mechanical for total 36* all-in by 3k RPM. It ran terribly and sounded like I was getting detonation at idle. Another oddity is that the idle pattern was completely and disappointingly smooth as a stock 350 would be and in all the videos I've watched of engines with this cam, it should have a noticeable lope.

Well, now yesterday I decided to play with the initial by paying attention to the idle pattern. If I set it to between 8 and 10 degrees initial, it has that noticeable lope and revs in a much healthier manner.

Not my car, but this link is exactly what my car sounds like if I run only 8 degrees of initial:


I'm wondering if I have something wrong with my cam or timing chain now because that doesn't seem right. I've verified my TDC mark and it's totally correct. Have also tried two separate timing lights.

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Last edited by shdggsdv; 07-07-2019 at 03:51 PM.
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Old 07-07-2019, 04:20 PM
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Was the cam degree'd in at install??

You are checking timing with vacuum advance disconnected from manifold port/carburetor, correct??
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Old 07-07-2019, 05:21 PM
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Sounds a lot like the cam is not timed correctly to the crank.


More detail on pistons, their deck clearance, head gasket used, the heads used, rocker ratio, the ignition system used, and timing light brand and model (hunting for dial back or not)


Bogie
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Old 07-07-2019, 05:54 PM
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was the timing mark/timing tab verified for accuracy?
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Old 07-08-2019, 10:51 AM
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Quote:
Originally Posted by ericnova72 View Post
Was the cam degree'd in at install??

You are checking timing with vacuum advance disconnected from manifold port/carburetor, correct??
I didn't install the cam myself so I can't really tell you.

And yes, vac advance is disconnected.

Deck clearance is unknown, pistons are stock 80s GM with valve reliefs unsure of CC, head gasket is MLS with compressed .025", Heads are aluminum 64cc from Promaxx, Ignition system is MSD Streetfire with a stop screw that was limiting it to 18 degrees of mechanical advance.

Timing lights used was a Bosch inductive light with no dialback, the other is a digital Craftsman dialback
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Old 07-08-2019, 10:52 AM
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Quote:
Originally Posted by BogiesAnnex1 View Post
Sounds a lot like the cam is not timed correctly to the crank.


More detail on pistons, their deck clearance, head gasket used, the heads used, rocker ratio, the ignition system used, and timing light brand and model (hunting for dial back or not)


Bogie
Oh also, rocker ratio is 1.5.

And yes, verified accuracy of the timing mark
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Old 07-08-2019, 11:51 AM
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The timing I was hunting for is the cam to crank which unfortunately is under the timing cover so can't be checked without considerable effort of either front end disassembly or with a lot of tooling and some front end (engine not car) disassembly as well as rocker cover removal.

The carburetor is a what or is this using injection if so whose?

The ignition is whose distributor and does it have a "power" box like an MSD?


When setting the base timing with the vacuum advance disconnected is the vacuum source port on the carb or injection plugged to prevent air ingestion?


What is the damper? The SBC has been fitted with several over the years that differ in diameter and mark location. It is necessary that the damper being used and the pointer on the timing case cover be matched or you can get anything from straight up position errors to, with wrong diameters, parallax errors do to eye angles across the distance. Another problem is manufacturing error in alignment of the outer marked ring to the inner key slot position; or with older units slippage in these positions as the rubber bond losses its grip between hub and ring.


Too bad about the factory round dish pistons. When compared to D dish pistons or step dish pistons for an otherwise identical engine with modern chamber heads; the old round style factory piston falls short by about 25 pound feet on the torque peak and about 20 horsepower on that peak which they also carry the power peak in a longer and flatter curve for about 600 more RPM. That is real power you can feel.


Bogie
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Old 07-08-2019, 02:49 PM
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I've run that cam a couple times. I know they say 2-6k but my experience is they don't turn on until a little higher and can benefit from a mild converter.

Were those 80's smogger type rebuilder special pistons? Those type of pistons usually have a shorter compression height to maintain stock compression ratios with a mild deck clean up.

I'm not thinking it's close to 10:1 as it doesn't sound as if it was zero decked. So power would be down from the expectations. 400 ft-lbs at just off 2k is a pretty tall order.

It's been a long time, but I'm wanting to say I ran around 10-12 initial. Give it what it likes.

If the cam ended up in a retarded position due to timing chain or cam machining inaccuracies it can take away a fair amount of power from the bottom end.

Last edited by Hipster_G; 07-08-2019 at 03:12 PM.
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Old 07-08-2019, 06:02 PM
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Quote:
Originally Posted by BogiesAnnex1 View Post
The timing I was hunting for is the cam to crank which unfortunately is under the timing cover so can't be checked without considerable effort of either front end disassembly or with a lot of tooling and some front end (engine not car) disassembly as well as rocker cover removal.

The carburetor is a what or is this using injection if so whose?

The ignition is whose distributor and does it have a "power" box like an MSD?


When setting the base timing with the vacuum advance disconnected is the vacuum source port on the carb or injection plugged to prevent air ingestion?


What is the damper? The SBC has been fitted with several over the years that differ in diameter and mark location. It is necessary that the damper being used and the pointer on the timing case cover be matched or you can get anything from straight up position errors to, with wrong diameters, parallax errors do to eye angles across the distance. Another problem is manufacturing error in alignment of the outer marked ring to the inner key slot position; or with older units slippage in these positions as the rubber bond losses its grip between hub and ring.


Too bad about the factory round dish pistons. When compared to D dish pistons or step dish pistons for an otherwise identical engine with modern chamber heads; the old round style factory piston falls short by about 25 pound feet on the torque peak and about 20 horsepower on that peak which they also carry the power peak in a longer and flatter curve for about 600 more RPM. That is real power you can feel.


Bogie
Carb is a Holley 760 Street Avenger. Dizzy is an MSD Streetfire without a power box. Damper is a SUM-161358 from Summit measuring in at 8", but I've verified TDC as correct so no slippage there.

And yeah, if I run through more troubleshooting I might just pull the front end apart and degree the cam if I continue to have this issue.

Wish I could've swapped to flat-heads but I couldn't afford the extra upgrade unfortunately. Running this engine build on a pretty limited budget.

Quote:
Originally Posted by Hipster_G View Post
I'm not thinking it's close to 10:1 as it doesn't sound as if it was zero decked. So power would be down from the expectations. 400 ft-lbs at just off 2k is a pretty tall order.

It's been a long time, but I'm wanting to say I ran around 10-12 initial. Give it what it likes.

If the cam ended up in a retarded position due to timing chain or cam machining inaccuracies it can take away a fair amount of power from the bottom end.
I know the block has ben decked at least once before by a performance shop, but the receipt doesn't show how much it was decked, so you could be right.

Short of taking the damper and timing cover off, is there any way to verify cam timing? I do have a degree kit I picked up at goodwill a few weeks ago if that adds any possibilities.
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Old 07-08-2019, 07:01 PM
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You don't have to take the damper or cover off. You can put the wheel on the damper or if you do take the damper off the cover can remain attached. You'll just have to get creative with the wheel and pointer. put your DI on the end of the push rod. Take care to make sure the pushrod and DI stem are inline for best accuracy.

I'd double check the damper marks with a piston stop.
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Old 07-08-2019, 07:51 PM
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Quote:
Originally Posted by johnsongrass1 View Post
You don't have to take the damper or cover off. You can put the wheel on the damper or if you do take the damper off the cover can remain attached. You'll just have to get creative with the wheel and pointer. put your DI on the end of the push rod. Take care to make sure the pushrod and DI stem are inline for best accuracy.

I'd double check the damper marks with a piston stop.
I'll definitely try that, thank you!
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Old 07-08-2019, 09:35 PM
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Quote:
Originally Posted by shdggsdv View Post
Carb is a Holley 760 Street Avenger. Dizzy is an MSD Streetfire without a power box. Damper is a SUM-161358 from Summit measuring in at 8", but I've verified TDC as correct so no slippage there.

And yeah, if I run through more troubleshooting I might just pull the front end apart and degree the cam if I continue to have this issue.

Wish I could've swapped to flat-heads but I couldn't afford the extra upgrade unfortunately. Running this engine build on a pretty limited budget.



I know the block has ben decked at least once before by a performance shop, but the receipt doesn't show how much it was decked, so you could be right.

Short of taking the damper and timing cover off, is there any way to verify cam timing? I do have a degree kit I picked up at goodwill a few weeks ago if that adds any possibilities.
Tools needed are degree wheel, a positive piston stop, dial indicator with stand, a sold lifter or lifter tool that replaces a roller lifter in this case, the cam's timing card and some heavy gauge wire like a coat hanger's.

What you will test for is first where is true TDC then once that is confirmed then test for where is the cam events compared to true TDC.

This is a tedious process, so before digging into the motor, I'd recommend trying another distributor. This could be something as simple as the MSD Street Fire is messed up. So lets winnow out the less intrusive things first before diving into the guts of the motor.

Bogie
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Old 07-11-2019, 09:55 AM
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Could the distributor be in a tooth off?
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Old 07-11-2019, 11:25 AM
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Quote:
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Could the distributor be in a tooth off?

That's pretty common on the SBC as the intermediate shaft rotates with the distributor shaft as it pulls through the cam gear on removal.


But if you time to the rotor to cap aligned to number one on the cap this should begin at number 1 firing.



The always worry-some thing is setting up the timing set. Chevy in its wisdom made number one firing as both the crank gear and cam gear being in the 12 o'clock position. This is a long way to judge if these marks are actually aligned and if using a multi position crank gear can easily get off if you don't have calibrated eyeballs. One way to help this is to set the engine up at number 6 firing which puts the timing marks facing each other with the crank gear at noon and the cam gear at 6 o'clock. Then of course you can button it up that way and set the distributor for firing number 6 cylinder or before closing up the timing case rotate the crank one turn to bring number one up to firing.



The TDC mark on the damper has two locations for alignment those being number one and a turn later number 6, it's easy to get lost especially if the crank has been turned for any reason after being set up. I've done this to myself and I know this stuff, poo happens.


An easy way to get back to original distributor position on the SBC and BBC if you marked the orientation of the distributor before removal is to set it to that mark. It will not engage the intermediate shaft and will sit high but the gears have some mesh. At this point rotate the crank in the clockwise (normal rotation) direction. When the distributor drive crosses the intermediate shaft's slot the distributor drops all the way down. Now continue crank rotation till the rotor aligns with the number one terminal. The damper should be aligned to the TDC marks, at this point and you are good to go.



Bogie
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Old 07-11-2019, 01:24 PM
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The timing is “180 out” . I have mistakenly driven my 1966 GTO about a mile with the cam gear timing 180 out.

The pre-1967 Pontiac Master Shop Manuals were issued with the timing shown 180 degrees out. The shop manual illustration showed the Cam gear dot at six o’clock and the crank gear dot at twelve o’clock with No.1 cylinder at TDC firing position. That was corrected in 1967 and later shop manuals

I replaced the timing set on my 1966 GTO according to the 1966 shop manual.
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