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Engine Detonation 350 Oldsmobile with Quadrajet

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rhinosoft007:
Hi All,

Not sure if I've posted here about my issue but maybe someone could assist on what I could look at next?  Not strictly speaking a carburetor issue...but maybe it is?

PROBLEM
Engine starts and idles great, zero noise or vibration. When I move off sedately, car changes gears smoothly, no bangs or flaring.

What I have been experiencing is detonation when I give the engine a lot more gas in first gear or when under heavy load and changing gear from 2-3. The valve train (I guess it is?) starts rattling really badly so a bit worried about that - need to back off right away. At cruise, perfect, I cannot hear any noise and power delivery is fine but possibly is pinging just can't hear it.

I removed the engine then pulled the camshaft and heads to inspect for damage - none found. Well, I did throw away the CompCam valve train - one rocker arm was jamming on the CompCam stud, back to standard valve train now. I did not check pistons or bearings, there was no metal in the oil filter. I wonder if one valve has been 'tweeked' from the CompCam jamming, causing the noise? Hmmm.

If you read below, I have changed distributors and carburetors (one item at a time and tested) and for the life of me I cannot get rid of that noise. Advanced/Retarded timing every test and used a lot of fuel to get nowhere.

Engine builder replaced rotators with standard springs and retainers - he is backing his choices. 73262K2 - Lunati KIT 1.495". Not sure if the noise may be valve float due to incorrect springs?

DETAILS

350 Oldsmobile had an engine rebuild, pretty mild  :

- block decked, align honed, .30th over etc....
- CompCams camshaft CL42-227-4 H252 .433 intake/exhaust valve lift, flat tappet
- standard valve train
- heads improved (needed it) - #8 heads with larger 2.000 intake valves
- 17057253  and 17059241 Quadrajet carbs. Both updated using recipe 1. 17059241 running long blue power valve spring.
- 9:10 compression
- running dual exhaust through Thorntons cast manifolds
- recurved distributor (original set 10 to degrees) and mechanical set to the same 10 degrees.
- automatic 350
- running high octane fuel (in Australia ULP 98) plus added some octane boost but I hate that stuff so no more.

I ordered the specific parts for both carbs a while back and they all look to be in good order.

Someone told me core shift may have come into play after the rebore? That could cause overheating and damage the pistons - sounded plausible.

Thank you for reading this post and if anyone has any ideas, please post back - I would appreciate the help and thank you very much.

Graeme

quadrajam:
Have you verified the timing mark on the balancer is located proper?
What is the total centrifical + initial timing?
Was the cam degreed in?
What is the cranking compression?
A 9:1 engine with a good quench distance should run fine on 87 octane.
Proper spark plug heat range?

.....???..  Could it be an exhaust leak at the manifold donut?
Are the motor mounts good?

QJ

Pav8427:
Just a thought. Did you use a timing chain set to make up for align hone?

rhinosoft007:
Thank you guys.

QJ - setup the timing mark with balancer using TDC with micrometer as per standard steps. Cam was degreed in as per cam card. New motor mounts,. Timing 34 deg total.

I will have to try a compression test next, will advise. Spark plugs look good, just standard ACDelco.
Exhaust leak maybe? I will jack it up and run engine to verify. 

Pav - used a double row timing set yes. 

May be something simple, I really hope so. Exhaust leak may be the next check - shop did whack a lot of orange sealant around the flange.

Will report back, thank you. 

quadrajam:
Sometimes it takes some engine load, AKA cylinder pressure to reveal a smaller
exhaust leak, so it might not show up reved up in the driveway.

I dont think you have valvetrain issues, that noise would be heard reving up at any load.



QJ

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