Hi there. I am from Australia and have a 1963 EJ Holden, our local version of your Chevy made by General Motors. She has a 138 cu inch straight six with a Stromberg carby. I am running HEI electronic ignition. My problem is that at around 2000 - 2500 rpm, 45 mph in top gear, the engine is a bit harsh, a bt “hammery” and noisy, not smooth. At rpm below this and above she is nice and smooth. At 3000 rpm she cruises nicely at 90 mph, no hammering or noise. Idles beautifully, quiet as a mouse. Have checked manifolds are tight, exhaust is brand new stainless system, timing is set right. Would appreciate any ideas. Thanks.
Have you checked the ignition timing and the timing advance mechanism in the distributor? Nice old car, btw.
Yes, total timing is at 36 BTDC, Vac advance working fine. Have tried advancingand retarding a little, no real change. Not getting any pinging at all pulling up hills in top gear. It does seem to run slightly better if I run on a slghtly lower octane gas. Over here we have 95 or 98 unleaded, engine a little smoother on the 95.
Have you run a compression test? Lower octane means the gas ignites easier, less compression is needed.
36 BTDC seems a little advanced.
What are the specs?
Do you have a shop manual?
I would check the mechanical advance for proper operation.
When you say “total timing is at 36 BTDC”, are you talking about initial distributor timing (at idle)?
Or are you talking about total advance at 2500 RPM with part-throttle? I could only see a number like 36 BTDC when both vacuum and centrifugal advance is factored in - and even then it sounds a bit high.
Thanks for the replies. Yes, compressons are OK, an even 145 across all 6. Total timing is initial plus mechanical (with vac disconnected) at 3000 rpm. Initial is about 10 BTDC. Car is not pinging at all, so iti cant be too far advanced.
It could be a problem in the low speed circuit of the carb, causing a lean spot.
Try turning the idle mixture screw a bit richer.
If it improves the situation then you know you’re in the right neighborhood.
Are the cam and exhaust manifold stock?
Hi, yes everything stock as far as I am aware. Can’t swear to the cam as i bought the car with a supposedly reconditioned engine, but cant see why anyone would change it. Here’s a pic of the engine for anyone interested.
I have to compliment you
That’s a real nice looking engine bay
It would be a pleasure working on that engine
Most of the engines I work on are absolutely disgusting
Does this come across as an engine miss or is it more of a vibration issue?
Just wondering if in the case of the latter if it’s a flywheel and/or clutch imbalance issue.
(Nice car by the way. I’m a fan of Austrailian cars and the Austrailian Super Car series racing which is superior to NASCAR; at least in my opinion. What I really want is a new Maloo, but… ) 
How about a stretched timing chain?
Tester
Thanks for the feedback and kind comments about the car. Just for the record here is a pic of the engine bay before and after I refurbished it.
In answer to questions it feels like its missing, slight vibration but clutch is new and flywheel has been balanced. Engine has timing gears, no chain. Its definitely rev related as I can accelerate ugh it and its not there at lower revs. And it is most pronounced unr acceleration, back off the pedal and it dissapears.
It has timing gears?
If the timing gears are the originals they may need replacing. There’s a reason they sell replacement timing sets for your engine. Because they wear out.
Tester
As a quick experiment, if you disconnect the hose to your distributor vacuum advance and then then it out for a test drive, does the problem still occur?
Also, if you rev the engine in neutral, do you get the same “roughness” in the 2000-2500 rpm range?
Joe, yes I’ve tried that, didnt really make a difference. And yes, its not as pronounced but there is still some vibration there when revving in neutral. I know everyone is thinking its tming related but the thing that confuses me is that it responds so closely to the gas pedal - light touch when cruising at 40 mph and the noise/vibration starts. Lift off and it stops. Its almost like its exhaust related, but I have a brand new stainless steel system. Have checked carefully for vacuum leaks and exhaust leaks and cant seem to find any.
1963EJ:
If I infer correctly, you can get the vehicle up to the 2000-2500 rpm range (~40mph) and the problem occurs. Then if you lift off the gas pedal, it suddenly stops (before the rpm and mph can drop). Then if you press lightly on the gas, the problem comes back.
Please clarify if the above is valid.
Yes thats about right Joe.
Could it be a problem with the harmonic balancer?