1963 Corvair Engine detonation

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edherba
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1963 Corvair Engine detonation

Post by edherba »

1963 Corvair Engine (102 hp with powerglide)). Used Dwell-Tach for measuring RPM (running at approx 850rpm), then switched to Dwell at 58 sec into video. Revved engine to see if dwell varied more than 3 deg. Remained at 32 deg with slight change. Not sure if distributor is good. You can hear the pinging (detonation) when giving gas. If I retard the engine anymore, it will overheat, difficult to start and stall at lights. If I advance, it detonates even more. Running 94 Octane gas. Engine was rebuild last year and has 200 miles on it. Compression checked showed 150psi at all 6 cylinders. Valve lash adjusted.
Questions: Is the RPM variation acceptable? It varies between 3 and 18 BTC. Why does engine detonate under load?
Please see video: https://www.youtube.com/watch?v=3oOwyaWWqD4

joelsplace
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Re: 1963 Corvair Engine detonation

Post by joelsplace »

Is the vacuum advance connected to the correct port and working?
Make sure the vacuum advance doesn't bind anywhere in the travel.
Does the mechanical advance work properly? If the distributor is assembled incorrectly it can retard the timing instead of advance.
What spark plugs are you using?
Maybe lean mixture?
Are the cylinder baffles installed?
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bbodie52
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Re: 1963 Corvair Engine detonation

Post by bbodie52 »

This diagram may be of help in confirming your vacuum connections. The vacuum advance MUST be connected to the vertical "spark port" on the right carburetor. The vertical port only applies vacuum to the vacuum advance mechanism as the throttle is opened, to bring the vacuum advance timing into play at low to medium throttle settings. The horizontal vacuum port is directly connected to full intake manifold vacuum, and is intended to activate the choke pull-off diaphragm to partially open the choke butterfly the moment the cold engine starts. Connecting the horizontal port to the distributor vacuum advance would engage full vacuum advance at engine idle, which is too much timing advance at the wrong time.

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edherba
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Re: 1963 Corvair Engine detonation

Post by edherba »

Hi, Thanks for the vacuum connection diagram. I checked every connection and it is in the correct place and the hoses are new and tight. The carbs were rebuilt and have new gaskets on each side of the phenolic insulators. How can I check that there is no vacuum leak?
I am experiencing rough idle and detonation under load. The timing is bouncing around between 8 and 16 deg BTDC. I hooked up a vacuum gauge with a "T" with one hose running to the right side distributor advance port and the other hose running to the left vacuum advance port (removed plastic cap). I increased the engine speed to 1200 rpm and the vacuum is only 4 inHG. When I squeeze off the right hose the vacuum increases to 10 inHG. When I squeeze the left, vacuum drops to almost zero. According the manual, I should only have a variation of less than 1 in HG. So I definitely have an issue here. Firstly, I need to find out why the vacuum is so low at 1200 RPM, then adjust the left throttle rod to balance both sides. Could this imbalance and low vacuum explain the detonation under load? How to check for vacuum leak. I would appreciate any advice you can give. thanks

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bbodie52
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Re: 1963 Corvair Engine detonation

Post by bbodie52 »

Are you certain there is no vacuum leak on the side with zero vacuum? Have you verified the hose connection on the right side between the metal balance tube and the short tube connection to the intake manifold? With normal vacuum on the left, and zero on the right, it sure sounds like a major vacuum leak — like the balance tube hose was split or not connected at all. Such a vacuum leak would cause a very lean condition, pinging and detonation , as you described. Also confirm that the vacuum break diaphragm is in tact and not allowing a major vacuum leak via the hose connected to the choke vacuum break diaphragm. If you have essentially no vacuum on the right side, covering the carburetor intake briefly with your hand would likely expose your hand to little vacuum, since air is apparently entering the intake manifold from some other location.

Also, was a cylinder compression test run on all cylinders?
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terribleted
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Re: 1963 Corvair Engine detonation

Post by terribleted »

If the carbs are way out of balance and the engine is struggling to pull a load on one side only with high timing yes it would detonate. The carbs need to be balanced before really final setting the timing, because you can not set a real curb idle to set the timing without the carbs being balanced.
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edherba
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Re: 1963 Corvair Engine detonation

Post by edherba »

Thanks guys for your advice. I will check again the balance tube and the hose connection to the carbs. Maybe I will plug the line that runs to the powerglide modulator just to eliminate that. Sounds like a leak on the RHS.
The compression test was run on all cylinders and gave 150psi for all (I just rebuild the engine and it has less than 500 miles on it).

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Re: 1963 Corvair Engine detonation

Post by flat6_musik »

terribleted wrote:
Fri Jul 31, 2020 5:43 pm
If the carbs are way out of balance and the engine is struggling to pull a load on one side only with high timing yes it would detonate. The carbs need to be balanced before really final setting the timing, because you can not set a real curb idle to set the timing without the carbs being balanced.
Exactly. The carb throttle plates on each bank need to be adjusted and "cracked open" equally so each bank of 3 cylinders can start pulling their weight from a dead stop.

I'm not sure what's going on with that timing jumping around, but if me.....I'd drop a brand new distributor in. After all, it's a new motor.

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Re: 1963 Corvair Engine detonation

Post by 61SuperMonza »

Based on what you have said about your engine I would say double check for a vacuum leak. The only thing that hasn't been rebuilt is the distributor. So I would consider a replacement if it's a problem.
I would first check the carb on the 0 vacuum side and make sure it's getting fuel. I mention this because my second Corvair(66 monza coupe 110/PG) had a similar problem.
Turned out to be the fuel inlet needle was stuck. I had re built the carbs not long before. Was an easy fix. Check the bowl for fuel.
Running on 3 cylinders will detonate like you described. At idle with a quite exhaust you wouldn't notice that it running on one bank.

edherba
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Re: 1963 Corvair Engine detonation

Post by edherba »

Hi Guys,
So an update.... (the problem of normal vacuum on left and almost zero on right)
I used a turn buckle to advance engine speed to aprox 1200 RPM, then attached vacuum gauge to left and right spark ports. I sprayed "Quick Start" at the base of each carb: When I sprayed the LHS, no change in RPM. When I sprayed the RHS (with low vacuum), RPM increased to 1400 RPM. I guess the plastic spacer and gasket are not sealing. Should I just try tightening down the carb a bit and recheck, or take the RHS carb off and replace the gaskets? Also I could check that nothing blocked in the jet or idle circuit or fuel inlet needle.
Oh, I also re did the compression check. Took all the spark plugs out, held open throttle plate and choke, cranked 5-6 times and got 170 to 180psi. I think the more times I crank the higher and higher the pressure indicates. Or does it level off after a certain number of cranks?

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Re: 1963 Corvair Engine detonation

Post by bbodie52 »

When I last attempted to carefully remove the plastic insulators from the intake manifold carburetor pads, they came off in pieces. They can develop hairline cracks that will permit a vacuum leak. I would recommend that you order a new set of insulators and gaskets for the 2-carburetor engine and see if you can reseal that apparent vacuum leak. (When you pull the carburetor, you might also take a close look at that short piece of hose that connects the balance tube to the intake manifold tube. The hose sometimes can become "cooked" from the engine heat. The heat can make the rubber hard and brittle, and a split can develop that lets air in. At first glance, the hose may appear to be OK, but it could be split on the underside).

:link: http://www.corvair.com/user-cgi/catalog ... ow_page=56

Part number C506A: CARB BASE GASKETS & INSULATORS (2 CARBS) (4 GSKTS & 2 INSULATORS) SET FOR 4 CARB=C506B

Weight: 0 lbs 6 oz
Catalog Page(s): 56(35)
Price: $ 14.30


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Brad Bodie
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Re: 1963 Corvair Engine detonation

Post by terribleted »

If you are getting rpm increase by spraying bottom of carb and head you should replace the carb to head gaskets and maybe the insulator as well?
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edherba
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Re: 1963 Corvair Engine detonation

Post by edherba »

is it a good or bad idea to put a bit of gasket sealer on each side of the carb gaskets?

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bbodie52
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Re: 1963 Corvair Engine detonation

Post by bbodie52 »

I never put any type of gasket sealer on them. Installed dry and properly tightened they normally seal just fine. But if an unnoticed crack formed in one of the insulators that could be an open path for a vacuum leak.
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joelsplace
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Re: 1963 Corvair Engine detonation

Post by joelsplace »

No sealer needed. Supposedly they didn't have the gaskets originally just the spacers. Hose failures are much more common than cracked spacers. It happens but I've never had any fail but the hoses will fail at some point. Be sure the crossover tube that connects to the hoses is pointing straight at the tubes in the heads so there is no stress on the hoses.
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flat6_musik
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Re: 1963 Corvair Engine detonation

Post by flat6_musik »

edherba wrote:
Thu Aug 06, 2020 11:00 am
Oh, I also re did the compression check. Took all the spark plugs out, held open throttle plate and choke, cranked 5-6 times and got 170 to 180psi. I think the more times I crank the higher and higher the pressure indicates. Or does it level off after a certain number of cranks?
WOW......that is some serious compression! I'm curious to see how high it would go if you keep cranking.

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