Overheating and backfiring

[Oliver S.]

[Timothy B.]

Re: Overheating and backfiring

Oliver,

The best thing you can do is to verify that timing and distributor are in spec. After that check all your carburetor adjustments, accellerator pump etc.

If your car is 300 HP does it have the correct gaskets and shield under the carburetor.

On my 300 HP car, after I stop the hot engine the fan usually stops in one turn. I was told that's the best way to test a fan clutch. Your car should run smooth and be very easy to drive from stop so something is amiss.

[Jim T.]

Re: Overheating and backfiring

With the information given on the higher heat at idle and previous owners modification to the distributor, is the vacuum advance still operating and connected to full time vacuum source? Not having full time vacuum will increase operating temps.

[William C.]

Re: Overheating and backfiring

I'm with the others here, I'd start with the basics, Timing, vacuum advance, and also check to see that the centrifugal advance is working (disconnect the vac advance, and with a timing light hooked up, bring up the rpm on the engine and see if the timing advances)

[Jack H.]

Re: High Temperature and stalling

Should the fan clutch turn at all (engine off) when the engine is hot an you jolt it?

Yes, the fan should turn. The profile of a fan clutch changes with BOTH ambient temperature and input RPM. The output is %-slip and it NEVER goes to ZERO (fan clutch FULLY locked) or 100% (fan clutch FULLY free wheeling).

May the timing be the cause of both observations?

Sure could be a critical factor along with correct operation of BOTH the distributor's vac advance and its centrifical advance...

What else should I check or am I hunting ghosts (I'm used to newer cars)?

Check the pressure integrity of the cooling system. Most mechanics have a Stant tester that has two modes of operation"

(1) With adaptor installed, allows the rad cap to be removed from the car and exercised to verify it hold pressure properly AND it releases at its 'pop' point as it should.

(2) Without the adaptor, it installs on the radiator in place of your rad cap and displays actual coolant system pressure. Plus, it has a hand pump to boost system pressure above normal to check the integrity of various hose/clamp seal points.

Even though you're in Europe, the same technology applies and you should be able to find a local auto shop who can check/verify your cooling system's pressure integrity fast and cheap...

[Oliver S.]

[Grant M.]

Re: Overheating and backfiring

You said that the radiator was replaced by the previous owner. Is it a correct replacement (ie: aluminum/same cooling capacity), or is it perhaps a replacement brass or copper rad? I've read of overheating problems with replacement rads.

grant

[Oliver S.]

Re: Overheating and backfiring

I believe it's an aluminium one.

Oliver

[Jim T.]

Re: Overheating and backfiring

Oliver to check the vacuum advance, when your Corvette is at idle remove the rubber hose from the vacuum advance nipple. If your engines rpm does not change (becomes lower) and the rubber vacuum line you removed has vacuum, then most likely your vacuum advance diaphram has ruptured.
If you have no vacuum pull with the removed rubber hose at idle, but have vacuum when rpm's are increased, your vacuum source is ported vacuum and not full time vacuum.

[Oliver S.]

Re: Overheating and backfiring

Here's what I checked:
Before engine start

• removed vac. hose from carb and suck -> was ok - at least a hint that the vac works somehow.

Start engine:

• open pressure cap to see if there're bubbles in the expansion tank in case of a (bigger) head gasket problem -> no bubbles

engine warm:

[Timothy B.]

Re: Overheating and backfiring

Oliver,

I would set the inital timing at 10* at the low idle then set idle to stable RPM and adjust air/gas emulsion screws to highest vacuum or highest RPM. You may have to reset idle after that.

Is the cooling system going under pressure when the engine gets hot, does the top radiator hose get hard? Below is a pic of the correct gasket arrangment for my 63 car. I am pretty sure they are the same as your car so double check as the carburetor may be getting so hot the gasoline is boiling. The sandwich is manifold, gasket, black heat spacer, stainless shield then carburetor..

[Oliver S.]

[Donald L.]

Re: Overheating and backfiring

To minimize EGT you need idle timing in the mid twenties. At 10* with the VAC providing 16* when hooked up will provide the ideal amount of lead.

[Oliver S.]

[Jack H.]

Re: Overheating and backfiring

That's not the right way to visualize the system, Oliver... Advance is the product of TWO components: vacuum and RPM. The vac advance changes its contribution to the distributor timing based on absolute vac pressure (that's why you set initial timing/dwell/RPM with the hose disconnected and plugged).

Each distributor has an advance profile that varies with absolute pressure AND with RPM. The distribuor spec tables called out in the shop manual and/or chassis service manual provide 'snapshots' of how much advance should be generated based on a specific vac pressure AND how much advance should be generated by specific engine RPM. Of course, those tables presume the engine is bone stock in all other respects.

But, 16-degrees (and higher) of advance from the VAC system typically corresponds max vac pressure operation--NOT what you'd expect to see at/near idle.

So, the tuneup proceedure forces you to set BASELINE conditions (no VAC advance and no centrifical advance) for the distributor/ignition. Once that's set properly, you re-connect the vac advance line to the distributor and PRESUME the two systems (centrifical and vacuum) work as they were designed to...

That's why you hear folks questioning the TOTAL advance profile (rev the engine briefly and watch/measure advance) as well as the car's curb idle characteristics. It's a means of GROSSLY verifying that the two advance systems of the distributor are alive and well!

But, when you question what should be measured at points in between idle and WOT, you're in a grey area that's HARD to specifically quantify. It WILL vary with engine specifics and the specific advance characteristics of that engines' distributor.