'67 427/435 overheat

[Ray Dattilo]

I am new to 427's and haven't driven this recent purchase much yet. It has overheated a couple of times and dumped radiator fluid with what I consider normal driving and reasonably cool ambient temps (60's). Before I go taking things apart, I wanted to make sure that overheating isn't a part of owning a 427/435. Fan works fine. I did drive it about 70 mph for about 20 min. today on highway at 3500 rpm (it has 4.11 rear) and it made a mess when I got it in the garage. Your thoughts are appreciated.
BTW, my '71 LS5 chevelle never overheats.
Regards,
Ray

[Joe L.]

Re: '67 427/435 overheat

Ray-----

How hot is the engine getting as shown by the in-car temperature gauge?

[Ray Dattilo]

Re: '67 427/435 overheat

Joe,
The gauge reads 210 to 220. I realize there are a number of things I will need to check. I do not currently own a IR temp gun but plan to get one (I am new at this). Before I start down the path of checking everything from the radiator to thermostat to fan clutch, etc., I wanted to be certain that the overflow I'm getting after routine drives is atypical. I should mention that the gauge gets up to about 230 to 240 or so after shut down from heat soak I guess and that's when it spills.
I apologise for not checking the archives first; you guys are probably tired though obviously polite about answering the same questions over and over again.
Regards,
Ray

[Jay Hastings]

Re: '67 427/435 overheat

Ray
Their is a bypass hose that goes from the intake manifold to the water pump
take that hose off and get a brass soft plug that fits the inside of that hose
drill a 1/4" hole in it put it inside the hose and reinstall. that bypass is too big and lets too much uncooled water flow back through the motor. A big block race car guy told me to do this to my 435 and it hasent overheated since

I used to try to keep the rad. full and it was always overflowing then I saw the full line way down at the edge where the fan shroud goes around the side of the rad. so I now keep it at that level and no more puddles.
now if i could just get it to stop dieseling when I shut it off it would be perfect.
jay

[Ray Dattilo]

Re: '67 427/435 overheat

Jay,
I only just noticed the "fill line" yesterday. I'll try decreasing the size of the bypass hose opening as you suggested and see how it goes. Thanks for suggestion.
Regards,
Ray

[Clem Z.]

Re: '67 427/435 overheat

the bypass hose deals works because we did that to all our BB race cars back in the 60s. since we used a restrictor instead of a thermostat we just blocked it completely. it bypasses too much water back to the block. also use a robertshaw thermostst as they work much better than the stock type.

[Clem Z.]

engie run on

try drilling a 1/16" hole in each of the center carb butterflys towards the front edge, so you can close down the idle speed screw to prevent fuel from being pulled into the engine either from the idle transfer slots or the boster nozzles after you shut it off and being fired by the hot spots in the chambers.you may have to go larger on the hole to get the correct idle speed.

[Duke W.]

Re: '67 427/435 overheat

We've been through this with the '66 L-72. It turns out that they have "ported" vacuum advance, so there is little or no vacuum advance at idle, which makes them run hot. You might want to check the archives.

Determine how much vacuum signal you have at the vacuum can at idle with idle vacuum checked from a full manifold source. If the vacuum can does not see full manifold vacuum, reroute the signal line to a full vacuum source.

Also, the L-71 vacuum can does not provide full vacuum advance at normal idle vacuum, so I recommend you swap it out with a NAPA/Echlin VC1810.

You might want to check the recent archive on this vis-a-vis the L-72. I think your problem is ten bucks and an hours work away from solution. The vacuum can may be swapped on the mag pulse distributor without removing it from the engine.

The following are the igntion map specs for a '67 L-71.

Initial timing - 5 deg (no range specified as on the L-72)

cent: 0@900, (no intermediate spec), 30 @ 3800

Vacuum: (no start spec) 15 deg @ 15.5"

Since the engie will only pull about 10-12" at idle, this is insufficent to provide full vacuum advance. The VC 1810 provides a max of 16 degrees at 8". Also, I would suggest increasing the initial timing to at least 10 degrees. SHP engines need 25-30 degrees of total idle timing (vacuum plus initial advance)to idle at peak effeciency, which mill minimize EGT, heat rejection to the cooling system, and and combustion chamber boundary surface temps. The later is what causes runon.

The above applies to overheating at idle and low speed driving. If it overheats at highway speed, the radiator likely needs to be pulled and rodded out.

Duke

[John Lokay]

Re: '67 427/435 overheat

Would someone explain to me the purpose of the by-pass hose? It sounds to me that the consensus opinion on the DB is to retrict the coolant flow to help reduce engine temps.

[Duke W.]

Re: '67 427/435 overheat

The purpose of the bypass hose is to allow quick and even warm-up prior to thermostat opening. Early SHP/FI 327s have the external bypass along with the internal bypass, but the external bypass was eliminated on later SHP small blocks. Medium performance 327s don't have an external bypass.

The heater circuit also functions as a bypass.

The bypass(es) also help reduce the thermal shock that occurs when the thermostat opens, especially in very cold weather when the radiator has coolant at ambient temperature. As the thermostat just begins to open the bypass keeps the extremely cold coolant from going directly to the engine because it is mixed with bypass coolant.

I don't think overheating problems with any engines have anything to do with the bypass. They didn't run hot or overheat when they were new!

Instead of trying to "re-engineer" the cooling system, find the root cause of the problem and correct it.

Duke

[Jay Hastings]

Re: '67 427/435 overheat

thanks Duke
I will try the new vac canister, tweak the timing and check for vac.at idle.

jay

[Jim T.]

Re: '67 427/435 overheat

You could try having one foot on the brake and at the same time of turning off the key with the car in gear let to clutch out just enough to stop the engine from turning.

[David D.]

Re: '67 427/435 overheat

Jim, That's the way we did it in the 60's. Dave