68 427/400 Fan Clutch Causing Vapor Lock?

[Jack C.]

Can a suspect fan clutch be a contributing factor to cause vapor lock?

[Jack H.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

I would think you'd be complaining of high temp gauge readings (warm engine at idle) or radiator boil over before you'd see vapor locking...

[Rich P.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

Jack,

a slipping fan clutch will add to heat under the hood. Which could add to vapor lock. But if you were using pump gas it will boil faster than you can make 3 WOT 1-2-3 runs. I was out in a 67 435 today which tend to run cooler than 68's and even as cool as it was today I was flirting with vapor lock. I know you sometimes use 100LL I would be suprised if that would vapor lock. But with no additives with todays gas you'll always be walking that fine line.

Rich

[Jack C.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

Thanks guy's. I am running 100% 100LL. I have nearly no resistence on the clutch. No boil over. Once it sits for 15 minutes it starts right back up. I'm running a 165 thermostat and no signs of any overheating.

[Clem Z.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

make sure the bottom rad hose is not touching the fuel line to the carb.

[Jack C.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

Yes, that's fine, plently of clearence.

[Jack H.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

So, alcohol doped pump fuel isn't an issue. The rating of your thermostat is a non-issue. That simply dictates when the thermostat opens on initial cold start and after that engine coolant temp is a function of the radiator, water pump, fan/clutch and coolant system sealing integrity.

If the fan clutch turns without noticeable resistance when it's cold, you've got a problem there. Expect it to 'bite' you under hot engine idle conditions (excessive slip and resulting lack of fresh air flow through the radiator).

When cold, a fresh fan clutch should exhibit noticeable resistance to being turned by hand... But, that's just one of several things that can contribute to fuel percolation and vapor lock.

Correct carb to intake gasket configuration, pump to carb fuel line configuration, and radiator seal dam components are other considerations. Sometimes these problems result from a combination of deviations where there's no SINGLE item to change out to see the problem come & go.

[Jack C.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

Jack, I'm pretty much to the book on all of the other items, however, the fan clutch spins about 3 quarters of a revolution with a pull of the finger.

Thanks, next stop, fan clutch.

[Ken A.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

Do you have a hard/no start situation after heat soak? If so, and it has a TI system, then I would suspect the old/early repop circuit board in the amplifier.

[Jack C.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

Once it cools for about 15 minutes it starts right up, no heat soak symptoms. It does have TI. It normally happens when I start to sit and idle in once place for a few minutes.

[Clem Z.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

Thanks guy's. I am running 100% 100LL. I have nearly no resistence on the clutch. No boil over. Once it sits for 15 minutes it starts right back up. I'm running a 165 thermostat and no signs of any overheating.

i would check for spark during the non start period and if you have spark you have a fuel problem

[Duke W.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

[quote=Jack Humphrey (17100);420264]
If the fan clutch turns without noticeable resistance when it's cold, you've got a problem there.
quote]

The fan should have more resistance to turning by hand when the engine is very hot than when cold.

Get the engine hot by letting it idle for an extended period in warm to hot weather - coolant temp over 200 - then shut the engine down and turn the fan.

It should have much more resistance than when cold.

Fan clutch tightening is based on the temperature of the air exiting the radiator, so getting the engine hot with extended idling should increase the radiator air out temperature to the point where the clutch tightens.
If not, the fan clutch is likely not performing to spec.

Duke

[Ken A.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

Once those old germanium transistors get to 200 degrees, then they fail to function correctly, a flaw of the 60's technology.

[Jack C.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

Thanks Duke, I had it out this morning and when it went into vapor lock at idle when I returned to the driveway, the fan clutch was very easy to turn 3 fan blades worth of revolutions with the pull of the finger. Now the the car cooled down, there is slightly more resistence. It seems I have exactly the opposite of what you're describing that it should be when fuctioning to spec.

[quote=Duke Williams (22045);420284]

If the fan clutch turns without noticeable resistance when it's cold, you've got a problem there.
quote]

The fan should have more resistance to turning by hand when the engine is very hot than when cold.

Get the engine hot by letting it idle for an extended period in warm to hot weather - coolant temp over 200 - then shut the engine down and turn the fan.

It should have much more resistance than when cold.

Fan clutch tightening is based on the temperature of the air exiting the radiator, so getting the engine hot with extended idling should increase the radiator air out temperature to the point where the clutch tightens.
If not, the fan clutch is likely not performing to spec.

Duke

[Duke W.]

Re: 68 427/400 Fan Clutch Causing Vapor Lock?

You are correct. The purpose of the fan clutch is to only engage the fan when greater air flow is required through the radiator to maintain coolant temperature within the normal operating range. This reduces fan noise and power absorption.

When radiator outlet air temp is relatively low (due to high flow through the radiator such as highway speed) the fan will only turn about 1500 at high engine speed. When fully tightened due to high radiator air out temperature (like at extended idle in hot weather when air flow through the radiator is low)it will spin at near engine speed up to about 3500, and this absorbs up to about 10 HP and makes a lot of noise Since fan horsepower increases with the cube of speed, at 1500 it is only consuming about one HP and generates little noise.

Think of the radiator as a heater core. For a given coolant inlet temperature and flow rate, the less air is flowing through the radiator the hotter the exit air will be.

The clutch fan was a pretty "high-tech" deal for its day. Most cars had simple fixed fans back in the sixties.

You can get an idea of how fast the fan is turning relative to engine speed by shining a timing light at it. The fan will usually appear to go "backwards" because it is turning lower speed than the engine, and when cold, its "backwards" speed will appear to be lower at 3000 RPM than when it is hot. Similarly at idle in cool weather the fan will appear to go slightly backwards. In hot weather at extended idle it will appear nearer to stationary.

You can also get an idea of fan clutch engagement by revving the engine to 2500-3000 and comparing air flow and fan noise when cold and hot- like when the temp is not quite up to thermostat opening temperature and after an after extended period of idling in hot weather.

Duke