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1974 L18 620 overheating

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Hey everyone, been reading alot of posts here and they have been alot of help. I picked up a 74 with a L18 put a weber 32/36 and it fired right up. As soon as it started though the fan broke off a blade. It dosnt overheat at idle even for 20 mins but as soon as i drive over 30 or more for about 5 minutes it overheats bad needle in red zone spewing coolant out of overflow everywhere..i pull over and repeat. It starts instantly every time and runs perfect but timing is retarded because it wont go any further counter clockwise to hit with timing light. Like i said it starts and runs awesome no choke ever hooked up and fires up in cold weather standing outside of truck. It does shutter when i shut it off and have to pop the clutch after turning off the key to get engine to stop because of timing. My question is would the retarded timing overheat it this fast? I have ordered a timing chain set and water pump but Wondering if it could be something else. Thanks

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watch this and get up to speed. if you cant center the timming then the oil pump is off a tooth or so.  Put motor on TDC. Its Ez open the oil cap and when the lobe you see right there is like poiting in the 9 o clock position then go to the crank pully then dial the timming mark up to zero on the timming tab or tooth what ever you have. then  you can pull the cap off and look where the rotor points too and see whchi way you have to go. But its in my vid to alighn a pump spindal and install. get new gaskets and make sure the pump dont loose oil and reinstall till the dist rotor is line up on #1  plug wire and you have movment in the timming to get what you need.

As for the over heating real bad I guess it could be it. I neever seen this myself. Try looking at the lower hose and rev up the motor is soft and rev motor up the lower hose will collaps.  how about trying a new stat, but hopefull you dont break the bolts if they rusted.


maybe you mean its too far advanced???????  I always noticed too retarded it dont even idle.



only get the japanese timming sets. Dont get the Mellig or Cloyes.!!!! why you get a timming set? Must be ready to change a head gasket


what spark plugs look like?


why change the water pump? unless it dont have any vanes on there its good. Most is just the bearing goes bad then leaks.

Edited by banzai510(hainz)
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Do NOT drive it again till fixed. Overheating is Kryptonite to aluminum head engines. You warp the head can't fix it scrap it. This sounds like a really nice '74 Look after it and you'll have tons of fun with it.


Fan blade fall off.... duh it over heats*.... maybe a new fan would help???? Missing blade will be badly out of balance and rough on the water pump bearing.



Take a good look at the timing marks on the crank pulley. The engine turns clockwise so the first notch to come into view is the most advanced. Zero or TDC will be the last one. I'm assuming there is a metal pointer on the timing cover???



*   Over heating tends to be one of two things not enough cooling air or not enough coolant. Other causes or over heating...


Air, lack of.

Slipping clutch fan

Missing fan shroud

Glazed or slipping belt

Radiator outer fins broken or plugged with leaf and bug debris

Broken fan

Grill obstruction like fog lights or spare tire mount, bush bar etc.


Coolant, lack of

Faulty thermostat or wrong heat range

 Inner radiator tubes blocked with hard water scale.

Soft lower rad hose that collapses from pump suction when revved up.

Leaking heater core so hoses joined instead of being plugged.


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Thank you guys for all this information you rock..will do all and report back..wont drive til fixed..have alot of parts coming for her..was sitting 20 years so you can imagine.

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Finally figured it out..ling story..the radiator cap has a place for an overflow hose that should go to an overflow reservoir..truck would nit overheat in park but when driving would blast coolant everywhere.. overheat pop hood see mess repeat..well today i put a hose from the cap to a two litre bottle remembering how i read a post by datzenmike that that hose is magic..well turns out it actually is magic because instead of throwing coolant everywhere this time i was able to see there was a leak in a small 3 inch long 1/2 inch 90 degree hose on the front passenger that was leaking ..an hour or more on the side of the road i had to scavenge and cut hoses and clamps from pcv and what not to make it work long enough to get to the auto parts and put my last beer in the rad.." gotta do what ya gotta do" ..im almost positive that tighting the alternator up to make the belt on the water pump tighter solved the problem ..it was hella tight but i think the combo of old belt old water pump and rpms was slipping at high speeds and water pump wasn't moving..i drove all day today in the heat and never passed the halfway mark..i have a good quality cast blade water pump coming as well as tsubaki timing set that should in theory help me sleep better... appreciate you cats and ratsun more than you know. 


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I would rinse out the radiator real good.

Aisin makes the better water pumps as they made in Japan or Atsugi marked ones. Most other are Chinese now on the market if under 40$  the GMB japan ones are OK. Im running a used one now as my Atsugi finnal gave up at about 180K miles.

If not over heating right now then the pump is GOOD. Remember the stat is closed most of the time so water is not really pumping too much. only thru the bypass hose if you have one.

Run the 50/50 mix from Walmart or where ever and should be fine.


Radiator cap? it depends on the cap you get. I use a flip up type dont need a resivoir but I have a bottle on there in case. if it over heats the lever should release.

My experince the non flip type run a little hotter. More or less it stop the pissing when I went to a flip type radiator cap.


check the cam timming marks and if with in spec and the sprocket theeth are not sharp I would leave it alone.  If anything is bad its the tensioer and the slack side guide.


If aint brok right now dont worry about it. IM TELLING YOU.  I would get a electronic ignition first, shocks,

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That bottle will catch any over flow but you have to pour it back in. If you get the proper coolant recovery rad cap it has a second valve that opens when the rad cools and the coolant in the recovery bottle will siphon back in always keeping the rad topped up. I used to top up every few weeks and by end of summer the anti freeze was weak. With the coolant recovery cap check once a year and always topped up.

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4 hours ago, Stoffregen Motorsports said:

Does the factory overflow bottle have a dip tube to siphon with? Or does it flow into and out of the bottom of the can? If dip tube, make sure there is one there or the system will never siphon.

I litterally poked a hole in a 1 litter water bottle lid and dropped a small tube down to the bottom and ran it to the cap..im super surprised how well this truck runs.. although i cant get gas to pump from the gas tank i haven't investigated as of yet..i just put a temp tank in the passenger floorboard and fill it up like every 10 miles..lol 

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I did the same but get the proper coolant recovery radiator cap.


Jesus, don't drive around with gas in the cab. Put it in the box and run a longer hose. It's way safer.



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If your going to clean or get that tanlk going I would get another stock gas filter and a another to run in line to the carb or before the carb

dont want junk and rust getting in the idle JET.  Do IT now.


blow air back to the tank maybe its varnish if sat of 20years. Or there is a cracked fuel line and its just trying to suck air as all the vacuum is being wasted before getting to the tank

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Working on getting the fuel done today..Yea i tried to blow through it as well as compressor.. previous owner said it has to do with the filler hoses or some smog stuff before the tank where you fill it that shuts of or plugs?..cant remember..also anyone know where to get a radiator fan?..its the clutchless type 4 blades..one is broke off. I have searched and searched and im thinking i might have to go electric.

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Btw Datzemike just ordered this rad cap for the 74 datsun 620 based on you advice from a different post most appreciative. Nissan part 21430-C0500


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