1971 Datsun 521 Heartbreaker

[Matt H]

Coolant Leak
OK so the good news is the T-Bolt style clamp stopped the leak which means the solder on the radiator neck is ok. Of course as soon as I fixed it the other side started leaking - I sense the T-Bolt clamp is ever so slightly too big on that side which should be an easy fix.

Question: If the radiator cap was faulty would that force as leak somewhere by over pressurizing the coolant system?

I'm also noticing a very subtle sort of a rhythmic whirring noise every second or so. I'm going to fire it up again tomorrow to investigate - its very subtle but not something i've noticed before. I'm hoping perhaps its just the noise of the leaky radiator hose or maybe the sound of a pump? 

Fluctuating oil level
I'm also noticing after the oil change the level is fluctuating quite significantly. With a cold engine the level sits perfectly on the high line but when its hot and has been running its right on the low line. Is that normal for it to fluctuate so much?

Also the color of the oil is nearly clear. I put it in a whole bottle (1 pint / 16 fl oz) of TB Zinc Plus as per the directions on the back of the bottle along with the new oil. Is that too much additive for the amount of oil i'm using?

  

 

[datzenmike]

The radiator cap has a pressure relief built into it for 13 PSI. Normally there is a slight pressure in the cooling system but the engine NEVER gets this hot. Look at your temperature gauge. If in the normal run range then you're fine. You may have had two small weak spots. Once you fix one the other takes over.

 

 

 

Maybe reading the dipstick wrong? The oil is reading at the low level point. The line just to the right of the H is the full mark. Between is about one quart.  As I said check the oil with the truck on level surface. A cold engine will always read higher level than one that has just been run because the oil hasn't had time to drain back into the pan.

 

As long as at the full level point when starting for the day you are fine.

[Matt H]

Hey @datzenmike So I managed to get the truck going today and took it out for a good hour or so. I think all the oil thing was is after I had cycled the engine the level was reading much lower. I topped it up and now its at the Hi line even after its been running. The temp gauge is stead and stays in the centre.

Unfortunately i'm still having quite a bit of trouble with the truck...

Rough Start
It has a horrible time starting. I'd say t took around 12-15 attempt to crank the engine before it eventually started up and kept running. After its hot it start up right away but if you leave it for say 10 mins it will start to have trouble again. Regardless to how I set the choke (Pedal down, Choke out, pedal down once more) or me feathering the gas pedal it just does not want to start.

Strong smell of gas
There is a strong oder of gas coming from the car. A lot of the smell is coming right back into the cab. It's always been a bit smelly as there are a couple of wholes that aren't plugged in the firewall but now its so bad I was literally getting light headed trying to drive the truck even with all the windows open. The smell is quite extreme

White vapor coming from the carb
I managed to trace the smell back to the top of the carb. I noticed when I was testing the choke that there is a white vapor coming from the under the choke plate. I opened the choke with the engine off just to test it and I can see the white vapor seeping out.

Choke is closed 
When the choke knob is pushed in on my truck the choke plate is closed. When I pull the choke the choke plate is open say 25%. Is this correct? I was under the impression the choke should be all the way open as standard?

 

[datzenmike]

Choke should be on when knob pulled out and closed when in. Something is connected backwards.

When choke is off the plate should be fully open. This is the cause of the gas smells.

White vapor is just evaporated gas from engine heat, it's normal.

[Matt H]

I think i'm missing the choke plate!! After looking at the diagram I think what I was describing to you was actually the throttle valve. The thing I don't understand is i've had the truck for nearly 4 years and its never really been a problem until now. It looks like the previous owner might of snapped it off as one of the screws is sheared off and stuck in the thread. What are my options? Am I better off looking for a replacement carb or can I just replace this top section?

               

[fiveoneO]

You are missing your choke plate. That will cause your hard starting situation. If your carburetor's still in good shape, your best course of action would be to find another choke plate. If looking to upgrade your carburetor, I really like the Weber 32/36.

[Matt H]

Thanks @fiveoneO So I managed to get a couple of spare OEM Hitachi Carbs off ebay for parts. I now have the choke plate but the issue i'm having is its near damn impossible to get the choke screws out without shearing them clean off. Unfortunately I just learnt the hard way...




Am I missing a trick here? I tried putting some WD40 on them but the just break off in your hands the minute you try and move them.

As a temporary fix do you think I could get away with running one single choke screw to attach the choke plate on my existing set up or is that asking for trouble?

Do anyone know the correct size of the choke screws?

 

 

[fiveoneO]

Heat is probably your best bet at getting those screws out. With them being snapped off like that, might be easier to just carefully drill them out. I'm pretty sure they are brass which is a soft metal. I wouldn't recommend driving around with only one screw holding on the choke plate, I could see it slightly rotating in there and getting bound up, but I don't see any problem just trying it out and seeing if that fixes your problem. Not sure what size those screws are.

[EDM620]

2 minutes ago, fiveoneO said:

they are brass which is a soft metal

Heat should help, and you may have to drill them out. Tiny drill bits and a hand drill if you have one, better yet to clamp the shaft in a vise and use a drill press. Not a lot of room for error.

I suspect the screws are a standard size and thread, M3 I think.

6 minutes ago, fiveoneO said:

I wouldn't recommend driving around with only one screw holding on the choke plate

I had a choke plate fall out while on the highway eons ago, stranded me right in a national park resulting in an expensive tow bill

 

Note that the choke plate doesn't close totally flat, angle remains at like 1-2 degrees. The choke plate edge has a slight bevel so watch that when you reassemble to put the plate back correctly.

[Stoffregen Motorsports]

Those screws usually have thread locker on them, and they peen them in the middle after assembly.

 

If you can't get them out without screwing up the threads in the shafts, just go to the next size larger screw. They don't ned to be metric. Buy some generic screws like these - https://www.summitracing.com/parts/qft-5-20qft

[Matt H]

Thanks everyone. I tried to drill one and managed to get through but the case is stuck solid. The other I tried to heat which did help however what was left of the screw eventually broke off. @Stoffregen Motorsports If I go with a larger screw are you suggesting that I re tap the hole or will the the larger screw naturally bite by itself?

 

[Matt H]

Hey thanks @Stoffregen Motorsports I didn't realize how easy it was to re tap something! I used a hand tap to bore out the holes to #6 32. It's not the prettiest but its secure! Below is a mock up on my spare Carb. Its actually been suer helpful to have a spare to mess around on and act as damage control! I didn't realize how fragile this things are... Very easy to shear the screws and also you have to be super careful not to bend the choke shaft.



 

My next challenge is removing the existing choke shaft as it look like the previous owner created a dimple to keep it in place. The sheared off screws are also protruding which prevents me from removing it. I'm going to try and remove the top section of the carb and see if I can file down the existing screws.

 

 

[datzenmike]

The choke plate will never go through the throttle butterflies but those new screws could. Trim them and pinch threads with vice grips or thread locker.

[banzai510(hainz)]

just make sure it dont come apart and swallowed in the engine.

Can believe that stock carb is still working!!!!!!

 

PS I love this truck and the color,keep the stock wheels

Edited January 11, 2025 by banzai510(hainz)

[Matt H]

ok so I managed to remove the old choke shaft after filing down the protrusions. I have to say it was an absolute ball ache to get this thing back together. You have to be miracle worker to get those hairpin clips back in the linkages. However, I'm pleased to say mechanically it is now functioning as it should. I'm going to start it up in the morning and see if its fixed the trouble it had starting. I'm a little skeptical this will have fixed the 'smell of gas' issue as essentially when the choke is open and the engine warmed up it would be the same as having no choke plate in there. We shall see...

@datzenmike I've put some blue thread locker in there. Is that enough? The bolts are long but they don't foul anything. When you say pinch them do you mean like squash the thread? 

I also saw these online - seeing as I now made the thread #6 32  

https://www.carburetor-parts.com/stromberg-throttle-choke-plate-screws-s131-57-set-of-10.html

https://www.carburetor-parts.com/Choke-Throttle-Screw-6-x-32-x-516--80-5-Set-of-10_p_6290.html?srsltid=AfmBOop9mn6MpciBzcXGpZDEtWYb_MmsWwKAGcDEGsskESMMsIsA1pM_imw

Do you recommend I change them for these? 

I would like to avoid having to take it all apart again is possible 😅

Edited January 12, 2025 by Matt H

[datzenmike]

Yeah squash the threads slightly so if loose it can't unscrew, but if you have thread locker that should hold them. No, leave it don't take it apart again.

 

 

 

Gas smell. Your PCV valve is functioning? There is a vent line from the gas tank to a thing called a flow guide valve. Should be below the brake master screwed to the body metal. Has three small hoses to it. One to tank, one to block vent pipe and one to the air filter. Looks like this...

 

  

 

Engine off, any evaporated gas fumes push through the flow guide valve and are directed to the crankcase for storage. Engine running the fumes are drawn out of the crankcase by intake vacuum through the PCV valve. As gas tank empties, air from the air filter is drawn into the tank by suction to replace the gas removed. It's a closed system so fumes will get out if hoses are off or cracked.

 

Gas leaks even small will stink a lot. Inspect all around the pump, filter lines, carburetor. Look for and seal off any openings from the cab into the engine compartment.

 

[Matt H]

Hey @datzenmike Is this the PCV valve? I've also been a little suspicious of this hose as it looks like its leaking

 

 

 

[datzenmike]

The pipe exiting the engine is the block vent. The large hose connects it to the PCV valve screwed into the intake below the carburetor. The tiny hose connects to the flow guide valve and allows gas fumes into the engine for storage when engine is off. Engine on, the PCV valve sucks fumes out of the engine.

 

Later set ups use a charcoal canister to store fumes when engine is off.

[Matt H]

Crankcase blowby hose was split... and absolutely filthy





Does anyone have any ideas on how best to plug these holes in the passenger side firewall? Is duct tape a viable option 😅

 

 

 

 

 

 

Edited January 12, 2025 by Matt H

[banzai510(hainz)]

On my weber the gas smellw was a crack accell pump diaphrame and was a slight leak . Most likely not your proplem but these are old carbed vehicles. Back in the day was kinda normal

[Matt H]

Thanks @banzai510(hainz) Yeah its always been a little smelly but last time was ridiculous. I was literally getting light headed and headaches trying to drive the thing with all the windows down. I noticed the accell pump dustboot is split but I haven't checked the diaphragm. I'm going to start it up later today and hopefully see if the work i've done over the weekend has resolved the issue. I think the split blowby hose might off been the cause but we shall see!

[banzai510(hainz)]

dont need to start it up just cycle the linkage(if the float bowl is ful)  if bad it will squirt out the plunger or side of it and work its way out sometimes to the exhaust manifold

 

Rare but if a manual fue pump is bad it can suck gas in to the crankcase. Smell the dipstick or maybe its filling up.

 

 

blow by smelll is mostly at idle  it will come out the crank case. when speed up it actually will suck air in.

Edited January 13, 2025 by banzai510(hainz)

[datzenmike]

Measure the hole diameter. 1 1/2 38mm???

 

 

https://www.amazon.ca/Flyshop-Grommets-Synthetic-Rubber-Protection/dp/B07841YZM5

[Matt H]

@datzenmike Truck started up first time from cold in less than 2 seconds!! I can't believe what a difference that has made... I didn't even need to touch the accelerator pedal. Just set the choke and it started the minute I cranked the engine. All those months I was out there cranking the gas pedal 😅

Thanks all thats a game changer 🙌 
 

 

[NC85ST]

On 1/12/2025 at 3:39 PM, Matt H said:

Is duct tape a viable option 😅

Sorry, no, you’re from the wrong part of the country. You’re not from the south. Haha. Maybe if you’re in Southern California.