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AlexDeLarge

Help with my 74 620....going nucking futs....

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Ok guys, really need some help/advice with this truck. This thing is popping/backfiring out of the carb. Starts up and runs and will intermittently backfire through carb, less when fully warm. Everytime I try to give gas it pops through the carb. Here is what I have done:

New weber 32/36 installed with tall adapter, no vacuum leaks at base, I have checked and verified.

Float adjusted and rechecked twice. Positive it is in spec. Fuel in bowl.

Accelerator pump sprays strong stream in primary, fuel pump working at 3lbs, fuel line not kinked, fuel filter has clean gas.

Primary and secondary jets cleaned. EGR blocked off with plate and sealant.

New NGK plug wires, new NGK plugs, new rotor, new cap.

Verified plug wires are hooked up correctly 1,3,4,2 counterclockwise.

New Pertronix 1741 and Pertronix 3ohm coil installed with ballast bypassed.

Prior to first start set engine to TDC and verified the mark by pulling spark plug #1 and feeling piston come up on compression stroke.

Rotor was pointing at cylinder #1. Pulled distributor and verified tang was at 11:28.

Got truck running and warm and popping was intermittent at idle and occurred less when fully warm. Was able to time engine at 12 degrees before TDC  with light.

Cannot give it any gas without loud popping through carb.

I have seen this has been discussed many times when I search Ratsun. I have never checked valve lash and saw where that was the solution for one guy. I'm at a loss.

Before I switched to the Pertronix I could not get the truck to start.

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A tight intake may not allow the spring to tightly push the valve closed and sealed. Cylinder randomly fires past the valve seat and explodes all the gas and air in the intake. Likewise a badly worn intake and seat will do the same thing.

 

A very lean mixture will also do this.

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 Many years ago I saw a motor that sounded similar to yours that had about half a container of carb cleaner(?) poured down the throat of the carb while the motor was idling. It was poured just fast enough that the motor just barely was able to keep running while it was idling. Once the valves had been thoroughly coated with the cleaner a LARGE amount of the cleaner was then poured into the carb, killing the motor. The ignition was then turned off and the motor was left setting with the cleaner in the cylinders and all around the valves. After about 30 minutes (maybe less) the motor was started and it was run real hard (on the road) to get all the goo and carbon out of the motor. I was surprised at how well this worked to get the motor running real good again. I am thinking the valves were gummed up and sticking and this helped get the valves cleaned up. 

 

I don't remember many details since it has been so long. Probably since the late 60's, maybe mid 70's. It was a farm truck that didn't get used every day, so that is probably why the valves were gummed up. I'm not saying this is your problem, but it is something to check out.

 

(I just read datzenmike's reply. What he says goes right along with what I'm wondering about, except that he is thinking possibly tight valves. Doesn't matter, it will give the same result, popping back.)

 

Don

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  If your engine runs well and is kept tuned up carbon build up isn't really a problem but for all others or those making short trips there's the Coke bottle tune up.

 

Ever seen the top of a piston when the head gasket blows and coolant gets into the cylinder?? Steam cleaned. The water absorbs huge amounts of heat and the sudden cooling flakes the carbon and crud off the pistons, valves and the combustion chamber. A buddy back in the '60s showed me how to do this tune up. He put cardboard in front of the rad so the engine got very hot the took the air filter off, revved it and slowly poured a Coke bottle full of water in. The sudden loos of heat makes the engine slow down so you have to hold the throttle more and more open till it's howling. Then let the engine idle and warm up, believe me the the temp gauge will drop into the cold range. Let it warm up to hot and finish the bottle. All that water turns to steam and blows out the back taking a spray of carbon with it. I did this in the winter once and left a black patch on the snow the size of a kitchen sink.

 

Today I would run the windshield washer hose into the air filter and go for a drive.

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9 hours ago, wayno said:

How old is the gas in it?

Hey Wayno, the truck has been sitting since Sept. of '18. I started working on it May of this year. It  probably had 2 gallons of gas in it all that time it sat. About June of this year I put about 4 gallons of non ethanol gas in it. Do I need to drain all that gas to verify that's not the problem? Does the tank have a drain? What should I drain it in? I'm still learning guys...lol Not giving up, I'm hearing it from my wife and kids..."youre never gonna get that truck to run"...lol

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Yes has drain bung. Plastic gas can and funnel if it will fit under there or plastic basin and then transfer to a proper sealed container. You may end up putting this back in the tank and using it later. Do outside if possible. It's easy to be complacent with gas but the fumes are heavy and will flow down hill or fill a room looking for an ignition source.

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28 minutes ago, datzenmike said:

Yes has drain bung. Plastic gas can and funnel if it will fit under there or plastic basin and then transfer to a proper sealed container. You may end up putting this back in the tank and using it later. Do outside if possible. It's easy to be complacent with gas but the fumes are heavy and will flow down hill or fill a room looking for an ignition source.

Ok Mike, I will try to do this later today and hope this is the problem. Does bad gas usually cause this issue?

Now, I've got to get back to making these damn stators!! Lol

20201006_095141.thumb.jpg.d20ef88305bdd49d42d72fb448b5e2a3.jpg

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A cold engine does not fully vaporize the fuel droplets exiting the carburetor and will act lean. A lean mixture will burn very slowly and there is a good chance that there are still some burning even after the exhaust stroke and the intake valve opens. Carburetors only supply gas in a mist and requires that the engine warmth of the intake, head ports, intake valves and combustion chamber heat will do this. Raw gas will not burn, it has to evaporate first. To get around this, a choke is used which forces an extremely over rich condition to assure that enough gas will evaporate and the engine run well till warmed up. Make sure that the choke flap on the carburetor is closed during startup and fully open after about 5 to 10 minutes, depending on the outside temperature.

 

Bad gas which has absorbed water from the air can separate out and collect in the fuel bowl. It's heavier than gas and will roll around on the bottom blocking the jets. This could also cause a lean mixture because gas can't get through. I've had this happen, I believe, and because the engine ran, though not very well, I romped on it using the secondary and it eventually cleared itself out. It happened after a fill up so maybe some water at the gas station?  Pouring some methyl hydrate (gas line antifreeze) into the carb directly will absorb the water back into the gas and speed clearing it through.

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I'm at work, so I can't verify yet. Is this diagram correct, because if it is I have my #2 and #3 hooked up backwards. I have the wires as 1,3,4,2  left to right. Is this causing my issue possibly?Screenshot_20201006-123926.thumb.png.55101902c3971c669e962956c26d3bc6.png

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If those 2 wires are backwards its definitely the issue.... 

That is the correct firing order and direction....

 

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And you should be describing it as clockwise or counterclockwise...

Left to right doesnt translate to well with a circle....

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10 minutes ago, Crashtd420 said:

And you should be describing it as clockwise or counterclockwise...

Left to right doesnt translate to well with a circle....

Youre right dude, I get ahead of myself sometimes. I am going to go home after work and switch those around and hope all goes well. Pissed off at myself for not seeing this sooner.

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16 hours ago, AlexDeLarge said:

Verified plug wires are hooked up correctly 1,3,4,2 counterclockwise.

 

 

 

This statement is correct. On a clock face that's 12, 11, 10, 9 and so on.

 

Regardless this is a super easy and cheap fix for your backfiring.

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3 hours ago, datzenmike said:

 

This statement is correct. On a clock face that's 12, 11, 10, 9 and so on.

 

Regardless this is a super easy and cheap fix for your backfiring.

Yep, that was the problem. Runs great now. I timed it again and set the idle to around 850. Pulled it out for the first time in 2 years. That shitty thing is that I still have no brake pedal after changing pads on front, wheel cylinders on the front, new springs, new hoses, new rear hose. Guess I have to try and bleed again...

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Just wanted to say that after sitting for 2 years and working on the truck since May of this year and replacing/waiting on parts, studying and asking about how to do things, the truck is now running great and ready for the road. Big shout out to my favorite member, although we don't agree politically..lol,  datzenmike who answered all of my stupid questions and who is always so quick to offer advice and help anyway he can. Forum would not be the same without him. 👍

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I've said this before... there are no stupid questions, ok there might be some, but very rare, something on the order of the Planck constant. There is only knowing and not knowing. If you don't ask it's like saying you don't want to know something... that would be stupid. Now get out there and enjoy before the bad weather

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6 hours ago, datzenmike said:

I've said this before... there are no stupid questions, ok there might be some, but very rare, something on the order of the Planck constant. There is only knowing and not knowing. If you don't ask it's like saying you don't want to know something... that would be stupid. Now get out there and enjoy before the bad weather

 

6.62607004 × 10-34 m2 kg / s

 

The headlight on my vacuum cleaner burned out, Should I upgrade to HID or LED?

 

.

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LED

 

I left the Panasonic M-wave door open all day and the bulb burned out. Replaced it with w $14 LED.... nice.

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