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Hard start when warm/stalling


JWray707

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Just bench tested the ignition coil...primary is at between 1.8 and 2 ohms. Secondary is not reading at all. Maybe due to my crappy multimeter.

Sorry, I know theres a bunch of different issues im bringing up. Still learning. Really appreciate this forum and all the informed folks on here though.

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Connect test lead to negative terminal. Check the + positive and the secondary output at the top.

 

The '78 should read 0.8 to 1.0 ohms between the + and - coil terminals. Looks to me like you have a points coil. Your EI ignition will draw so much current through a points coil it will over heat it. Replace with a '78 or newer Nissan coil. Truck, car it doesn't matter.

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There's also one more thing I can't find anything about in diagrams or the manuals. (This is a photo before I replaced the carb.)

 

KF7EfX.jpg

 

What is that little unit?

I know that T connection is wrong but what is supposed to connect there? I've been putting the vacuum for the AB valve on there.

That large tube is running from the master vac directly into the manifold.

 

Your 620 has/had an air conditioner, right? I believe that item with the ? is the fast-idle solenoid to idle-up when the compressor is energized. If you don't have operating air you don't need it. Just be sure there is no vacumn leak.

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whats the brass tip on lift side of carb near the bottom?

That was the old carb, thanks to datzenmike, found out this was the wrong carb for this year/model. Trying to figure the vacuum system with the wrong carb was a damn headache! Hah.

Got the right carb on there and the vacuum system made much more sense, but,thats when these other issues started...

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  • 4 weeks later...

So, I'm exhausting all my resources on this one. Figured I'd post on here before I call a mechanic. Would love to be able to figure it out on my own, but I'm pulling my hair out trying to figure out what's going on.

Three main issues.

1. Carb tuning.

2. Hesitation on slight acceleration.

3. Heating up hotter than normal.

I replaced the carb recently and have been trying to tune it, but it's hard to tell what the afr is or if it's correct. I know the procedure for tuning, just trying to get it set right.

-Bought a timing light and set the timing to 12.

-Replaced the spark plugs.

-Tested the resistance on all the plug wires and the only one that is off is #1. All the rest measured 7.2 ohms, #1 was 6.8...would that be enough of a difference to cause the hesitation?

The only other thing I could think of was the float bowl. It's not filled up to the line on the sight, it's about 3/4 of the way up. Does the fuel level NEED to be at the line on the sight glass? Would this cause the hesitation?

No matter how I tune the carb the hesitation still happens.

Also, hooked up a vacuum gauge to the manifold and it's reading at a steady 20, so I know there's no vacuum leak.

Any ideas would help. This is a 1978 620 L20b

Sorry for writing a damn novel here, just trying to cover all my bases.

So you replaced that wrong carb with another one? What is it Hitachi? Weber? Maybe post a picture of it.

 

 

 

Ignition timing at 12 but idle must be below 900 rpms.. is it? Any higher and mechanical advance will creep in and you will be setting retarded.

 

Pull vacuum advance hose of carb and suck on the end while watching under the distributor cap at the rotor. The rotor should turn slightly clockwise and hold till you release it. If it moves weakly twist by hand to see if it was just sticky. If still weak check the advance hose for cracks and leaks.

 

What spark plugs? Should be NGK BP6ES for points and gap 0.032"

 

Wire resistance is likely higher and you are reading the wrong scale or multiplier. I would expect over 10,000 under 30,000 ohms. https://www.ngk.de/en/technology-in-detail/ignition-cables/diagnosis/testing-of-ignition-cables/

 

+ and - coil terminal wires are clean and tight, nothing loose.

 

Wire to distributor from coil - terminal is well and securely connected to the points or if from California, do you have breakerless electronic distributor???

 

Pull high tension wire off coil and inspect the coil plastic top specially the underside or hard to see part. Look for any cracks or carbon tracking. Carbon tacking is when a spark travels across a surface and leaves a spider web fine mark. This is a weak point in the insulation and a spark is more likely to jump across it again. Spark ALWAYS looks for the easiest path to ground so you must assure that your wires, nipples, distributor cap, rotor and spark plugs are clean and in good insulating shape. Check each wire individually for damage to the outer insulation. You can also run the engine at night or a dark garage and look for any blue flashes indicating an escaping spark. If they are all good, the easiest path to ground will be the plug gap, which is what you want.

 

 

Reading plugs....

 

Take for a drive on the highway warmed up and for 15 min or so. Take your spark plug wrench. Pull over and check the plugs. Creamy light brown is ok, darker is richer, black is really a problem, as is very light and white White shiny porcelain indicates a very lean mixture.

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So you replaced that wrong carb with another one? What is it Hitachi? Weber? Maybe post a picture of it.

 

 

 

Ignition timing at 12 but idle must be below 900 rpms.. is it? Any higher and mechanical advance will creep in and you will be setting retarded.

 

Pull vacuum advance hose of carb and suck on the end while watching under the distributor cap at the rotor. The rotor should turn slightly clockwise and hold till you release it. If it moves weakly twist by hand to see if it was just sticky. If still weak check the advance hose for cracks and leaks.

 

What spark plugs? Should be NGK BP6ES for points and gap 0.032"

 

Wire resistance is likely higher and you are reading the wrong scale or multiplier. I would expect over 10,000 under 30,000 ohms. https://www.ngk.de/en/technology-in-detail/ignition-cables/diagnosis/testing-of-ignition-cables/

 

+ and - coil terminal wires are clean and tight, nothing loose.

 

Wire to distributor from coil - terminal is well and securely connected to the points or if from California, do you have breakerless electronic distributor???

 

Pull high tension wire off coil and inspect the coil plastic top specially the underside or hard to see part. Look for any cracks or carbon tracking. Carbon tacking is when a spark travels across a surface and leaves a spider web fine mark. This is a weak point in the insulation and a spark is more likely to jump across it again. Spark ALWAYS looks for the easiest path to ground so you must assure that your wires, nipples, distributor cap, rotor and spark plugs are clean and in good insulating shape. Check each wire individually for damage to the outer insulation. You can also run the engine at night or a dark garage and look for any blue flashes indicating an escaping spark. If they are all good, the easiest path to ground will be the plug gap, which is what you want.

 

 

Reading plugs....

 

Take for a drive on the highway warmed up and for 15 min or so. Take your spark plug wrench. Pull over and check the plugs. Creamy light brown is ok, darker is richer, black is really a problem, as is very light and white White shiny porcelain indicates a very lean mixture.

So, I did replace the carb. The new one is a remanufactered Hatachi. I will post a picture tomorrow.

 

Just downloaded a stroboscope app to help determine RPM so I need to try and determine the speed tomorrow night (the light is not that bright)

 

I will pull the vac advance hose and follow your instructions. Shouldnt be any cracks in any of the vacuum hoses as I replaced them all, as well as all the fuel hoses. I will inspect anyways though.

 

Spark plugs were NGK-BPR6EY and I set the gap to 0.040" (via specs from davidmcmurphy.com)

I have another set of new plugs that are NGK-BP5ES-11, would you advise putting those plugs in and setting the gap to 0.032"?

 

I had the multimeter on 200k on the ohm section and it was coming up with those readings. Either way they all read the same other than #1 and the wire from distributor to coil was 4.6 (because its shorter, right?)

 

Coil is brand new and I cleaned the connectors on the wires, both are clean and tight.

 

I dont know if I have a breakerless distributor, how would I tell? The previous owner said it was a newly re-manufactured distributor.

 

I will test all the wires for carbon tracking. Also saw another method of grounding a screwdriver and running it along the wires to check for an arc somewhere. Will do that tomorrow aswell.

 

Im a bit nervous of driving it on the highway as its heating up just going back and forth in my driveway. Also the hesitation makes me hesitant.

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I think the California trucks were breakerless but for now that doesn't matter much. I don't think the early ones were high output so a 0.040" gap will be too much. Try 0.032"

 

Carbon tracking is more evident on the cap, rotor and the coil plastic surfaces than the wires.

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uFRzOZ.jpg

 

There are some pics of the new carb.

 

This morning I pulled all the plugs and reset the gaps to 0.032" and checked all of them for spark. They all had a bit of blue but the majority of color was yellow. Considering they're new plugs I'd guess that has to do with the wires or distributor?

Also checked the wires for arc with grounded screwdriver and later water and didn't see anything. They look dirty and a few minor shallow cuts but nothing enough to cause arcing apparently.

 

Also checked the vac advance hose and it did turn slightly clockwise.

 

Gonna go pull the plug wires off and inspect in more detail one by one.

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dude you time this motor?

Plug wires just buy new NGK wire set NE64.  Unless the ends come off most are good.

 

if motor is mechanical timed then time the distributor than adjust the carb.  distributor could be worn and carb out of adjust to make it look worse than it is.

Im 8 years on same distributor cap and use mismatch spark plugs!!!!!!!!

 

how about take photo of inside the distributor cap so we don't have to guess if this is a point or electric distributor

 

 

what is the exact proplem now after carb swap?  choke still on? not adjusted?

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Yes, I timed the motor with a light to 12.

I did just order a new set of wires and will be picking them up tomorrow. See if that helps anything.

 

pYL95u.jpg

 

ZDDHe1.jpg

 

The choke is working fine.

The main issues are that the engine is heating up more than before the swap (was running between 1/4 and 1/2 and now 3/4 or more usually) and the hesitation when lightly hitting the gas. If I don't press hard and fast enough on the pedal (from a stop) it will hesitate and some times even shut off.

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Yeah, I was planning on looking at the timing again tonight and checking the RPMs when it gets a little darker. I did set it at twelve when I timed it last. There's even two little lines on the distributor mount that lined up almost perfect at 12. I will try to snap a picture tonight.

Also have read that the AFR can affect engine temp. Just replaced the thermostat (and tested before install) so I figure it has something to do with timing or AFR.

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Just noticed you have a '78 so YES you have EI system. Plugs MUST BE NGK should be BP6ES 11 and gap 0.042" sorry for misunderstanding.

 

 

 

U7HQTe.jpg

 

The two hoses from the base of the carb. Make sure the front one goes to the distributor. The other goes to the temp vacuum valve on the side of the thermostat housing and then to the EGR.

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A issue I was having timing it was #1 and #2 wires weren't lighting up the light as consistently as #3 and #4 were. Since the plugs are new I assumed it was the wires. I'll put the new ones on tomorrow test the light with those, see if it has a more constant flash.

If not the wires it would have to be the distributor, yeah?

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Just noticed you have a '78 so YES you have EI system. Plugs MUST BE NGK should be BP6ES 11 and gap 0.042" sorry for misunderstanding.

 

 

The two hoses from the base of the carb. Make sure the front one goes to the distributor. The other goes to the temp vacuum valve on the side of the thermostat housing and then to the EGR.

OK, I will reset the plug gap. Probably just put in the new set I got when I put the new wires in. They are NGK BP6ES11

 

Those two hoses are going to the right places. The only vacuum port I'm not entirely sure about is the AB valve which is currently connected with a t from the intake manifold teeing to the charcoal canister.

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Yeah, I was planning on looking at the timing again tonight and checking the RPMs when it gets a little darker. I did set it at twelve when I timed it last. There's even two little lines on the distributor mount that lined up almost perfect at 12. I will try to snap a picture tonight.

Also have read that the AFR can affect engine temp. Just replaced the thermostat (and tested before install) so I figure it has something to do with timing or AFR.

 

Often an owner says running hot but this is just that they think or expect. Not saying you are wrong but what makes you think it's running hot? I've seen pages go by and then find out a 'problem' is actually something quite normal. Anywhere in the run range is safe but about half way in the run range is normal. If it normally runs about half way and then begins running higher it may be an indication that something has changed. Summer driving may run a little higher.

 

 

Some things that may cause warmer than usual running...

 

Is the heater hose from the rear of the head by-passing the heater and connected directly back to the lower rad hose? Block the flow or fix the heater which has a shut off in it.

 

Drain a couple of quarts out of the rad to expose the tops of the cooling tubes inside the top tank. Take the rad cap off and look. Are they crusty with dissolved minerals like the inside of an electric kettle? A crusty rad will not transfer heat properly.

 

Feel the lower rad hose.  They may look OK on the outside but rut from within and become soft. Whatch the lower hose while you rev the engine. Suction from the water pump can collapse the lower rad hose.

 

Do you have a fan shroud? It increases fan efficiency.

 

Engine off. Turn the fan blades by hand. There should be firm but smooth resistance. You should not be able to spin the blades... only turn them. The looser the clutch fan the more it will slip and the less air moved through the rad.

 

Retarded timing dumps huge amounts of heat into the exhaust ports and from there into the cooling system.

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Try this, set your timing at TDC, make sure you know where TDC is in the pulley or tab, I think later ones had timing tab. Take a picture and show.

This model does have a timing tab and as far as I can see is set at twelve. Can't really get a good picture (tough angle.) Either my timing light is defective or my plug wires are toast. Not getting enough light strobe to clearly see the tab or the mark.

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