620 misfiring

[620connor]

I have a 74 datsun 620 with the stock L18 engine and for some reason the truck is misfiring really bad and i cant figure out why. I have installed a new cap, a new rotor, new points and condenser, new spark plug wires, and new spark plugs and it made no difference at all. I removed and checked one of the spark plugs for spark to see how it was firing and it didnt look to good, it will fire normal and then it will fire really weak and then it wont fire at all. I took a wire and jumped it from the battery directly to the coil and it still did the same thing. A few months back i installed a new Weber carburetor when it started this misfiring problem and it ran okay for a while until it died recently. If anyone has any info that could help me with this i would greatly appreciate it. Thanks.

[dat77sun]

my 620 just started doing the same thing, i replaced all the stuff you did, and i determined that its the carb, its letting too much gas to the cylinders. is yours dumping black smoke out of the exhaust?

[620connor]

No black smoke is coming out of the exhaust, i have a fuel regulater on there. I dont think its a fuel problem.

[Skib]

have you tried replacing the coil?

[620connor]

have you tried replacing the coil?

Yes i did. i forgot to mention that. i tried a newer coil and it did the same thing.

[Zeusimo]

Might Be Your Carburetor.

My Accelerator Pump Is Shot And It Is A B1t3H To Drive Until Completely Warmed Up.

It Misses When Trying To Drive Right After Turning On.

[datzenmike]

Pull the plugs and read them. This one of the best methods to see what is going on inside the cylinder. They should be light brown or tan, even creamy colored. Black dry and sooty would indicate a too rich condition and carbon is a conductor that will leak high voltage to ground causing a missfire. Black oily could be oil fouled on one or more plugs.

[banzai510(hainz)]

If Datzenmike is correct then running new plugs right off that bat would change the situation..But you said you put new sparkplugs and it was the same.

However when you put a new coil on there make sure you go the coil correct for a point vehicle. Dont just buy a coil for a unknown car. It does matter.

 

I would ck the points again to make sure they are set right. Regap them. if burnd up already then you have a low ohm coil in there.I assume the Ballast resisitor is still there(the white ceramic block looking thing)

also maybe do a valve lash ck. maybe the lash is close enought to give a cylinder lost. No matter how good a spark if to dont detonate(no compression) then Nothing will happen.

 

hard to believe new weber is dead. unless the bolts are not tight(dont over tighten as can crack the adapter if the thin ones)

 

this is a ez fix.also ck to make sure the distributor bolt didnt fall out causeing the distributor to move alot making the timming of out of wack

[620connor]

I have done/tried everthing that everyone has suggested and it still wont start.. Maybe its an electrical problem?

[datzenmike]

You removed the coil so look there first for a loose wire.

 

You replaced the points so go back and see if there is a loose wire there.

 

Don't assume! Check carefully!!!

[620connor]

You removed the coil so look there first for a loose wire.

 

You replaced the points so go back and see if there is a loose wire there.

 

Don't assume! Check carefully!!!

Okay i'll go back and recheck everything. Thanks for the help everyone.

[dat77sun]

check and make sure the carb float isnt stuck, mine is and its making it misfire because its giving the motor way to much gas

[datzenmike]

check and make sure the carb float isnt stuck, mine is and its making it misfire because its giving the motor way to much gas

 

True but he did say...

 

I removed and checked one of the spark plugs for spark to see how it was firing and it didnt look to good, it will fire normal and then it will fire really weak and then it wont fire at all. I took a wire and jumped it from the battery directly to the coil and it still did the same thing.

 

He says that sometimes it won't fire at all. Which could be a loose wire on the points or coil?

[620connor]

Okay guys i went back and checked every wire there is to check, and i pulled out the distributor to see if anything was wrong with it but everything looks 110 percent perfect. My dad has been helping me work on my truck since this happened and he has been an auto mechanic for almost 30 years. It should be running... but its not. Something else has to be wrong with it. By the way, the other coil i tried was a brand new L20b coil and resistor. Is there anything else that can happen to make it misfire like this?

[banzai510(hainz)]

take the coil wire aND PLACE NEAR THE CHASSIS and see if f sparks when you turn it over.

IF yES THEN CK THE FIRE ORDER the valve lash

 

NO spark

ck wire blk wht wire at 12 Key ON

Ck Hot START wire when key at ON position.

 

 

I have seen where the hot wire was before the resisito instead at the coil + side.