Argh, 620 wont run right

[wildmaninid]

Drving home last night, running about 85 or so on the home stretch in the ol' 620 (l20b, hitachi, all emissons still hooked up *uck*) and then the truck just loses power....... At first i thought i lost ign. power but i held the pedal to the wood and actually limped the truck home about 1/4 mile. Seems to idle just fine as frogs hair, but as soon as you start to push the pedal it just bogs down like it wont go......feather and lightfoot the pedal and coax her up over 4000 and it starts to run out fairly good with a slight stutter.....let off the pedal and she backfires like a shotgun.....(pisssed my mother in law off so that much is good LOL).

 

Checked fuel delivery this morning (electric pump) and that checks out. Cap rotor and dist (matchbox) look ok and seem to be functioning properly...pretty sure i have spark since it does run. Accelerator pump seems to be functioning as it squirts a nice amount of fuel when i operate the the linkage...

 

Anyone have any idears? Prolly need to either figure it out/fix it or find another rig to drive to wrk at 4 am tomorrow morning.

 

Thanks guys

Bill

[datzenmike]

Back fire out the exhaust is often a slight loss of ignition from a loose wire. Unburnt gas goes out the exhaust and when the power comes back on it lights it off. BooM! Very short duration will just cause slight loss of power and the gas just burns up in the manifold, but if the interruption is long enough the pipe fill with fumes and the bang is very loud! I've split a muffler open from this.

 

 

First take the cap off and look on the under side for dampness and signs that the spark is jumping to ground or between terminals. Check the center carbon contact that conducts the spark down to the rotor is not broken off worn out or making bad contact. Look under the coil wire nipples (both ends) Check the top of the coil where the wire comes out. This is where the spark can jump down to the neg terminal. Look for a small track like a spider web or crack. This is where the spark has burned a track. Now check the points for loose wire connections.

 

Let it idle and poke around all the wire connections on the coil and ballast with a wooden stick. Stay away from the high volt wires, we know they work. Now wiggle the plug into the back of the ignition switch. If the idle acts up you have found the loose wire.

[I'm BLUE]

Test your coil ? Bad Coil

 

Burned points / Bad condensor ?

 

ignition ballast resistor any good ? ( some you can take off ,,, look at backside to see condition of the "element" wire )

 

check operation of distributor vacuum advance ?

 

Timing Chain Stretch ( long shot )

 

Block Fuel Filter(s) ?

 

Blocked Exhaust ?

 

Burnt Exhaust Valve ?

 

What do your Spark Plugs Indicate ?

 

IDK sorry for your troubles ,,, just throwing some ideas out there !

[wildmaninid]

Got a matchbox so no points/ballast resistor etc..........I have a good coil in town.......also the tool to pull my plugs out etc. I am now leaning toward it being an electrical problem since it just happened out of the blue. I am due for a new cap although my rotor and wires are brand new along with the plugs. Mike is right on about the backfiring issue.........also i just started it up and I noticed when i am revving it up mid range it is smoking pretty bad with real rich smelling exhaust...

 

I will tackle more on it tomorrow, today i will have to go borrow a ride

 

Thanks for the input guys

 

Bill

[herculesinwyoming]

symptoms dont all relate but maybe just maybe your cat converter has

gone bad.

[deadmonkey]

I had something similar, but it ended up being my electric fuel pump. I changed out fuel senders and in the mix must of got sediment into the fuel pump. It would barely idle, then die. I took the fuel line off and turned on the power (pump starts pumping) but nothing comes out the end. Looked and fuel filter was bone dry. I did an on the road patch and bypassed the fuel pump, praying the datsun pump on the engine still worked. Amazingly id did and I got her home! I've been using the built in datsun pump ever since.

[wildmaninid]

I do have great fuel flow and float level int he carb stays nice and level. I have no cat converter so no worries there. I am leaning towards spark/ignition............the backfiring after i let off the gas seems to make me think improper spark. I mean the only other time i ever had a datto run this bad it had tons of wate rin the fuel........wich isn't likely but i ran some heet through it anyways. Gonna tackle wiring connections/coil/dizzy tomorrow.

 

It was just like someone flipped a switch when this happened. Just going along fine at a steady pace then bam, i thought like i lost ignition power at first, like if you shut the key off under power...........but like i said when i gave it more go it would barely keep up..........advance in the dizzy maybe?? I dunno gotta start getting back into it tomorrow..

 

Thanks for the ideas......keep em coming

 

BIll

[Iowa Mark]

Almost sounds like the choke is getting sucked closed, or some other intake restriction.

[wildmaninid]

Ok so update time. Tonight i did the following. New cap/rotor, no change. New Coil, no change, Swapped on another known good matchbox dizzy, no change.

 

Spark plugs are all fouled black with soot, like it is running very rich. Choke is not looso nor is it getting sucked closed.

 

I do have an idea though. Truck came with factory a/c therefore has an electric fuel pump. It went out about a month and a half ago and i replaced it with a very similar one from napa (a round carter brand pump) Is it possible that the fuel pressure it too great and it is overpowering the float, or possibly the float has gotten heavy and is sunk a bit? My dad had his float replaced on his 620 back in the day because it got too heavy and it acted similar to this......or too much fuel PSI.......i need to check this I know.......

 

Spark and fuel problems are so similar they are hard to differentiate, but either i am getting insufficient spark to burn the fuel present, or I am getting too much fuel for the present spark...........

 

Revving up the truck produces wonderful clouds of billowy, black.blue smoke that reeks of dead dinosaurs. It no longer runs that great at higher rpms like it did yesterday, and it is idling worse.......

 

There it is boys and girls. I'm gonna get to the bottom of it sooner or later LOL

 

Keep the thoughts and ideas coming

Bill

[I'm BLUE]

since after market electric fuel pumps pressure designs VARY greatly ,,,,

 

you need a fuel pressure regulator ,,, I put an allen head adjustable Holley fuel pressure regulator on mine looks like this

 

 

Holley makes this kind in 1-4psi and a model in 5-9psi I believe ? ( Holley 390cfm 4bbl calls for 5.5-6psi I believe )

 

I have found that ,,,Aftermarket cheapy electric fuel pumps vary greatly on pressure's , even across the band ! !!!

 

Has worked fine for a while ! if your carb is a webber ,,, with plastic floats then yes ! they can fill with gas or "bleed" gas into ,,, holding weight of the float ( I don't know politically correct term for the process :P ) My carb guy "anytime you can get ahold of brass floats do it !"

 

Too much fuel pressure can just wreak havoc on some carbs ! On my 390cfm 4bbl Holley , like 4-5 minutes after installation of a carter electric fuel pump ,,, just started pushing all kinds of gas straight through it :blink: I wen't and bought one of these ^^^ too correct it and has been good since !

 

Are your metering rods stuck in the carb ???

 

Any other thoughts guys ???

 

 

Also maybe you shouldn't be driving 80 all the time :P ... :lol: ( but then you wouldn't be enjoying the datsun ;) )

[mike]

This sounds silly but check your vacuum lines and see if any are cracked or if you have any of them capped off. I had a similar issue on the 74 when I was trying to tune in the carb after rebuilding it...

 

It was running pig rich and I had to adjust the idle screw all the way out and it was still above a grand at idle when warm. The fuel level was about a quarter inch above where it should be in the sight window. I messed with it for DAYS and I finally said to hell with it and put the aircleaner back on. I hooked up the vac lines and the damn thing died. I readjusted everything with the aircleaner attached and the vac lines hooked up and I can seriously barely hear the damn thing run now and its smooth as can be from idle on up to freeway speeds...75 with the stock 4 speed, I wont push it much more than that.

 

I dont know the difference between the 74 and 77 carb but they should be pretty damn similar. Another thing I did was pull all the emissions stuff and clean the carbon build up out of it and clean up the BCDD and all the solenoids. Be careful with the diaphragm thingy on the BCDD...

 

With all the emissions crap hooked up and functioning its still got good power... its just way the hell quieter and smoother than a weber and it gets just shy of 30 mpg with the L18.

 

Hope this helps

[mike]

Oh yeah, the FSM on http://www.olddatsuns.com//620tech.htm has damn near everything youd want to know about the hitachi... its kind of spread tout through the carb and emmissions sections I think, but its all in there. Its got a handy troubleshooting section too.

[datzenmike]

If all new wires, cap, rotor, coil, dizzy it can't be electrical or loose wire.

 

Black plugs, running rich. Sudden onset is odd. If the wrong pump it would have acted up right away. You say fuel level is steady and correct so what would cause it to flood or run rich????

 

 

Take air filter off and look down inside below the choke butterfly at the venturi. Is it clear of obstruction. Has anything clogged it. Use a flash light.

 

Start and idle. Look down inside... is there any gas dripping from it or any where else?

 

 

Backfire from exhaust or intake????

[wildmaninid]

Exhaust backfire. Timing is right on. Looking through the sight glass on the front used to show the fuel level being just above 1/2 on the window, now it fills the whole window up. I don't see any obstructions in the venturi, and the accel pump works. God the stupid thing misses like it has a dead hole but compression is 170-185 spread. I am gonna put on a fuel pressure regulator, and 4 new ngk plugs and see what happens now.

 

Bill

[I'm BLUE]

Exhaust backfire. Timing is right on. Looking through the sight glass on the front used to show the fuel level being just above 1/2 on the window, now it fills the whole window up. I don't see any obstructions in the venturi, and the accel pump works. God the stupid thing misses like it has a dead hole but compression is 170-185 spread. I am gonna put on a fuel pressure regulator, and 4 new ngk plugs and see what happens now.

 

Bill

 

Pressure Regulator FTW ! ( if not help your symptoms will help prevent future problems )

 

Your fuel bowl use to 1/2 level ,,, now Full level ?

 

Is your needle stuck open or full of dirt inside the float ? ( causing it to stick open ? )

 

Bent Fuel level Float Arm ? Possibly need to check/adjust float height ( I don't know how to do that on those carbs , never owned one )

[wildmaninid]

Well i pulled the front panel off of the float chamber, It was nice and clean in there so i am not thinking contamination, and honestly i don't know if i am even on the right track, i am just eliminating things as i go. I screwed up a rubber fuel line so I am gonna replace that tonight along with 4 new spark plugs.

 

We shall see i will post anything that happens here.

 

I sincerely thank everyone for their input, 3rd party advice just helps me think of something that i might not have!

 

Bill

[mike]

Check the routing of your vacuum lines off of the carb and check the condition of the rubber lines. The raised fuel level is what had me messing with it for so long. Putting the vacuum hoses in the right spot is what cured it... I dont remember which one it was but it is one of the ones that comes out the side towards the fender.

 

Check for obstructions in the vacuum lines too. Fire it up, unplug the vac lines one by one and plug them with your finger for a few seconds, you should hear a change/rise in RPM (more than it should)on one of them when its capped/plugged, this is the one that you need to make sure is hooked up right or its just going to keep pissing you off.

[datzenmike]

Exhaust backfire. Timing is right on. Looking through the sight glass on the front used to show the fuel level being just above 1/2 on the window, now it fills the whole window up.

 

Bill

 

That's your problem right there. Over full will spill into the primary and run rich.

 

Hold off on the regulator for a while. Or have the fuel pressure tested. This could simply be a float adjustment, bad or clogged needle valve. A regulator won't cure that. Remember it was working and suddenly went bad.

[wildmaninid]

Mike,

 

I just got in from working on it. I put in new plugs just in case. At idle the fuel level fills up the entire window of the carb. If i rev it up real high it drops almost enough to see the level in the very very top of the sight glass.

 

I am remembering that it was a sudden thing..........I am gonna test fuel PSI (BTW what should it be) and maybe just over the past couple months the excess pressure finally made something give (needle/seat something). I may need to go ahead and kit the carburetor and adjust the float.

 

 

I think i am on the right track, just so odd that it happened so quickly.

 

 

Also, when adjusting the mixture i only see 1 screw. Is that the only adjustment? Is tightening it making it rich or lean?

 

Thanks Mike

 

Bill

[datzenmike]

The screw on the left of the spring is the idle speed. It just adds more throttle. (air) In is increase speed, out is reduce idle speed

 

The screw to the right of the spring is the idle mixture screw. It adds more or less idle fuel. In is less, out is more.

 

 

If possible turn the idle speed down so that mixture adjustment is more noticeable. Turn the mixture in or out until the idle quality decreases then turn the opposite direction until the idle quality decreases. Select a position about half way between where it idles best. Likely it will rev up slightly so use the idle speed screw to turn it down and then adjust the mixture screw again for best running. If you have to reset the idle speed again. Keep doing this until you cannot improve on it..... you are done.

[wildmaninid]

Ok,

 

I got rained out this afternoon trying to get the carb off of there and swap it with one from my 79' parts truck. As I took the carb off and tipped it slightly, this piece fell out of the venturi.........I don't know what it is, but I know that like when I push on the throttle with the truck not running fuel sprays out of it.....It appeared that it was glued in or something at one point.

 

I guess my question is would this cause the problem I am having, or is this normal. On the 79' hitachi carb, this piece appears to be one with the body of the carb.

 

Anyone have any ideas? Hopefully tomorrow i will have the carb swap done and that will fix it, 1 week without driving my truck is killing me.

 

Bill

 

[datzenmike]

 

Lolz that's the primary venturi. Only the California models look like that IIRC. If you look on the back of the carb just to the left of the idle cut solenoid there's supposed to be a set screw to hold it in place and another just over from it for the secondary. These are NOT supposed to be removed when rebuilding it..... friggin previous owners!!!! Idiots.

 

 

.

[wildmaninid]

So did I find the culprit of my problems? Is it possible to repair this? Is it worth it or should I just use the 79 carb and forget about it?

 

I agree P.O's have caused some of the worst troubles I have ever had with vehicles.

 

Thanks Mike

Bill

[datzenmike]

My confidence is shaken. better use the '79. No telling what's missing or loose on that one. Did you look on the back for the screw that holds it in? Pic?

[wildmaninid]

Mike,

 

Yes I found the set screw that holds it in place, and it is seized up tighter than a tick. I am looking at this carb very closely and I see absolutely no sign of anyone even removing a screw on it. As I cleaned off the mild layer of buildup, I could see no evidence of it being taken apart before.....

 

If I could get that venturi back in right I would probably try to run it again, but since that doesent seem to work I am gonna bag it and run the 79' carb.

 

Thanks for your help all along, glad we have you here at Ratsun!

 

Bill