78 620 Carb, Trans or Both?

[Daryl]

K, 620's are all new to me, but I have had 510's and 521 before. Maybe someone can put me into the right direction to er road worthy once again.

 

1978 dats 620 KC L20B, Auto Trans. 132k or so miles.

 

 

Here's the Mystery Problem #1: Stock carb has a hesitation when pressing throttle before half way. Once pressed in between half and full throttle it halls ass.  It'll drive even with it but sounds like it will stall if I don't step on it past half way quickly. 

 

What I have found or done: Distributor diaphragm doesn't seem to be functional in sucking or pushing air out. All vacuum lines are connected and in good condition. I replaced the accelerator pump and it made no difference. Rev's ok but dosen't sound right.

 

 

Here's the Mystery Problem #2: Trans sometimes doesn't grab 1st gear or motor once put into "D" just rev's for no reason.

 

What I have found or done: Maybe I have to mess with the vacuum diaphragm, the piping for it? The other possibility that I can think of is the Downshift solenoid or kickdown switch?

 

 

 

Carb mishap has been there for a week. Trans issue just began today. So far I've put 600 miles it with no issues. It'll go 65+ no prob and doesn't overheat. No leaks or anything or the sort. Any info is greatly appreciated.

[datsunaholic]

Problem 1 sounds like a clogged primary main jet.

[datzenmike]

Not much you can do for a transmission but look at it from the outside. If not dropping into gear, or revving when in gear STOP driving until corrected!

 

You can check the transmission fluid level and be sure it is near the full mark BUT NOT OVER. There will be a mark on the dipstick for fill level. Over full is worse than too low. Warm it up in neutral and check the dip stick. ATF (auto trans fluid) should be see-through clear and bright ruby red. Foam indicates over filling. Discoloring can indicate over heating and age, black or dark can be clutch material worn off.

 

The carb, vacuum advance and kick down switch do not tell the transmission to 'get into gear'. It knows when to down shift into first and always starts off in first when stopped.

[Daryl]

K, carb is out! PO put transparent glue all over gaskets. Hell, never even noticed it. Going to remove that and put proper new gaskets in it as well as a good blow job. 

 

Warmed it up this morning and noticed that when I put in in any gear (R, D, 2, or 1) motor would lower rev sound and engage into the gear, but rev's up by itself and goes back into gear, rev's up by itself again and goes back into gear. It only does it at a dead stop. Real weird. Once it lowers rev back into gear in drives fine when ya step on the gas.

 

Anyway thanks for the info, Im making some progress now.

[distributorguy]

Going to remove that and put proper new gaskets in it as well as a good blow job. 

 

Vacuum leaks, in places like the vac advance cause lean fuel mixture and hesitation.  So do clogged emulsion tubes and low float levels (adjust your float).  Verify the accelerator pump works.  And sign me up for your service listed above!   :rofl:

[Daryl]

 

 Going to remove that and put proper new gaskets in it as well as a good blow job. 

Only now did I figure it out... Haha. 

 

Well, said "blow Job" went great. Carb now rev's way too good. Vacuum advance or Distributor vacuum diaphragm is not sucking as it should be (Cracked inside). Where about's do I get a new one?

 

Trans may need a replacement sometime in the future... Would a Auto Trans out of a 80-82 720 be a direct bolt in? What  others would work?

 

620's aren't as simple to work on as 520's in my book.

[datzenmike]

Check the fluid level!!!!!! It's not operating properly and you WILL need a replacement if you keep driving it. Christ, I suggested this for a reason. Low fluid might cause this so destroying a good transmission because you didn't check or can't be bothered is kind of stupid.

 

 

There is only vacuum advance vacuum when the throttle is above idle. Take the cap off so you can see the rotor. Take the rubber vacuum line off and suck hard on the end. What you should see is the rotor turn about an inch clockwise and it should hold position until you release it. If it does this it's presumed to be working. 

[distributorguy]

Actually the plate will only turn 1/8" or a little more.  An inch would be 50+ degrees of advance and that would stall out your motor.   :poop:    If it goes back to starting position while you hold vacuum on it (leaks) its bad.  Nissan is the only place I know of that you can get one - around $30.  

[Daryl]

I'll call up NISS in the am and get one underway  :thumbup: .

 

 

Trans has clean pink fluid, no smell of any kind and fluid is at the middle of low and high.

At a dead stop the motor revs in and out as if engaging and disengaging the trans or something of the sort. Once engaged it drives good and has no hiccups of any sorts.

[datzenmike]

Is it the engine that's revving intermittently? Does the idle remain constant if in neutral? If the idle jumps around then if in gear the transmission will seem to jump also.

 

 

What does it do when idling in gear with brake off? Just rolling along?

 

 

Lets assume the engine is idling erratically. Like a bad miss. If in gear, this will be transmitted to the car and could be perceived as jumping in and out of gear.

[Daryl]

Yup, Idle is constant at 1k. in neutral and in park as well. Once In gear it's at 800-750. 

 

When in gear it sometimes rolls and sometimes just sits revving. 

 

-- I think I solved the problem. Filled up the trans fluid to the max level and the jumping gear situation disappeared completely.--

 

Drove er for 30 miles and continually made stops, and trans grabbed good and hard. Changed gears good and at correct RPM's. 

 

I'll give a test once again tomorrow and hope she's road worthy. Datzenmike ya awesome!

[datzenmike]

Maybe the dip stick isn't seated properly. Usually you can be close to the fill line with no ill effects. Well all's well the ends well.