Carb rebuild?

[FauxRoux]

That would be 1 flat old o-ring

 

although the new one fits and helps.

............

I think a monkey worked on this thing before me......prying shit open with flatheads....

 

 

[datzenmike]

definitely didn't remove it by choice. it was loose and literally  fell out :crying:

 

Did not intend to make it sound like you did. I was wondering why gaskets would be supplied for something all manuals say not to remove.

 

datzenmike - Do you have anything in your bottomless notebooks on the hollow screw(s) that hold the carb base to the body? I took another carb off a manifold and it has two hollow screws. Of the four carbs I've looked at so far, three of them have two hollow screws, not just one like I thought was correct. Maybe I was wrong about there being two blind holes taking solid screws. Any opinion on number and location of hollow screws?

 

Len

 

One hollow screw is to allow intake vacuum up into the power valve. At part throttle high vacuum holds the power valve closed against a spring. Heavy throttle... low vacuum would see the spring push down on the valve opening it. The power valve adds additional fuel to the primary during full throttle acceleration, making the mixture richer. All carbs would have one of these so this would explain at least one of them. I have only see the one hollow screw. Maybe this is something only on California carbs??

[LenRobertson]

When I took the loose venturi out of the carb on my 200SX, the O-ring was kind of pushed down below the bottom of the recess it is supposed to fit in. Maybe it had been pushed slightly out of place when it was installed, either at the factory or by a ham-handed rebuilder. Hitachis tend to not be loved by Datsun owners."Get a Weber!" is the common advice which I don't think is doable in Cali and maybe some other smog controlled locations. Or maybe all Datsuns are old enough the smog inspectors just wave them through. Anyway I suspect Hitachis tend to suffer from two problems - old gas dried out when the Datsun is parked for years and sleazy rebuilds, either amateur or professional.

 

When you get your carb back together and the set screw tightened as best you can, it may be worth reaching down the carb throat every few month to be sure it is staying tight. And let us know how it goes as far as tightening that screw. I'm curious.

 

Len

[FauxRoux]

Did not intend to make it sound like you did. I was wondering why gaskets would be supplied for something all manuals say not to remove.

Ahhh....makes sense. Unfortunately while my kit does have an exploded diagram and adjustment info...it doesnt say what ea h part is for....it assumes I know..so i do have 2 small o-rings ive yet to find a use for....

[FauxRoux]

When you get your carb back together and the set screw tightened as best you can, it may be worth reaching down the carb throat every few month to be sure it is staying tight. And let us know how it goes as far as tightening that screw. I'm curious.

 

Len

Well my o ring was sheared flat!....and im afraid of stripping that setsrew...ive soaked it a few times and its holding fast

[LenRobertson]

 

 

 

One hollow screw is to allow intake vacuum up into the power valve. At part throttle high vacuum holds the power valve closed against a spring. Heavy throttle... low vacuum would see the spring push down on the valve opening it. The power valve adds additional fuel to the primary during full throttle acceleration, making the mixture richer. All carbs would have one of these so this would explain at least one of them. I have only see the one hollow screw. Maybe this is something only on California carbs??

 

 

 

Thanks for the explanation. I'm puzzled that the screw nearest the float chamber that I thought was the correct location has the head covered by the base gasket. But maybe there is a tiny air passage over to the carb throat I couldn't see in the fading light just now. I'll look tomorrow in bright light.

 

All four of the carbs I've been looking at today have the venturis held in by two screws from the top, hence non-Cali I'm thinking. One thought on how a carb gets two hollow base screws is rebuilders having a bucket of screws beside them when they reassemble the carb. They may know to put a hollow screw in the correct hole. But then maybe they know it doesn't matter if a hollow one ends up in another hole. Seems like they would run short of hollow ones eventually. And as I said, one of the carbs has a solid screw in what I think is the hole a hollow one needs to be. I read a book last year on "Chaos Theory". I'm thinking it is time to reread it.

 

Len

[LenRobertson]

Well my o ring was sheared flat!....and im afraid of stripping that setsrew...ive soaked it a few times and its holding fast

 

When I thought I was going to have to loosen that set screw I considered something like a dental burr in a Dremel tool to grind away the staked area around the threads. But I ended up going with the other style venturi carb so never had to get it loose. I remember trying to loosen it and realizing it was really tight.

 

Len

[FauxRoux]

Man oh man....4 soaks and maximum effort and I got it out....now to clean and back on track!

[LenRobertson]

Man oh man....4 soaks and maximum effort and I got it out....now to clean and back on track!

Way cool! (As us old guys used to say) I've wondered if Loctite on the setscrew threads would be a good idea, but don't really know. Seems like it should be more secure than trying to restake the threads. I don't know if gas has any adverse affect on Loctite.

 

Len

[FauxRoux]

Way cool! (As us old guys used to say) I've wondered if Loctite on the setscrew threads would be a good idea, but don't really know. Seems like it should be more secure than trying to restake the threads. I don't know if gas has any adverse affect on Loctite.

 

Len

I found myself contemplating a drop of the blue myself....the set screw IS pretty slutty now (he said with mildly misplaced pride)....probably a good idea....especially since I reused the original o-ring cause the other 2 were popping out slightly...I may try to go resize that o-ring and see if I can find the right fit...

 

Ive spent this long at it...whats another day?

[distributorguy]

I found one of my old carbs in the shed.  The hollow screw is the one between the throttle blades, on the float chamber side of the carb.  Didn't even notice it was hollow until I was able to stick a dental pick into it.  What a mess.  

[datzenmike]

That is exactly where the power valve is above it.

[LenRobertson]

I've been puzzling how the hollow screw could be a vacuum passage. With the carb I just pulled off my 510 there is a solid gasket and phenolic spacer underneath the hollow screw head which blocks any air passage through the screw. Finally this morning the obvious occurred to me. Could there be a different phenolic spacer with a passage for air to the screw? So I went to the parts stash and the first manifold I picked up had a spacer with a cutout over to the secondary bore. For years I've been running a solid spacer from what I think is a DCH328 Hitachi (which has a cutout in the cast iron carb base, so doesn't need one in the spacer). In addition to not having a functioning power valve, I've had a mild restriction in the primary bore since the 328 spacer is a couple of mm smaller diameter than the one I found with the cutout. So other than feeling a bit stupid, I can say I'm at least learning a few things.

 

Len

 

 

[LenRobertson]

Here is a pic of the two style phenolic spacers just in case the information may help someone.

 

https://www.flickr.com/photos/78824315@N00/26504041962/in/dateposted-public/

 

Sorry I can only post the link. Since I'm using Flickr, the procedure I used last time to find the URL for the "Image" box here on Ratsun isn't working today.

 

The spacer on the left I believe is for a DCH328 Hitachi, probably from a 1600 engine. That is what I've been running on my L20b with a DCH340. There is no cutout area to allow manifold vacuum to reach the hollow screw the goes up through the cast iron carb base. The 328 carb has a cutout in the carb base so doesn't need one in the spacer.

 

The spacer on the right has a circular cutout from the secondary bore for vacuum passage. This is need with a 340 since it doesn't have any sort of cutout in the iron base.  If I hadn't been following this thread I probably never would have figured all this out.

 

I got a different DHC340 installed on my 510 yesterday (with the correct spacer!) and it started right up. I didn't road test it yet but just driving back and forth in the driveway I can tell the accelerator pump is working properly on this carb. I don't know how long I drove the old carb with the hesitation every time I accelerated. I had learned to kind of feather through the hesitation, so it was more annoying than anything. Now I need to retighten the carb base bolts and find a switched 12V source of power for the idle cut solenoid and electric choke. With the old carb I removed the needle from the idle cut solenoid and killed the engine with the brake and clutch. But I'm thinking an operating cut solenoid is a luxury I'll treat myself to.

 

Len

[datzenmike]

LEN!!!

I've been planning to remove my carb ('76 710 L20B with U67 and co-joined intake/exhaust manifolds. First thing I noticed was the bottom throttle plate mounting screw with the hole in it was in the wrong location!!! It was on the outside and a solid screw in the power valve position.

 

Car has always run ok but too much throttle at too slow a speed in too high a gear acts like the accelerator pump is not working right. It balks above idle in gear and has to sort of 'clear it's throat' before it starts pulling when you 'step into it'. I just use a lower gear if possible and allow it to rev up a bit before flooring the gas. The tear down today was to check all the passages on the accelerator pump... something that is not easy with it on the car. I will be doing this as planned but the vacuum signal blocked to the power valve means someone had this off before. 

[LenRobertson]

When I realized having the wrong phenolic spacer had been blocking the hollow screw from getting manifold vacuum, it crossed my mind that may have had something to do with the throttle hesitation I'd been experiencing. But the cutout in the correct spacer is in the secondary bore. I assumed there wouldn't be appreciable manifold vacuum on the secondary side until the secondary plate was pretty well open. But I was entirely guessing on this idea. I considered doing something like this guy is doing so I could watch when the secondary opens, but I don't think I can get my wife to drive the 510 with the hood up:

 

 

I know in the interest of science I should re-install the old 340 now that I have the correct spacer and see if the hesitation is still there. But I have so many other things to do to the 510 I'll no doubt just stick with the "new" carb.

But let me know if you solve your hesitation problem and what you think the cause was.

 

Len

[LenRobertson]

So I'm sitting here drinking a celebratory beer! I took a test lap around town in the 510 with the "new" carb. Came home and shut it off. Restarted and went back uptown to the post office. Shut it off and it restarted. By then I was feeling really reckless. I stopped at the grocery store and bought a six-pack. Restarted and came home.

 

It is really weird driving without that throttle hesitation. It almost feels like something is wrong. Even what I thought was my weak clutch works way better. I can now let out the clutch at really low engine speed and just glide away.

 

Now on to fixing the 100 other annoying little things on the 510. Starting with the fuel and temp gauges pinning on the high side (which wasn't happening the last time I drove it). I know there was a thread awhile back on the care and feeding of the voltage regulator for the gauges. I need to see if I can find that as a starting point.

 

But I'm extremely happy this afternoon!

 

Len

[datzenmike]

LEN!!!

 

I've been planning to remove my carb ('76 710 L20B with U67 and co-joined intake/exhaust manifolds. First thing I noticed was the bottom throttle plate mounting screw with the hole in it was in the wrong location!!! It was on the outside and a solid screw in the power valve position.

 

Car has always run ok but too much throttle at too slow a speed in too high a gear acts like the accelerator pump is not working right. It balks above idle in gear and has to sort of 'clear it's throat' before it starts pulling when you 'step into it'. I just use a lower gear if possible and allow it to rev up a bit before flooring the gas. The tear down today was to check all the passages on the accelerator pump... something that is not easy with it on the car. I will be doing this as planned but the vacuum signal blocked to the power valve means someone had this off before. 

 

Well finished and installed and hesitation is worse but more power when floored mid range... probably the power valve working.

 

Tearing a '79 620 carb down and rebuilding it.

 

Jets... main.... secondary

710-...... 98..... 160

620-.... 108..... 150

[LenRobertson]

I thought about checking the jet sizes on the carb I swapped on, but didn't do it. It seemed yesterday when I drove it I may have had a bit less power, but I was so agog at the lack of hesitation I probably wasn't thinking clearly.

 

One thing on my old carb with the hesitation - I believe I originally did a "spray can overhaul" (didn't soak it, just sprayed carb cleaner through everything and blew with compressed air). Now that it is off I may tear it down and throw it in the bucket of carb cleaner. If it has a partially blocked discharge passage, soaking it may clear it out.

 

I'm also uncertain about the proper set up for the accelerator pump discharge check ball/springs/weights. I've read various posts on weight or no weight, one check ball or two, etc. It makes me wonder if it is that critical.

 

Same confusion on the accelerator pump arm stroke adjustment. Two holes on the accel. pump shaft and 3 or 4 holes on the pump arm. I've never had any idea what the ideal adjustment is. Seems like once I tried different stroke adjustments trying to help the hesitation and couldn't tell any difference, but if I wasn't getting good gas flow through the accel. pump circuit I suppose the stroke adjustment wouldn't help.

 

Len

[datzenmike]

There is a light spring and ball under the pump. It allow fuel in on the up stroke but the ball jams the hole preventing fuel returning to the fuel bowl under pressure on the down swing. On the pressure side is a spring and a small weight under a sealed brass cap. Pressure from the accelerator pump lifts the ball and fuel squirts into the barrel. On the up swing the ball blocks air being sucked back while fuel is being sucked from the fuel bowl.

 

I found some sludge under the BB in the bottom of the accelerator pump that may have prevented it from stopping fuel being forced back into the bowl. When I was done I filed the accelerator pump with carb cleaner and pumped it and watched it squirt into the primary.... it looked good and strong. But it drives worse that it was.

 

 

Spent the PM rebuilding a '79 620 carb. There was some dried powder in the fuel bowl and the secondary jet was plugged for sure. Cleaned it all up. The choke and idle cut wires were cut it seems. The throttle is cable operated so I still have to remove that part because the 710 is linkage, but the plan is to take the 710 carb back off again and substitute the 620 in it's place.

 

I don't see how I could have made it worse so I'm just going to swap it and see. The 710 has always had a good strong idle but never run right just above idle. Figured it was the accelerator pump and threw a new one in years ago but nothing, now I've cleaned it and it's worse.

[LenRobertson]

Yesterday morning I set off across town in the 510 to get a haircut . I made it about a block, U-turned and came back. It seemed like the choke was sticking on and running super rich. Later in the day I took the choke cover off to see what I could see. Everything in there looked fine. When I reinstalled the cover I backed the adjustment off quite a bit. I sprayed carb cleaner on the choke shaft and linkage to make sure everything was loose. This morning I took it out for another test and it seemed much better, enough so that I may quite screwing with the carb and move on to something else.

 

The pinned fuel and temp gauges responded to being unplugged and the wires wiggled. They are both working now. The speedometer on the other hand decided to go on strike - the needle isn't moving at all. So that may be the "something else".

 

Your description of the accelerator pump circuit is pretty much the way mine is. I've had it apart a few times trying to find a problem. Next step is the carb body getting a soaking in carb cleaner. 

 

I was going to tear that carb down today, but decided to put the carb back on the F10. Every so often someone alleges they want to buy the F10 but they always want it running. What fun is that? When I was putting the carb base nuts back on the 510 I was thinking it was more fiddly than I remembered. But trying to get the base nuts onto the F10 is a genuine horror story. The only thing that kept me going is I knew I had done it before. I don't have the nuts tight yet. Tomorrow I need to heat and bend my 10mm carb nut wrench to a different angle. I still may need to remove the valve cover to have room to swing a wrench. That may be what does in F10s - if they have carb problems a mechanic takes the carb off but can never reinstall it.

 

Len

[datzenmike]

My 620 carb was an absolute ordeal. The 710 seemed straight forward. They are the same carb. Truck sits higher, maybe that had something to do with not being in a comfortable position.

[FauxRoux]

im embarrassed to report that try as I might I cant get this carb to idle...First the fuel level was right at the dot but when I ran the engine to adjust the idle it would shoot to halfway above the viewing window dot while idling waaaay too high and then die pretty quick....seemingly flooding the carb. Also hitting the accelerator just made the carb go

"flub" and die.  

 

So I readjusted the float to the specs given and now the fuel only comes halfway up to the dot...... doesn't idle at all, just dies... So clearly I need to adjust again...

 

This thing has me second guessing my competency in a major way :no:

 

.......ouch....right in the manhood :rofl:

[datzenmike]

As long as the fuel level is in sight it's good enough. Take the top of the air filter when idling and look. It's only flooding if you see gas dripping inside.

 

 

Turn the ignition key to ON (don't start) then OFF, ON, OFF, ON, OFF while listening near the back of the carb. You should hear a soft clicking sound. This is the idle cut solenoid and it needs to be working or there is no fuel to the idle circuit and you won't be able to idle below 1,200.

 

If the air filter is off, or been off make sure that all the vacuum lines to the air filter housing are connected and in place. It won't idle with any vacuum leak.

 

Carb bolts snug?

[LenRobertson]

If the idle cut solenoid isn't clicking or you suspect it may not be working, you can unscrew it, remove the internal needle and spring, then screw it back in to fill the hole. The carb will work just fine - until you turn off the engine. Then it will run on and you will have to kill it with clutch and brake, or floor the gas pedal just as you turn the key off. I did it this way for several years (out of laziness). I finally have a working idle cut solenoid and I highly recommend it (although somewhat boring compared to a fight to the death every time to get the engine to quit running).

 

I can't recall the approximate setting of the idle mixture screw in the base of the carb. You screw it all the way in, then back it back out some. I'm thinking 1 1/2 turns, but maybe that is Briggs & Stratton. Anyone have a better memory on this?

 

Len