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Carb advice for 620 w/ L20b

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Working on a new 620, swapped by PO, the L20 seems to run strong...when the carbs will cooperate anyway, they behave differently every time I try to drive it. It currently has the old dual SU's that need to be rebuilt, synced and tuned, throttle linkage needs some custom work as well. I'm thinking of replacing them with a weber...or maybe dual webers...I have experience and the tools to sync/tune hif44 SU's I run on my 240z, Ive done it several times but they have integrated floats and don't require me messing with jets or float levels or anything like that, I just sync them together and then adjust fuel with my colortune......I have absolutely zero experience tuning or syncing webers so I would like to hear some opinions from those running them.

 

I am by no means chasing hp number with this truck, but it already has headers and I might want to throw a cam in down the line...really I just want it to run well, be reliable, and be able to sync/tune whatever carbs I end up with at home....so the simpler the setup the better. So I guess right now its between the 38/38 dges, 32/36 dgv or 40 dcoe. Any opinions? Which of these is the easiest to tune/maintain for someone new to webers? Or should I just skip webers all together and just go with an oem replacement hitachi?

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Why not spend the money and have the Su's professionally rebuilt? 

Reliable and run well is probably the 32/36.... some say the 38/38 is too big... dcoe's opens up other issues, like no distributor vacuum port,  but are fun and relatively easy to work on and adjust... 

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2 minutes ago, Crashtd420 said:

Why not spend the money and have the Su's professionally rebuilt? 

Reliable and run well is probably the 32/36.... some say the 38/38 is too big... dcoe's opens up other issues, like no distributor vacuum port,  but are fun and relatively easy to work on and adjust... 

 

I have nothing against the SU's, just thinking since this truck is going to be a driver it would be more reliable/easier/and probably cheaper in the long run to just buy a brand new single carb and sell the SU's to somone that wants them.  

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I have SUs on every gas engine vehicle I own except for my 320 trucks, it is going to take some thought to put SUs on them as there are clearance issues.

SUs are easy to adjust if they are serviceable(not worn out), I adjust them the way the Chilton's manual described to adjust them and have never looked back.

First I would remove both bowl tops carefully as it sounds like you have it running with them and check to see if both fuel levels in the bowls are the same, basically your making sure both floats are still floating.

Next disconnect the throttle cable/linkage and make sure both sides/throttle shafts move at the same time, adjust as needed.

Now start it up and check with an air flow meter, they both should have the same flow, adjust as needed, try to have the idle at around 900rpms for now, check to make sure both throttle shafts move at the same time again when you move the throttle lever.

Now start it up and turn the screw/disc on the bottom of the carb till it bottoms out counting how many turns it takes to close it, open it back up to where it was, now do the same thing to the other carb, when closed the engine should either idle roughly, open it up the same amount of turns as the other carb but remember how many turns.

Now while idling look at both the carb pistons, they should be both floating the same height/have the same gap, now lift the back carb piston up without touching the needle inside(there is a pin you can press up on on the bottom of most carbs for this purpose), you now adjust the front carb by turning the adjustment nut/disc on the bottom in till it just barely idles, if you can turn it in all the way and it still idles then the carb you are adjusting is not serviceable as it should die before you bottom the adjustment, if it dies then start it up again and turn it out till it barely idles, now do the same thing to the other carb, they should both have the same lope when the other carb is disabled, you then are done.

Now there are some on here with very complicated ways of adjusting SU carbs that involve needle changes and such, I have never had to change a needle in the 20 years of having SU carbs and my engines run great until one of the carbs wears out.

Keep in mind that I started with a set of brand new SUs from Nissan back in the mid 90s, I adjusted them and then took the truck to a Datsun race guy and he said I had them almost perfect, since that day I do what I said above to start/check a set of new SUs to me, after that I adjust according to spark plug color, I look for a tan color, not white, not black, I look to have a tan color although slightly tan/black is better than white.

 

I had a Weber on an L20b once a long time ago, I find downdrafts hard to adjust, I suppose if one knows what they are doing with downdrafts it likely is easy to deal with them, I would never buy a used Weber unless I personally seen it was good before removing it from an engine.

 

Edited by wayno

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They also need damper oil in them or checked or replaced. Th damper oil viscosity sets the opening speed of the slides and this is equivalent to the accelerator pump in a regular carb.

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You will never get the truck to run as well with a DGV or DCOE as it will with SUs.  They are very simple, and a re-bush job with rebuild kits that you can install are cheaper than buying a DCOE setup. Or call any one of the pros who can rebuild them, such as Joe Curto (.com), Z-therapy, Paltech, etc...  Its worth the money and time to rebuild them.  

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Alright guys, you talked me into at least considering just keeping these su's. I'm going to see if I can get them synced and dialed in a little more today. I do like su's, I love the new integrated float hif44 su's on my 240z, but I have never had good experiences messing with old ones. I thought about z therapy but I really don't feel like waiting 2-4 months to get the carbs back. May look into rebuild from your other sources.

 

On a side note, redline recommends the 38/38 webers since I have headers/exhaust and am considering a cam down the road. I can get a new genuine weber kit with their performance manifold for 380 shipped...so that's a little tempting. 

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The 38/38 is a 6 or small V8 carb. On top of this it's synchronous, meaning the primary and secondary open together. You will have to learn how to drive this off the line even when fully warmed up. It's a shitty poor carb for a 2 liter engine. Twin 38mm SUs are constant velocity carbs and can be floored right off the line... and get better mileage. In addition the SUs offer a more direct flow into the head. Down drafts have to make a 90 degree turn.

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From my experience, the 38/38 with a manual transmission on a 2L engine is a good solution, but takes additional tuning FAR beyond the sub-$400 price tag.  If you think the SUs are hard to tune, the 38/38 confronts you with a far more significant learning curve.  The 32/36 is used on any 4 cylinder from a 998 cc Mini up to a 2.4L Ford.  One size fits all?  There's no such thing.  I think its too small for anything bigger than 1500 cc if you're looking for drivability more than economy.  If you want easy, learn to tune your worn carbs and call it good.  Personally I'll take the old SUs over HIFs any day.  The fuel bowl and floats are accessible at all times - which helps diagnose drivability issues when they arise, plus they are further away from the exhaust.  

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12 hours ago, distributorguy said:

You will never get the truck to run as well with a DGV or DCOE as it will with SUs. 

 

I'm getting tired of the DGV steadily going off, and have dealt with SUs before, both UK cars, and Datsuns.

Never seen a carb wear the choke shaft holes oblong like the DGV has done.

The SUs definitely ran more consistent, and didn't seem to eat themselves as quick as the Weber.

Thought about the 38/38, but it's kind of silly putting that much carb on a stock manifold, and throwing a new aftermarket manifold on, with only slightly better flow didn't seem worth it.

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My l20 has SU's and I daily it just fine. As long as you do your normal maintenance (damper oil, lubricating the linkage, occasional tuning), its a great setup. My setup runs too ruch since I have the incorrect floats to float bowl covers, but i still get 19mpg in the city and 28 highway. 

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Alright guys, made some progress and need more advice. Finally had a chance to really dig into everything today.

 

It seems to me I have a late l20b with a crank pulley that only has a single 0 degree timing mark...but I think they swapped the timing pointer from the L16 over to it, so I don't have the saw tooth timing indicator I should have and having no luck finding one. Checking the timing its showing the mark on the pulley perfectly lined up with the pointer, so seems to me the PO had the timing set at 0 degrees. Trouble shooting and using my timing light advance I believe I have adjusted it to 12-14 degrees now but hard to verify 100% without the saw tooth indicator. Car certainly runs smoother, so that may have been a big part of the problem.

 

Now onto the carbs, they were way out of sync and very unbalanced, got them perfectly synced and balance now, and got all the linkage issues sorted. I use this meter to sync, same one I use on my 240z su's. http://a.co/d/92hUctm

 

Now for the two problems, these are probably related to each other.

 

First issue, the idle will not stay were in needs to be, I sync them to 700-900rpm and in a few minutes its idling at 1500rpm, so I sync back to 700-900 and a few minutes later its stumbling trying to idle at 400. So basically re-syncing up and down and idle won't ever stay where I put it.

 

Second issue, its running rich, was running super rich before I changed the timing and doesn't seem much better now. Plugs are black but no smoking. I have tried to adjust fuel on both carbs using my colortune and with the adjustment nut cranked all the way up its still showing me very rich mixture, so fuel adjustments on both carbs are pretty much not working at all from what I can tell.

 

So whats the next step here? I have no experience troubleshooting these external float su's. Is it time for a rebuild or should I check the floats or something else. Carbs seem to be working fine other that the fuel issue. Thanks for all the help guys, really appreciate it.

 

 

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Set the fuel level by adjusting the float. I think clear vinyl tubing can be used to see where the gas level is.

 

If over rich is the choke free and moving? Is it turning off? Stuck?

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29 minutes ago, datzenmike said:

Set the fuel level by adjusting the float. I think clear vinyl tubing can be used to see where the gas level is.

 

If over rich is the choke free and moving? Is it turning off? Stuck?

 

I'll have to do some searching and see if I can figure out how to set the float level tomorrow. Is there a write up on it anywhere?

 

Choke is moving and I'm pretty sure it's all the way off, I'll double check that as well.

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Alright, more progress, good and bad.

 

First off, floats both look good but chokes were not fully closing, and will not work properly at all. I can disconnect the cables and manually push them closed but as soon as I reconnect and use the cables they will get jammed back up and I have to manually close them again...this is still the case after cleaning and lubing the hell out of them. With the chokes closed and the motor warmed up I can actually set the fuel mixture now though, so not running so rich....but, motor is still randomly bogging and stuttering, mostly above 3k rpm, which I'm thinking is likely throttle shaft leakage....sometimes it pulls fine, sometimes bogs and looses power, could potentially be timing or an ignition issue but the randomness of it makes me feel like its a carb issue.

 

So at this point I'm kind of back to where I was in the beginning...this kind of stuff is why I hate messing with old carbs. As much as I would like to keep the su's at this point there are a few problems. If I was to get them rebuilt I would want ztherapy to do it so the carbs would be fully remanufactured and have their roller throttle shafts. However, my carb's bore measures at roughly 36mm, so I'm thinking these are the sss 510 carbs and if they are, even ztherapy doesn't recommend using the sss su's on a motor over 1800cc. On top of that it would cost 700+ and I'd be waiting months to get them back.

 

So I'm reluctantly back to thinking the weber 38/38 w/ the redline performance manifold is my best choice. I would agree with what distributorguy said above after doing more research, I think the 32/36 are too small for an L20. The dcoe are nice but pricey and much more complicated to tune. So really not sure there is a better option than the 38/38 at this point.

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Floats looking good is not the same thing as having the correct fuel level in the float chambers. The fuel level is critical to how it is metered by the rod and jet.

 

Throttle shaft wear might affect idle but not much else.

Timing wouldn't randomly change and affect running.

 

First I would replace the fuel filter. Maybe the random drop out is the carbs running out of gas from a plugged filter. Basically using fuel faster than it can be replaced.

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26 minutes ago, datzenmike said:

Floats looking good is not the same thing as having the correct fuel level in the float chambers. The fuel level is critical to how it is metered by the rod and jet.

 

Throttle shaft wear might affect idle but not much else.

Timing wouldn't randomly change and affect running.

 

First I would replace the fuel filter. Maybe the random drop out is the carbs running out of gas from a plugged filter. Basically using fuel faster than it can be replaced.

 

Filter is new. Didn't look into the floats anymore as it didn't seem warranted...mainly due to the choke problems I can't fix and because ztherapy doesn't recommend using these carbs on this engine to begin with. My timing comment was directed toward the timing mark/pointer issue I talked about a few posts up, not that timing is randomly changing or anything like that. I have a late L20b with a single timing mark on the crank pulley and it looks like they installed the L16 timing pointer on the block instead of the saw tooth indicator...so I need to dig into it more and make sure the single mark on my crank pulley is a 0 TDC mark and that the the timing pointer is lined up with it a 0 TDC in order to 100% verify the timing is correct....but I'm pretty sure I'm sitting at 12 degrees right now and I don't really think that's part of the problem.

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If you run a smaller carb than necessary it will give good mileage and perhaps not give as much performance as a larger one. I don't know why they would recommend not using them. Smaller carbs always run better than carbs that are too large. Besides, two 36mm SU carbs (I think the SSS SU's are 38mm... measured on the intake side diameter) are way larger than the 30/34mm Hitachi 2bbl that came on the L20B.

 

Turn the advance up in increments till it pings under load and then back it off till quiet. See if it runs better. The late L20B and the later Z series engines all used the saw tooth timing indicator. The single notch is TDC.

WQdyxvc.jpg

 

The TDC mark has to pass the advance marks before getting to 0.

 

 

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19 minutes ago, datzenmike said:

If you run a smaller carb than necessary it will give good mileage and perhaps not give as much performance as a larger one. I don't know why they would recommend not using them. Smaller carbs always run better than carbs that are too large. Besides, two 36mm SU carbs (I think the SSS SU's are 38mm... measured on the intake side diameter) are way larger than the 30/34mm Hitachi 2bbl that came on the L20B.

 

Turn the advance up in increments till it pings under load and then back it off till quiet. See if it runs better. The late L20B and the later Z series engines all used the saw tooth timing indicator. The single notch is TDC.

WQdyxvc.jpg

 

The TDC mark has to pass the advance marks before getting to 0.

 

 

 

Yeah, the 36mm thew me off a little but I found another guy with sss carbs that measured 36mm as well...maybe a variation or something, were there ever any other datsun dual su's made that small? I thought 38mm was the smallest.

 

This is the plan I have in my head to verify the timing, if I have something off and this won't work or theres an easier way please correct me. The question I have is if the pointer that's currently on the block is oriented correctly to line up with the pulley mark at 0 tdc. Once I have the orientation between the single pulley mark and the single pointer sorted I can just set my timing light at 12 degrees advance (or 32 degrees total timing @ 3k rpm if I do it that way) and line the pulley mark and pointer up while setting the timing. My plan was to pull the first plug, watch the piston top through the hole and turn it by hand to get as close to tdc as I can, then verify where the pulley mark is in relation to the pointer. If they line up at tdc then great, if not I would need to make a new O tdc mark on the pulley to be able to use the pointer I have accurately. Then I can just set the timing with my advance light as mentioned above.

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Any idea if the saw tooth indicator on an L28 is the same as the one on an L20b? If so I may have one I can swap over.

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I am confused, I just searched Datsun 36mm SSS carbs and everything came up as 38mm, now I am not saying there is no such thing, but a search shows nothing for 36mm SU carbs, it did show a bunch of 36mm motorcycle carbs.

Also I am wondering/confused on the choke thing you are talking about, the SU carbs do not have a conventional choke valve like a down draft carb where you pull the choke on and it closes a metal butterfly valve, when you pull the SU choke on it drops the seat on the bottom of the carbs and that makes the fuel mixture way richer instead of cutting off the air like most chokes, the seat that slides in and out of the fuel adjustment nut is what drops when you pull the choke on, it has a rubber hose connected to it.

Are you talking about the piston in the dome of the SU that goes up and down when the throttle is opened and closed, is that what you are calling a choke?

Are you driving this vehicle when you talk about it bogging down/stuttering, or are you just revving it up in your driveway?

 

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24 minutes ago, wayno said:

I am confused, I just searched Datsun 36mm SSS carbs and everything came up as 38mm, now I am not saying there is no such thing, but a search shows nothing for 36mm SU carbs, it did show a bunch of 36mm motorcycle carbs.

Also I am wondering/confused on the choke thing you are talking about, the SU carbs do not have a conventional choke valve like a down draft carb where you pull the choke on and it closes a metal butterfly valve, when you pull the SU choke on it drops the seat on the bottom of the carbs and that makes the fuel mixture way richer instead of cutting off the air like most chokes, the seat that slides in and out of the fuel adjustment nut is what drops when you pull the choke on, it has a rubber hose connected to it.

Are you talking about the piston in the dome of the SU that goes up and down when the throttle is opened and closed, is that what you are calling a choke?

Are you driving this vehicle when you talk about it bogging down/stuttering, or are you just revving it up in your driveway?

 

 

I'm confused on the measurement as well, but I found one other post where a had sss that measured 36mm on the intake bore..and these look identical to the sss so I'm assuming that's what they are.

 

I am driving it. When the choke cable is pulled the lever on the carb pulls up and pushes the rod with the little rubber hose under the mixture adjustment nut down. The rod on both of my carbs will not retract all the way back up after the choke is turned off, it has to be manually pushed back up, regardless of how much it's cleaned and lubed. 

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OK, that is likely a linkage issue, that is a known issue that Ztherapy fixes when they rebuild the carbs along with the known bushing issue that they install sealed bearings to fix that.

As I said, if you can close/turn that fuel adjustment nut closed all the way on the back carb and disable the front carb by lifting the piston and if it keeps on running the carb is not serviceable and needs rebuilt.

I drove my worn out not serviceable carbs for several years that way, but when I started losing power on the freeway and was not able to keep up the speed limit on the hills because of it I replaced them and some day I will take them to Ztherapy and have them rebuild them as I bought them new from Nissan back in the 90s when they had a clearance sale, they lasted 20 years of daily driving.

Edited by wayno

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6 minutes ago, wayno said:

OK, that is likely a linkage issue, that is a known issue that Ztherapy fixes when they rebuild the carbs along with the known bushing issue that they install sealed bearings to fix that.

As I said, if you can close/turn that fuel adjustment nut closed all the way on the back carb and disable the front carb by lifting the piston and if it keeps on running the carb is not serviceable and needs rebuilt.

I drove my worn out not serviceable carbs for several years that way, but when I started losing power on the freeway and was not able to keep up the speed limit on the hills because of it I replaced them and some day I will take them to Ztherapy and have them rebuild them as I bought them new from Nissan back in the 90s when they had a clearance sale, they lasted 20 years of daily driving.

 

That's pretty much exactly what I was thinking...but like I said before, z therapy doesn't even recommend running these carbs on anything bigger than an L18, and I trust their recommendations. Plus I don't want to spend 700+ and wait months to get my carbs back...so I am back to looking at 38/38 webers now. 

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