Replaced flat tops with round tops off ebay, need help setting up

[Josh Slade]

As title says, I have a 74 260z that came with flat tops, so i immediately changed them to round tops. The carbs look like they have been completely taken apart and steam cleaned, (i had to change both needles as they were bent), and I have no idea how to set them up even to start the car. the car bursts into life absolutely fine with starter fluid, but can barely even get one cylinder to fire off without it. The carbs definitely have fuel, the front carb drips out fuel slightly by the plunger, and the rear carb shoots out quite a bit (might be an issue with the float).

Is there a "default" setting that i can work from, or should I just get a specialist to come and have a look?

 

[Rob Gaskin]

Josh, there is a lot you can do with these carbs 'on the bench' and that would be the best way to start. I THINK you need someone with more knowledge than you currently have otherwise it will be a struggle to get the car started and to run well.

I can link you to a Thread but if you have limited skills it may be a bridge too far for your first attempt.

Supplying fuel to a cylinder in the correct quantity is a very technical thing which carbs can do and Fi can do better.

I'm not being patronising I'm just trying to save you time and avoid a fire.

 

[Rob Gaskin]

The link:

https://zclub.net/community/index.php?threads/carb-tuning-advice.22965/#post-265706

 

[Josh Slade]

No no not patronising at all. I really appreciate the honest advice. I've got a family friend that has worked on a lot of SU type carbs, I'll ask him to come and have a look. Thanks

 

[Rob Gaskin]

Do you have a Haynes manual or similar?

 

[uk66fastback]

That thread is bookmarked for the future, thanks Rob,

Is the choke mechanism working properly? There are a lot of springs and levers on these carbs, are any weak / bent / missing? Are the floats okay with the heights set correctly. These carbs are a doddle to dismantle on the bench and check ...

 

[Josh Slade]

Yeah i've got a Haynes manual. I think the carbs are missing a few bits, for instance they came with no springs, and the the arm that connects to the jet to adjust choke.

 

[Rob Gaskin]

Well what I would say is even if the carbs are complete and 'more or less' tuned correctly it can be hard to start a car that hasn't run for a long time and may have other tuning issues.

Only this week I rebuilt and adjusted my 3 Screw Hitachi carbs. It was difficult to start because it's a modified car and I had set the jets as the Haynes which was too weak. Once started and warm I was able to tune it ok.

 

[Farmer42]

Buying carbs off e-bay is a bit of a lottery and you could easily end up wasting your money (speaking from experience). Most of the round tops are over 40 years old and unless you can either see/hear them running or have receipts for a full rebuild then you could have worn parts which, even with countless hours of tuning, will never run right if at all. The problem tends to be with worn throttle shaft bushes which will let in air. The way to check is to hold the throttle shaft and move it side to side and up/down to see if there is any movement. it should be minimal. The same with pulling it back and forth. It is an awkward job to replace bushes unless you know what you are doing because you have to drill them out very carefully.

Assuming that you throttle shaft is ok on both carbs, the first thing you need to do is set the float height. Take the lid of the float chamber and turn it upside down. bend the metal tang so that the gap between the float and the lid when it is sat on the float valve is the correct height (sorry I cant remember what that is but its in the Haynes manual). If the gap is wrong, the carb jet will flood or the chamber will overflow or it will starve of the jet of fuel. Either way it wont run. Then make sure the float valves are working by blowing through the inlet pipe whilst pushing the float up and down carefully. It should stop the air when you push it up

You will need to get some return springs if you dont have any as this will return the throttle and stop it over revving. They connect to the heatshield so if you don't have one, you need to find somthing that will allow the spring to have tension. A lack of heatshield may also cause the carbs to overheat so it would be best if you can get hold of one.

Assuming everthing is now in place, the next thing to do on the carbs is to wind the jet adjustment screw all the way in and then wind it back out around 2.5 turns on both carbs. This will give you a good starting base to tune the carbs. You will probably need to adjust the throttle stop screw or idle screw. Basically follow the advice in the earlier link but as it said, you will need an airflow meter.

Just a quick question, you say you changed the needles, have you got the right ones. For British SUs they are SM or DX needles but I don't think it is the same for Hitachis.

 

[Josh Slade]

Buying carbs off e-bay is a bit of a lottery and you could easily end up wasting your money (speaking from experience). Most of the round tops are over 40 years old and unless you can either see/hear them running or have receipts for a full rebuild then you could have worn parts which, even with countless hours of tuning, will never run right if at all. The problem tends to be with worn throttle shaft bushes which will let in air. The way to check is to hold the throttle shaft and move it side to side and up/down to see if there is any movement. it should be minimal. The same with pulling it back and forth. It is an awkward job to replace bushes unless you know what you are doing because you have to drill them out very carefully.

Assuming that you throttle shaft is ok on both carbs, the first thing you need to do is set the float height. Take the lid of the float chamber and turn it upside down. bend the metal tang so that the gap between the float and the lid when it is sat on the float valve is the correct height (sorry I cant remember what that is but its in the Haynes manual). If the gap is wrong, the carb jet will flood or the chamber will overflow or it will starve of the jet of fuel. Either way it wont run. Then make sure the float valves are working by blowing through the inlet pipe whilst pushing the float up and down carefully. It should stop the air when you push it up

You will need to get some return springs if you dont have any as this will return the throttle and stop it over revving. They connect to the heatshield so if you don't have one, you need to find somthing that will allow the spring to have tension. A lack of heatshield may also cause the carbs to overheat so it would be best if you can get hold of one.

Assuming everthing is now in place, the next thing to do on the carbs is to wind the jet adjustment screw all the way in and then wind it back out around 2.5 turns on both carbs. This will give you a good starting base to tune the carbs. You will probably need to adjust the throttle stop screw or idle screw. Basically follow the advice in the earlier link but as it said, you will need an airflow meter.

Just a quick question, you say you changed the needles, have you got the right ones. For British SUs they are SM or DX needles but I don't think it is the same for Hitachis.

Thank you! I will try all of this. The carbs visually look very very nice, but i think the throttle shaft may have a bit of play. I'll double check the float heights and throttle shaft later. The jet adjustment screws on both carbs were both wound down almost all the way, the rear one most so, so i imagine that would be why it was gobbing out fuel. Also, this was the carb rebuild kit i bought, that came with needles https://www.ebay.co.uk/itm/DATSUN-2...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

Thank you so much for the reply

 

[Rob Gaskin]

Thank you! I will try all of this. The carbs visually look very very nice, but i think the throttle shaft may have a bit of play. I'll double check the float heights and throttle shaft later. The jet adjustment screws on both carbs were both wound down almost all the way, the rear one most so, so i imagine that would be why it was gobbing out fuel. Also, this was the carb rebuild kit i bought, that came with needles https://www.ebay.co.uk/itm/DATSUN-240Z-1972-KEYSTER-CARBURETOR-REBUILD-REPAIR-KIT-OEM-carbs-16010-E4301-2/382828350557?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

Thank you so much for the reply

I can see what attracted you to that site.