Yet more carb woes

john-h

john-h

Well-Known Forum User
I'm trying to set up my standard carbs for my 260z but I have a problem and need some advice. The carbs have been on and off lots of times and the car runs O.K. when warm but is a pig to start. I set the choke aperature gap as per Haynes, balanced the carbs and set the mixture. It was still poor starting I adjusted how much the throttle is opened by the choke to 0.6mm as per Haynes again because the throttles where virtualy shut. But now the car is still tricky to start and the engine races at 2500rpm with the choke out and is hunting and it is very tricky to work the choke in without stalling the engine. Can someone tell me what the throttle opening should be, it is clearly too wide now. Is there a book on this type of carb? Help please they are driving me daft.
 
Lots of good stuff on the net re setting up SU's, and am sure you'll get some good advice on here too (regret I can't remember exact settings). Have you got stock flat-top 260 carbs or 240 domed-top jobbies? (Hopefully the latter!)

YL
 
I've got the stock flat top carbs which I understand are tricky to set up.They are of the last design of S.U. and have the mixture adjustment by adding air into the carb rather than by adjusting the jet position to control fuel. I've had every problem possible and my workshop manual is poor. The problem now is just associated with the choke.
 
John: Don't quite know what you mean about carbs mixture being adjusted by drawing air into them. If they are Hitachi's then they should be almost identical ti S.U's. Are you sure your jets are correctly centred and have you checked for air leaks on the inlet side? Have you got the requisite oil in the dashpots? Are both your choke cables working? Have you checked that the pistons in the dashpots are free for their full length of travel and drop down with a little clunk when released?
 
The 260Z has the HMB46W emissions carbs which, although SU in principle, have no parts in common with a traditional SU carb. The HJG46W on the 240Z is like a traditional SU made under licence, but still has virtually no parts in common with a British made SU carb. Both types were made by Hitachi.
 
Probably this is not the advice you want, but I just throw the emission type carbs away( i must have scrapped 6 sets). Try and find a useable set of 240 carbs or get some 1 3/4 SU carbs and fit them to your 260 manifold. A little fabrication required but not rocket science.
I know some one will post up saying they have a set of 260 flat tops that run really well, but the other 99 sets(out of 100) are useless.
Just my opinion, of course, based on my experiences.
 
I thought I'd leave it to a more experienced voice to suggest ditching the 260 carbs. I think the most generous comment I've heard about those things is that they make good boat anchors! Now, in all fairness, my brother-in-law has flat-tops on his 260Z 2+2 and they work ok (although the car is waaaaaaay down on power), but he's already said that if they ever go wrong then he won't try to get them sorted because it's not worth it.

I think 240 carbs are the way to go.
 
I want to stick with the stock carbs for the time being because my car is totaly standard with no modifications at all. With regards to the mixture adjustment There is a screw on the front carb which allows additional air to be drawn into the arb bodies. This controls the mixture by controling air into the body rather than on a standard S.U. where you control the flow of fuel past the needle. With regards to the problem I have I adjusted how far the throttle opens on full choke to .6mm but this is not correct. It seems it should be open this far at the full extent of the lever which is not the same as the choke cable travel. In effect the I have adjusted the throttle to open too wide which is why it is racing. Why are these carbs poor on power?
 
I think they were designed as a response to tightening pollution laws in the US, and hence there's a lot of exhaust recirculation and crancase emissions stuff on them. All of this didn't help the car when it was new, and 30 years later will be even worse. Just compare your 260 with an under-bonnet photo of a 240, or a 260 fitted with 240 carbs. Every time I pop the bonnet on my brother-in-laws 2+2 with flat-tops I see a load of extra pipework that simply isn't there on mine!

I don't think any of my links to carb pages on websites address the flat-tops, but there must be the knowledge out there to keep the more "original" cars running.

Good luck!
 
The official Nissan manual quotes 0.59 - 0.64mm for the throttle butterfly gap (measured with carbs. off the manifold, obviously), so I would suggest your problem lies elsewhere. The power valves are often a source of problems with these carbs. making them run over-rich.

Have you set float levels - fuel overflow could cause too much enrichment when hot? Is the choke (suction) piston operating properly - rod may have been bent or diaphragm damaged?
 
John
Just been on a couple of american websites ref carbs & they both concur to throw away your carbs & fit 240's, although one of them does a kit to do away with most of the emmision gear which would rather defeat the object in your case. If you are interested I will have a pair of 240 carbs in the next month or so.
 
Any 260Z owners thinking of switching to 240Z carbs. should be aware that the nozzle (jet) assy., the inlet valve assy. and the original needle are all now unavailable from Nissan in the U.K. Standard SU needles can be used, but no other parts are interchangeable. You need to ensure that any carbs. you buy are in a "bolt on and go" condition......
 
Thanks for all the help. I can now update you on the latest developments. The problem I started with was that it is bad to start when it is cold and has been stood for a day or so. It is as though there is no fuel until it has turned over several times. Then I have poor throttle response for a few seconds until it has run a short while. With the choke in it runs like a dream and lots of throttle response so I think the throttle pumps are working. I adjusted the opening of the throttle on the choke incorrectly (now set as Mike says to 0.6mm) Now it is still tricky to start from being stood - it starts fine when cold after it has started once. It ticks over a bit fast with the choke fully out but this just requires the choke cable adjusting tho alter the amount of cam opening the throttle. I have a couple of questions if anyone can help.
1. Do the cars need full choke to start them from cold but in a garage?
2. On full choke what tick over speed shoul I expect?
3. How easy should it be to start from being stood for some time?
4. If I am waiting for fuel to get up to the engine what is the cause - faulty fuel pump valve? split hose? carb float closing too soon?
Again thanks for any help.
 
1. I would expect to use full choke with the ability to go to half / three quarter choke after a minute or two.
2. On choke engine speed 1500 - 2500 rpm (at a guess from memory)
3. Hard starting from cold after long lay-off is quite common.
4. Does fuel fill the filter assy. quickly? I'm assuming you have a nice fresh plastic filter fitted where you can see the fuel level in it. An electric pump will quickly prime the system before turning the engine over, but the standard mechanical pump has to be primed for a bit to fill the system. A weak pump or poor pump valves would slow this down and may be a contributory cause. Check cranking fuel pressure - should be 3.4 psi.
 
The fuel pump and filter are brand new. The problem I think is I am expecting too much from a thirty year old design. My next plan is to turn the engine over with the coil disconnected and time how long it takes to get fuel up to the engine. This will give me an idea how long I should expect it to turn over before it will fire.
 
My 260 usually only gets used at the weekend and with a new fuel pump, plenty of choke and good battery charge I would expect anywhere from five to ten seconds of turning over before she fires. And she doesn't always fire all six cylinders straight away either!

After use and then being put away she'll start almost instantly for a day or two, but the turnover time becomes progressively longer.

TL
 
I think that this is the point I am at. Ten/fifteen seconds of turning over with progressively more firing until it starts, then a few seconds on the choke when it doesn,t respond to the throttle (it always fires briefly first turn, which I think is the fuel in the carbs). As it runs and the choke is worked in it gets better and with the choke fully in it is fine. The engine hasn't run for more than 30 minutes in the last 14 years so there will be a lot of settling down. I'm begining to feel a lot better now thanks again.
 
That sounds much like mine, although your turnover time is longer. I think these engines and carbs respond very well to being used and not left standing around. Crud and petrol varnish are real problems otherwise.

One thing worth noting is that if I leave mine idling for a long time she'll foul the plugs (even with no choke). You need to get your engine hot and get some revs going!

Good luck!

TL
 
Thanks Tony,
Another slight thing I failed to mention is that the fuel tank had loads and loads of crud in it. It was like brown sugar to look at, hard, like sand and insoluble even in petrol. Although I got as much as possible out it I will take some getting rid off.
 
Aha...... The real problem has come out at last !
I hate to be the prophet of doom ( I'll leave that role to Pmac !), but as long as you are trying to get fuel out of that tank, you are going to get nothing but grief in attempting to get the Engine running properly.
What you have in there is the residue of evaporated petrol, and whatever the proper name is for it, it will continue to contaminate any fresh fuel that you pour into the tank, and 'confuse' the carbs from running properly. Been there, done it,.... and wasted dozens of hours going round in circles.
The only way to sort the tank out is to either try to buy a good one from a 'Z' Breaker ( and we've run out of them ! ), or bite the bullet and take it to that firm in Leeds which another member has already recommended, for a Recondition job. Not cheap, but the only way for a long-term cure.
If you think that you can clean it out effectively yourself, then, well.. lots of luck, and enjoy yourself in the process. We've tried it on many an occasion , and still the scale keeps falling into the fuel pickup pipe. At least two of us Rally guys have been stranded with fuel blockages in this way.
Do remember that the tank is full of fuel vapours which are highly explosive, so do not try to flush it out with thinners or anything similar. If I hear a big boom from the wrong side of the Pennines next weekend, I will know what it is !
Not want you want to hear, perhaps, but hopefully this will be constructive in problem-solving.
Good Luck.
 
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