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240Z Carb tuning - No suction on the front carb


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#1 e9

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Posted 20 March 2009 - 11:51 PM

After some playing under the bonnet I managed to get the idle down to 1k but I suppose I didn't do it properly because it's pretty unstable, even when warm.  My guess is I adjusted the screws in the wrong order which would be a pretty good assumption as illustrated in the diagram below (it doesn't help that I can't a good 240Z twin SU diagram).  I got the picture from http://phreakmonkey....ay/IMG_2331.jpg if you want to make corrections.  Also the pic shows a pushrod set up for the throttle but mine is a cable with another screw at the top.

Any pointers?

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#2 Zeddophile

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Posted 21 March 2009 - 09:45 AM

Check the fluid level in the dashpots on the carbies.  On the twin strombergs on a rangie this can make the idle rough, and give it a miss when revving up.  If too low, top up with ATF or similar weight oil.

Cable operated throttle sounds odd, I thought they were all pushrods - maybe yours has been converted for some reason?  Check that its not holding the butterflies slightly open even when pedal not pushed.

Check your ignition timing too, roughly balance the carbs, then set the mixture screws under the carby, then finally fine tune the idle and balance.

#3 e9

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Posted 21 March 2009 - 06:12 PM

I think I've sussed the problem, I'm getting no suction from the front carby (which would explain the fumes as well).

Can someone please suggest something other than loss of compression in one or more of the cylinders caused by scoring or busted rings?

#4 Zeddophile

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Posted 21 March 2009 - 10:43 PM

I think I've sussed the problem, I'm getting no suction from the front carby (which would explain the fumes as well).

Can someone please suggest something other than loss of compression in one or more of the cylinders caused by scoring or busted rings?


No vacuum at all would mean all 3 cylinders which that carby feeds aren't firing, which is very unlikely to be caused by rings etc.  I'd also be very surprised if the idle would not go below 1000 rpm under this condition, it would struggle to stay running.  I'd say you mean you have less vacuum than the rear, but there would still be some there.

Couple of possibilities:
1.  Front carby mixture way out.  To check this, pull all the spark plugs out (keeping them in order so you know where they came from), and see if the front 3 look different on the electrodes in comparison to the back ones (example, the back ones might be really black and the front really light coloured).  Then put new plugs in, if you haven't already - cheap, and always a good idea - especially when chasing these sorts of faults - since you started with 6 shiny clean spark plugs, after fiddling with it some more you can pull them out again and have a look, any differences will be much easier to spot.

2.  Front carby butterfly way out of sync with the rear one (meaning its open either much less or even much more than the rear one).  Check the clearance between the bottom of the butterfly and the carby body with a feeler guage for a rough balance.

Personally, I'd be starting at option 2, and putting new plugs in so I could get an idea of whats actually happening.

#5 silverz

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Posted 22 March 2009 - 07:49 AM

Hi,
When I had a bent needle (measuring rod)  in my front carb the slide would often jam partially open and disable the carb. Could this be occuring?

Regards
Peter

#6 Scoota G

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Posted 22 March 2009 - 08:13 AM

Can someone please suggest something other than loss of compression in one or more of the cylinders caused by scoring or busted rings?


Sounds like you need to check your valve clearance.

#7 e9

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Posted 22 March 2009 - 08:18 AM

No vacuum at all would mean all 3 cylinders which that carby feeds aren't firing, which is very unlikely to be caused by rings etc.  I'd also be very surprised if the idle would not go below 1000 rpm under this condition, it would struggle to stay running.  I'd say you mean you have less vacuum than the rear, but there would still be some there.


It's quite reluctant to go below 1000, it gets rather rough but it has had cam work which could partially explain that situation.

1.  Front carby mixture way out.  To check this, pull all the spark plugs out (keeping them in order so you know where they came from), and see if the front 3 look different on the electrodes in comparison to the back ones (example, the back ones might be really black and the front really light coloured).  Then put new plugs in, if you haven't already - cheap, and always a good idea - especially when chasing these sorts of faults - since you started with 6 shiny clean spark plugs, after fiddling with it some more you can pull them out again and have a look, any differences will be much easier to spot.


I changed the plugs a couple days ago and it looked rich all round so I tried resetting the mix (screwed it all the way up and loosened 6 turns

2.  Front carby butterfly way out of sync with the rear one (meaning its open either much less or even much more than the rear one).  Check the clearance between the bottom of the butterfly and the carby body with a feeler guage for a rough balance.

Personally, I'd be starting at option 2, and putting new plugs in so I could get an idea of whats actually happening.


When I had a bent needle (measuring rod)  in my front carb the slide would often jam partially open and disable the carb. Could this be occuring?


I'll check those out today.  Thanks for the suggestions.  What number is the carb slide on the diagram?
Posted Image

Also heard it may be a vacuum hose issue.  Is there any likelihood of that?

Sounds like you need to check your valve clearance.


You're talking about the air valve? What am I looking for, if there's low or no clearance what do I do?

#8 DevilZ

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Posted 22 March 2009 - 08:25 AM

I think I've sussed the problem, I'm getting no suction from the front carby (which would explain the fumes as well).

Can someone please suggest something other than loss of compression in one or more of the cylinders caused by scoring or busted rings?


How are you checking if you hae suction from the front carbs? The first thing youshould be doing to check if its your carbs that are at fault is this:
Take the entire air cleaner assembly off so that you can look right into the carbs. Then start the car and open the throttle while looking ino the carbs to see if the needle moves up. As the throttle increases along with vacume, the carbs should open up obviously. If they dont then use a screw driver to slide it up and see if there is any resistance or if it can move freely.
Once I know this then I can tell you what to test next.
I was also curious as to wheather you just had an idle problem or if the car is not driveable? Is it fine at full throttle? If the carb isn't opening, the car wont be able to move really.



#9 silverz

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Posted 22 March 2009 - 09:56 AM

The carb slide I was talking about is no 12 in the diagram. Have you done the clunk test?

That is raise both slides with your fingers and let them drop together, they both should fall at the same rate and make a similar clunk sound when they hit the bottom. If they don't, they are fouling on something.

Regards

Peter

#10 Scoota G

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Posted 22 March 2009 - 10:28 AM



You're talking about the air valve? What am I looking for, if there's low or no clearance what do I do?


The valve clearance under the tappet cover. If the valves aren't closing all the way it would explain a loss of compression. My Zed used to do it on the rear three cylinders until i installed new rings (which were all broken).

#11 chris240

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Posted 22 March 2009 - 02:08 PM

yeah I had this before...in my case it was carb sync (as mentioned) and my "clunk" rate was very different..
in my opinion, its not cylinder related, but your carbs need a decent balance and a kit put thru them...also check the gluggy-ness of the oil.

#12 xa1973

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Posted 22 March 2009 - 05:12 PM

yeah I had this before...in my case it was carb sync (as mentioned) and my "clunk" rate was very different..
in my opinion, its not cylinder related, but your carbs need a decent balance and a kit put thru them...also check the gluggy-ness of the oil.


Agreed   ;)

( This was also a similar issue on an old Rangie 3.5 lt I had prior to dropping a 410 Chev into it )

:)

#13 e9

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Posted 22 March 2009 - 05:53 PM

Checked the carbs and they weren't the problem (though the front was clunking a little softer than the back) and I'm not game to try the valves myself so I'll get a mobile mechanic to come and have a look at it and pester him about everything so I'll learnt a few things.

Is the oil in the carbs any special type, or just regular engine oil?

#14 xa1973

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Posted 22 March 2009 - 06:00 PM

Checked the carbs and they weren't the problem (though the front was clunking a little softer than the back) and I'm not game to try the valves myself so I'll get a mobile mechanic to come and have a look at it and pester him about everything so I'll learnt a few things.

Is the oil in the carbs any special type, or just regular engine oil?


There will be many many varied opinions on this, some use ATF, I have seen sewing machine oil used, I have used a very light compressor oil

:)

#15 silverz

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Posted 22 March 2009 - 06:08 PM

Everyone has opinions on the oil to use. Most engine oil may be too heavy and cause a lack of responsiveness. a lot of people use ATF.

I have been using Valvoline 2 stroke mower oil! it is lighter than engine oil and seems to work fine in my car. I have also heard that really worn carbs may require heavier oil.

The original owners manual for my 240Z states to use "SAE, MS#20 or 10W-30 for damper oil. Do not use SAE #30 or higher weight oil"
the oil volume is only 3cc

#16 Scoota G

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Posted 22 March 2009 - 06:23 PM

I used to use sewing machine oil but that's only cause my mum and dad are tailors :S.

#17 chris240

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Posted 22 March 2009 - 07:06 PM

you can extract the old oil using a syringe (chemist $1.50)..or a  bit of tube and suck on it !!

#18 silverz

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Posted 23 March 2009 - 07:17 PM

E9,
How are you getting on with the carb/compression problem. have you worked out what it is yet?



#19 e9

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Posted 25 March 2009 - 09:13 PM

It's in at the shop now and they've said that the compression is okay and it's the carbs that need to be rebuilt.  This was confirmed by the plugs which I put in last week that have completely sooted up in the time I've been trying to identify the problem.

So the carbies were whisked away to the magical place where they charge lots of money to come back good as new.  Funny thing is they rang me and they turn out to be 180B 38mm carbs and not 44mm 240Z SUs which could explain why I have a cable throttle instead of a pushrod now.  My guess is that this will mean less power but better consumption.  On the other hand I'll probably have to take it back to the guys that rebuilt the carbs to get the jets adjusted once they're back on the car.

I'm hoping after this I'll finally get what I thought I paid for (a 260Z in good working order).  Oh well, hopefully things will run smoothly for a while after I get a replace the radiator, hoses and leads.

#20 Zeddophile

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Posted 26 March 2009 - 06:18 PM

So the carbies were whisked away to the magical place where they charge lots of money to come back good as new.  Funny thing is they rang me and they turn out to be 180B 38mm carbs and not 44mm 240Z SUs which could explain why I have a cable throttle instead of a pushrod now.  My guess is that this will mean less power but better consumption.  On the other hand I'll probably have to take it back to the guys that rebuilt the carbs to get the jets adjusted once they're back on the car.


Don't bother rebuilding them!  Track yourself down some with the correct size throats, and chuck them on instead - 38s on a car that's supposed to have 44s will seriously strangle it, and I reckon you'll get worse fuel consumption from having to keep the boot into it all the time.




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