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Advice for an Amateur refitting SU carbs


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#41 DoctorZ

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Posted 12 May 2009 - 10:56 PM

This might help you guys with tuning the round top carbs.
It's a bit like watching Al from Home Improvement on their "tooltime" segment but the dude knows what he's talking about.




#42 stevo_gj

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Posted 27 May 2009 - 04:45 PM

Pushed back carby swap til the 6th.

Has anyone ever heard of/used colortune? Here is an example of how it works on a bike:

When the flame is 'bunsen blue' the mixture is correct.

I found a thread discussing them too:

"Re: Gunson Colortune
That old thing was great for carbs like SUs or Strombergs, where basically, if you set the mixture at tickover, it was correct for the rest of the range."

Quote stolen from http://www.cobraclub...-colortune.html

I'm seriously considering buying one for when I do my carby swap.

#43 Lynton

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Posted 27 May 2009 - 07:53 PM

Steve I replaced my flat tops (that were running fine) with 240Z SU's and manifold I also left the carbon cannister in place and used the standard 260z, 3 line fuel rail, although I had a 240Z spare fuel rail just in case I also used the standard power brake hose to the carbie manifold, if you need any pics let me know?
The other thing is if you are removing studs holding the carbie manifold put plenty of penetrine on them a couple of days before dismantling so you don't break the studs and use proper good quality spanner and sockets so to try and avoid breaking 30 Y.O studs, if this happens they will have to be drilled out which I left for the professionals. Does your car have extractors? I also purchased new fuel rubber hose lengths and cut to fit I also used the original heat shield foil around the fuel hoses to help stop fuel evaporation when engine hot. I have also used a 240Z air box. The job is just doing it carefully and don't be in a hurry I was able to do it, so I am sure you can too!! Lynton 8)

#44 stevo_gj

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Posted 05 June 2009 - 12:05 AM

Well I watched the educational videos that we got from the states and they are very helpful. I am 90% confident that I'll be able to do the conversion to 240z carbs on sat.

:o :o I just realised that I don't have a gasket to replace the intake manifold gasket that is currently on the car! Is this something I can pop down to Repco and buy? The old one is looking pretty wrecked!

#45 gav240z

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Posted 05 June 2009 - 04:06 AM

Well I watched the educational videos that we got from the states and they are very helpful. I am 90% confident that I'll be able to do the conversion to 240z carbs on sat.

:o :o I just realised that I don't have a gasket to replace the intake manifold gasket that is currently on the car! Is this something I can pop down to Repco and buy? The old one is looking pretty wrecked!


Should be able to get 1 fairly easily. I'm fairly certain the same gasket is used for exhaust and intake from memory. So you'll need to unbold the exhaust manifold also.

#46 stevo_gj

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Posted 05 June 2009 - 10:10 AM

Cheers Gav, if you hadn't said they were the same they would have had no idea what I meant on the phone! I called a supercheap and repco to no avail, but Lawnton Autobarn said they didn't have it in stock but could get it by this afternoon! :)

#47 stevo_gj

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Posted 06 June 2009 - 01:59 AM

Well I'm just about good to go. The only question I have is whether to use the 240 or 260 inlet manifold. The Z therapy video Shaun bought states that the 260 has markedly improved airflow, but it looks uglier with all the emissions crap sticking out of it. Hmmm...

The work I did on the carbs tonight included:

Setting both the mixture and idle screws to their correct settings before adjusting after fitment.
Shortened one needle to be the same length as the other.
Adjusted both floats to be the 5mm distance from top. (they were really far out)
Adjusted both choke linkage to open both butterfly valves the same tiny amount. (the butterfly valves weren't opening at all)

Two days ago I would have read the above list and had no idea what any of it meant. These Ztherapy videos are pretty bloody good. I'm tempted to make a couple of my own during the fitment and tuning process.

Oh and no I don't have extractors Lynton :P

I'll let you guys know how everything goes tomorrow :)

#48 gav240z

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Posted 06 June 2009 - 02:12 AM

Well I'm just about good to go. The only question I have is whether to use the 240 or 260 inlet manifold. The Z therapy video Shaun bought states that the 260 has markedly improved airflow, but it looks uglier with all the emissions crap sticking out of it. Hmmm...

The work I did on the carbs tonight included:

Setting both the mixture and idle screws to their correct settings before adjusting after fitment.
Shortened one needle to be the same length as the other.
Adjusted both floats to be the 5mm distance from top. (they were really far out)
Adjusted both choke linkage to open both butterfly valves the same tiny amount. (the butterfly valves weren't opening at all)

Two days ago I would have read the above list and had no idea what any of it meant. These Ztherapy videos are pretty bloody good. I'm tempted to make a couple of my own during the fitment and tuning process.

Oh and no I don't have extractors Lynton :P

I'll let you guys know how everything goes tomorrow :)


I'd stick with the 240z intake manifold. What's important is port matching more than the manifold. I can't see any major difference between the 240z / 260z intake manifolds other than the number N63 / E88 from memory.

They might have slightly wider diammeter. Measure the openings on both ends and see how big they are? If the 260z manifold is larger it 'should' flow better, but the only way to know for certain (Dyno!).

#49 stevo_gj

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Posted 08 June 2009 - 12:36 AM

Well it went quite well, the job is 99% complete and I drove the car home this afternoon. Josef's dad Paul was a great help, but since he had a sprained wrist I did 100% of the work myself under his direction - and you guys know that I am not a mechanically experienced person. it just goes to show that these engines are really easy to work on. :)

I do need to get a set of choke bits though, as the little plates I rigged up from the 260 manifold aren't exactly ideal. I'm going to PM greg for them.

As mentioned previously I set the carbs up before the install as specified in the video and they didn't need too much work to get running nicely. The mixture was really rich for both carbs initially, but it is running great.

Probably the biggest plus for me is the improvement it has made on the braking. I have the hilux front calipers, stock rear drums and a reconditioned stock master cylinder, and there was quite a bit of travel in the pedal when I had 260 carbs. I never found the floor but when you compare it to what I have now it was not ideal. I think the brakes are better because the 240z manifold has a much better vacuum seal (doesn't have like 100 emissions hoses running off it using the vacuum), and therefore it is ensuring much more vacuum is located at the MC. The pedal feels firm and the car stops very quickly.

Driving the car is beautiful, with the carbs opening up really nicely. They pull in low revs quite well and can rev highly without a problem. The only things I still need to do are:

Replace fabricated choke mechanism with original one off another manifold.
Perfect tune (front carby is still running a little rich according to colortune spark plug)
Tighten throttle return springs (the springs I had to add to the throttle to close the butterfly valves when I release accelerator are a little loose, this is an easy fix)
Source another wingnut for the airbox :) I only have one

I have a bunch of photos/videos but I can't find the camera cable just yet, so when I'm ready to procrastinate study some more I'll throw them all up so you can see the procedure.



#50 2+2er

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Posted 08 June 2009 - 09:13 AM

Good stuff, can wait to see the pics and video. My colortune arrive on Friday so I'll be tuning mine latter today with it.

What needles did you end up using?

#51 stevo_gj

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Posted 08 June 2009 - 10:50 AM

Thanks mate. I just used the needles that came with the 240z carbs in the end. The only adjustment I made to them is to shorten one so that they were both the same length.

#52 stevo_gj

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Posted 08 June 2009 - 08:51 PM

...well I've been having some issues tuning it and I need some help. :-[

This is the tuning process I have tried:

Start engine, runs rough, put it to an acceptable idle and then use unisync meter to balance air flow at idle. Then adjust the mixture screws until I achieve a blue light on the colortune spark plug which indicates that the carbs are running at the right mixture.

It is at this stage that I am struggling. It backfires when it is idling, even when both carbs are sucking the same airflow and the mixture is appearing to be the right color using the colortune. The only way to fix it seems to be to adjust the idle mixture screws to a higher RPM but that's hardly an ideal solution since I'll be idling between 1500-2000.

Does anyone have any information on how to troubleshoot a low RPM backfire? I'd really like to learn to fix it rather than take it to a shop.

I would really appreciate some help as I need to get it running before I drive to uni tomorrow  :-\  :-[

#53 Zedman240®

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Posted 08 June 2009 - 08:58 PM

Are you checking all the cylinders with the color tune? Some cylinders may need a little extra fuel than others; Not all cylinders are equal. Unless you have cc'd the heads etc.. If it wants a slightly richer mixture just for the idle, I think that should be ok.

#54 stevo_gj

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Posted 08 June 2009 - 09:05 PM

I haven't tried checking all the cylinders. I'll have to give that a go in the morning. I was using cylinder 3 for the rear carb and cylinder 5 for the front carb. (assuming the rear of the engine is the cylinder one).

I might just try and get it running rich and see if that stops the backfiring.

Anything else you can think of?

#55 stevo_gj

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Posted 10 June 2009 - 07:21 PM

Well here are the pics from Saturday:

Here you can see what I started with, eugh:
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Unplugged all relevant hoses and unbolted the carbs to expose the exhaust manifold:
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Comparing the two sets of Carbys:
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Cleaning the crap from the old manifold gasket:
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What we ended up with:
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Exhaust manifold and new gasket (note sheared bolt on far right of manifold, was like that when I bought it):
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Bolted new carbs on:
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Oh crap, no choke brackets (that pin I'm holding was one idea we had, but ended up using 260z plates. I still need to find some 240 brackets or something):
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Removed water lines that fed into carbs and created a temporary bypass of the T junction for water:
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Here are the accelerator linkage springs I bought from repco and installed using a couple bits of metal that I drilled holes into:
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Here is the final product:
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Here is what I started with, as a reference:
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Most of the emissions stuff is gone, as you can see. I took out the carbon canister and the plastic bottle too as well as heaps of little bits and unnecessary hoses.

It's running quite well currently, if a bit rich. I have tuned it a couple times and while I feel like every time I do I am learning something, it is as much by luck as by skill that I am getting it working. For instance: I had the idle mixture screws in too far initially which was causing the backfiring problem. I would have had no idea since it seemed to be idling acceptably, however it was by chance that I backed both screws out heaps to restart the tuning and see what made the difference.

Currently the car is putting smoke out the exhaust when i accelerate under load. Combined with the significant increase in the petrol smell makes me think it is running very rich when accelerating. It's very drivable though, and there is definitely more power there. I would like to fix it though.

The choke mechanism is only coming out half way, due to the 260z plates that are holding the cable causing it to have a weird angle and restricting movement.

Can anyone show me a picture of what their round top carb choke mechanism looks like? I would really appreciate it.

#56 Thomo260

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Posted 10 June 2009 - 08:28 PM

When you say you removed the 'plastic bottle' are you referring to the vacuum tank? If so, how well do your demist controls work with it gone? Also here is a pic of my 260 with dome top choke mech. for reference.

#57 stevo_gj

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Posted 10 June 2009 - 08:31 PM

When you say you removed the 'plastic bottle' are you reffering to the vacuum tank? If so, how well do your demist controls work with it gone? Also here is a pic of my 260 with dome top choke mech. for reference.


Hmm you could be right, my mates Dad said it was emissions related since it was connected to the solenoids but I haven't tested my demister controls. You're referring to all of the different airflow settings on the aircon? They haven't been working in a while hey. It's sitting in a box atm so worst case I just bolt it back on.

Thanks for the pic, it appear as if the choke cable doesn't connect to any bracket at all, it just sits in that hole.


#58 Thomo260

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Posted 10 June 2009 - 08:38 PM

The tank gives you a reserve of vacuum to move the controls. Mine wasn't working either until I fiddled around a bit and now they work great. The choke cable now just has a hook at the end into a hole on the lever as I got sick of it binding up and limiting how for you could pull the lever back. Now I have full choke with little effort and pressure on the console from the choke lever.

#59 2+2er

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Posted 10 June 2009 - 08:50 PM

A sticking needle will cause backfiring, is it from one or both carbs? Did you align the jets at anystage?

#60 stevo_gj

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Posted 10 June 2009 - 09:35 PM

I didn't align the jets, I constantly tested to see if any sticking was occurring but the piston always fell nicely with a clunk. Should I still check the jet alignment in that case?




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