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stevo_gj

Advice for an Amateur refitting SU carbs

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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.

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...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  :-\  :-[

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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.

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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?

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Well here are the pics from Saturday:

 

Here you can see what I started with, eugh:

IMGP0440.jpg

IMGP0436.jpg

 

Unplugged all relevant hoses and unbolted the carbs to expose the exhaust manifold:

IMGP0444.jpg

 

Comparing the two sets of Carbys:

IMGP0446.jpg

 

Cleaning the crap from the old manifold gasket:

IMGP0451.jpg

 

What we ended up with:

IMGP0453.jpg

 

Exhaust manifold and new gasket (note sheared bolt on far right of manifold, was like that when I bought it):

IMGP0454.jpg

 

Bolted new carbs on:

IMGP0456.jpg

 

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):

IMGP0458.jpg

 

Removed water lines that fed into carbs and created a temporary bypass of the T junction for water:

IMGP0464.jpg

 

Here are the accelerator linkage springs I bought from repco and installed using a couple bits of metal that I drilled holes into:

IMGP0465-1.jpg

 

Here is the final product:

IMGP0472.jpg

 

Here is what I started with, as a reference:

IMGP0436.jpg

 

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.

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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.

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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.

 

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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.

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

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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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sorry, I just re read your post and realised the backfire was at idle and not while opening the throttle.

 

Is the backfire happening in the exhaust or through the carb(s)?

 

Have you cleaned the sparkplugs and check the gaps? They can foul up when your mucking round with rich mixtures and are the only other component you would have altered during the swap.

 

Also, if your vacuum has change it may have effected the vacuum advance on your distributor.

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Well I'm assuming its a backfire. The carb makes a loud popping noise and fuel spits out the open mouth. This is followed by a massive drop in RPM.

 

I haven't cleaned the spark plugs, I assume there is a special chemical I should use for it? I haven't check the gaps either so I will do that as well. Is it as simple as measuring the distance?

 

How do I determine whether the vacuum change has affected the vacuum advance on the distributor. I don't think it has had a very significant effect, however it might be part of the reason the idle isn't smooth. I have done nothing about adjusting the distributor, do you have any links to tutorials that would help me with it?

 

Not sure if this is relevant but: I did notice a slight vacuum leak at one stage when I heard a hissing near the intake manifold at idle. This was right after I had done some heavy braking. The brakes developed more travel for a while but this eventually disappeared so I assume the leak closed up somehow.

 

Thanks for the help man, this is just the kind of info I was looking for.

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Just realised something after watching the Ztherapy video: One of my pistons dropped slightly quicker than the other, which according to the video is a bad thing.

 

If I take some 400grit sandpaper to the edge of the piston where it slides against the vacuum chamber I should be able to remove enough material to ensure the correct clearance, and therefore the correct sliding speed for both carbs.

 

EDIT:

 

Well I was wrong about the piston being the problem. I think it is the jet alignment on the rear carb.

 

Using either piston still shows more resistance and a scraping noise as it slides down in the rear carb. Using both pistons in the front carb does not display either the scraping or the resistance. I will have to watch the Z therapy videos and see if he shows how to adjust this alignment.

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the plug gap is easy to check with a feeler gauge and should be .8 - .9mm. The plugs will probably be dry, black and have sooty deposits if the carb(s) is running rich and is most likely the cause.

 

What oil did you use in the dash pots?

 

RE the pistons, the Youtube video suggested you try swapping the pistons between the two carbs first to get them both falling at the same speed. Some one may have mixed them up.

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the plug gap is easy to check with a feeler gauge and should be .8 - .9mm. The plugs will probably be dry, black and have sooty deposits if the carb(s) is running rich and is most likely the cause.

 

What oil did you use in the dash pots?

 

RE the pistons, the Youtube video suggested you try swapping the pistons between the two carbs first to get them both falling at the same speed. Some one may have mixed them up.

 

I need to go to supercheap and grab a set of feeler gauges!

 

I used 5-30W oil, which is slightly thicker than tranny fluid but it is recommended by the Haynes manual.

 

Yeah check my edit in my previous post, it is the jet alignment I'm pretty sure. Both needles are encountering resistance when they slide into the rear carb jet.

 

Edit:

 

Well I just mucked around with the carbs (quietly because i didn't want to wake anyone) and ensured that both pistons now rise and fall at same speed.

 

I also readjusted both needles to be the correct depth by loosening the screw,  slightly pulling them out of their housing, tightening the screw until it was just touching the needle, and pushing them into carb until they contacted the jet which pushed them into the correct position. I did this while both jets were screwed in to their highest (leanest) setting.

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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.

 

I just realised that you are running British SU carbs there Thomo  :P

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Pic 1 Choke bracket

Pic 2 I used a Ford Falcon heater hose 5/8th inch one end and I think 3/4 inch the other and got rid of the three way metal pipe intersection and used a bracket to support the one piece hose. Lynton 8)

post-24-144023524627_thumb.jpg

post-24-144023524634_thumb.jpg

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Pic 1 Choke bracket

Pic 2 I used a Ford Falcon heater hose 5/8th inch one end and I think 3/4 inch the other and got rid of the three way metal pipe intersection and used a bracket to support the one piece hose. Lynton 8)

 

Thanks Lynton! I have one of the 4 screw SUs, do you have a 3 screw? I don't seem to have the same holes to mount that bracket as you do. That's a good idea with the hose I think I'll do the same thing :)

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Steve, by the looks of things since I don't have my 260Z with me where I am they are 3 screw see pic.

Lynton  8)

post-24-144023524665_thumb.jpg

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Ahh there we go, cheers 2+2er now I know what bits I need. :)

 

Well I've got some bad news, and some worse news.  :-X

 

Bad news: The exhaust bolt that was sheared has got a significant exhaust leak going on... Meaning I have to get it drilled out soon or it will burn through the gasket. I'm booking it in atm to get this done.

 

Worse news: The two centre cylinders might have a leak in the head gasket between them. This was pointed out by a mate who pulled out the lead for each spark plug while the engine was running, and noted that there was no significant drop in RPM of the engine for these two cylinders. I need to steal a compression tester and see if there is a leak between them. I've tried replacing the plugs and these centre cylinders still show the same problem.

 

I really really don't want to have to do get the head machined, and am open to suggestions of other causes of the problem.  :-\

 

Sorry to keep pestering you guys with questions like this...

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Its not looking good  :(

I'm about to ad a compression tester to my toolkit but have to move this weekend.

 

I suspect your friend might be right about the head gasket leaking and a compression test is the way to go.

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A quick test to see if your head needs machining, brace / clamp it so it is level, wipe some bearing blue over the edges, get a sheet of 12" x 6" x 5mm thick glass,( or similar dimensions, offcut free from glazier ) cover it with a sheet of wet and dry and rub back and forth a few times, as you rub off the bearing blue this will show you if you have any issues.........

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Hey xa1973, I thought that if you removed your head its a rule of thumb that it needs to be machined before you put it back on?

 

I'm going to hopefully get a compression tester this weekend and have a go with it. Depends how busy my mechanic is. I feel like its pointless trying to tune the carbys past drivable in these circumstances.

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xa1973 has mentioned a good method, don't need to machine the head before reinstalling it if its not warped and or pitted, 320 grit (of finer) wet and dry will prepare it nicely for a new gasket.

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