Advice for an Amateur refitting SU carbs

[stevo_gj]

My mechanic has said he thinks it is more likely an issue of vacuum leaks in the inlet manifold, and that it is not necessary to remove the head to get it running well.

 

A new symptom has appeared: My car started backfiring when I turn the choke off at normal operating temperatures. I put the choke back on and it seemed to run ok. This is after running fine for a few days and no tinkering.

 

I noticed that when I put my head in the engine bay: if the electric fuel pump that I have in the rear is running, then I can hear a high pitch whining noise (not a hiss) which I am unable to locate the source. I have not noticed this before and it might be related to the backfiring issues. Has anyone ever heard of this before?

[Thomo260]

My mechanic has said he thinks it is more likely an issue of vacuum leaks in the inlet manifold, and that it is not necessary to remove the head to get it running well.

 

A new symptom has appeared: My car started backfiring when I turn the choke off at normal operating temperatures. I put the choke back on and it seemed to run ok. This is after running fine for a few days and no tinkering.

 

If the carbs are running way to lean then it will idle very rough if at all. When you pull the choke on, the carbs richen the mixture in effect correcting the ratio which is why it idles better. I just found that out when tuning mine on the weekend.  The balance pipe accross the top of the manifold, these can leak where they mount. Just a thought.

[stevo_gj]

Zedback (Mike) came over this afternoon and was a fantastic help troubleshooting my carby problems.

 

Here are the things we did:

 

The jet on the front carb would stick when i put the choke back in, meaning unless I had the choke on the front carby would be really rich, and the rear normal. I think this was lending towards the fuel smell in the cabin and the backfiring issues. We fixed this using a bit of INOX (lubricant) on associated parts, and by taking the piston/cylinder assembly out and putting it back in making sure when we tightened the screws that it did not cause misalignment.

 

We changed the 5-30W oil for tranny fluid, just some el cheapo stuff from supercheap. Mike commented that it the cylinders were really hard to lift, and that this might be causing issues with their movement. Hopefully with the new tranny fluid in there this will make for smoother operation.

 

Adjusted the choke cable so that they engage simultaneously, allowing for ease of running on startup.

 

We also put a piece of trimmed hose on the accelerator linkage to stop it moving around so much and reduce the amount the idle speed varies by.

 

Then we tuned the system using the following process:

 

• Put it to idle speed and use UNISYN airflow meter and the idle adjustment screw to ensure both carbys had the same airflow
  
• Then we plugged the colortune in to the 2nd cylinder (corresponding to front carb) and adjusted the mixture screw until the color went from a deep yellow (rich mixture) to a bunsen burner blue (ideal mixture). I should note that it seemed to run better when it was blue with a hint of yellow.
  
• Then we plugged the colortune into the 5th cylinder and and adjusted the rear carb mixture screw using the same method outlined above.
  
• Between adjustments, to test the mixture we increased RPM to a steady 3000 and expected to see a nice blue colour. Under acceleration the carbys should be rich.
  
• It might be necessary to repeat the past steps a couple times to ensure one carby doesn't lose its tune when the other one is adjusted
  

 

I realise now that I had made a mistake when working on the carbys. I had use a 200 grit sandpaper on one of the contact surface of the rear carby cylinder to dashpot. Now it is noticeable that this cylinder falls more slowly and is easier to get stuck or 'bind'. After a quick google search Mike discovered that if an SU carby is 'binding' then the carby runs lean which causes it to 'spit' out some fuel and create a significant drop in RPM. Similar to a backfire. To rectify this problem I will get some 1000-2000 grit sand paper and see if I cant correct my mistake.

 

Well I ended up driving the car to a freinds house tonight and it was running really well! It did not backfire once, and the acceleration was smooth. Apart from the fact that the idle still wanders between 650-1000 RPM the car is running great.  Not sure why the idle keep changing. There is a little bit of smoke out the back under acceleration but its better than it used to be. Mike pointed out that it's probably just a case of worn needles/jets, but that it was not a serious issue.

[garvice]

you guys know that I am not a mechanically experienced person.

Can I ask what degree you're doing again?

[Zedback]

Can I ask what degree you're doing again?

 

LOL.  I have flash blindness from the light bulb going off over Stevo's head all afternoon!  All I did was act as brick wall to bounce ideas off. 

[stevo_gj]

LOL.  I have flash blindness from the light bulb going off over Stevo's head all afternoon!  All I did was act as brick wall to bounce ideas off. 

 

Haha don't be so modest mate I would have been pretty screwed without your help.

 

Update:

 

My mechanic came over today and together we pretty much fixed everything.

 

The first thing he did was go over the entire car and ensure that it was fit to drive 1000km to Sydney. Apart from the rear tyres he gave it the thumbs up. Then we dropped the oil in the engine and replaced the oil filter at the same time.

 

After I had told him how good the engine was running the previous day thanks to Mike, it barely started and when it did it was backfiring out the exhaust and spitting out the carbs consistently. He said straight away: One of your cylinders ain't firing, lets check your plugs and leads.

 

All of the leads to the plugs were A-OK for resistance, but when he tested the coil->dizzy lead he found it had far too high a resistance. But replacing this did not fix the backfiring or spitting. He looked at the plugs and said, 'NGK HP6ES? You need hotter plugs!'. So off we went to supercheap to find some NGK HP4ES plugs. In the end they didn't have any in stock at supercheap or Repco but Repco had Bosch plugs that were equivalent to them so we got 6 of those. With these new plugs the engine ran much better with all 6 cylinders firing, the backfiring out the exhaust stopped and the spitting out the carbys stopped as well (or so we thought).

 

I then found that if the accelerator was engaged quickly the carbs would start spitting again. My mechanic cleaned the points and adjusted the 'dwell' to be the correct level. He then adjusted the timing from 8 degrees to 10 degrees advance.

 

After this the engine ran beautifully with no flat spots even at low RPM. We took it for a test drive and it felt like a new car. It still 'spat' once, but we were on a steep hill and at a low RPM. Otherwise I am very happy with the way it is running and hopefully it won't reset and run like crap tomorrow morning.

 

[stevo_gj]

Well after driving it today it drives beautifully when the choke is on, but when the choke is off the carbs begin to spit unless I accelerate really really slowly.

 

I am pretty certain that the carbys are binding (piston getting stuck or rising slowly), and are causing it to run lean and spit. I think I need to sand down the contacting surfaces between the pistons and the vacuum chambers on the carbys, and ensure that they fall at the same speed. I was gonna get some fine grit (between 1000-2000) and sand down the sliding surfaces.

 

Does anyone have any other recommendations about how I can stop the carbys binding?

[garvice]

I think you just need to go fuel injection, this would have driven me nuts by now, especially since it is a daily driver.

[stevo_gj]

Haha yeah I did think about that. I've had to fix just about everything else so when I do get it sorted its going to run really well.

 

On a completely related note I dreamed you were parked at a shop I pulled in to in a green 240z which you had painted red with spray cans. I asked you why and you said you regretted painting it green. Then you got in a fight with the guy who pushed in line. I have weird dreams. 

 

Anyway, anyone got any ideas about fixing the binding issues?

[chris240]

I thought you bought new SU's ? This is becoming a soap opera 

I had a local carbie guru install new su's on mine 2 years ago and theyve been perfect from day 1..

You "may" need to dyno-it once & for all, to get it running 100%....

Properly set-up SU's will give you less headaches than injection (there, I said it !!!)

[Zedback]

That rear carby was definitely slower to descend and was the only one I saw spit.  It's also the one Steve got a little too "creative" with when cleaning.  200grit and precision machinery doesn't mix!   What qualification are you studying for again   

 

Steve, I'd get the rim as smooth as you can with 800, 1200 and 2000 grit and then metal polish, unless someone has a better idea?  If that doesn't fix is then carby shop time.  When are you going to Sydney?  You could come around this weekend and we could try a different dashpot. 

[garvice]

I'm a bit worried Steve. We have only met twice and I am already in your dreams.

 

Properly set-up SU's will give you less headaches than injection (there, I said it !!!)

[sexual_sushi]

hmmm I have a complete fuel injection setup sitting in my garage.....

[stevo_gj]

200grit and precision machinery doesn't mix!  

 

Haha yeah I must have missed the lecture on sandpaper grades. Damn! (until a few weeks ago I didn't know what a higher grade of sandpaper meant) It's funny how many basic things we don't get taught.

 

I'm a bit worried Steve. We have only met twice and I am already in your dreams.

 

Haha don't worry, I think everyone from our Z meetings was there.

 

Steve, I'd get the rim as smooth as you can with 800, 1200 and 2000 grit and then metal polish, unless someone has a better idea?  If that doesn't fix is then carby shop time. 

 

That sounds like a good idea. I intend to have a good crack at it this afternoon and tomorrow. Will let you know how it goes. 

 

hmmm I have a complete fuel injection setup sitting in my garage.....

 

Damn you for that temptation. But seriously if I have this much trouble with carbys imagine the trouble I would have trying to set up EFI... eugh.

[Lynton]

Are you sure it is not ignition timing or a worn dizzy? By the way electronic ignition makes a huge difference IMO on the S30's. Lynton

[stevo_gj]

Are you sure it is not ignition timing or a worn dizzy? By the way electronic ignition makes a huge difference IMO on the S30's. Lynton

 

Hey Lynton, yeah I went through all that with my mechanic, the timing is perfect and the car starts and runs beautifully with the choke in, the minute I put the choke off it starts spitting.

 

I just had a thought, if there was some misalignment on the needles the problem would be hidden when the choke was on because the jets would be lowered, and when the choke was off the jets would be rubbing against the needles again and causing friction. This rubbing would result in the pistons struggling to rise and would cause the spitting!!

 

I picked up 800 and 1200 grit sandpaper from bunnings, they didn't have 2000, and I will sand and then polish the contacting surfaces on the pistons and vacuum chambers until they both slide smoothly and fall at the same speed. When I reassemble the system I will do some drop testing and ensure they fall at the same speed. Then I will see if there is significant rubbing of the jet with the needle and if I can do anything about it.

[stevo_gj]

Ok I'm officially stumped. 

 

The carbys are tuned using colortune and unisyn, Mike will back me up when I say they are reasonably well tuned.

 

After my sandpapering efforts tonight, the pistons are falling at EXACTLY the same speed and are more free to move than before.

 

If anything, it has made the problem worse.

 

There just seems to be a flat spot at low RPM that if you try to accelerate through you will cause BOTH carbys to spit SIMULTANEOUSLY.

 

If I pull over and look into the mouths of the carbys and engage the throttle quickly, both of them spit fuel at the same time.

 

If the choke is on, the car runs reasonably well and does not spit out the carbys.

 

Anyone got any ideas?     :-\

 

 

[sexual_sushi]

I don't know how it affects it but what about your compression test results?

 

140,150,115,110,120,110

 

Not the greatest, would this just effect overall performance or could it cause havoc on tuning?

 

I'm not sure about internals, maybe someone can put their 2 cents in, but what about checking valve clearances etc? Once again, don't know if this will have any affect on your problems...

[stevo_gj]

When we took those compression results we didn't know my car had barely any oil in it. Those centre cylinders were slightly higher compression after the oil was replaced.

 

Also, my mechanic said that he didn't think that those figures were low enough to be causing any problems. Not ideal, but yeah.

 

Hmm, perhaps fixing the falling speeds of the carbys requires me to retune them since the pistons don't require as much vacuum to rise?

[Zedback]

If anything, it has made the problem worse.

 

Maybe not, at least they are both now consistent  :-\

 

I'm out of my depth here but try:

1. disconnecting and plugging the vacuum advance -> better/worse/no change

2. putting heavier oil back in the dashpots -> better/worse/no change

3. email Greg and ask him for contact details of Robyn or one of the other Z club qld guys who might know a mechanic who specializes in SUs

 

Are you sure it is not ignition timing or a worn dizzy?

When we checked the timing the markings were dead steady so I don't think dizzy wear is the issue.  However when we checked the timing we did not disconnect the vacuum advance.  It was at about 8 degrees with vacuum advance connected, so could it be badly retarded.  SU Carburettor High-Performance Manual By Des Hammill states "Engines that do not have enough idle speed advance frequently 'spit back' while idling (backfire through the carburetor) and hesitate when initially accelerated, but then pick up and go."

 

Steve, did your mechanic set the timing with the vacuum advance plugged?

[stevo_gj]

Steve, did your mechanic set the timing with the vacuum advance plugged?

 

I think he did.

 

What can I do to check if this is the cause? Increase idle RPM? Increase advance without vacuum advance connected?

[Zedback]

Increase idle RPM?

That helped when we were messing around didn't it, probably worth trying as short term fix.  Hopefully someone with some more expertise will step in about now...  You could mess with the timing but you need a timing light, should have left you the one I had, I've got another.

[stevo_gj]

Yeah roger that I'll increase idle RPM and see if I can get a timing light from somewhere.

 

The other thing that might work as a short term fix is richening the mixture slightly, since engaging the choke somehow hides the problem.

[stevo_gj]

With both mixture screws richened by 1 full turn, and the idle RPM set from  ~700 to ~1000 the problem is significantly less noticeable. However when I'm doing hill starts it still spits and nearly stalls... It should be ok to drive to Sydney, but its just not as much fun as it used to be around town.

 

I wonder if Greg has a timing gun

[stevo_gj]

Car currently at Carb and Gas getting checked by Scott. He will see if he can diagnose what is causing the problem and let me know.

 

I noticed a red 240z with triple carbs in their workshop in very nice condition, does that belong to anyone on here?