SU Carb / L28 help

Post by **Jabo** » Wed Jan 04, 2012 8:41 pm

So here's the situation, After finally sorting a series of smaller problems I finally thought I had my Su's running well on my Z with flows...... but it would appear my joy was short lived, cylinders 3 & 4 run rich but 1,2,5 & 6 are perfect, I can't for the life of me find why as they are on separate carb's. I cant find any vacuum leaks its had brand new plugs, the carbs a balanced and I didn't have this problem before I put the carbs and flows on.

If anyone has any Idea what the cause could be please help im pulling my hair out and have run out of ideas

Cheers Jay

85_Z31 · Post by **85_Z31** » Wed Jan 04, 2012 9:51 pm

Do a compression test maybe ?

I have no idea, sounds like you have covered most things it could be.

Post by **Bartman** » Wed Jan 04, 2012 10:40 pm

How balanced are they?
Throttle butterflys synchronised?
Dashpot return springs all equal lenths?
Linkages all in perect sync?
I'm no SU expert. I got a mates dad to tickle mine up a while back. And it sang.

Post by **160jsss** » Thu Jan 05, 2012 7:44 am

Bartman wrote: Re: SU Carb / L28 help Reply with quote

is that the guy buy the park buy Lincoln rd that likes playing with minis he made my mates brick run sweet

kk-kolonel · Post by **kk-kolonel** » Thu Jan 05, 2012 8:55 am

Jabo wrote:So here's the situation, After finally sorting a series of smaller problems I finally thought I had my Su's running well on my Z with flows...... but it would appear my joy was short lived, cylinders 3 & 4 run rich but 1,2,5 & 6 are perfect, I can't for the life of me find why as they are on separate carb's. I cant find any vacuum leaks its had brand new plugs, the carbs a balanced and I didn't have this problem before I put the carbs and flows on.

If anyone has any Idea what the cause could be please help im pulling my hair out and have run out of ideas

Cheers Jay

Couple questions to help diagnose.

Have you got pyros on each cylinder or are you just looking at the plugs?

I'm guessing you are just looking at the plugs so is it fouling the plugs up and running like shit?
Have you done a "proper" compression check?
You say you put the carbs on? Did you go from a single carb?

I had a problem with my old one fouling 1 and 6 (mainly 6) and I fixed it by running an MSD ignition and being careful with the amount of choke I ran on startup.

When you say balanced - how have you balanced them?

Post by **Jabo** » Thu Jan 05, 2012 11:36 am

Hey cheers for the quick responses guys.

kk-kolonel Yes I am just checking the plugs and I have gone from one carb to 2, a copression check is next on my list, also I was thinking of investing in a msd ignition coil and I balanced them using one of those carb balancing tools with the float on the side that measures the draw on each carb.

I hope that helps you help me Cheers Jay

Post by **Jabo** » Thu Jan 05, 2012 11:37 am

Oh and also its slightly worse on No. 3 than No. 4

kk-kolonel · Post by **kk-kolonel** » Thu Jan 05, 2012 2:22 pm

OK for a correct and accurate compression test you will need to follow this instruction I wrote a while back for Old School..
If you just jam a compression tester in the hole and wind it over then write down the compression then you are doing yourself a dis-service..

My technique for compression test is this.

1 - Get engine up to operating temp
2 - Remove HT leads and crack off spark plugs a quarter turn each, reattach HT leads in the correct order.
3 - Run engine for 30 seconds at around 2-3000rpm (this gets rid/burns off any carbon that may have been in the spark plugs threads and eliminates carbon chunks getting jammed under the valve seat when you are performing compression test)
4 - Remove the spark plugs
5 - Disconnect the distributor to eliminate the spark
6 - Jam the throttle wide open (best to have someone sit in the car with their foot held down) make sure the throttle stays WOT until the entire test is complete (opening and closing the throttle between compression checks causes bore wash and can alter the compression from first to last)
7 - Start on number 1 and thread the compression tester in till it is firmly seated
8 - Crank Engine over so the engine turns over 20 times (10 compression strokes)
9 - Write down the result from the gauge (generally between 100-180psi) for each cylinder
10 - Back to stage 7 but move to next cylinder and continue
11 - Compressions shouldn't alter more than 10% between worst and best (up to 20% at most)

Once you have compressions report back and we can take a look at whats going on.

As a theory for right now - you say number 3 and 4 sparkplugs are your problem but as they are running on different carbs as such then I doubt carb balance is your issue. If the first 3 cylinders or last 3 cylinders were running rich then yes it could point to carb balance being a problem.
My best assumption is that because the carb balance tube runs between cylinders 3 and 4 (I am assuming you are running a 240/280Z factory twin carb manifold) that the mixture is richer at that point in the manifold and the firing order is 153624 so the engine turns an entire 360 degrees between cylinders 3 and 4 firing.

As I said before I fitted an MSD and my L26 just stopped fouling plugs. I was changing plugs every 500-1000Km or so before I fitted the MSD and after I fitted it I didn't change any and the guy who owns it hasn't had to change the plugs ever.

I'm not saying its going to fix YOUR problem but it did fix mine..

Another thing are you running Round Tops or the Flat Tops carbs?

Hitachi Flat Tops

Hitachi Round Tops

Post by **Jabo** » Thu Jan 05, 2012 2:38 pm

thanks yea I'll get onto the compression test as soon as I can get my compression tester back the guy I lent it to is on holiday, I'm running round tops and your therory about the spark had crossed my mind but I wasn't sure if it was a possibility

cheers for your help

Post by **Jabo** » Thu Jan 05, 2012 2:41 pm

heres a pic of my setup just incast you can spot a instant error I've made except the flows are now heat wraped