10 replies to this topic

[shutdown98]

Hi there,

 

Another weird one on my mini. Since I bought the car a few months ago, I have been doing some suspension work and solving some issues. I am currently trying to sort out the mixture on the carb, I always had the feeling of low power on higher rpm and thought that it should go up above 3000rpm more easily. 

 

It is a 1275 with single HS4 (fixed needle), RC40 exhaust,  maniflow freeflow, minispares inlet manifold and cone filter

 

I took the car to a local shop that works on classic minis and did a rolling road test using a O2 sensor to check the mixture. Turns out the car was running too rich on idle/low rpms and lean under load. Interestingly, if I floor it, the mixture stays nice for a second and the starts to drop.. Also, if we open a little choke while accelerating it fixes it.

 

What we have tried/checked:

- Timing checked using a strobe light

- No apparent inlet leak, all gaskets were renewed.

- Changed to a richer needle (fixed needle - BQ) and tried to lower mixture on the carb (because of rich idle) --> This improved a little bit but not much..

- Correct SU oil on the damper 

- Also tried a new damper spring 

 

Anyone has suggestions on what to check next? 

Edited by shutdown98, 18 May 2022 - 01:24 PM.

[cal844]

Try a heavier oil.

Correct SU oil isn't what that carb came from the factory with, it should have had 20w50 engine oil but what I suggest is to try a 10w40 then the 20w50 oil (you'll only need a cap full at most)

It's worth a try, for 20 minutes running on the dyno and changing the oil in the dashpot between runs. (you'll know if its richer or leaner under load with the different oils)

Hope this helps

Cal

Edited by cal844, 18 May 2022 - 02:23 PM.

[Chris1275gt]

Change the dash pot oil to engine oil 20w 50 and have a look at this chart.

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[sonscar]

I understood that the idle stations on most needles were the same?Opening the throttle starting OK and leaning off semms to indicate the dashpot and oil is doing its job,slowing the piston rise.If it is still too weak a different or reprofiled needle is needed.Steve..

[cal844]

I understood that the idle stations on most needles were the same?Opening the throttle starting OK and leaning off semms to indicate the dashpot and oil is doing its job,slowing the piston rise.If it is still too weak a different or reprofiled needle is needed.Steve..

Oil thickness does make a difference, I believe that different needles have different fuelling, whether idle or under load, knowing the difference is what matters... (happy to be corrected though!)

You are correct that lean or rich means another needle is needed.

Edited by cal844, 18 May 2022 - 07:36 PM.

[ACDodd]

The problem here is it needs a tune up, the needle profile is clearly incorrect for what the engine requires.

Ac

[shutdown98]

 

I understood that the idle stations on most needles were the same?Opening the throttle starting OK and leaning off semms to indicate the dashpot and oil is doing its job,slowing the piston rise.If it is still too weak a different or reprofiled needle is needed.Steve..

Oil thickness does make a difference, I believe that different needles have different fuelling, whether idle or under load, knowing the difference is what matters... (happy to be corrected though!)

You are correct that lean or rich means another needle is needed.

 

From what I understand, needles have different thicknesses across all course of the throttle. That is why we went with the richer needle thinking that it would solve the issue while accelerating. As for the oil, I was told not to use engine oil since it varies its viscosity with temperature, I believe the SU "correct" oil is mono-graded. 

 

Here is the chart that i followed.

 

Edited by shutdown98, 18 May 2022 - 10:46 PM.

[shutdown98]

The problem here is it needs a tune up, the needle profile is clearly incorrect for what the engine requires.

Ac

That is what we are attempting to do, we changed the needle from the STD one to a the richer one, but barely noticed any difference. 

[sonscar]

It is a bit more scientific than changing needles,I believe they are tapered every one sixteenth of an inch( stations?) and it is getting the right station at the jet at the right piston lift,correct spring.The damper is almost entirely there for acceleration enrichment though it also damps any tendency to fluttering.Monograde oils alter their viscosity with temperature too.throw into this that the timing also needs adjusting to fire off the mixture at the right time to fully exploit the flame spread...and it is a miracle that it works at all.Steve..

[shutdown98]

Bump up on this thread, I am still after this issue but the car has been parked for a while now. I did a little more digging and road tests and it backfires through the carburetor now and then (not every time) when going uphill in fourth gear. It is a huge bang, and sometimes it even pops the dashpot oil cap off, and I have to screw it in again. I also notice that it starts to go down in power just before it pops. However, it idles very well

Head gasket has been changed since then, valve clearances checked also condenser and points.

What do you guys think? Could this be timing related? Fuel starvation under load (maybe fuel pump)? (I actually think it still might be running a little lean under open throttle, but I don’t have a AFR meter to verify that).

I really appreciate your help.

[ACDodd]

The problem is that you are not monitoring what is going on while driving, a wideband gauge install will let you understand how the fueling is when driving. Secondly using a chart to determine what needle is required is also very misleading. The trouble is due to wear and production tolerances you may need a significantly different profile to fuel your engine properly. This will mean measurement and only then suitable correction. There is also a school of thought that thinks you can use computer programs to determine profiles for your engine, that simply is not the case, engine of the same specs and carbs don’t usually need the same needle profiles, due to wear and tolerances in the carbs and engine. If you want to correct your fueling you need to measure where you are now and and then correct this by change the spring/needle/oil combination to shape the fueling curve as YOUR engine requires. Any other method is just guessing.
This is why I offer training as it’s not as simple as people on forums try and promote.

As regards timing, this is fairly straightforward, measure the advance curve of what you have now and post back the results here.

Ac

Edited by ACDodd, 29 September 2022 - 01:12 PM.