20 replies to this topic

[Tornado99]

Just a quick update, due to weather I’ve not been able to take the car for a spin.

I have done the drop test and it takes 3.5-4 seconds ish which is less that the SU website quotes so I tried the HIF off my other mini and it’s about the same.

I have got a yellow spring to try though which may help if the piston is lifting too quickly. The yellow is 8oz compared to the standard 4.5oz red.

Interesting both different carbs give similar drop time despite what guide states. Perhaps guide is incorrect? How does it compare to those times in the two videos I provided?
If you put a few drops of engine oil on the piston sealing area, does the drop test change significantly (it should)?

I have not heard of any fixes for poor drop test times other than replacing the dashpot and piston as a matched pair.

[mkw]

Interestingly both videos have similar times to me, although the SU site quotes 5-7 seconds for a 1.5 inch carb.

 

I've had a look through Vizards yellow bible and he quotes 3-5 seconds so i'm in that window which i'm taking to mean my carb is ok and it's a misquote on the SU site.

[mkw]

Latest update - running great.

I swapped the spring to yellow, still ran a bit off so played with the fuel mix. Got it running better but still not perfect.

I changed the damper to a LZX1511 I had knocking about and it’s now brilliant! No hesitation, no bogging down. Up hill in 4th under load and no pinking so I’m very happy. I may try the red spring again with the different damper just to see what happens as I’m not up on what the damper differences are.

[Chris1275gt]

Probably a bit late but on a SU carburettor diagnostic chart it recommends when hesitation on acceleration the oil in the dashpot is to thin put thicker oil in it. I noticed you've only tried thinner oil.

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[mkw]

Thanks, I’ll keep a copy of that as it looks useful.

I’ve tried a few oils, anything from 3 in 1 to gear oil plus various engine oils. I’ve heard that proper su oil is sometimes a little thin too, although that doesn’t seem right.

[Chris1275gt]

Thanks, I’ll keep a copy of that as it looks useful.

I’ve tried a few oils, anything from 3 in 1 to gear oil plus various engine oils. I’ve heard that proper su oil is sometimes a little thin too, although that doesn’t seem right.

I thought it doesn't seem right but it worked on my refurbished hs4, it stops the piston rising too quickly when you put your foot down.
Su carburettors tuning tips and techniques by Brookland books is a very useful guide, eBay has numerous copies for sale.