9 replies to this topic

[twign]

Hi.

I've recently had my 1380cc mini tuned up... and I'm VERY happy with it. However, the only niggle I now have is that I get a slight hesitation on changing gear. If I rev it all the way round the clock in 3rd, change into 4th, and then boot it... it throws you forwards slightly before it puts you back in the seat, and it's quite noticeable.

It's a 1380, kent 286 fast road cam, single HiF44 carb, approx 90bhp at the wheels.

I have a yellow spring in currently (provided by Avonbar), which I recently purchased along with some needles. We're not sure what spring was in it previously as there was no colour marking.

Now, am I right in thinking there are 3 different springs - red, green and yellow? And could someone explain the differences?

The guy who did the tune-up has told me to put the old spring back in the carb to see if this makes a difference. If I'm right, I think what was in the carb previously was a weaker spring than the yellow spring we've fitted. What effect would this have?

Also, would different thickness of oil in the dashpot have a similar effect as fitting different strength springs? I currently have 20w50 in the dashpot. Could this be anything to do with it? The tune-up guy says the mixture is spot on now, so it must be something trivial (hopefully).

Any help, as always, is much appreciated.

Thanks

Twign

[Kerrin]

The advice from winsu for my 1380 was to fit a yellow spring.

But after posting on ML Motorsport's website AC Dodd gave me a guess at a needle profile and also said to use a standard red spring. Once I had reprofiled the needle and changed the spring back over the car went really well.

It's worth a try, and if it works another RR visit to check.

I use 3 in 1 oil in my dashpot. As it is on the shelf and seems fine.

Edited by Kerrin, 20 January 2010 - 06:13 PM.

[bmcecosse]

Yes 3 in 1 usually works best, the 20w50 may be holding the piston down for half a second - hence the hesitation. The spring is there to make sure the piston doesn't go fully open until the air flow is near maximum (ie full revs at full throttle) . Too heavy spring -it may never lift right up (so like having a smaller carb!) - and too light a spring means the piston lifts too quickly and once it reaches the top the control of the mixture is lost from the needle profile, although it will still meter out depending on the air speed through the venturi. So - you want a light spring - but not too light! I would start with whatever spring is the lightest - and if that's no good - move on to the next one. What needle is in the carb ?

[twign]

Yes 3 in 1 usually works best, the 20w50 may be holding the piston down for half a second - hence the hesitation. The spring is there to make sure the piston doesn't go fully open until the air flow is near maximum (ie full revs at full throttle) . Too heavy spring -it may never lift right up (so like having a smaller carb!) - and too light a spring means the piston lifts too quickly and once it reaches the top the control of the mixture is lost from the needle profile, although it will still meter out depending on the air speed through the venturi. So - you want a light spring - but not too light! I would start with whatever spring is the lightest - and if that's no good - move on to the next one. What needle is in the carb ?

It's got a BCE needle in it, which has been slightly reprofiled. It drives REALLY well, the best it has done, it's just this slight hesitation (I'm thinking of my 1/4 of a mile times here! lol).

Previously it had a BCA needle (the only one available at the time so I had to make do) and it was spot on on cam and no hesitation on gear changes, but it was really boggy pre-cam, making it VERY tricky to drive. Horrible. (This was when I had my high-lift rockers on. I've since removed them)

Prior to this, it had a BCP in it that had been over-machined, and it was massivly down on power AND had the hesitation on gear change.

Hmmm... well the spare spring this guy has recommended me trying is stronger than the one I have in it at the mo (we sat the dashpot on it and observed how far they sank). The one in it currently is the one we're unsure of the "colour" of, and the once I have spare is a yellow ordered from avonbar - the tougher/stronger one of the two.

Maybe changing the dashpot oil is worth a try then for what it would cost... just to see what effect it has.

Thanks for the help

Twign

Edited by twign, 20 January 2010 - 08:24 PM.

[bmcecosse]

Thin oil - weak spring - and maybe try a BDL needle.

[twign]

Thin oil - weak spring - and maybe try a BDL needle.

I've got a new BDL needle that I ordered along with the BCE, but the guy who did the tune up got his book out and said that, I can't remember exactly, it would either be too lean or too rich. one or the other lol.

I'll try the oil first and see how I get on with that, and then pop this stronger spring in just to satisfy my curiousity, and then it'll be back in for a tweak more than likely!

Thanks

Twign

[Kerrin]

The last off the shelf needle I had fitted was a BBW and it went pretty well, but seemed slightly too rich.

The one in at the moment Started off as a BDL and has been carefully modified by sanding with wet and dry in a pillar drill, and checked with a micrometer. I had to sand it down along its whole length, suggesting it is likely to be too lean as standard

My motor is a 1380 but has a SW5 and 1.5 Roller Rockers and is fitted with a ITG stub stack filter.

My needle spec is on the MLforum if you want a go at it. But if your fueling has been checked and is right the spring or oil may be the solution.

Also I think the Yellow is a stronger spring so swap it back to the red and see how you go, I am not sure what spec of engine needs a stronger spring but it appears to be the higher tuned or cammed motors.

Edited by Kerrin, 21 January 2010 - 10:02 PM.

[twign]

The last off the shelf needle I had fitted was a BBW and it went pretty well, but seemed slightly too rich.

The one in at the moment Started off as a BDL and has been carefully modified by sanding with wet and dry in a pillar drill, and checked with a micrometer. I had to sand it down along its whole length, suggesting it is likely to be too lean as standard

My motor is a 1380 but has a SW5 and 1.5 Roller Rockers and is fitted with a ITG stub stack filter.

My needle spec is on the MLforum if you want a go at it. But if your fueling has been checked and is right the spring or oil may be the solution.

Also I think the Yellow is a stronger spring so swap it back to the red and see how you go, I am not sure what spec of engine needs a stronger spring but it appears to be the higher tuned or cammed motors.

Hi, thanks for the replies.

It's the weaker (unknown) spring that it has in it at the minute. The stronger yellow spring is the spring that the guy who did the tune up is suggesting I try. I suppose if the oil were too thick (as bmcecosse has suggested) then a slightly stronger spring may compensate for this? I'm going to give it a try tomorrow anyway so I'll let you know how I get on.

Thanks again

Twign

[gazza01]

I had a similar issue a couple of years ago. Saw a thread somewhere which said to sand down the damper stick end.

By this I mean one part of it slides a little and the other is static. Sand down the static bit so its smaller and wont have any effect any longer. Worked a treat.

[ACDodd]

Your problem is not related to oil or carb spring or needle.
Modify the damper rod as mentioned in gazza1's post or Fit an LZX 2085 piston damper rod. The flat spot between gear changes will be gone.

AC

Edited by ACDodd, 07 December 2013 - 09:34 AM.