18 replies to this topic

[Simples]

Model:998
Year:1986
Description of problem (please be as in depth as possible): Every time I set the carb up it goes out of tune within 48 hours

I'm currently running a 998 with stage 1 kit (Exhaust/K&N element/drilled airbox/AAU needle) and HS4, every time I set the carb up within 48 hours it starts running really lumpy, and won't pull away, it's as if the dashpot's empty (although it isn't!). If I remove the dashpot/piston and reset it again it's OK for a little while. The last time it happened I stripped it right down and cleaned everything with carb cleaner. I'm running electronic ignition and everything seems to be OK, tappets were checked about 500 miles ago. I have tried 3 in 1 oil, 20w 50 and a mixture in the dashpot without it making a difference. I can't hear/feel any exhaust blowing and there's no longer any excessive engine movement. Is it carb wear? Is a HIF 38 any more reliable?
Please help as I'm at my wits end with it!!! >

Thanks

Chris

[lawnmowercrap]

Its nothing silly like you're choke / waxstat is sticking is it? I had this on my girlfriends mini and every time I set it up, it would cool down and put it way out.

Also, you are setting your carb up when the engine is warm arent you?

Lee

Edited by lawnmowercrap, 24 August 2009 - 05:15 PM.

[Simples]

Its nothing silly like you're choke / waxstat is sticking is it? I had this on my girlfriends mini and every time I set it up, it would cool down and put it way out.

Also, you are setting your carb up when the engine is warm arent you?

Lee

Chokes definitely off and yes it's warm when I'm setting it up, didn't think HS4 had a waxstat? or am I being a simpleton??
Just took it out now and it's doing it again, put a smidge of 3 in 1 in it and richened it a bit and it's ok (for now!)
It seems to be worse if you manage to wind the revs up and then go back to pootling around. Also it's almost at the point where the little spring is off the nut!?!?! Should I try a heavier spring/ different needle? I don't want to throw money at it for no reason!!

[lawnmowercrap]

The girlfriends has a waxstat some do and some dont though, my mate bought the SU rebuild kit and it totally removed the waxstat.

Are you just adjusting the carb or are you playing with the ignition timing as well? Have you tried fitting a richer needle?

The girlfriends ran amazing when I put the richer needle in it but my HIF44 hated the richer needle so had to keep the standard one.

[Simples]

The girlfriends has a waxstat some do and some dont though, my mate bought the SU rebuild kit and it totally removed the waxstat.

Are you just adjusting the carb or are you playing with the ignition timing as well? Have you tried fitting a richer needle?

The girlfriends ran amazing when I put the richer needle in it but my HIF44 hated the richer needle so had to keep the standard one.

Just playing with the fuelling, timings about right. Haven't changed the needle. I thought the AAU was about right, but it might not be.

[Simples]

The girlfriends has a waxstat some do and some dont though, my mate bought the SU rebuild kit and it totally removed the waxstat.

Are you just adjusting the carb or are you playing with the ignition timing as well? Have you tried fitting a richer needle?

The girlfriends ran amazing when I put the richer needle in it but my HIF44 hated the richer needle so had to keep the standard one.

Just playing with the fuelling, timings about right. Haven't changed the needle. I thought the AAU was about right, but it might not be. How do I tell if it's a waxstat??

Edited by Simples, 24 August 2009 - 09:35 PM.

[Simples]

Right, it's not a waxstat and there's no play in the butterfly spindle (except a small amount side to side)
Anyone got any other ideas?

[mintom]

Its nothing silly like you're choke / waxstat is sticking is it? I had this on my girlfriends mini and every time I set it up, it would cool down and put it way out.

Also, you are setting your carb up when the engine is warm arent you?

Lee

Chokes definitely off and yes it's warm when I'm setting it up, didn't think HS4 had a waxstat? or am I being a simpleton??
Just took it out now and it's doing it again, put a smidge of 3 in 1 in it and richened it a bit and it's ok (for now!)
It seems to be worse if you manage to wind the revs up and then go back to pootling around. Also it's almost at the point where the little spring is off the nut!?!?! Should I try a heavier spring/ different needle? I don't want to throw money at it for no reason!!

Can I ask for a clarification about your statement of "the little spring being almost off the nut?"
Are you meaning the mixture screw on the jet? in my mind, with the carb in the car, this would be turning the screw clockwise, lowering the jet and richening it?

[jaydee]

The spring you need is the red type, try an AAA needle, AAU doesnt sound quite right for your spec.
Check the condition of the float needle jet and the carb jet and look for any play in the butterfly. Those parts usually wear down with the time, and have to be renewed using a rebuild kit.
When you reassebly the carb, take an SU tuning giude as a reference for the initial settings.
During the mixture set up process, its essential that you let engine run at about 2k rpm for about 30 secs beetween any tweakening, to burn all the fuel residues.

Exactly which parts of your carb have you stripped down?

[lrostoke]

Are you certain its not waxstat, obviously could have been changed but I would think 1986 should be waxstat as standard



Also I'd also agree with trying an AAA needle, your setup sounds same as mine, runs great with AAA

[Simples]

[Can I ask for a clarification about your statement of "the little spring being almost off the nut?"
Are you meaning the mixture screw on the jet? in my mind, with the carb in the car, this would be turning the screw clockwise, lowering the jet and richening it?

Yes I mean the mixture screw, thats where it seems happiest (sometimes!)
Whenever I reset the carb/take it to bits, etc I use the initial set-up guide and run it at a higher RPM for 30 seconds. It's definately not a waxstat.
I may invest in an AAA and a red spring then, I just don't understand how it runs sweetly for a few days then goes off.
Will check the butterfly next time it's apart. Are HIF's any better as I may keep my eyes open for a cheap one! Do they use the same needles?
Thanks for your help!

[jaydee]

The Hif carbs are better evolution of the old HS type, and yes they use the same needles.
In my opinion theres no need to swap a carb because it goes out of tune, and the HS4 can be fantastic carb if well tuned. The use of a fuel filter in line with the carb will be good to avoid this kind of troubles.
Setting carbs sometimes require a bit of time, especially if this is your first rebuild/overhaul: you do the settings, test drive the car, then adjust settings again.
As i said before, try an AAA needle and a new float needle jet, check the butterfly for any play.

[Simples]

The Hif carbs are better evolution of the old HS type, and yes they use the same needles.
In my opinion theres no need to swap a carb because it goes out of tune, and the HS4 can be fantastic carb if well tuned. The use of a fuel filter in line with the carb will be good to avoid this kind of troubles.
Setting carbs sometimes require a bit of time, especially if this is your first rebuild/overhaul: you do the settings, test drive the car, then adjust settings again.
As i said before, try an AAA needle and a new float needle jet, check the butterfly for any play.

Have an in-line filter already. It gets set up properly runs well for a little while and goes off song withing a few days normally. I expect to have to tweak it every oncein a while but at least once a week? I've got an AAA and spring on my list, and I am going to check the float needle/butterfly over the weekend over the weekend.
Thanks for your help

[Simples]

Update on this, float seemed ok. Took it all to bits no scoring or anything on the needle, cleaned it all up and put it back together, no it's throwing fuel out through the overflow rebuild time!! I think I'm going to buy a HIF and try it with an AAA and spring. Just to clarify I popped the fuel line off the pump and it chucked it out constantly, does this mean the pump diagphram (sp) is on it's way out? Or was it gravity pushing it through (it was on a slight slope)?
Any thoughts or comments appreciated!
Thanks

Edited by Simples, 07 September 2009 - 08:09 PM.

[bmcecosse]

Could well have been gravity if on a slope ? SU carbs don't go 'out of tune' - sure it's not sticking throttle cable ? Or sticking choke cable ?