Carburettor troubles

[gs.davies]

I’m struggling to get my car idling nicely so that I’ve got enough confidence to get it to a rolling road.

Last night I pulled the carbs apart again and whilst the needles are both GZ’s, I’ve got one longer than the other, I’ve got springs at different lengths and I found that the pistons weren’t sliding as smoothly as some others in the parts pile.

I’ve cleaned the pistons up again and suspect they may have been swapped over, as swapping them from one suction chamber to the next has left them both moving freely.

i don’t have any flooding issues and float heights seem ok and consistent from side to side.

I now need to start fresh with needles and springs and I don’t have time to experiment with endless combinations.

I’ve had various recommendation for needles and springs and I’m interested to know what other folks who have a similar engine spec to mine have used successfully.

It’s a 1098CC, 9.3:1 CR, 295 of standard sizes, Cooper 997 profile camshaft, Maniflow Freeflow, Maniflow 1.75” twin box, twin HS2 with standard air box and filters.

So far I’ve heard;

M - Blue
H6 - Blue
D6 - Red

I want it to idle properly and run ok on part throttle at up to 50 mph so I can get to a rolling road where they can sort it.

Help!

[timmy201]

Most of the usual needles have essentially the same profile at the idle end, so getting the idle correct is almost independent of the needle you pick

My 1098 is a higher spec than yours, I started at M, then AH2 then ended up with HV3 needles and blue springs. I was using an O2 sensor for checking the mixture under load

[gs.davies]

Thanks Timmy, I'd forgot your suggestions. I presume those needle combinations are just progressively richer and richer?

I'm leaning towards M, if only on the basis it's available now (!)

[timmy201]

Yes, they got richer as I went along, particularly at the low load end

[Oneball]

If you’re struggling with idle. Have you checked timing? Have you checked for vacuum leaks, PCV, manifold gasket, servo etc?

[gs.davies]

If you’re struggling with idle. Have you checked timing? Have you checked for vacuum leaks, PCV, manifold gasket, servo etc?

Yes, I've been over timing several times, no leaks (there's no PCV, servo)

Tonight, I'm having the jet bearings out - I've got new ones to go with new needles and springs, will recentre the jets, set carbs up again from scratch, have been all through the suction chamber and pistons, cleaned them, found that perhaps they've been mismatched..

I'll see what it's like when the new needles arrive.

Currently a list of over 60 jobs to do before Saturday morning's rolling road session..

[Oneball]

If you’ve had them on and off is the manifold gasket ok?

[Dan Tweed grey]

Check the inlet manifold is not fouling on the exhaust manifold and not allowing the seal onto the gasket.
Also are yopu using locating rings?

[gs.davies]

No, inlet isn’t fouling and seals well. No locating rings as the manifold doesn’t have the groove for them.

[floormanager]

Probably isnt it but as it happened to me, I'll mention it. The core plug on a twin carb manifold fell out causing it seriously lean and would not idle. You can always check for air leaks using a bit of brake cleaner. If the engine speed increases, then you've got a leak.

[BAD942B]

problem I had a couple of days before a sprint at Goodwood, car started running poorly, checked everything including brake fluid level. it was getting worse s i pulled the servo hose off to start taking the inlet manifold off & there was just a slight trace of damp in the hose, it was brake fluid, checked the level in the reservoir & it was just slightly lower.
The servo had failed so I re-kitted it.
At Goodwood I won the class but blew the head gasket between 3 & 4 cylinders coming out of the chicane, even through the crash helmet I could hear the engine pinking.
only time it came back on the RAC truck. I replaced the head gasket the following week.
I think this eventually caused the piston failure on no.4 cylinder that made me take it off the road

[gs.davies]

Probably isnt it but as it happened to me, I'll mention it. The core plug on a twin carb manifold fell out causing it seriously lean and would not idle. You can always check for air leaks using a bit of brake cleaner. If the engine speed increases, then you've got a leak.

This is a good shout. I’ll check it in the morning.