Sprint - carb misbehaviour

[payload]

Hi all,

Hopefully someone can help me with pointers of where I might be going wrong with my carburettors.

The car has been layed up for a long time and this summer I have been working towards getting her back up and running.

The symptoms are, when I start the car with choke there is an immediate super scary high rpm (I'm not touching the throttle at all) and can only be brought down by pushing the choke in but then the car will stop, as it won't idle with the choke pushed in. I notice that the choke linkage is not getting anywhere near the fast idle screw in any part of its travel so I am sure it isn't a throttle issue.

The things I have done prior to this are the following: stripped both carbs almost completely but did not touch butterfly or spindles. Replaced float chamber cut out valves, jet assembly using SU kit from Andrew Turner. Kept the original needles, they looked ok to me. Set the jets to the initial 12 flats from fully wound up. Also have fitted alloy mounts replacing rubber. My limited knowledge suggests a jet issue but I just can't work out.
The only non-standard thing I did was used some oil I normally use for lawnmower instead of damper oil in carb..
.that wouldn't be the cause would it?


Can anybody suggest where I could have gone wrong or how to start diagnosing?

[dollyman]

Get someone to operate the choke from inside while you are looking at the carb linkage. You will then see the choke opperating a snail cam which can be adjusted on the bolt and locknut. It has probably been set to high.

Tony.

[payload]

Thanks Tony, If I understand you correctly, I have already tried that. The cam doesn't go anywhere near the fast idle screw. It is pure mixture setting that is causing the problem I think, throttle not involved.


Right now I think I am going to pull the carbs out again and double check needles and jets...and put some 20w50 on the dashpots rather than lawnmower oil....!

[GrahamFountain]

Have you taken the dashpots off and pistons out, and looked to see if the butterflies are closed or if some strange crosslinking effect moves them when the choke is pulled? If the car revs lots with the butterflies closed, that would be odd, wherever the jets are set.

I try very hard to keep the pistons and dashpots as mated pairs. So I've not seen problems if they get swapped. But I was once told the effects can be odd.

Do you have butterflies with overrun poppet valves? Is either of them stuck open?

Will it idle with no choke if you screw the jets down further and screw the idle screws in?

Graham

[payload]

Thanks Graham. Pretty sure the butterflies are closed when no throttle applied, but then logically as you say air must be getting in from somewhere if they are to cause that kind of rpm. I will check tomorrow and report back.

Thanks so much to all for your help

[Carledo]

If it's been stood a long time, are the rubber breather hoses intact and not split? Are they even still present? Damaged or missing breather pipes will give you exactly these symptoms.

Steve

[xvivalve]

There are two 7/16" AF bolts which hold the throttle bracket onto the inlet manifold; slacken these and you'll find there is slight adjustment in the alignment of the bracket. If it is not aligned properly, you'll find it can bizarrely hold one of the butterflies slightly open!

[payload]

So, I checked the butterflies, they look ok, not stuck, and they move as expected. No fast idle actuation with full choke, so I think throttle issues are out of the reckoning.

I replaced the two carb breather hoses and also changed out the cam cover one also.

Tried to start again and exactly same issue.

Based on the advice here I guess the big question is where is the air coming from to allow the engine to rev that much with no throttle applied and the butterflies pretty much closed?

To rule out the breather hoses could I block off the carb breathers what should happen?

Another possibility maybe air is coming in post the butterflies via the new alloy mounts? I did not use gaskets here as I thought the rubber o-rings meant no gaskets required on either the inlet manifold or the carb face side?

Any other ideas on what I should look at/check?

[xvivalve]

and the butterflies pretty much closed

Do you realise how small a gap is required to cause the revs to rise?

It will take you less time than it took me to type it, to test what I described: I have lived this very problem.

[payload]

Thank Alun, will try this. Presumably I need to have both idle adjustments on no gap when I try your suggestion?

[payload]

Thank Alun, will try this. Presumably I need to have both idle adjustments on no gap when I try your suggestion?

[payload]

Two words; INCREDIBLE FIX!

Alun you were right it is amazing how little a displacement of that bracket can cause such an over rev. I slackened them off as you suggested and saw the linkage move so very slightly. But fired her up and she runs so sweet now.

Thanks again to everyone for tips and especially Alun for that bang on tip. Maybe we should add this to the tech tips collection as I'm sure other people could be caught out to in this way?

Is it ok to leave the bracket nuts backed off in normal use, as I noticed that when I tighten them the revs start to rise? Or is there a more permanent solution such as bending bracket or replacing it?

Ian

[Carledo]

Two words; INCREDIBLE FIX!

Alun you were right it is amazing how little a displacement of that bracket can cause such an over rev. I slackened them off as you suggested and saw the linkage move so very slightly. But fired her up and she runs so sweet now.

Thanks again to everyone for tips and especially Alun for that bang on tip. Maybe we should add this to the tech tips collection as I'm sure other people could be caught out to in this way?

Is it ok to leave the bracket nuts backed off in normal use, as I noticed that when I tighten them the revs start to rise? Or is there a more permanent solution such as bending bracket or replacing it?

Ian

Not a good idea to leave the bolts loose, sooner or later they will fall out! It's possible that there is just a tad too much tension on the throttle cable, back off the adjustment a bit and try tightening the bolts again.

Steve

[xvivalve]

No need to leave them slackened, just make sure when tightening them they don't cause the bracket to rotate.

[GrahamFountain]

As well as making sure the cable isn't tight with foot off pedal, have you looked at the two arms that clamp on the throttle linkage spindle. These are the arms that actually operate the butterfly spindles through a pair of forked arms attached to the butterfly spindles of the carbs.

With the throttle cable slack and the idle screws fully backed off, the square pin at the end of the arm wants to be ever so slightly slack in the fork that actually operates the butterfly. Otherwise the butterflies can't ever close properly. Ideally, each should be close to but not pressing on the tine that opens the butterfly.

I know this is not yet your problem, but you want to set those arms so the cable operates both butterflies together. I find that's a bit tricky to get exactly right, as the arms have a habit of moving on the linkage spindle when you try to tighten them. But it is an issue of carb set up that's worth spending much more time on than balancing the carbs with the idle screws, which some go on about for hours. Setting the butterflies to be open the same maximizes power when you're driving. Whereas setting the idle screws really only affects idle.

Graham