Twin SU’s full throttle

[Wagon Sailor]

Originally Posted by packers1712

Good morning all,

I've finally managed to attach some photos of the butterfly positions when operated from the pedal compared to directly at the carburettors, photo 1 is pedal and photo 2 is direct.

Doug.

They look OK to me.

[packers1712]

Originally Posted by Wagon Sailor

They look OK to me.

Do you reckon they are open far enough, it just seems odd that the linkage can't open them fully when only the pedal is pressed, I would assume the linkage would be able to achieve the condition seen in photo 2?

Doug.

[Derek UK]

Work backwards from the carbs to the pedal to see where you are losing the movement. I suspect that it is the vertical link in front of the bulkhead. Check that the nylon grommet in the bulkhead bracket is good. It may even be missing. You say that you have some interference problems with the carpet etc under the pedal so cure that first. Originally the rubber mat, which is thin, didn't cause a problem but with that mat the pedal arm should still come close to the floor. Felt and carpet can raise that position can raise the arm a lot, maybe 1/2". It shouldn't compress the felt/carpet. The further the pedal is from the floor the more strain you put on the linkage as you accelerate. Bottom line, how often to do you floor it to the max?

[packers1712]

Hi Derek,

I have already done what you suggest, I'm conscious of not inducing any idle hang up by dialling out all of the free play but by the same token want to ensure I'm not loosing vital HP's if full throttle is never being achieved.
I have sorted the carpet underlay issue and now the pedal contacts the floor albeit through the carpet but that alone isn't thick so I'm confident it isn't an issue.
I haven't checked the nylon grommet in the bulkhead bracket yet but will this evening and as previously mentioned in the thread I'm beginning to think the vertical link isn't long enough, oh and yes unfortunately I am guilty of having a "heavy right foot"!

Doug.

[packers1712]

As promised here are some pictures of my linkage.
I checked to make sure that the bulkhead grommet was in good condition and it seems to be, there is no excessive play anywhere within the linkage that I can identify, however what I did spot after studying Alan's photo of his linkage was that the rest point of mine was quite a bit different to his so I've adjusted that and it seems to have improved the situation to a point that I'm happy its probably as good as it needs to be!
Its getting a run down to the Ace Café this evening for general classics night so I'll see if there is any noticeable driving experience difference!

Doug.

[Derek UK]

I would suspect that the link rod in picture #1 needs adjustment. I think it needs to be made longer going by my linkage. My link rod actually touches the cross rod but I believe it is more correct if there is a few thou clearance here. Mine is due to wear and there is a bit of slack in the system but works fine. Make sure that the upper lever moves the carb rod lever through 90º. Someone else on the throttle pedal would help.
Edit: Looking at your pic again I would think that the central part of the shaft with the lever on could be twisted round to make the lever end closer to the ground. i.e. the end would would go downward. Just loosen the clamps and twist a bit, then retighten the clamps. You would then need to readjust the link rod.

[packers1712]

Hi Derek,

Do you mean loosen the nuts circled in red and move the lever with the green arrow pointing to it towards the ground?

Doug.

[Derek UK]

Yes. You have to slacken these to enable you to balance the suck on the carbs individually. This is the first thing you do when tuning the carbs. Equal suck means the pistons both rise at the same time as the vacuum generated when you open the throttles is equal. Best to use a proper plug in vacuum gauge but "clever" tuners use a a length of small bore hose from ear to carb throat to get the same sound from each. Have you ever tried to tune your carbs to the book?

[Wagon Sailor]

Originally Posted by packers1712

Do you reckon they are open far enough, it just seems odd that the linkage can't open them fully when only the pedal is pressed, I would assume the linkage would be able to achieve the condition seen in photo 2?

Doug.

I've had Solex carbs do the same thing due to a burr on a butterfly.

If they open further by hand, ie: when not connected to the linkage, then the linkage is probably at fault. If not, it may be that they're a bit skew or catching somehow and the bore is preventing them rotating fully.

[packers1712]

Just a quick update, yes Derek I have previously set up my carburettors using the SU how to guide and am happy that they are performing close to if not on the money as Volvo had intended, the car drives nicely and returns acceptable MPG.
Having made the previously mentioned adjustments to the rest point and linkage there is a noticeable difference in throttle response but probably nothing else so if I was sacrificing HP's by not managing to open the throttle fully before then my feeling based on the drive to the Ace Café last night is that it was immeasurable!
I'm happy that its as good as it can be so will make no further adjustments until the driveability suffers if at all.

Thanks for everyone's comments/suggestions and help they were invaluable and to echo Alan's comments from other threads within the forum what a valuable tool this place is for maintaining these sometimes fickle old motor vehicles!

Doug.