Amal carbs on Speedwell 7 port 1098 engine - sticking throttle.

[L7MAD]

Sorry posted in wrongfolder first!

Not a mini but an A series engine. After a few years I have finally got my Speedwell 7 port cylinder head - brought through this Forum - on an engine and up and running in my Lotus Seven Series 1.

20200719_110205.jpg

Thanks to Rod and Simon at Southam for the engine build and after an initial rolling road setup by Peter Baldwin and John Lee at LynxAE the issue I'm having is the throttles are sticking open slightly so after driving round and roling off the throttle slowly the tick over is up to 2000 rpm, if you blip and snap shut the throtle it will sometimes return to 800 - 1000 rpm. There is no sticktion in the linkage or throttle cable - but it appears to be increased resistance on the air slides in the carb bodies when the engine is running caused by the induction suction. Before the rolling road I changed the Amal internal throttle springs to there "light weight" ones as the throttle pedal was excessively heavy, these are alot longer then the originals so we cut them down thinking they could be binding, but no better. We then put the original springs back in and it appeared slightly better but pedal weight was too heavy - Peter and John had never come across this issue before. After the rolling road I spoke to Burlen and they had also never heard of this issue. After several months I have re-engineered the throttle setup to go to pull/push rods on the air slides with external return springs and throttle stops in various forms/posistions/combinations/weights but all to no avail, the issue remains and I have only managed just over 100 miles in the car. Sorry for the long post but has anyone running Amals on an 8 port engine come across a similar issue and if so what is the cure or is in something that will go away once all is bedded in?

The engine is a 1098 Midget bored +40 so about 1130cc, 10:1 compression, SW5 cam with 1.5:1 rockers and the carbs are 4 new Amal 932 Mk1 Premium Concentrics on a fabricated steel manifold without link pipes.

I have considered chamfering the bottom edge of the air slides but not tried it yet? Would link pipes between each inlet tract help? and if so what sort of diameter? (I believe they help improve tickover) or could the carb size be causing the issue (is 32mm too big for the engine cc/spec?)

Any thoughts/suggestions greatly received as it is seriously effecting the drivablity and there for the fun factor of the car.

Best regards Paul.

[L7MAD]

20200719_110205.jpg

Photo which should be in above post.

Regards Paul.

[Gary Schulz]

Not entirely familiar with Amal carbs but in looking at your picture, it would seem that the linkage connected to the slides will of course follow an arc and as you raise the slides the linkage will start to push outward on the piece that is connected to the slides. I would think that would naturally cause some binding. Is the linkage just pulling on a cable that is attached to the slides? Maybe if the cable were a bit longer with the arms raised up higher the resultant angles wouldn't be so severe.

I know space is limited on a Seven though...

(It is a beautiful thing to look at!)

[L7MAD]

Hi Gary,

Thanks for your reply and yes you are correct that the arms move in an arc, but to conteract this they are attached, not by cable, but a rod that is anchored in the air slide with model helicopter rose joints to allow for angle change so there is no binding at this point. And yes space under the bonnet is extremely limited...

(It is a beautiful thing to look at!)

- Thanks, but they are normally so much better to drive

Paul.

[Oneball]

What have you done with the standard throttle stops, were you using them initially and still having the problem?

What have you done with the choke?

Have you tried it with no linkage?

Have you tried any different cutout slides? Did Burlen say anything about this? Could it be that at the idle air volume you’re asking for you’re right on the transition between the pilot and needle jets.

[L7MAD]

What have you done with the standard throttle stops, were you using them initially and still having the problem?

It had the individual throttle stops in on the rolling road but I've since removed/blanked them and the issue was there before and after.

What have you done with the choke?

I have left provisions to install them if cold starting is an issue but at the moment they are left out.

Have you tried it with no linkage?

If I disconnect the return springs and the cable without the engine running and open the throttle, on letting go the linkage drops to the throttle stops and all carbs close, with the engine running it sticks minutely open with higher revs than tickover and if you push each airslide down with your thumb the revs drop back down - this seems to be on different carb each time I try, so not the same carb sticking each time.

Have you tried any different cutout slides? Did Burlen say anything about this? Could it be that at the idle air volume you’re asking for you’re right on the transition between the pilot and needle jets.

The carbs came from Burlen with their recomended No.3 cutouts, and they didn't suggest this as a possible reason for the issue also neither did Peter Baldwin. The intension is to get some miles on the engine and then have another rolling road session to check further setting. This is my first experiance with Amals and thanks for this suggestion as I had not considered this as a possible reason, how do I know which way to go with the slides, No.2 1/2 or No.3 1/2, as I have not taken it over 3500 rpm yet and other than the tickover issue it seems fairly drivable at the moment?

Cheers Paul.

[Oneball]

If it’s actually sticking it’s not going to be related to the cutouts as I’m sure they just affect the mixture.

[Oneball]

How have you attached the carbs to the manifold?

Did Burlen say anything about carb warpping and over tightening to you?

Could it be sticking on the needle? Maybe try putting the clip on the bottom groove to raise it and see whether it sticks.

What have you done with the air valve spring and slide?

[L7MAD]

I have read about issues with over tightening and warpping (from Burlen website and others) and to be honest I thought this could have been the issue during the rolling road (I read about this after) because I could not get the tickover below 1800 before the rolling road and thought there maybe air leak issues so made sure everything was tight! since it has all been apart a few times and is not as tight now. There are phenolic spacers Hylomared onto the steel inlet manifold face and then the original O ring between these spacers and the carb body - held on by K nuts. I will check again for movement and I have considered using thackery washers as on Webers but will need to get longer studs.

If it was needles I would not expect this to be effected by the engine running but I can try this - they are on the middle grove at the moment.

This photo shows how I have mounted the pull/push rods in the air slides and the internal springs have been completely removed (this mod was done after the rolling road session)

20200606_1306071.jpg

And the tops of the needles have been shortened to clear the rose joints.

Cheers Paul.

[Oneball]

If you can take a carb off put it in the freezer for a couple of hours and see if the slide is still free. Try heating one up and see if it’s still free.

Have you drilled the slides to attach those linkages?

My experience is with old bikes and the usual permanent fix is to go buy a new one but that’s not much help here!

[mk1]

Hi L7MAD,

First, please let me say what a joy it is to see the 7 port that I found so many years ago, finally on a Working engine! It's been a long time coming, but goodness me! It is worth the wait. Very well done Sir!

Now, onto the problem in hand. Problems like this can be immensely frustrating & the trade off between an acceptable pedal pressure & the slides working as expected can be a very fine one. It has to be said that Amals are NOT the most technically marvellous bits of equipment, but at least the new ones are no worse (and possibly a bit better) than the old ones.

One possible solution would be to alter the lever ratios of the pedal to enable you to go back to the original heavy springs. Remember these are the "normal" springs for these carbs and the weight, was presumably originally calculated to efficiently close the slides, in what were admittedly single or twin configurations. the light springs carry out the same purpose, but with a further aim in mind, to make the throttle easy to operate when used in 4's. I suspect that the pedal pressure issue has been prioritised over their original purpose. If this is not a possibility than my next suggestion would be to try and polish the outside of the slide & the inside of the body to try & reduce friction, this is however a shot in the dark. As you suggested it may also be worth trying to very slightly chamfer or round the bottom edge of the slide to do the same. The danger of this though is that it will affect air flow through the carb & screw the mixture up a bit. A final solution & I may be inventing something that doesn't exist here. Would it be possible to get the slide & body treated with some sort of PTFE coating to reduce the friction between the two components?

I have used Amals twice, once a pair of twins on an A series & once a big single on a JAP 500, in both cases getting the balance between throttle feel & efficient operation was a fine balance, the issues you have are at least 2 or maybe 4 times as difficult to sort.

I hope that the above provides a bit of food for thought even if I can't offer a solution.

Good luck & please keep us posted.

[mk1]

I didn't make it up!

https://www.plasticcoatings.co.uk/plast ... gI1zfD_BwE

[Oneball]

Aren’t the dark grey slides already coated as opposed to the normal shiny ones?

[AndyB72]

That looks fantastic, the IMM seems like a long long time ago now.

[LarryLebel]

On motorcycles they are mounted with O-rings. The use lock nuts and don't tighten them too much. Having the carbs move a bit on a motorcycle is not a bad situation but I don't know how compatible that would be with your linkage.

[Octanecars]

i have a set on my 8 port mk3 s , the pedal is quite hard but really acceptable!

[Octanecars]

my only trouble is to get a total opening on the slides as they actually open 3/4 with the crap linkage! your rigid setup is really interesting!