Twin Carb Problems.

[LJ.]

Hi LJ, Do I see an anti-bias spacer? I have pretty much the same bike and twin carbs also and I am having no end of trouble getting the cylinders to run evenly. What I mean by that is that one side always runs a bit hotter than the other indicating perhaps it is working harder. If you have fitted an anti-bias what led you to do that? Cheers, Dave

Hi Dave... I've started a new thread for this enquiry you've made so that others with twin carbs might be able to chip in and help.

I've not used Induction bias gasket spacers as you ask. What you see is just ordinary tufnol spacers which I had to use because of a small ridge around the port holes on the manifold. I am using 276 carbs whereas from what I understand from Brian the twin carbs should be 275s because those carbs have a cut away just inside for the manifold to fit snuggly in to. I did not know this until after I purchased my second carb thinking that a pair should be identical to the single one I already had fitted. There is a bit of discussion about that here somewhere.

I re-newed all the internals apart from the jet block as obviously we need to get both carbs identical to each other. Obviously one of yours is running hot, is this while ticking over or on a good ride? You might find out by doing a plug chop at 45mph and checking the plug colour if they are both identical then that would point to the fact that pilot mixture is lean. I have had my left pot running hot at one stage and this was due to the throttle slide open just a fraction more than the right side. I also have found that it is crucial to have both float bowl needles set identically too whearas on a single carb this is not so important. Having twin carbs at first seems quite weird as the engine behaviour is very different, if I tickle one side and not the other the bike will start only on the tickled side, the untickled will not run until it has had that tickle!

I must say at this relatively early stage after fitting them that I realise just how lucky I am with the measure of sucess that I have had as by no means am I an expert on this. I have been dreading the amount of petrol around the area like I have on my M21 it's a horrid mess which is why I carry my fire extinguisher   I also thought that there would be lots of wastage of petrol from tickling but I've found that I only need to give two quick taps of the tickler plunger on both sides and the bike will start but I have to use choke. Once started choke is imediately removed. Blimey.... this is turning into a book!

Obviously it helps with having a carb tuning gauge (see picture of the one I have)



Also double check that your tappets are set correctly and that ignition sparking is the same both sides. It does seem that everything with twin carbs need to be spot on. I've quickly come to realise how much of a hassle it is and probably why BSA decided not to persue with them, But... I've had a lot of curious comments made when at bike meets. In all... I'm pleased that I've fitted them, keep at it cuz its highly satisfying once they are correct.

Let us know how you get on... Cheers!


I've re posted the carb pictures here to use as reference.

[Brian]

I guess this is as good a time as any to have a bit of an update on how my Star Twin is going. I have to say my carby's have performed faultlessly from day one. I built two new carby's from nos parts which no doubt helped. I have only done about 1500 miles on the bike so far as its the middle of winter here at the moment and I ride my plunger A10 most of the time.

There are quite a few things that can influence how the two carby's work, mechanical condition, maggy condition and of course the carby's themselves. Assuming the engine and maggy are ok then the carby's must be set up the same, same float level, jetting etc. If one side is running leaner than the other then its a matter of double checking everything. I would start by checking the balance. Also take into account the wear in the carby's, if they are secondhand then one may be worn more than the other so it may be necessary to run one slightly richer than the other to compensate. Never forget we are dealing with things that are 60 years old so there are a lot of variables.

I'm actually quite impressed with how the long stroke motor performs, its a lot better than I thought it would be. I have done one longer trip about 400k's and all at a steady 60mph without a problem. I also have a 51' A7 and have been able to compare them side by side, the 51' has a slightly better top end performance but the longstroke is definitely torquier. I would say the longstroke is the better bike to ride. I have a pair of Armours mufflers on it and it sounds great. Now all I need is a single carby model to add to the stable, preferably a plunger, hmmmm, I wonder where I can find one.....................................................

[olev]

Brian, Its just up the road from you.

http://cgi.ebay.com.au/BSA-ZA7-1950-500cc-TWIN-RIGID-MOTOR-GEARBOX_W0QQitemZ300330018083QQcmdZViewItemQQptZAU_Motorcycle_Parts_Accessories?hash=item45ed106523&_trksid=p3286.c0.m14&_trkparms=65%3A1%7C66%3A2%7C39%3A1%7C293%3A4%7C294%3A50

cheers

[Brian]

I saw that one Olev but I already have a spare motor. Actually I bought the spare motor I have long before I had a bike, it was offered to me very cheap about 10 years ago. At the time I didnt need it but being BSA I took it anyway. Turned out to be a very good score, excellent condition still with std bores and crank etc. I gave away the rocker boxes and it doesnt have a cam in it, but otherwise complete.

The one on flea-bay at the moment looks to still have the cam in it and if its near new as he claims it might be worth looking at.

If I bought another longstroke it would have to be complete or very near to, I have built bikes from parts but its just too hard to find all the parts these days.

[muskrat]

G'day Blokes,
                 To balance carbs without guages take air filters off, open the throttle all the way. put finger in and slowly close throttle till you feel the slide enter the venturi, hold still and feel the other slide. Use the cable adjusters to make even. Then back the idle screws right off, making sure the slides close completely. Open a little an place a piece of thin wire under the slides and close. Now screw the idle screws up till the wire moves. Any future idle adjustments must be made equally. If everything else (magy, tappets, comp etc) is right, tell wife you have to test it. 200 miles should do it.
Cheers