Hitchcocks Forum

Hi.
After many years away from Brit bikes I have acquired a `54 350 Bullet.One of (the many) problems I have is that having set the engine tickover when I rev the engine it will not readily return to idle.Particularly when warm after a period at higher revs ,shutting the throttle is not accompanied by the engine doing as it`s bloody well told and ticking over.Not good.Applying some load via the clutch soon brings it into line but I need to fix this.
I have rebuilt the carb with an Amal rebuild kit but problem is still there.
Any suggestions

First things to check are:
Air leaks on the inlet (spray with WD40, if the engine smokes when you do, it's sucking air somewhere)
Worn throttle slide. Amals they are notorious for this. A new slide won't fix it as the body will be worn as well. There are people who will bore and sleeve the body but a new carb may be cheaper.

Ok
Thanks I`ll try that now.My problem is not knowing any history and condition of the thing.By the time I had forked out for the rebuild kit and"stay up float" (Which by the way doesn`t as it sticks on one of the float bowl screw bosses,way to go Amal)I could have put the money to a new carb.
The slide is shiny chrome which is not as I remember them on my old concentrics(BSA days)so is the carb newish or has someone tried a bodge?.I noticed silicone sealant (clear like B+Q stuff) around the carb flange so maybe more bodgery indicating an air leak.

Well I wanted something to tinker with.

Hi Rob, I have held back from answering here until now, as I was in the middle of dealing with the same symptoms on a Continental GT 250, but this may help you. As your machine is 1954, I assume you have a manual advance magneto for ignition, or a later type engine? For what it's worth, in my case I tried various slides and pilot jets in the Monobloc carb, but couldn't get rid of the 'lag' when shutting the throttle. The racing tickover would gradually slow and eventually, the engine would stall. I tried another, reconditioned Monobloc [rebored, oversize brass slide, etc], but no change. In the case of my machine, it has an automatic advance / retard mechanism and, although it was correctly statically timed at both full advance and full retard, testing by strobe revealed that it wasn't returning to the full retard position when the throttle was closed, but it actually paused about halfway for a while, before slowly dropping to full retard. On this machine's auto advance unit, there are two sets of holes for the spring ends and both springs were on the softest settings, so I moved one spring end to the further hole and this sorted it. Checking with the strobe showed a good, smooth advance and retard action right through the range and throttling off led to an immediate drop to a steady idle, although, at present it idles best on the prop stand, so I am going to lower the float height a little to get round that. As I understand it, your machine's ancillaries may be very different, but the same basic principles apply and if you have a manual advance magneto, make sure it drops back to full retard when you back the advance lever off. Hope this helps, cheers, Paul.

Rob,



I'm guessing that, as a '54 Bullet, yours will have the magdyno, but if whatever advance and retard mechanism you have operates freely, and you have eliminated carb flange air leaks as the cause, the other thing I would want to check (if you haven't already done so) is the routing of the throttle and choke cables making sure that particularly the throttle cable isn't snagging on something, or that a damaged outer cable (eg under the tank) isn't rubbing the inner and preventing the throttle from closing fully when it should. A nylon-lined outer cable might be a good idea.



It sounds like someone has fitted a concentric carb at some stage. The chrome slides are sold by Surrey Cycles as being more wear resistant, and I would definitely have one if I had a concentric on any of my bikes, though if the carb body is already worn, it's a bit late. No doubt our hosts will gladly sell you a new 376 monobloc or 276 carb, if you're concerned about originality, but Amals are not the only game in town...



A.

Hi
Thanks for your help.Informative and as I`ve just seen on the money as the cable for the manual advance /retard is shagged.Also the carb while definitely a monobloc has this shiny looking slide so definitely looks like someone has attempted to stave off inevitable Amal erosion.
So 1.New cable and check the a/r unitworks ok.
2.New carb? Amal make an Aluminium Concentric now which should not be prone to wear.It was that zinc alloy they used that was the problem,best left for toy soldiers not engine parts.
I have more experience of Concentric so would go for that over a monobloc if I go down the Amal route but I am interested in any alternative.Adrian left on a "see next weeks thrilling episode" saying "Amals are not the only game in town" then didn`t say what was.No I am not interested in originality.I am not a collector/investor or old Brit Bike bore and miss the days when nobody wanted this old stuff and would consider it a favour if you took it off their hands.I want something I can ride,work on ,improve and enjoy.

Rob

Rob, sounds like you're on the right track. I think what Adrian means about carbs is that the Micarb VM24 (Indian made Mikuni) as fitted to the 350 Bullet will fit straight on and works well, especially if you're not bothered about scintillating performance! There are plenty of these on Ebay as folk take them off when they tune the bike. Amals are good reliable carbs but they made what the British industry wanted and in the case of the Concentric the industry wanted cheap. Good luck,

REgards, Mark

While I`m thinking about it,does anybody know what the rest condition of the manual advance/retard unit is?
I mean is it fully advanced and operating the lever retards the ignition or t`other way round like an auto unit where it is naturally fully retarded at idle and then advances as the revs rise.

I didn't specifically have the Mikarb in mind, though as Mark says it's a bolt-on replacement, and I'm sure our hosts have some tuning notes for it. Check the prices carefully as some sellers put their used ones up for more than the cost of a new one. I was rather thinking of a genuine Mikuni VM26 or 28, or even a Dell'Orto PHBH 26 or 28.



I can't remember which way the magdyno A/R lever works, but think of it this way and you can work it out for yourself by checking on the bike. Looking at the magdyno from the points end with the point cover off, the points will rotate in a clockwise direction when the engine turns over. To retard the ignition, the A/R will move the cam plate clockwise for the points' heel to hit the cam plate later, to advance the ignition the cam plate will rotate anti-clockwise for the points' heel to hit the cam plate sooner. Check and see what the lever does when rotated.



A.

The 24mm Mikcarb is a good item that will bolt straight on, and other than fitting a 110 main jet will work fine 'as-is'. But, the bike will perform much better with a 26mm choke, or larger.


The common problem with the Mono is from too large a section O ring in the flange face (There are two sizes) and over tighten manifold nut. Make sure the flange is not warped.

Make certain that the pilot out-let and the pilot by-pass passages are both clear. You will have to remove the jet body to fully clean the passages...