Brother got his IIa up and running today, however he tells me that its running like a bag of spanners!! He says that it ticks over fine, but under acceleration it misses and chuffs out black smoke. Has turned the mixture screw right in, and it will continue to run and accelerate with black smoke. Economy diaphram has a small tear in it, is this likely to be the cause?? He also seems to think ther is excessive wear around the hole through which the shaft holding the flutter valve sits (apparently petrol comes out rount here, but he thinks its a result of the huge amounts of fuel being dumped into the system).
Should he get a repair kit? Is it something he's missing, or shoud he just throw it in the hedge and buy a new one?
I had a lotta trouble with a Zenith/Stromberg carb on my IIa, despite a rebuild with an overhaul kit, it still did strange things. I replaced it with a brand new Weber, not had a single problem since.
Apparently they can have problems with warped carb bodies, something to do with problems during casting.
...and Alex spake unto the tribes of Usenet, saying...
The original Zenith on the 2a was well knackered, so I replaced with a "Zenith type" (ie pattern) carb from Cra/Paddocks. It lasted a few months, by which time it was so warped it would hardly run at all. The gap between the lid and body on the front edge was over 1mm in the centre. Flatting the mating surfaces on fine emery on a glass plate helped but didn't cure it. Alex is right - it's something to do with the heat-treating during manufacture, and there are a lot with this problem AIUI. It would not idle evenly, even when set to idle at >1000rpm, ran extremely rich, and turning the mixture screw in and out made no difference. I bit the bullet and got a new genuine Zenith (now being manufactured by Burlen
Thanks, I've got a fair bit of rust to exorcise before I can start putting the shiny bits on, I can feel one coming on in the spring though (hopefully).
As Alex and Richard have said, a replacement 'other make' carb is a very worthwhile investment. My S3 never ran properly with a zenith, but did with a weber.
there are a few things you can do, as you suggest, the repair kit might work, the flatting of mating surfaces might work, a damn good clean of the petrol pipes, tank, sedimenter and carb might also help, as would checking that air can also get in. A screwed air filter might also make one think it could be the carb when it isn't ... ;)
The oil-bath air filter is pretty free-flowing, though it's worth cleaning out. I've had the semi-flexible air-hose on a diesel suffer an internal collapse, which is serious black-smoke time.
...and Paul - xxx spake unto the tribes of Usenet, saying...
No, what I suggested was a genuine Zenith, manufactured on the original tooling by Burlen, who have bought all the tooling and rights to the name etc from ?Solex?. Mine runs well and is showing no signs of failure so far. If you have a look at the Series 2 Club forum
formatting link
you will see that it's about 80/20 against Webers, mainly for their propensity to ice up and also starve of fuel on steep slopes.
On or around Fri, 16 Dec 2005 20:33:49 -0000, "Paul - xxx" enlightened us thusly:
ah, been working on this on the minibus - removing the restrictive inlet to the airbox created more inlet noise and noticeably more power. Next investigation is into ram-air effect...
Been considering the speed of the air in the inlet tract:
So, the engine swept volume per rev. will be half the capacity, i.e. 1.25l.
6cm pipe has a cross-sectional area of about 28 cm² so the afore mentioned
1.25l of air occupies about 45cm of the said pipe. Suppose now that the engine is doing 2200, that gives you 2200*0.45 = 990m/min. make it a tad over 2200, and you have 1km/min which is 60 km/h
Now, suppose I duct the inlet to the front of the vehicle and point it forwards, and do 60km/h at 2200 rpm, the incident air on the end of the pipe is at the same speed as the air flowing into the engine, and as such, must increase the inlet pressure slightly as compared with running at 2200 rpm in still air. If I now attach a funnel, of say 12cm down to 6cm diameter, to the pipe, I've got a 4:1 decrease in CSA of the pipe. I'm not daft enough to believe that this makes the pressure at the small end 4x that at the large end, because there are doubtless lots of reasons why not. But it must increase the pressure a bit.
At lesser road speeds, obviously, the ram effect is less - but at any speed there will be *some* effect.
All this can be done by potching about with bits of otherwise worthless tqt, and if it makes the motor go faster or better, well and good.
The main thing you've missed is that the air temperature will rise, tending to reduce density. So the mass flow will be reduced, and it's the mass of air which matters to the combustion process.
That's why intercoolers make a difference to a turbocharged engine (or supercharged). It's also why water/alcohol injection makes a difference.
But it's probably going to be masked by the detail effects of the intake design.
Your reasoning seems sound enough Austin - when I tried the same sort of thing with my motorhome there was a marked improvement in performance, although no real gain in economy as the extra oomph tends to be translated in a higher speed. As you say, there's nothing to lose playing with it.
Re-work the maths for the diameter of the exhaust, factor in the thermal expansion of the gas and then fit a decent bore exhaust - you'll prolly make a similar gain there too.
Acksherlly it's not. Look back to the ram-air Suzuki 2 strokes of yesteryear ... or the way that almost any racing car has similar ducts to channel a positive pressure of air into the inlet. We used to do it on our racing sidecar 'cos they had no airboxes to speak of, so central heating flexible duct was used. The wider the better, we found.
What does help _a lot_ if you do make some positive pressure at the inlet is to also do something to the outlet ... ;)
It was ... but some people, I believe Suzuki did on the racers, also used it to channel 'extra air' into the airboxes, especially the production racers ... ;)
On or around Sat, 17 Dec 2005 10:45:11 -0000, "Paul - xxx" enlightened us thusly:
yeah, thought about that. I rather think the exhaust may be the same for a turbo engine, though - if it is, then it's got the additional capacity. but it's easily checked.
hmmm. Bosal list 2 exhausts, one turbo, one non-turbo... But, the only part that's differnt is the front pipe. The main box, intermediate pipe and rear box/tailpipe are all the same part numbers. SDo I reckon the exhaust shouldn't be too restrictive.
Many thanks indeed to everyone that has offered input, have passed it all on. Emphasis has shifted slightly today; he wasn't getting far with the carb so he thought he would finish off the electrics. For some inexplicable (atm) reason the side light that has worked up until thus point, caused a short. this failed to blow the correct sized and installed (checked both) fuse and promptly melted the brand new wiring lume that he fitted some months previously. He was almost crying at this point. So he has had to pull the lot out and set about bypassing the damaged wires.
Incidentally, we have been offered a workshop with enough room for 4 lifts, heated, offices, changing room, kitchen, parts store and alarmed. Whats it worth to rent? We have been struggling to decide what a sensible figure for such a place would be.
Most modern big sports 'bikes have some form of ram-air induction system. There is a sort of test of how well it works by simulating it on a dyno here:
formatting link
Not sure how accurate it is compared to on the road!
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.