Trying to understand BVF carburettor....

[keleto]

In the BVF BVF 30N3-1 carb on an ETZ251 am I right in thinking that in principle OPENING the air bypass should increase the idle speed, and closing decrease it, and that opening the mixture screw should richen the idle mixture ?

The tickover on my machine is awful and the overrun is worse than awful. One friend has suggested using the absolute minimum settings to barely allow any kind of tickover may help control the awful overrun (at the expense of having to blip the throttle all the time in traffic), and on a few forums I have seen suggestions that replacing or drilling the pilot jet to 0.5 mm and using the cable adjuster to achieve some kind of tickover is the way forward. If I try this method what settings should I use for the air-bypass and mixture screws ? Also, the pilot jet is the one hidden under a blanking screw behind the main/needle jet, right ? NOT the one in the float bowl (which seems to relate to the choke).

Furthermore, the standard needle clip position (4th from top) seems a bit rich and even a hotter B7HS plug comes out black. Is there much risk in trying a weaker mixture (ie dripping the needle one notch) ? the engine seems to run pretty cool so it seems to me the whole thing is rich rather than weak at the moment.

Thanks.....

[Blurredman]

As long as the jets are clear there is no need to change them from standard. I've recently come into the use of an 30n3 as a replacement for my 30n2.5 and although it took a little while to work out the differences, the principle is easy.

To tune a carburettor's idle properly you must:

read the owner's manual, mine said the mixture screw should be 4 turns from in and 4 turns also of the air screw.

Set it accordingly. They may not be correct for your carburettor's current state of wear but they are a good starting point.

Start the bike and let it warm
Now you need to find the highest and optimum mixture for the idle speed. You do this by screwing the mixture screw either in or out slowly. you must find the point where the idle becomes highest, turning out will richen the mixture, in will lean it. It is best to reduce the idle slightly every now and then in order home the precise mixture and also a revving engine is annoying and can become a little uncontrolable if you're a beginer. When you find the highest point to which it will idle, test that theory by screwing in and out slightly to see. Once you reach that highest point do not keep on unscrewing because that will make the mixture too lean, if you think you've surpassed the point of best mixture, screw in slightly and double check. Minor adjustments make a lot of difference!


Once you have the highest idle through the mixture screw simple tune the idle down (roughtly 1200-1300rpm) and you're done. You may have to compensate and increase idle speed due to clutch drag depending on oil type.

As for needle position. I was never very happy with my Bikes power delivery when on the 3rd notch. It really did mean that was 'nothing' under 3k rpm which is often noted for the ETZ models. I always put it on the 4th notch. It is richer yes but gives more user friendly power delivery, esspecially under 3k.


Yes the idle jet is behind the blanking screw.

In my experience surging over-run is also a cause of improperly set or worn points. I have gone down the wrong roads before and it has caused me no end of greif! You know what they say.... 90% of carburettion issues are ignition faults. I am guilty of being a statistic.

[keleto]

Many thanks for the detailed reply, and sorry it took me so long to get back to you. So you recommend to get the idle mixture sorted first with the factory default air screw settings (4 turns out) THEN screw in the air screw to get the correct tickover RPM, without further adjusting the idle mixture screw ?

Regarding the surging, would you suggest that this is a symptom of over advanced ignition timing, or over retarded ? The points are new but they might not be the best quality......

I was surprised that my bike seemed to prefer 2nd weakest needle position, but I found that it might have been related to an exhaust that had been messed with, because with a different pipe I got a good plug color at 4th position, which I think is the standard setting and is in line with your recommendation.

Thanks again !!

[Blurredman]

It took me a while to get my head around the 30N3 carburettor as I hadn't had one before, But I think I prefer it to be honest..


Yes. Adjust the mixture and idle according to the manual for a good base.. Then find the highest steady idle with the mixture screw. I think it'll get up to around 3k. Adjust the idle back down to accomodate.

And yes, for best results you could keep adjusting the mixture although it will be a bit like a half life effect in progress to get it right. Infact if you do it right first time you'll be pretty much spot on anyway..


As far as surging is concerned I've never really been bothered by it and just put it down to an MZ thing. Many brands have their own quirks, this one is MZ's.. Though I have noticed a properly set up ignition timing does reduce this. Prior, I had very bad over-run surging (and strange running issues that I put down to the carb and totally followed the wrong enquiry...!) this was down to the points being fully worn. I suppose promoting erratic spark. I replaced them and all was well. I wasted a lot of time messing with the carburettor which ran perfectly smoothly for three years without any adjustment.



Matt

[DAVID THOMPSON]

make sure the spark system is in good order on any motor

I have a friend with an 1963 R60 bmw i did a total check and and reworked his magneto adjustments

he says all his weird carb and idle problems went away

on the old bmw idle at a slow speed sounds great but they have a very small oil pump

and do not oil correctly when set too slow
always pay attention to correct idle on any motor

[keleto]

Still having hassle getting a good tickover, and still getting some bad pickup, misfiring and surging

After getting the idle mixture right the Haynes manual says to 'lower the idle speed by backing off the by-pass air screw', but surely that is nonsense ?? surely backing off (ie opening) the air screw should INCREASE the idle speed ?? or am I being dense ??

I am getting tempted to just disable the idle system and use the throttle cable adjuster to get an idle instead.......

With regards to the ignition system, I have tried 2 different coils. The HT lead, plug and cap are new and there is no significant arcing over the points, so *presumably* the condenser is OK (Although I might try my spare one anyway, just to rule it out. I know they can be funny buggers). The points are only a few hundred miles old, but I am not sure about the quality. They are FACET brand, anyone have any experience with these ? I have checked, and re-checked the gap and timing about a million times.....

[DAVID THOMPSON]

i note comment "Haynes manual " bringing from the uk i am surprised.
most uk riders refer to it as a "little book of lies"
i hide the 2 i have from the BMW riders i know so they do not read them
way to many errors
dave

[Blurredman]

I always try to find original operating/service manuals. Unfortunately I don't think MZ made one for the 251. I can't find it anywhere (if someone has it scanned in can I have it please?). Haynes is okay for some things, but I do become frustrated by either lack of details sometimes and the seemingly off figures or adjustments. For example concerning the Haynes says the pilot screw should be 2-2.5 turns. The official book says 'About 1'. The official one is the correct one.


Screwing the air bypass screw inwards leans out the air.

Screwing the pilot screw inwards leads out the fuel.

My current setup with my 30N3 is 4 turns out from in on the Pilot screw, and 5ish on the air bypass screw.

Misfiring is normally associated with the ignition. And once again these strange 'carburation' issues could most likely be down to the ignition. I currently have a problem with my MZ 251 where sometimes the points stick open. CHEAP INDIAN CRAP! I am on the lookout for contact-less not really for the fact that the points are poor (i could make them better or replace with more expensive) but for different reasons concerning a seemingly unaligned lobe.

If you have a spare carb just to check try it out just to prove to yourself it is not the fuelling, esspecially if the spark plug is a nice brown colour.. Definitely check the resistence on the coil. The HT lead could be cracked and losing spark, also the cap itself could be gone. Equally the spark plug could be defective. Don't dismiss new items. Just swap the items with known working ones from friends or what you have spare.

I have (so far) luckily not had ignition problems where the coil or lead were at fault. I don't believe a defective condensor would even matter anyway, they just decrease the residual effects of the spark which is mainly interference with other devices that depend on radio waves, TV, Radio etc.

If I was close. I'd come and visit. Where you? I'm in Cardiff, S. Wales.

[keleto]

Hiya, I am in gloucestershire so not toooo far away if you find yourself passing On the open road it behaves pretty well, just on overrun and tickover it is lumpy. I am probably being too much of a perfectionist but of course, in town and living in the cotswold hills I probably spend 3/4 of my time on the overrun so it affects me more than it would if I spent all my time on the open roads. I will get some new points soon, and I will also check for leaks on the exhaust, manifold, airbox etc. So the correct reference settings should be 4 turns out on the air screw and 1 turn out on the pilot jet ? That is very interesting.

[Blurredman]

Unfortunately I work the normal hours in the day, so I don't really 'pass through' Gloucesteshire

But I guess we can do somehting in a weekend at some point.