V50 II Flat Spot

Oh yeah, that old chestnut. Now I’m pretty much of a view that this somewhat notorious issue can be mostly tuned out with a bit of care. The story for this one goes like this;

We’re talking the Black Pearl here and I don’t tend to use the bike at all during the summer months. This poor old girl, though much loved, is my main winter ride these days. Pulling her out for use back around November time and I only had single cylinder running. After a couple of false starts I found this to be down to a dead ign box and I replaced it with a second hand item from the bay of e. So far so good, except I have a real bad flat spot now in the usual place – around 2.5-3k revs.

The bike has done maybe around 65k miles.
I spent a good deal of time setting up the ign sensors a long time back after which all was well. I’ve not been in there since.
I replaced the coils with new this last few months when tracking the ign failure. So that included the HT leads which are integral on these.
Std airbox and carb set up. Replaced all the jets with new maybe 10k miles ago or so.
I only ever run the bike on high octane.
I put in new plugs Saturday morning, and whilst she ran just peachy at 70/75 per all the way back from Lincoln (so around 120 miles at good speed, you’d think that would blow away the cobwebs) the flat spot remains.

Could it be that the bike is always this bad and I’d just forgotten? – I don’t think so. And I was really struggling to drive around this out in the snow yesterday.
Is the replacement ign box in some way different? - I don’t see how, it has the same Bosch ref code emblazoned upon it.

Where now?

Well I am new to this, so I can’t say how typical it is, or if it can be “tuned out”. I have spent some time making sure the timing is well synchronised on the two cylinders - at least that should avoid the flat spot being wider than it needs to be, due to one cylinder advancing before the other. However, having spent far too much time lately with a strobe in my hand, you can’t get away from the fact that somewhere in the 2000 to 2500 rpm region, the ignition suddenly advances by some 15 degrees in a step. Of course you only notice it at that point, where the engine suddenly surges or dies, depending on what you are doing. I find it most noticeable either in traffic or when doing a U-turn or similar low speed manoeuvre. As soon as you get up past 3000rpm, all is well.

I can believe that if you ride the bike a lot, you may get so used to this that you don’t really notice it.

As for the question of is the new box different to the old one - well they are both 40 years old, so I guess anything is possible. I can only suggest carefully strobing each cylinder and comparing the result. At least you’ll be able to give the ignition a clean bill of health if they are both identical. What’s that old saying? Something like 80% of ignition faults are caused by the carburetter - and vice-versa!

Best of luck!

Yeah. I pulled the carbs down first - that’s a job I prefer doing. They were clean as a whistle despite being steeped in their own juices for probly 6 months prior.

I have a modern electronic ignition system on my V50 and I have no problem. I can time either cylinder with a strobe and know the other will be good too. I suspect that on the original there is a small voltage rise (efficiency) at a certain speed, or the Bosch boxes have limitations.
I know my V50 is a mk3 but my system is crank mounted, no different to mk2.

As a matter of interest, which modern system did you go for? I am highly tempted by the “Silent Hektik” system, just put off by the rater scary price. I am well aware that this is the only complete answer. I can fiddle about with the Bosch system for ever and it’ll still have that step in the advance curve. Hmmm.

I went for Silent Hektik. Did a lot of research and felt this was the best. Not let me down.

It’s just that my bike was OK. Then I had to replace an ign unit and now it is not. Options;

. It was never OK and I just forgot whilst it was laid up.
. Something else during the layup has had a detrimental effect.
. The replacement box is performing differently to the original.

I’ve lots of boxes you can have one for postage

I got Sachse. IMO standard maps not ideal for small blocks. Have to send box back with own maps to be loaded. Silent H maybe best as programmable

Is that an open offer as a spare would always come in useful…

Interesting! What do you think is the problem with the Sachse maps? Just looking at their leaflet, it looked as if there was quite a good range from pretty steep to pretty mild. It sounds as if you’ve done a bit of work on this, how do you feel the ideal V50 curve should look? On the Sachse graph, I thought the light green line looked good. It starts at 0, idles at 8 deg btdc, starts to advance at about 1200rpm rises almost linearly and it’s all over just before 4000. I think that line is no. 5 but it’s not that easy to read! I did wonder if it needs to be a bit steeper initially, but it hits 25deg at around 2500 rpm which doesn’t seem too bad. With the standard ignition, the bike starts well, idles well and works fine above 3000 rpm. It’s just that 2500 region that’s not so good. I don’t want to spend £300+ and just end up with a different problem!

Yeah, so first a health warning. I have only tried the standard maps.to date; second I may have coil issues and third I am not an expert just having a go. So on this last point I am unsure of the significance in variance of curves and its effect on performance.

All that said I think the standard map that looked the best match (5 from memory) felt like it was a little sluggish to pick up; the advance started lower than the stock Bosch map. It also climbed less steeply and felt like it was holding back a touch when going up through the rev range. I got a bit anal and perfectionist about it - the bike is quite rideable but I thought that if I’ve spent this much I really want it to be as good as it can be.

Also need to consider the Bosch curve was (according to Guzziology) the same on various bikes that used it so how can it be ideal for all? It is insanely steep and has that mad step that causes a flat spot.

So, to test of the Bosch curve was any good (without the tricky variables of the pick ups and boxes) I sent my Sachse box back with a request for 4 custom curves that I plotted on Excel. One was the standard Bosch curve, and one was the V50 curve given in Guzziology. The other two were home brew variants on these - but not much different. 60+ Euros later I have my box but we have winter and salty roads…

I’ll write up my results in a separate post. Also there was a write up in Gambalunga saying V50 flat spot was cured by Sachse using standard map. It is a nicely made system with good components for sure and I fitted it with basic garage skills…


All very interesting, many thanks. I live in a cul-de-sac, so my quick road test is a sharp turn onto the road, up to the end, about 100m, a U-turn and back again. My first impression with this bike was that something was making the clutch suddenly bite when it had been slipping nicely and smoothly coming off my driveway. That’s how it felt but in fact the clutch is innocent, what I’m feeling is that step in the timing. As I come off the drive, the engine is pretty gutless until it hits that magic 2500rpm, when it suddenly takes off. The same thing happens on the U-turn, first it dies and then tends to snatch. Maybe I’m over-analysing now, but I think with the standard Bosch setup, the advance starts too late, making the bike weak at low revs, the famous flat spot, then it catches up all of a sudden at around 2500rpm. If you are finding the response a bit weak at low revs with Sachse curve 5, perhaps it needs to get advancing even sooner.

On my 1978 750 Bonneville, I have an Australian “Tri-spark” system and the advance curve is really pretty steep. It starts at zero degrees and it’s up to 38 deg btdc by 3000rpm. - with no further advance after that. It’s pretty gutsy at all revs - of course that engine couldn’t really be more different! That system has “anti kick-back”, I’ve no idea how it works, but it’s a godsend for skinny blokes like me!

I seem to remember reading somewhere that the Silent Hektik system can be programmed by the user (?) - now I can’t find that reference, though I don’t speak more than a few words of German and of course their website is all in German, so I may be missing something. My son is a German teacher - he doesn’t know it yet, but he’ll be doing some translating later! There is no mention of user programming in the English version of the installation instructions.

I’ll dig out my ignition maps but yes the Later v50 curve for Mk3 bikes is pretty linear but climbs from 2000 or 2500 rpm apparently. I think my modified V50 I changed it to start to advance earlier. The Bosch I think I smoothed the curve but kept much as standard otherwise.

One item I had come across in a search for a replacement box is listed here;


Interesting perhaps, though horrible money for a pair. Note the list of bikes that this will fit - and hence were presumably fitted with the same Bosch units originally - clearly considered a one size fits all by Italian manufacturers at that time.

Yes I also stumbled across that on my internet travels. I could see no point, you still have the same pickups and coils and it’s more expensive than any other option I’ve found. There is very little information on their website so I have pretty well ignored it.

Off to take the Bonnie for MOT this afternoon, so a temporary suspension of Guzzi operations - soon to be resumed!

SO these are the bespoke curves I had programmed on my Sachse box.

  1. Standard Bosch (blue)
  2. Modified Bosch - smothed curve (yellow)
  3. V50 MKIII (grey)
  4. V50 MKIII modified (orange) - primarily by starting to climb at lower revs so pick up is quicker. Wish I’d started even earlier say 1000RPM.

I am hoping No.2 or No. 4 will be pretty good and will report back
V35 Curves Snip.JPG

Your Bosch curve is very different to mine. Mine was around 10deg at idle and then didn’t change until 2300rpm when it suddenly jumped to about 25deg. This causes the revs to rise dramatically, so it’s difficult to chart exactly what happens next, especially as the “step” seems to come at slightly higher revs when the engine is slowing down. However, my impression is that the timing retards slightly after that step (rising revs) before continuing to advance smoothly up to the max of 34 deg. at something over 4000 rpm. I would have expected your blue curve to work quite well.

I spent a bit of time on the Silent Hektik website last night with my son translating. The “Powerblock” ignition is not user programmable as I originally thought, I’m not sure where I got that from. They make a big thing of explaining that user programming is not appropriate for road vehicle engines unless they are in development. They do make user-programmable units for racing and aircraft use, but I didn’t pursue that.

I am still very tempted by the Silent Hektik product. My bike still has the original ignition coils, which are working fine, however the plug leads and caps are pretty tatty - are the plug leads moulded into the coils? They don’t look replaceable.

I took my 1978 Bonneville for MoT yesterday, after which I took it for a celebratory blast around the lanes, the roads being dry and it wasn’t particularly cold. I enjoyed it immensely and it reminded me that the whole idea is to ride these things! Maybe the Guzzi will get let out of the garage over the weekend.

Sounds good, enjoy the Bonnie. Leads not easily replaced in coils but you can cut them and use a connector block to join on a new one

Muzzi moto bumph says Hektik curves adjustable. I assumed this was by the user but apparently not. Beware hidden costs of tweaks. For comparison Sachse charge about 12 euros for a bespoke map supplied by the user. Plus postage etc…

I expect this means the curves are adjustable from the pre-installed maps.