SOHC.co.uk Forums > CB350/400
The elusive surge
Alexrayz:
Hey Julie, finally got time to search for it, I knew it was buried somewhere in my mailbox.
The plugs are NGK D8EA and the caps fitted on it are XD05F and VD05F. The leads are originals, same as the coils.
Ted: I did do that, I have a daytona electronic ignition trigger, there is an LED that turns on to show when the sparks will happen and I'd say it is right on the dot. It was a static test though.
You'd recommend getting a stroboscopic light?
K2-K6 I have bench synced them yup. Some lad at work advised me to do a running sync to make sure everything is tip top on that front. Even with a bench sync you can get inbalances that throw off the whole thing.
Daz-J did you check float heights?
Thanks for your help guys, means a lot!
Take care,
Alex
allankelly1:
Question for the group rather than Alex but may help
With the original coils there are two HT leads but can you lose one HT output but the other HT output be OK?
I.e 1 coil does cylinders 1 and 4 and the other does 2 and 3 so can 1 coil intermittently feed a spark to say cylinder 1 but the spark to cylinder 4 is consistent as on the wiring diagram it look like all plugs “go to earth†unlike my RD where both HT leads make up 1 circuit so if you disconnect 1 cylinder you lose everything as you have broken the single complete HT circuit
Hope that makes sense
Just that an inconsistent spark on 1 cylinder will imitate this surge as when all 4 are running the revs will be X but if you lose a cylinder the revs will drop to Y as the other 3 cylinders have to carry the “dead†cylinder and then when it decides to run again the revs will rise
Just a question for the experts as had this issue on a old 125T where a dodgy CDi was dropping in and out so it would run on two then drop to one cylinder for a short period of time with a big drop in revs to where it would die if at tick over and then it would decide to work again with again the rise in revs back to correct idle
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Alexrayz:
Hey Alan,
I guess that could be possible with old cables. As it gets old with many manipulations and service the copper wires can fray and cut which could lead to intermittent connections.
I have never encountered the case in motorcycles or cars or else but I have in houses and offices with old electric installations.
Soo maybe?
allankelly1:
--- Quote from: Alexrayz on January 10, 2022, 05:43:45 PM ---Hey Alan,
I guess that could be possible with old cables. As it gets old with many manipulations and service the copper wires can fray and cut which could lead to intermittent connections.
I have never encountered the case in motorcycles or cars or else but I have in houses and offices with old electric installations.
Soo maybe?
--- End quote ---
Hi Alex
Yes that why I asked the question
When I did my Boyer system I bit the bullet as replaced the complete ignition system (As in theory the whole system was over 40 years old) So that as well as the magnet pick up system that replaced the points system, it also does away with the mechanical advance and replaces it with a “mapped†CDI (so you have an advance curve programmed in) and two new coils etc
But on the old Honda system I am not sure if both HT cable make up the HT circuit(Like my RD). Or as I suspect on the Honda,one end of the HT winding goes to earth but the other end is split to two HT leads, and then goes to earth via both spark plugs, hence one lead could be good and the other bad / damaged giving a intermittent spray to that cylinder.
Let’s see what the experts say
Best wishes Al
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400 Cafe Racer:
--- Quote from: Alexrayz on January 10, 2022, 05:43:45 PM ---Hey Alan,
I guess that could be possible with old cables. As it gets old with many manipulations and service the copper wires can fray and cut which could lead to intermittent connections.
I have never encountered the case in motorcycles or cars or else but I have in houses and offices with old electric installations.
Soo maybe?
--- End quote ---
Hi Allan and Alex,
To the best of my knowledge the spark performance on each coil provides for simultaneous sparking on both cylinders served by the coil - ie 1 + 4 for one coil and 2 + 3 for the other coil. This means in fact that the cyinder that is not on compresion stroke has the spark "wasted".
Unless there is a specific problem with one HT lead or cap, the spark performance should be the same.
My comments below assume that there are no carb or inlet manifold air leaks.
Having looked at the issue you have got Alex, I encounted a similar situation during tuning my carbs after doing only a bench sync. I found that spending time afterwards with close attention on the vacuum guage balancing and mixture sync method, this unevenness and surge disappeared. It only needs one of the cylinders not contributing equally to the tick over, and a little off with the mixture to cause this. When the off cylinder decides to clear a little and then "comes in and contributes", this causes a rise in engine revs for no apparent reason. You will be supprised when making adjustment with the vacuum guage, even - say 5 degrees - rotation of the balance screw for 1 cylinder will make a big difference to all the other cylinders. Just keep repeating the sequence until they are all the same.
Hope this helps. ;) :)
Dennis
PS Just thought I would add a small note to explain that for example in the same period (1970 - 1980 )that with a 6 cylinder car engine with 2 or 3 Carbs, to overcome a tendency for imbalance on idling the manufacturer would tend to put a small balance pipe connected across the manifolds so that all cylinders could suck Air/Fuel mixture, even though the individual Carbs may not be set spot-on.
In the case of the 400F all manifols are individual and have NO interconnecting balance pipe, so this makes the correct balancing and mixture for each Carb and Cylinder hyper critical to get best running and performance.
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