“Fluttering” at full throttle

[Stubaker58]

Hi All,

My Bantam is a bit of a mongrel but the top end is a rebored D7 barrel with one of Rex Caunt’s big pistons fitted. The head is D10/14 ie with the squish band.  It’s running a Monobloc which has been well cleaned; pilot 20, needle 105, needle notch two and main currently 180.

It starts easily and runs perkily up to three quarters throttle (around 45mph) but then starts to “flutter” and won’t accelerate further.  I suspected too small a main jet and have gone from 150 to 160 to 180.  There has been a small improvement but it still reaches a limit beyond which it will not go.

I’ve raised the needle which was worse and gave lumpy acceleration.

Should I go for an even bigger main jet? (How big do they Go?) or could it be ignition related? It runs a D7 crank, points on the left, and a separate remote coil under the seat.

Any suggestions gratefully received.

[AdrianS]

The last bit of the throttle opening is the main jet.
Have you tried a smaller main jet?
You could compensate by raising the needle 1 notch to keep the mixture in the mid range.
You could mess with the timing but if it’s running sweetly up to that final bit I would leave alone.
It may just be the characteristic of a bitsa engine!

[bluebell]

you dont say what size ex system you have ,,is it wipac points,,,isit magneto or battery/coil or full electronic,,,is it the early or late d7 cam if it is wipac,,,, what air filter do you have,,,, are the fuel filters clear especially if you have the small spigot  push/pull tap,,,,,also if the mag surprt bush is worn it can flutter as you put it at higher revs,,,,,if its maneto try cleaning the points with wet and dry and some petrol soaked cloth and reset to  012 thou,,,

[Stubaker58]

Hi Bluebell,
It’s a small bore exhaust. It’s on the original Wipac points but with a separate coil ie not magneto.  The electrics are simplified with full output via a reg/rec to charge the battery, still 6v though.
The centre bush on the generator is good.
It is a small spigot tap, I’ll look at that first I think.
Could a silencer in need of a decoke bring these symptoms?

[AdrianS]

The standard d7 ran perfectly well with the small bore exhaust up to its max speed of around 60mph.
I would be surprised if the exhaust was the problem.
I would still play with fuelling.. you could try messing with the timing - just mark where the points plate is now before moving it. Try advancing a little and see what happens. If you get it too advanced, the kickstart will let you know when starting!

[royblackburn1@btinternet.]

Like bluebell says could be fuel starvation I once had same symptom and filter was part blocked at fuel tap 

[Stubaker58]

Thanks guys,
I’ll have a go at the fuelling this week and report back.

[bluebell]

have you measured the mag plate bush diameter against a known crank thickness,,get a set of dividers or similar take a reading off the crank,,  get some small wooden dowel and reduce in a drill and sand paper so thy are the same dia,,,,there should be virtually nil radial play,,,  at 3/4 to full revs if there is radial play the end of the crank actalluy vibrates and interferes with the spark  and stutters the  engine,,,,or measure a new bush against the old one  fitted in the stator,,,,also  if it has an earlier  lumpy cam fitted  that limmeted the rpm a bit  compared to the smooth lift later cams from  d5 onwards,,,,,,,check the strength of the points  spring against another set  if its weak  this could also give the fault,,,

[bluebell]

have you measured the mag plate bush diameter against a known crank thickness,,get a set of dividers or similar take a reading off the crank,,  get some small wooden dowel and reduce in a drill and sand paper so thy are the same dia,,,,there should be virtually nil radial play,,,  at 3/4 to full revs if there is radial play the end of the crank actalluy vibrates and interferes with the spark  and stutters the  engine,,,,or measure a new bush against the old one  fitted in the stator,,,,also  if it has an earlier  lumpy cam fitted  that limmeted the rpm a bit  compared to the smooth lift later cams from  d5 onwards,,,,,,,check the strength of the points  spring against another set  if its weak  this could also give the fault,,,  is it the long type of silencer,,,the short ones  dont seem to rev as high