Gold Star misfire

[VintageBike]

Folks,
after a bit of help...  Got a 58 DBD34 that has a slight problem.  Starts well & pulls well but misfires when "pushed" at a sustained speed.  Anything around 5K rpm in top for more than a couple of minutes and the bike misfires.  It will rev quite happily past this in the gears, but not in top at speed (65-70mph).  It feels like fuel starvation, but I've got a good flow from both fuel taps.  Has anyone got any ideas where I should start looking?  I've changed plugs and leads etc just in case, but it just doesn't feel like a spark problem.
help....!
Nigel.

[zigzag]

The fact that it will rev ok when going through the gears does not indicate an ignition problem,but a sustained high throttle opening does indicate that the fuel flow can't keep up with what the engine requires,maybe the float level is slowly falling at sustained high revs until it can no longer keep up with the engines need,resulting in a misfire,you're going to have to go right through the petrol feed,and carb to eliminate fuel as you're problem.

[VintageBike]

i guess that is what I was suspecting...  I'll keep you posted..
N

[willse]

I'm guessing you've already checked things like petrol tank breather , and globs of water in the floatbowl . Just a thought . Hope it is something simple .

[VintageBike]

thanks for the input.  Yes I've already check the breather - and  the float bowl is coming off tomorrow....

[VintageBike]

Mmmm.  No progress.  I've been right through the bike's ignition and fuel system, but still the same problem.  Will rev freely up to 6000rpm (haven't got the courage to take it past that!) in the gears, but in top will misfire at 4000 rpm - about 70mph.
GP card stripped and cleaned.  Breathers checked.  HT lead, plug and cap replaced.  Mag rebuilt last year.  Points and timing adjusted.
Someone a while ago suggested that it could be the fuel frothing in the float chamber due to vibration at a given rpm.  This seems to make sense, as rapid acceleration past the resonant frequency in the lower gears would not give the fuel a chance to froth.  The big question is what can be done about this.  Apart from rebuilding the engine to reduce vibration!  Someone mentioned anti-frothing baffles for the float chamber, but I can't find such a part anywhere.  Any ideas?

regards

Nigel.

[geoffus]

Nigel
if you are sure the magneto is not at fault you can try
a few another things one  thing that Cumto mind is what sort of float bowl you have fixed or remote if remote lift it a 32nd next if carby as being a part check that the paper gasket is not broken betwen the body and the mixing chamber if it as renew as this gives a seal or may not be a clean cut.if this does not work trying putting the coke on this should remedy any lean mixture and therefore tell you if it is running lean  .
you must be happy to  get 6000 with no happy snappers around the only time I get 6000 is when I missed a gear   .             regards geoff

[geoffus]

   ;DNigel,
Sorry, read your tale of woe without looking too hard & thought you had troubles at 6000 rpm, but you say on deep reading that it is at 4000.
1.  Lift the needle a notch
2.  Go for a slide with less cutaway
3.  The anti-frothing baffle is a piece of thin aluminium in the top of the float bowl.
Regards, Geoff
PS  You will now notice the spelling is better as I am no longer typing this.

[VintageBike]

thanks for the update geoff.  I have a separate float bowl.  In fact I spent a while yesterday checking the fuel level.  It was a little low, so I have set it to the correct level (to the bottom of the circle on the blanking plug).  I was sure this was goling to fix it....but no.  It is now pulling even more willingly, but still the same problem.  Weather permitting, I'll try the choke idea tonight.  Also thought I'd change the fuel hose between the float chamber and the carb as it seems very old and stiff.  Thought it might be transmitting unnecesary vibration to the float chamber.  Perhaps a softer pipe may help.  I'll try the needle too.  As for the baffle, there is already an aluminium plate at the top of the bowl - I presume this is the baffle.  I am starting to doubt the mag now.  It was rebuilt last year, so should be OK, but I'm running out of ideas now!
N

[lsdt]

Nigel,
What throttle opening does the misfire start?
Dave.

[VintageBike]

Full throttle.  and only when held at full throttle for a sustained period.  Its about 4500rpm (70mph) in top.  It's a serious misfire and only picks up again when I back off the throttle a bit.  Same symptoms in third, but I believe I can accelerate through it.  I certainly can in 1st and 2nd.
N.

[willse]

Hi Nigel,
Sorry to see you still have problems . From what I've read you have pretty much covered the fuelling , so have a word with Tony Cooper m/cs about your mag . Does the problem seem worse as the engine warms up ? My Red Hunter had a similar fault some years ago , and was cured by a mag rebuild .
Best of luck ,
Will

[geoffus]

Nigel
Exactly what revs does this occur at?  Full throttle is not 4500 revs, which you should peak at about 7-7500 when you are in the red line.  If this is happening when you are ticking over in top at about 36-4000, do try a lower cutaway on the slide, as I have had problems with Gold Stars over the years with flat spots around the 40-50 mph mark.  This is 50/50 to take off more slide or another slide with less cutaway (I make my own slides).  If it is spitting back through the carburettor, throttle slide too high.  Guess you're using a no. 5 cutaway - try a no. 4.  However, I am beginning to believe your magneto may be breaking down.  In which case, you will be for ever chasing this problem.  Get it checked.
Regards, Geoff

[VintageBike]

Mmmm.  Just changed the fuel line between the float chamber and the carb and there does seem to be an improvement.  Not such a serious misfire now.  I'll have to recheck as it started to rain, but it seems to be leaning more towards a fuel problem.  I'm riding it around Mallory Park this weekend in the Festival of 1000 Bikes, so I'll be able to give it a good old run!  If anyone is there and they see/hear a ropey sounding (but nice looking!) Goldie struggling to reach 70mph - then give me a wave! I'm bike no 4100 riding in session 24 on Sunday morning at 9.40am.  
Will update afterwards.

N.

[lsdt]

Nigle,
If the problem is with the carb and I think it is, its the main jet only. Try a size down and see what happens, If its worse go up two sizes.
What type of inlet are you using,  filter or bell mouth if its a bell mouth has it a mesh?
Dave