New rebuild problems

[admin]

I got my WNG engined Anstey linked finally registered and have been road-testing it today.

I put a new concentric 626 on it as it came without a carb. I put a 180 main jet and a 25 pilot in (original was a 150 main on it's pre-monobloc)
The idle is good and the mixture screw does what it should.
It's got a magneto that sparks ok ( or seems to). timing is about right.

Bike storms from idle but seems to misfire when opened up.
I am thinking too rich a main jet as opposed to ignition ? (Closing the air slide didn't help any; which is why I thinking too rich a jet )
Do concentrics run a leaner or richer jet vs pre-monoblocs ? I didn't have a leaner main jet to try .. (the richer ones I had didn't help any ... )

[keith.mettam]

Hi John,
The main jet does not normally affect the mixture below 3/4 throttle opening. The area you appear to be describing has its mixture controlled by the needle jet and needle position i.e. you are off the pilot jet, slide cut-away and onto the needle jet. Are you running any form of air filter as this will affect the settings ? Does the new carb's bore match the inlet tract correctly (don't really want the internal step a smaller inlet tract bore would give making good carburetion even more difficult to achieve) ? If it is any help to you 1955-58 NH's standard set up (with Burgess filter, which has virtually no restrictive effect at all) is :- 1" Monobloc carb 25 pilot jet, 106 needle jet, needle clip position 3 and 200 main jet, 3.5 slide. From memory Mk1 Concentrics have the exactly the same type of main jet as the former Monobloc carbs. I am guessing that you will have played with the ignition advance/retard lever whilst riding the bike in an effort to narrow the problem down.
From what you have said I wouldn't have thought you were too far off a reasonable setting. If you run a filter, try it without. Whip the carb off, strip it down and carefully check everything, particularly the needle jet etc. Even new carbs can have manufacturing faults ! My new carb had a 30 pilot jet instead of the 25 I requested (human error in this instance). Double check the ignition timing and state of the points as might still be an ignition issue. Any clues from looking at the spark plug ?
All the best with it,
Keith Mettam.

[nevhunter]

Check also weak valve springs or sticking valve guide, and I suggest that you remove the points assemlly from the magneto and strip the assembly completely and assemble it with particular care with the points spring tension, and the fibre push rod not worn in the diemetal body or sticking in any way. Nev

[john.nash]

To be fair, it's pretty much 3/4" to wide open (i.e. around 50mph) that it starts with the problems.
I have gleaned that concentric jetting is around 20% less than monoblocs; so I should really be starting around a 160 ...
I'l buy a few more jets around there and see ..

otherwise ..........as Nev says it's a magneto check ..........

The engine is a brand new build. The valves weren't fitted and needed a regrind. Guides were ok. I bought some new springs and keepers.

[brenton.roy]

Hi John, congratulations for getting it running and registered.
Your jetting may well be out, but the symptoms you have don't point to too rich a mixture. Unless it's really rich, it will more make the engine feel sluggish than cause a misfire.
A lag rather than a stutter.
You could drop the needle a bit to see if it improves at the problem throttle opening.
Is it doing this just up the road, or from a cruising speed? - if the latter, it could be starving for fuel. Your float level could be a little low?
it might also be worth checking flow from the tap with the cap on (vent).
Nev's probably on to something with spark issues. If the bike is newly built, a bit of crap on the points, mag pickup, HT lead ends, plug cap could all lead to a weak spark under load.
Just out of interest, you could try a different spark plug.
Brenton

[john.nash]

I tried a couple of plugs. Same result

This is just "up and down the road". Maybe a half mile of flat, into a forest track, turn round and back home. I live on the edge of the forest so the road is quiet and there are no houses, that way (just as well).

Hmm float level .. the concentric does sit at an angle; I looked at that and wondered.
It looks like the original has an angled float chamber ? hmmm .. I wonder ...

[dave.owen]

John , it may be that the carb is just too big, the WNG ran a smaller carb ,so you may be over fuelling the engine at wide openings of the carb.

rejetting to the same as s

[john.nash]

160 main jet has improved things

still missing/fluffing when on a steady throttle at low openings - maybe the slide.

Been down to clubnight and it's actually quite good thing.
Much faster than my enfield 350 ....

[dave.owen]

John,

have you got any leaks on the maniflod or exhaust

Dave

[martin.casbon]

good thinking dave,my nh ran so much better with a tiny smear of silicone on both sides of the carb gasket,and easier to set the mixture.