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Allan G, Gordon Gray, Jon W. Whitley, kommando
Total Likes: 6
Original Post (Thread Starter)
#853478 07/11/2021 12:10 PM
by RichT
Hi. My first post so here goes.
I’m trying to tune my carb, at the moment after each ride the plugs are black. I’ve fitted a new monobloc carb.
Bike starts ticks over and runs great until I give it full throttle, then it misfires and if I open up further it cuts out completely. Same throttle opening in each gear. I’ve reduced the main jet size down from 160 to 140 and it still cuts out. I have a 130 jet still to try.
As I say the bike starts and ticks over great at about 1000rpm when cold and little faster when hot, it pulls great through the gears and from standstill. I have lowered the needle one notch so it’s now on 2nd from top. Standard air filter is fitted and choke which is all the way up.
I’ve checked the tappets and strobed the ignition which is Pazon with two 6 volt coils.

Any help with ideas appreciated.
Liked Replies
by Allan G
Allan G
Originally Posted by RichT
Originally Posted by Allan G
Sounds like a lean mixture at full throttle, if it was rich it would 8 stroke, it would still run ok and not cut out. If it was too lean it may cut out and going leaner will only risk you holing a piston or seizing the motor.

Because the plugs are so black I’ve been assuming it’s running rich, thanks for that I’ll put the 160jet back in and test for fuel flow as above. Cheers

What makes you think the issue is the main jet? the main jet is only effective from half throttle, having a limited effect to full effect at wide open throttle.

You need to fit some new plugs, mark off the throttle in 1/4 incriments and go for a ride, note which throttle position is the first point where it doesn't want to pull cleanly.

Your problem could be anything from having the wrong slide, to having a worn or incorrectly sized needle jet.
1 member likes this
by kommando
If it fails the compression test ie one side is lower than the other by 10% then to find where the compression is being lost you perform a leak test.

1 member likes this
by RF Whatley
RF Whatley
First Question
Have you read and fully followed the instructions at the top of this forum entitled: Tunning Your Amal Carb ??? If not, why not ?

If you read that instruction you'll see that NO carb tuning can be done until every other bit of tuning has been completed and verified. That means ignition, valves, compression, spark plug wire test, spark plugs, new fuel, etc, etc. To this end, often times HUGE assumptions are made without any supporting facts. One of these is to ASSUME that since a Pazon ignition was fitted, that the ignition timing was properly set and verified. Other new owners have made basic mistakes like fitting automotive spark plug wires, when the 5TA requires metal core wires. It is also common to see resistor type plug caps and spark plugs, when the stock 5TA called for non-resistor.

Also worthy of note is the fact you have altered the engine with new pistons and valves. According to which pistons were fitted, the timing specs may have changed from 5TA to T100 specs. All this has to be checked out.

I see where you attempted to strobe time the machine. May I ask how that was done? The 5TA (at least the models we got here in the States) were all pre-strobe window. Therefore, the bolted-on degree wheel has to be verified before and after timing. I'd like to know how the verification was done.

So as you can see, "The devil is in the details."

You have not stated where the carb you fitted was a brand new unit, or a good used unit. The manufacture date of the carb is of great importance. The NEW Amal carb company had a LOT of quality issues, and I generally need to rebuild these carbs right out of the box. This includes fitting the viton-tipped Concentric float needle and checking the float level before fitting. Then when fitting, the age-old issue of sealing is always present.

All this to say things are NOT straight forward. Double checking and verification are the watch-words. The simplest things can (and will) trip you up.

All the best.
1 member likes this
by RF Whatley
RF Whatley
To continue...
  • There's no mention of the plug age, brand or number
  • You mention the TDC crank locating hole, but did not say how you got from there to a strobe timing mark, which we assume you added yourself since the entire cover had to be taken off.
  • Assuming you used the marks on the alternator rotor to strobe, but did you verify the health of the rotor itself?

The devil is in the details and it can be maddening. But just think of all the things you know it's NOT ! laughing
1 member likes this
by kommando
Well you know know what 8 stroking is, it kicks in when the main jet is too large by 2 or more sizes.

I am just concerned you seem to mix up or do not quote the exact throttle openings.

eg from post 1
until I give it full throttle, then it misfires and if I open up further it cuts out

If you are at full throttle then you cannot open it more.

I opened the throttle and it eight stroked, this was different to the misfire I got with the jet in, but as I opened the throttle further the engine cut completely

Again you should be quoting 1/4 1/2 3/4 or full

The amal carb is really 4 carbs in one, a pilot circuit, throttle slide cutaway, needle and needle jet, main jet. Which one is the issue is determined by the throttle opening, so mark it in 1/4's and report your findings quoting which 1/4 zone the symptoms are.
1 member likes this
by kommando
So if we take it the misfire is weakness then go larger on the main jet, I test on successively larger jets on a weak mixture in jumps of 2 sizes as too rich is not an issue in the way a weak mixture will cause a seizure, when it starts to 8 stroke then go back down 1 or 2 sizes and you will not be far away from the correct main jet mixture. Your next step is to set the needle position based on how well the bike pulls between1/4 to 3/4 throttle.
1 member likes this
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