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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.
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Has it ever run well with you?

If you remove the main jet cover from the bottom of the carburettor and open the petrol tap, what volume of fuel flows out in a minute?


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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.


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Originally Posted by triton thrasher
Has it ever run well with you?

If you remove the main jet cover from the bottom of the carburettor and open the petrol tap, what volume of fuel flows out in a minute?
It has never run over about 60 without misfiring, I swapped the old Concentric for new Monobloc and it does the same. I’ll put the 160 jet in and see how much fuel comes through later and report back. Cheers.

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RichT Offline OP
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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

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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.


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Originally Posted by 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.
I’ve changed the plugs and marked the throttle, the problem is at 3/4 to full throttle, hence me thinking main jet. The carb is brand new and specced for the bike. I’ll check the full flow later today.

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Originally Posted by triton thrasher
Has it ever run well with you?

If you remove the main jet cover from the bottom of the carburettor and open the petrol tap, what volume of fuel flows out in a minute?
Well I put the 160 main jet back in and fuel flow was good with just over 1/2 pint in a minute! What to try next?

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Some more history then, how long have you owned the bike, when you bought it what work had been allegedly been done on it recently.

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Originally Posted by kommando
Some more history then, how long have you owned the bike, when you bought it what work had been allegedly been done on it recently.
I got it running about 2 years ago, after a re-bore, valves and guides, pazon fitted. It had a worn Concentric on it and I was hoping the new Monobloc would sort the problems.

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Cam timing needs looking at, some Pazons came with the trigger wires reversed internally so the advance function is disabled so check the ignition timing advances with revs, if not reverse the 2 trigger wires from the rotor at the Pazon box connections. Compression check and leak test would be next.

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Originally Posted by kommando
Cam timing needs looking at, some Pazons came with the trigger wires reversed internally so the advance function is disabled so check the ignition timing advances with revs, if not reverse the 2 trigger wires from the rotor at the Pazon box connections. Compression check and leak test would be next.
I strobed the ignition and advance works fine, I don’t have a compression tested so not done that, what do you mean by leak test?

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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.


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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 ?

Secondly
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."

Further
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.


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I’ll try to answer all the questions above:-
The Carb is brand new and came with stayup float and viton tipped fuel valve specced for 5ta. As I understand it Monobloc carbs don’t suffer the same problem as Concentric regarding fuel float height. It also came with AMAL’s pretty comprehensive setting up instructions. The build quality I’d say is excellent compared to the Concentric it replaced.
The ht cables are copper wire and the plug caps are resister caps, I checked with Pazon and that’s what they recommend with their system. Pistons are standard compression.
I fitted two 6 volt coils in series and had the re-bore done myself, so I’m not relying on hearsay.
I strobed the ignition with the primary chain case removed, bit of a messy process!

I’ve put the 160 main jet back in now I’ve raised the needle to 4th groove from top, not had a chance to try it yet.
It has been starting and running great until about 55/60mph then it misfires on both cylinders and with a bit more throttle it dies completely and cuts back in when I reduce the throttle. I was hoping the new carb would solve the problem as the old Concentric was quite worn.
I’ve also fitted a new battery and I have electronic Smiths dials which stay on so I’m pretty sure the pazon is getting good voltage all the time.

Perhaps I’m chasing my tail, obviously not looking in the right place.
I appreciate all the comments, I will do a compression test when I get a tester.

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Any carb will suffer with fuel height problems. The monoblock is just easier to adjust, the pip on the float chamber door is the fuel height point.

Also if you have raised the needle you will have made it richer, think if it this way. If your lifting the tapered needle further out of the needle
Jet it will be richer at a lower throttle opening. If you lower it, it will take more throttle opening until the needle reaches that same
Point on the taper.

When you follow the anal carb tuning instructions. It lists the order as follows

Set the idle mixture screw
Verify/Adjust the main jet
Verify/Adjust the needle/needle jet
Verify/Adjust the slide
Verify/adjust the idle mixture screw.

The float level has an impact across the complete range and should be verified first. Don’t take anything to chance.

The main jet can only be verified at 3/4-full throttle. In 3rd up a hill or preferably 4th gear. Back in the days when my local motorway section was quiet I’d do the 10 mile route to warm the engine up then blast down for 5 miles to the next junction at full throttle, I’d set the carb from there. Now the motorways and faster roads are getting busier it is getting harder to set a carb up properly. Until you get that main jet spot you’ll be peeing in the dark. You might have to drop the needle a touch until you can clearly adjust the main jet. Working with non standard setups I’ve found that a rich mid range can foul the plug enough to have you struggling to clean it at WOT.


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I can see the previous posters have covered just about everything.

My 650 has a tdc slot in the crankshaft. How did you verify the piston position on your 5TA for marking the rotor?

You can check the float height by removing the pilot jet cover and fixing a tight fitting tube to the jet and curving it up next to the float bowl. The level in the tube should coincide with the dot on the float bowl cover.

Monobloc carbs are so simple that just fitting it with all the right jets, correct needle position and air filter the bike should just run well. The carb can be very worn and still run ok on the road but would just have idling issues.

My bike does not have very good fuel flow and with one tap open it wouldn't run properly at wide throttle. Opening the other as well cured this.

Dave

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Originally Posted by dave jones
I can see the previous posters have covered just about everything.

My 650 has a tdc slot in the crankshaft. How did you verify the piston position on your 5TA for marking the rotor?

You can check the float height by removing the pilot jet cover and fixing a tight fitting tube to the jet and curving it up next to the float bowl. The level in the tube should coincide with the dot on the float bowl cover.

Monobloc carbs are so simple that just fitting it with all the right jets, correct needle position and air filter the bike should just run well. The carb can be very worn and still run ok on the road but would just have idling issues.

My bike does not have very good fuel flow and with one tap open it wouldn't run properly at wide throttle. Opening the other as well cured this.

Dave

5TA also has a tdc hole in the crank. I’ll check the float height as you suggest, but I can’t see how it could be far out. I need to check the compression and I’ve not checked cam timing marks, fuel flow is good so it’s looking less likely a carb problem?

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You can pin down if it is carb related or not, that it occurs at the same 55 to 60 mph could mean its electrical or mechanical not carb related. If at 40 mph you open the throttle fully do you get the same symptoms or does it carry on ok until 55mph and then the symptoms start. If it does it at 40 mph with a full throttle then its carb, if it does not play up until 55 mph with the throttle full open at lower speeds then its electrical or mechanical.

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Originally Posted by kommando
You can pin down if it is carb related or not, that it occurs at the same 55 to 60 mph could mean its electrical or mechanical not carb related. If at 40 mph you open the throttle fully do you get the same symptoms or does it carry on ok until 55mph and then the symptoms start. If it does it at 40 mph with a full throttle then its carb, if it does not play up until 55 mph with the throttle full open at lower speeds then its electrical or mechanical.
Cheers I’ll try that.

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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


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Originally Posted by kommando
You can pin down if it is carb related or not, that it occurs at the same 55 to 60 mph could mean its electrical or mechanical not carb related. If at 40 mph you open the throttle fully do you get the same symptoms or does it carry on ok until 55mph and then the symptoms start. If it does it at 40 mph with a full throttle then its carb, if it does not play up until 55 mph with the throttle full open at lower speeds then its electrical or mechanical.

BINGO! It did it at 40-45 and even at 30 going up hill, so larger main jet? What do you reckon?

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What throttle position are you in at that time?

If your at 3/4 to full throttle to do 30mph up a hill then you have many more problems than carburation. That’s worse than a D1 bantam.


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Originally Posted by RichT
Originally Posted by kommando
You can pin down if it is carb related or not, that it occurs at the same 55 to 60 mph could mean its electrical or mechanical not carb related. If at 40 mph you open the throttle fully do you get the same symptoms or does it carry on ok until 55mph and then the symptoms start. If it does it at 40 mph with a full throttle then its carb, if it does not play up until 55 mph with the throttle full open at lower speeds then its electrical or mechanical.

BINGO! It did it at 40-45 and even at 30 going up hill, so larger main jet? What do you reckon?

As long as it was a true full throttle then it just tells you your main jet is wrong, could be too small, could be too large.

If its too large you get what's called 8 stroking where is fires intermittently like a misfire, if its too weak the engine goes flat and lacks power.

To find out what 8 stroking feels like remove the main jet and leave the main jet position empty, the bike will start and run up to 3/4 throttle and then past 3/4 it will 8 stroke. If that feels the same as your misfire then you need a smaller main jet.

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Cheers Kommando

I’ll try that tomorrow if I get time, I’ve already tried smaller jets so I think I need larger.

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