1971 Fastback 750: the right exhaust valve dont close well?

pierodn · Oct 24, 2015

Hi All.
At beginning of septmember i started first time my rebuilt bike.
With the bike at the minimum idle for 2/3 minutes the only right exhaust pipe (stainless stell), on the curve of the tube after the collar, became red red fire!.
But Riding over rpm idle the bike worked fine (no fire to the pipe again).
Today, after have changed the pipes with stock one rechromed, only the right pipe again, at the minimum of the idle, became yellow "metalflake" (the chrome super heated)!
The bike rode well over the idle (no more super heated).
It happens at the minimum idle and only when the bike is started from cold!.
I think the right exhaust valve dont close well (clearance is 0,20/.008 like x manual).
Or the problem could be the carburation? In fact the right spark plug is always very very thin (at the idle and after a fast riding!).
What do you think about?.
What do you think about?.
Thanks.
Piero

comnoz · Oct 24, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

The most common reason for a glowing exhaust pipe is slow ignition timing on that cylinder. Jim

pierodn · Oct 24, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

comnoz said:
The most common reason for a glowing exhaust pipe is slow ignition timing on that cylinder. Jim

Hi Jim,
Thank you.
Please, could you kindly explain me what means "slow", sorry, but i lost me in translation.
What should I do to adjust the slow ignition cylinder?
Ciao
Piero

comnoz · Oct 24, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

pierodn said:
comnoz said:

The most common reason for a glowing exhaust pipe is slow ignition timing on that cylinder. Jim

Click to expand...

Hi Jim,
Thank you.
Please, could you kindly explain me what means "slow", sorry, but i am lost in translation.
What should I do to adjust the slow ignition cylinder?
Ciao
Piero

I mean the ignition timing is late. What kind of ignition do You have? Jim

pierodn · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

comnoz said:
pierodn said:

comnoz said:

The most common reason for a glowing exhaust pipe is slow ignition timing on that cylinder. Jim

Click to expand...

Hi Jim,
Thank you.
Please, could you kindly explain me what means "slow", sorry, but i am lost in translation.
What should I do to adjust the slow ignition cylinder?
Ciao
Piero

Click to expand...

I mean the ignition timing is late. What kind of ignition do You have? Jim

Hi Jim,
Thank you.
I use a new Pazon Sure Fire setted to 31 BTDC with a degree disc that coincides perfectly with the ignition timing degree scale on triplex cover.
I didnt used the flash light to 4/5000 rpm.
A question please, is possible that the two timing, right and left are different? In case what i should to do?.
The bike starts at first kick and ride fine and fast, but the right spark plug (that has fire) is very very wet, like new!
Ciao
Piero

comnoz · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

It is possible for the timing to be off on one cylinder if the ignition pickup is not centered on the end of the cam. I would suggest checking it with a timing strobe light.
It is also possible that the problem could be caused by a lean mixture or intake air leak. Jim

pierodn said:
Hi Jim,
Thank you.
I use a new Pazon Sure Fire setted to 31 BTDC with a degree disc that coincides perfectly with the ignition timing degree scale on triplex cover.
I didnt used the flash light to 4/5000 rpm.
A question please, is possible that the two timing, right and left are different? In case what i should to do?.
The bike starts at first kick and ride fine and fast, but the right spark plug (that has fire) is very very wet, like new!
Ciao
Piero

Peter R · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

As Jim said, a lean mixture or an airleak at the carburator side can also cause the problem that you describe.
Are you sure the idle jet is not clogged ? It may be clogged, because yo say that the engine runs well at higher rpm.
Also make sure to check the inition timing on both cylinders with the strobe.

pierodn · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

Peter R said:
As Jim said, a lean mixture or an airleak at the carburator side can also cause the problem that you describe.
Are you sure the idle jet is not clogged ? It may be clogged, because yo say that the engine runs well at higher rpm.
Also make sure to check the inition timing on both cylinders with the strobe.

Hi Peter,
Thank you.
But dont is almost impossible for the left and right cylinders to fire at different times with electronic ignition, as both plugs are linked and both fire even when one is on the exhaust stroke?.
Airleak: where i should check?
Idle jet: main jet 220, needle jet 106, jet needle with the clip at the central notch, all new!
I fear for the valves (Jim says intake air leak, i thought the exhaust dont close well).
Ciao
Piero

o0norton0o · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

the fact that the bike rides well "over idle" speeds and the problem is confined to one particular cylinder, tends to point to the fuel delivery as the problem since it's a dual carb set up.

You could have an air leak on your intake manifold which you could test by starting the bike up and spraying carb cleaner around the intake manifold to see if the bike stumbles. If spraying the intake manifold has no effect, you probably don't have an air leak...

You could also have a clogged idle jet or clogged idle chamber holes but I'm not sure why that would make your exhaust pipe hot. If anything, it would make your exhaust pipe cold at an idle because that cylinder won't be getting fuel.

Peter R · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

Piero, what Jim means with airleak is leakage between the carburator and the cylinder head. Air leakage here causes lean air/fuel mixture with overheating as result.
I assume that you have set the clearance of the inlet and exhaust valves correct, so I do not expect that that that the valves are causing your problem.

pierodn · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

o0norton0o said:
the fact that the bike rides well "over idle" speeds and the problem is confined to one particular cylinder, tends to point to the fuel delivery as the problem since it's a dual carb set up.

You could have an air leak on your intake manifold which you could test by starting the bike up and spraying carb cleaner around the intake manifold to see if the bike stumbles. If spraying the intake manifold has no effect, you probably don't have an air leak...

You could also have a clogged idle jet or clogged idle chamber holes but I'm not sure why that would make your exhaust pipe hot. If anything, it would make your exhaust pipe cold at an idle because that cylinder won't be getting fuel.

Hi and thank you.
Please what you mean for "idle jet" and "chamber holes".
Ciso
Piero

pierodn · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

Peter R said:
Piero, what Jim means with airleak is leakage between the carburator and the cylinder head. Air leakage here causes lean air/fuel mixture with overheating as result.
I assume that you have set the clearance of the inlet and exhaust valves correct, so I do not expect that that that the valves are causing your problem.

Hi Peter,
thank you.
Sure, there is a lean air/fuel mixture because the plug is very very clean like new!
But there is not air leak from the head/manifold/carburator.
The problem is the lean air/fuel mixture, but why?
The valve have the right clearance (.06/.08) but maybe dont clode eell?.
Ciao
Piero

Peter R · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

I would suggest to check if the idle jet is open. This idle jet is difficult to see, you have to remove the idle air scew, and than you can see the idle jet inside. It has a very small hole, use a small drill (0.3mm) and twist this drill by hand into the idle jet to make sure it is open.

o0norton0o · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

Below, you see a schematic of the amal carb passages. Behind your idle mixture screw is the "mixing chamber" which is shown in yellow highlight. It has 2 tiny openings which feed fuel to the carb's intake at idle. They have to be clear of any "crap" to feed the proper amount of fuel for your bike to idle well. Behind the "Mixing chamber" is the "pilot (or idle) jet". That's the orange highlighted part of the drawing below. IF that jet clogs up, your bike will not idle. Do NOT poke it with a toothpick or you will clog it up. Most people use a guitar string or a sewing needle to poke it to make sure it's not clogged.

1971 Fastback 750: the right exhaust valve dont close well?

The image below shows the body of the concentric carb with the main jet in the center and the 2 tiny holes that come from the mixing chamber into the body of the carb's intake. These holes must be free of any blockage for your bike to idle well.

pete.v · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

Also, make sure the yellow/black and the white/black pickup wires are not reversed on the pickup plate connection block. IT MATTERS!

If the are, switch them, restrobe the timing and give it another try.

pierodn · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

o0norton0o said:
Below, you see a schematic of the amal carb passages. Behind your idle mixture screw is the "mixing chamber" which is shown in yellow highlight. It has 2 tiny openings which feed fuel to the carb's intake at idle. They have to be clear of any "crap" to feed the proper amount of fuel for your bike to idle well. Behind the "Mixing chamber" is the "pilot (or idle) jet". That's the orange highlighted part of the drawing below. IF that jet clogs up, your bike will not idle. Do NOT poke it with a toothpick or you will clog it up. Most people use a guitar string or a sewing needle to poke it to make sure it's not clogged.

The image below shows the body of the concentric carb with the main jet in the center and the 2 tiny holes that come from the mixing chamber into the body of the carb's intake. These holes must be free of any blockage for your bike to idle well.

Hi.
I am sorry but the idle circuit is well clean.
I have used a guitar string and the carb cleaner go out from all the holes!
The bike has a good idle!!!!!
Thank you.
Piero

pierodn · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

pete.v said:
Also, make sure the yellow/black and the white/black pickup wires are not reversed on the pickup plate connection block.

If the are, switch them, restrobe the timing and give it another try.

Hi Pete,
I mean.
The pick up wires are well connected.
The bike starts at first kick and ride fast and well!
The plug is very wet!
Thank you.
Piero

Rohan · Oct 25, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

Have you checked and strobe timed the ignition for EACH cylinder,
including that the spark is advancing and retarding correctly ?

Each exhaust valve still has correct tappet clearance ?

No air leaks into the carby or manifold ??

Synchronised the carbs. ?

If one pipe is glowing red hot, it has to be one of the above.

Brooking 850 · Oct 27, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

Make sure the you crank shaft is not out of phase, by this I mean if the crankshaft was apart recently was it bolted up correctly so the big end journals are in exactly the same position either side of the flywheel, I found this recently on a crankshaft that I sent to have ground, we found it was 0.015" out of phase , which means you can never get the timing right for the opposite cylinder, no matter which side you check your timing for.
Just a thought
Regards Mike

pierodn · Oct 27, 2015

Re: 1971 Fastback 750: the right exhaust valve dont close we

Brooking 850 said:
Make sure the you crank shaft is not out of phase, by this I mean if the crankshaft was apart recently was it bolted up correctly so the big end journals are in exactly the same position either side of the flywheel, I found this recently on a crankshaft that I sent to have ground, we found it was 0.015" out of phase , which means you can never get the timing right for the opposite cylinder, no matter which side you check your timing for.
Just a thought
Regards Mike

Hi Mike,
Thank you.
Could you please explain to me what does it means "out of phase"?.
Cio
Piero

1971 Fastback 750: the right exhaust valve dont close well?

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