backfiring & hesitation

HI All,

New member and a proud owner of a 74 Commando 850.

So I have recently replaced the old carbs with a set of Amal Premiers as well have replaced the headers ( a whole bunch of other stuff as well.)

I am getting some backfiring, just seems to be coming from the left side. as well as a slight hesitation.

I had a mechanic tune the carbs and it runs a bit better but still a bit of backfiring and hesitation.

Thought it was maybe the exhaust seal washer on both sides thinking it was a leak, replaced them on both sides.

Is it possible the valves need to be gapped (someone mentioned to me).

Looking for advise or suggestions on what might be a solution.

Thanks in advance for any help.

The valves do NEED to be set to the correct clearances.
Inlets 0.006" , exhausts 0.008"

Could still be a leak in the exhausts, check around the muffler area too.
If the pipe doesn't quite fit squarely into the exhaust port, you may not get a good enough seal.

Could just be swarf in a new carb too, pop the float bowl off and check for debris and blocked jets ??
Does altering the idle mixture screw change things, or have no affect.

Do you know what jetting are in the carbs, and what groove the needles are in?

I just tuned an 850 (a '74 MkII) with new Premiers and found that the air screws ended up about a tad less than one turn out. (260 mains, .106 needle jets, #3 1/2 slides, needle clips in the top most position).

Loose headers and tight valves can cause popping. If the motorcycle ran well before replacing the carbs and the headers, then the solution is in what you changed. How are your plugs?

Thanks all,

It did not run well before the new carbs, hence the change. the problems did seem to start when I put the new headers on. I did put the old headers back on, with new seals and it still seemed to backfire a bit. Gentleman who did tune the carbs for me suggested it may be the valves, in particular the exhaust valve. I am going to pull the covers off and see how the are set. Thanks will also check the float bowls. Appreciate the help. If all goes well will let you all know.

Hey Kislo-
Glad to hear there's a decent 850 in Calgary, my old home town.
If by "backfire", you are referring to a popping on "deceleration", I don't think that's a problem. Mine (same model/year) runs pretty good, and it always pops when I come off the gas.
I do it on purpose now, 'cause I like the sound. :twisted:
Definitely you must check the valve clearances. And if you re-torque the head, you have to do it again!
BTW, there have been some reports of brand new Premiers with loose parts inside...

Thanks. I imagine the riding season is a bit longer where you are now vs Calgary!! I am jealous!

It does pop on deceleration (honestly I like that as well!!!), but also a bit when it starts. I am going to check the valves.

A point I missed from before is that it did get slightly better, less popping or backfiring, when I adjusted the idle up as well as when I richened up the air
/fuel mixture.

popping on decelerations is a pretty sure sign of leanness in the idle and right off idle circuit

try first screwing in the air mixture screw on the suspect side a half turn

standard Premier mixture setting is 1 1/2 to two turns out from seated

That popping back through the carbs will also cause the throttle slides to break up. I suggest you fix it if that is what is happening, or ride the bike so it doesn't happen. I get the problem on mine every now and again because of lazy spark plugs, new plugs just seem to foul up just enough to cause the laziness. I have buggered two slides now as a result if this problem. Even Iridium plugs go lazy so I have gone back to standard plugs. Found a box of N7Y plugs which i will use up now. I tend to ride a lot and dont have enough time to spend on the bike trying to find out why this is happening. This problem of mine happends just above idle, on overrun and slow cruise, so i tend to throttle up or throttle of to stop it happening. That might help you diagnose the fault. It could be a slight leaness in one carb, but that is going to take time to sort out. As mentioned above, changing the position of the main jet needle is too big a change.
Incidentally, any good mechanic would check tappets and timing before he worked on carbs. When something needs fixing I do it straight away but this wee problem is going to take some microscopic adjustments of the offending carb, which might be the righ hand one ??????.
Dereck

kerinorton said:

That popping back through the carbs will also cause the throttle slides to break up. I suggest you fix it if that is what is happening, or ride the bike so it doesn't happen. I get the problem on mine every now and again because of lazy spark plugs, new plugs just seem to foul up just enough to cause the laziness. I have buggered two slides now as a result if this problem. Even Iridium plugs go lazy so I have gone back to standard plugs. Found a box of N7Y plugs which i will use up now. I tend to ride a lot and dont have enough time to spend on the bike trying to find out why this is happening. This problem of mine happends just above idle, on overrun and slow cruise, so i tend to throttle up or throttle of to stop it happening. That might help you diagnose the fault. It could be a slight leaness in one carb, but that is going to take time to sort out. As mentioned above, changing the position of the main jet needle is too big a change.
Incidentally, any good mechanic would check tappets and timing before he worked on carbs. When something needs fixing I do it straight away but this wee problem is going to take some microscopic adjustments of the offending carb, which might be the righ hand one ??????.
Dereck

Click to expand...

Sounds like a leaky manifold seal?

I would check for air leaks on the inlet side, in particular the side popping but no harm in both.

Popping out the exhaust on the down throttle is usually an exhaust leak at the exhaust sealing washers or sometimes a sticking auto advance unit if you still have a points ignition.

Hesitation on the early throttle is caused by mechanics who don't know shit about amal carburators...

This is my method to adjusting idle air screw position, and it's hard to explain... but I'll give it a shot...

When you set the idle air screw, usually there's a range of positions for it where the bike idles ok, but turning the screw in that range seems to do very little to the idle. In actuality, what I believe happens, is that the bike will idle similarly as the mixture changes when you turn the screw either way, up to a certain point when turned in the lean or rich direction. Then, once it's out of that workable mixture range the bike will begin to stumble.

The fine tuning of that idle screw position can't be set only using the bike's idle speed because the idle circuit also is in play during the early stages of lifting the throttle as well. SO,...

First, At an idle only, I try to find the idle screw position where the bike stumbles as I turn the air screw in. Then I do the same thing to find the position where the bike stumbles when I turn the idle air screw out. The position half way between them is the "starting position" for finer adjustment. I do this on both carbs. I was surprised at how little turning the idle air screw effected the engine when it was only idling, but how much turning the screw a tiny bits either way effected the way the bike transitioned from the idle circuit only to needle jet & idle circuit.

Then, I gentley lift and lower the throttle slightly as I micro-adjust the air screw positions to obtain the smoothest transition from the bike's idle speed to about 1800 rpms. This method of adjustment takes into account that the mixture is changing due to the needle jet interaction as you lift the throttle off of the idle stop screws where it's primarily running on only the idle circuit.

If you can't get a smooth transition from idle ~900 - 1000 rpms to 1800 rpms using this method, You may have more than a single issue besides carburation effecting your bike's performance. One of the things guys sometimes brag about is that they have a "One kick bike". Well if you retard a bike's timing it will start a lot easier and idle a lot better too,... (but it won't have the same top end power), so maybe retard your timing a few degrees and see if that helps you work out your carburation.

This is just my method. I'm sure someone will say I am either crazy, wrong, or both...

I will have to get my ultrasonic tester out. That can find air leaks or restrictions inside manifolds as well as out side. It can find blockages/restrictions in hydraulic pipes as well. If it is a manifold leak it will stick out like a sore thumb. I was lucky enough to keep it aside when I sold my Automotive business. A great tool which hasn't been used much by me since selling.
Dereck

If it is popping or backfiring on throttle roll off it is likley to be exhaust gas reigniting in the exhaust. This happens as the exhaust gas which is part unburnt fuel, (no oxygen left) is expelled into the pipe. If you have an air leak then oxygen is reintroduced, if the gas is still hot enough it will ignite and you get combustion and associated noise.

Hesitation as you accelerate is (possibly) due to not enough fuel. I have found that there appears to be a link between float level and needle position. A lot of advice out there on setting float levels. I have settled on 2 seconds pushing tickler to fuel flow. Make sure bike level on centre stand and both petcocks open with float bowls full if using this method. Then change needle position and see the effect, I am currently running on the bottom groove of 3. Experimentation is part of the fun and every bike is different.


Edited to correct Apple presumptive spelling

1up3down said:

popping on decelerations is a pretty sure sign of leanness in the idle and right off idle circuit

try first screwing in the air mixture screw on the suspect side a half turn

standard Premier mixture setting is 1 1/2 to two turns out from seated

Click to expand...

+1
Fatten up the idle mixture 1/4 turn to test.

When I tune my bike, I usually lean off the jetting until I get the popping back through the carbs and get hesitation, especially when I ride the bike and do gear changes up and down. Then I slightly richen the jetting. The guy who jetted your bike, might have done it when the air was warm. The bike will run leaner if the air is cold.
If you lift the needles by one notch, you might solve your problem. Most needle jets change in size by a thou of an inch as you go up and down in number. The needles have five positions for the clip. However a thou of an inch is a very big step for the needle jets when you are using petrol. Even when using methanol which needs double sized jets half a thou increase in needle jet size makes the difference between flying and sluggish.

Thanks to all for your help. It turned out to be a couple of things that fixed it, the slides we not set equally on the carbs and the most important thing was that it looks like I had some damage to the exhaust port on the left side, I needed to triple up the exhaust gaskets to tighten it up enough to stop the leak. Now it runs like a bat out of hell, no hesitation or backfiring!!!

Kislo said:

Thanks to all for your help. It turned out to be a couple of things that fixed it, the slides we not set equally on the carbs and the most important thing was that it looks like I had some damage to the exhaust port on the left side, I needed to triple up the exhaust gaskets to tighten it up enough to stop the leak. Now it runs like a bat out of hell, no hesitation or backfiring!!!

Click to expand...

It takes a lot of experience to know how to troubleshoot amals. It's kind of a lost art. If you don't have all the right parts set up correctly for your application, turning the adjustment screws isn't going to do much to help you. Personally, I am used to them after 40 years of owning my commando, but I can understand why people switch to a single mikuni for simplicity's sake. I'm much too stuborn for that. 8)

If you aren't going to run your bike regularly, drain the fuel out of the carbs so it doesn't evaporate and leave behind a gummy mess to clog your ports...

The popping in the exhaust on the down throttle happens in all kinds of vehicles that have air leaks in their exhaust manifolds. You must have heard cars driving along on the gas sounding normal and then when the driver lifts off the gas, the car goes, "pop, pop, pop pop...". His exhaust gas is reigniting in the manifold because of his air leak... just like your bike.

Don't be surprised if you have to tighten your exhaust nuts again after a few heat cycles when the exhaust washers compress a bit. An exhaust nut wrench is a good tool to own. Congradulations on getting your bike going well enough to get some riding in. It makes the battle of learning about these old bikes a rewarding endevor.