750 misfires and cuts out

It could well be fuel starvation.

Hi Neil,

I have just got my renovated 850 on the road and suffers from a similar problem to what you are having. It will start misfiring then going onto one cylinder, if I ease the throttle off the fault will go away until I start opening the throttle hard again. The fault is more pronounced when the throttle is opened hard for longer periods, say on a long stretch of road. I found opening the reserve tap seemed to cure the fault. Ensuring the tank was full the same fault recurred, again cured when the reserve tap was activated. When I got home I emptied the carb float chambers and checked the petrol was clean, it was. Next I opened the main tap with the drain plug out of the R/H carb and checked the flow of petrol into a container, there was flow there but it was slow, I then did the same for the L/H carb and here it was just dripping out. I then did the same with the reserve tap and found there was a good flow out of both carbs. The inside of the tank is clean with no residue on the main tap filter. Both taps were renewed during the rebuild and initially tested OK so I can only conclude that the main tap has become faulty, so at the moment I am running with both taps on and the fault have not recured. It looks like I will have to buy a new main tap, this time from a different supplier, I think!

I would suggest you check your petrol tap in a similar manner as above.

Hope this is of help,

Cheers, Paul.

PS :- I cant see or find a gallery on this website, where is it? I think it would be a good idea to have a gallery so members can post their bikes on it.

Hi Paul

We've very carefully hidden the Member's Bikes Gallery under the 'Membership' link - When one day it's all grown up, we may give it it's own button!

To upload a picture of your bike, first go to your Dashboard (click on your name inthe top right of your screen).

Then click on 'Membership Details'.- (You can alter these if we've got them wrong?)

Then click on the 'Click Here' (To add and edit your Bike details) Link at the top.

Not as intuitive as we would like but - We're working on it?

Best regards

Peter

Previously wrote:

PS :- I cant see or find a gallery on this website, where is it? I think it would be a good idea to have a gallery so members can post their bikes on it.

Previously wrote:

Hi Neil,

I have just got my renovated 850 on the road and suffers from a similar problem to what you are having. It will start misfiring then going onto one cylinder, if I ease the throttle off the fault will go away until I start opening the throttle hard again. The fault is more pronounced when the throttle is opened hard for longer periods, say on a long stretch of road. I found opening the reserve tap seemed to cure the fault. Ensuring the tank was full the same fault recurred, again cured when the reserve tap was activated. When I got home I emptied the carb float chambers and checked the petrol was clean, it was. Next I opened the main tap with the drain plug out of the R/H carb and checked the flow of petrol into a container, there was flow there but it was slow, I then did the same for the L/H carb and here it was just dripping out. I then did the same with the reserve tap and found there was a good flow out of both carbs. The inside of the tank is clean with no residue on the main tap filter. Both taps were renewed during the rebuild and initially tested OK so I can only conclude that the main tap has become faulty, so at the moment I am running with both taps on and the fault have not recured. It looks like I will have to buy a new main tap, this time from a different supplier, I think!

I would suggest you check your petrol tap in a similar manner as above.

Hope this is of help,

Cheers, Paul.

PS :- I cant see or find a gallery on this website, where is it? I think it would be a good idea to have a gallery so members can post their bikes on it.

Paul

I too had a faulty main tap recently and bought a replacement from RGM. After a couple of weeks the flow reduced to a trickle. I returned it and RGM sent a new one, but it had the same problem. I tried a third one from RGM and it too was faulty. Seems there is a bad batch of taps in circulation where the rubber seal inside seems to move out of position and block the flow. I have now found a different type of tap from L P Williams and so far it seems OK!

David

Peter,

Thanks for your info on the members gallery, I have just found it following your links and have uploaded 2 photos of my restored Commando with Thomas Telford's famous aquaduct at Trevor, North Wales in the background.

Thanks again, Paul.

Previously wrote:

Previously wrote:

Hi Neil,

I have just got my renovated 850 on the road and suffers from a similar problem to what you are having. It will start misfiring then going onto one cylinder, if I ease the throttle off the fault will go away until I start opening the throttle hard again. The fault is more pronounced when the throttle is opened hard for longer periods, say on a long stretch of road. I found opening the reserve tap seemed to cure the fault. Ensuring the tank was full the same fault recurred, again cured when the reserve tap was activated. When I got home I emptied the carb float chambers and checked the petrol was clean, it was. Next I opened the main tap with the drain plug out of the R/H carb and checked the flow of petrol into a container, there was flow there but it was slow, I then did the same for the L/H carb and here it was just dripping out. I then did the same with the reserve tap and found there was a good flow out of both carbs. The inside of the tank is clean with no residue on the main tap filter. Both taps were renewed during the rebuild and initially tested OK so I can only conclude that the main tap has become faulty, so at the moment I am running with both taps on and the fault have not recured. It looks like I will have to buy a new main tap, this time from a different supplier, I think!

I would suggest you check your petrol tap in a similar manner as above.

Hope this is of help,

Cheers, Paul.

PS :- I cant see or find a gallery on this website, where is it? I think it would be a good idea to have a gallery so members can post their bikes on it.

Paul

I too had a faulty main tap recently and bought a replacement from RGM. After a couple of weeks the flow reduced to a trickle. I returned it and RGM sent a new one, but it had the same problem. I tried a third one from RGM and it too was faulty. Seems there is a bad batch of taps in circulation where the rubber seal inside seems to move out of position and block the flow. I have now found a different type of tap from L P Williams and so far it seems OK!

David

As a footnote to the above; I heard from RGM today and they say the tap supplier rep reports that "fuel additive" (ethanol, I presume)is to blame and that they are working on the problem.

Hi David,

Although RGM is my preferred supplier, I in fact got the fuel taps from Norvil about 3 or 4 years ago ( Yes it really has taken me that long to finish! ), so whether both companies use the same supplier I don't know. I don't think the 5% ethanol currently in today's petrol should have such effect on the tap seals in such a short space of time ( about 1 month in my case ).

I have a fibre glass fuel tank and despite some of the horror stories I have read, so far this is not suffering any ill effects from the ethanol additive, although I am keeping a constant eye on it, the petrol is clean with no signs of contamination, the inside of the tank is still hard and shiny ( so Far! ). I decided to use the fibre glass tank because second hand steel fuel tanks on Ebay go for about £500 and there is no way I would pay that sort of money for something rusty and leaking. Surprisingly new tanks don't seem to be available considering virtually everything else for a Commando is ( even dodgy fuel taps! ). Maybe there is a opportunity for somebody here to fabricate replacement tanks.

Cheers, Paul.

Previously wrote:

I have one other bike, a 650 unit T'bird that has been converted to 12V with Mk III Boyer, solid-state rectifier and normal Zener and I used to have to run that with the ignition switch in position 2 (ignition & lights) to stop it cutting out. A friend has suggested this would be employing more of the coils in the alternator, boosting the charge tothe levelthat the Boyer demands?? This bike has it's original 376 Monobloc that is in good condition, resleeved and has an air filter.

I could live with riding with the lights on but this dodge does not work on the Commando. As spring is here any help ASAP would be gratefully received.

Neil

In reply-to the T bird problem, if you have done the conversion to 12V properly there should not be any switching of alternator coils left. The alternator should be connected for full output (GRN/BLK connected to GRN/YELLOW) It is possible that you have too high a voltage (Zener wrong voltage or disconnected?)and putting the lights on can cure a high voltage missfire. Another way to check this is to disconnect the output of the rectifier and go for a few miles. If the battery is reasonable well charged you can do 50 miles plus.In this situation any battery charging gremlins will then not be seen.

Hope this helps

Al Osborn.

If the problem lay with the carbs then I'd expect it to go onto one cylinder unless you were unlucky enough to have the same problem on both. Fuel starvation should give the same symptoms as going on to reserve which in my experience is a brief loss of power followed quickly by the motor cutting out. I wouldn't describe it as a mis-fire.

The symptoms sound like a typical Boyer low voltage situation. Is your wiring from the pick-up new as well ? If there is an internal breakage there, it will cause this problem dependent on motor shaking and temperature increasing. Is the battery (even if new) really OK ? Does it go lumpy or really pop and bang ?

The removable pilot jets were replaced by the pushed in bush very early in Mk 1 Concentric production. If yours has them then it's probably a modification. I have to say that despite sometimes leaving mine for long periods, I've never experienced the blocking pilot jet problem.

Can you borrow a spare Boyer from someone and hard wire it ?

Hi Neil,

Its good to see you may have cured the misfire fault. Dodgy plug caps can of course produce misfires, however as far as I am aware whether the caps are suppressed or not should not make a lot of difference. When the Commando was originally produced it would have had standard copper HT leads with standard caps. I bought a set of standard leads with rubber caps from RGM during my rebuild and do not give any trouble. An alternator charging system will have no influence on what type of plug cap is fitted, the charging and ignition systems are completely separate circuits and the only effect one has over the other is in so far as the battery is kept charged up enough for the ignition system, and other circuits, to work affectively. There should be no interference from the charging circuit that will coarse problems with the ignition circuit whether its standard points or electronic, the only thing you should ensure of, when using electronic ignition is that the wiring from the stator plate ( formally were the points were ) to the electronic module is run separately away from the rest of the wiring harness. This is to prevent any electro magnetic interference being induced that could course spurious signals that could interfere with the ignition module.

Hope this makes sense,

Paul.

On some after market taps, the rubber gets cut by the sharp edge of the hole which the fuel flows in the middle of the tap. Thecut rubberthen get pushed into the hole and reduces the flow of fuel. I've seen this a couple of times on the cheaper taps you can buy at around £10. The fix is to buy those that cost £20,

Regards - Paul.

Previously wrote:

Hello Neil,

I had cuto uts or missfiring with nice flame comming out of the silencer and was searching for weeks. The Boyer ignition has its welded-in cables and is too stiff and the cable was broken with a clean cut inside of the plastic cover. Pull on the cables may be yours will come out as mines...

A nice NOC member advised me to change this system to a welded in part where you can screw the cables on, so if it happens on the way you can just cut cable and fix again.

good luck

Kind regards

Martin, SPAIN

Rebuilt 750 misfires &/or just cuts out after a mile or two. It will restart quite easily and run for ? to 1 miles until I get home.

The bike has been rebuilt over a long period with most things being fettled in some way or other but is otherwise a pretty standard Combat engined Roadster (see orange '72 bike in the gallery. Electrical mods include: Mk III Boyer, A REG ONE (rectifier/zener) solid-state gizmo (retained capacitor), solid-state Lo Voltage Warning Device (instead of Warning Light Assimilator), rebuilt loom with all unnessesary cables removed, new 6V coils, Champion plugs etc.Carb mods include Amal velocity stacks (jetted to suit) and extended air/mixture screws only.

I've checked the connections and continuity of the harness several times. I've checked that the battery (12N9B) is being charged at various tick over speeds.

I know it will probably be a simple fault to rectify but could do with some suggestions.

I do intend to check that the new Monza type tank cap is breathing (I think the symtoms could be caused by fuel starvation from a vacuum in the tank).

I have one other bike, a 650 unit T'bird that has been converted to 12V with Mk III Boyer, solid-state rectifier and normal Zener and I used to have to run that with the ignition switch in position 2 (ignition & lights) to stop it cutting out. A friend has suggested this would be employing more of the coils in the alternator, boosting the charge tothe levelthat the Boyer demands?? This bike has it's original 376 Monobloc that is in good condition, resleeved and has an air filter.

I could live with riding with the lights on but this dodge does not work on the Commando. As spring is here any help ASAP would be gratefully received.

Neil

Neil

I had a similar problem after work on my tank and found it was fuel starvation caused by a faultytank cap.

Try opening the cap as soon as it starts to stutter. If the engine picks up again the cap needs attention to clear the breather holes.

Sounds too simple I know. Iain

hi my 750 commando has developed a misfire ,it runs ok on idle but spits and misfires when setting off ,does enyone have any ideas what it could be .

Donald could you give more info, type of ignition fitted etc as per post 1.

Do the symptoms go away after the engine is warmed up, if so fit a choke or give carbs a second tickle as it could be weak mixture on a cold engine. Also the pilot jets could be furring up, remove the pilot mixture screw and with a #78 or 16 thou drill mounted in a WD40 plastic tube clean out the pilot bushes.

If so, check the leads going to the pickup coils which often fracture. Mine led me a merry dance on one trip to France.

I would go with John Holmes idea's.