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750 commando firing on left only

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My 71 commando started cutting out on idle so I serviced the twin carbs (new jets, floats etc). The problem remained and then I noticed that the LH exhaust was hot fairly quickly after starting but the RH exhaust was cold. I swapped over the HT leads and the problem stayed with the RH exhaust remaining cold. While running at 1500 I pulled off each plug cap in turn and removing the RH one made no difference but the engine stopped when the LH cap was removed. So it looks as though it was only running on the LH cylinder which is why it won’t idle. The valve clearances are set correctly so i’m Thinking compression loss in the RH cylinder does this sound logical based on anyone else’s experience? I have Pazon ignition, twin 932 Amals and a fully charged battery. Advice appreciated.

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Compression loss is about the last thing to consider,and an easy test is to stand on the KS and time how long each cylinder takes to go over compression (about 5 secs! each side)?. Dud plug?.Blocked pilot?.

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If the RH chimes in at higher revs then drill out the pilot with a 16 thou or #78 drill, as Robert suggests they get blocked very easily and there is then no fuel for that cylinder until the carb moves onto the needle circuit. Dud plug is easily checked with a new one fitted.

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Have you swapped the spark plugs over to the other side?

If and when you swap them over, does the lack of spark stay on the RH side?

Also try swapping the spark plug leads.

Let us know the results of the tests

Mike

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You did not say weather you have spark to both sides - only that it doesn't run on one cylinder. If you do have spark to both sides check needle in in RH carb . I had the exact same thing happen to me some years back . Turns out the needle was not in slide correctly resulting in an extreme over rich condition.

Thanks for all the advice, i’m currently waiting for nice mr amazon to deliver a spark Indicator and compression kit. I will post the results. 

By holding the plugs against the head I could see a spark but it was yellow and I couldn’t see if it was consistent. 

There was a lack of combustion in the RH cylinder over a wide range of revs which made me think it wasn’t the carburettor?

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With the needle displaced it will not run at any RPM - we now know you have spark outside the combustion chamber however weak or inconsistent . Worth a look at the needle . Also, it should be able to idle on one cylinder as that is the old school way of setting idle - set idle on each cylinder by removing leads on opposite side . When both leads are back on idle will be high so back off on both sides equally to obtain desired rpm.

Well, the compression seems fine and is approx 7bar on each cylinder and I’m getting a spark across a 1cm gap. 

So, back to the carbs which have new pilot and main jets and needles to the same specs as the originals. 

Firstly I pushed a 16thou wire probe through the pilot, then checked the needle operation into the needle jet and that seemed fine, and blew it through with air and carb cleaner. I attached my bowl level monitoring tube and blow me it ran ok on both cylinders. I stopped the engine to set the slides properly and take off the bowl level kit and it wouldn’t burn on the RH cyl again.

i’m wondering If I’ve got the needle retaining clip in the wrong groove. The needle identifier is two rings and the clip is in the centre groove of three? Just seem to be going in circles at the moment. 

 

 

 

 

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So, I stepped back and had a good think. One thing I had noticed was it seemed to be happiest after the carburettors had been tickled, so wondering if the pilot was too lean I started the engine up on one turn of the pilot air screw instead of the customary 1.5turns. It worked and now ticks over happily at about 900, no blow backs and the exhaust temps are even. 

Here’s my theory (sure it will be shot down). 

The pilot air hole has probably become a bit oversized due to years of probing and screwing the pilot airscrew in a bit further than normal has negated the effect?

 

 

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The screw meters the air, further in means less air is needed as your fuel supply is lower, so your pilot jet is getting blocked and needs drilling out not probed. Probing sends the crud back behind the pilot jet ready to come forward with the fuel supply, the drill brings the crud out in the flutes to remove it completely.

Thanks for the tip John, i’ll get a #78 drill bit to make a better job of clearing the pilot jets. 

Running is definitely improving, especially after I cleared out the old fuel which I think was 95 not 97. 

It’s been an interesting learning curve. 

 

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