power loss

Only time one could get a back fire over 2500 rpm and increasing throttle is d/t interminent spark cut out or mis fire timing from electronic ignition issue. Highly tuned race engine will back fire igniting unbunt mixture in exhaust on let offs. Your darker jug may be clue to trace to carb, oil sealing or sparking issue and of course all at once each giving similar symptoms. Air leaks possible too as I've had on loose manifold fasteners.

Ta.

Hi By tweaking the mixture screw on the left hand cylinder I am looking to get the colour of the plug / mixture right before I move on to other possible faults. The exhaust looks clean no excessive smoking . Once the engine has been running and used the backfiring stops

The biggest problem was leaving the bike sitting at idle for 5 minutes.

By adjusting the low speed mixture screw, you did nothing to correct your overly rich carb settings.

First, make sure your air cleaner is CLEAN (new is even better). Dirty filter can contribute to richness.

Next, pull the carb tops and LOWER the needles (raise the CLIPS) one notch, then install a new set of plugs; keep the plug wrench in your back pocket along with a rag.

Start the bike and ride straight out to an open highway with good shoulders, get it into 3rd gear and run it up to 50-60 MPH and hold steady for a mile or so, then snap the throttle shut as you pull in the clutch and kill the engine all at the same time (or nearly so).

Coast to a stop and pull the spark plugs with the plug wrench and rag.

Plugs should look much better than the last set you removed. If they are still sooty, drop the needles one more notch.

Should be just fine after that.

Hi Before I took the bike out for its last run I removed the plugs and cleaned them with a wire brush both plugs were sooty I adjusted the mixture screw on both carbs by about 1/8 turn and this has resulted in the right hand one looking ok but left hand still sooty so I have cleaned them off again and weakened the left hand carb a little more. I have yet to run the bike to see how its effected.

ashman said:

You can have worn slides and the bike will still run, if your plugs are sooty black after a ride of 1 1/2 hrs then maybe that is telling you its running a bit rich, maybe your jets and needles need replacing, but just loosing power like that could be a few things to look for, you just have to trouble shoot everything a broken wire in your Boyar, the only way to find out is to check everything out that makes it goes bang.

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+1. If it was running OK before the 5 min warm up, and then started to backfire and lose power, I would check all the ignition wires and connections, and the earth (=ground) cable. A loose or bad earth will give the symptoms you describe. Give yourself more of a chance with new plugs, and toast the old ones with a blow lamp until the carbon has burnt off. Old batteries that are not holding their charge, or having the lights on at low revs may not allow a good enough spark at the plugs, especially with an analog Boyer system. If the plugs are good, they should give a fat, blueish spark when you kick it over.

After you are satisfied the ignition is not at fault and the earth connections are sound, take the air cleaner off and check that the needles are not loose in their slides and that both choke slides are retracting fully. Drain the float bowls and then turn the taps on to see if there is a steady flow of petrol. Then take her for a spin without the air cleaner.

Report back

Jk34 said:

Hi Before I took the bike out for its last run I removed the plugs and cleaned them with a wire brush both plugs were sooty I adjusted the mixture screw on both carbs by about 1/8 turn and this has resulted in the right hand one looking ok but left hand still sooty so I have cleaned them off again and weakened the left hand carb a little more. I have yet to run the bike to see how its effected.

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Turning the mixture screw(s) on the carbs WILL NOT fix your problem!

Please read my previous post; it is highly likely that if you take the steps clearly outlined, your bike WILL run better.

Just so that you understand, the mixture screws are for LOW SPEED (idle) adjustment, NOT regular running mixture which is dependent on main jet, needle jet and needle position (besides slide cutaway which is highly unlikely to be your problem).

Hi Just a post script. After reading report on two into one manifold using Amal mk1 carb on RGM web site I went ahead with the work . Reading spec for my 750 it should of had, 930 carb 220 main jet with needle with two scribe marks at top. I had 932 carb, 260 main jet and needle with 4 scribe marks at top. I now have 932 carb with 240 main jet ( as recommended for single carb by RGM) and needle with two scribe marks. Bike is now preforming very well, easy to start no missing and great low down power and acceleration. many thanks for all your help and comments

Jk34 said:

Hi Just a post script. After reading report on two into one manifold using Amal mk1 carb on RGM web site I went ahead with the work . Reading spec for my 750 it should of had, 930 carb 220 main jet with needle with two scribe marks at top. I had 932 carb, 260 main jet and needle with 4 scribe marks at top. I now have 932 carb with 240 main jet ( as recommended for single carb by RGM) and needle with two scribe marks. Bike is now preforming very well, easy to start no missing and great low down power and acceleration. many thanks for all your help and comments

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Glad to hear it! My test for sloppy slide to body fit is to have the bike idling at 1000 rpm or so, then turn the twistgrip just enough to take the slack out of the throttle cables, but not enough to actually raise the slides. If the idle changes significantly the bodies are worn. After 10,000 miles or so of use taking the slack out of the throttle cables will slow the idle enough to kill the engine.

Two wires from black box going into timing cover, check for contact with exhaust pipe. Ask me how I know, 43 years ago !
What say you Windy?

All good advice........but it's almost for sure an electrical fault. I would start there. Start by looking at ground (Red) wires. A very high percentage of all my problems have been attributed to one of those. Good luck.

Glad you got it sorted, I have Boyer ignition and would always charge battery 1st.