tuning problem

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Hi all, I have just completed my first complete retoration of a 75 mk 3 ( I have done 3 others but never the motor or elec). The bike starts up great first kick however it sputters (misses) a bit near full throttle. It has a boyer and a mikuni 36mm carb. It has a 280 mj in it (stock). When I check the plugs with the 280 mj they look like the bike is running really rich (black, sooty) when I switch to a 270 the plugs look a little lean (almost white)

any suggestions on what else to try?

Thanks

Mark
 
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This was discussed not to long ago . You can find the information on page 2 and look for " "Jetting a 36mm Mikuni"

I am using a 280 main and the needle clip position is second from top and works great.
 
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With the 270 in the carb, I think I'd try raising the needle height 1 notch on the E-clip to enrichen it at full throttle. (E-clip 1 notch closer to the tip)
It's easy to do and it's easy to return to your previous setting too if it doesn't work.
Just a thought.
 
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o.k so I checked the float height and it is correct. I have the needle and mj set to how they came from sudco and what seems to be the general concencus as to whats right (280 mj and e clip 2nd from the top). I am starting to thinbk it might be an ignition thing. I have a boyer with a dyna coil 5 ohm coil. What I found is when I static time the bike to 30 deg btdc and then I try to dynamic time it with my strobe, the marks dont line up (they are not even close at 5000 rpm).

I am running an exposed primary so I made a scribe mark on my alternator at 30 deg btdc. with the strobe, the witness mark on the rotor is quite far off from the mark when it was static timed. I cant get them to line up at all (there is not enough room for adjustment on the boyer plate.

Shouldnt the static mark be really close to the dynamic?

thanks

Mark
 
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OK you have a mark three the only Commando with a proper timming hole. You need to use it little round hole in the timming cover use service tool lock it up. Now re-set the magnet block in the end of the cam so the screw head shows centered in the anti clockwise hole in the coil plate and the lock down screws are centered in the slots at the same time. You don't need a degree wheel like the rest of us. Now check the gage in the primary side to see what 31 degrees is it may read anything just note it for light timming. Now you must light time it no options but you have a corrected number to time to. Have a partner sit on the bike no stands and rev to 5000 the mark should move up to the number and stop on the new number. Stadic timming is only so you can start it. Untill you have done this all other adjustments are nothing.
 

L.A.B.

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norbsa48503 said:
OK you have a mark three the only Commando with a proper timming hole. You need to use it little round hole in the timming cover

The MkIII timing aperture is not in the timing cover but in the crankcase below it, the small access plug hole in the timing cover is for checking cam chain tension.
 
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norbsa48503 said:
OK you have a mark three the only Commando with a proper timming hole. You need to use it little round hole in the timming cover use service tool lock it up. Now re-set the magnet block in the end of the cam so the screw head shows centered in the anti clockwise hole in the coil plate and the lock down screws are centered in the slots at the same time. You don't need a degree wheel like the rest of us. Now check the gage in the primary side to see what 31 degrees is it may read anything just note it for light timming. Now you must light time it no options but you have a corrected number to time to. Have a partner sit on the bike no stands and rev to 5000 the mark should move up to the number and stop on the new number. Stadic timming is only so you can start it. Untill you have done this all other adjustments are nothing.

thanjs Norbsa, I do use a degree wheel because I have no primary cover. I made an exposed belt setup. That is why I used a deg wheel to locate 30 deg btdc. When I try to use the strobe light the mark I made at 30 deg is way off.

what am I doing wrong?

also could this be what is making the bike run crappy from 1/2 to full throttle (missing)

thanks

Mark

here is a link to what the setup I made looks like

http://homepage.mac.com/fourperf/PhotoAlbum17.html
 

L.A.B.

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Are you sure the pickup wires are connected the correct way around? As that will put the timing well out if they have been reversed.
 
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L.A.B. said:
Are you sure the pickup wires are connected the correct way around? As that will put the timing well out if they have been reversed.

if you mean the b/y,b/w wires they are. The b/y wire goes from the plate with the pickups in to the b/y on the boyer box. same with the b/w.

Mark
 
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Are you using a dead stop tool in the spark plug hole to find TDC? If not how are you setting up and using the degree wheel? And of course you are right L.A.B. timming side case not cover. If I had this locator I would use it for timming. If it works like BSA's and Trumph's it locks the crank. But it may lock it at 28 degrees for points instead of 31 for the Boyer in this case back to the degree wheel. Getting this right is the first thing.
 
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I found tdc by using a dial indicator in the left spark plug hole then set my deg wheel at 0 degrees then rotate the crank cw to 30 degrees. I then put a scribe mark on the alternator that lines up with the mark on the rotor. I thought that when I purt the strobe on it at 5000 rpm it should be close.

Mark
 

L.A.B.

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norbsa48503 said:
If I had this locator I would use it for timming. If it works like BSA's and Trumph's it locks the crank. But it may lock it at 28 degrees for points instead of 31 for the Boyer in this case back to the degree wheel.

A locating tool isn't really necessary as the crank timing slot on the crankshaft is easy to see through the timing hole and the slot can be positioned in the centre of the hole, this will set the crank to 28 degrees BTDC, then check the position of the alternator rotor mark against the degree plate and adjust the crank position to 31 degrees, (allowing for any error in the position of the degree plate of course). The new position of the crankshaft slot can then be marked on the case (I use a paint mark) for future reference.
 
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Fourperf, L.A.B. has good advise use the mark on the crank to double check your work. There is a reason that you need to use a dead stop tool and not an indicator. There is about 15 degrees of crank shaft rototion durring TDC. So if you were only useing one side of TDC you are for sure 15 out of phase. If you use the indicator twice and split the differance to set the degree wheel you would be closer. But it is much better if you stop the piston about 30 degrees before and after and split that difference. On a Commando the tool needs a pin 3/16 in dia. coming out of the bottom of the old plug and needs to be 1 1/4 long. It has to be thin or the valves will hit it. Because it is thin you must touch it gently with the piston.
 
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thanks Norbsa and L.A.B, could someone please explain the timing hole method again(I assume it is the bolt beneath the timing cover). Thanks a lot guys. This is the first motor I ever built and while it is stock the bike is quite different from a stock one with no primary cover. It does start up on the first kick and run really well up to 1/2 throttle.

Mark
 
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You start on the timming side do that piston first. Plugs out, fourth gear, rear wheel is your lever. Finger in the plug hole bump over the motor till you feel gas excaping around your finger.Let this happen twice in case you happen to be too far along when you start. Continue turning the wheel till you see the slot thru the hole, nice and slow if you go past back up and try again you need this lined up just right while turning the rear wheel in the normal direction. Ok now your 28 degrees BTDC for that piston. Because you have no gage on the primary side there's not a way to work out the math to get it to the 31 you need. So fit a degree wheel set it at 28 degrees to your piece of wire pointer. Your going to have to strobe to the degree wheel so get it snuged well. Still have to work the math to get to corrected 31 degrees. With the wheel in the way it's not posible to put on your marks to be used later. If I were you, buy one of the 720 degree wheels off Ebay fits on the cam now you can mark your stator and rotor for use later. Fabing a dead stop tool is in an earlyer post still a better way to get were your going I think. In any case you need the motor set at 31 so that you can get the range of the adjustment slots on the Boyer. Light timming to an inacurate mark is a waste of time. Can you temp mount your primary cover? It would help. You have to have a good mark at 31 to time it has to be right.
 

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The timing plug is the 3/4" hexagon just below the timing cover.

Remove it and rotate the crankshaft until you see a slot. As you do not have the degree plate in the primary cover you need a degree disc as norbsa has explained but after setting my own bike to 28 degrees with the slot and then 31 degrees with the primary degree plate this moves the crank slot to a position where the lower edge of the slot is central to the timing hole. So just positioning the lower edge of the slot in the centre of the hole should give the 31 degree position with no need for a degree disc.
 
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thanks a lot for all the info guys, I will try this weekend again. I will post the results

Mark
 

L.A.B.

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If you intend to mark the rotor position at 31 deg. BTDC I suggest you then set the crank to TDC and mark that too.
When the engine is idling the rotor mark should be somewhere close to the TDC mark, as the engine is revved then the rotor mark should move from the TDC mark to the 31 degree mark.
 
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L.A.B. said:
If you intend to mark the rotor position at 31 deg. BTDC I suggest you then set the crank to TDC and mark that too.
When the engine is idling the rotor mark should be somewhere close to the TDC mark, as the engine is revved then the rotor mark should move from the TDC mark to the 31 degree mark.

Thanks a lot, I will do that.

Can I get the slot lined up in the timing hole which should be 28 deg then put the deg wheen on the drank set it to 28 deg then rotate to 0 deg/tdc mark that then rotate to 31 deg and mark that?
 
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