ET: Engine (Twin) Still Can't Start My Twin, Replaced Mag, But Not A Pop

[ACD]

Carl, you’re getting both cylinders to run so it’s got to be something that’s common to both.

You’ve eliminated, Magneto and contaminated fuel. Cam timing you haven’t moved so no reason for it to be an issue. I’m guessing if you have points cover with a kill wire you have already disconnected it and tested to see if it’s the cause, so that’s out. Plugs and HT leads are individual and you'ed have to be pretty sad case to have both sets of new ones fail straight up.
Only 2 things left on the list really; contact points circuit continuity or some kind of fuel starvation that is somehow affected both carbies i.e. a jetting issue (not likely if it was running fine 12 months ago & not been played with) or there is still some residual gumming hidden in one of the passages, possibly acting like a little flap on a check valve.

 

[brian gains]

back to basics; take fuel line off and run into container to show good uninterrupted flow. next check float bowl and float, etc, etc.

 

[vibrac]

Shout out! but 36-32 BTDC it should still run at this stage not critical my Comet BTH stop hole is worn to 10 degrees I guess centre sweet spot
I would be looking for a mechanical problem by now with a squeaky clean mag and carbs just start by checking both rockers look equal opening at TDC I have had an Aerial single with a sheared cam shaft run for 10 seconds at starting repeatedly till the 'friction drive' slipped but the shear was irregular enough to return on stopping

 

[oexing]

Carl, maybe I have come upon something last night: The standard carbs, pre-monoblocs, are handed on Vincents - and the jet blocks as well !! So maybe you have swapped them without noticing. When you just look at positioning the pin in the jet block to go in the slot of the carb body you can mix them up as they are handed. In consequence the air intake hole of 1/4 in the jet block facing the bellmouth side then looks to the inlet valve with half a turn wrong. Also the pilot jet will point to the bellmouth side then. You should be able to stick a 1/4 drill (a bit smaller) way down the carb hole at bellmouth side onto the needle jet in center of carb. Otherwise you´d possibly look at the recessed pilot jet hole when having a torch at hand and seen from bellmouth end.
Another handed detail is the small hole of around 2 mm close to the pilot jet, in my photo below at right side and a bit lower. This is handed depending on the pilot system air adjusting screw. When the screw is on right side, the hole has to be on left side. Otherwise no air can enter the pilot sytem and poor starting is observed. So when checking your carbs see for the air holes of pilot system to align with each other, jet block and drilling in carb body. Yes, you´d have to knock the jet block out the carb for seeing the holes.
Amazing that you can even start the thing occasionally when the fault is actually the swapped jet blocks. Anyway, there must be something affecting both carbs at same time, unlikely just a blockage in both carbs at same place. Seems both cylinders can run a short time, not just one, so my idea may be worth checking, symptoms may point this way.

Vic

 

[CarlHungness]

Carl, maybe I have come upon something last night: The standard carbs, pre-monoblocs, are handed on Vincents - and the jet blocks as well !! So maybe you have swapped them without noticing. When you just look at positioning the pin in the jet block to go in the slot of the carb body you can mix them up as they are handed. In consequence the air intake hole of 1/4 in the jet block facing the bellmouth side then looks to the inlet valve with half a turn wrong. Also the pilot jet will point to the bellmouth side then. You should be able to stick a 1/4 drill (a bit smaller) way down the carb hole at bellmouth side onto the needle jet in center of carb. Otherwise you´d possibly look at the recessed pilot jet hole when having a torch at hand and seen from bellmouth end.
Another handed detail is the small hole of around 2 mm close to the pilot jet, in my photo below at right side and a bit lower. This is handed depending on the pilot system air adjusting screw. When the screw is on right side, the hole has to be on left side. Otherwise no air can enter the pilot sytem and poor starting is observed. So when checking your carbs see for the air holes of pilot system to align with each other, jet block and drilling in carb body. Yes, you´d have to knock the jet block out the carb for seeing the holes.
Amazing that you can even start the thing occasionally when the fault is actually the swapped jet blocks. Anyway, there must be something affecting both carbs at same time, unlikely just a blockage in both carbs at same place. Seems both cylinders can run a short time, not just one, so my idea may be worth checking, symptoms may point this way.

Vic

View attachment 59873

Thanks so much for the concern. No possibility I have swapped them as I tried to remove the rear first and it was very tight..thus I heated the body of the carb with a blow dryer and managed to JUST wiggle the block down with a screwdriver about an eighth of an inch..then I heated it a bit more and was able to wiggle the block free by gently yanking on the octagonal jet. The front jet block fell out on its own. I took photos of the blocks just as they cam out and know where the holes are..and scribed #1 on #1. Still just getting a pop (but did start for about ten seconds..then I re-timed it..again)...but now I'm having a time getting the ATD set..I've set it 4-5 times in the past week, and now it seems when I DO finally get the bolt started, and then screwed in, the ears are stuck in the OPEN position...(after I remove the wooden dowel I use to prop them open). So it's one hiccup after another. If I screw the ATD on, then use a socket to 'set' it, the ears are 'stuck' open...so I fiddle and fiddle and loosen the bolt, and TRY to keep the plot on the tapered shaft at the same time...and Lord, what a journey! I just did it all again and for the first time ever I got the kickstarter to kick back..never happened in 32 years..so my timing may be off...will try it again in a few minutes. I'm now in total despair as I have over 50 hours in trying to make the plot run..whew, am at wit's end. Just did a compression check, and it's 90 on both cylinders..did one years ago and seem to recall the same number.

 

[CarlHungness]

back to basics; take fuel line off and run into container to show good uninterrupted flow. next check float bowl and float, etc, etc.

I've gone so far as to remove the fuel tank, all lines. Have brand new fuel, new plugs, three times.

 

[CarlHungness]

Carl, you’re getting both cylinders to run so it’s got to be something that’s common to both.

You’ve eliminated, Magneto and contaminated fuel. Cam timing you haven’t moved so no reason for it to be an issue. I’m guessing if you have points cover with a kill wire you have already disconnected it and tested to see if it’s the cause, so that’s out. Plugs and HT leads are individual and you'ed have to be pretty sad case to have both sets of new ones fail straight up.
Only 2 things left on the list really; contact points circuit continuity or some kind of fuel starvation that is somehow affected both carbies i.e. a jetting issue (not likely if it was running fine 12 months ago & not been played with) or there is still some residual gumming hidden in one of the passages, possibly acting like a little flap on a check valve.

Put in ALL brand new jets, took carbs off and cleaned all holes. Bought the #76 drill and made sure the pilot hole is clean. Will do it all again..and again for about the 4th time. I'm in agony over this one.

 

[Terry Martin]

You seem to have tried everything else, is the exhaust clear? Maybe try starting without the silencer fitted, hopefully you have understaning neighbours.

 

[oexing]

Sorry for another question that may have been asked before: After a successful run of some seconds and early death (engine) did you check ignition timing then ? Just suspecting slipped mag ATD on taper. Do you degrease the tapers when setting ignition timing ? Dry tapers should not slip much when tapping with a small hammer and bolting the lot down.
Anyway, I would widen the three mounting holes of mag case for fine adjustments after fixing the ATD on taper. It is absurd hoping for exact timing by just relying on taper seat by tapping the device on. One millimeter per side widened in these holes will give you 3 degrees each side range for fine tuning - when my maths were correct. So afterwards this will save you a lot of bad language and nerves - and later generations will be very thankful for your efforts - if there will be any messing around with old iron . . .
See my photo below, an aircraft mag with wide slots for fine adjustments. No way can you achieve exact timing on both mags - dual sparks - with other tricks. These engines are timed by dual timing buzzer for exact same degree , otherwise dual spark plugs would be pointless.
Anyway, seems you are having a mess with fixing the ATD on the mag shaft. Maybe better have the mag out for checking all components of ATD for seating properly on taper resp. binding flyweights - don´t know about them , no such ATDs in my home.
Vic

 

[CarlHungness]

You seem to have tried everything else, is the exhaust clear? Maybe try starting without the silencer fitted, hopefully you have understaning neighbours.

Hi:
Have had exhaust off 3-4 times, all is clear. I'm worn out, first time in 32 years this bike has beaten me...
am giving up for now, have well over 40 hours invested and can't put in any more time.