It’s a confusing story, but suspect richness on the pilot screw on the side that soots up the plug.

Rich idle can mean even richer over-run, which can foul a plug, making it hesitate when you open the throttle after slowing down.

 

Sorry to be confusing but I wanted to get it all out there. My idle mixture screw is 1.75 turns out. I agree the left side is over rich. I had suspected that the idle jet may have been cleaned out and become oversized. Not much I can do for that.

 

I’m wondering if your throttle cables to the left carb are binding?

 

Hi Jim,

plug, gapped to 30 thou

Why? Should be 25 thou. unless the coils have very high maximum HT Volts (30 KV? 35 KV?). For testing, the gap can be as little as 20 thou.

Thought: if the drive-side coil's maximum HT Volts are low, could be the reason for the problem you're dealing with. Consider testing coils as well as the HT leads. Do the HT leads have separate screw-in plug caps, or plug terminals crimped to the ends of the HT leads (only covered by separate plastic caps)?

left side
suspected that the idle jet may have been cleaned out and become oversized.

So verify rather than "suspecting"? Standard original Concentric pilot jet orifice is 0.016" ID; that happens to be the same as a #78 drill bit. Next size larger is #77, that's 0.018" ID. Buy one of each size (pennies) (if they aren't long enough to reach from the air screw hole, stick the fluted end in an aerosol can 'straw'), attempt to pass the unfluted end of each bit through the pilot jet. Then you'll know if one has been enlarged.

As long as a Concentric pilot jet has not be enlarged bigger than 0.020" ID (#76 drill bit), the carb. is usable - John Healy has posted in the past that, 'back in day', both US importers supplied dealers with a set of #76, #77, #78 bits for enlarging standard pilot jets if required.

My idle mixture screw is 1.75 turns out.

If one carb's pilot jet orifice is larger than the other one's, by definition:-

. Any pilot jet flows a given amount of fuel, a larger jet orifice flows more fuel than a smaller one.

. A given carb's idle mixture screw is for adjusting that carb's idle air to match the amount of fuel coming through that carb's pilot jet. If one carb's pilot jet has a larger orifice than another carb's, if the pilot air screws are set the same in both carbs., something else must be going on in at least one of those carbs.?

Hth?

Regards,

 

Hi Jim, Sleeved carbs are often bad news. Trust me on this. Here is a test. Your tune up & mixture sound spot on.

The test is warm motor very well. A good 20 miles. Pull over. While idling turn the throttle as slow as you can. The motor should not stumble, slow down or die. The rpm should track throttle exactly. I mean exactly. rpm should ever so slowly raise. Do this test several times over several test rides. You should see repeatable pattern.

However opening throttle more quickly to pass the bad spot the rpm tracks the throttle as expected.

Snap throttle opening is a different test. Determines slide cutaway. I'm not talking about snap throttle.

I have found this test to 100% reliable on concentric MKI carbs. Has not failed yet. My good friend had carbs sleeved. He finally gave up & bought 2 new Premiers. They cost a bundle. Order carbs from Amal direct & tell them what jets, slide, carb top, bango fittings you want. Generic is generally a bad value as they often need the above parts. A few USA retailers will custom assemble. Often a 019 pilot jet works better than the normal 017. If you have both on hand you can determine which is best on your motor.

Sleeved or not the slow throttle test is the real deal. Yes indeed when you are riding, once you know what to look for you can feel this hesitation while riding. Mostly in stop/go traffic & on slower curvy roads where you fully back off throttle, then slowly add throttle. New slide might help a little. Key work little. Counter intuitively cheating idle mixture leaner than you'd like can help a little. Root problem is wear in the groove of the floor of throat where slide enters it.

New Premier will likely last at least 50++k miles. Once tuned, they stay tuned & last a really long time. The needle & needle jet wear at 12-15k miles. That's the fast wearing part.

Try the test & please let us know what happens.
Don

 

Try the test & please let us know what happens.
Don

Hi Don.
By the way, it is great to see you back in the saddle, so to speak. I have had my share of scares and issues as well. If anyone told me how painful getting old is I may not have bothered. Anyway I am still here and intend to ride till I die. Not anytime soon.
Once I get reliable fire back to the plugs I will try your methods if still needed. I did a version of it, by pulling the outer cables (sheath?) gently out from the twist grip one at a time. The right side cylinder accelerated smoothly. The left side immediately misfired, then caught with increased throttle. No amount of idle mixture adjustments cured it. I ordered number drills to check the pressed in pilot size but they were useless. Actually they were all the same size. Maybe I will shop locally. I CAN say that a .0195" drill bit is a no go. So not WILDLY oversize.

 

OK a bunch of things to answer. Someone here told me to gap the plugs at 30. They were 25. Today I tried 30 and no discernible change.

Don, I bought two new premier carbs, trying to fix my "just off throttle" miss. The right premier carb is fine. The left premier carb is not. I twice went through it with a fine tooth comb, and it just runs crap. Out of frustration, I put my old left carb back on. NOW I have this very low (like 800+) miss on that side. Other then that, the bike runs smooth and pulls sweet, all the way up. When I say miss, I mean it just cuts out, then back on. So yes I strongly suspect electrical, but it is funny that it has been fine for years, and swapping carbs around has caused an electrical problem?

Stuart. Good idea with the "go, no go" gauge. I have the "go" already and it does go, but maybe it is too big. The "no go" will answer that. I should have the number bits tomorrow.
The coils are from tri spark and I don't recall what the wires are, but I had crap wires when I got the bike and definitely replaced them with robust wires and caps. The tri spark system has worked perfectly, so far. That said, I must pull the tank and inspect everything. If I don't find a smoking gun, I will switch the coils/wires and try that. I will also test the coils. I don't know the process but I have good equipment and about half a brain left. What could go wrong?

Ultimately, I want the left side premier on as well, but I will first rule out any other issues before I pick that carb up again.

Dave, both carb pilot jets are #17. As I said, I couldn't get the left carb to behave so I have my old Mk1 non premier on the left side for now. Actually had a great 50 mile ride today but the little niggles still exist.

More to come.

 

Hi Jim,

test the coils. I don't know the process

Ohm-meter or multi-meter set to Ohms:-

1. Measure between the two threaded terminals, '12V' coil (with points) should measure between 3 Ohms and 4.4 Ohms, '6V' coil (with EI) should measure between 1.5 Ohms and 2.2 Ohms.

2. Measure between the HT terminal and each threaded terminal in turn, I like between 5,000 and 7,000 Ohms, can be higher but bear in mind higher looks more and more like 'no continuity' ...

3. Measure between each coil casing and each of its terminals, should be very high Ohms (no continuity).

Plug wires and caps
I loathe separate caps that screw into the ends of plug wires; the connection always goes wrong sooner or later. Measure Ohms between a wire's coil terminal and plug terminal, it should be zero Ohms; if it isn't, throw the wire and any separate cap away, buy wires with both coil and plug terminals crimped on the ends of the wires. If the bike has a component with specifically digital electronics, use resistor plugs (Champion RN3, NGK BR8ES).

Hth?

Regards,

 

I was having running problems and after trying many things out, regapped the plugs at 30 thou and i have a much easier starting engine. That may not have been the cause of poor running but with other work, using 30 thou made it much better to start.

 

Hi Jim,

tri-spark coils

Missed that earlier.

send this information to Steve Kelly, at Tri-Spark.

+1.

Threaded terminals to output terminal reads 9.1K on right side

If Steve says this is OK, no argument from me.

Threaded terminals to output terminal
no reading at all on left side. Small amount of oil leaking from positive terminal on left side.

I'm pretty-certain Steve will say both of these aren't right.

Aside, any coil with a single HT terminal, that's one end of the HT windings within the coil, the other end is one of the LT terminals (original Lucas coils, it was the positive LT terminal, but I'm not sure it is on every make of coil). Is why a resistance is measurable between a coil's HT terminal and each LT terminal.

Plug wires on the right slam right to 4.9 K

If this is between the wire's coil terminal and the plug terminal inside specifically a resistor cap, resistor cap should be the ~5 KOhms. You can check this by unscrewing the cap from the wire, using the meter to measure between the wire coil terminal and the plug end of the wire, should be zero(ish) Ohms; similarly, using the meter to measure between the cap's two terminals should show the "4.9 K" Ohms.

Plug wires on the
left side is all over the place and never settles down. Sounds like a bad screw in resistor cap

... and/or the resistor making intermittent contact and/or a break in the plug wire's conductor ... If you're interested, unscrew the cap from the wire and check as in the previous paragraph?

So I need 1 new or logically 2 new coils

Mmmm ... first see what Steve says about the coils?

So I need
bomb proof wires.

Uh-uh, just bog-standard insulated flexible multi-copper-strand plug wires. I do recommend ones with both coil and plug terminals crimped to the wires ... certainly in the US, Mitch Klempf, CBS and The Bonneville Shop appear to advertise these complete with caps. With resistor plugs imho only if absolutely necessary specifically for digital electronic components.

coils
No dents, no signs of damage.

Mmmm ... if your bike has the coil mounting parts illustrated and listed with "T120R" on the '68 650 parts book pages 76/77, I hate to rain on your parade but ... many ignition coil internal parts are delicate, not designed or intended to be vibrated (was true of original Lucas and Siba ignition coils when your bike was new); however, your bike vibrates when the engine's running, the illustrated and listed mounting parts, all being steel, cannot absorb any vibration whatsoever.

Contrast with the "TR6R" coil mounting parts ... the correct #15 Rubber grommets are a tight fit both in two holes in standard frame brackets and around the two #14 Bolts (I've had studs made for my T100's coils mountings), the Rubber grommets isolate the coils and brackets from vibration. The missing bit is, certainly here in GB, we can get coil mounting brackets shaped like those mounting the "MA" coils, but for 40 mm. OD coils like the Tri-Spark. Post if this interests you and I'll post links to the brackets in GB.

Hth.

Regards,

 

Mmmm ... first see what Steve says about the coils?

Hth.

Regards,

Stuart everything you say helps. That goes for everyone as well.
My bike came to me with one Siba and one Lucas coil. In 55 years of riding I cant remember having a bad coil. When I switched to the 6 volt coils for use with the tri-spark ignition I noticed the coils were loosely mounted in bracket #27. Also there were globs of a clear adhesive material holding them in place. I installed the new coils, and promptly tightened the coils in the brackets tight (overtight). I now notice on the failed leaky coil, the very slightest indentation from the two bolt brackets, previously UN-noticed. My bad. Lesson learned!
Also, the readings I got on the primary windings, and the one good secondary winding do fall into Tri-Sparks specifications. That said, I ordered a pair anyway. We will see soon enough if that somehow fixes my fueling problem. Eventually, sooner that later, I hope to get my "faulty" brand new premier left carb into service.

 

Hi Jim, This carb thing has become most interesting to me. 2 friends got new Premiers , '69 & 73 Bonnies. Both motors had recent full overhauls. Both bikes ran decently according to owners. Both bikes suffered detonation on long up hill grade & did grave damage to piston. On damaged top ring land area, the other destroyed piston completely.

Other side of cylinder & piston were totally fine. I did not personally ride bikes or examine carbs. After much fussing with the carb on the '73, it couldn't be made right, but problem were not obvious. He ended up getting new carb for that side. Was good ever since. The '69 is currently in the process of repairs. I'd like to get my hands on this carb & see what is going on.

My experience is even though the static float level setting, just above bowl surface & level is good on most premier, so demand the actual liquid level to be set using the manometer tube. But, I think there is more going on that just float level.

I'd like to compare left/right carbs. Not just float level but actual size of main jet, needle jet & cross drilling in needle jet, as well as actual jet heights & the floor drilling of carb. Anything else on the casting I could see/find. Something is causing this but what?

On a side note bad coils is a bummer & can be hard to diagnose. My personal experience with Trispark is you don't actually need resistors or resistor plugs. (never both at same time of course).

Depending on who knows what Trispark can run perfectly with solid copper wires & normal N3C plugs. (now numbered Champion 801), Doing experiments both ways, it made no difference whatsoever. The '73 Bonnie runs perfectly on N3C for several thousand miles now already & solid copper leads. 6v Emgo coils. I personally have ridden many miles with the N3C (801) plugs.

At this point no telling what all your problems are. Carb & spark at the same time, YIKES! That's tough. Generally getting spark perfect first is best, so you're on the road to fixing this.
Don

 

At this point no telling what all your problems are. Carb & spark at the same time, YIKES! That's tough. Generally getting spark perfect first is best, so you're on the road to fixing this.
Don

Hi Don. Threw the new coil on this morning and went for a nice 50 mile ride. Started first kick again. I loved that! Ran just like it has for the last few years. Still have the old Amal Mk1 on the left side. I modified the coil mounting though. I added shims to the bracket so they don't squeeze the coil. It is now a tight slip fit. I put some silicone cement
in them just to keep them from wandering. I will revisit the other premier some time. No hurry. It starts easy and runs like a bear. Thanks to all for the help though. It means a lot!
Jim