Many of you have been hearing about this of late. My 68 rebuilt Bonneville has always, since I have had it, had a very slight stumble off idle or otherwise closed throttle. Coasting etc. Always idles fine. Always runs smooth and quick, except for that slight stumble off idle. So I went through most everything. Tri spark ignition timing was off about 1 degree. Fixed it. Did the valve adjustments. Spot on. Compression very even at about 145. Went through the carbs. Found I had sleeved slides. Looked top shelf. Very little play. Low jets clean and open. Had replaced main and needle jets with stock size amal bits. Also new needles on center groove. Balanced slides. Plugs looked pretty good and never fouled. Never did a throttle chop. Don't fancy reaching for the key at classic triumph hyper speeds. Experimented with different float levels on brand new stay up floats with alloy needles. Nothing cured or changed that little stumble. So I said screw it, and got new Premier carbs. Key info: I pulled the carb balance tubes and manifolds off the head. They looked great. Cleaned them up, added new gaskets and new phenolic blocks and reinstalled everything with the new premiers. Balanced the slides etc and fired it up. Ran crap. Removed carbs. Adjusted float heights (way off), sprayed everything with carb cleaner and air. Reinstalled and it ran better but left side was awful. Went through it carefully, reinstalled and still crap. Removed it and put the old carb on and it ran nice, but STILL had my little stumble. Also another new development is, at very low revs, as when I am leaving from a stop in first gear and the revs go down for a moment, it stops firing on the left cylinder. Picks up the moment I pull in clutch and increase revs. Looked at plugs. Right side is fine, if not a touch lean, and left side is sooty and black. Said "Hmm". installed new plug, gapped to 30 thou and restarted. Same ****. Double Hmm. So either the spark is getting interrupted, or the mixture is very bad. I am out of time right now, but I intend to remove the left plug wire, stick a spare plug in it, start it on the right plug and observe the spark, looking for to see if it is steady or erratic. My plug wires are too short to swap sides. Also this bike has been 1st kick every single time. Not so much any more. If anyone has any better, or other ideas, please shout it out.
Jim Gregory
Joined
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15,668 Posts
Hi Jim,
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)?
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.
. 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,
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)?
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.
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,
Joined
·
15,668 Posts
Hi Jim,
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,
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,
Joined
·
15,668 Posts
Hi Jim,
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.
With resistor plugs imho only if absolutely necessary specifically for digital electronic components.
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,
Missed that earlier.
+1.
If Steve says this is OK, no argument from me.
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.
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.
... 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?
Mmmm ... first see what Steve says about the coils?
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.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,
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