Resistor spark plugs

Hello All,

It is getting impossible to find non-resistor plugs here in Victoria. I can still find them online but I was thinking of switching to zero ohm boots and using the resistor plugs instead.

Does anybody have an opinion on running the resistor plugs with the resistor boots? It is 5k ohm out of spec when you do this.

 

The resistance of secondary circuit sets the burn time of the spark, as well as its intensity. If you run resistor plugs with resistor caps, you'll have a longer duration spark with lower intensity. Whether that will translate into a real problem.... I don't know. But it's not "correct."

 

yes you can do that benifit is you are refreshing the resistor every time you change the plug.
if you read the xj4ever catalog he talks about this in the sparkplug cap and wire section.

when you do the math in the specs of the ignigition coils with caps you are in the correct ball park

XJ700 air-cooled models:

Pick-up coils:
120 ohms +/- 20% = 96 ohms to 144 ohms acceptable range


Ignition Coils:

Primary side (input from main wiring harness):
2.7 ohms +/- 10% = 2.43 ohms - 2.97 ohms acceptable range

Secondary side (spark plug wires, without their end caps):
12K ohms +/- 20% = 9,600 ohms - 14,400 ohms acceptable range


Spark plug caps:
1985 N/NC models: 5K +/- 20% = 4,000 to 6,000 ohms per cap acceptable range
1986 S/SC models: 10K +/- 20% = 8,000 to 12,000 ohms per cap acceptable range


Spark plugs:
1985 N/NC models: 0 ohms per plug
1986 S/SC models: 5K ohms per plug

so why would you be out of spec with an 85 xj700?? non resistor plugs and 5k resistor caps = non resistor caps and 5k resistor plugs.

is your bike an X which has different specs?

 

Really, though, how often are you guys replacing plugs? They should be good for 10000 miles at the very least, which is a long time for most of these bikes.

 

at a 1.63$ each I replace them every year.
"
you could ask how often people replace the caps. I think most would answer" when they fail".

 

When (if) they fail seems like a fine time. There are a lot of things that don't easily wear out.

 

over the years, I've run every combination of plug, cap, wire you can think of. Only Dynatek coils might have maybe made a little differance

 

I'm running Hitachi coils out of a Saturn (SC2, I think). The wire is Accel 7mm copper core silicone, and the caps and plugs are stock (NGK). I probably should check what resistance is expected for the secondaries on those coils. My setup may be too low with just the resistor caps. I can say that I get a much brighter and bluer spark than the stock setup produced, but I don't know if it has made any difference in performance or reliability... there are too many other variables with the fuel injection setup, different intake, different exhaust, different timing...

 

All Canadian motorcycle had resistor plugs by law. Use the BPR7ES with 5 ohm caps

 

Couldn't find any NGK 5 Ohm caps. Only 1K Ohm, 10K Ohm, and 5K Ohm. In the shapes needed for these engines I only find the 5K Ohm ones.

I'm thinking, though, that you actually meant the 5K Ohm stock caps. So you would be doubling the resistance of the secondary. Doubling the burn time, but reducing the intensity of the spark. Not sure that's a good idea. I expect it will still run OK, though.

I also couldn't find details on this law. Searching Canadian statutes for spark plugs returned a few hits, mostly about off-road vehicles and emissions, but nothing about requiring resistor plugs. Closest thing was that Google returned a few hits of people saying that CA had or would soon have such a law. They were similar comments to yours, not articles or legal references. Can you link to something authoritative on this?

 

law or not it has been stated by many of our members north of the boarder that they use resistor plugs in their xj bikes because that was the spec

 

Interesting. Yes, NGK.ca says BPR7ES (with a 5K Ohm cap), while NGK.com says BP7ES (and doesn't specify the cap). I suppose it's possible that Yamaha had different coils for the Canadian bikes, but it doesn't seem that likely they would do that (Len?). Maybe it just didn't make enough of a difference for Yamaha to fight with CA authorities over it.

I've been doing some research on this, but I'm not finding a huge amount. Most I've found is that fuel economy suffers in the cruise zone for unknown reasons with long spark durations, and that spark duration with Kettering coils should be 1 to 2 milliseconds.

There are probably some engineering books out there that describe all the results of incorrect choices in secondary resistance, but I haven't found anything from them online. Just trying to theorize about it, it seems to me that, by raising the secondary resistance, at some point you're going to start reducing the duration as you exceed the energy capacity of the coil... i.e. the voltage dropped across the cap and plug resistors will eventually exceed what the coil can supply while still keeping the plug gap ionized.

Anyway, I'm going to measure my burn time. If it's less than 1ms, I'll probably try some resistor plugs to see what happens with that. I'll also probably measure my XV750, which is (US) stock, and see what the durations measure there.

 

US 750 are supposed to have 5k caps on outer 2 cylinders and 10k caps on inner cylinders

 

Why? And where do you get that information?

 

I tried that once but i passed out before i could finish

 

1. Not sure, but I suspect to alter spark duration to allow for the increased temperature of the inner cylinders.
2. The Yamaha Factory Service manual, as well as the Haynes.


As to the need for resistor plugs; remember that resistor plugs on motorcycles were fairly new at the time (as were TCI ignitions and AM/FM radios on motorcycles), and were required in some places (sometimes by local, state, or provincial law) to reduce radio interference. I know that in Oregon we were required to run resistor plugs (or caps) on dirt bikes if we were riding in areas that had active radio-controlled blasting going on (for quarrying or tunnel buliding), and that my dad had to turn his 2 Meter radio off when driving near a blast zone.

 

first heard it here from long time member then found it in the haynes manual

why? who knows? maybe the same idea as to why yamaha went to different jets in 2 cylinders in 84

 

Wouldn't be the first time (swapped jets anyone?) that these manuals got a detail wrong.

I just went up in the attic and pulled my stock coils out of a box. They still have OE caps on them. The 2/3 caps are Tokai Denso TD T-133s. They measure 5.7K Ohm.

 

Yes, the manuals are full of errors, but my U.S. model Seca 750 had the original plug caps on it when I bought it and #2 and #3 were 10k Ohm caps. This is the correct spec, and has been confirmed by several of the long-timers here. It is entirely possible (as with many other instances of such discrepencies) that the worker needed to put bikes out to fill an order, and used what parts were available if there was a shortage of the correct caps.

Back to the OP's question. It likely won't hurt much to run resistor plugs with resistor caps (you may find cold starts to be a bit more difficult), but new non-resistor caps are inexpensive and many on here have swapped to them in order to run resistor plugs.

 

5.5k caps are spec in UK

 

Did you buy your RJ new? Mine was not new. So I can't be sure those T-133s are original, but they certainly look like it. My bike had 14000 miles on it when I got it.

What make and model are your caps?

And which ones are you suggesting were the incorrect caps? The 5.5Ks or the 10Ks?

 

5.5K caps are spec for the outer cylinders for US in the Haynes manual. 5K is what NGK says is spec (for Canada, at least). 5K is what Len says is spec.

My bike is not a UK model

 

Haynes manual

 

OK. So now the next step for me is picking spark plugs so I can replace them. (I've changed the engine oil, the shaft oil, replaced the brake lines with Galfer SS lines, rebuilt the calipers, replaced the front pads (surprisingly, even though the calipers were leaking and I had basically no front brakes at all, both sets were totally evenly worn) replaced the air filter, and as soon as I get the clutch rebuild kit I'll replace the clutch.) I just want the best plugs for my US XJ750 1983 Maxim. (They pulled one of the plugs and it had BP7ES stamped on it, if that helps. Of course, I don't know if there are 2 different pairs, of which the BP7ES only represents one pair.) So do you happen to know which ones fit the bill? Thanks, as always, in advance. (Also, the bike has 30,000 miles on it. Should I replace the wires AND the plugs, or just the plugs?)

 

No. but the specs are well documented by the service manuals, parts fiche, NGK, Len, and dozens of posts that predate my presence on this forum.
Note two part numbers for the plug caps. One for #1 and #4, and another for # 2 and #3.
https://www.yamahapartsnation.com/oemparts/a/yam/500421f3f8700209bc78746b/electrical-1

 

Haynes manual is wrong (and not just in this instance). There are 2 different part numbers for the outer vs. inner plugs caps because of their shape/orientation, not necessarily due to different resistance values.

 

all 4 of them BP7ES

 

I did get a factory service manual, and it does indeed say 5K outer and 10K inner. On the other hand, the only measured evidence we have is contradictory. Mine with what I'm pretty sure are OE caps on my 5G2 bike that measure 5.7K, and k-moe who's also maybe OE ones measure 10K. They're both '82 750s. Might be interesting to compare S/Ns to see which was built first and how far apart.

Len, do you have stock of any of the OE inner caps that you could measure? And/or could you tell us any OE part numbers on them?

I also did finally bring the 'scope into the garage and try to measure spark duration. It's a pretty messy signal to look at. Best I could tell it's about 750us on all cylinders. That's a bit short, but not dramatically so. One of these days I'll probably try a resistor plug, maybe one of those fancy Iridium ones (BPR7EIX) and see what effect that has on burn time, and whether there's any other change. I would expect that since I'm using a very modified intake there's a good chance my bike is more prone to missing ignition of the mixture with too short a spark... I don't hear any popping or other evidence of unburnt charge igniting in the header, though... but the idle has never been very even and maybe a longer spark would help with that.

 

HCP7437
OEM spark PLUG CAP 256-82370-00-00
#1/4
90-degree, short cap
5K ohms
1.330" on plug leg
2.135" on wire leg
some marked Taiko Denso TD T-134
some have no number cast into them
---> cannot be taken apart

Applications:
XJ650 all
XJ700 non-X
XJ750 all non-X
XJ750RL
XJ750-D
XJ750-P (24L)
XJ900RK, RL, N/FN, F
XJ1100
XS1100 all



HCP7436
OEM spark PLUG CAP 1J7-82370-00-00
#2/3
90-degree tall cap
5K
NOTE: 1980 650 svc. manual, XJ750J svc. manual, and Haynes manual lists 10K for #2/3 plugs?
Haynes lists 5.5K for all 4 plugs on UK models..........
2.442" on plug leg
1.230" on wire leg
some are marked Taiko Denso TD T-133
some have no number cast into them
CAN be taken apart, and resistor, spring, and disc are replaceable.

Applications:
XJ650 all
XJ700 non-X
XJ750 all non-X
XJ750RL
XJ750-D
XJ750-P (24L)
XJ900RK, RL, N/FN, F
XJ1100
XS1100 all