1970 Triumph T100 Spark Plug Gap

Hi,

Just checked the plugs and the gap was close to 0.040" and the manual states it should be 0.020"

OK, the question:

1. If the spark plug gap was too big (0.040" instead of 0.020") could this result in the engine feeling 'fluffy' (rich) coming off idle and onto throttle?

Overall the 'look' of the spark plug is good, light biscuit brown on the porcelain but dry black soot on the ring.

Just curious if the wider gap than spec. could cause this issue?

Yes. Widening the spark plug gap will effect the timing. Not sure if it advances or retards. Someone with a better memory than me will be along shortly.

Rule of thumb would say retards. But I do NOT know.
I do know that wider gap means weaker spark if you don't have a big coil to drive that jump.
And no doubt the saturation time on the coil may change too.
I set it 18 to 22 and let it go at that.

Hi Guys,

Thanks for the replies. I've reset the spark plug gap to .020-.022"

I'll see how things go next time I'm out & report back

Well, did the smaller plug gap cure it ?

Hi
Not sure on retards or advances, but one gives a hot spark and the other a cooler spark from memory, not sure this would advance or retard the spark, me I think not!

I would assume the bigger gap would give a hotter spark?

Couldn't be bothered even checking, just set the gaps correctly as the manufacturer specified unless you have changed a lot of things?
Burgs

Fast Eddie said:

Well, did the smaller plug gap cure it ?

Click to expand...

Got distracted with other things!! then work got really busy!!

My plan is to take the bike for a good spin over the weekend, I'll be back with an update then

Burgs said:

Hi
Not sure on retards or advances, but one gives a hot spark and the other a cooler spark from memory, not sure this would advance or retard the spark, me I think not!

I would assume the bigger gap would give a hotter spark?

Couldn't be bothered even checking, just set the gaps correctly as the manufacturer specified unless you have changed a lot of things?
Burgs

Click to expand...

Fair enough, on this occasion I've only changed the plug gap, curious myself if it makes any difference, better or worse!

OK, had a great spin this morning, approx. 30 miles, mainly pottering speed, 30-60mph (don't have a rev counter but at a guess 2-4000 revs with the odd blast up to 5-6K revs)

Recap, newly built motor, complete overhaul top to bottom (SRM Wales) new Premiere carb., Pazon Surefire EI, engine has about 2K miles on it. Tappets adjusted, timing done etc. etc.

I've been chasing an annoyance rather than an issue, if you know what I mean. The bike runs really well but I had to change from BP7ES to BP6ES (hotter) as the plugs were fouling (probably due to the pottering around I'm doing!)

I also changed the pilot jet in the Premiere carb to a leaner one and moved the needle in the slide to lean the fueling.

I still get very little difference in the pilot air adjuster, I can turn it out 2-3 full turns & it does not seem to make any changes, it only seems to make a difference from 3/4 out to fully in, at the moment I have it set to just over 1/2 a turn out.

Overall the bike runs great, light biscuit brown on the porcelain but dry soot on the round 'rim'. That's the bit that was annoying me, thought it should be running a bit cleaner but as others have stated in a previous thread, the pottering around I'm doing doesn't help matters!!

So, good running bike, slight annoyance on the fueling of the pilot circuit but ticks over OK & runs great.

When checking the bike I noticed my plug gap was just over .040", the recommended is .020" hmmmm I thought!

I decided to change the gap to .022" to see if it made any difference, good or bad.

Just looked at the plugs and the porcelain is probably a slightly darker brown, not black & I still have the dry soot on the 'rim'. Bike ran well couldn't really make out any difference.

I'll put the gap back to .040" as I think the porcelain part of the plug was a bit lighter.

So my scientific findings are:

The smaller gap, in my case, did not seem to burn as well as the larger gap from looking at the porcelain part of the plug. Overall it made little or no difference to what I could 'feel' on the bike.

I might even experiment going in the other direction & make the gap a bit bigger & see what that does.

I know each bike can be quite different in its starting procedure & running due to so many factors regarding the individual bike. This one does not want any choke & settles down to a tickover fairly soon after starting (which to me indicates a slightly rich setting low down but as I explained above I've been attempting to lean the settings by changing plugs, needle position & PJ.)

The Norton on the other hand needs plenty of choke to start & takes a while to 'warm-up' & can be rough & snatchy until he warms up but then runs really well.

I’d say some soot on at least part of the end of the threaded portion is almost inevitable.

Triton Thrasher said:

I’d say some soot on at least part of the end of the threaded portion is almost inevitable.

Click to expand...

Starting to think that, I'll put a new set of plugs in next time I'm out, I'll re-do my plug-chop test just to ensure everything is OK.

Onder said:

Two overhauled engines and they both have sooty plugs. Not fouled so they won\t run
but black entirely. Also note the idle screws don't seem to have a decided effect as they
would in the past. Carbs new and stripped and inspected. Both run fine but are not happy to idle. Riding is the same mostly but not entirely low to medium speed.
Blame that on the incomers. Oh, gaps are on the tight side.

Click to expand...

Thanks for sharing your info., good to know it's 'normal'

click said:

I still get very little difference in the pilot air adjuster, I can turn it out 2-3 full turns & it does not seem to make any changes, it only seems to make a difference from 3/4 out to fully in, at the moment I have it set to just over 1/2 a turn out.

Click to expand...

The pilot air screw is the air metering screw, 3/4 out best running means you are needing to restrict the air due to a small pilot jet. The right sized pilot jet will put you in the 1.25 to 1.75 turns out range for best idle when tuned on a fully warm engine. Your pilot jet could be the right size but blocked, what size is in their now.

kommando said:

The pilot air screw is the air metering screw, 3/4 out best running means you are needing to restrict the air due to a small pilot jet. The right sized pilot jet will put you in the 1.25 to 1.75 turns out range for best idle when tuned on a fully warm engine. Your pilot jet could be the right size but blocked, what size is in their now.

Click to expand...

I've been down this rabbit hole & totally agree with what you're saying.
I'll check my notes later this evening.

OK, got some info. The 'standard' Premiere carb came with a 17 PJ(2 rings).

I'm now running a 15(1 ring) which is the smallest PJ Amal supply, so this should give less fuel which equals a leaner setting.
Needle clip is on the highest setting, so needle is at it's lowest.
Float height has been checked & is set as per Amals recommendations.

click said:

OK, got some info. The 'standard' Premiere carb came with a 17 PJ(2 rings).

I'm now running a 15(1 ring) which is the smallest PJ Amal supply, so this should give less fuel which equals a leaner setting.
Needle clip is on the highest setting, so needle is at it's lowest.
Float height has been checked & is set as per Amals recommendations.

Click to expand...

That doesn’t make sense. As Kommando explained above, your current set up suggests your pilot is already too small.

A #15 pilot and a needle clip in the top grove does not sound right at all to me.

You‘re either going in the wrong direction, or there is something else wrong that’s forcing you to do this to offset it.

In addition to you going smaller on the pilot jet than normal (17 premier pilot is same size as the old 25 pilot bush used on virtually al 4 strokes) the premier pilot circuit seems to run leaner than the old pilot circuit. Hence why Burlen have standardised on 19 pilot jets on 850 Commando carbs they supply.

Hi Lads, Really appreciate your help & advice.

Interesting comments, I understand what you are both saying but my experience is the opposite of what should be happening!

I did strip the carb at one stage & gave it a good clean, carb cleaner + compressed air etc.

As I said its running quite well & I’m enjoying the bike.

I'll look at the carb again & see if I missed something when cleaning it.

Would a partially blocked pilot jet circuit negate any changes to the PJ size? i.e. if it was partially blocked, putting a smaller or larger PJ in would make little to no difference? Just curious

Circuits normally get blocked at the narrowest spot is the jet, but can get blocked elsewhere but to affect the circuit the blockage needs to result in a hole smaller than the jet.