Triumph MC dual plug heads?

stephen thomas · May 26, 2008

I'm not there yet, too many other projects in line, _but_:

Recent gas price spikes make it interesting to consider getting one of my basket case Triumphs left over from a previous life, back on the road. Of course the type of gas we got back in the late 60's early 70's is no longer available except at the airport, and if economy is a big part of the factor, it would be nice to run cheap swill anyway.

So, besides lowering the compression ratio to, what, 8.5? 8.0? does anyone have experience with a _successful_ dual plug set up on 9 stud T120 heads? Is the improvement in antiknock worth the effort? What about head cracking between the plugs? Did you use a pair of Honda dual coils, or? Did you stagger the timing? How? (I used to run single Honda dual coils as the main coils on some of these, the extra spark supposedly helped scavenge on the exhausting cylinder. Mainly at the time they were more available, cheaper, & more reliable.

)

Thanks!
smt

donie · May 26, 2008

Stephen, I can only relate to Harleys, it cant be too far off. The old XL models were 9to1. Many still run them stock, but they work out much better today at 8to1.
Dual plugging them is a money maker for some, but the timing needs to be retarded because of two flame fronts causes pinging in its self.
I dont know the Brit bikes, but HD had had mild and hotter cams for various models. The mild cams and low compression works best for pump gas, without the dual plugs.

Paul Farley · May 26, 2008

SMT,
Are you talking about a two cylinder 750cc Bonneville ?

PJF

stephen thomas · May 26, 2008

Donie, thanks. It's always good to get your opinion on MC matters.
Paul, they're T120's, not 140's. So 650cc stock.
Although I have an unused Routt 825 kit here....with H & C hot cams...
Always wanted to build one with that & a 90 degree crank. Then decades flew by.

But as Donnie says, keeping it mild works better today for a pump gas regular driver.

So, 650 Bonnevilles. For now.

smt

aboard_epsilon · May 26, 2008

Don't you have super unleaded ...or shell optimax in the USA ....

that stuff should stop it pinking

you also have to have hardened valve seats fitted ...if it don't have already ...
if it's an alloy head it should have already.

all the best.markj

moonlight machine · May 26, 2008

I have done the dual plug deal on t150 tridents, I'm not convinced it amounts to much. True, the old triumphs have a bad chamber design, but mine all run fine with stock compresion ratio on pump premium gas. The trade off I think is the amount of cooling fin area you lose for the second plug. the main benifit to running the second plug is when running high compression pistons the big dome blocks flame travel. the standard c/r pistons on a t-120 realy don't have much dome as compared to say an iron head sportster. I have about 30 triumphs from the 60s-70s and was a mechanic at a triumph, Norton, BSA. and kawasaki dealer in the 70s. jeeez that was a long time ago!!!!!. I still work on them if you need a hand with any thing.

donie · May 26, 2008

From my very limited experience on those bikes, they have what looks like cast iron valve seats.
The tricks I have learned to make old bikes work on bad gas is, low compression, mild cams, set valve to guide clearance at upper limit, three angle valve seat grind, piston clearance to cylinder at upper limit, and ring gap a little beyond upper limit.
I really cant see much difference between super unleaded and regular. Its seems we have crap and more expensive crap.
I seem to get less ping from Conoco and Shell, with Chevron and Gippos being the worst. I was told the the stations "here" anyway get their gas from the same source, just the brands additive package is added.

moonlight machine · May 26, 2008

yes the seats are some sort of iron. not very hard for sure, when I grind seats I just use stones for iron heads and they cut way easy, You have to be careful not to sink the valves. If the valve gets sunk because of wear or poor workmanship you may find that you need to use oversize valves because as the valve sinks deeper it will not leave enough room for the seat to be cut as the valve gets to close to the edge of the seat. I try to run sunnoco 94 octane. works fine. lots of sunnoco stations around here. I have a trident that I bored and stroked to 1000 cc, it has 11.5-1 compresion ratio, I use cam 2 racing gas, 105 octane and avalible at a local marathon station. just drive up and pump all you want for 5.85 a gallon.

Langanobob · May 26, 2008

smt,

I have a 66 Bonneville. Not sure what the exact compression is since I put in some new pistons years ago and don't recall exactly. Somewhere around 9 to 1 and it runs OK but I don't ride it hard anymore. I haven't had any problems with valve seats. I don't see any need for the dual spark setup and as someone pointed out you lose some cooling fin area. No problems with any head cracking either.

I do remember pushing the envelope lightening the replacement pistons and the vibration is a lot lower than a stock engine.

I have a dual Dyna coil and it works fine. The coils were one of the biggest improvements on the bike and really helped.

The bike is just a lot of fun to ride.

Michael Moore · May 27, 2008

On Guzzis and bevel Ducs you'll normally retard the timing 4-8 degrees when going to dual plugs. Air head BMWs benefit a lot more than the big Guzzis because the BMWs often had really open chambers, where the Guzzi kept the smaller displacement chamber size and got some squish effect (after you set it to the right clearance) with a largely flat topped piston.

When I was importing Lucas Rita electronic ignition kits the manufacturer of the kits told me that if you dual-plug a twin with a pair of dual lead coils, run one lead from each coil to each cylinder. They referred to that as "balancing" the coils.

You might be better off just getting some custom pistons to give some squish (like the factory squish pistons in the triple racers) instead of dual plugs, if the extra plug is going to be a chore to fit.

cheers,
Michael

GregSY · May 27, 2008

I'd just run booster if it concerns you. Are you really going to log a lot of miles on an old Triumph?

BTW I'm about to sell my old 77 Bonneville. It's not a basket case but it's not a runner either.

jim rozen · May 27, 2008

The air-head bmws do benefit greatly from dual plugging. The chamber is large and
the single plug is way off center. On 93 octane gas they do pre-ignite and I've seen
the results: a cracked pin boss and the teeny termite holes on the top of the piston
crown. What that says is that the boundary layer is being blown off by the detonation
and bad things happen.

On my bike it was most noticeable during hot dry days, when running uphill at full throttle.

PUtting a half tank of cam2 fuel in smoothed out things noticeably.

Because cam2 is not available around here now I simply put thicker base gaskets
in. That solved the problem. Things that did not work:

1) octane booster

2) stronger advance springs

Dual plugging allows less advance which is a *good* thing - what it means is
that the flame front can be started at two locations and smooth combustion
can occur in the entire chamber before detonation has a chance to happen.

The one problem is if the second ignition system happens to quit on the road, you
can get burned valves.

If you are serious about this then you will want to fit an aftermarket (non-points)
ignition system, and you need to make some inquiries in a triumph/brit bike site
to get the particulars, for example where best to send the heads to have the
work done if you are not doing it yourself. They will also probably go into detail
about the potential pitfalls.

Jim

JunkyardJ · May 27, 2008

You should be good to go on premium.

Compression should be OK up to 9.5:1 as long as the heads aren't prone to detonation like the beemers jim mentioned. There's lots of things you can do to tune the engine to prevent detonation. Less advance, cooler plugs, richer jetting, so I dunno if a dual plug head is worth it or not. On my one of my old DT 250s I hooked up both plugs, but it didn't make any real difference. I used a car spark coil, and a PVC T fitting filled with silicone and solid core spark plug wires soldered together. It worked, the car spark coil made the biggest difference. It was an OEM mopar points type coil, no balast resistor or anything.

If you do go the dual plug route, get a 3ohm dyna dual output universal coil. They don't need balast resistors, the lower resistance primary ones DO. I dunno if they make an electronic ignition for those bikes, but it seems they're pretty easy to adapt into anything if you're willing to tinker a little.

:cheers:

superalloy · May 27, 2008

Another thing you might consider if economy is foremost: replace the dual carbs with a single carb like the Tiger.

Motomoron · May 27, 2008

Add a base gasket, jet it a little fat. Use 93 octane if need be.

Or, buy an unwanted early 80's UJM (universal Japanese motorcycle) for $400. Yamaha Seca 550s and Visions, Honda Nighthawks and CBs, Suzuki GS 550Es, etc are all dirt cheap and work just fine for getting from A to B.

Sure, I've got plenty of cars and bikes which are WAY more interesting and exciting, but as the precious "vitamin G" edges toward what it cost in Europe a few years ago, I'm starting to look for one of the ment free UJMs I turned down the past few years.

jim rozen · May 27, 2008

The UJM doesn't have the same cache as the 60s-70s
brit bike. Mostly folks are impressed when you show up
driving one of those, means you must be a mechanical
superman to keep it running long enough to actually
*go* someplace. Also an wizzard electronics engineer.

Nothing like a norton for looks I always say.

But then, somehow nobody ever is suprised when I show
up on a '59 bmw. Oh. Those things always run. No
wow factor.

Jim

Michael Moore · May 27, 2008

If you've got a functioning squish band, adding a base gasket kills that and moves you into "more prone to detonate" territory.

On my Kawasaki vintage trials bike (250 two-stroke single) I welded around the edge of the combustion chamber, milled the different head/cylinder surfaces to set the squish and then deepened the chamber bowl to get the compression back down. That let me retard the timing quite a bit as the squish added turbulence for better combustion but it is still nice and smooth and not prone to spin the rear wheel in low-traction situations.

cheers,
Michael

CarbideBob · May 27, 2008

Is there enough meat in the piston to take some off the top in the center like a typical low-compression auto piston? Just a thought.
I played around with dual plugs a lot on my drag bikes in the 70s and never got any advantage from them. Probably depends on chamber shape though.
Bob

aboard_epsilon · May 27, 2008

jim rozen said:
The UJM doesn't have the same cache as the 60s-70s
brit bike. Mostly folks are impressed when you show up
driving one of those, means you must be a mechanical
superman to keep it running long enough to actually
*go* someplace. Also an wizzard electronics engineer.

Nothing like a norton for looks I always say.

But then, somehow nobody ever is suprised when I show
up on a '59 bmw. Oh. Those things always run. No
wow factor.

Jim

what about the yamaha 650 twin ...
that had the looks

all the best.markj

Michael Moore · May 27, 2008

On my CB160 trials project I went with XR100 pistons (brings it out to about a 185) and then dished them. Squish around the edges, but not a huge amount of compression:

cheers,
Michael

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