Triumphrat

dmc71t120r

Here's a bit of a mystery for someone; Since a rebore, piston and ring replacement the right cylinder of my 71 T120R has raw fuel pouring out the exhaust. Here's what I've done to date;

-the bike came with 11-1 pistons and finding fuel was a nightmare. Also fitted with 30mm Mikuni VM carbs. I decided to make the bike more rideable and go to 9-1 pistons.
-the right side seemed to idle poorly originally and the bike would miss-fire at 6000rpm
-Stripping down to replace the 11-1's with 9-1's I discovered the bores were scored and required .20 over re-boring (my 6000rpm 'miss-fire'?). This was done and we ended up with a 6 thou gap. This was by design as the machine shop suggested I would be less likely to seize although I might get a little slap on start-up.
-new hastings rings and 9-1 pistons
-fired it up and it runs on the left side and pours fuel out the right
-swapped leads...same problem.
-swapped coils and tried a new coil on rights side (6V in series) ...same problem
-removed rh carb, checked float level and it is within spec (24mm from top gasket face).
-float needle tested using a hose to blow into the inlet and it is opening and closing fine. Note: there is no fuel coming out the overflow tube when it's flooding.
-checked and blew out all jets. Put the mid-range needle c-clip at the top position to lean as much as possible. Idle air screw at 1 1/2 turns.
-replaced carb...same problem.
-swapped carbs, rh to lh...same problem.
-checked spart by holding the plug grounded to the cylinder head and switching the Boyer MK111 of and on. Nice blue spark which matches the lh side.
-New battery, 13+volts (keep it on a battery tender)
-Checked valve clearance- valves both at 1.5 thou when closed. Both push open and closed.
-static compression checked at 155psi both sides.

Ideas?

Snakeoil

Assuming nothing mechanical wrong with the jug/piston/rings/head, a few things to check.

It is firing on the correct stroke. Not sure if the Boyer is a lost spark design that fires on compression and exhaust stroke. If so, forget that. If not, make sure it's firing on the correct stroke and is timed correctly.

If you have the original points set up and you cannot get to the bottom of this, you might want to put the points and condensers back in, static time it and see if it will start and run on both jugs.

You never said you swapped the plugs or tried new plugs. I would try that. Plug that fires outside the cylinder does not mean it fires under compression.

Do you have a choke on these carbs. If so, any chance whatever controls the choke on that side is not properly adjusted?

I was going to say sticking float, but swapping carbs indicates that's not it.

I know you said you have 155 psi in both cylinders, but I'd squirt some oil in that jug thru the plug hole to see if that gets it to fire. I know it is not recommended to put oil on new rings, so you may want to save this for last.

You have all the right things for it to run. So one of them is not right. Don't assume anything. Confirm everything.

Not familiar with the carbs you have. But is it possible to put the slides in backwards. If so, check them for proper orientation. Don't ask me how I know this one.

regards,
Rob

dmc71t120r

...thanks for the help.
-Yes, I tried new plugs...NGK B7RES (hoping for a little more current) and then Champion N3C's ...no change.
-Swapping coils and plug leads doesn't affect the lh cylinder so I'd say the Boyer has wasted spark
-The Mikunis are keyed so you can't reverse them
- I was very careful during the whole rebuild to keep things locked in place and marked the Boyer rotor and stator and the lh side seems happy.
- doing the thumb in the plug hole idiots version of compression test gives you a good pop consistently plus I used a screw in compression gauge several times and consistently got 155.
-one thing I haven't done is swap the slide needle valves. To make life was with several carb swaps and cleanings I swapped carbs but left the slides on the same sides...maybe I'll try swapping those but I'm not sure what that could do...the needle is clearly seating. I'll also try running with the rh plug removed and grounded to see if something strange happens when it's running as opposed to static.

Caulky

"Checked valve clearance- valves both at 1.5 thou when closed. Both push open and closed."

Please explain this!
The inlet valves should have 0.002" gap and the exhaust 0.004" tappet gaps.

Snakeoil

Good catch, Caulky. I should not try to help after the wife and I have shared a bottle of wine.

Be careful with the Boyer, DMC. I understand you can damage them if you do not have both plug wires properly grounded thru a plug. I know on Miata factory ignitions, if you crank the motor with the plug leads off you destroy the ign module.

regards,
Rob

Mr.Pete

You can ground a plug lead to the head.It's safe and keeps the coil voltage low.
Just don't leave a plug lead dangling and not connected.It should be grounded to the head or connected to a grounded spark plug.

henryanthony

Hi dmc,

As far as my sketchy knowledge goes, your diagnostic tests are comprehensive. The only thing I can think of is a sticky valve that is opening but not closing on compression. But, with a strong spark and fuel passing out the exhaust, I would at least expect a huge backfire at some point. However, in a condition with the spark plug flooded with fuel, a spark may not be possible while running but only while being tested outside the cylinder.

Hopefully you will find the answer to your problem and post the solution to the forum.

All the best,
Henry

RII

Sounds like carbs aren't the issue, if you can swap from L-R and the problem remains. Lends itself to ignition, or lack thereof. Boyer's are wasted spark, so as other have said, if it is firing you should be getting one heck of a backfire at some point, bare minimum.

If the plug is firing outside the cylinder when grounded, and the plugs are new, try this experiment:

Find a ring terminal that will fit around the plug body. Put a short lead on it, and run the plug through it into the head. Run the lead to a head bolt, or similar ground location and see if you get fire. This will elminate a problem with the ground side of the equation for the plug.

If you still are dumping gas, it almost has to be a the carb. I would "bench test" the carb just for kicks, and as you said they are mikunis, make sure the enrichment circuit is closing properly. If it is flooding through the enrichment circuit, you may not get any fuel flowing out the overflow tubes.

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dmc71t120r

Thanks all. I've revisited things. Tried running with a plug stuck to the head to ensure spark as before I just switched the boyer on and off. I get a very nice spark (careful to make sure it's always grounded). I tried isolating the carbs before I swap them back again by pinching the carb connecting fuel line and I hear signs of life occasionally. I've had two plugs on hand to try and keep a 'not fouled' plug handy. I tried using the petcock on the RH side as a variable valve of sorts to see if i could get some action by limiting feed but that had mixed results. As the RH cylinder seems slightly warm and I see signs of smoke occasionally out the exhaust I'm going to tackle the carbs again. I did have a very stiff choke valve (enrichment circuit) so I may well have an issue with this. I would have thought that it's a mechanical push pull affair and it would work if the plunger works ...but I doubt everything and anything at this point. The carbs were set up for 11-1's so the jetting may be way off. Oddly enough it runs beautifully as a 374cc single although very rich and starts first kick. With all I've done I may have fixed one problem and introduced another and thus all the confusing symptoms. By the way, for those of you thinking of a Morgo. I finally sorted out the blown seals and leaks. Note that I have a QDP belt and dry clutch and that side vent has been plugged and holes drilled in the right side and a vent drilled out the timing case. I went back to a stock pump and attached a Ford PVC valve to only allow inlet breathing and it works great. I also disconnected an oil cooler that was doing nothing but acting as a high resevoir that drained into the sump when the engine shut down and made things worse for wet dumping. It's now very dry and I have 70psi at 3000rpm and about 35 at about 1000. The valves are set sloppy so 1.5 thou is an estimate. i'll check again but they are nice and rattly. Again, compression 155 dry, lots of pop at the plug hole and bang at the exhaust. I'm leaning back to carbs. I'll talk to Sudco about what I should start with with VM30's, 9-1 pistons and 20 over and see what they say. There are signs of hope. If the RH side ever get's running like the left I'll be walking on air.

Caulky

Switching the ignition on and off doesn`t check the operation of the rotor/stator.
Take the plugs out, earth them [not near the plug holes] and kick it over.
You have spent a lot of time playing with the carbs.
The ignition timing should be done first.
It may be very retarded.
The valve timing is right with the correct tappet gaps.
[if the timing pinions are set properly].
Sloppy (wide) gaps makes this timing late.

I have no experience of mikuni`s, but they appear to be very different to Amals.
http://www.650motorcycles.com/XScarbs.html