LeMans carb setup

Technical Big Block Tonti and Loop Frame models
1

Hi all,

I stripped, ultrasonically cleaned, painted & rebuilt my carbs using the Dell’Orto refurb kits. I’m reasonably sure that I put everything together correctly, and the bike runs, but it’s not right. Before I strip the carbs again, I’d appreciate any experiences or pointers anyone can offer.

The plugs are black (right) & black /wet (left) so it’s obviously stupidly rich.

It’s idling on the right cylinder, the left fires occasionally, it starts when cold, it will start when warm if I don’t touch anything other than the starter button. Opening the throttle it picks up on both cylinders & revs freely, but starts popping & banging at constant throttle & 5k rpm. Don’t want to do that too much, we have nice neighbours and I’d like them to stay that way.

When idling, the left carb adjuster screws make no difference, the engine does change with the screws on the right carb.

There is slack in the “choke” cables.

The throttle cables are approximately synched, I haven’t put the vac gauges on it yet.

Tappets are correct, timing is also good.

My thoughts are

1 float valves not seating (I don’t think they included new ones in the kit, so I presumably put the old ones back in)
2 floats set too high, causing flooding (or at least incorrect fuel level)
3 I’ve reassembled something wrong

Hopefully Gutsibits have float valves in stock, I’ll order a pair & fit them.

Any other ideas?

Ta
Simon

1 Like
2

Hi Simon,

Just make sure that the throttle cables are lifting the slides at the same time.

There should be a bit of slack in both cables which ensures the slides are fully seated on the tick over screws at idle.

With the engine not running open and close the twist grip and with your ear close to the carbs you should hear a slight “click” when closing the throttle. If there are two clicks then one cable is tighter than the other. Loosening the tight cable will reduce the time between the clicks. Keep loosening the tight cable until a single click occurs. From your explanation, whereby adjusting the tickover screw on one carb is having no effect it would suggest that this carb has the tight cable.

Good luck.

Phil

3

Thanks Phil,

The cables are both loose (just) when the throttle’s closed. I’ve not synched them yet, but that’s, unfortunately, not the issue.

I’ve ordered new float needle valves, so hopefully, I’ll be able to sort it later in the week.

Cheers
Simon

4

Check the condition of the rubbers in the bottom of the choke plungers. They can perish over time and let a bit of fuel through. Double check the brass barrels are sliding up and down freely and are dropping fully onto the seats. Oiling the choke cables is often overlooked and they can get a bit dry and stiff.
My old Le-Mans was terrible for spitting and popping back when you opened up the throttles. I tried everything. Eventually took it to Windy Corner and they cured it by by resetting the ignition!

5

I put a loose feeler gauge under each carb slide with the bellmouths removed so I could see them move when the slide started lifting, adjust cables to make them open together. :smiley:

6

Thanks guys,

New needle valves and choke rubbers due tomorrow, so carbs coming off and to bits again.

Advice on the GSers forum suggests double checking for leaky floats, blocked passages, and the timing.

I shall redo the carbs, ensure every drilling is clear, check the chokes, fit new needle valves, verify the float height, make sure the floats are not sticking anywhere, put it all back together and see where we are. I’ll measure the plug cap resistances, I already tested the coils. The plugs look ok when I clean them, but I’ve got a spare pair, so I’ll throw them in too.

Assuming all well, actually, assuming it’ll run, I’ll strobe the timing and adjust if required. Once that’s sorted, I’ll vac synch the carbs, which will ensure they open together.

Should keep me occupied over the weekend.

Thanks again for the suggestions
Simon

7

My money would be issues on reassembly. Floats mostly don’t leak, float heights don’t change unless you’ve been real rough with them. Choke (enricher) plungers are often a problem part and are cheerfully cheap so should be treated as a service item. Noted that you have new ones on order.

Oh, and 75% of fuelling issues turn out to be ign (and vice versa). But best to start off with the thing you ‘fixed’ last.

8

Yes last thing fiddled with is the carbs and that is where my suspicions firmly lie, but I’ll keep an open mind. I have new needle valves & choke slides to fit, we’ll see where we are after they’re in.

Thanks
Simon

9

Strange goings-on.

I took each carb off, and changed only the fuel filters & float valves. The ones that were fitted had two fibre washers, I put the one new one in.

The old needles had black rubber tips, the new ones are red.

I checked the float height, maybe marginally under 18.5mm.

I poked out the jets with a strand of wire, just to check, tho’ I was confident they’d be ok. Blew compressed air through the carb passages too.

Double checked that the chokes are seating down. I have new plungers, but haven’t fitted them. It whistles when the choke is out, and doesn’t when it’s not, so I think they’re sealing. Easy to change later anyway.

Reset the idle mixture screws to 1.5 turns out, and got it to start. At this point, it was running on the left cylinder, but not the right one. This is a curious reversal of the situation last weekend.

Now, something I’d forgotten, when I was tidying things up, I changed many of the screws for stainless, and I painted some of the bits of steel around the battery tray. I wonder if I had somehow failed to ensure the ignition system earth connections were good enough, so I made up a lead to connect the earth point back to battery negative. This certainly seemed to help.

Well, it was running, but not happily, until I covered the intakes, one at a time, with my hand as I opened the throttle a bit, which led to some serious spitting and coughing, and some quite big flames from the exhausts, and it then settled down to idle quite nicely. I wonder if I had an air-lock in one of the carb passages. I wonder if I had an airlock in the other carb before.

It’s not right, but it’s much better. It starts off the button, choke if cold, gentle twist of the throttle if warm, and runs evenly on both cylinders, not yet idling without slowly puttering out, but it’s now useable…

Then I checked the timing. If the indication on the strobe is to be believed, it’s about 60 degrees out. That can’t be true. Puzzled.

Well, I wouldn’t want to be bored tomorrow.
Cheers
Simon

10

Did a bit today, despite the crappy weather.

Firstly, finding TDC. The recommended approach, using a timing wheel on the alternator end of the crank, and dead stops in one cylinder, did not work quite as planned. I had a bit of an issue laser cutting a protractor, finally fixed that, took off the alternator cover and looked at fixing it to the crank. It’s a special bolt… So I made one, with an extension on which to mount the protractor, fixed by an M8 nut. Tightened the nut, and the plastic protractor exploded. The dead stop isn’t a great idea either. The angle of the spark plug to the axis of the cylinder means that rather than simply gently stopping the piston dead in its tracks, it simply bends the stop. I suppose I could have made it tougher, but it seems like a better solution is called for. I put an insulating sleeve in my modified spark plug, and slid a length of piano wire through it. A multimeter show when the wire touches the piston, and some fiddling proved that the D & S marks are one tooth out. I marked up the 9th tooth before the corrected TDC for both sides, and if the weather permits tomorrow, I’ll be able to strobe it and hopefully get something sorted.

First, static check. Well, the timing hadn’t “drifted”, but the screws that fix the reluctor baseplate to the distributor were loose. Fixed that and regapped the reluctor from the cam. It looks like they hadn’t touched, despite the screws being loose, as the cover prevented the plate moving much, but I’d bet it had moved a bit. Well, it’s tight now. Fit & forget, but only if you put Loctite on the screws…

I had the tank off to do the distributor adjustments, so I couldn’t run it, but gave it a prod and it fired up straight away, so that bodes well. More soon.

Cheers
Simon

11

Sounds promising. A bit odd that the timing marks are one tooth out? Are you sure they are TDC marks and not static timing marks? It’s been a while since I looked at mine so the memory is a bit hazy about what’s what. I tend to ignore the first ones and go for the full advance marks, 38 degrees from memory and check it against those at full advance, about 5K revs with a decent strobe light.
If you can still read enough of your degree wheel, it would be a good idea to check those marks against your new TDC marks.

12

It’s not right, but it’s much better. It starts off the button, choke if cold, gentle twist of the throttle if warm, and runs evenly on both cylinders, not yet idling without slowly puttering out, but it’s now useable…

I used to find, need to ride it maybe up to 10 miles before it’s got warm enough to idle properly. Until then, it won’t idle but poop out. If you force it to, e.g. by raising the throttle stops, then the idle will be too high when it is properly warm. My answer to that was to have a screwdriver handy the first time I’m out on it. :smiley:

13

and some fiddling proved that the D & S marks are one tooth out

The ‘D’ and ‘S’ letters are not the timing marks - the marks are a scribed line, maybe very faint or even rendered invisible, beside each letter - or in other words, the letter is only an identifying label for the mark. I think the letters are placed after the line, in direction of rotation. Ahead of that (in direction of rotation) is the static or fully retarded line, at 8° BTDC. Before that is the full advanced line (can’t remember how far offhand).

Tip - I found 8° BTDC for the static timing was a bit too rough, it was smoother and more tractable if backed off a bit, say 4° - I used to loosen the distributor and gently tap it round a bit either way until it felt right when riding it. Hope this helps. :smiley:

14

Checking the owners handbook, the marks D and S are tdc and the static timing marks are just less than 1 tooth round. Initial advance 0 to 2 degrees and full advance 31 to33 degrees.
Note- Those figures are for a Spada (1978) rather than Le-Mans so this could vary. You’d best check the figures for your bike.

15

Thanks both.

Weather horrid. Will finish fixing wiring and refit nosecone, and then replace the brake discs & fit new number plate.

If the monsoon has abated by then I might take it out for a run. If it hasn’t, I’ll do something else!

Re the timing marks. Yes, it’s ridiculous. There is quite a bit of chat on other web discussion groups about it, and it seems that it is not uncommon. What I will do next time I have the heads off, is get an absolute position using a metal protractor and a DTI.

I hope the heads are not coming off for at least a year! It may be possible to clock the piston through the spark plug hole with a lever type DTI but I don’t have one, I have a couple of plunger ones. Might see if I can get one.

More later
Simon

16

Checked timing, looks like RTBM, re the Gunson timing light. Stupid 60 degrees of advance would have been 30 ish plus the indicated, giving 90, which would be about right for t’other cylinder, I think.

Had a brief ride as I wanted to put fresh fuel in. Bike now starts easily, and is idling nicely, and pulling like a train above 3500, with possible pinking on left pot. Flat below, so difficult to pull away, needs a handful. Transition clearly not right & needs sorting, it’s surely carbs, not timing. I have not yet synched the carbs, so that’ll help. Didn’t get a chance to look at the plugs.

Then had to go for a walk to a local pub with Madame. She’s a hard task master.

Didn’t feel like doing any more today, watched French Moto GP. Good race.

Thanks to Phil, Cyclobutch, Chris, Don & Mike for their interest & replies - it all helps.

Cheers
Simon

17

My memory is rubbish - Don appears to be right -

Annotation 2021-05-09 191558.jpg

You think you got problems, on mine ‘D’ and ‘S’ were back to front. Would you like to guess what happened the first time I tried to do the valve clearances at TDC? Why the pushrods didn’t drop off the rockers will remain a mystery. The slack was several mm.

18

Thanks Mike,

The Rita does not lend itself to static timing, the instructions say to set the reluctor core 5mm behind (ie advanced of) the cam tooth with the crank at the full advanced position, if that makes sense. I didn’t measure the radius but it’s about 15mm, which would be about 18 degrees, so the Rita trigger point has a fixed total advance of maybe 48 degrees. It presumably makes the necessary “calculation” to generate the advance curve from this info. There are no Bob weights or springs.

I’ve marked the flywheel with white paint on the S & D, and on the stamped timing marks, and yellow on the TDCs that I measured, and 9 teeth before. The 9 teeth would be 3.75 * 9 = 33.75 - call it 34, and quick strobe at Max advance showed the yellow teeth bang in the centre so the Rita is doing what it should there. I guess the advance at idle should be around one tooth but it was not obvious. I need to check that again.

More soon, but I have to go to work!
Cheers
Simon

19

Advance at idle is too advanced I think.
Also reaching theoretical full advance at about 3500 rather than 6500.
Have ordered a set of crows feet so I can adjust it.

20

If you have the tank off, you should be able to get the distributor clamp bolts with a short 13mm spanner if you get your hand right in between the barrels.
Watch the gasket, last time I adjusted my timing it tore and I ended up removing the whole thing to fit a home made replacement.