V7 ignition problems

Technical Big Block Tonti and Loop Frame models
1

Continuing the discussion from Loop frame electrical gremlins:

Re the comment about swoping the coils out there is only one coil on the loops, try checking the distributor as the rotor arm can be out of alignment with the cap, when on full advance on no 1 make a mark on the rotor where the cap lines up, turn the engine over to number two and check that cap lines up with the rotor.
I have discover that if you are one tooth out onto distributor it runs lovely on no 1, but number 2 completely misses the cap hence the reason the rotor arm has such a wide sweep area. Also it is worth looking at the u tube article about V7 electronic ignition, there is a fella in Norway that shows how to convert yor loop to electronic using VW golf parts,
Well it works if you have pacience and it comes out about 40 pounds, simples.

3 Likes
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Thanks.
It’s No. 1 (right) that’s not firing on idle. Left cylinder is fine.
Right cylinder does seem to start firing if I rev the bike up a bit.

Have taken carbs off and cleaned jets several times now, and switched carbs left to right, but same problems with right cylinder. Could be distributor related. Revving the engine and the advance gets it firing.

Newbie question - How do I find TDC on No. 1 cylinder when there are no marks for this on the pulley ?

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Are you saying there are no marks, or just no marks for no1 cylinder?
I’ve never come across no marks /cutouts on the edge but if that’s so I would take the plug out, with a torch wait for the inlet valve to open, keep turning the crank until the piston rises
Then with a rubber tipped pencil feel until the piston rocks over tdc, not the most accurate way but will do for timing

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There are the usual marks for No 2 (left) cylinder.

Nothing for right cylinder.
Yep, I had considered just putting something in the spark plug hole and doing as you described.

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Just make a second set of marks 90 degrees round. You can take the pulley off to do it if it’s easier.

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Had a free hour or two this afternoon.
Guessed TDC on right cylinder using the pencil in the hole method.
Timing seems fine, as far as I can tell.

Might get a new cap and rotor and see if it makes a difference.

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Do you have a link?
thanks

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One cylinder not firing at idle isn’t always due to anything too complex on the original V7.

Over a few years I have experienced this for various reasons and invariably it turned out to be something fundamental. examples are below. Not trying to teach grandmother to suck eggs or anything, just listing various reasons (and cures) I have experienced, this a summary of the same problem at different times:

Carburation, e.g. after a period of non-use. I realize you cleaned / soretd the carbs though so this info. is probably not needed:

I experienced one cylinder not firing (until revving) a couple of times due to a slightly leaking choke-valve (on VHB 29 etc.). The rubber choke-piston face gets compressed over time - sometimes the choke cables can be slightly too short to accomodate flattenting of the rubber face on the valve so the choke piston is effectively lifted (albeit very slightly). If this happens, it will miss on one or other cylinder at idle. Quick sanity-check, back-off the choke-cable adjuster ensuring there is about 1/8" - 3mm free play - But, check tension / free-play of the cable at each carb as well as at the choke-lever on the bars. A problem in the cable-splitter was causing one choke to remain slightly open even with free play at the choke-lever.

Carb balancing - I set-up a U-Tube manometer connected to the inlet manifolds (nice visible green 2-stroke oil in the u-tube) and found VHB’s more sensitive than expected to very small adjustments at idle. A fair bit of difference on settings between L / R carbs to achieve balance. Patience here made a big difference to smooth firing at idle and on picking-up the revs.

Ignition problems - which you are now concentrating on:

The V7 single-points and coil makes static-timing easy to set-up.
For fine tuning I also use a strobe but my experience is: once the timing is set-up and the bike runs well, don’t adjust anything - if a problem happens adjusting can cover-up the problem and not find the root cause.

Reading what you have done - the timing is OK. Double check the obvious: Cam (any sideays play?), the points-gap and that points-contacts really are clean - no grease or oil on them at all (but a tiny bit of lube on the cam / points heel).

You are replacing the distr. cap, I did this and found the original cap is in fact still 100% OK. Same with rotor-arm, slightly burned, it cleaned off but replacement made no difference.

Plugs: Simply put a fresh set of plugs in. I had a low miles NGK plug give exactly this problem - it would not fire at idle - only with revving over approx. 1500 rpm. New plugs - problem cured immediately. No explanation at all. It has happened to me twice with NGK’s now.

Plug caps: - check or simply replace if they are old - especially if original nice looking Bosch resistor caps. I spent way too long chasing this problem and eventually went back to basics - tested the metal plug caps with a meter and found one of them was not OK. New NGK non-resistor plug caps solved the problem immediately.

HT leads: can also give problems. I replaced them all, with new ends, simply to eliminate them as a source of potential trouble.

I hope you find some sense in all this - perhaps it’ll be easily resolved.

Giles

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Thanks Giles.
Plenty of things to check there.

I don’t have choke cables, just the small levers on the carbs, but that’s definitely something I can have a look at tomorrow.

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As surmised earlier in the topic, timing marks on the dynamo pulley are only tied to the LH cylinder as it’s a single-points set-up and the distributor-cam opens points at the correct position for each cylinder providing the static timing is set correctly. There;s no means of deviation, the RH cylinder points-opening and gap has to be correct providing static timing is correct and the distributor is functioning correctly (no undue-slop in the shaft, no lateral movement in the cam, and auto-advance working correctly).
If the condenser fails neither plug will fire on a loop frame ignition system.
On Tonti frame twins from V-7 Sport / 850T generation onwards ignition system evolved into a more precise twin-points twin-coil set-up where timing can be set for each cylinder. The V-7 Loop frame ignition is simple and effective (similar to car ignition of the period, distr. cap seems to be Fiat 500).
It’s worth checking the distributor internals, ensure the cam does not have sideways play, and the advance mechanism is correctly assembled (check the tension springs are correctly installed). Check fully-open points gap is same at each cylinder - 16 to 18 thou, this checks the cam is rotating without any side-play. When setting timing, the distr. cam should be right back at it’s static advance position (5 degrees for 850GT, 10 degrees for V700 / 750), The workshop manual has full details of the distributor in the V700/750 section, and the revised spec. for 850GT timing in the supplement.
Hope this may help a little.
Re. carb settings, bear in mind the idle-screww initial settings are different for left and right carbs.
Hopefully a couple of info sheets are attached from the workshop manual, but you probably have this anyway.
Regards, Giles

850GT specification
850GT specification1920×2714 390 KB

vhb 29, 850GT
vhb 29, 850GT1181×755 132 KB

850GT ignition spec.
850GT ignition spec.1920×2499 516 KB

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I recall once fitting the pulley in the wrong position on mine. As it has 3 holes at 120 degree spacing and the firing interval is 270 degrees, it’s possible to get it to run on one cylinder. To confirm this, you need to check TDC on left cylinder puts the mark in alignment with the arrow on the case.