This problem is exasperating.The bike has done only 18k from new, has been laid up for a year or two. It cranks over but will not fire up. It has an excellent battery, kept on an Optimate. Old petrol drained, new installed. Carbs have been cleaned by sonic and soaking in solvent + compressed air. Compressions on both cylinders 175+. Healthy looking sparks at both plugs (when out of the engine of course!). Umpteen new/cleaned spark plugs fitted , including a pair of NGK Iridiums (=no difference). If new plugs are fitted it will occasionally start but not often enough to be relied on. Once warmed up it will start ok. Operating the āchokeā when engine cold floods the plugs very quickly, when switching off ignition thereās often a loud bang from the exhaust ! Any ideas please? Are the carbs normally this finicky ?
The comment āOperating the āchokeā when engine cold floods the plugs very quicklyā makes me think it is carb related. I would strip and rebuild them again, checking float heights etc. Try running some fuel off into a jam jar and look for water at the bottom. Make sure it is flowing well from the taps.
Where abouts are you? Maybe there is someone localish who could take a look?
maybe worth checking the ignition timing?
hi sorry dont know much about the small blocks- but can i ask is it fitted with electronic ignition? if so over time the pickups might be failing causing weak or more likely intermitent sparking under compression?
Thanks for your reply. Yes, it is fitted with electronic ignition, hence the loud bang in the exhaust when I turn the ignition off after one of my start attempts which has ended up flooding the plugs.
Thanks for your reply. The thing is is that on the rare occasions when it does start and get warmed up it runs faultlessly and is a pleasure to ride. If the ignition timing was out then I imagine I would notice some drop off in performance ?
Thanks for your reply Don, these suggestions are encouraging me to think that the carbs need their float heights checking because it always seems so easy to drown the plugs, even when the choke has been operating for 1 say 3 second burst on the starter motor. Then disengaging the choke completely usually results in one or two thuds as one cylinder fires but does not pick up. Then I discover both plugs are utterly wet with fuel. Surely the choke operation should not be as critical as this (or am I lacking in finesse !) ? Do you know the data for measuring my float heights, does adjustment mean bending the tab on the float I wonder ?
Where are you buying your plugs from? I have heard of a lot of fake NGK plugs out there and they donāt like getting soaked. There is a good article hear about plugs. This is also the company that I always buy mine from.
Thanks. No, Iām not buying my plugs from the internet (Iāve heard of fakes) so I donāt think thatās my problem. Thatās an interesting article about āunglazedā plugs for modern ignition systems. Iāll look back in the archives and see if I can find previous posts about float heights, Iām confident that itās a topic that must have cropped up time to time !
Hi JonnieJ
Iāve had Guzziās in my workshop with similar problems. My 1st question would be is the fitted Ignition system original, ie, Motoplat or is it aftermarket, like Lucas AB11 or Boyer Bransden as this will effect the coils you should have and from the symptoms you describe i would suggest your issues are more with Coils, Leads and Plug Caps.
Ok, so and Ignition Coil suitable for Electronic Ignition will typically have a Primary Impedence of about .3 Ohms and a Secondary Impedence of about 7-8 K Ohm together with the Plug Cap of 5 K Ohm so about 12-13 K Ohm.
Aftermarket Ignition Systems are typically designed to operate using the std āpointsā coils so would have a Primary impedence of around 3-5 Ohms. Also the Boyer system uses 2, 6v coils in series.
One of the reasons for my thinking is the comment about the fact that it will start much better from cold with new plugs but starts and runs fine when hot. A new plug requires a much lower voltage to actually produce a decent spark and obviously that quickly rises as the plug ages and then fouls and a cold plug require much higher voltage to operate than a hot plug.
I assume you have a Multimeter. So to check Primary Impedence, using the Ohms Meter on the lowest scale, typically 50 Ohms connect the meter Leads across the 2 small terminals on the back of the coil. To check secondary impedence: use a higher at least 50 K Ohm scale: connect 1 meter lead to the HT Lead: remove the Cap 1st: and the other Lead to either one of the small terminals on the back of the coil. To check the cap poke a meter lead into either end it.
Please donāt think im being rude, just trying to give clear precise help!
If you are on or near the south coast feel free to call in.
Regards, Nik
Hi Jonnie, similar to my small block 750 Strada, I fitted BP6ES plugs, they are the ones with the electrode protruding, less chance of fouling. Also check your fuel heights as E10 weighs different to E5, needles can corrode, use a magnifying glass to check them, all this I did on my Strada and it now runs better than new.
If you have a Motoplat system then it needs proper earthing, let me know if you have this system as I can go through with you on how to earth it correctly.
Regards Bob
Hi Nik, thanks for this help. Firstly my ignition system is original Motoplat, plug leads are plain copper and plug caps are NGK, type LZFH. Iāve just done the multimeter tests as you recommend, but the lowest scale on my meter is 200 so thatās the scale I used for the primary tests. For the other tests I used the 200K scale. The readings I got were : LH cylinder, Primary 04.2 Secondary 1, plug cap 00.0 RH cylinder Primary 04.4 Secondary 1 plug cap 00.0
The secondary readings on both cyls are āopen circuitā I assume, and the other readings do not show much correspondence with your figures that I can see ? As you can tell Iām not an expert in reading my āCirkitā multimeter for ohms, but itās been reliable for voltage readings over several years . I await your interpretation please and any further recommendations as to my next step. Iām in Norfolk so not able to call in, but thanks !
Jonnie
Thanks Bob, Iāve not got around to checking my fuel heights yet (and have avoided using E10 so far). Can you run me through the process of checking the fuel height and what figures should I be looking for and where, please ? Iāve tried several different plugs (including Iridium NGKs from a reputable dealer) but theyāve all got ādrownedā very quickly. Some instructions on how to earth the Motoplat ignition would be most welcome. Cheers, Jonnie
Hi Jonnie
Ok, so 2 dead Coils and 2 Plug Caps. So before you go putting your hand in your pocket thereās a little trick that occasionally works for the Coils. Cut the Leads back 15mm to make sure the Copper Core is making contact as the screw thread in the caps tend to break up the strands, then retest. Also when testing the Caps i should have mentioned to make the probes from the meter contact the brass insert where the plug goes and the screw thread where the lead goes. Again not being rude, just making sure!
If you still get the same ādeadā results then its new Coils and Caps.
I will check in the workshop on monday to see if i have suitable new or s/h and also replace the plugs again, BP6ES or Iridium equivalent.
She should then fire up a goodun! 
A couple of thoughts from my long protracted āplayā with my 1979 V50. One on line plug retailer, well used in our local network, states the plug to be, or did, as B8, it should be BP and project. Mine came with incorrect and badly fouled plugs, doing about 30mpg.
I found poor starting/flooding and running very rich (Inc the random lost spark bang from one flooded Pot) was at least part down to worn/hardened choke plungers. Available for a few quid from Eurocarb. There should be a slight even indent all round where they seat on the brass jet. Mine, 40 plus years old didnāt show this.
My starting procedure, maybe still under evaluation is one tap off near home when convenient, 2nd (LH and easily accessed on the move) about 500 yards from home. Aim no fuel left standing in the carb, then fill fresh from tank.
I suspect but do not know definitively that as oil prices have rises the proportion of ethanol in E5 has increased.
Hope these thought help. G
Hi Nik,
Iāve tried cutting back the HT leads to get rid of the damage to the copper core where the plug caps screw in. The multimeter test result is unchanged, (i.e. open circuit) so it looks as if I need 2 new coils + plug leads. Please let me know if you can supply them, cost + postage etc. Regards, Jonnie
Unfortunately i donāt have any with the correct Impedence, and that value is important. There are plenty aftermarket ones online just make sure you get the correct Primary Impedence!
Also, i need to correct myself. You should be using a BP7ES not a BP6ES. Its the Big Blocks, ie T3 etc that use a 6.
Regards and good luck. Let me know how you get on.
Nik
Hi Jonnie,
The float heights are a bit difficult to say what is correct but to get an idea put the bike on a centre stand, remove the float bowl, push up the float until the float just touches the taper needle and the needle contacts itās seat, the measurement should be from the bottom of the float to the underside of the carb (the part that the float bowl would be in contact with the main carb body when assembled) The measurement should be 23.5 to 24.5, but they should both be the same.
Check you have no water in your fuel tank, a bike laid up over winter will always sweat in the tank, it might be worth draining it dry, allow the fuel to settle to see if there is water present and refill.
The Earthing bit. If you have a Digiplex system then when you remove the front plastic alternator cover where you would expect the points to be there is a clock type hand that rotates close to 2 pick ups, if yours has points then check that they both are set at the same distance when open on the points cam the gap should be 0.35-0.45 mm.
The digiplex system has no points and the plate that you can slide round a few degrees to get the timing correct needs to be secured with 2 cap head stainless screws and make sure they are tight, the gap between the pick ups should be 0.2-0.3mm.
Let me know how you get on, you can ring me on 07989 612877.
Regards Bob.
Hi Bob, thanks for this info. Iām pursuing suspect ignition coils at the moment, neither of them has passed the multimeter test for their impedence. When I have been lucky enough to get the bike to start it runs pretty well so Iām not suspecting the ignition timing to be the culprit. Iāve already drained the tank once and found no water in it, neither in the carb float bowls. But knowing how to check the fuel height is very useful : I take it you mean that the petrol supply through the needle should cut off when the float bottom is at the distance you give ? Thatās my next step if the new coils donāt do the trick. Regards, Jonnie
Hi Nik,
Iām happy to report that having fitted a new pair of coils (as you suggested) my V65 engine is starting from cold with regularity, dare I say it yet, reliability ! Iām very grateful for your advice. A couple of queries : the new coils have the layout of their terminals different from the old original ones, the new ones have their primary terminals at one end and h.t. lead at the other end. Does it matter which of the two primary terminals carries the + feed wire? I have fitted new plug caps and find I get a much better spark with non-resistor type caps, does this signify any other weakness in the ignition system ? Again, thanks very much for your help.