Hitchcocks Forum

After thousands of miles riding, my Big Head Bullet has started playing up. It has never run properly after a rebuild which was compounded by the epoxy liner in the tank being dissolved by ethanol. That has all been cleaned up now, but the bike will only start with the carb. flooded and the air lever in the fully closed position. I have cleaned every item in the carb. (Mk1 Concentric with new body and slide)with spray cleaner and the pilot jet passages were definitely clear as demonstrated by the spray coming through both from the hole for the pilot adjuster and the internal passage. When it does start it will tick over (rather fast) but no amount of adjusting of the pilot screw makes any difference. When the throttle is opened there is spitting in the silencer, and if the air lever is opened the engine dies. Classic over-lean symptoms. I have checked the carb. flange joint and cylinder head joint for air leaks, but found everything OK.
I must be missing something. Any ideas what?
Thanks
Don

Don, you know they say 99% of ignition problems are in the carb and 99% of carb problems are in the ignition? I think you may be looking in the wrong place, your symptoms could also be of an engine that is over-advanced.

REgards, Mark

You say it was rebuilt, Valve timing and cams correct for head? Compression not too high/low? Ignition timing plus advance mechanism working.. If its never worked right from the start then back to the drawing board as they say and check everything again..

I think Mark and Gwilly are on the right track. Symptoms point back to the rebuild, rather than simply the carb. If all was good before - and all is wrong since - then the rebuild is suspect number one. (Just a thought - why rebuild if all was going so well? Just a question, NOT a criticism).

Hi Don,
When you checked the pilot passageways, did the three holes all connect in both directions? I ask this, because the MK I's are terrible for getting blocked [or being blocked from new] where the passageways meet at 90 degrees to each other. I have unblocked several new ones at this point by carefully pushing the smallest gas welding nozzle cleaning needle from a set of BOC nozzle cleaners right through from both ends of the passageways. It is fiddly, but possible to get this into the passageway from the underside of the carb body with the float bowl removed [the little hole drilled vertically in the body face is the one you want to get into], also remove the pilot screw and go in this way, making sure you go in all the way, by checking the length of nozzle cleaner you can insert, against the underside of the carb at that point. Sometimes, I have felt a dead stop when doing this, but pushing harder, I have felt a sudden extra bit of movement and that is when the obstruction has moved. With all three holes interconnected, the engine should idle and start more easily and the carb will respond to adjustment of the pilot screw. I even had one case of a series II Interceptor which would only idle on one cylinder, while spitting and popping on the other, with brand new MK I's fitted. The above procedure sorted it, though. Cheers, Paul.

Just a note of caution - if you do remove the pilot air screw, to spray or clean down there, be careful because on your carb that is where the delicate pilot bush will be located - a brass bush with a very small drilling [17 thou].

Thanks for all the replies - sometimes you go round in circles without someone else's input. Mark, I retarded the ignition after after it gave me an almighty kick back (the bruise is starting to fade)so it is now at 5/16" BTDC whereas earlier it was 3/8"BTDC. Sorry, forgot to mention it has a Lucas SR1 magneto. When altering the ignition timing the advance retard unit seemed to be working freely enough with no broken springs. Gwilly, the cams are the same as it has always had but it might be worth checking the valve timing again. Gwilly and Presto, I am sure you are right that the problem dates back to the rebuild, but I can't think what I have done differently that would account for the situation. The rebuild was needed due to the big end needing to be replaced with a floating bush after the earlier replacement roller bearing broke up (appears to have been due to incorrect hardening). The problem with the ethanol and tank liner may have been a red herring but coincided with the conclusion of the rebuild. I have the old Concentric carb. body which I can use to experiment with the suggestions made here.

Thanks once again for the help

Don

This afternoon checked the valve timing. All the dots lined up as they should. Had to take the oil pump worm off as it was obscuring the dots on the timing pinion, so I also took the timing pinion off to check that the key on the crank was in place and in good nick. Everything looked OK. Put it all back together and checked that the inlet pushrod had clearance so that it was not holding the valve open. Now it won't run at all, but just coughs and wets the plug. Too knackered to even despair!

If you haven't already done so you might want to check the magneto, HT lead and plug (+ plug cap if you have one?) are still doing their stuff. I love the SR1 magneto, but if I had one on any of my present bikes I'd want to make sure it's 100%, especially if it's the 50+ year old original. 3/8" BDTC fully advanced doesn't sound over-advanced for the timing figure, I think it stayed the same when coil ignition replaced the magneto. The fact that you're getting the plug wet suggests something definitely IS getting through the pilot jet. Nil desperandum. (when you get your energy back).



A.

Following this with interest.