Can anyone help with the diagnosis on my Velo MDD? Re-bored, new piston, rings, valves, guides, cams etc.. I have covered about 450 miles since the re-build with no problems. The last three times I have been out it has ran perfect for about 20 miles then it starts to run perculier, it jerks and wants to die unless I roll the throttle off to just open. if I try to increase the throttle it jerks in a stop/start manner then dies. I have had to limp home on slight throttle in 1st. The first time I stripped the carb thinking a jet was blocked, the 2nd time I checked the timings and auto/advance unit and now I am at a loss. Could it be trying to seize?

email (option): alistair@jimpeet.co.uk

I always check the simplest things first- is the tank cap venting OK? When it starts to lose power, stop, then open the cap. If you hear an inrush of air, the tank may be sealing and creating a vacuum, inhibiting the flow of fuel. Or just open the cap a crack while running and see if it improves.
How about a clogged fuel filter? Similar symptoms , I had an MGB once that had a rust choked fuel filter , worked fine until I got on the highway and it would lose oomph at speed.
Bad coil or condensor? Is it a mag or coil ignition?
Valve clarances tightening up?
Might be trying to seize- but I'd think that 450 miles should be suitable break in, have you changed riding style in the last few rides? Before you pull the engine apart though I'd work down from simplest to hardest checks first. Best of luck,
Vincent

email (option): vinver@ns.sympatico.ca

Could you be running over rich?
Do you have a heat insulator (Tufnol or similar) between the carb and the head?

email (option): pvlietstra@gmail.com

Sorry I didn't reply to your email Alistair. It's possible your piston is getting tight in the bore as it gets hot. What piston skirt clearence did you get at the rebore?
I had done well over 500 miles of carefull running in on my M20 before the piston started to nip up. I had to have a new piston and re-hone.
But as Vincent said, start with the easy things and work from there. Ron

email (option): ronpier@talk21.com

Peter, I put a carb on the bike to the original handbook spec, it had a 6/4 slide with a 130 main and I dropped this to a 120 main as spec., but I do think it is running rich. The spark plug is quite sooty when I investifgate it. I've checked and double checked the needle height slot, unless it could be the float bowl height? I do have a heat insulator between the carb and head.
Cheers

email (option): alistair@jimpeet.co.uk

Thanks for the advice, I'll start with the easy stuff 1st. The clearance on the re-bore was about 5 thou. On the mag issue, it does have that feel as if there is no spark then it cuts in giving a jerk, but can the mag give a problem only when hot? Thanks, Alistair

email (option): alistair@jimpeet.co.uk

Could it be a tank liner breaking up and partially blocking the tap, I've seen a few of these lately and they all seem to run 15 to 20 miles and then stop, only restarting after it has had chance to refill the float bowl

email (option): davmax@ntlworld.com

Dave,
Thanks for replying. I don't have a liner in this tank and it's looking quite clean. I've just tried the petrol cap to see if it is venting which it is. The fuel filters are o.k. I was wondering if the float height could affect the running like this, but I've done over 450 miles without a problem. Alistair

Alistair I don't think it will have anything to do with your problem, but when I restored my MDD the carb was rebuilt by Martyn to the WO spec. However I found the bike would only run with choke. So I discussed it with Martyn and the conclusion was that the WO in there quest for fuel economy were strangling things a bit. So he rebuilt my carb to the spec of a pre war iron head MAC. ie 6/3 slide and 130 main jet as opposed to the WD 6/4 slide and 120 jet. The bike ran perfectly after that. Of course your bike runs fine for 20 miles so it can't be that. Ron

email (option): ronpier@talk21.com

Ron,
When you converted to the 6/3 slide and 130 jet etc., did yours run rich with a sooty plug? Alistair

To be honest Alistair, I haven't done enough milage to tell. It just wouldn't run with the WD settings.

Ron

email (option): ronpier@talk21.com

To update, I've now been through all the easy tests, so I went for a spin tonight and it showed the systoms immediately! Damn!

hi,whats your spark like at the end of the H.T lead?could be the condenser breaking down,cheers rick

email (option): richardholt@rocketmail.com