Hitchcocks Forum

Hi, I have been faffing about and trying to fix the reason why my 2004 65 keeps on nipping up at around 55/60 indicated. I have had it all apart and used new piston and matched barrel, new oil pump and changed the fuel to be richer. But still keeps nipping up. So I want to go back to the original carb. I have the Amal and it's too confusing for me to set up. So what I need to know is what do I need to get to revert back to original ? Anyone got the kit I need who has converted to Amal? It's the last chance for this bike !!!! I have no confidence in it, short ride today, 10 miles into Cambridge - all ok, then on the way back, hour later, it nipped up twice , both at similar speeds. 50/55. Any help on returning back to how it was would be great. The conversion was done when new. Thanks Andrew

Unless the bike is running extremely weak it could be a simple clearance problem .On a new top end 10 miles sounds just about the right distance required for a ring gap problem to make itself known. When the gap is too small the ends butt together and best case scenario is they expand outwards tight up against the bore. In the absence of fitting instructions a minimum 0.015 for the top and a bit more for the second/third, say 0.018 to 0.020. If the fitting instruction state otherwise use those when setting the end gaps.

I agree with Rick, first thing I thought of reading your post was ring gap. You say you have fitted matching piston and barrel but did you measure them to check the clearances? If the bike is running ok until it nips up I don't think the carb can be far off. Have you taken temperature readings when the engine tightens? It might show if there's a heat problem.

I suggest you take a close look at the fuel tap. I've had a "high speed" weak mixture problem on my low mileage but neglected, 2004 iron barrelled 350 ever since I bought it a year ago.

It suffered exhaust valve seat recession and a damaged exhaust valve (tappet was closing up almost every time I rode it), despite having rebuilt the Mikcarb 24 mm carb and ensuring it had the correct jets (it was very clean inside). It would also misfire if held on full throttle for more than a few seconds.

I had a new valve seat fitted and put in a new valve. I also fitted a larger main jet and a richer valve slide an attempt to richen it up. All that did was to cause it to misfire sooner on full throttle. Briefly closing the throttle allowed the engine to pick up and run again. On a hunch, I rode the bike till it misfired then immediately switched off the fuel tap. The engine then died completely. The penny dropped - the engine was able to empty the float chamber so there was a fuel flow restriction.

I fitted a two foot length of plastic tubing to the tap outlet and watched the tank drain into a can - the flow was slow, but constant over time. I took off the fuel cap but the flow didn't improve (confirming no partial vacuum caused by a blocked vent on the cap).

I took out the fuel tap to check it over. Perfectly clean, apart from a few paint flakes on the filter on the standpipe. I then took it apart to check for an internal blockage, there was none. I drilled out all the fuel passages to a slightly larger size - no better. I then realised there was some moulding flash on the second, lower filter inside the little plastic bowl under the tap body. I carefully cut this away with a Stanley knife blade. I also noticed how very fine a mesh this filter had. This didn't seem to make much of an improvement on the road.

I was beginning to despair and was about to look for a different type of tap but then in a flash of inspiration questioned the fuel system design, in that were a total of three filters between the tanks contents and the carb because there are the two in the tap and a third, canister type filter in the fuel line itself (which I believe is standard). I took the tap apart again and completely cut out the mesh of the bowl filter (the canister filter will catch anything in the fuel line).

Problem solved; the fuel flow improved quite noticeably and if anything the smaller main jet can now be put back in, which will reduce the fuel demand anyway.