I have a 230cc Tiger Cub motor with a Dellorto carb, Armac exhaust and PVL style (UPB) ignition. The slow running and mid range power is okay but on long climbs where it requires high revs it misfires very badly and often does not make it up the hill. I have spent the best part of a year trying to get the motor to run correctly but I am getting nowhere and going around in circles. I have asked many fellow Cub owners for help but it mystifies us all. I have checked and double checked the ignition timing which is spot on. I have tried all sorts of spark plugs, main jets, needles and pilot jets but still the problem persists. Currently I have a 35 pilot, 105 main jet and a D23 needle set in the third groove from the top. Mixture screw is 2.5 turns out. Can anybody help? Thanks Jim

How is your air filter connected to the carb? I once had a similar problem but it was not until I looked very closely when revving whilst stationary that I noticed the air hose was being sucked in on itself (soft rubber and a bit perished). Of course once flow was stopped it opened up again to repeat the process.

How is your air filter connected to the carb? I once had a similar problem but it was not until I looked very closely when revving whilst stationary that I noticed the air hose was being sucked in on itself (soft rubber and a bit perished). Of course once flow was stopped it opened up again to repeat the process giving the impression of a misfire.

How is your air filter connected to the carb? I once had a similar problem but it was not until I looked very closely when revving whilst stationary that I noticed the air hose was being sucked in on itself (soft rubber and a bit perished). Of course once flow was stopped it opened up again to repeat the process giving the impression of a misfire.

Thanks. The air filter is a 1/2 moon SM type with a Twin Air foam filter. It has a wire cage to stop it from being deformed or sucked into the inlet. It is connceted to the carb by means of approx 1" length of car radiator hose which is tough stuff.

Sounds ignition related to me.

Is it related to throttle opening....or revs....or both? Does it miss under full throttle right from the very start - or does it only happen at the higher rpms?

Generally when an engine misses under heavy loads and full throttle - it is and indication of weak ignition if it happens at all rpms when applying full throttle. The spark has more throuble jumping the plug gap when the cylinder is full of air and fuel (while it may be able to run OK at lighter loads when the thottle is only partially open). If the engine just misses and backfires out the exhaust it would lead me to believe it was spark - if it misses and backfires through the carb I would think the fuel mixture is lean.

It might also be that your valve guides are too tight and the valves are not closing fast enough at higher rpms. I once had this problem with an engine that had new guides installed and the machine shop did not ream the exhaust guides with enough clearance.

Since you have checked all the obvious likely problems, it sounds as though it may be something a little out of the ordinary. Is it the same whether the engine is hot or cold or does it get gradually worese as the engine gets hotter? I had a a grass track JAP engine once that was fine for the first half of a meeting but as the engine got hot the tappet clearance closed up such that it ran fine up to half throttle but would miss fire at full throttle.

Since you have checked all the obvious likely problems, it sounds as though it may be something a little out of the ordinary. Is it the same whether the engine is hot or cold or does it get gradually worese as the engine gets hotter?

Is it a recent 230 build? Not wanting to "name names" incase this suggestion is wrong, however, I have heard that some "non-Serco" alloy barrels can "ovalise" due to the position of the pushrod cut-out. Just wondering if it might be this, exacerbated when the engine runs hotter, thus losing compression? Just a wild thought!

Is it related to throttle opening....or revs....or both? Does it miss under full throttle right from the very start - or does it only happen at the higher rpms?

Generally when an engine misses under heavy loads and full throttle - it is and indication of weak ignition if it happens at all rpms when applying full throttle. The spark has more throuble jumping the plug gap when the cylinder is full of air and fuel (while it may be able to run OK at lighter loads when the thottle is only partially open). If the engine just misses and backfires out the exhaust it would lead me to believe it was spark - if it misses and backfires through the carb I would think the fuel mixture is lean.

It might also be that your valve guides are too tight and the valves are not closing fast enough at higher rpms. I once had this problem with an engine that had new guides installed and the machine shop did not ream the exhaust guides with enough clearance.

Thanks. The engine is fine at low revs and mid range. Also I can crack it open to full revs no problem. It's only at high revs when blasting up a long climb that it misfires. When I shut off throttle it backfires once. No spitting in carb at all. No smoke or burning oil. Engine had new guides fitted in December 09 and they were free. It might be that I need more clearance in tappets. Currently 2 and 3 thou.

Is it a recent 230 build? Not wanting to "name names" incase this suggestion is wrong, however, I have heard that some "non-Serco" alloy barrels can "ovalise" due to the position of the pushrod cut-out. Just wondering if it might be this, exacerbated when the engine runs hotter, thus losing compression? Just a wild thought!

Thanks. Engine was rebuilt in December 09. Still plenty of compression when its hot or cold. No burning oil.

Thanks. The engine is fine at low revs and mid range. Also I can crack it open to full revs no problem. It's only at high revs when blasting up a long climb that it misfires. When I shut off throttle it backfires once. No spitting in carb at all. No smoke or burning oil. Engine had new guides fitted in December 09 and they were free. It might be that I need more clearance in tappets. Currently 2 and 3 thou.

If it runs fine for a little bit and then acts up - It sounds like you have a fuel flow problem. You may be running your float bowl dry when under full power. Check for a restriction in the lines running from the tank to the carb, a low flow filter, or a restriction in your fuel tank vent.

Edited January 17, 201115 yr by 1oldbanjo

I might be miles out but I once repaired a Greeves, which was having exactly the same problems (it ran very sweet up to ¾ throttle)

The carburettor was a new item, all the jetting was correct but it had a 4stroke spray tube

Is there any chance yours could possibly have a 2stroke carb fitted?

Edited January 17, 201115 yr by Monty_Jon