Cyclone 340 Misses At Full Throttle

[Liquify]

We built a '78 340 Cyclone for ice ovals. Engine uses stock pipe, intake, carb, and ignition. New seals and gaskets were installed when the engine was rebuilt. Our problem is the engine will begin to miss at full throttle when it fully warms up, usually one lap into the race. Back the throttle down to half and the miss will disappear. The reeds are fine and the plugs show our jet settings are good. We assume this is an ignition issue but not sure where to start looking. We do have another ignition available but wanted to seek your advice before replacing the entire ignition system. Thanks for your help.

[harleysportster]

Do you have a good or new choke plunger? Also, do you have the carb jetted for your altitude?

[Liquify]

Choke plunger is good but not used. The engine requires higher octane fuel so we actually start engine with a squirt bottle containing lower octane gas. Plug color is tan/light brown, burn line on electrode is where it should be. The sled takes off like a rocket and runs the correct rpm. It continues to run strong for about a lap then suddenly develops a full throttle miss. The engine was pressure tested and there are no leaks. Thanks for the tips.

[sledhead16]

Disconnect kill switch or tether once, easily could be that, sounds ignition to me

[sledhead16]

Also could be coil failure after they are warmed up

[HoosierDeereMan]

I'm struggling with the you let off the throttle and the miss goes away being electrical. Usually with electrical issues once they get hot they continue to fail until they cool down. To me it sounds like you are starving for air on top end. You mentioned the engine requiring higher octane. So I take it some mods to the engine have been done? I'm assuming you are running the stock carb with a pod air filter? You also said the plugs look good. I think you need to increase your carb size. I don't remember what the Cyclone uses but if it's a VM-34 you may have to switch to a VM-36 or a even a 38. Depending on what has been done to your engine you may be exceeding the cfm delivery capability of your carb.

Troy

[Liquify]

Thanks for all the tips. Will get back to everyone with what I find. Also, spoke to a guy who runs a small engine shop and he said he has had a number of older bikes come through that had good spark when first started but would develop a miss at high rpm when warmed up. He would check a coil by warming it with a hair dryer and watch for change on his meter.

[Liquify]

The CDI was the culprit! We changed everything to eliminate the high rpm miss (coils, trigger, flywheel, jetting), nothing worked. Did not think the CDI was the problem as they usually either work or they don't. Lastly, we found another CDI, installed it, and behold, the high rpm miss disappeared. Never would have guessed the CDI was the problem as the sled would run normally except for the miss at WOT after running some distance. Learn something everyday.

Thanks for all the suggestions.

[sledhead16]

Glad the electrical gremlin got figured out