Full throttle performance problem

[Toys4me]

My carburated twinkie falls on its face at full throttle and about 4500 RPM. Heavy missing and surging at that point. Engine has Stage 1 filter and exhaust, with SE 203 cam, carb has Dyno Jet thunderslide w/200 main jet. Checked petcock and line for vacuum leaks and fuel flow. Slide diaphram checked for condition, and reinstalled carefully. All jets and passageways clean. Did manifold leak check at all 4 points, no leaks found. Noticed improvement when pulling choke. No improvement when running with loose fuel filler cap. Don't force me to go to the Stealer!! Help.

[Toys4me]

Not that it matters much, but this is a 2000 Fatboy with 30K. Also, there are no engine light codes, current or historical. Appreciate any comments from anyone that has run across this before.
Bob

[Coolmaker]

Sounds like a piece of trash restricting the fuel flow in the carb, or the flow to the carb is restricted.

[Toys4me]

Originally Posted by Coolmaker

Sounds like a piece of trash restricting the fuel flow in the carb, or the flow to the carb is restricted.

Yeah, that has been the thrust of my trouble shooting. I did pull a vacuum on the petcock vacuum line, and got a heavy fuel flow from the carb fuel line, say a half a pint in 20 seconds even with the fuel filler cap on. I also blew through the carb fuel inlet fitting with the float needle removed, and it seemed unrestricked. I just wonder if some ignition system gremlin could cause the same symptoms.
Thx
Bob

[Coolmaker]

Could be coil or wires, however, I would think you would have a code showing if it was that. My mind still goes to fuel, because it does not start until 4500. And pulls good until then...

[kelly]

Originally Posted by Toys4me

My carburated twinkie falls on its face at full throttle and about 4500 RPM. Heavy missing and surging at that point. Engine has Stage 1 filter and exhaust, with SE 203 cam, carb has Dyno Jet thunderslide w/200 main jet. Checked petcock and line for vacuum leaks and fuel flow. Slide diaphram checked for condition, and reinstalled carefully. All jets and passageways clean. Did manifold leak check at all 4 points, no leaks found. Noticed improvement when pulling choke. No improvement when running with loose fuel filler cap. Don't force me to go to the Stealer!! Help.

um... ...rev limiter set a 4500RPM?

[Toys4me]

One additional check I did over the weekend was to gradually run up to about 5000 RPM or so in 2nd gear with partial throttle. No problems. That might close the rev limiter idea. Only heavy load, full open throttle at high RPM does this show up. Keep in mind I don't have a tach, but have used many tachs before so my estimate should be in the ballpark.

[kelly]

Q/A first. Not fixes but exploration on symtoms... (and some shots in the dark...)

How long has it been acting up; is their a correlation of gas station you fill up selling blended fuel? (Not that blended fuels will have significant performance decreases but, you will feel an ever so slight decrease as timing advances and blended fuels are not optimally synced)--I seriously doubt this but, it is worth considering because of the "heavy load, WOT" ill handling motor.

Have you had your gas filter replaced lately?

Does it do this when your tank is full? Half, Near empty?

What about fuel pick up tube degradation inside the tank and the fuel pressure required for "heavy load, WOT?"

The absolute worse case thought is stretched rods, although I've never heard of HDs experiencing this--It is quite common on the 80's Chevy V6s though, and what you describe sounds exactly what happens to Chevys when a rod or two stretches. Don't Panic--I don't think it's that. You'll have to wait to the "experience" shows up here.

Oh yeah--if you dare--go second gear HEAVY load, WOT--Check your speed. Then do it gently light load and check your speed. If you feel/see a difference--prolly slippy clutch--and that is allowing you hit the rev limiter when the bike is under load.

Stick to simple first--pull your fuse cover off sun-dry the panel, put the cover back on. And go for a spin--dern electric gremlins.
(every now and again when I ride in HEAVY rain, for an extended ride, more than a couple of hours, the water can accumulate inside the cover onto the panel and start bridging stuff as it takes days for the water to dry inside that panel).

[Toys4me]

Since it has been a few months since I really wound it up, it could have been several months since it ran perfectly. I have tested it several times in the last few weeks, and get the same response, even with varying fuel levels.

I have not changed the filter, but since I got a good heavy flow from the petcock line under vacuum, it must be good (famous last words). Clutch is strong, and not slipping. The engine clearly runs rough when I hit that point of high load, WOT.

I did have the engine shut down during low load cruise conditions a couple of months ago, but it started right back up and has not caused that problem since. I had just filled up with gas, and figured I picked up some water. That is one reason I have checked the vacuum line and petcock so closely.

I just wonder if I am loosing vacuum at WOT high revs for some reason.

[hardluk1]

Are you sure that missing and surging isn't your rpm limiter??? You could help to by try to find another stock carb. Stock ,no thunder slide. That set up is impossible to get right run well and get any mpg's. No tach, buy one. Bet you can't tell 4500 from just before the limiter kicks in when your in 4th gear. Still bet it's that carb job kill'n it. Take your air filter off and see if runs better, run just the backing plate. It could run good in second but by 4th or 5th start run'n out of air. If it is wash the filter ,also check it for excess oil from breathers in it . If it runs better get a larger filter, will need the 8" round cover. A stock jetted and sportster needle carb will handle 100 hp easy with good mileage and great throttle responce, a thunder slide setup leaves some all along the table. Go to nightrider for some good hd tuneing and info .

[Toys4me]

An update: I pulled the fuel shut off valve and filter, and it had a lot of sh-t in it, but not full blockage. Flushed tank several times to clean up and replace filter. Thought I might have it wooped, but it still misses under heavy load full throttle. This time, the enricher did not improve it however. Full tank of fresh fuel. Also, to eliminate the rev limiter issue, WOT up a hill in 5th caused the same missing at 60MPH. Loosened gas cap, but made no difference.

[Kwikt]

Check the plug wires again. What do the spark plugs look like? Are they the original wires?

[Toys4me]

I will double check the plug wire resistence and visual, but they were good. Coil readings seemed OK at the time, but I know that can be misleading. Regarding plugs, current plugs have 15K on them, and look good. Electrode tip is gray brown, light black coating on outer rim. I did a WOT (only) run with new plugs, and basically very little color picked up.

Have read some trouble shooting (HD manual for one) that says to check the carb vent for obstruction. What carb vent? I know I'm blind and stupid, but I see no such thing.

[Kwikt]

If it's not plugs, wires, carb. My next step would be a compression test then a leak down test. Very strange.... but it really sounds like a plugged slow jet.

Kevin

[Toys4me]

I did recheck resisitence on plug wires and coil, all within the manual specs.

From what I can tell from my CV carb research, the main jet/needle is the primary circuit at or near WOT. I would not think the slow jet circuit would matter much at that point. But all circuits looked OK when I went back thru the carb 3 weeeks ago........but then I haven't figured this out either. I'm still puzzled by the vent reference I referred to before.