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Old 09-09-2016, 09:14 AM   #1
fuckwinter
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Default very odd issue with fxs not responding to throttle

I've been hunting on here and all over the web for something similar and I can't find it - so here goes:

on my 76 fxs chop the bike fires up (e start) great. idles great. compression in both cylinders is 135 (done on a cold engine), plugs are a nice tan/brown. electrinic ignition, new coil, wires etc.

super e carb. had 295 / 72 jet combo and accel pump when I got it. been running it but after 10 minutes it totally bogs down in 1st gear when trying to accelerate from a stop. in other gears it will bog but not as much. eventually it will catch up but it still feels like there's a limiter on the bike. the rev limiter on the ignition is set to 5500 - perhaps that's too low? But why would that affect 1st gear take-offs? It's literally unsafe to ride in traffic because i may not be able to accelerate up a hill.

i dropped the jets to 28/70 and while it felt a little better in the powerband, sure enough 10 mins later as i tried to start from a stop it just died. The accelerator pump is working and is about 2 3/4 turns out.

Checked primary chain tension and rear chain tension. Adjusted clutch per spec - again. Bike does not surge or drag regarding the clutch nor does it buck or spurt or pop regarding fuel. It sounds great and the throttle response is right on - its just that the bike doesnt GO anywhere in first?

The only way I can really descibe it is as if you started off from a dead stop in 3rd gear.

any ideas?
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Old 09-09-2016, 11:58 AM   #2
farmall
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Default Re: very odd issue with fxs not responding to throttle

Does your engine run better if you use the enricher?

Do cold compression check for reference. Warm up until it malfunctions. Perform hot compression check. What do you see?

Always begin t-shooting with a compression test even if problem is likely unrelated. A few minutes tells ya your engine condition and eliminates surprises.

May be a thermal related electrical problem.

Swap in the points ignition wise bikers keep for troubleshooting. Swap in the spare coil with wires and fresh plugs wise bikers keep for troubleshooting. (Ignition parts, like toilet paper, are best bought in advance.)
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Old 09-09-2016, 01:10 PM   #3
Blackbetty
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Default Re: very odd issue with fxs not responding to throttle

I had a similar issue - bike ran fine but would break up under throttle. Was unusual because it was fine and then the problem appeared - traced it to a loose ground. Was making enough power to keep it running but when vibes and rpms went up the ignition wasn't getting the juice.

Outside of that I'd check to make sure the timing is right. They'll bog off throttle if it's excessively retarded.

Where is your accelerator pump set? You'll want to turn it completely off to help diagnose the problem.

295/68 is a good baseline for 74" shovels
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Old 09-09-2016, 01:43 PM   #4
joe49
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Default Re: very odd issue with fxs not responding to throttle

What size air bleed jet you have in there? May be to small.
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Old 09-09-2016, 02:57 PM   #5
fuckwinter
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Default Re: very odd issue with fxs not responding to throttle

thanks all - no air bleed as its an older Super E. ill double check the grounds and add an extra just to be safe. also realized that i added some 10w30 to the primary and am wondering if i am causing the plates to stick once warm. although the shifting has always been fine. bike shifts into/out of all gears inc neutral with no issues. just doesnt respond to the gas.

i tried with accel pump off, baselined, 3 turns and 4 turns. i'm hoping its as simple as a ground and or the oil in the primary. since the plugs were a nice tan/brown I thought the timing was pretty close?
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Old 09-09-2016, 03:19 PM   #6
Blackbetty
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Default Re: very odd issue with fxs not responding to throttle

I doubt it's the oil in the primary - if there was a condition where the plates were sticking the bike would likely lurch w the clutch pulled in and would be difficult to shift.

Plugs probably aren't an accurate barometer for this problem either. Get the super e back to baseline settings with accelerator pump off and then I'd bump the timing up a few degrees and see if anything changes. Really sounds like a timing thing to me unless there's a bad ground somewhere along the line.

You could also potentially have a massive intake leak that only presents itself once the rubber is warm.
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