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No performance past 5k RPM

7K views 54 replies 9 participants last post by  OleDirtyDoc 
#1 ·
Backstory

Bought this bike in June, knowing it needed a little love. Turns out it needed a lot more love than I expected. I've replaced: wiring harness, battery, R&R, front brakes, key ignition, junction box. Just when I thought I had everything running, bike broke down on trip to NY from DC. The way it was acting I assumed it was carb related (last thing I haven't touched). After pulling the carbs decided to also perform the ear shave procedure. After finding nothing wrong with the carbs I checked the gas in the tank and it turns out I had bad gas (water in the gas) from a mom & pop gas station that I stopped by on my trip.

Problem

The ear shave went off without a hitch. Airbox removed, K&N fliters installed, I cleaned the carbs while I had them off (and re-jetted per the instructions). Shimmed the needles, etc. Re-assembled everything. Bike starts great, tweaked the air/fuel mixture needles since I don't have stock exhaust and it was burning a little lean.

Now, when I get out on the road, in fifth gear, I can open the throttle all the way and get no response. It's hard to get the bike much past 65 MPH/5K RPM. This saddens me since I ride on the highway a lot. I'm not sure exactly what needs to be addressed or fixed. It appears the air/fuel screw is really only for the pilot jet, and this is a problem with the main jet. I can't tell if maybe it's getting too much air during high speeds and not enough gas?

I'm really not sure how to proceed and I can't reverse the procedure since the airbox was cut for removal.
 
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#2 ·
The vent line that went to the right ear needs to be tucked into a place that doesn't get air turbulence. Some people stick a filter on the end of it and then shield off the front side of the filter from the wind, or others (as myself) have just tucked the hose into the frame under the gas tank, while even others use the gas tank vent.
 
#3 ·
Do you have a picture of this vent hose? There was the crankcase filter line that connected to the front of the forward cylinder that has a filter on it now, and the carb vent has a filter as well. I didn't see any other lines, but it's been about two weeks since removal. Where on the bike does the line go to?
 
#5 · (Edited)
It was attached to an Emgo Crankcase filter (amazon.com/Emgo-Crankcase-Vent-Filter-14-20900/dp/B0022ZZHG8) that was attached to the side of the frame where the right ear was. I pulled the tube off the filter and placed it under the tank with the opening facing the back of the bike, while the performance was slightly better still wasn't able to get the bike past 55mph/ 4.5KRPM with the throttle fully open.

Could the shims contribute to this at all? I may try removing/adding more to see if that has any affect.
 
#6 ·
You still may not have the vent in 'dead' air.

Yes shims + too large of jets can delay the main/intermediate circuit long enough to bog out. I recently was asked to help with exactly that situation, however they had went way farther than the normal advise given here.

What is your combo, mods, etc.
 
#10 ·
.....from prior experience.... that can do it .

:smiley_th
 
#11 ·
another member on here used the emgo filter and put it on the right ear and had hell getting over 65 we took the vent hose and stuck it on the plastic piece that goes over the fuel sending unit under the center of the tank and he had no issues after that
 
#13 ·
So, as far as mods go. The original owner put a custom exhaust on the rear cylinder, it now snakes under the frame and is routed right below the forward cylinder exhaust



I'm not sure the brand or type this is. It doesn't seem to affect much, but when I originally got the bike it was popping on decel in that cylinder, the aluminum caps were still on the carbs and the jets were stock so it appears they never turned it for the change in exhaust.

I'm going to try to re-locate the hose based on other posts I've read here to see if that helps.

RE: vacuum leak, that could be, I bought new boots to attach the carb to the cylinder, we had a hell of a time trying to get the orientation right so that still might be the case. If so, how could I got about diagnosing that?

I only bought one shim with my jet kit, then put a second "not official" shim which is just a metric washer I had that was about the same size only it's 0.7mm thick. I'm going to pull those out and return to just one shim for troubleshooting purposes.

After all this I'll ride and if it's still acting up I'll pull the plugs and take pics. Thanks for the help so far everyone :pepper:
 
#14 ·
Those pipes are made by Cobra or Vance and Hines, I would need to see the rest of the pipe to tell for sure exactly what you have, but regardless, since you rejetted, you did the right thing. you should be running two #4 washers for shims, and 142/42 jets. pilot screws 2 1/2 to 3 turns out.
 
#15 ·
I thought those pipes were Mac brand. Saw a set on ebay at $300.

Spray bottle with water, squirt around boots, listen for stumble, watch for steam out the pipes.

Propane or flammable spray works, but it can get pulled through pod filters and give you a false positive.

Does it ever run? Like before it's warmed up all the way? Just in case it's POOGS. Didn't sound like it, but ...

I was also thinking fuel starvation. So you're on the right track.
 
#17 ·
The bike runs with lower RPMs, it's not POOGS from what I understand (at least I didn't have the problem prior to ear shave). I've been doing small local trips after each adjustment to see if that helps. Raining today, but I'll take a water bottle down to look for a bad seal.

I'm probably also going to pull the carb and make sure I've got 142/42 jets then re-install the carbs using the alignment markers I learned about earlier today.

Here's a picture of the other side of the exhaust:

 
#18 ·
those are not vance and hines pipes cause v&h have one pipe on each side i would say they are mac or the old jardines that no one can find
 
#22 · (Edited)
Yeah, the original owner never tuned the carb after installation. They came blued like that. Now that I've got it, I've adjusted the mixture and it no longer runs lean. Now I just need to get back to performance at higher speeds/RPMs and we'll be good to go :D Once I get that resolved, I plan to come back through with some Blue Away, or similar, to clean up the pipe.
 
#23 ·
marco, guys I have known would use a plastic plug for the carb vent hose and drill a 1/16" or so hole in the middle of it. If the plug is long enough and you can drill straight enough, you should be able to mount it anywhere but in direct airflow. I never tried it.

Look at the extremes the factory used to find dead air.
 
#24 · (Edited)
Okay, the rain let up for a bit so I decided to try moving things around and applying everyone's suggestions.

Pulled the tank and did and checked the fittings on the carb to the cylinders, they looked good. Removed one of the shims from the front carb (leaving one on) just because I had a strong belief that was contributing to issues. Re-tightend the boot connections, then relocated the carb vent (and EMGO filter) to where the "VALVE, AIR SWITCH" (16126-1139) for the reeds used to sit, right above the rear cyclinder ignition coil. None of these worked.

I guess this could just mean that I still haven't found a dead air spot. It's a '96 750A so I'm not sure where the clips under the tank is, but I'll keep trying to move the vent tub around until it fixes itself or I get tired of trying. I'll also give a smaller plug a go and putting a small hole in it. I may also put the right ear back on and place the hose back where it was to actually verify that is the problem! Doh, just thought of that.

I'm still open to all suggestions at this point, will keep banging my head until I break down and slink to a shop for assistance.

:itchy-scr
 
#25 ·
Just tightening the clamps may not fix a vacume leak on the carb to head boots. It didn't on mine. By really looking hard I could see there the boot to my rear cylinder was a little twisted and tightening the clamp didn't fix it. I had to instead loosed the clamp and wiggle things around until the boot twisted a bit ans settled better on the head side before re-tightening the clamp.
 
#27 ·
I didn't need to pull the carbs but to see where the boot was not squarely on the head intake tube I about had to lay on the ground and peer up with a flashlight. Wiggling the rubber boot around would not make it pop on well so I ended up using a screwdriver tip slid in an wiggled it a bit. Once it popped in place I could see that it fit much better than before.
 
#30 · (Edited)
Checked the boots, they seem sealed pretty good. Routed the carb vent in to the sending unit hose port. Same thing as before. Added the second shim back to the front cylinder, noticed the that the diaphragm/slide doesn't fit all the way across the head, beginning to wonder if maybe it's not sealing properly. I'll take a video of each slide for feedback, I can't tell if it's sealing properly or not at this point.

Anyways, took it out without much improvement, as I was riding around I kept opening the air/gas mixture screw and was able to, when going full throttle, get it to sputter a bit around the 5k, like it was desperately trying to go (almost like it was running out of gas). Then I feared POOGS, so I opened the tank cap for a bit and drove around to no avail.

I really do hope it's not that, as 16126-1173 doesn't look to come cheap. Since I'm running out of ideas though I'm not really sure what to check/look for now.
 
#33 · (Edited)
Have you ever replaced those shakey Vacuum Lines ?
I use 3/16" clear hose off the roll from Ace or True Value. (Do not use Fish tank hose's). The 3/16" fits snugly w/o leaks.

Does the rear carb vacuum port (right side of bike) have a Non-Leaking plug/Cap ? (is it blocked off)

Are you sure both cylinders are firing - ALL the time ?

:smiley_th
 
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