Third post, and boy howdy, we got a fun problem. - Honda Shadow Forums : Shadow Motorcycle Forum
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post #1 of 28 (permalink) Old 04-28-2019, 07:23 PM Thread Starter
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Model: VT750CD2 ACE
Year: 1999
Location: Texas
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Third post, and boy howdy, we got a fun problem.

Hey again, all.
Been working on this bike for about five, nearly six months straight now, and so far your help has been invaluable.
I started working on some of the final bits before I can actually ride the old girl around, but I'm stumped on this one problem.

My bike's a 1999 VT750CD2, and I've rebuilt the carburetors, and the jets look stock, at least from what I can tell. I made sure the floats were set to 7mm per manual spec using the mechanical test method, the one where you blow in the fuel line and push it until it closes. Everything seems to be working well with both, both diaphraghms showed no problems, jets and bowls clean as factory.

Here's the problem. Rear cylinder adjusts with the pilot screw no problem, bogs down, chokes, all that goodness. However, it doesn't do that on the front cylinder. Additionally, when I rev the engine, at around the >60% mark the engine starts bogging down. At full throttle, it sounds like it runs, then cuts out, then runs, then cuts out. Like a Vrrrrrrrrrrrrmmm! ... Vrrrrrrrrrrmmm! ... Once the throttle is released, it falls back to a nice happy idle. It also sounds like there's some escaping gas somewhere, but it's not coming out of the spark plug ports. I figure it's a cracked carb boot, but from what I'm reading the symptoms don't match up. One is cracked and I ordered a new one, but I'm not sure it'll fix the issue.

https://www.dropbox.com/home/Camera%...5+20.36.31.mp4 Here's a quick video of the problem. I cranked the throttle to full in the start of it, so you can hear the issue.

Mind, this is my first bike, and I still don't know exactly what it should sound like. Best approximation I've heard is from videos, since Harley owns the motorcycle game in the town I'm in. I do know that what it's doing definitely sounds wrong, though.

Any of you gearheads got an idea what it might be?

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post #2 of 28 (permalink) Old 04-28-2019, 10:51 PM
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Put a rag over the exhaust and rev it to see f there s an exhaust leak you hear.
Did you sync the carbs after they were off so both throttles are opening the same amount?


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1983 750 Shadow
From the past=
1951 Cushman scooter
1962 Honda 305 Dream
1965 Honda 305 Dream
1971 Honda 175 scrambler
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post #3 of 28 (permalink) Old 04-29-2019, 02:36 AM Thread Starter
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Model: VT750CD2 ACE
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Yup, did a bench sync before I put them back on. I just replaced the exhaust gaskets - bike didn't have them when I got it and the pipes were on finger-tight. No idea how the PO rode it like that. I'll try the rag tomorrow, and will recheck the sync as well. New boot's coming in two days, so if nothing else, one bit of preventative maintenance out of the way.

Last edited by Aphexangel; 04-29-2019 at 02:53 AM.
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post #4 of 28 (permalink) Old 04-29-2019, 12:28 PM
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Quote:
Originally Posted by swifty2014 View Post
Did you sync the carbs after they were off so both throttles are opening the same amount?
Do you have to re-sync every time you pull the carbs off?

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post #5 of 28 (permalink) Old 04-29-2019, 01:34 PM Thread Starter
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Okay, checked the exhaust, no leaks. Boot was seated wrong, engine stopped bogging out at higher rpms. Got the carbs pulled to sync, and noticed this.
Left carb is for the front cylinder, and both are completely at 0, decoupled. More fuel holes are exposed on the front carb than the rear carb - is this normal, and if not, how do I adjust?
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post #6 of 28 (permalink) Old 04-29-2019, 01:56 PM Thread Starter
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Fully closed. Am I just gonna have to deal with one cylinder running rich?
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post #7 of 28 (permalink) Old 04-29-2019, 05:46 PM
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Those are transistion ports that allow a smooth change when opening the throttle slowly from idle. As long as the throttles are synced and working in unison it should be fine. If the throttles can snap back closed they are good

If one cylinder is richer it would be another reason such as a wrong jet, loose jet, clogged air bleed passage somewhere.


You only need to sync the carbs if they are taken apart from each other and the linkage is worn and moved on the adjustment.


A final actual running sync is best however with a gauge, even home made. Restrict the hose to smooth it out.




Only a biker knows why a dog sticks his head out of a car window.
1983 750 Shadow
From the past=
1951 Cushman scooter
1962 Honda 305 Dream
1965 Honda 305 Dream
1971 Honda 175 scrambler
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post #8 of 28 (permalink) Old 05-03-2019, 10:04 PM Thread Starter
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Okay, did some testing, cleaned the front (problem) carb again to make sure everything's working and all passages are clear, got the new carb boot on, ran the bike about 5 to 7 minutes... Main exhaust was hot, front exhaust merely warm. We're talking instant singe to "I can hold onto this for about a minute then it's too uncomfortable" difference. This was after I synced the carbs as best as I could.

I rebuilt the airbox, so that's clean and there's no blockage there, the filter's fine otherwise the main cylinder wouldn't run, there's gas getting to both of the carbs because both of the bowls fill, both are calibrated to spec, all passages are clear, didn't hear an exhaust leak when I crammed a rag in each pipe, pulled the spark plug wires while the engine was running and it sounded like they were sparking across air, so the ignition coils SHOULD be fine (I'll be taking an ohm meter to them later)...
The only thing I can think is that maybe the exhaust is blocked for the front cylinder? Figure I'll try pulling or loosening the pipes tomorrow and see how it runs.
Since it used to run with the pipes off, maybe that's the issue - just needs new pipes or a good cleaning. Dunno. I am baffled.
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post #9 of 28 (permalink) Old 05-03-2019, 10:38 PM
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Have you adjusted the valves? That can cause a mismatch cylinder.

Only a biker knows why a dog sticks his head out of a car window.
1983 750 Shadow
From the past=
1951 Cushman scooter
1962 Honda 305 Dream
1965 Honda 305 Dream
1971 Honda 175 scrambler
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post #10 of 28 (permalink) Old 05-04-2019, 11:52 AM Thread Starter
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Nope, haven't touched the valves at all, or the timing. I got compression test results of 150psi per cylinder, so I figured those were fine. Do those sometimes go out of spec?

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