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Third post, and boy howdy, we got a fun problem.

3K views 27 replies 8 participants last post by  swifty2014 
#1 ·
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 Uploads?preview=2019-04-25+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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#2 ·
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?
 
#3 · (Edited)
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.
 
#5 ·
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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#7 ·
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.



 
#8 ·
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.
 
#11 · (Edited)
Just tried testing one of the ignition coils on a hunch. Did per manual specifications, and now the bike won't start up at all. Charging it on the trickle charger, but it wouldn't start with a jump box attached. Figure that may be the issue, either that or the pulse generator.

- edit - realized it wouldn't start because I'm stupid and left it in first from when I gave it a quick test ride last night. Stalled out unless I had throttle more than half open, so figuring the problem persists.
 
#13 · (Edited)
Thanks, man, I'm hoping I can get it sorted out. I'm still pretty green myself, this being my first bike and still not having either a license or a permit to ride. XD
That and the MSF course closest to me is around a hundred miles out, so a little ridiculous, might have to see if there's something to get around that...

Update : Bike did run on both cylinders (though weak on front) with the old mufflers on finger tight. Just to rule it out, I pulled both mufflers and ran the bike. There was obvious exhaust fumes coming from the rear cylinder, but none from the front. Compression felt like the same coming out each cylinder, but one was hot, one was not, and there was the smell of gas from the front header exhaust. From this, I can assume that it's not the air, not the fuel, and not the compression as issues, but most likely the spark's weak. We'll see next week once the new ignition coils come in. If it's not those, it's probably the spark generator, at least so I think.
Or it could be a timing thing.
I'm leaning more towards a spark issue because when I pulled the front left spark plug boot, I felt it grounding through my hand. I'm figuring that it's grounding to frame and also interfering with the other spark plug in some way, maybe by lowering resistance/potential between surfaces or some crap, I dunno.
 
#14 ·
To prove a bad coil you can switch them and see if the problem follows the coil.
It could be one of the spark plug end caps also. They have a 5000 ohm resistor and can burn out. Measure from end to end of both wires through the coil secondary and it should be about 35,000 to 40,000 ohms.
 
#16 ·
Wasn't the coils. Replaced them, got the same problem again.
Also threw brand new stage 1 jets on the carburetors, along with the shim that came along with them.
Now the bike will start, but stalls when given ANY throttle.

The spark plugs are brand new, and should be seated properly. Think I just have a bad carburetor assembly?
I'm freakin' baffled here. I'm going to tear the carbs completely down again when the weather gets a bit better (going to rain all week) and see if it's a blockage or something somewhere.

When I got this bike, it barely ran. I got it running decently for a bit, but the front cylinder wasn't running so hot even then, and now it doesn't run at all, despite good compression and spark. It's just been a slow, downhill decline, and I really don't know what to do about it, especially since I can't afford to take it to a shop.
 
#17 ·
Go back to square 1. You need air/fuel compression, spark and timing. You have checked compression, spark and fuel .

Are you running it with the air intake tubing on? They don't run right when you have it open.
You mentioned it shocked you some when you pulled a plug wire off. Spray the wires and end caps with water when it is running to see if it arcing over to ground.


Are the 2 carbs the same ? It looks like one is a different color . Check the part numbers on the casting near the float bowl seam. See if they are a matched set.
If you pull the carbs look carefully into the intake and look at the intake valves. You might turn the engine over slowly and watch the operation of both valves. See if they both open the same amount.
 
#18 · (Edited)
Pulled them, they're both marked as 11. Soaked the front carb in Chem-dip for half an hour, then cleaned all the passageways with bore brushes, compressed air, and standard berryman carb cleaner. Put it all back together, did a bench sync, and had the same problem.
When I was tearing down and rebuilding the carbs, I noticed that the air-fuel mixture screw was missing its spring, washer, and o-ring. Could this be the problem? I think it might be, since it idles beautiful now, just bogs down and stalls when throttle gets high.
 
#19 ·
Pulled them, they're both marked as 11. Soaked the front carb in Chem-dip for half an hour, then cleaned all the passageways with bore brushes, compressed air, and standard berryman carb cleaner. Put it all back together, did a bench sync, and had the same problem.
When I was tearing down and rebuilding the carbs, I noticed that the air-fuel mixture screw was missing its spring, washer, and o-ring. Could this be the problem? I think it might be, since it idles beautiful now, just bogs down and stalls when throttle gets high.

You don't have all the parts for your carburetor?
 
#20 · (Edited)
Looks like it. Just ordered new screw assemblies.
I think I damaged a couple of the o-rings when I was trying to install some after-market mixture screws and figured I'd worry about it later. I'm gonna say that this bit of dumbassery was because it's been pretty hot lately and I've been doing all the work in my driveway since we have no garage, and likely scrambled my brains. Busted weeks ago, forgot to replace.

At the same time, the front cylinder wasn't running on idle. Now it is.
While I'm still the mayor of dumb**** island, I did get that main problem fixed, and I bet it'll run a treat once the new OEM screw assemblies are in.
 
#21 · (Edited)
Back once again.
Got new screw assemblies in, chem-dipped the carbs and cleaned all passages with compressed air @150 psi. Problem still persists. (I've got fully assembled carbs now, also completely disassembled and removed all the rubber bits before I went at it with the air.)
Front cylinder's still misfiring or firing intermittently, even after carb synch. Synched them up, then ran the bike, riding it up and down the street. Pulled out fine at first, needing only a little throttle, but after about ten minutes it began bogging down heavily and required full throttle in order to not stall when releasing the clutch in first. Rear cylinder exhaust was scalding to the touch, front cylinder exhaust was only mildly warm, as in you could comfortably grab it and leave your hand there for five minutes.

I checked the fuel pump, figuring maybe it was weak and not delivering fuel equally to the carburetors. Tested at 1650 Cm3 over manual spec's 900 Cm3. This was after I installed a brand new fuel filter, just to rule that out as the cause. I also have new ignition coils and wire installed.

When I was disassembling the carburetors (again), I noticed that there was some residue buildup on the inside of the chamber cover and on both sides of the slide diaphragm for the front cylinder. I'll be replacing it when I can afford the part and will update once it's in. As it is, I'm running out of parts I can replace. Really wish I knew what I was doing, and there's no techs that work on these within 100 miles, it's all Harley, all the way.
 
#22 ·
The fuel supply from the pump should be enough for both. If you are in doubt run it and shut off and loosen the bowl drain screws to see if both carbs have about the same amount of fuel. One guy got so frustrated trying to figure out if one carb was bad that he flipped the carbs so the front carb was now feeding the rear cylinder and the problem changed to the other cylinder.
I didn't even know that was possible but he did it.
 
#23 ·
Yep, they both have the same amount in each one.
I'm working on getting the cash together to pay for new slide assemblies, but it's gonna be a little bit since I'm a work-from-home freelance illustrator and I've got a trip coming up, funeral I gotta get to in two weeks. I'll see about updating on progress once I can get those parts in, because by then the carbs will have been completely rebuilt from the tops to the bottoms.
 
#24 · (Edited)
Somebody may have already said this but you can replace just the diaphragms with aftermarket replacements and reuse the slides.
That saves a bit of money versus buying complete new slides from Honda.

I've heard good things about these folks products:
http://jbmindustries.com/K-72-S.html

There are others out there for various $ outlays and delivery times.

There are even complete replacement slides with diaphragms from China, but the low cost, 4 week delivery and loss of riding time would make me say "Wait a minute".
 
#25 ·
Somebody may have already said this but you can replace just the diaphragms with aftermarket replacements and reuse the slides.
That saves a bit of money versus buying complete new slides from Honda.

I've heard good things about these folks products:
JBM Diaphragm for Honda VFR400R , VFR400RR, NC-30, VTR250, PC800

There are others out there for various $ outlays and delivery times.

There are even complete replacement slides with diaphragms from China, but the low cost, 4 week delivery and loss of riding time would make me say "Wait a minute".
lol yeah, I'm a little afraid of some chinesium components. I've heard some horror stories about how often and spectactularly they can fail.
I got some new slide assemblies from TJ Brutal Customs and threw those in earlier this morning. Just took my first test ride (I haven't done full calibration on the carbs yet or the air/fuel mixture screws. I'll be doing that later tonight when it's not 95 degrees out), and there's a definite difference. While I still have to crank the throttle quite a bit to get it to start from first without stalling, requiring now about 3/4 instead of completely wide open, and I've got a bit more acceleration than before.
I'm figuring at this point it's going to come down to getting the carbs synchronized at temp and then doing the fuel screws. Otherwise... I really don't know. It's rideable, but no quick starter. Then again, still a newbie here and never owned nor worked on one of these before. I may be trying to solve a problem that doesn't really exist, but I'm not sure.

Should a Vt750 require about half a turn of the throttle or so to get started out of first without stalling, or should it be able to do it on idle? What are your experiences?
I'll post an update after the wife and I get out there tonight and do a carb synch and drop idle procedure.
 
#26 ·
I can start out slowly with just idle speed and then throttle up and not kill the engine. But when I lowered the speed (to sound real cool) I did kill it a couple of times. Raise it up if it is dying.
But 1/2 turn of throttle is a bit too much I believe.
If the engine is not running smooth you are losing power.
 
#27 ·
Okay, went out to work on 'er today since it was a nice cool 70 instead of 100 with 40% humidity...

Decided to pull the spark plugs and do a compression test before I was going to start warming up the bike for carburetor synchronization. Here's what I found.
Front cylinder spark plugs were extremely black and fouled, also wet with fuel (figure this was from the last time I gave it a ride.) Extremely rich mixture, so I reset the fuel screw to factory 2 1/8ths turns.
Rear cylinder spark plugs had a small coating of black on them, but it seemed a bit old, mixture not too far off, so likely just needs a little tuning there.
However, when it came to compression readings, I got a front cylinder reading of 130, rear of 105.
Squirted some oil in, rear cylinder came back up to 120-125 range.

I think I'm getting there, but I'm definitely going to have to replace those rings. Like with most of the repairs on this bike, this is something I've never done before. Does anyone have any good tips for pulling the cylinder and replacing the rings? Should I address the valves on both cylinders while I'm at it?
 
#28 ·
First did you open the throttle wide when compression testing? It makes a big difference. I did the same thing and got low pressure till I tried it again with open throttle and it went up to 180 #.

Run the mixture screws in GENTLY, and back them off to 2 turns and go from there back and forth between them till you get the fastest smoothest idle . Maybe that will correct some of the over rich condition.
 
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