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Discussion Starter · #1 ·
Ok guys, I am really needing help on this, I must be halfway slow. I bought this 1983 shadow vt500c with rear cylinder inoperable. Upon bringing it home, I drained the bowls to discover that the rear cylinder carburetor is carrying orange gasoline! So I had the carbs cleaned thoroughly and put fresh premium gas into the whole system. I then continued to reassemble the bike. To my dismay, the rear cylinder is not firing at all (or firing rarely, or firing slow, or firing some other way but correctly). I checked the coils by swapping them... no difference. Checked the resistance in the wires, it is ok. Replaced spark plugs with the manufacturer recommended NGK plugs, no difference. Tested the spark generators for ohms, checked out. Swapped the CDI on both sides with one another and then in every combination I could with a different one that I purchased, nothing! Furthermore, I checked compression, it is ok! The front cylinder just keeps starting right up and the thing sounds like a lawn mower!!! If anybody can think of anything I missed during my trouble shooting, let me know I am pulling my hair out!!
 

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Sounds like the only thing you haven't touched is the wires. I'd start messing with one at a time.
 

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Discussion Starter · #3 ·
The wires as in the spark plug cap wires or the wires in general? The CDI units each have 5 pin connectors; 2 come from spark generator (continuity OK) 2 go to spark coils (continuity OK) and then there is a green one... I figured green was ground so I checked the green's continuity to the other CDI's green and found continuity.... what else should I check do you think? Also, is there a way to quantify the spark voltage? Such as, can I use a voltmeter and stick it right into the cap and ground it and find readings while the bike is running? I know it sounds dangerous, but this issue is driving me nuts!! Lol. any help would be much obliged
 

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Any time someone posts a problem like this, more than 9 times out of 10 it's a carb problem. It doesn't matter if they've been "professionally" cleaned, re-tuned, re-synchronized, etc, etc, etc. It's almost always a carb problem.

Don't tear into your electrical system too much. Work on the carbs.

--Justin
 

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. . .
Don't tear into your electrical system too much. Work on the carbs.

--Justin
Justin knows Hondas way better than I do; I'd listen to him.
 

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Discussion Starter · #6 ·
Justin,
After referring to these forums in great detail over the past few weeks I do trust you alot and your opinion. You are usually spot-on when it comes to fixes, so I would like to know... What do you think I should do to check them? Should I open them back up and reclean everything? The thing is... the rear cylinder works if I rev it because I feel the pipe getting hot... not as hot as the front cylinder but hot none-the-less. In fact, I thought the thing was firing normally until some drunk guy stopped me and informed me that my bike sounded like a moped and he proceeded to pull the plug wires... which had no effect :?. So I pulled my bike back into the garage and started tinkering again. I think I am going to check valve clearance and carburetor sync, but I am not sure if I can sync the carbs if the cylinder is not working properly... Can I? If so, I think doing those two things will eliminate at least a bit of uncertainty that I have with the system... Let me know what you guys think...
 

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Probably obvious, but I didn't see you say you'd tested the plugs to make sure they were sparking. Pull the plug out, hold the thread against your engine head and hit the starter. If you see spark, you're ok. You should only need to pull one to test.

If you don't, I think I can help you solve your problem, since I had the same issue last week. Coils check out, wires checked out, had a bad connection in the wiring harness.
 

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If you pull the plug, wire it and ground it to the motor, do they spark when you crank? I suspect yes and that Justin has hit it. My kid's 88 VLX carbs didn't work right until after the third cleaning.
 

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Discussion Starter · #9 ·
It sparks when I place a plug against the motor, but the spark is small... What I do not understand is that it is the same even if I switch the coils... All I can imagine is that maybe the pulse generators are not connected properly? I did not mess with them but I would think that there is some sort of degradation... Is it possible that I can swap the pulse generators or test them for actual voltage? Because the resistance checks out on them as I have seen justin say in earlier posts (within 20% of one another and similar in value to the theoretical put out by honda). Furthermore, the rear cylinder spits the smallest burst of grey smoke and revs when I spray starting fluid in the air box so I know it must be getting some sort of spark if it is igniting the starting fluid...
 

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Furthermore, the rear cylinder spits the smallest burst of grey smoke and revs when I spray starting fluid in the air box so I know it must be getting some sort of spark if it is igniting the starting fluid...
Your orange gasoline makes me suspect corrosion in the carb, and your last statement may indicate no or very low fuel flow into the cylinder. Even the weak looking spark should be enough to ignite a good mixture. So Justin and others give very good advice about focusing on the carbs. Clean it out thoroughly with gumout and blow all the ports and needles with compressed air. DO NOT use a wire or any abrasives. If you must you can soak the entire carb in gumout overnight. You do need a solvent as gasoline will not dissolve the varnish or dislodge corrosion. Good luck to you.
 

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The quality of the spark isn't much of an issue. Hondas use what's called a "fast spark" and it is very good at igniting gasoline, but looks pretty pathetic when you see it. if you're expecting a fat, blue "Briggs and Stratton" spark, you won't see it on a Shadow, ever. The spark you should be seeing will be pale yellow, maybe a little purple, and you'll have a tough time seeing it at all in daylight. If you have that, you have what you want.

Since the rear cylinder seems to be firing at higher throttle positions, I'd suggest a few tracks to follow.

First: Did you clean the idle and slow jet passages in your carbs? the idle jet is the one with the needle valve. Did you unscrew the needle valve, remove the o-ring, spring and washer and clean it with carb cleaner and compressed air? if not, your bike's cylinder has almost no chance of running at idle. That idle jet is all the bike runs on up to about 1/8 throttle. The slow jet powers that cylinder from about 1/8 throttle to around 1/2 throttle. Make sure you removed it and cleaned it good as well.

Next, sync the carbs. You can do this with a cylinder that isn't firing without trouble. If your carbs are out of sync, the bike will be running on whichever cylinder has the most air at the idle position. (when they are in sync, they will be equal). The other cylinder won't have enough air to fire, so it just freewheels. At higher throttle positions, this isn't as much of an issue and both cylinders fire fine.

Do your carb diaphragms have any tears or holes in them? That will affect higher throttle positions mostly, but can cause vacuum leaks as well, killing a cylinder.

Speaking of vacuum leaks, do you have any? Those are effective at killing cylinders too!

Lots of things to check, I know, but the VT500s are great bikes and well worth the effort!

--Justin
 

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Discussion Starter · #13 ·
It worked!

Well guys, today I was able to break away from obligations to re-examine the bike and take justin's advice to clean the carbs once more. Sure enough, the carb for the rear cylinder had a jet clog that was pretty intense. After soaking in brake cleaner for an hour or so it sprayed just fine! After installing the carburetor and putting the bike together I gave it a good start. It puttered and smoked and smelled terrible, but the rear cylinder breathed life! I was very excited, I cannot thank you guys enough. However, now that I got the guy running, I rode it for about an hour after putting SeaFoam into the gas tank and filling with premium. I still have an issue with puttering at 3/4 to full throttle. As soon as I pull back it sounds great, but as the throttle pulls further it starts coughing and puttering, so I am guessing this is a carburetor sync issue now? I am not sure, if anybody has input let me know. Thanks again guys, this was extremely, EXTREMELY exciting and I cannot thank you enough! :D:D:D:D:D:D:D:D thank you thank you thank you!
 

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Glad to hear you found the culprit!

Carb sync problem always show up in the bottom 1/8 of the throttle and pretty much go away completely when you're up past 1/2 throttle. It doesn't sounds like a sync problem to me. However, the 3/4-WOT range is where the main jet and diaphragm-operated jet needle are working their magic. Did the carb slides move freely all the way up and down their bores? If they move hard or roughly, or if they get stuck, you'll have exactly the symptom you describe. Also, be sure that the diaphragms have no holes or tears, and that the atmospheric vent tube that hands down between the carbs is tucked neatly between them in a place that won't get wind blowing across it. That messed up the pressure reference for the carb and it won't meter right.

That being said, good places to also check will be a dirty air filter, not allowing enough air through at high throttle. Also check for good fuel flow. A clogged fuel filter or weak fuel pump can cause fuel starvation at high throttle also. But, I'd put my money on the carb slides.

--Justin
 

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Just as a note.... Premium gas will not clean, repair, otherwise help with any issue you are experiencing except for detonation. Premium gas is for high compression performance engines or turbocharged engines. with a 9.5:1 compression ration your honda does not fit that category. Using premium to make it run better is a waste of good money.
Regular is fine.
 

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Discussion Starter · #16 ·
Ok, I am going to check up on those things you are talking about Justin. I will get back to you and let you know if it was either or all of those things. Out of curiousity, however, isn't there something I can add to the fuel system to help with the final cleaning of the carbs? I put half a bottle of sea foam and then (as I was told) filled the tank with premium. Doing this did not seem to give me a difference though... so I was wondering what you would try?
 

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Seafoam and other "tune-up-in-a-can" solutions work fine when the problem is something small, like a rough idle or a touch of hesitation off the line. The issue in your carbs is not something that Seafoam will help. What I would try is going through the carbs again.

However, in your reply to cl65ame, you mention that you have detonation. This shouldn't be happening in a bike with less than an 11:1 compression ratio at normal temperatures with standard timing. If you're really getting detonation, you have a deeper problem than we may have originally suspected. Your bike was designed to run best on 87 octane. What symptoms suggested you were getting detonation?

--Justin
 

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Agreed justin

Detonation is characterized by a Pinging or rattling under load. The higher the load the worse the rattle. The noise normally diminishes at cruise.
Detonation can be caused by several factors including carbon buildup, incorrect plugs (heat range), extremely lean mixture and a few others that escape me at the moment. These issues would be very bad to make a shadow detonate since the compression ration is like 9.5:1 or lower.
 

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Discussion Starter · #20 ·
I do not think that "detonation" is happening... I think that maybe I will go through the carbs one more time. What it sounds like (more or less) is a hesitation to respond. It happens around 50% or higher throttle (originally i said 75% but I started riding it more and noticing that it happens fairly frequently.) It is anytime I try to give it more throttle, the bike will sound good starting off, then start puttering and jerking a little bit from loss of power, then it will accelerate well, then start puttering about 2 seconds later. That continues throughout the ride as long as I am trying to throttle back. Sorry for the confusion, I thought cl was referring to detonation as in a cylinder detonating the fuel (like working), not detonation as in some sort of terrible issue. So when I take the carbs apart this time, what should I specifically focus on to remedy this issue?
 
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