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Discussion Starter #1
So I finally put old lady back together and have some more issues I am unable to solve yet.

First, the engine fires right up with full choke, revs good on neutral. But rear cylinder is backfiring no natter how I set the pilot screw.

So I pulled the carbs out to check them /again/ and clean them. Bought carb repair kit, cleaned them both once again, checked diaphrams against the light to be sure there are no holes, when I blow air from the compressor info carb intake the needles jump right up and return so both are OK I guess. Both floats are set to 7,5 mm like shop manual says.

Carbs were cleaned with carb cleaner, every hole, passage, jet is clean and carb cleaner flows / blows there with no issues.

New fuel filter, new fuel lines, petcock OK.

So I mounted carbs back, synced them, made a new sleeves from 40mm diameter fuel hose to be sure there is no leak between carb and head, tightened them and installes KN filters. Pilot screws set to 2,5 turns out, new O rings.

Started the engine again, fired right up with same issue, front cylinder looks OK, rear one still backfiring but a little less than before. So I took her for a little ride.

And here is where my issues start. As long as I am in low RPM the engine runs kinda good but at higher RPM / 1/4 throttle maybe / is sounds like its fuel starving. After I give her full throttle engine dies. I am able to drive only in 3 gear at 50km/h maximum. When the engine is warm and on neutral if I give her a quick full throttle the engine is caughing pretty hard and does not rev like when I give her throttle slowly.

When I gave her half a choke the engine starts to go a little but only for about 80km/h in 4 gear. As long as rpm drop to lower it runs good and has some power.

I swapped stock intake back with same results. I also noticed that tachometer goes crazy while riding. sometimes it shows real rpm, sometimes 0 and sometimes constant 4000 rpm.

Any ideas what should cause this behavior? After engine rebuild the timing is OK, tappets and shims are OK, engine has good compression with no head gasket leaks or oil consumption. Spark plugs are brand new. Thank you for any ideas.
 

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Discussion Starter #3
Battery is brand new. Ill measure the coils tomorrow. Check the grounds too and let you know. Fortunately I have a spare pair of coils, spare fuel pump + relays and 1 spare igniter.
 

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1983 Honda vt750 Shadow
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Disconnect the yellow signal wire inside the tach to see if there is a short causing the coil to short out. If that shows nothing, the tack is needing soldering on the circuit board. I did it to mine when it was erratic and now it is fine.
The tach would be a separate issue from the missing if the yellow wire is good.
You can pry the chrome cover off instead of cutting as in this video.
And try loosening and tightening the screws on the back where the wires connect as a try first before opening it. That may fix it.


 
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Discussion Starter #5
Gotta try this today too


Is it normal that when I pull the clutch in gear neutral light came on?

I know the ground connector is under the seat. Are there any others to look for?
 

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1983 Honda vt750 Shadow
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Not normal for the neutral light to come on unless it is in neutral.
The purpose of the clutch switch is to allow you to start in gear if you have to.

That is only the starter circuit, so why is it interfering with the ignition???
The diodes may be causing the neutral light problem but I still can't see the missing from that.
 

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OK here is what I have learned about these ignitions. And some of what i can't figure out.

The 1983 model in the manual shows 2 ignition units.

The 1984 shows 3 units, one in-line with the others. It has a yellow/red wire from the start solenoid to the extra unit.
Some guys have had spark problems and took it out of the circuit and it cured their problems. Very confusing.

What makes it more confused is mine is a 1983 and I have that unit on it also.

The 1985 has 2 units but they have an extra yellow/red wire from the start solenoid to the spark units.

Since you are having an issue with misfire and it ties in with the clutch switch, take that 3rd unit out of circuit to see if that fixes it. Or disconnect the yellow/red wire as a test.

Maybe we can learn a little bit more of the interaction.
I have been studying this secret for a while.
 

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Discussion Starter #8
There is no 3rd unit on my bike as far as I know /83 year according to VIN/. As I understand wiring diagram, the clutch switch only grounds starter relay coil while engine is in gear and clutch is pressed.

When there is neutral the ground for starter relay switch is made through clutch diode.

But that does not explain /or I dont understand it yet/ why my neutral light comes on everytime I press the clutch while shifting gears.

Edit: When the clutch diode is blown / current flows both directions / the neutral light comes on while pressing clutch in gear.

So I start with replacing clutch diode first I guess...
 

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Just for curiosity could you double check to see if yours does have spark unit #2?
I didn't think I had one but found that I did. There is a small box behind the starter but that is the tail light sensor for the bulb out warning light.
Under the air box there is the spark unit #2 . Maybe if yours is a early production it won't have it.
It is good to know however, if you loose spark and that thing is the whole cause, and you can take it and ride.
 

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Discussion Starter #10
Yup, I have that one but it says STOP & TAIL HO-841-01-TL NIPPON SEIKI JAPAN. So imho it is open circuit warning unit with 6 cables going into it and the color of the cables exactly match OC unit.

Anyway, both diodes I have are blown so I am starting without it with pulled clutch with no problems. Coils seems good, measured resistance and its in limit. I also chaged the fuel pump and disconnected tacho yellow cable. Still the same problem.

Measured all the grouds I was able to find following green wire all the way arround the bike, connection seems OK. Main grounding is on the same bolt as negative battery terminal.

When I put the bike on stand and add full throttle quickly I can hear the carbs sucking air /and fuel maybe/ but engine struggles to rev at all. Fuel pump works.

Next thing will be: Measure spark plug wires with and without spark plug caps, change spark plugs, change spark units... oh my.
 

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Discussion Starter #11
So here is my problem, made a little vid how it looks like


And a view from carbs


Can anyone help with this? Any help will be appreciated.
 

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That mist is fuel... I don’t recall mine producing that much during my tests, but I don’t think I did a full-throttle rev. What seems wrong to me is the mist coming backwards out of the carb throats. There should be enough vacuum sucking into the cylinder that nothing that close should want to go the other way. Watch yourself on the backfire, but what happens if you partially cover the throats when you rev it? Does it get better or worse?

It seems to come up fine slowly, so that tells me ignition is working fine. I’m not aware of any special advance used for full throttle...
 

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Discussion Starter #13
Yup, that is fuel for sure. I dont get it either why its going backwards in carbs. Carb boots were original, still good ones, I made another ones of 40mm fuel line, they are super tight with no vacuum leaks. If there wasnt enough "suction" into the cylinder the diaphragms wont go up, am I right?

Ignition is working fine, I have one from ignitech installed now, both pick up coils are good and induction coils too.

It kinda revs while adding throttle slowly so I assume the timing is good too. The engine starts at 1st crank so there is no issue with compression too.

Full advance is 26 deg @ 4000 rpm. I need to try to partially cover the carbs and let you know.

I just simply dont get it what the hell can be wrong.
 

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Discussion Starter #14
Looks like spitting mixture into the intake is pretty common thing in CV carbs especially while engine is gaining RPMs


 

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1999 Shadow 750 ACE VT750CD3 Modified
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My take on it and it is worth 5 Canadian cents.
It looks like fuel stand off with a high performance engine, but it is not, although the overall effect it is similar and performance will suffer while this is happening.
A couple of things going on here caused by no intake filter tubing.
The bikes intakes are tuned as a whole from the air intake snorkel to the back of the intake valves, you mess with that flow profile you need to compensate with the slide response amongst other stuff.
1. You get slide flutter as there is no relatively stable column of air feeding the carburetor intake horn and the slide spring is fluctuating metering fuel in this unstable air condition it is not tuned for, and the engine has yet to move enough air for the amount of fuel that is being fed to it.
2. With no intake tubing or long velocity stack or plenum you get to see this happening. (-:
 

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I agree. Put the air tubing on and see if it reacts better on quick acceleration. Other wise it looks pretty good revving slowly. These don't have accelerator pumps so any increase in air/ fuel suddenly can cause poor performane with the air tubing off.
 

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I am not sure but maybe a lot of that spray back may be from valve overlap in that engine.
With 2 intake valves that is a lot of area for any pulsation to blow back.
 
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Discussion Starter #18
I agree. Put the air tubing on and see if it reacts better on quick acceleration. Other wise it looks pretty good revving slowly. These don't have accelerator pumps so any increase in air/ fuel suddenly can cause poor performane with the air tubing off.
The intake tubing was there on 1st video posted. Only after that I made 2nd one with fuel tank and intake off. There is pretty much no difference if there is original tubing, K&N filters or nothing. Engine response is always the same.

Well, it looks like it revs by slowly adding throttle but there is absolutely no power, bike looses it all the moment the diaphragms should kick in.
 

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CSR, I agree it would be rougher, but I didn’t get that when running mine with the intake off. Proper operation, but it got WAY smoother when I had the intake on briefly...

Easy starting does not indicate good compression, though. Can you get your hands on a gauge to do a test?
 
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