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1984 Honda Shadow 750 stalls with throttle

7K views 17 replies 3 participants last post by  RaithM 
#1 · (Edited)
Good morning all,
I'm writing in because my 1984 Honda Shadow 750 stalls out whenever I give it throttle. I've had the bike almost 2 months, and it ran great up until about 2 weeks ago. I went away for a week, and when I came back, the bike would start fine, but as soon as I gave it any kind of throttle it would stall out.
At first I assumed that it must've gotten water in somewhere, as it rained while I was away. So I checked the plugs, cleaned the water out of the one that had some in it, and drained the float bowls on the carb. No change.
So I removed the carbs and cleaned them thoroughly. No real change. After that I noticed that the idle air screws were backed out so far that fuel was leaking out past them. I found the factory setting for them in the service manual, set them, then adjusted the throttle stop screw as well. The bike idles great now, but when I give it throttle, it will rev up for a second or two, before wanting to stall right out again. When it does stall, the rpm's will drop considerably (no tach), probably to around 500, even after I let go of the throttle, and take 3 or 4 seconds to build back up to normal idle. I have noticed that I can increase the rpm to 2 or 3 thousand by messing with the throttle stop screw position.
Anyone have any thoughts? I'm thinking maybe its a weak fuel pump and the carb bowls aren't being refilled fast enough?
 
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#2 ·
Well just a short update. I checked the fuel pump today, had power to the connecter. So I started the bike up (which requires a boost at this point), and put a fuel line on the drain screw nipple, then opened the screw up wide. Fuel pump pumped fine, and a steady stream of fuel came out. Didn't seem to affect the running of the bike at all.
So I pulled the spark plugs. Spark plugs are black around the bottom of the threads and the electrode, but white around the electrode arm. I can only assume that the bike was at one point running lean, and is now running rich, or vise versa. At this point I'm thinking maybe a weak ignition system, providing bad spark. Any thoughts would be welcome.
 
#3 ·
Did you check the fuel flow out of both carb float bowl screws?
Maybe one is getting fuel and the other is not. Make sure the fuel filter is clear also. You may need to sync the carb shafts.
Have you tried to add choke while holding the throttle up and it is faltering, to see if that added fuel helps?
 
#4 ·
No, only checked the fuel at the one. I have tried leaving the choke on to see if it made a difference and unfortunately it doesn't. I assume that the filter is clear since its on the suction side of the pump. The only thing I'm not to sure of is exactly how much fuel flow I should be getting out of the drain screw there. It was a consistent flow but not a high volume.
As for syncing the carbs, I don't think that would be the issue as it ran fine until I left it sitting for a week.
 
#5 ·
If the pump is working but weak output at the carbs maybe there is some fuel in them but not enough to keep the main jet supplied. If you pull the output hose off at the pump and run it for about 10 to 15 seconds it should fill a coffee cup by then. Not sure of the exact specs.
 
#6 ·
Well I think I'm back to weak fuel flow. I had thought that it was fine since it seemed to be supplying the carbs enough to maintain a constant flow of fuel when the bleeder screws were cracked. However, after consulting with the guy at the local salvage yard, I decided to try a used carb just to cross that off the list for sure. So I went and picked one up, cleaned the jets, and then threw it on the bike. It actually runs worse with the new carb. It only started once and then it just kind of ran for 3 or 4 seconds and died as if fuel starved.
I notice now that when I crank it over for 5 or 10 seconds, after I stop cranking I hear a sort of "glug glug" air through fuel kind of noise coming from the auxiliary tank.
I also pulled a spark plug after cranking it, cranking it with choke on, and cranking it after giving it a few turns of the throttle. Spark plug looked completely dry and a bit of white carbon, which to me would indicate lean. So if I'm cranking repeatedly without the bike actually starting, that plug should be covered in fuel should it not?
I clamped off the fuel line running from auxiliary tank to pet **** and removed the pet **** and fuel filter. Pet **** seems to operate properly and be free of obstruction. There is some nasty looking gunk inside the fuel filter, but its hard to tell if its obstructed or not. I'm thinking at this point of completely draining and cleaning both fuel tanks, as well as replacing the fuel filter. The pump looks relatively new to me, and seems to operate as it should. I would suspect that the fuel pump would be ok at this point, as everything I've read seems to indicate that the bike should run on a gravity feed. That tells me that whatever is obstructing fuel flow is obstructing gravity fed flow as well.
As a side note, because it was mentioned earlier… I re checked my choke for proper operation and realized that the cables were pretty close to seized in the choke off position, which explains why I've never had a strong choke on this bike.
 
#7 ·
Well another small(unsuccessful) update. I removed the fuel filter and bypassed it with a fuel hose just to see if that helped at all. It didn't really, but I did manage to get the bike running again with some throttle stop position adjustments. So I noticed that one cylinder was running hotter than the other. I also noticed a bit of a weird noise coming from the vicinity of the air box. So I opened it up, and pulled out the filter. I noticed that some air mixed with fluid was coming out of a hole in the back left corner of the air box. Looks like oil with water in it, kind of that coffee with cream in it kind of colour. Does anyone have any idea what that hole is, and why there would be fluid/air coming out of it?
 
#8 ·
There is a breather hose off the rear head that goes to the air box and then drains down below the bike in a clear plastic tube . It has a plug in it that is to be pulled out to drain any fluid in there. Then re plug it. See if that is full of liquid and drain it.

What does you oil look like, and is it over full?

Did you check how much gas your pump can deliver? It is necessary because even with gravity feed, the carbs are high up on the engine, and the filter and pump are at the bottom, so it needs help to get pressure up to the carbs.
 
#9 ·
Well, I drained the fluid from the air box. I checked the oil, it was a little low but clean and not fuel contaminated or anything. I checked how much fuel the pump is pumping and it seems ok.
The bike is only running on the rear cylinder. I had thought that I wasn't getting spark in the front cylinder, so I picked up a used coil. Installed it, didn't help. So I switched the coils and still the same condition. I pulled the spark plugs and found my front plugs to be covered in fuel as you would expect. I pulled the spark plug and cranked the bike over with the spark plug touching the engine and had spark. Which is strange because I had checked last week and was sure there was no spark.
The only thing I noticed that seems out of wack is that I got a shock off the exhaust of the dead cylinder, and also off the spark plug wire. So I have to take this to mean that either somehow the electrical is shorting itself out when the spark plug and wiring and everything are connected, or something along those lines maybe?
The only other idea I have at this point is that the compression in the cylinder is not good, but I don't have a compression tester. It feels like a lot of air is coming out of that pipe, any idea how to test with no tester?
 
#10 ·
Well just a slight addition to that. I found the source of the shock. Its the short wire on the first cylinder. I don't have a multimeter so I can't really test, but I can say that with everything connected as per normal, if I put my hand on the wire for that cylinder I get a shock. In fact, if I leave the spark plug installed, and disconnect the wire, the current will jump from the wire to the engine, without having a spark plug installed. Is it possible that I've got a short between my two power wires for the coil, causing to much voltage/amperage on one plug, and not enough on the other, or something wacky like that?
 
#11 ·
It sounds wacky alright. These bikes have a dual plug spark system that fires the 2 plugs in series. The coil has both wires for each cylinder and when the coil fires it goes in a forward direction to one plug, and through the head as a ground, and then in a reverse direction out the other plug back to the coil. It is a loop. So all the wires and connection s have to be good or you get a poor spark.
You may have a bad plug wire end on the one that is shocking you. They are a resistor end cap and if it is burned out will cause the spark to jump some where else.
If you take that one off by unscrewing it, and connect any type clip on the end of a nail or screw in the end of the wire hooked to the plug, see if it runs better.
Also if you see a cut or scraped area of that wire use electrical tap a few wraps to insulate it. Probably needs new wires then.
 
#12 ·
Well.. If it is the wires I just have astronomically bad luck. I purchased a use coil earlier tonight, and got plug wires with it. No combination of any of my three coils with the original wires or the wires that came with the used coil changed the running condition of the bike, nor did switching the wires on the igniters. Tommorow I will switch the plug wires from the rear cylinder to the front cylinder and see if I still get shocked.. I think it must be bad wires or something messed up in the wiring harness itself.
 
#13 ·
I have an '83 750 and checked my wires and found they are solid wires not the resistance wire like on a car. So the ends are where there is 5,000 ohms of resistance as well as using resistor plugs. You can take off the ends and run them straight for more spark, but they are there for reducing radio frequency interference in radios and TV's, etc.
You can also go the local auto parts store and get regular plug wire that are resistor wires and connect them without the Honda ends.
 
#14 ·
Also did you check the plugs lately because they may be badly fouled and not wanting to spark in the cylinder. So you would be fighting the spark issue all over again.
 
#15 ·
Well if I could retitle this thread I would title it "Circular Troubleshooting". I checked and swapped every possible thing I could on the wires/plugs/igniters/coils, with still the same result.. Rear cylinder firing, front cylinder dead. Finally I removed the gas tank, and the air intake, and got the bike started with some starter fluid, since it didn't want to start with no intake system in place. when I finally did get it to start, I was able to get both cylinders to fire with some light application of starting fluid directly into the carb.. Thats when I noticed it. Fuel was just steadily pouring into the front cylinder from the carb.
You may recall, I actually switched carbs just in case it was a carb issue. Both my carb, and the new carb, had the exact same issue. So I cleaned my original carb really really well for a second time, and made damn sure the needles slide easily. Reinstalled the original carb on the bike, fired it up, and voila, both cylinders firing and capable of applying throttle without stalling the bike. Now I just have to wait for the air filter I ordered to arrive in the mail, so that I can set the carbs to idle properly.
I guess in the long run, I came right back to where I started. But I never would have guessed that two separate carbs would present with the exact same issue. Especially since the issue turned out to be that the needles weren't seating properly, and the bike shop that sold me the carb specifically pointed out that this set of carbs had been checked over for proper operation.
I am still having a slight issue that the front cylinder is backfiring, and seems to be getting a little bit to much fuel.. But I hope that I will be able to correct that once I get the air filter and adjust the idle air screws properly.
Thanks for all your help, hopefully this is the end of my issue.
 
#16 ·
Good to hear of a happy ending. Not only is it running much better but you have an education on the ignition system and carbs. Will be valuable in the future.
 
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#17 ·
Yeh might be a weird timing thing going on with your sparks, but I was wondering if you'd noticed or checked if your bowl floats are at least level, set wrong would mean fuel starvation once revving.
 
#18 ·
Well at this point looks like I might need to rejet the carbs as well. After cleaning them all out, I ordered a K&N filter to replace the sponge the previous owner had installed as an air filter. The Previous owner had also installed a set of aftermarket pipes with no baffles. Unfortunately, that leads to a lean condition no matter what I try. I even tried covering 3/4 of the K&N filter in duct tape.
Basically I can get it to a point where if I ride around the block, the first street it'll cruise down ok, second street it bogs and sputters and wants to stall, 3rd street it runs fine, and 4th street it bogs and sputters and wants to die all the way home.
Get home, pull the plugs, and white carbon all over them(brand new plugs).
Its very frustrating and a little odd because the front cylinder is running very hot, while the rear cylinder is very cool. The rear cylinder responds very quickly to the idle air adjustment screw being adjusted, while the front cylinder seems to have very little response.
Any thoughts on what else I may be having a problem with before I drop $125 on a Dynajet Stage 1 Kit? Or even if there is a better option than a Dynajet kit?
 
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