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Old 06-24-2011, 01:35 PM   #1
ShadowRider55
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Default Only Runs When Choke is On

Been troubleshooting an 84 Honda Shadow. Made some progress today and was able to get it to idle good. It originally had some bad gad in it, but since I put some fresh gas in it, it appears to start much easier.

Now the issue is that it will only run when the choke is on. Also if I rev it too fast it will stall out. But it I rev it very slowly it will increase in RPMs.

Waited a while for the bike to heat up and still same result.

I checked the float bowl and they are getting plenty of fresh gas. Check for spark and I get good spark to Both Cylinders. Good Compression at 120 psi.

Adjusted the air/fuel mix screws to see if the micture was to rich or too week, but it had no affect.

Have not inspected the Jets as I would rather not pull apart the carburator if not needed.

Cleaned the fuel filter and it does pump fuel to the carb.

Perplexed at the moment. Anyone have any ideas.
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Old 06-24-2011, 02:55 PM   #2
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It's probably just dirty carburetors. If it will only run with the choke on, that indicates rich conditions. The fact that you adjusted the mix screws and didn't get any results further indicates that the carbs are not properly functioning.

If you've got it running, and you don't want to pull apart the carbs, dump a whole can of seafoam in the tank. That MIGHT clean the carbs well enough to where you don't have to pull them apart, but you should be prepared to do so anyway, for a good cleaning if nothing else.
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Old 06-24-2011, 06:54 PM   #3
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Default Still Not Running Right after Carb Cleaning

I pulled out the carb today and cleaned the jets with carb cleaner and blew them clean with my air compressor. Put it all back together and still the same symtoms remain. Also ran seafoam through the gas and air intake.

Only runs when choke is on. Will rev up, but only after about 5 minutes when the engine is warm. Will stall out if I rev it too quickly. Stalls every time I take the choke off.

So far I have checked the following.

- Good Compression in each Cylinder at 120 PSI
- Fuel is making it to the carb bowls
- Spark on all 4 spark plugs.
- I tried adjusting the fuel/air mix with the mixing screw but no luck with
that either.

Not sure what else it could be.

Some questions I have are as follows.

- Is there only 1 fuel/air mix screw on a honda shadow vt700c or are there two one for idle and one for the main jet?

- Is there a specific pressure that the fuel pump should operate at, could these symptoms be caused by a weak fuel pump?

- Could this be caused by bad timing and is there an easy way to test this?

I did notice that some liquid, I assume gas is leaking from one of the tail pipes, The one that comes from the front cylinder. Not sure if this is an indication of something.
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Old 06-24-2011, 07:47 PM   #4
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I'm not familiar with the shadows, but generally speaking there should only be one fuel/air mix screw. They're not difficult to identify, so if there are two it should be easily identified.

Not sure on the fuel pump pressure, but I find a weak pump an unlikely cause. I want to correct my earlier statement and say that if it's only running with the choke on, that indicates lean conditions, or in other words it's getting too much air. So it's just a matter of finding out where the extra air is coming from.

What is the condition of the carb-to-cylinder gaskets? If there's any doubt, just replace them.

Do your carbs have diaphragms? If so, carefully remove the covers and inspect the condition of the diaphragms and cover gaskets and sealing surfaces. I want to stress CAREFULLY removing the covers, because they are not cheap to replace if one or both tear.

Also, are you sure the fluid leaking from the tailpipe is gas? You should really try to make a positive ID on the fluid. If it's gas, it could be an indication that a cylinder isn't firing. If it's oil, could be piston rings. If it's water, some condensation is normal.

Last edited by Gumby73; 06-24-2011 at 07:51 PM..
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Old 06-25-2011, 08:12 AM   #5
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This sounds like something seen repeatedly at virtually any bike shop every spring. It may not be the case in your situation, but the usual culprit is when gas is left in the carbs over the winter. It dries out and leaves residue in the float bowl. When gas gets to that residue it flakes off and turns into gummy stuff. It usually plugs the pilot/idle mixture circuit. In other words your pilot circuit, aka idle mixture circuit, is plugged up with dirt or gunk. It won't be easy to just blow out with an air gun. It requires more than that in many cases.

Don't root around wit wires in all the holes, scratching up the surfaces affects fluid flow. I don't know how it would work, but maybe some heavy gage fishing monofilament might just work well to poke and prod around in the circuits since it can bend easier than wire. Maybe some 20 or 40 lb test line would work. Never really thought about that till now.

The thing is the pilot jet and the circuits through the carb are tiny and can easily plug with any sort of residue. If you pull out a pilot screw and compare the hole size with that of a main jet you will see what I mean. Makes the eye of a needle look huge. It is possible you might even find one or both pilot jets are clogged when doing this. After all that is the smallest part of the passages. If clogged, clean very carefully with a pin and cleaner. A carb washing machine is the best way to clean out the circuits in a carb body set, letting the carb bodies slosh around for an hour or two. Otherwise try to fill the pilot jet hole and circuit with cleaner and finally with water and air. Yes, wash them out under warm water, then blow them out.

In addition, there is one idle mixture screw per carburetor and I'm thinking they're at the back of the float bowl on the bottom between the float bowl and the intake, but it's been so long I don't remember for sure. There is a screw on the linkage for balancing the carbs and a large screw that adjusts overall idle for rpm setting also on the throttle linkage, don't confuse these with the idle mixture screws. The idle mixture screws may be capped off if they were controlled by EPA back then. I'm thinking they were. If so, a little heat from a soldering gun will melt the glue if the cap is over a protruding screw and it can be pulled off. That or grind off the stops if possible. On some bikes the screws are in a recessed hole and the cap has to be drilled and pulled out.

After making sure all is clean in the pilot circuit try again.
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Old 06-25-2011, 08:16 AM   #6
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Quote:
Originally Posted by Gumby73 View Post
It's probably just dirty carburetors. If it will only run with the choke on, that indicates rich conditions. The fact that you adjusted the mix screws and didn't get any results further indicates that the carbs are not properly functioning.

If you've got it running, and you don't want to pull apart the carbs, dump a whole can of seafoam in the tank. That MIGHT clean the carbs well enough to where you don't have to pull them apart, but you should be prepared to do so anyway, for a good cleaning if nothing else.
Only running with choke on actually indicates a lean condition, the choke blocks off the air making the mix richer, open will be leaner.

I did need to include after my early comments, that the reason it stalls when the throttle is opened up quickly is because of the lean condition and the fact that going from idle up to about 1/4 throttle includes the transition from the idle mix circuit to the slide and needle circuits. If the idle mix is leaned out due to blockage, opening the throttle too fast will make the engine gulp too much air without any fuel... stalling or stumbling as a result.
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Old 06-25-2011, 08:23 AM   #7
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It's easiest to check for an air leak by using some spray carb cleaner around any suspect area while the bike is idling, like the aforementioned carb boots. If any air is getting sucked in, the engine should rev up.
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Old 06-25-2011, 06:54 PM   #8
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Quote:
Originally Posted by markk53 View Post
Only running with choke on actually indicates a lean condition, the choke blocks off the air making the mix richer, open will be leaner.
Yeah I corrected myself in my second post. Sometimes my brain just flips words around and the crap that comes out is in the form of bunk engine advice. But hey, at least I'm not the only one who thinks it's dirty carburetors.
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Old 06-25-2011, 07:26 PM   #9
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Default I think I have it solved

I noticed a fairly signifigant crack in the air intake tube to the carburators. As soon as I taped it up she ran much smoother and was able to run it without the choke on. Thanks for all the good advice.
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Old 06-26-2011, 08:58 PM   #10
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Cool deal. Start looking for a good pair of replacement boots.
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