83 VT750C misses - backfires above 3K after running for 1 mile - Honda Shadow Forums : Shadow Motorcycle Forum
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post #1 of 42 (permalink) Old 06-14-2019, 02:51 PM Thread Starter
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83 VT750C misses - backfires above 3K after running for 1 mile

My trusty 1983 Shadow 750 has just developed a problem where it starts missing/backfiring at RPM's above 3K about a mile into the ride.
I have had the bike for almost 6 years, put on 20K miles, many 500+ mile trips, no issues.

Yesterday I took it for a spin with my daughter, 1 mile into the ride it started missing / backfiring about 3K rpm, we limped it home. I drained the tank, checked the carbs (did not remove them), checked the air intake for blockage, cleaned re-oiled the air filter, checked the plugs (looked good, light tan/gray, plugs have 4 years, 11K on them), checked for loose wires.
Put 3/4 gallon of premium in the bike, waited 20 hours because of rain.

Took the bike out this afternoon to test it. It started fine, gave it some power in 3rd, 4th, 5th gear, 7K rpm, 80mph, seemed fine. Got about a mile in into the ride and the same symptoms appeared.
Misses / backfires above 3000 rpm, seems to have enough power, and runs fine below 3000 rpm.

I'm at a loss.

Next plan is to replace plugs (I doubt this is the issue, but cheap fix, and I did once have an issue with a spark plug under load in a piper warrior airplane).
Pull the carbs, completely clean them maybe a blockage?

I have not yet verified the timing (can't seem to get the timing mark cover off, it is stuck, and I would think that it would be bad from the start, not after 1 mile).

I did rebuild the carbs on 5/23/2018 at 29,626 miles used the outlaw carb kit, so I have 2,500 miles after the rebuild of the carbs. Shadow currently has 32K



Update 6/14/2019 4pm
The bike sat for 6 hours so it should be "cold".
Since spark plugs are only $2/each I replaced them hoping for a miracle.... It was not to be.
Problem exists immediately after hitting around 3K, so I think the 1 mile thing to warm up might have been a false symptom.

I can rev the bike in neutral, and I get a good response, going up to 7K no problem.

Once the bike is under load, even in 1st gear, it misses / backfires above 3K.

Thinking that this might be related to fuel starvation (fuel filter looks fine), I closed the fuel petcock and did a bunch of loops up and down my 250' driveway.
I got the same symptoms, problems above 3K, after about 4 loops the engine quit. I open the fuel petcock, got it started again, problem still remains.


My next plan is to pull the carbs, clean them see if that helps.

Any suggestions appreciated.
Thank you.
-andy


Last edited by AndyFromWI; 06-14-2019 at 05:27 PM. Reason: More information
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post #2 of 42 (permalink) Old 06-14-2019, 03:22 PM
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I wouldn't mess with the timing. Most likely fuel or spark related.


http://2007 Shadow Spirit VT750C2, Ride bell by h13man, Ride cross by Capt D
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post #3 of 42 (permalink) Old 06-15-2019, 12:15 AM
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I had a brand new plug go bad before. I would try new ones first if that doesn't fix it you have a spare set. Occasionally plugs crack but it is rare and when it happens it will drive you crazy.
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post #4 of 42 (permalink) Old 06-15-2019, 12:50 AM
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Get it warmed up and in a dark area spray water on the coils and wires to see if they are arcing over.
Maybe check ll the connectors for the 2 spark units under the seat for corrosion.

And if you have the third spark unit like mine does, it can be disconnected out of the circuit, and the main connectors plugged back together of the 2 units and it will run. They go bad after years and interfere with the spark.

Only a biker knows why a dog sticks his head out of a car window.
1983 750 Shadow
From the past=
1951 Cushman scooter
1962 Honda 305 Dream
1965 Honda 305 Dream
1971 Honda 175 scrambler
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post #5 of 42 (permalink) Old 06-15-2019, 12:35 PM Thread Starter
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Update 6/162019 Still have issue.

Ran perfect for 4.5 miles, then problem returned


I pulled the carbs last night, removed float bowl covers, jets, float valve. Sprayed with carb cleaner, blasted with compressed air.
Put it all back together in the morning, and it was running great, plenty of RPM, no back fires, I was smiling.... drove 3 miles to a gas station, filled up with premium I thought I was home free......
Drove another 1.5 miles from the gas station, and the issue returned, no power about 3K rpm, will rev great in neutral, idles great, starts great.

Limped back home, replaced fuel filter, tested fuel pump (pump tested at about 24 oz/minute within specification)

So I don't know if it ran for a bit because it was cold, and once it warms up coolant is causing an issue? Or if I cleared something in the carbs, and it returned.

Tried Swifty2014's suggestion of parking in a dark spot, sprayed water on the coils looking for arcing, I could not see any.




Summary:

Problem: No power / backfires / misses above 3K rpm
Adding choke helps a tiny bit, might give me 3.1K rpm


Attempted so far:

Drained fuel tank, replaced fuel with premium.
Replaced spark plugs (Thank you kensanders for the suggestion).
Tested pulse unit with ohm meter (in spec)
Cleaned carbs (did not separate), ran great for 4.5 miles.
Tested fuel pump (in spec)
Replaced fuel filter

Next step is to do a compression check, but I need to get an adapter for my compression tester.

Other suggestions?
Thanks
-andy

Last edited by AndyFromWI; 06-15-2019 at 12:42 PM. Reason: Typo
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post #6 of 42 (permalink) Old 06-15-2019, 01:09 PM
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Does it feel like the whole engine is cutting out on both cylinders, or just one cylinder is missing and running weak and sputtering?



Why not try to switch the connectors for the 2 spark units under the seat? Sometimes just changing the connections may reveal a fault from one cylinder. Look for loose fitting connections at the coils too.

Just a wild guess but that is what electrical diagnosis is all about.


Compression should not suddenly change from running good to bad. Not a bad idea to have the knowledge of how healthy the engine is however.
These have hydraulic valve adjusters so they don't require manual adjustment.

Only a biker knows why a dog sticks his head out of a car window.
1983 750 Shadow
From the past=
1951 Cushman scooter
1962 Honda 305 Dream
1965 Honda 305 Dream
1971 Honda 175 scrambler
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post #7 of 42 (permalink) Old 06-15-2019, 01:35 PM
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Try running with the fuel cap loose and see if it reoccurs.

http://2007 Shadow Spirit VT750C2, Ride bell by h13man, Ride cross by Capt D
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post #8 of 42 (permalink) Old 06-15-2019, 04:35 PM Thread Starter
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I appreciate all the wild guesses.

I pulled the rear plug wires, did my driveway loop.
pulled the front plug wires, did my driveway loop.
I seemed to have a bit more power on the rear cylinder.

I did try running with the gas cap off. No change.

I drained the tank, pulled the carbs again, checked / cleaned the the needle valves / vacuum cylinders, blasted everything again with compressed air.
Got 1/2 mile, then the problem kicked in again.... Crazy.

I will try swapping switching connectors on the spark units.

Thanks.
-andy
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post #9 of 42 (permalink) Old 06-15-2019, 04:36 PM Thread Starter
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Good suggestion, I just tried running with the gas cap off, no change.
-andy
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post #10 of 42 (permalink) Old 06-15-2019, 05:47 PM Thread Starter
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I tried the spark unit swap, no difference. Than tried running just on one spark unit, not much of a difference from the left or right spark unit.

Crazy.

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