Engine Barely Fires Up, But Doesn't Stay Running... - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 11 (permalink) Old 04-11-2012, 07:01 PM Thread Starter
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Engine Barely Fires Up, But Doesn't Stay Running...

I've recently taken on a 1993 VN750 "fixer-upper" project for a family member, and I can't seem to figure out what the under-lying issue might be. I'm well versed in motors from an automotive standpoint, but a bit new to bikes. So allow me to start from the beginning (and I've been through a lot, so bare with me here):

When I got it, it wouldn't start at all with the push of the start button. However, with a short "tow" through their front yard in the country (which after looking back on, I don't recommend), we discovered it would run via a "push-start". No starter action at all could be heard when pushing the start button, only a clicking noise. I first changed out the battery. No improvement. I did some research and found that one of the most common culprits was the Starter Relay / Solenoid. I tested this theory by jumping it with a flathead screwdriver (also not recommended in hindsight) and the starter began to crank, but no firing on the motor.

I picked up a brand new Relay, and "Voila!" It worked. Now the starter would turn over, but the motor would never actually fire up and stay running. The most I could get out of it was a few sputters, and then back to the starter whining as it turned and turned. I knew the bike was a bit aged, and had sat for awhile, so I thought "It likely has dirty carbs and aged plugs" so I picked up a can of Sea Foam, some Carb Cleaner, and some brand new NGK plugs.

I proceeded to change the plugs, and then empty half of the can of Sea Foam into the gas tank (as I had read others do so many times on here) and try to shake the bike up a bit to get it mixed in. Then I proceeded to crank it up a few times to get the Sea Foam through the carbs. On top of that I sprayed in just a tad bit of Carb Cleaner into both sides of the carb.

I turned it over and (likely because of the highly volatile Cleaner) it fired up loudly, with the RPMs shooting up, and then gradually coming back down. After doing this a few times it began to start easier and easier, but if I held the throttle open to it's maximum, the RPMs still would just hover between 4,500 - 5,000, and would never climb any higher. It didn't seem to sound right, (kind of like a go-kart) so I placed my hand at different spots on the motor, and what I found was that it's only firing on the front cylinder. A friend that was well-versed in bikes said that the next thing to change out would need to be the Regulator/Rectifier which apparently is another common trouble item on VN750s. I bought a brand new one, changed it out, and it still has the same problem.

I checked the plugs, and both of the front plugs are grey/white as they should be when firing properly. Both of the back plugs are dirty and greasy. I'm fearful that there may be some internal issues, such as oil leaking into the chamber that's not allowing the air/fuel mixture to ignite.

Any suggestions as to what the next move should be?
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post #2 of 11 (permalink) Old 04-11-2012, 08:25 PM
..have a vulcan good day!
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Check your coils with a Volt/Ohm Meter. Also, old bike...maybe bad spark plug cables.
You should consider buying NGK plug ends if you haven't already. They are
great looking and my directions are based on using them.
Dennis Kirk:
[email protected] or 1-800-328-9280

2 each Part# 20306 – NGK Spark Plug Cap XD05F $3.30 a piece.
2 each Part# 20789 - NGK Spark Plug Cap LD05FP $4.80 a piece.
72" spark plug cable.....you will only need about 60"
******** All NGK caps are made for Stock 7 mm wire. *********
Make sure you use copper core wire and not carbon core. 7mm is stock size, you can use 8mm but is harder to install with little to no gain.

I lost the rear cylinder spark, swapped out the Ignitor Box, replaced sp caps and wire, performed 12V to coils mod ---(Direct wiring the coils to the battery using
a relay to bypass the wiring harness and give more voltage to the coils).......problem resolved.
Good Luck

'86 VN750 13,700 mi -Stock
-Purchased 2008 w/8800mi
Replaced ALL Cables . Kuryakyn LED Voltmeter
SilverStar Ultra 9003/HB2 H4 Headlamp, Jardine Fwd Controls,
Iridiums DPR7EIX-9 & Wires, Tuxedo Mod, Coil Mod, P/U Sensor Mod, Fork Seals,
Splines Lubed - 11/4/2012 - MF AGM Battery
Additional Flashing LED Brake Light on Trunk
Dampers went out @ 13+K !
After TOC MCCT's..... so amazingly quiet I discovered a rattling heat shield on my stock exhaust !
...have a vulcan good day!
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post #3 of 11 (permalink) Old 04-11-2012, 10:10 PM
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Do a compression test on the cylinders.

Welcome to the fun.

02 honda sabre 1100
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post #4 of 11 (permalink) Old 04-11-2012, 10:20 PM
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Yea, definately compression checks on both cylinders. Also, check for a nice fat blue spark on all four spark plugs. Clean them up of course.

You might also check to make sure that the petcock is delivering fuel to the rear carburator.

Open both carburator drains, and see if any junk comes out.

Have you cleaned, checked, and re-oiled the filters?

~~C8> Ratt
'91 VN750 - Cosmic Splendor/Candy Wine
Done: Coastered, .020 ign., Iridiums, VM
Plexistar II, MCCT's, Relocated MOSFET
Next: LED's
Will Do: Horns, Shave(maybe)
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post #5 of 11 (permalink) Old 04-12-2012, 04:23 PM
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I didnt see anyone else mention it, but the Regulator/Rectifier (R/R) has nothing to do with starting the bike or how the cylinders get there spark. The R/R in conjunction with the stator keeps your battery charged.
The VN750 needs a good charged battery to operate the starter motor and the ignition. If the battery starts getting weak it can be very hard if not impossible to start. When you can get it started check out your charging system. You could even check the resistances of the stator at any time.
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post #6 of 11 (permalink) Old 04-28-2012, 04:42 PM Thread Starter
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Okay, so I bought a brand new set of pre-assembled wires with NGK plug ends (you were right - they look awesome). I installed those. I also took the stator cover off and performed the pickup coil mod (filing the screw holes to bring the coils closer to the magnetic field).

I changed the oil, filter, and also removed the gas tank and removed all existing gas/chemicals. I replaced the fuel lines with new ones, double checked several carb diagrams and photos and made sure I was hooking everything back up to the petcock correctly, as well as to the carb.

I then proceed to fire it up, and STILL, the same problem is occurring! At this point I have both intake plenums off of each side of the carb, so I can see right down into the carb and watch the butterfly open up when I pull on the throttle. Now I realize that this isn't a good idea long-term, but I'm just trying to get an idea for what everything looks like when it's trying to run.

So what I noticed is (and please forgive me for not knowing the technical terms for each of the moving parts - my background is 100% electronic fuel injection, so I know very little about carb systems), at each carb intake, there is a little "shield" that slides up and down right in front of each of the throttle plates.

The right side, when pulling on the throttle, shoots up and out of the way, and you can see the throttle plate open wide as well. On the LEFT side of the carb, that "shield" wasn't moving. So I thought to myself "maybe it's stuck". So using a flat head, I very gently nudged it upwards (all the while my other hand has the throttle wide open). The RPMs drop, and it almost dies, but it manages to keep firing off little bursts of ignition, just enough that it keeps going.

So I continue to hold the throttle like this for a minute or two, and the RPMs just keep shooting up sparatically (just a little bit - not high at all) and almost dying again... I can't seem to get much more out of it other than that...

Thoughts, anyone?
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post #7 of 11 (permalink) Old 04-28-2012, 05:28 PM
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it sounds like the back carb severe vacuum leak, probably caused by a bad slide diaphragm. Part number 16126
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post #8 of 11 (permalink) Old 04-29-2012, 07:46 PM Thread Starter
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Just out of curiosity, is there anything in particular that would contribute to a part like this wearing out? Also, I know that in the automotive motorsport world, you can get aftermarket performance parts to replace anything and everything. Many times these parts perform better and are cheaper than OE parts. Does anyone manufacture an aftermarket carburetor for these bikes? I've already dropped around $300 into this "project" and I'd like to try to keep this cost effective. Any suggestions?
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post #9 of 11 (permalink) Old 04-29-2012, 08:05 PM
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the diaphragm wears out over time... the only possible aftermarket set up that might work is going to a single carb set up... but that is uncharted territory.
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post #10 of 11 (permalink) Old 04-29-2012, 10:07 PM Thread Starter
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Considering all the things that I've done up to this point, plus the symptoms it's continued to show, do you think that this diaphram has been the culprit the entire time?
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