07 750 Aero Weak spark, Out of time ?

[wallismotorsport]

I have a 2007 VT750 Aero. Sat for about 5 months (I have 2 other bikes). Charged the battery and tried to start it but it would not run. I thought the fuel went bad and maybee the carbs were gummed up so I drained the tank and cleaned the carbs. Strangely the fuel seemed to be ok and the carbs were clean. Now it wont start and it sometimes backfires when cranking both thru the exhaust and carb at times like the timming is off. I checked for spark and have a very weak and inconsistant spark, checked the signal to the coils and dont have a good pulse signal to the coils, this explianed the poor spark. Now I dont know what to do, is it the ignition box thats failed or something else. I dont even know what type of pickup or crank sensor this bike uses, I imagine its probably a hall sensor with an exciter ring on the crank but not sure.
Can anyone HELP me out with some insight on this, has anyone seen this problem before ?

[Easy Rider 2]

Originally Posted by wallismotorsport

Can anyone HELP me out with some insight on this, has anyone seen this problem before ?

No "insight" but a suggestion. Make sure you don't just have a bad battery. Jumping from a larger car battery is a pretty good test. You could also charge the battery and have it load tested.

If you have a good ear for the starter and the cranking speed is normal then not likely that is the problem......but lots of folks can't tell when the cranking is slow.

You are using some choke when trying to start it.....right ??

A tiny bit of starting fluid (ether) can be helpful in deciding if you really have a fuel or spark problem.

[wallismotorsport]

I'm 99% sure its a spark problem, put a spak tester on it and tested with new plugs also, spark barely jumps the gap and is inconsistant. The battery is charged and seems to be OK.

[Coolride]

I second the battery, if it's not up to par, the starter and lights may be drawing it down past the required voltage for the ignition. You could not use the starter and have someone push you, and drop it in second gear to test the theory. Just don't fall over in the process. Or scratch that, and just jump it off a car batt.. Batterys do some funny things, I just replaced the one in my riding mower, it started the mower fine, but it would blow the fuse to my ignition after 2 minutes of mowing. I replaced the ignition ,$75, then found out it was the battery all along(which seemed good) by swapping it with my Dad's mower, same one.

[tubes_rock]

it sounds like you have access to equipment, and judging by your handle, it sounds like you've been in this business for a while. I won't second-guess your assessment, but I'll provide a little info that may help you in your diagnosis, in case you didn't know these things already.

The Shadow line of bikes (being that they have two plugs per cylinder) use what's known as a "fast spark." it's efficient at igniting fuel and uses less energy than standard ignition. However, when working properly, it produces a pale yellow/purple spark and not a fat blue spark like most wrenches are expecting. Your spark may indeed be working as designed, just not what you are expecting to see.

The inconsistency of it may point to a bad battery, as already stated. the ignition module in these bikes had some components in its design that date back to the early Shadows that had the 10.5:1 compression ratios. Those bikes had a habit of rotating the engine backward if the battery was weak because the cylinder would fire with advanced timing and the slower RPMs of a cranking engine with a weak battery would not getting the piston past TDC before it was under combustion forces. The ignition module was designed to kill the spark if the engine RPMs were below a certain threshold. If your engine isn't cranking as full speed, you will have an inconsistent spark.

An ignition module on these is most often pass/fail. if you get a spark at all, it's probably fine, so start looking elsewhere.

The pulse generator on these is a full inductive coil, like a tape recorder head. It should measure a few hundred ohms on your meter and put out a few millivolts of pulsed voltage if you have an oscilloscope in your shop to test with.

Hope this helps!

--Justin

[MCCVIII]

i'm having the same problem as this, today. the spark on my bike is somewhat weak. it was having issues before, and has been sitting for a month or two now. the battery has been indoors keeping a full charge (it's been sitting for a week or so with a full charge now), and is only 5-6 months old. AGM style.

all 4 plugs get some sort of spark and all the gaps look fine. the spark doesn't seem healthy or robust, but it's still jumping the gap on each line. a bit of starting fluid in each cylinder helps the engine jump to life for a few cycles, but it definitely can't sustain anything once the starting fluid is used up.

i think i'm going to go and finally buy a radioshack multimeter so i can test voltages, but where might be the place to check? i had a brief introduction to using a m.meter, but it was in application to a clavinet, not a motorcycle. red should find the voltage, and black should be grounded to the bike frame, right?

happy thanksgiving to all. here in cincy we've had thanks-giving worthy weather for riding the past few days! carpe diem n stuff.

[93gc40]

Battery or fuel, is the more likely culprit. Ignition sounds OK, for both MCCVII and the OP, bikes issues..

LOAD TEST battery, check ALL battery and starter conections, CLEAN the run/stop switches, do you have fuel in the carbs, pump pumping, choke on and working. IF you have spark at all, on BOTH cylinders, the bike should start. Timing is fixed, so unless something inside the motor is Broke, timing should be OK.

[MCCVIII]

thanks man. i'll make some checkings out of those things.

[vonh]

Get a good manual and check the timing points yourself it's easy enuf, but prolly not the problem. Get the battery tested at a shop, it's free and if that specks out you most likely have a fuel issue. And thats my guess prolly a plugged jet or 2. I've had to clean carbs a half dozen times befor the whole system was right. Flushed tank and reserve more than a few times, new filters, yes filters 1 after each rebuild and new fuel lines befor it finally ran properly, it can be a real pain!!! You can gravity feed gas from a can directly into your carb and by-pass your whole system to verify dirt. Unplug the fuel pump and turn off the fuel **** when you try it if needed. Get a ladder to put the gas can on, disconnect the fuel line at the carb and connect the the gas can and start away If that works well then you have a fuel system issue.

[C.W.]

One possibility.

07 & 08 Aero's had a recall on the petcock vacuum diaphragm. It was defective and would eventually fail. But, just before failing it will starve the carb of fuel. Check for drips of gas at the petcock. I would suggest at least checking with your dealer and have them run the VIN to see if your Aero was in the recall list and if it had been repaired. It would be in their repair history, also checked with VIN, if it's been done.

Good luck, C.W.

EDIT: Ooops! Old post. Sorry!