83 VF750f rear cylinders

So I bought the bike a week ago and rode it 3 and a half hours with no problems.. It's been in a heated garage the past few years*. Right before I got home, the tach cut out along with the rear cylinders.. After some googling, I assumed it was a faulty cdi box and bought another one. (I forgot to mention I can ride it for 10 - 15 min with all 4 running strong and then the rear cut out; repeat daily) anyways, new box installed and same problem. Swapped cdi box connectors and no change.. Took a spark plug out and tested it on both wires for coil feeding the rear cylinders and no spark at all. Bad coil? I'm about out of ideas...

 

The tach runs off the rear coil.Check your connections at the rear coil and ground from coil bank. Coils can be finiky and work till they warm up then fail. Swap the connections and see if you have spark, swap coils and see if you get tach back.

 

Yeah I'm betting its the coil too

 

CDI's tend to just fail. As unlikely as it is, it does sound a bit like a coil. Pretty sure you can swap the two oot on gen 1 and duplicate the failure on the front. Just a suggestion.

 

Good, because I have a coil being shipped to me already. Should be here Wednesday or Thursday.

 

Yep, did this earlier before I posted the thread. Just wanted some reinforcement since I'm only 19 and although I've rebuilt a few bikes already, I like to be sure.

 

You said it cuts out after 10-15 min of riding. That's enough time to get the engine up to operating temp,hot. I'm betting on the pulse generator cutting out when it gets hot. I can't really see the coil getting that hot in such a short period of time. I could be wrong though. Yes swap your coils first before spending any bucks. Sent pm on the box. Thanks.

 

The pulse generator would only affect cylinders 2/4 though? I can't check the wiring diagram to see how it works because my manual is at my buddy's house..

*scratch that, looked up a diagram. I have work in 45 min, but I'll check it out more tomorrow..

One thing I noticed today after the mention of pulse generator is that the polished engine cover that houses the pulse generator is leaking oil around the rubber grommet/seal on the pulse generator wiring.

 

There are two pulse generators. One for 1/3 and one for 2/4. As far as the oil leak that's standard. Squirt some silicone in the rubber boot to seal against the wire jacket.

 

#1 Rule: if you think it's the coil, it's probably not the coil. I really doubt that will help you. It's almost certainly one of the pulse generators. I'd check them (when hot) and I think you'll find they are out of spec. Coils hardly ever go bad but the pulse generators are known to fail. Good luck!

 

I just said screw it and bought another pulse generator

 

Got the coil in the mail today when I got home from work. Too dark outside to really do a whole lot and tomorrow is my day off, so that'll be on and whatever else I can do will be done. Pulse generator should be here on Thursday/Friday, so that will be done this weekend some time.

Regarding pulse generator: I know the removal process (unhook battery, unplug pulse generator/oil pressure switch, etc etc..), but besides check gap before removal as well as after, does anybody have any other advice? I was going to order a new gasket, but since it was $28 plus another $27 to have it here before next week and I'd like to have this all together rather than wait till the 11th-15th as is the quoted arrival, I'm just making a gasket if it tears. If not, hey, I'll make sure the old one is still in decent shape and try to reuse it (but most likely I'll just make a new one).

As I said though, any other advice? I would like to only do this process once

Anthony

 

This is just me, but I like to swap things one at a time. It helps me figure out what the offending component was. I find that if I replace a bunch of things at the same time to fix a problem, I don't really learn which symptoms went with which solution. For giggles, I'd swap the coil and see what happens. It's easy and no gasket needed. If that was it then you have a spare set of generators on hand and you may need them some day. If not then you know your next step, one you've already planned on.

Good luck!

 

Alright, so coil installed (love how easy working on bikes is compared to my daily job of wrenching cars lol). Went on a test ride and bike psyched me out by running until one traffic light from home. Then, right back to cyl 1/3 only. Soooo, as figured, pulse generator will be here today or tomorrow and then hopefully I'm done.

The other day I was looking through my shop manual and it listed how to check the generator (ohms), and sure enough cyl 1/3 were right in range, whereas 2/4 were about 150 ohms over. I'm thinking that the air gap between the lobe and the magnet is too much due to being worn over time (and 26,000 miles) and it's just malfunctioning and would probably do so more rapidly over time if not fixed. So at this point, it's more of I'm 100% sure that it's the pulse generator and not just... "what else could it be?"

Thanks for all the help and I'll post the result after the part arrives/is installed.

 

have to check the pulse gen w/o the rotor on or the mags will give u a slightly wrong reading if they are positiond over or around the pulse generator

 

Fixed and on the road . Old generators were ridiculously worn and had a huge gap..

 

Interesting, I've only ever seen the pulse generators fail for electrical reasons but never from a mechanical failure. The rotor doesn't (or should I say, isn't supposed to) touch the pulse generators. In the end I suppose it doesn't matter, the most important part is that you have it back running again!

 

Alright! Another one on the road. Good job.:thumb:

 

Good to hear. :thumb: