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Ran great, now suddenly like crap (78 GS750)

  • Thread starter Thread starter Speedo
  • Start date Start date
S

Speedo

Guest
Just have carb work done 6 months ago (gummed up, running but very bad) and the bike - "Suzi" (yeah, real original I know) ran excellent. Running 29mm Mikuni Smoothbores, which if it makes any difference, have never worked quite as well as the old stockers which were jetted spot-on by a Columbus Ohio guy using a dyno (who is now crew chief on some race team). Anyway, the Smoothbores ran ok and the bike was happy except a sudden snap to full throttle.

Last week, when I tried to start, battery was almost dead. Kick start didn't work. Run and bump did (flashback to my 20's). Ran like crap...like almost as bad as when one set of points broke on my old 78 GS but not quite. Running on 3 cylinders? I checked for spark and all 4 cylinders seem to be getting good spark (Dyna S/Dyna coils).

Here's the wierd thing: when I pull off one plug wire at a time to see if subtracting a cyl makes a difference to isolate what cylinder is sick, it seems to make no difference. It runs equally crappy - with seemingly no change to tone / idle - no matter which plug wire I pull off.:confused:

BassCliff's site has a great doc for how to tear apart and rebuild VM carbs but I hope it is something simpler. I mean it was running REALLY well after the carb cleaning.

What kind of diagnostics can I perform? What could cause such a sudden loss of performance (ie what can break from shut down to start up and/or be caused by battery? Or is that just a coincidence?

I've read a lot of the threads and there appears to be some REALLY knowledgable mechanical folks out there. Kinda made me feel pretty dumb and amateurish, but hey....

The older I get, the faster I was....:o
 
Generally, I would say you have some sediment coming from the tank, but your tank is lined.

I'd say pull the float bowls and see what's in there
 
Thanks for reply BigT. further rookie questions:
1. Do I need to pull off the bank of carbs to remove float bowls?
2. Suppose I find sediment, now what? Totally drain/flush tank, reclean carbs?
3. Can I clean anything (jets, needles, etc) once float bowls are removed or do I have to disassemble to do so?
4. Will I need to have various gaskets on hand or anything?
5. Can it be something simpler like a fuel filter or something maybe? Maybe a coil crapped out? (no idea how to test but willing to learn/try)
 
I will check voltage at battery manyana. For those of us idiots (like me) that don't know, why? What difference does it make?

Yes the bowls have drain plugs. That much I know from winterizing back in Ohio. I used to drain float bowls every winter.

But sediment's not necessarily going to impeed the function of a float bowl (I'm thinking like toilet tank float), so who cares? Wouldn't sediment, if the problem, be clogging the main jet or causing the needle(s?) to stick if the problem? Can sediment cause a float or floats to stick? Come to think of it, last time I went to start it, I had to take choke OFF to have it fire up suggesting a flooding condition. Clue or just summer?
 
Sedment gets in the float valve and impeeds fuel or floods the oil.
Low battery causes low voltage at the coils and a weak spark.
But who cares?
I'm from Canton
 
How long has it been sitting?

And,

Put your old carbs back on, sell me the smoothbores, they are no good at all.
 
rode it to my bud's to help him with his slate floor for 3 weeks in a row. ran like a champ. apx 12 miles each way. then last week rode there, ran perfect. went to fire up to go home, didn't start. run and bumped it, ran like crap. charged batt when got home. next day, ran the same (like crap).

old carbs gone, traded as part of smoothbore deal. still 400 bucks with trade. VERY hard to find 'em these days. so if they suck, why do you want them:lol::D?

Chef - I've been through and to Canton many a time. Best man at my bud's wedding in Canton.
Grew up in Cleveland and was a Columbus/Cleveland guy for 20+ years b4 moving out west. Ignatius grad. Think I'm bsing now?
 
I would pull the spark plugs and atleast clean them if not replace them, and I would also inspect the points, maybe clean them or file them and set the timing.
 
points long gone (dyna s). plugs are new (3 months) and look ok. now i did have a dyna s module go bad about 6 7 years ago that caused it to run on 2 cylinders (just like a set a broken points). but if that was the case, I shouldn't be getting good spark at all 4 cylinders -and I am.
 
probably just crap in the fuel . Disconnect the fuel line & vacuum (plug it) . Run the bike until the carbs are dry. Hook up the fuel line / prime to fill them, disconnect & run them dry again . Do it 3-4 time. May help flush the crap. Otherwise pull the carbs off and blow them out with some carb spray...
Also drain some gas into a container & check for water
 
I have an issue that points out that MY bike will ONLY run properly with those smoothbores. :D
 
andrewpogany said:
But sediment's not necessarily going to impeed the function of a float bowl (I'm thinking like toilet tank float), so who cares? Wouldn't sediment, if the problem, be clogging the main jet or causing the needle(s?) to stick if the problem? Can sediment cause a float or floats to stick? Come to think of it, last time I went to start it, I had to take choke OFF to have it fire up suggesting a flooding condition. Clue or just summer?
Click to expand...

Sediment is unlikely to cause the float to stick, but could very likely cause a needle not to seat correctly which would cause that cylinder to flood. Are you dropping any gas with the engine runninng like this? Is there fuel in your airbox? Have you looked at your plugs to see if they are wet? These are symptoms of a flooded carb/cylinder.

One of the best ways I have found to narrow down which cylinders are not running well is to run the engine for a while a take the temperature with a good thermometer of the exhaust pipes near the head. Cylinders that are running rich are cooler than cylinders that a running lean. Cylinders that are not running at are stone cold by comparison. The same test can be preformed less scientifically by spraying of water from a spray bottle on the header to see how quickly it cooks off.

If the bike has never been hard to start on choke in the summer before, I doubt that it would suddenly start now. To me, this this indicates some that something has gone wrong in you air/fuel system (as opposed to electrical).
 
charge the battery for several hours and make sure its hot, make sure no one has played with the firing order by swapping wires on you as I have seen this done. replace all plugs because compression fouled plugs can spark fine outside the cylinder but once installed they do not. if this doesn't help, pull the carbs, remove the main and idle jets, pull one bristle from a typical 8-10 inch wire brush and slide it completely through each jet and spin the jet on the wire a few times to completely clean all sides of the inside of the jet and to insure a crud free perfectly round hole. then spray through each jet with carb cleaner such as b12 chemtool and then blow through them all with compressed air . hold each jet up and verify that you can see day light through them all and reinstall. withe the entire rack of carbs standing on their sides (so no weight is put on the needle and seat from the floats)re set all floats to where they are level with the flat bowl gasket area of the body of the carb and all look the same then reinstall the bowls and carbs and try again. I have removed carbs to find a main jet laying in the bottom of the bowl. that sure makes a motor run funny:) if it still misses I would probably do a compression check. if that's good your dyna ignition is suspect and swapping a points plate back in for testing would be my next step.
 
andrewpogany said:
points long gone (dyna s). plugs are new (3 months) and look ok. now i did have a dyna s module go bad about 6 7 years ago that caused it to run on 2 cylinders (just like a set a broken points). but if that was the case, I shouldn't be getting good spark at all 4 cylinders -and I am.
Click to expand...


Replace the plugs.. 3 mos old? Replace em anyway, they're 2 bux.
Still dont do it, remove em and put a few drops of fuel in the cylinders. Place the plugs and fire it up.
Post the outcome..
If you have fire and compression all ya need is fuel to make it run. If it runs good for a few seconds your carbs need work.

Been there done that.
 
supersonictoys said:
charge the battery for several hours and make sure its hot, make sure no one has played with the firing order by swapping wires on you as I have seen this done. replace all plugs because compression fouled plugs can spark fine outside the cylinder but once installed they do not. if this doesn't help, pull the carbs, remove the main and idle jets, pull one bristle from a typical 8-10 inch wire brush and slide it completely through each jet and spin the jet on the wire a few times to completely clean all sides of the inside of the jet and to insure a crud free perfectly round hole. then spray through each jet with carb cleaner such as b12 chemtool and then blow through them all with compressed air . hold each jet up and verify that you can see day light through them all and reinstall. withe the entire rack of carbs standing on their sides (so no weight is put on the needle and seat from the floats)re set all floats to where they are level with the flat bowl gasket area of the body of the carb and all look the same then reinstall the bowls and carbs and try again. I have removed carbs to find a main jet laying in the bottom of the bowl. that sure makes a motor run funny:) if it still misses I would probably do a compression check. if that's good your dyna ignition is suspect and swapping a points plate back in for testing would be my next step.
Click to expand...


Wouldnt it make more sense the check the compression first?
Do the simple stuff first..
 
Thanks for all the advice folks. I will be a busy guy over next few days and will def post my results. Couple thoughts /notes to now:

* bought comp tester today to do test
* no way are plug wires hooked up wrong (confirmed)
* battery is brand new (1 month) and fully charged now. starter spins fast and healthy
* Dyna S module possible but highly unlikely as there is all around. When I pull wires off plugs one at a time, every one of them starts jumping spark to top of plug like crazy if I get within 2 inches. I still need to pull plugs and run test with plug against cylinder to make sure actual spark is fat/blue.
* no EGT (ex gas temp) tester. Is there a cheapo workaround? If not, I'll do the water spray test
* no gas spilling out of carbs. no fuel in airbox since no airbox(K&Ns)
* will try few drops of fuel in cylinders test and post results
* If everything points (no pun intended...well maybe:lol:) to carbs I will give my best college try at cleaning carbs.

I think that summarized all the advice / my "to do" list.... Thanks again.
 
Octain said:
Wouldnt it make more sense the check the compression first?
Do the simple stuff first..
Click to expand...

basic tune up is the simple stuff and the most sensible approach in my humble opinion. if the bike ran fine when parked and then when restarted ran like crap, I doubt it scored a cylinder or bent a valve from storage. make sure you have the basics that "can" change from sitting. a hot battery, good spark, clean new plugs and a clean proper functioning fuel system.
 
Last edited:
Healed itself - how? Miracle?

Healed itself - how? Miracle?

Again, thanks for everyone's help.

The good news: Suz (my 78 gs750e) runs great again!
The bad news: I did NOTHING to it! Nothing. I was getting ready to pull the plugs, etc and I decided to fire it up one more time just to warm her up a little.

A "miracle"? What the?

Review: ran great to my buddy's house 15miles. 5 hrs later, battery dead and wouldn't turn it over. I bump started and ran terrible but got me home. Next day, with fully charged batt (new, 2months old), turns over fine and evenly (WV compression check) and fires right up but continues to run terrible (like on 2 or 3 cylinders).

Then I post to you guys. Then I'm busy until today to try all this stuff you all recommended, fires right up, ran perferct, just returned from a 10mile ride. Perfect.

Theories anyone? Stuck slide or float bowl that magically unstuck? I'm 99% sure that, not having touched the Dyna, coils, or plugs that I can rule out that it was a spark issue.
 
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