Two of four cylinder died

[Plaid_Bagger]

I bought a 1978 gs 750 e about a month ago.   Needed it's lights rewired but other than that it was in good shape.   Rode it for a couple days than all of a sudden it bogged down.    I can get the old girl ta start, but she is only firing off the first two cylinders.   Cylinders 3 and 4 don't even get warm.  They are getting fuel, [spark plugs are whet when I pulled them]  and I switched the plugs around to make sure they are ok.    All the plugs work.   I'm certain it's getting compression, but don't actually have a meter to gauge properly.  Have no clue what to look into next, and it's my main mode of transportation at the moment.  Please, if any one could throw some ideas out there.  I'd be very thankfully.  

Edited January 29, 2018 by Plaid_Bagger
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[johnr]

have you tried the basics like changing the plugs? doesnt sound lik an ignition fault as the coils are paired 1+4 and 2+3 so an ignition fault on either circuit would take out that pair. but you have one from each pair running which suggests fueling. carbs could be flooding, but tbh there are a lot of other variables that need to be taken into account. so,  compression test first, then if thats ok, fresh plugs, then carbs.

[BigT]

Try swapping the spark plug leads for each coil.

Take 1 and swap it with 4. Swap 2 and 3.

 

Does the problem now move so 1 and 2  aren't firing?

 

Start it up in the dark and see if the spark plug caps are shorting out while its running

 

Does your bike still have points?

[Plaid_Bagger]

So the bike does have points.    The plugs are good.   Compression is good.   I will try switching the plugs around.   But as was said.   The coils wouldn't go in that order.   I changed the plugs, and oil and basic maintenance before operating.  

[Blubber]

- Have you drained the float bowls of the carbs?

I know you stated the plugs were wet when pulled but maybe the float level is low /  sticking  /  clogged /etc

[wraith]

So, just to make sure I've got it all 

1. All 4 cylinders are getting fuel.

2. Have good compression on all 4 cylinders (above 100psi)

3. All 4 plugs are giving  a good spark.

So if all the above is wright, try 

1. Get 4 new plugs put them in and see, you may have some duff plugs (had this myself the plugs fail under compression but look OK otherwise).

2. Put HT and caps from 1to4 and 2 to 3 see what happens, if it stay with 3/4 cylinders or goes to 1/2. May have 2 bad caps.

3. Check 3/4 cylinder carbs are not flooding, are you running air box or pod filters?

But my bet is on duff plugs, you sometimes find they give a good spark out and against the head but when put in they go coupt. 

[Plaid_Bagger]

I will check that.   It is the original air box.  New filter.  I bought a compression gauge this morning so I will take a look at that.   Thanks will post what I find.  

[Plaid_Bagger]

So I switched the wires around.   And still the same.   1 and 2 fired.   Have good compression.   So I tore the carbs out.  Ordered a rebuild kit, going to bench sync the floats again.   Has to be something going on in the carbs.   Was wondering about the t section where the fuel splits off. Thanks for the help guys.  Apprcost a that.  

[Plaid_Bagger]

So it has finally been above 20 degrees and I have been back to playing with the bike.   I started it up out of curiosity and the pipes for 3 and 4 had a little heat to them.   So I took the carbs off and of course the carbs had a bunch of fuel.   So somewhere something is sticking or flooding.   Be working on the rebuilds.  Anything specific I should be looking for?

[BigT]

Deep clean the carb bodies, don't just swap parts

 

Make sure all of the passages are clean

[Sheep]

Have you tried new plugs yet?...I find that plugs nowadays do not tend to last half as long as they used to and they give up the ghost at the faintest wiff of over-fuelling..I find only a good cook in the oven brings them back to life.....

Have you tried increasing/holding the revs and seeing if the warm downpipes get hotter....over fuelling on the idle circuit will wet your spark but could clear with grater throttle positions.Rich at idle could point to sticking float needles.

[BigT]

http://members.dslextreme.com/users/bikecliff/images/vm_carb_rebuild.pdf

[Plaid_Bagger]

I have tried at higher revs.  And I think that's how the pipes warmed up.   Bot it bogs out and dies.  I just bought the spark plugs.   And I did buy new one and tried again.  Same thing.   So I'm going to deep clean the housing and the fuel pipe that goes to the carbs.   That link is awesome.  Thank you.   I've done lawn mower carbs.   But this is different.  Like I said, now that it's above 30 degrees during the day I'm able to get some work done.  

[Plaid_Bagger]

So carbs are rebuilt.   Put accel coils on, new wires and air filter.   All four cylinder heated up.  Till the battery died.    Thanks for the help guys.   Next step is to rebuild stater and do timing on the points.   

[Plaid_Bagger]

I have got a new battery.   Went to fire up and it just back fires out the exhaust.   I let it sit for a moment.  But the no results.    Getting fuel.   It sparked over for a second but than it backfired.