Topic: 750's 1 and 4 cylinders running cold ? Normal ???

[dboblet]

Just got my 750 running smoothly after many years of sitting and I've noticed that the 2/3 cylinders exhaust pipes are very hot (don't worry, the blister on my finger will heal) while the 1/4 cylinders are still cool to the touch.  Admittedly, this was only after about 5 minutes of idle time, but is this normal??  If not what should I be looking for?  All else 'seems' normal, point gap, valve adjustment etc.  The idle speed is fine, and the engine is running smoothly (though I haven't added a muffler to the header yet).

So, is this normal? 

[Jay B]

No. These have ignitions that spark 2 plugs at once, 1 and 4 ,2 and 3. If you have 2 cylinders cold chances are good you have an ignition issue on the 1 and 4 side.

[crystalhelix]

Could also be they need balanced.  I had a similar issue but balancing the carbs helped a lot.  I think the arm that moves the slides isn't perfectly straight anymore and a balance helped me out.

[markjenn]

If the headers remain cool enough to touch after five minutes of running, I doubt you're firing on these cylinders at all.  A bike that is only firing on 2-3 will run and will idle if you bump the idle high enough, although it will be dog slow, sound awful, and rev like it has molasses in the cases.

1-4 and 2-3 fire off the same ignition circuit (which fires a waste spark on the cylinder that is not on the combustion cycle), so probably ignition related.

Definitely not normal.

- Mark

[kghost]

Sorry I've looked at this thread since last night....

WHY would you think that was normal? 

[dboblet]

I've checked the 1/4 for spark and it's definietly fireing a spark on the plug.  I've checked the carbs as near as I can and they all seem to work well.... but I don't have a throttle cable yet to play with reving except by hand.  I'll try to swap out the coil and see if it makes a difference (since I have an old spare).  Is there a way to static check the coil?  Resistance or something.  I'll also re-check the points for those cylinders and see what I get, but it all checked ok a couple weeks ago when I checked it out.

[TwoTired]

Have you taken the carbs apart?

Are you confident that all the carbs have the same slide height?

[markjenn]

Again, if the pipes are completely cold on both 1-4, that really speaks to an ignition issue since these two are paired.  Like you're doing now, I'd start swapping parts side to side to find out what changes with the swap.

A key thing here is whether the outside pipes are stone cold or just not was warm as inside.  A pipe running cooler than the rest points to possible carb issues and mis-sync, but a stone cold pipe points to no ignition (or fuel) whatsoever.  And a scenario where 1-4 get absolutely no fuel and 2-3 get enough to run fine -- strictly by chance -- seems hard to come by.

One remote carb problem that comes to mind is your idle jets are completely clogged on all the carbs such that the only way it will run is with a lot of throttle on the main jets.  And if the carbs are badly mis-synced, perhaps the 2-3 carb slides come up way before 1-4 and thus 1-4 get no fuel whatsoever.  Certainly sticking your fingers in the carb throats and getting some feel for whether the slides are coming up equally is worth doing.

If this bike has been sitting for a very long time, it almost certainly needs to have the carbs pulled, cleaned and sync'd, so I'd just do it now since you have to anyway.  And sort the muffler situation and throttle cable.  You're dealing with too many variables right now.

- Mark

[dboblet]

Yeah, despite the fact that I've had both carbs off the bike, and rebuilt them both and everything 'looks' ok...  AND I can see spark on the plugs for 1&4, I'm still leaning to an electrical problem.  I'll check it out in the next few days and let y'all know what I find.

As for warm vs cold... the 2/3 pipes can fry an egg and I can grab and hold the outside pipes and it's barely warm after about 4 minutes.... so... not fireing, right?  I see no problmes with the carbs though.

Is there a way to synch them easily without complicated gear?  My bud, an old racing enthusiast says all I'll need is a tube and a sensitive ear, then listen for the same tone or sigh when the tube is placed at the carb opening.

[TwoTired]

Yes, if you can grab hold of 1&4 after 5 minutes they aren't firing and it is likely an ignition issue.

Swap the 1&4 plugs with 2&3 and see if the problem moves.  Otherwise its a points issue.  Spark must jump across the electrode not just to the center insulator.


There is a senario where it is carburetion.  IF you only idled. And the slides were open too far on 1&4. Or the idle jets were plugged on those carbs, or the float bowls empty, etc.  But, you don't want to hear this, so...

The slides height can be mechanically synch'ed to get them relatively close on the bench.  Manometers are best for synching 4 cylinder engines.  Bud has probably done twins with the tube.   But teaching yourself to do 4 carbs by ear with a tube will require a bit 'o luck.

[paulages]

Yeah, despite the fact that I've had both carbs off the bike, and rebuilt them both and everything 'looks' ok...  AND I can see spark on the plugs for 1&4, I'm still leaning to an electrical problem.  I'll check it out in the next few days and let y'all know what I find.

As for warm vs cold... the 2/3 pipes can fry an egg and I can grab and hold the outside pipes and it's barely warm after about 4 minutes.... so... not fireing, right?  I see no problmes with the carbs though.

Is there a way to synch them easily without complicated gear?  My bud, an old racing enthusiast says all I'll need is a tube and a sensitive ear, then listen for the same tone or sigh when the tube is placed at the carb opening.

both carbs?

[mlinder]

Yeah, despite the fact that I've had both carbs off the bike, and rebuilt them both and everything 'looks' ok...  AND I can see spark on the plugs for 1&4, I'm still leaning to an electrical problem.  I'll check it out in the next few days and let y'all know what I find.

As for warm vs cold... the 2/3 pipes can fry an egg and I can grab and hold the outside pipes and it's barely warm after about 4 minutes.... so... not fireing, right?  I see no problmes with the carbs though.

Is there a way to synch them easily without complicated gear?  My bud, an old racing enthusiast says all I'll need is a tube and a sensitive ear, then listen for the same tone or sigh when the tube is placed at the carb opening.

both carbs?

heh.

[dboblet]

OK, all 4 carbs including BOTH carbs from BOTH cold cylinders.  Now I'm going to go to the fridge and drink BOTH beers... and don't I wish there were a quad of 'em.

[scunny]

I've used the poor mans manometer before(tube sytem) it's alright if you have a good ear for tones and pitch.

[paulages]

Ok, now i'm pretty sure the beers come in sixes...

[UnCrash]

I recently had 2 and 3 stop firing on me.  Turns out it was poor condition points on 2 and 3 coupled with leaking carb boots on 2 and 3 and fouled plugs on 2 and 3.

This trifecta left my 2 and 3 pipes cold. 

New plugs, and electronic ignition cured most of my woes.  I have new carb boots going on this weekend.

Best of luck.

[dboblet]

Tell you what, come on over and you can have the two extra beers... BOTH of them.
 

[uptworedline]

I recently had 2 and 3 stop firing on me.  Turns out it was poor condition points on 2 and 3 coupled with leaking carb boots on 2 and 3 and fouled plugs on 2 and 3.

This trifecta left my 2 and 3 pipes cold. 

New plugs, and electronic ignition cured most of my woes.  I have new carb boots going on this weekend.

Best of luck.

nice. that's what worked for me too!

[paulages]

Tell you what, come on over and you can have the two extra beers... BOTH of them.
 

 

seriously though, i'd check your left side points. get a 23mm wrench and turn the engine until the points are closed. assuming you have a test lamp, manually open the points while touching the left points assembly with the probe end of the test lamp (while grounding the other end). it should light when open, but go off when closed. when they get pitted, sometimes they won't ground (the light would stay on). i'd put my money on that as the culprit.

[dboblet]

Thanks, I'm leaning towards your analysis as the most likely (the points).  I'd reallllllly hate to find it's the coil.  The carb boots are in great shape.  But I swear, if I need to pull the carbs off again, I'm going to buy a case, not a six pack, before I try to put them back on again.  I curse the boots and the rubber tree they came from, though they look and feel less like rubber than some kind of poly synthetic material.  However, as I said before, I am getting a spark, I'm just not sure it's enough of a spark to fire the cylinder.

[paulages]

Thanks, I'm leaning towards your analysis as the most likely (the points).  I'd reallllllly hate to find it's the coil.  The carb boots are in great shape.  But I swear, if I need to pull the carbs off again, I'm going to buy a case, not a six pack, before I try to put them back on again.  I curse the boots and the rubber tree they came from, though they look and feel less like rubber than some kind of poly synthetic material.  However, as I said before, I am getting a spark, I'm just not sure it's enough of a spark to fire the cylinder.

i've seen plugs foul often from a weak spark (rather than no spark), when the points just weren't providing consistent dwell. i know it statistically happens (which you basically represent on here), but i have never seen a coil fail on a four. lots of bad points though.

[dboblet]

Results:  It be working now mate!!  Tuns out (call me dopey, go ahead, I deserve it) I left the wires loose on the points that fire the 1&4 cylinders and of course, it was grounding out... so intermittent spark at the very best.  I guess It's what I deserve for being excited and trying to rush it to completion at
0-dark-thirty with only a 60 watt bulb to light up the work area... and that on the wrong side of the bike. 

Anyway, all working great now.... but I still need to work on the carbs a bit.  With the choke closed, it fires and runs great, but open the choke and she dies within minutes and won't rev without shutting down.  Lots of adjustments needed there I guess. 

I'm going to go all the way back to square one on this though and start with re-gapping the plugs, adjusting the timing and valves, then I'll begin with readjusting the idle mixture screw...  They would have to put it in one of the most in accessable parts of the carb.