Colder left cylinder

[NYSingh]

I recently installed some Trail Tech CHT gauges (with mount designs shamelessly ripped off from 1976RD400C's bike) to give me an objective measure of what's going on in my engine since I don't have the experience to know from feel/sound.  The left cylinder seems to be running about 30-40 degrees cooler than the right (keep in mind this is city riding, so no long straights).  Replaced the plug today and the old one was a bit oily/black. New plug runs better, but still seems like it's 4-stroking and still running colder.

Any ideas about what would cause this? Haven't had a chance to troubleshoot yet, but things that come to mind are timing, coil, ignition system generally. Battery is a Yuasa with less than 7 months on it.

Carb setup: Dave F mod, 200 main, 40 pilot, 5DP7 needle in middle

[Greaser Greg]

I had a similar situation with my r5 and the one set of points was wet/oily. Thinking I had too much oil on the cam felt so sprayed points with brake clean and whoosh! All better!

[NYSingh]

Forgot to mention, but I'm running the Vintage Smoke Dyna S ignition, so no points 

[forexer]

check carb sync and make sure handlebars are pointing straight when doing so

[Alain2]

Air leak? Are the mufflers packed the same?

[Striker1423]

Start with simple tests first. Can of fogging oil preferably (yes its messy, but hey its lubrication, and doesn't explode like ether). Spray around base gasket and carb boots. If the motor rev's you have an air leak there. If that passes, check fuel level (or float height if a tester isn't available to you.) and carb sync.

The cylinders will almost never match up on temps 100%. But, you can indeed get them closer and it's usually something simple.

[1976RD400C]

The left side seems to be 20 degrees hotter on my bike after it runs for a bit. I wonder that since it is reading the spark plug base temperature the difference has something to do with how matched the plugs are at dispersing the heat. 

[forexer]

There used to be a member on the old site named ENGINAIR who was a brilliant and very helpful guy.

Here's what he told me when I had this same issue of dealing with different cylinder head temps.....

The cylinder head temps should be within 10 degrees deg. F

ALWAYS Start by checking the timing.  About 2.0 to 2.3 mm but match both sides to within 0.1mm.

POINT HANDLEBARS STRAIGHT AHEAD. Set carb sync using index mark on slides, make sure you have the specified free play in cable, set idle speed by momentarily (no more than 30 sec) grounding one cylinder at a time and setting idle to about 900 each side, both sides running will be about 1200 to 1500 RPM.  Check carb and throttle cable free play. 

Hope this helps............

[jmendoza]

First off, be sue both of your carbs are exactly the same when it comes to
1. Float height
2. Pilot air screw setting
3. Slides open the same amount at full throttle
4. Needle clip positions
5. Idle speed screws are with 1/4 turn of each other.

[Arrow]

NYSingh, don't be afraid to look 'outside of the box'.
You may well find that adjusting the carb cable on the colder cylinder; tighter by 1/4 of a turn at a time may well rectify the issue. I always set up my bikes dynamically, not staticly. Airscrews included. You have the MASSIVE advantage of combustion temperature gauges (whether cylinder head or exhaust). Jet engines use the latter, its a most useful method of combustion control.


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[retaRD]

NYSingh, don't be afraid to look 'outside of the box'.
You may well find that adjusting the carb cable on the colder cylinder; tighter by 1/4 of a turn at a time may well rectify the issue. I always set up my bikes dynamically, not staticly. Airscrews included. You have the MASSIVE advantage of combustion temperature gauges (whether cylinder head or exhaust). Jet engines use the latter, its a most useful method of combustion control.


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I agree, and do the same thing.
Once I know all other settings are the same... airscrew, idle, timing, floats, etc.. I'll set the carb cables to match EGT between each cylinder, holding it just off idle. 
It's often a very small adjustment.
I also adjust the idle screws to balance the cylinders out at idle as well, using EGT.

My EGT split throughout the RPM and throttle position range is usually within 10 degrees, and sometimes dead nuts the same.

[NYSingh]

Getting back to this after work distractions and a few weeks of chilly riding through the NE and Acadia on the Guzzi.

Here's what I did, with no change in temp difference (still 30-40 degrees different):
- Re-checked timing, set at 1.9 BTDC using dial gauge, confirmed w timing light
- Checked for air leaks using starting fluid (sprayed around manifolds, spark plug hole, head gasket, base gasket) - no change in engine speed
- Measured resistance on iggy coils - both seem to be w/in spec
- Checked carb sync via the "line up mark on slide through view hole" method, handlebars pointing forward
- Confirmed both air screws set to 1.25 turns (per recommendation of Chuck after rebuilding the top end)
- Confirmed floats are set at 23mm (measured from gasket surface, with carb held sideways and float just touching the valve spring)

My gut is something is up with the left carb, or maybe there's some electrical issue.  The right (hotter) exhaust has the nice 2 stroke crackle sound; the left (colder) exhaust has more of a percussive "POP POP POP" sound.

Re the carb sync:  the Yamaha workshop manual and Haynes favor the 'lining up the slide marks' method. Others favor the 'make sure slides start moving at same time using popsicle sticks' method.  Is either one better than the other?

Re adjusting idle speed:  Haynes recommends the approach forexer explained (get running at higher than normal RPM, pull boot off right cylinder, adjust left idle screw until it feels like it's just about to die, repeat for other cylinder). Would this work if you have a crossover tube or boost bottle?  If I take off my boost bottle, plug the manifold holes and adjust using this method, would that work, or will the jetting be all screwy without the boost bottle?
                                                                                                                                                                                               

[Arrow]

You may need to remove the balls and clean out the air jet. Try adjusting the air screw on the left side, if it doesn't change the running then this may be your issue.

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[NYSingh]

Carbs are Dave F modded so no brass balls.  But I'll adjust the air screw a tiny bit to see how the temp/exhaust pulse is impacted.  I was reluctant to diverge from Chuck's recommended settings so haven't tried that.

[rodneya]

Carbs are Dave F modded so no brass balls.  But I'll adjust the air screw a tiny bit to see how the temp/exhaust pulse is impacted.  I was reluctant to diverge from Chuck's recommended settings so haven't tried that.

Air screw settings are a suggestion only. Ideal setting will vary bike to bike and even with temp or humidity changes.