Right Header Significantly Hotter than Left CB175

B

BPM144

New Member
I bought a 72 CB175 a couple weeks back, pretty damn clean, almost completely original, but I noticed today when I was syncing the carbs and resetting the idle, that the right header was much much hotter than the left. I also noticed it became hot more quickly than the left. The left header is also much more "blue'd" than the right. I don't have an IR thermometer, but if I put my hand on the left muffler after riding, its only slightly warm. The right side however, is almost too hot to touch in the same place.

Outside of all that, the bike runs well. Syncing the carbs eliminated a lot of handlebar vibes and throttle response seems much crisper all around. I'm just concerned that this temperature differential stems from something serious, but I am just unsure what I should check first. I had a 1974 BMW 2002 and many other maintenance/tinkering intensive vehicles before this, so i'm pretty handy, I just lack experience with antique bikes specifically.


Thanks for any help in advance,

Prescott
 
You are likely running on one cylinder. I was also fooled by my 175 when i first got it. The float was stuck closed.

Remove the spark plug lead from the hot cylinder and see if the bike runs. That should confirm it.
 
I'm pretty convinced its running on both cylinders, both headers get very hot after riding, just one more so than the other. The exhaust smells roughly the same between pipes, meaning im not spewing unburnt fuel. The exhaust note is also nearly identical from both sides, with the cooler cylinder probably being the louder of the 2. It also doesnt feel like im dragging a dead cylinder. I've had her cruising at 60-65mph a few times and theres still a decent amount of pull left. which I doubt would be possible on only one cylinder (top speed is reportedly 80mph new) I would check definitively, but it just started raining cats and dogs, so the bike's all covered up.

I definitely agree with the idea of pulling and cleaning the carbs though, never know what you may find in there. If weather allows, I should be able to pop them off tomorrow and give them a thorough cleaning/once over. When riding the bike, my butt seems to think that its running a little rich. Sometimes it will stumble/bog slightly when opening the throttle after cruising at a constant speed or after a decel where the slides are fully closed. Other than that though, she runs smooth and content, only other symptoms being the obvious temperature difference between pipes.
 
Its likely a lean condition as stated. A few things to double check:

Make sure the carbs are CLEAN and synched. Also make sure they're functioning properly and that the floats are set correctly.
Make sure you're getting proper fuel flow to both carbs. That means making sure the petcock is functioning correctly and your fuel lines aren't kinked or routed improperly, allowing air bubbles to form.
Adjust your valves.
Set timing.
 
Oh, and make sure that both exhausts are correctly set up. No holes or leaks. Also look at the airbox/filters. If the left has an air leak you need to fix it. Same goes for the carb boots.
 
Switch plugs and wires to determine it is no ignition problem. ( i assume its carb related, but just to be sure.)
 
The weathers been pretty dismal the past couple days, but I was able to do some fiddling today. I was basically able to determine that the hotter cylinder is indeed running on the lean side. Inspecting the right plug showed a very light sand color with a little bit of crusty grey/white while the left was a perfect mocha color. Ive definitely seen worse plugs, but it confirms the suspicions that one side was running lean. Also verified that she's firing on both cylinders as pulling either plug wire while she's running essentially shuts her off. I picked up some carb cleaner, so hopefully tomorrow i'll pull them both and see what the damages are. I'll be sure to keep you guys posted.
 
If you pulled one of the plug wires while its running and it shuts the bike down then it sounds like its a good time to clean up the carbs and get some new plugs.

Generally speaking It should be capable of idling on one cylinder.

Glad to hear you've decided to fix her up. Good luck.
 
She would keep running if I gave her more throttle, but she sputtered out without any intervention. Plugs actually looked to be in good condition. Carbs are coming off in a few minutes though, hopefully I find the culprit.
 
Pulled and cleaned the carbs. Fair amount of varnish and general gunk but no obvious clogs. Both carbs had 90 mains and 38 pilots with the needle in its lowest position. The stock jetting for the 20mm's was 98/38, so thats kind of interesting.

The cleaning had no measurable effect on the headers' temp differential. I even swapped the fuel lines' position at the petcock with no change. No visible leaks in the intake tract either. I'm pretty stumped as to whats causing this.
 
I'm not sure what youre getting at there, but no need to be condescending. My mixture screws are adjusted damn near identically left vs right and while syncing them I found that adjusting them in either direction makes the bike misfire/stumble if I go more than a half turn either way. If theres some sort of mixture screw trick im clueless about, please fill me in.
 
Not trying to be condescending, no need to get defensive. If you want to change the fuel/air mixture a little to richen one carb, there you go. Adjustment screws don't have to be turned out identically in terms of number of turns out (although they should be fairly close), but they need to be identical in terms of the fuel/air mixture. Reread post before I wrote that, and you hadn't mentioned adjusting that screw. Sometimes people overlook the most basic shit, and sometimes people don't read manuals, don't know how carbs work, don't know the parts in a carb, etc. I have no idea where you fit in that scheme, but if you don't want help, don't ask for it.
 
You may want to perform a compression test as well.
This should tell you if your valves or pistons rings are not sealing properly.
 
Wasn't intending to be too defensive, and I obviously do want help, but I am familiar with the basic functions of the various carb components. Valves/compression are my next things to check, but I am stuck at my parents house without my compression or feeler gauges, so i'm flying blind in that regard until I can return to my house.

One thing that I can do before I go home is adjust my needle clip positions. The hesitations I am experiencing only occur at partial throttle and the bike seems happy as clam when its at idle or over 60-70% throttle. With the left plug looking spot on/maybe slightly lean and the right side looking lean, I figure raising the needle/dropping the clip position on both sides couldn't hurt. I'd rather be slightly rich and load up/foul plugs occasionally than be slightly lean and be constantly trying to weld my piston to my cylinder. I also figure with my main jets being 90's and the stock main size being 98's, adding a little more fuel isnt a bad idea, especially considering im right at sea level.

I'm gonna raise the needles here in a little bit once the sun goes down some, hopefully i'll have some good news for you guys later. I figure that at the very least, it should tell me if I should order some larger mains to play with too.
 
So I dont know if I was drunk yesterday or just stupid, but the needles were in their middle position, not the lowest as stated before. I pulled the carbs, double checked their cleanliness and float height. Passages were clear and the floats were at the recommended 21mm. I then raised the needles 2 clicks and reinstalled everything. The bike ran ridiculously rich and I could never get it to idle properly. Plugs were a very sooty black.

Pulled the carbs again and then lowered the needles by one position. Very easy to set the idle at 1100rpms, fiddling with the mixture screws got it run smoothly. Exhaust smelled appropriate and pulling the plugs revealed a very nice dark brown, not black, but very brown. Plugs looked even and the temp of my headers seemed even as well, albeit both stayed much cooler than usual considering I was idling for the better part of 15 minutes. Deemed it worthy of a ride to see how everything felt.

It definitely feels better at lower throttle inputs, but the hesitations that I experienced before, remain. The last 1/2 mile of the ride to my parents house is a 50mph, stoplight free, flat section of road. 50mph is also where I typically experience the hesitations and burbles, so I got up to speed and then kept the throttle constant. I killed the engine turning in the driveway and coasted on in and pulled the plugs.

Both plugs showed symptoms of being lean, the "hot side"/right seemed slightly leaner than the left. The right side was also hotter than the left again. This leads me to believe its my mains causing all of this. Stock is size 98, mine are 90's. I live at sea level, but i'm thinking this bike didn't always. The small jets are 38's, which is the stock size, and she's really content to just idle all day. I guess my butt mislead me and I've been more in the throttle when she stumbles/sputters than I thought.

I'm gonna order some 92's, 95's and 98's and see how far they get me.
 
The local Honda dealership supposedly has 95 mains for $4 each. I figure for $8, its a good place to start. Hopefully i'll have some optimistic news by this afternoon.
 
Old jets are often damaged from a PO "cleaning" them with a wire or small pick and who knows what size they really are.

Pros recommend new jets every time as a starting point.
 
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