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1976 XL350K2 - An Off and On Project

stl360+450

Veteran Member
Joined
Aug 6, 2021
Total Posts
2,876
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357
Location
St. Louis, Missouri, USA
I have a few maintenance tasks to do on my XL350 during some vacation time next week. I thought it might be helpful to post here in the Other Bikes forum to get some advice.

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To-do List:

  • Paint the rear fender to match the front fender.
  • Repair damaged mount for rear fender.
  • Paint right side engine cover.
  • Replace piston rings? Hone cylinder?
  • Eliminate rattle of homemade exhaust hanger. The downpipe header comes from XLint Performance (great resource for XL250/350s) and I have ordered adhesive silicone rubber strips to dampen the inside surfaces of the exhaust hanger. Unfortunately, they won't arrive until the end of March or early April. I might be interested in a quieter muffler, although I do like the sound of this one.

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I am seeing some smoke in the exhaust right after start-up and the bike bogs down if I try to take off under load without letting it warm up. Once it warms up, the bike feels fine and I see no smoke in the exhaust. The video below shows some smoke coming out of the exhaust when I blip the throttle right after start-up.


The next video shows that the exhaust does not contain smoke after the bike is warmed up. In this case, it idled for a few minutes and then I rode it around a city block in my neighborhood.


Once I recorded the videos, I shut the bike off and removed the seat and tank to make it easier to do a compression test. The compression test on the warm motor came in at slightly over 150psi. (The specification is 170psi.) Sometime soon I plan to do a cold compression test and check the valve clearances.

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Wow, I'd forgotten how low the redline is on that engine. It's possible the smoke is caused by excess richness from using the choke on a cold start, surplus fuel washes a little oil away. Hard to be sure of the compression reading being totally accurate too because of the variances in gauges and hose lengths (looks like a HF gauge, Pittsburgh name). If it was mine and it ran well, I'd leave it alone for now. Wait until it really needs the top end refreshed, they're simple enough to do when the time comes.
 
Thank you both for the responses. It will make my life easier to postpone the top end work. XLint performance actually has a 412cc kit that I've been considering, so if I wait to do the work, I may go that route.

It's possible the smoke is caused by excess richness from using the choke on a cold start, surplus fuel washes a little oil away.

I'm running a Mikuni VM32 on it and I do need to use the starter circuit to start it, but I can immediately close(?) the starter circuit once it fires up and it will idle fine. I have issues if I try to run it right away under load, however.

It is a HF gauge. I have an old Craftsman as well and can try it with that for comparison. The hose is about 16" long ish.

What little smoke I see looks to be valve seals leaking a little oil after shutdown. Compression isn't bad enough for rings yet, maybe this winter.

Interesting, I hadn't considered that possibility. Would this also explain the sluggishness before the motor has warmed up? If I try to take off cold, as I open the throttle it will start to rev up and then fall flat, sometimes leading to a stall. If I creep up the RPMs slowly it will eventually overcome that flat spot with a big puff of smoke and typically it runs fine afterwards.
 
How is your XL350K2 as far as starting;does bringing it up toward TDC using the compression release and then 'kick through' do ok ?

Edit:the K2 is the 'center port' head with the exhaust port more centered?
I look at that head-pipe,the way it's designed/exits and it makes it look like that's an early model engine.
 
It is a HF gauge. I have an old Craftsman as well and can try it with that for comparison. The hose is about 16" long ish.
With ~16" of hose, the reading WILL be at least 10 to 15 PSI below what it actually is inside the cylinder.....You are fine there.......

Can be corrected if you want to do the math.......
 
How is your XL350K2 as far as starting;does bringing it up toward TDC using the compression release and then 'kick through' do ok ?

There is no compression release on the XL350, although I believe some owners have added them. Starting the XL350 isn't too bad, but requires a bit more coordination than my other bikes, all of which have a center stand. I have even read that some owners end up bending their side stand due to the kick starting. I keep the bike upright and use the kick start to locate the beginning of the compression stroke and essentially fall on the kick start while balancing the bike momentarily. Works well enough.

With ~16" of hose, the reading WILL be at least 10 to 15 PSI below what it actually is inside the cylinder.....You are fine there.......

Can be corrected if you want to do the math.......

The cylinder volume works out to 348cc and the compression ratio is 8.3, so I figure the compression chamber has a volume of 348/7.3=47.67cc. (Solving 8.3x=348+x.) What I don't know is the volume of the hose, because I don't know the inner diameter. If I had two hoses of different lengths I think I could figure it out. However, as an example, if we assume the hose has a volume of 5cc, then the pressure reading could be corrected by multiplying by the ratio of volumes: 52.67/47.67=1.10 which would put my 150psi up around 165psi. If the hose volume was 3cc the ratio becomes 1.06 and the compression measurement would correct to 159psi. Do these seem like reasonable guesses for the hose volume?

In trying to research the hose volume, Google led me to a post on sohc where you are mentioned, Steve. Apparently you fashioned a short hose extension for just this reason?
 
Hard to say if a leaking valve seal would cause that type of problem, I've only seen excessive carbon build up one the valve stem from that.

Or 2 valve stems in this case since it's 4 valves per cylinder.

Yup. The spark plug has been in the bike for a while and although it works, it also appears to be carbon fouled. I know I had trouble with ethanol gunk plugging up the float needle at one point before I switched to pure gas, so I'll be curious to see if my start-up symptoms go away with a fresh plug.
 
Do these seem like reasonable guesses for the hose volume?

In trying to research the hose volume, Google led me to a post on sohc where you are mentioned, Steve. Apparently you fashioned a short hose extension for just this reason?

Yes..... Hose volume of one inch (25.4 mm) of 1/4" hose (ID=6.35mm) works out at : 3.14159 x 3.175 x 3.175 x 25.4 = 804.397 or .8 cc/inch.....
About 12.8 cc for ~16" of hose.....
SO....60.47/47.67 = 1.2685.... x 150 = 190.27 PSI .... SO, apparently you are severely carbon fouled or you have smaller ~3/32" ish ID hose........;)

And, Yeah, I'm on a lot of motorcycle groups/forums.....The shorter the hose, the more accurate.....(y)
 
Yes..... Hose volume of one inch (25.4 mm) of 1/4" hose (ID=6.35mm) works out at : 3.14159 x 3.175 x 3.175 x 25.4 = 804.397 or .8 cc/inch.....
About 12.8 cc for ~16" of hose.....
SO....60.47/47.67 = 1.2685.... x 150 = 190.27 PSI .... SO, apparently you are severely carbon fouled or you have smaller ~3/32" ish ID hose........;)

And, Yeah, I'm on a lot of motorcycle groups/forums.....The shorter the hose, the more accurate.....(y)

Thank you, Steve. The compression was comparable when I last had the top end apart with the same gauge/hose. It was not carbon fouled then, so I'm hoping it's just the hose volume that we haven't nailed down completely. I'm tempted to get a hose of another length, which would allow me to calibrate without knowing the ID, provided it's the same on each hose.
 
I have to admit that I've taken compression gauges for granted for a while. I didn't quite get why it would take a series of kicks to achieve the maximum pressure, since the air in the cylinder starts at the same pressure on each compression stroke. So, I texted my little brother (Ford tech) and, after a few exchanges, discovered what I was missing: there is a Schrader valve at the end of the hose nearest the engine. This valve is only open when the pressure in the cylinder exceeds the pressure in the hose. The first kick will cause the valve to open almost immediately and thus exposes the cylinder to the volume of the hose early on, resulting in a low compression reading. On each successive kick, the valve opens later, allowing the pressure in the cylinder to increase beyond the level of the last cycle. Ultimately, the pressure achieved in the cylinder will be more or less equal to the pressure within the hose and the valve may not open at all, which explains the point of diminishing returns while using a compression gauge.

So, while it is true that the spark plug displaces more of the volume within the combustion chamber than the fitting on the compression gauge, I do not believe that the length of the extension hose substantially decreases the pressure achieved within the hose. The 3-5cc range for the difference in volume caused by the sparkplug seems reasonable and likely does cause substantially lower compression readings, particularly on smaller cylinders, but I think the hose length is immaterial.
 
I The 3-5cc range for the difference in volume caused by the sparkplug seems reasonable and likely does cause substantially lower compression readings, particularly on smaller cylinders, but I think the hose length is immaterial.

The compressed dome volume of the smaller cylinders DOES show a greater effect....The dome volume of a CB/CL 175 for instance is approx 19.5 cc's....
Even a 5cc hose would show as a ~20 to 25% error, reading maybe`~130 to 136 ish PSI when the dome (without attachments) is actually compressing to 170 PSI....
 
The compressed dome volume of the smaller cylinders DOES show a greater effect....The dome volume of a CB/CL 175 for instance is approx 19.5 cc's....
Even a 5cc hose would show as a ~20 to 25% error, reading maybe`~130 to 136 ish PSI when the dome (without attachments) is actually compressing to 170 PSI....

Those numbers look right to me. It's too bad I didn't realize this when I measured 90psi on a friend's "gently used" CT70. Without checking the compression ratio, even a 3cc difference would put that measurement at a somewhat respectable 120psi. It ran fairly well, so he didn't really care about the number, anyways.
 
IIRC, 125 PSI is the "normal" compression on the smaller singles, so that 120 would be reasonably good......
 
I took two cold compression readings today, one with my HF gauge (165 psi) and the other with an old Craftsman gauge (154 psi) that my brother gave me. At this point, I don't plan on pulling the motor to do any top end work, but still have a couple of other tasks that I plan to tackle this week.

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You're fine, I'd leave it alone and ride it. The cold running issue aside, a small puff of smoke is the least of any worries. Was the replacement Mikuni the same size as the original carb? The issue with Mikunis from a cold start standpoint is the enrichener method they use, there's no "partly on" position like a real choke on the OEM Keihin and if you leave it full on the idle is raised a fair amount (horrible for a 450 before oil gets to the top end). When you get it running and turn off the enrichener, it's trying to run on a fuel/air mixture intended for a warm engine which would explain some of the sluggishness until it warms up. Oh, and you don't need to pull the engine to do the top end on the XL250 and 350.
 
Was the replacement Mikuni the same size as the original carb? The issue with Mikunis from a cold start standpoint is the enrichener method they use, there's no "partly on" position like a real choke on the OEM Keihin and if you leave it full on the idle is raised a fair amount (horrible for a 450 before oil gets to the top end).

Thanks. What you mention about idling cold on a mixture intended for a warm motor makes sense.

XLint has a kit for the XL350 based on the Mikuni VM34, but I already had a pair of VM32 carbs that I had used briefly on my 450, so I decided to use a VM32 instead. The OEM carb is a Keihin PW-32. I will report on my jetting and consider some small adjustments depending on how it runs with a fresh plug.

The FSM is not crystal clear about head removal in the frame, but described it after the removal of the motor. As I recall, I tried it once and could not clear the studs when I lifted the head. XLint has an article that suggests the same, in the specific case of the 1974 model. Perhaps I'm missing a trick here?
 
Thanks. What you mention about idling cold on a mixture intended for a warm motor makes sense.

XLint has a kit for the XL350 based on the Mikuni VM34, but I already had a pair of VM32 carbs that I had used briefly on my 450, so I decided to use a VM32 instead. The OEM carb is a Keihin PW-32. I will report on my jetting and consider some small adjustments depending on how it runs with a fresh plug.

The FSM is not crystal clear about head removal in the frame, but described it after the removal of the motor. As I recall, I tried it once and could not clear the studs when I lifted the head. XLint has an article that suggests the same, in the specific case of the 1974 model. Perhaps I'm missing a trick here?

Maybe not in the case of the 350, I've never actually torn one down but I do know the 250 was possible as I've done that for a big bore kit back in the early '70s for a friend who raced one in motocross. I wasn't sure of the carb size and wondered if the Mikuni was larger.
 
I finished painting my rear fender and right side cover and put everything back together with fresh oil and a new sparkplug. The smoke at start up was the same as it was with the old plug and still goes away as the bike warms up. Once warm, it runs and idles well.

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My can of Duplicolor silver metallic spray paint came with a cracked spray nozzle, which I discovered after instantly painting several of the fingers on my right hand.

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Current carburetor settings: Mikuni VM32, 230 main, 35 pilot, 6DH7 needle with clip in second slot from the bottom, 3.0 cutaway slide, and the air screw at about 1.75 turns out.

Finally, I include a picture of the old plug, which, despite heavy carbon deposits, was working fine.

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If a valve seal is leaking and allowing oil into the cylinder after shutdown, would I be able to see evidence of this by looking in the sparkplug hole? I have a cheapo Android boroscope, so I may have a look-see.
 
I'd say at least on the valve stem, and you might see it on one of the valves through the plug hole. Best way would probably be to look into the ports. The carb and pipe are pretty easy to remove on that bike. Did you get the exhaust clamp situation cleaned up? That pipe really reduces ground clearance, I wonder if you can find an upswept pipe for it.
 
I'd say at least on the valve stem, and you might see it on one of the valves through the plug hole. Best way would probably be to look into the ports. The carb and pipe are pretty easy to remove on that bike. Did you get the exhaust clamp situation cleaned up? That pipe really reduces ground clearance, I wonder if you can find an upswept pipe for it.

Thanks, Tom. I'll try to have a look at the intake/exhaust ports and the plug hole as my next step.

I ordered some adhesive rubber strips to try on the mount, but they haven't arrived. So, my first iteration of damping treatment amounted to several coats of VHT Epoxy Paint (rubbery). It led to some improvement.

This bike will probably not see anything more extreme than a farm trail in my hands, but it's a good point about the loss of ground clearance.
 
The color of the sparkplug looks to me like the carb jetting is a bit rich.

Thanks - I agree it's pretty dark. I'm not sure if that's coming from this oil burning that happens when I start it cold or the jetting. I'd really like to solve the oil burning issue, which would make it easier to assess the jetting. Interestingly, that sooty and dark plug was still having no problem firing.
 
Oil burning? all I saw in the video was a momentary pulse of half-burnt fuel/air mixture blow out the exhaust. That's to be expected with a cold engine, carburetors don't behave like a modern computer controlled fuel injection system. Before you start messing around with carb jetting, try one step warmer on the spark plug heat range.
 
Oil burning? all I saw in the video was a momentary pulse of half-burnt fuel/air mixture blow out the exhaust. That's to be expected with a cold engine, carburetors don't behave like a modern computer controlled fuel injection system. Before you start messing around with carb jetting, try one step warmer on the spark plug heat range.

I am open to suggestions, so thanks for the input. I will also record a longer video to hopefully give a better sense of what I believe is excessive smoke in the exhaust, my points of comparison being my CB360 and CB450 which do not smoke excessively even when cold.
 
If an engine is running rich it also washes a bit of oil out of the cylinder before firing so that could be contributing.
 
Well, I think I'll have to concede to paranoia about the smoke I'm seeing at startup. It's over 70 degrees here today and the bike still gave a [big] puff of smoke when I opened the throttle as it warmed up, but it warmed up much more quickly than it had in recent memory and subsequently responded to the throttle well under load. It has been cold here until very recently, so I think I was reading too much into the fact that it needed time to warm up.

I established the jetting about three years ago with a suggested starting place for the pilot (25) and main (210). I ended up with slightly larger jets in both cases (35 and 230) and I have now ordered a 30 pilot for testing. It's easy enough to swap them out. I don't have a lot of tuning experience, either, so I'm sure the jetting can be improved.

I also plan to pick up a D7EA plug to give that a go as well.

Thanks for the suggestions and the reality check.
 
Small update: I recently dropped down to a 220 main jet and went back up to a 35 pilot jet. The bike runs much better than it had. I may try dropping the main to a 210 at some point and will report back if I do.

I also still need to get and try a D7EA plug.
 
I realized a couple of months ago that my XL350 originally had a toolbox and then found a rusty specimen on eBay. I've removed most/all of the original paint, inside and out, so now I plan to repaint it. I'm wondering if anyone has any suggestions for painting the inside. I plan to use a rattle can for the outside, something like VHT black epoxy.

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Powder coating would probably get it done the best without runs. Nice bike by the way. I have a XL250K2 which is the sideport engine. It’s a lot of fun to ride. Would like to get a matching 350.
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Thanks, 2wheel. Yours is so clean! I am envious.

I hadn't thought about powder coating, thinking it might be overly expensive for such a small piece. Does powder coat do well with internal surfaces? Or does it depend on who's doing it?

(Anyone else having trouble getting reply with quote to work? I'm experiencing intermittent problems with it.)
 
(Anyone else having trouble getting reply with quote to work? I'm experiencing intermittent problems with it.)

I'm still a couple experiencing little things like the scroll bar not appearing on pages sometimes, but I've had no trouble with the reply with quote function. I suspect it's still remnants of our server migration and possibly some little stuff we might be stuck with until we upgrade software at some point.

As for your toolbox, I'd simply get the inside of it as clean as you can and just fog the interior with the same paint you're going to use on the outside, tip it up on end and let the excess drip out if any and let it dry. Then paint the outside afterward, since the appearance of the interior is far less important than rust prevention a few runs that might remain will be unnoticed by those who hunker down to look inside it.
 
I'm still a couple experiencing little things like the scroll bar not appearing on pages sometimes, but I've had no trouble with the reply with quote function. I suspect it's still remnants of our server migration and possibly some little stuff we might be stuck with until we upgrade software at some point.

As for your toolbox, I'd simply get the inside of it as clean as you can and just fog the interior with the same paint you're going to use on the outside, tip it up on end and let the excess drip out if any and let it dry. Then paint the outside afterward, since the appearance of the interior is far less important than rust prevention a few runs that might remain will be unnoticed by those who hunker down to look inside it.

Thanks, Tom. I think the inner fog approach may best fit the budget for this project. And, on cue, I can reply with quote again.
 
if it were mine I would follow AncientDads advice. Powder coat does work well but is more $$.


Edit; mine is only that clean because I was asked to show it in a vintage bike display at Gettysburg bike week last year. Lol.
 
The clutch mechanism on the XL250s and XL350s is a finicky contraption in my experience. The exploded view of the left cover is shown below to illustrate how it works.

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A few years ago, the magnesium case started to disintegrate around the clutch actuator (part #6), which presented itself (to me) as an improperly adjusted clutch cable. I tightened the cable in order that the clutch wouldn't grab and, in the process, caused more deterioration of the magnesium until it was no longer possible to get the clutch plates to disengage.


I learned about an XL specialty shop in Tennessee called XLint that offers a repair service for the clutch cover and that solved the root problem for me. (The owner has been very helpful to me, in general.)


However, even with the repaired case, I've always found this particular clutch mechanism difficult to adjust because the adjustment is made with a 3mm square tappet and locked with an 8mm nut. When I tighten the nut, the tappet position would be disturbed and I'd have to do it all over again, crossing my fingers and toes. Somehow, I never knew that there was a special tool for this purpose, but I just received one today and to borrow a phrase from our UK contingent, it works a treat. It could be a little longer, but was up to the task when I adjusted my clutch earlier this evening.

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Mounted the toolbox this week. One of the catches for the rubber lock strap was broken off and I thought for a while about how to secure the door. I originally was thinking about a pin of some kind, but this little keychain cord spoke to me at Home Depot this morning. It cannot reach far enough to touch the tire, despite maybe looking like it could in this photo.

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Creative thought. The only XL350 I ever saw up close was the one owned by the crazy kid who did odd jobs and ran WFO for parts for us when my cousin and I had our small Honda repair shop in the early '70s. For whatever reason I never paid attention to where the toolbox was located on that bike so at first I wondered if you put it there yourself. A quick trip to the fiche for it showed it to be the stock location, interesting
 
Creative thought. The only XL350 I ever saw up close was the one owned by the crazy kid who did odd jobs and ran WFO for parts for us when my cousin and I had our small Honda repair shop in the early '70s. For whatever reason I never paid attention to where the toolbox was located on that bike so at first I wondered if you put it there yourself. A quick trip to the fiche for it showed it to be the stock location, interesting

Don't ask what I paid for that rear bracket that slips over the rear loop in the frame. it was NOS and the only one I could find. I should've just fabricated one, but thought: who else is going to buy and use this part? So, I bought it and used it.
 
Don't ask what I paid for that rear bracket that slips over the rear loop in the frame. it was NOS and the only one I could find. I should've just fabricated one, but thought: who else is going to buy and use this part? So, I bought it and used it.

And like so many NOS items in short supply, I'm sure it wasn't cheap. That might have been one of the easier things to fabricate to save a little money, but convenience is valuable too... as for your toolbox door "latch", I might have considered some of the smaller, narrower battery hold-down straps out there but what you've done is certainly secure. And neither will keep anyone from pilfering anything that's in there when you're parked somewhere near anyone with ill intent.
 
I had hoped that the new tappet adjuster for the clutch adjustment screw would bring an end to a recurring shifting problem, but, unfortunately, that is not the case. I've performed the following procedure a few times in the last couple of weeks:


  • Relax clutch cable adjustments at the perch and on the case;
  • Loosen tappet screw nut;
  • Turn tappet screw in until resistance is felt;
  • Back tappet screw off 1/2 turn;
  • Tighten tappet screw nut.

Initially, everything is fine in that I can shift through all gears and when I reach stoplights I am able to disengage 1st/2nd and put the bike into neutral. If I take a longer ride, however, I will eventually lose the ability to disengage first gear while stopped (with the motor running). I can continue to ride and shift in motion without any trouble, but, once stopped, there is too much resistance to lift the shifter and disengage first gear, even if I roll the bike forwards or backwards. If I use the kill switch to stop the motor, I can instantly disengage first gear and put the bike in neutral with no resistance whatsoever. A couple of additional clues are that (1) when I initially shift into first after a cold start, the clutch will drag slightly and (2) on a recent ride I was also noticing clutch slippage under hard acceleration.

I'd like to understand what the symptoms above indicate and replace any components that could be responsible.


  • The combination of dragging and slipping at different times has me suspicious of both the steel plates and friction plates.
  • I hope that the shift mechanisms are eliminated given that it shifts fine with the motor off. Is this wishful thinking?
  • I once had a lifter rod break on my dohc 450, so I was suspicious of a bent/broken rod here, but that is now ruled out.

I have both crankcase covers off and disassembled the clutch components for inspection and I'm including the parts diagrams below for ease of reference. I plan to follow up with photos of the various components and inspection notes, but initial thoughts, suggestions, and advice will be most appreciated.


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