1971 honda downhill commuting question

Hey everyone, I have an old bike riding technique question for you. I recently picked up a 1971 Honda SL350. I love the bike, came from the factory super stripped down and light so it's a blast in the city and I can't wait to do some moderate off-road riding in the future. Really the perfect bike for me as a guy getting back into it after a long time not riding.

When I got it I had it fully tuned up, carbs rebuilt, all the chassis stuff inspected - everything short of pulling the engine. It's been running well enough for an old bike, it has its quirks which I'm learning and staying attuned to so it gets treated the way it prefers. I've been able to start commuting to work which is a mix of city and suburban riding for about 12 miles. I've had the same issue a couple times around the same point in my commute home. There's an extended decline for 2 or 3 miles before getting back into city limits (Portland, OR) at about 35-40mph, so when I sit through a couple stop lights after cruising down the hill the bike stalls out. It's a kick start only, so it takes about 10-15 minutes of waiting and periodic kicking to finally get it going again, then it runs pretty strong again the rest of the way. It hasn't been too hot in the afternoon lately, maybe around 75, but that's about 10-15 degrees warmer than the morning when I have no issues getting to work whatsoever.

I'm under the impression that the top end needs a rebuild which I'll do when the rain starts this winter, but I'm wondering if there's something in my downhill riding technique I can do to keep it running until then. I usually keep it in a lower gear and give it some throttle, keeping the RPM's around 5500-6000. Should I be off the the throttle completely and in a higher gear at lower RPM's to maintain speed but not go out of control? Any other ideas what may be causing the bike to stall out after a downhill ride? I'm not doing any coasting with the clutch disengaged and the brakes on - that's the dangerous way to do this, right?

It does pop a bit when it's warming up so I think some fluids are just generally getting into places they shouldn't be, hence the rebuild, but based on it's stronger running moments I do think there's some things I can do with my technique to keep it going. Anyone have any thoughts?

you could have a float needle valve problem that is aggravated by a blocked vent in the gas cap

i would try this next time its hot and you are headed down the hill

loosen the gas cap make sure you dont slosh fuel out

but crack it open

see if this makes a difference if it does the vent in the cap is plugged and it is over pressurizing the tankl and forcing fuel

to the carb andpast the needle and seat flooding the motor

the other thing that could be a issue is one of the float levels is to high get them checked

Cool! I'll try popping open the cap on my way down. I've tried popping it open after it has already stalled out to see if I can get it started easier, but not while riding.

I can check the floats again but they've been adjusted a couple times and I've still had this somewhat consistent stalling issue.

Appreciate it!

Grab a manual and do the full tune up procedure and clean your carbs. I'd bet you're good to go after that.

what you could do is carry a plug wrench and teh next time it craps out on you see if its flooded

and when you have a plug out check for spark it may not be fuel related

but before pulling plugs ect make sure you are off the road and safe

So I took the same route home to recreate the problem and see what's going on. The bike stalled around the same place it usually does. I checked the plugs and one of them was definitely wet. For the first time I noticed some weeping on the cylinder head so I'm sure that can't be good. The bike rode pretty rough the rest of the way home. I'm assuming again that a top end rebuild is in order. Is that wrong? I'm pretty handy but this would be my first time giving this a shot. I have the Clymer manual for this bike, is it doable?

i would still try to actually diagnose the problem and i am not sure you have

it would be a bitch to strip it all put it back together and still have a undiscovered problem

question when you saw the plug was wet did you check that cylinder for spark?

Have you tried switching to reserve? When I go down a long steep hill my bike runs out of gas because it all goes to the front of the tank. My reserve tube is kinda long.


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Flugtechnik said:

My reserve tube is kinda long.

Click to expand...

I bet you say that to all the girls.

cxman said:

question when you saw the plug was wet did you check that cylinder for spark?

Click to expand...

I had a hard time checking as it's a kickstart and I was by myself on the side of the road. As I rode home from there it felt bogged down pretty bad in first gear so it could've been running on the one cylinder. So let's say there wasn't a solid spark in the left cylinder - what are my options then?

Could the coil be overheating? Would that cut the spark? And then the cylinder floods? I may sound totally ignorant right now...

next time it happens simply pull that plug wire off once you re start running

if the running of the bike does not change then the cylinder is dead

check the points and timing coil resistance charging rate and voltage

I the engine still pulling on this hill? or are you using the back pressure of the engine to slow you down? Running extended long sessions on back pressure with poor spark will definitely flood you engine in in itself.

I would pull the clutch at each 1/4 of the hill, coast and give the engine some revs. That will clean out the unburnt gas and clean your plug as well.

Maybe you have to do this every couple of blocks as long as you dot run out of control. While some of the other comments say do the tune up, I understand you want to get some riding in before we go inside for 5 or 6 months.

I could be crazy, but it does seem pretty damn consistent with riding down this hill. A couple strong revs with the clutch disengaged is good advice to burn the gas off.

This bike has had a neutral light on the fritz as well as a bulb out in the tach gauge since I got it. Is it possible the eletrics are grounding in the wrong places and screwing up the spark? Everyone I talk to tells me it's a fuel issue so I've been fussing with the carbs this whole time - never really thought the electric could be part of the problem...

this is why you need to stop guessing and start checking and measuring with a multi meter

figure out what is going on

start with charging voltage and static battery voltage low voltage=low spark

check coil resistance and check coils for cracks

check your points 12-16 thou wider is better not burnt or pitted

and static timing

Thanks, cx. I'll definitely do more testing.

Thanks everyone!

Hey guys. For anyone interested (or more likely just bored and blankly scanning the forums), I think I found the problem. Several weeks ago after going through the carbs a few times and feeling a bit overwhelmed, I took the bike into a shop that assured me they could work on a bike this old. They rebuilt the carburetors for me as well as went through most of the other standard tune-up stuff. I assumed their work was solid so I didn't do any checking up on what they'd done until I started having this downhill issue. After going through all the electrical as I mentioned I would and finding no issues, I took apart the carburetors again. The shop had never set the floats to the proper level .These slide type carburetors want to be at 25mm but they had left them at something like 22 or 23. So if I'm correct on this, I think I've been leaving a trail of gas all around town through the overflow tube in the bowls, which is no problem going uphill on the way to work. But going downhill on the way home, instead of through the overflow tube, gravity was pulling the gas into the cylinders and flooding them.

I'd love your thoughts, does that sound plausible to anyone else???

I haven't ridden the hill again yet so I'm not totally positive, but this is my guess at the moment. I have it running like a top in my garage right now which is saying a lot considering the state it was since I last posted, so I'm gonna take it out tomorrow on my normal commute for a test to see what happens. Wish me luck.

Were you overflowing before? And after setting float height are you no longer overflowing? I think while that sounds plausible it is unlikely the problem. I tend to agree with everyone else that you may just need to give it some gas to burn off some fuel and keep your plugs from fouling. Engines aren't designed to run for long periods in that manner

Let us know how your next run goes. Try it once with some throttle and once without. I'm curious myself