Runs fine for 10 miles... and then dies

[Crackers61]

Hi Guys,

I had a similar problem years ago with a V8 engine, turns out it was the condenser failing from the heat. I could drive for about 10 minutes, then the engine would fail. Turns out that a $2 part stopped my car.

Try changing your condensers.

Crackers

 

[sixfivezip]

My advice would be to solve the known issues first .

You say that the engine is over-revving and not returning to idle for up to 10 seconds or more. That alone could be the issue right there!. With over-revving and weak mixture the engine has to be overheating to some extent and could cause a range of problems including partial seizing. If you continue to run a 'tight' engine with a weak mixture and high idle you're end up with seized pistons for sure .

Anyway you cannot hope to test the bike properly if your idle is up near 1500+rpm every time you rev the engine.

Use a can of contact cleaner or carb cleaner etc to check carefully around the inlets. Any increase in revs would indicate a significant air leak.(If you cannot reduce the idle at all, this will probably not work.) If it were me I'd just remove the carbs thoroughly clean and adjust them and refit with new gaskets with 3 Bond and apply Hi-Temp silicon to the carb stubs to eliminate any possibility of air leaks.

I'm assuming that with new carbs the bike will now have aftermarket pod type filters ? if so you may need to increase the fuel flow at idle using the pilot (air) screw. If you run out of adjustment you'll need to go for a larger size pilot jets . The "air screw" meters the amount of AIR in the idle mixture. You'll need to find a Keihin tuning guide to download. I believe turning the Air screw OUT (Anti-clockwise) will weaken the mixture and turning the screw IN ,will enrich the mixture.

A good starting point is to set both air screws 1.5 turns out from lightly seated and then adjust from there for best idle and throttle response.

Once you have the idle speed sorted you can test the bike properly knowing that over-heating and air leaks are not part of the problem.

Thanks Peanut!

You make a good point. I got my hands on some BS 38's and from what I read they are better carbs and easier to dial in. I'll (try to) rebuild those and properly dial them in. The other plus to using the 38's is that there is a ton of tuning and repair info for them. the "Keihin" carbs from Mikes are harder to find info for.

I did spray some carb cleaner about the boots, tons of air leaks. I'll order the rebuild kit and some other odds and ends.

Again, thanks for the advice.

-Jake

 

[5twins]

Don't get rebuild kits. Half the parts in them are either wrong or you don't need. After teardown, just buy any individual parts you may need, for instance, a float bowl gasket. But if you're careful, you can usually re-use most of the gaskets. Don't pull the choke bodies off, just pull the plungers out of them. What you will probably need and want to replace are the needle jet o-rings. They're pretty much always toast. The size is 1mm thick x 4mm I.D.

Yes, those PWKs from Mike's are junk. Replacement is your best option. They were a nice concept with very poor execution (pretty much zero quality control).

 

[sixfivezip]

Thanks for advice 5twins. Good to know the rebuild kits are worthless. I agree, these PWKs are crap.

 

[grizld1]

We don't disagree too often, 5twins, but IMO the bored-out PWK28 knockoffs are anything but a nice idea! The reason for boring out a 28mm. carb to 32mm. is this. The nearest PWK's to our venturi requirements are the 33 and the 35. Since PWK's are dedicated 2-stroke carbs, those sizes are available only with needle jets you could drive a truck through. The only way to achieve anything close to a viably lean NJ for a 4-stroke street application (and for those of you who don't know, that's the heart of your carburetor) was to punch out the 28's, which still results in a fat NJ. It was a lousy concept from the git-go, and cheesy manufacturing just made it worse. BTW, Mike's XS and Heiden Tuning were not the first to market that junk to the unsuspecting. JRC Engineering had been selling the bored-out PWK knockoffs to Brit bike owners for years, with similar results.

Many thanks to Mrriggs, who painstakingly documented the costly hoops you'd have to jump through to get that junk to work, instead of tossing it in the recycle bin.

 

[XSLeo]

If you want Keihin carbs the ones off a Kawasaki EX500 Ninja work well with not much tuning. They can be bought relatively cheaply. Follow the thread on the EX500 carbs.
A few bucks for some jets and needles. And you get a nice running set of carbs that are much better than either the BS38's or BS 34's. I've ran both on my 75 and the EX500 carbs are the best of the bunch.
Leo

 

[sixfivezip]

I'll keep an eye out for those EX 500carbs, thanks for the advice. Good to know they can be made to work well. I suppose the PWK's from Mikes could also be made to work well with enough time and tinkering, but the materials used to build these carbs is cheap.

I already have the BS38s torn apart and some new parts on the way so sticking with the 38s for now. Once I get them put back together and dialed in I'll repost. Hopefully fixing the carbs solves my problem.

Also tested the rotor while hot, 8.4 ohms. The battery keeps charged though. Could increased from the rotor at operating temperature be causing weak spark despite the fact that battery is at +12 V?

 

[XSLeo]

I have a rotor that tests ok cold but climbs to around 8.5 hot. It still charged but not very well. Battery voltage dropped slowly so it took longer to discharge the battery. It did drop enough after awhile so it ran crappy from low voltage.
I have installed a voltmeter on the bike.
It tells me just what the charging system is doing at all times. This is what indicated poor charging. Once I knew there was a problem I did the testing to figure it out.
If your charging system is working well it should keep the battery voltage about 13+ volts.
If your only maintaining 12+ volts your system isn't working well.
I would replace or rewind that rotor.
Leo

 

[XSLeo]

Your Bikes a 79? If so it has points. They are not as voltage sensitive as the later TCI.
On the stock points ignition they used two coils rated for 10k volts at 4000 rpms. This rating is with a fully charged battery and fully functioning charging system. This is about 13.5 volts.
That was when new. After 35 years they don't put out as much.
A good spark plug take around 8k volts to fire.
A 12 volt input is about 13% less, so the spark out put will be about 13% less or about 8700 volts. Just over the voltage needed on fresh plugs. Dirty plugs need much more.
You can see how important proper voltage can be.
One thin g you can try is fully charging the battery. A 1.5 amp charge rate is just right, NO car chargers. They destroy motorcycle batteries.
Now unplug the regulator. This stops the charging system from working and stops the charging system from drawing the battery down. Ride the bike, does it stay running longer than 10 minutes.
If so this help prove the charging system isn't charging as it should.
Leo

 

[sixfivezip]

Thanks XSLeo! The tip about unplugging the regulator is just what I needed to help pinpoint the problem. Bike is a 79 and has points.

Going to wait until I get the BS38s dialed in then I'll give running without the regulator a shot.

I may try riding until the problem occurs then unplugging the regulator. If problem persists at least I know my problem is outside the charging system. One thing I did notice while tinkering with the electrical system this last time is that after riding the fuses get hot. Is that normal?


Jake

 

[XSLeo]

No, If it still has the stock fuse holder I would change it out for an inline blade type fuse holder.
Just a sec, some bikes in your year range used just one fuse, the standards. The Specials used four fuses. Not exactly sure what years got what.
One the single fuse do as I mentioned above. On the 4 fuses bikes they all need changing.
The stock round glass fuse holders were crap when new and 35 years didn't improve them any.
As they age the current flowing through them heats the hold clips. This weakens the clips, which creates more heat, a vicious circle that eventually fail. The excess heat you feel is the sign yours are very close to failure.
I like the ones Radio Shack Sells. cheaper than the auto parts stores and have a cover, not totally weatherproof but keeps out the big stuff.
If you wait to do the unplug the reg test after the battery gets to low to run right, it won't do any thing, the batteries already dead.
Do the test as I talked about. Fully charge the battery then unplug the reg and ride it. It should run ok for an hour or more if it's a low voltage problem.
Leo

 

[sixfivezip]

4 fuses here, 79 special. On closer inspection it looks like one of the hold clips for the main fuse was broken off and the fuse is just jammed in there. Blade type fuses it is.

Will do the unplug the reg test as you suggested.

 

[XSLeo]

Fixing the fuses might improve the way it runs. A weak or bad connection at the fuses can cause enough voltage drop to effect the voltage at the coils.
A good first step.
On your fuses a weak connection can get worse after running awhile. The excess resistance on the fuse from weak connections causes excess heat build up, this creates more resistance. This keeps circling around till you get so much resistance the voltage drop is just to much.
Leo

 

[sixfivezip]

Still waiting on ordered carb parts. Will post an update when I've put on the bs38s and new fuse holders.

 

[sixfivezip]

Much thanks for all the advice!

Finally got BS38s on the bike replaced fuse holders. The problem is gone!