Boxerworks.com

Hello everyone,

I have a 1974 R90/6 and it's been acting odd. I need some help in problem solving.

I took a week off from riding to go on a trip. Came back and set off for a ride. While riding to my destination, the engine lost power and started to bog down. I pulled over to check things, and everything was fine, just wouldn't go when giving it gas. Got back on the highway and everything was fine. Apparently pulling over for a second helped. It did the same thing when I left to head home. I pulled over again, sat for a few minutes with the bike running, then everything was fine, no bogging down.

A few days later I ride again and everything was perfectly fine. The very next day the bike would start but not hold its' idle. Even with the choke fully on. It was making a bit more noise as well and vibrating badly. No engine lights came on or anything. When the motor is running, the right hand side (as you're sitting on the bike) gets hotter a lot quicker than the other side. Not sure what the issue is. Possibly not getting fuel due to dirty carbs? Bigger problems inside the cylinder/head?


Here's a link to some video i shot of the issue...

https://www.youtube.com/watch?v=guW5dJ8j5qE


Any help would be appreciated!

Reading the first paragraph, temporary fuel starvation comes to mind on account of some jet or the float valve. Maybe the floatbowl setting too low which only shows on the highway? Simple non-scientific test: open both petcocks for a minute. Close them. Remove the flowbowls carefully and observe how much gas in there. The bowl should be about 3/4 full and similar levels on both side. It is possible that one side will be half empty or even less ... .

The problem described in the 2nd paragraph: from the Youtube file it sounds like running on one cylinder, the hot and cold exhaust are symptoms for that. Your carb problem from above could be the issue.

Your guess: possibly not getting fuel due to dirty carbs? You got it!

It's most probably nothing worse than that.

When were the carbs last disassembled soaked in some laqueur thinner or carb cleaner? Maybe it's time. Replace gaskets, o-rings and the lot at the same time and they're good again for 10yrs.


First post ... welcome to the asylum!

Be sure the valve clearances are properly set. A bike that won't idle well is a symptom of valves closed up.

Completely holed carb diaphagm? Remove the air horns and watch what happens to the slides from the rear. Maybe one of them is not rising with the demand of the throttle.

If you suspect electical issues, start swapping components side-to-side and see if the problem switches sides.

Kurt in S.A.

One possible problem that has has been addressed by the fuel flow test, but not specifically mentioned is a possible clogged vent in the gas cap. The test is to open it up when the problem occurs, and if the bike suddenly starts running right you've found the culprit.

Once on my R100/7 a problem would develop when the engine would stop running right and threaten to stop. I chased my tail trying some of the above tests, but the culprit wouldn't be found. Eventually I pulled both carbs and cleaned them up, looking very carefully for any debris. I didn't find any, so the only thing to do was to put things back to gather and see if I had fixed the problem. I had, as it turns out, but it took me a while to trust the bike any further from home than walking distance.

There is a possibility that your problem may not be carb or fuel related. Your /6 has a starter relay under the tank on the L/H side of the top frame tube. Especially if you've had brake fluid leakage issues there is a tendency for the starter relay pins to have bad connections with the mating relay connector. The red +12V wire coming from the starter motor cavity has to go through the relay before it ends up terminated at the ignition switch. When the connection at the relay socket goes bad the bike will quit running and all indicator lights will not light. The fix is to thoroughly clean the contacts both in the socket and on the relay. That is easier said than done.

I wouldn't be worried about head issues.


Ken

Thanks everyone for the replies and welcoming me to the forums. Sorry for the delay in getting back. I'm an architect student with little free time. As of tonight, the bike is back running!

I checked the levels in the bowl and the fuel level was fine. The float bowls were super clean as well. I didn't want to tear into the carburetor as this is my first carbureted bike. I've owned the R90 since this past November, and the PO had meticulous service records for the bike. I looked at the paper work and the carbs were last looked over, serviced and adjusted in late October, so I figured they would still be good to go.

I buttoned everything back up and started her up. She ran smooth again, although the choke wasn't bringing the idle up, but the bike ran with the choke off and I manually gave it gas to raise rpm's. went for a ride around the neighborhood and everything seemed fine. There was quite a bit of smoke coming out when first running but I'm assuming that's normal for an old bike that's been sitting for 2 weeks.

After my first highway hiccups, I looked up the issue and came across the gas tank breathing issue. The article mentioned it can happen after filling up (which I had done) This time though the bike was way worse and vibrated badly. I'm guessing by taking the fuel line off, it allowed the tank to breathe and that solved the problem. I still plan to check valve clearances as just precaution. And will stay close to home on rides until feeling more confident and keeping an eye on things.

Is there a modification that can be done or is typically done to allow the tank to breathe better?

There's probably a technique for cleaning the fuel tank's cap's vent mechanism, but I'd be pretty doubtful that it will solve your one-sided running problem. I bet on electrical/ignition issues, most probably a failing resistor in an aged spark plug lead. A fresh pair of plug wires and caps might explain and solve all of the intermittent problems/symptoms you've described.

I am inclined towards the ignition side of things too. Whenever my bike has 'bogged down' on the road it has always been due to dirty points. I can quickly file the points and adjust the gap and get going again.

The gas venting issue is easy enough to check, by turning off the fuel tap when it bogs down and checking fuel levels in the carb bowls.

If your bike has the stock points ignition you will need to familiarise yourself with maintenance of same. With a new-to-you bike, fitting new plug leads and caps is a good place to start, as Khittner1 states. I would also get a new set of points and condenser, a timing light and feeler gauges. Checking, cleaning and adjusting points gap and ignition timing is a normal part of the maintenance cycle. I do it each time I adjust the valve clearances every 5000 kms or so, at oil change intervals. There are numerous articles available on the net, and it is good to have the Clymer manual by your side.

If your bike still has the original coils, they can go bad and give trouble too. The spark will tend to be weak and yellow rather than fat and bluish. If this is the case, then any other weakness in the system will be exacerbated. As a time-poor and probably dollar-poor student it will be a burden to replace the whole lot. I would start with checking the points. Give us a shout if you need more info about how to do this - we don't know how familiar you are with tools and where things are on the bike.

ps: Be sure to remove the battery negative cable before opening the front timing cover, to avoid frying the diode board.

It is not proper procedure to file points. They should last 20-25 k miles on the /5 and later. If not, then do a night check for amount of arcing across the points at night. Maybe the condenser is the wrong value.

Uh oh: Duane said "condenser."