Another guy that needs carb help

[clintinga]

Hello all. I have posted on here a few times and have gotten some great help so far so I am hoping that is the case here. First off, I have a 75B with BS38's on it. I have been working on it slowly over the past few months and finally got it to run last week. It ran like crap, (static timing is set, cam chain tensioner adjusted, valve lash set, points good, battery good and have a good blue spark when checking plugs) and the PO said the carbs needed to be cleaned so I did just that. They received an ultra sonic bath then were cleaned again with aerosol carb cleaner and compressed air. I put them back on and the bike fired up and idled well. I took it for a spin around the neighborhood and it ran 10X better than before. When I got to the end of my neighboorhood I turned around, heading back to my house and went around a long sweeping right hand turn and the right cylinder died. When I got through the turn and got the bike upright the right cylinder starting working again. After I pulled in and turned off the bike I thought that it must be a fuel problem and remembered that in my haste to get the carbs back on the bike, I forgot the set the float heights. So, I pulled the carbs again, set the float heights and now I can't get the bike to start. It tries to start but..... Also, now when the I am trying to start the bike, fuel comes comes out of the overflow tube on the bottom of the bowl, when I let off the starter button, it stops. Vacuum? So, I took the bowl off the left carb and with the fuel line attached made sure the float wasn't sticking and the needle valve turned off the flow of gas. It all works properly but the "brass tubes" with fuel line connecting both carbs together that are just below where the fuel line from the petcocks connect, the one on the left carb pours out fuel when the petcock is on, regardless of float position. ( I am unsure if this is supposed to happen or not?)

So, any and all help would be appreciated. Sorry for the novel post. I just wanted to make sure all details were covered.

Cheers,

Clint

 

[grizld1]

First: reset your float levels to the max height for your carbs (25 mm. if memory serves, but check the Carb Guide) and use a caliper when you do it. The +/- 1 mm. specified is not a tolerance, it's an adjustment range; try for an accuracy of at least +/- .005". Examine the floats: one or both may have taken on fuel. Examine the float needles and seats. If there's any visible wear, replace.

 

[5twins]

Those brass nipples for the fuel crossover hose are a press fit into the carb body and should not leak. Maybe the left one has come loose. You may need to lightly tap it back in so it's tight again.

 

[clintinga]

Grizld1- thanks for the response. I took the float out and checked it and one side was taking on gas so that problem is fixed.

5twins- sorry I reread my post and it was confusing. The brass nipple didn't come out of the carb, the hose connecting the two came off and fuel was pouring out and I was wondering if that was normal.

Now that it runs again, I have to figure out why there is a miss in the left cylinder. above idle it runs but at idle the left cylinder misses. I pulled the cap off while it was idling to make sure what I thought I was hearing was correct (i was) and as I was moving the cap away from and towards the spark plug it would occasionally fire.

Clint

 

[scabber]

Did you balance the carbs I lost a vacuume cap off the rightside carb intake boot on mine witch caused the left side exhaust to backfire and miss acting like ignition problem. carb balance is so important on these bikes. I replaced the cap rebalanced the carbs runs like a top again.

 

[5twins]

If your caps and wires are originals, replace them. That's one of the first things I do to any old bike I get. If you can't get new stuff right away, shorten the wires 1/4" or so to reach fresh wire and re-install the caps. Fan the wire out in a radial pattern before screwing the cap on .....

 

[clintinga]

Scabber- it might be a vacuum leak but my bike, unfortunately, does have the the threaded bungs so I can balance the carbs.

 

[clintinga]

5twins- Thanks! I just walked out to the garage, unscrewed the cap and the wires were down in the wire so hopefully that is the problem.

Out of curiosity, if the spark plug wire is the problem, why would it not spark well at idle but would spark well above idle?

Clint

 

[scabber]

I'm not saying you have a vacuume leak just an example of what unbalanced carbs can do.the lost cap caused the right cylinder to run lean. Before I realised what the cause was on my 1980 the two carbs are linked by a common throtle shaft with an adjustment screw in the middle to balance right before I discovered the problem I monkeyed with the screw witch made things worse puting the cap on solved the vacuum leak and lean right carb. since the carbs were still unbalanced did not cure the backfire. I rebalanced and presto the bike runs great again. The 1975 carbs have two cables I believe and a differant balance procedure. The point is unbalanced carbs can fool you and make you think its ignition.

 

[5twins]

As the RPMs increase, the spark frequency does too. That more rapid sparking may be able to push through a bad connection easier.

Measure the cap resistance. The originals have a rather odd rating of like 9K ohms. They can and do go bad. When that happens, the resistance starts to increase. This will choke off the spark eventually and may be why it was firing better at higher RPMs. Most of us switch to NGK 5K ohm caps. Get the LB05F version .....

 

[clintinga]

Thanks 5twins! I stripped back a bit of sheathing, fanned out the wire as in your picture, put it all back together and she runs on two cylinders at idle now. Thanks you for your help. It seems that it is the simple things that elude me, but I always learn from them.

Clint

 

[scabber]

I've got to hand it to you 5twins that sparkplug wire did the trick That answer to to the Question why does miss at idle and not at higher RPM was pure genious. Spark Frequency Rapid Sparking I would have answered since the charging system puts out more volts the reason the headlight gets brighter the coil puts out a hotter spark in laymens terms. clint good job on those carbs they can be a probkem at times. My 30 year old plugwires were still pumping out voltage cut down twice.