1976 Running Rich

[Wilsonsk]

I put the 120s in this morning, the mid stumble is gone. Seems to run great, it’s a little cantankerous on startup, it wants choke longer and spits and sputtters, that lasts a minute or two, then it will idle on choke, then I can take it off choke, once fully warm, runs great, I have the idle around 1,000, idles super smooth. I’ll ride it down to Dennis Kirk at lunch, that’s 11 miles one way.

 

[Wilsonsk]

Seems to be good, the trip down to Rush City went good, it's very happy around 65 mph, need to get my 17 tooth drive sprocket back in, thought I needed the low speed gearing for cow paths and fire roads, it has plenty of torque. I'll run it like this for a while and check the plugs.

 

[Wilsonsk]

Runs great hot, starts hard now though. I need to nurse it for a couple minutes til it warms up, then it idles fine. Until then it burbles and sputters, needs choke a lot longer than it used to. Before it would fire immediately with the choke on and I could turn the choke off 10-15 seconds after that and it would idle smooth the whole time. Since everything else has wound up being basically stock, should I go back to those 25 pilot jets?

 

[Wilsonsk]

I've been riding it some more. I tried an experiment, I pulled out the sink strainers and confirmed 2 things:

1. It is still too loud without them.
2. With the current jetting, it COMPLETELY eliminates the midrange stumble.

I was about to order the Commando mufllers and happened to look at my rusted out stock exhaust, it has a crossover pipe, there is an old Triumph in the lobby at Dennis Kirk, it has a crossover pipe, my CM 400 still has the chamber under the engine that is common to both cylinders. This got me thinking, putting a crossover before my mufflers should reduce backpressure/increase flow by sharing both mufflers for both cylinders. I wouldn't think it would halve the pressure/double the flow due to the twist and turns it makes to get out of the mufflers, but would that improve flow by say, 40-50%?

Just a thought....

 

[Wilsonsk]

I put the 25 pilot jets back in, runs good hot, it was already partially warm when I put them in so I am not sure if it helps the cold start or not. We'll see

 

[5twins]

Many times, a larger pilot makes cold starting easier and lessens the required choke time. If your old 25's do that, it's probably a good indication that they are eroded away and now larger than the new 27.5's you tried. Then again, with those new 27.5's being aftermarket jets, it's possible they're not sized right.

 

[Wilsonsk]

Many times, a larger pilot makes cold starting easier and lessens the required choke time. If your old 25's do that, it's probably a good indication that they are eroded away and now larger than the new 27.5's you tried. Then again, with those new 27.5's being aftermarket jets, it's possible they're not sized right.

It's a little weird, the hard cold starts started with the new needle jets but I also changed to the 27.5 pilots, seems like its lean because it wants more choke than normal, but even with the choke on it doesn't run great, a fair amount of popping (probably also an indication of a lean condition), requires nursing to keep it running until warm. The issue seems to all be at idle or just off idle, if I nurse it off of idle, it runs great and then it's warm and idles great too. Maybe the old 25s are the same as the 27.5s? We'll see if the cold start improves. Seems odd that I would need fatter pilots with more restrictive exhaust. Maybe drop the needle 1 notch? (it's in the middle now) Does it do anything at idle?

Just went out and tried a cold start, it still runs rough cold at idle, can't really decide what it wants, won't run without nursing the throttle choke on or choke off, I find myself going back and forth. The popping sometimes sounds like gas burning in the exhaust.

 

[5twins]

Changing the needle doesn't really affect the idle but it will have an effect just off idle, on the idle to midrange circuit transition area. That's why you often need a larger pilot if you lean the needle a step. It leans the idle to midrange transition area and can create a flat spot.

 

[Wilsonsk]

Acetone should get it out. Was there possibly some type of liner in that tank?

Changing the needle doesn't really affect the idle but it will have an effect just off idle, on the idle to midrange circuit transition area. That's why you often need a larger pilot if you lean the needle a step. It leans the idle to midrange transition area and can create a flat spot.

Once it’s hot now, everything is working really well, idles at 1,000 rpms, snappy throttle off of idle once it’s hot. The midrange stumble is gone. I started it up an hour ago or so, the best way I’ve found to keep it running cold is to start it on choke, then turn the choke off and on, it’ll start to die and I’ll change the choke off or on, it’ll pick up. Nursing it with the throttle just results in pops and bangs. Once warm, pretty much perfect. It used to start so good, just a little choke, then idling with no choke. I could walk away from it. Could I have something going on with the choke circuit(s)? I haven’t had any of that apart since it was running good, but the weirdness is all from cold through warming up, then fine. Given the two scenarios, I’ll take this one over perfect idling and the midrange sputters. It would be nice to get this warm up smoothed out though.

 

[5twins]

I'm guessing it won't idle cold because your hot idle speed is set too low. Remember, the idle spec for these is around 1200 RPM, and they like it, and I feel they need it. Besides overcoming the random stalling at stops, this higher idle speed will also allow the bike to idle cold. It'll be slower of course, but fast enough so it won't stall.

 

[Wilsonsk]

I'll bump up the idle. I left the throttle alone this morning, choked it, it fired right up and idled for a few seconds, sounded like it was going to die, I turned the choke off, it picked up for a few seconds, I had to do that 3 or 4 times to keep it running then it would idle. I did check and make sure that my choke plungers were both moving up and down and seating all of the way, at least visually they are.

 

[Wilsonsk]

Any thoughts on adding an H-pipe? Seems like it should reduce the back pressure, may even quiet it down without the sink strainers?

 

[5twins]

Honestly, a small muffler can't be made quiet without literally plugging it up. It doesn't have the volume to "muffle" very well. I told you earlier what you need to do - get 1.5" aftermarket headpipes and the Commando mufflers. Both are on sale now so you can get the entire system for like $220. That's a real bargain for a complete exhaust system.

 

[Wilsonsk]

Honestly, a small muffler can't be made quiet without literally plugging it up. It doesn't have the volume to "muffle" very well. I told you earlier what you need to do - get 1.5" aftermarket headpipes and the Commando mufflers. Both are on sale now so you can get the entire system for like $220. That's a real bargain for a complete exhaust system.

That’s true, you did tell me that, I was just curious if there was any benefit to a crossover pipe. I got the order in, they were out of gaskets,had to order those from Mikes, I’ll try the shorter mufflers on my CM400, they have more volume than the shorties I have on there now, it’s not very loud with that chamber that’s the size of a car muffler under the motor.

 

[Wilsonsk]

I suppose when I get my pipes and mufflers on I’ll be changing jets again?

I’ve done 2 or 3 more cold starts, something seems wonky with the choke, choke on, first start of the day, left cylinder never misses a beat, right cylinder picks up and drops out until it’s warmed up, then all is great. I’ve had the bowls off a couple of times, can the gaskets cover any ports or anything like that when reassembled? I can’t see real well when I put them back on but they pop right back in place. I rebuilt the carbs about a month ago.

 

[5twins]

Well, the gaskets do have punch-outs that sometimes don't get removed, so you may want to give them a good look. Here's the various punch-outs. The one on the lower left is part of the choke circuit .....

Yes, with the new exhaust you will need to jet up again. With the new (and now good) needle jets, you can most likely follow the original jetting suggestions I gave you way back at the beginning of all this (27.5 pilot, 130 or 132.5 main, needles leaned a step). This should work well if you have pods too. If you still have the airbox, you may not need mains quite so big.

 

[Wilsonsk]

Well, the gaskets do have punch-outs that sometimes don't get removed, so you may want to give them a good look. Here's the various punch-outs. The one on the lower left is part of the choke circuit .....

View attachment 248917

Yes, with the new exhaust you will need to jet up again. With the new (and now good) needle jets, you can most likely follow the original jetting suggestions I gave you way back at the beginning of all this (27.5 pilot, 130 or 132.5 main, needles leaned a step). This should work well if you have pods too. If you still have the airbox, you may not need mains quite so big.

If I’m going to go to pods, I guess now is the time. I believe you have recommended UP4200, is this the one?

 

[5twins]

Yes, that's the one. The 2" I.D. mounting flange fits the BS38's like a glove .....

And there's no need for the special foam filter oil, in fact it's too thick and can clog them. Instead, just use a simple 50-50 mix of gas and motor oil. I bought a bottle of Walmart straight 30wt. specifically for this .....

 

[Wilsonsk]

Yes, that's the one. The 2" I.D. mounting flange fits the BS38's like a glove .....


View attachment 248953

And there's no need for the special foam filter oil, in fact it's too thick and can clog them. Instead, just use a simple 50-50 mix of gas and motor oil. I bought a bottle of Walmart straight 30wt. specifically for this .....

View attachment 248954

How often do you oil them?

Do I need to put a crankcase vent filter on the vent tubes? My Sportster always burped a little oil and the filter would catch a little and it would run back in when it was not running. Looks like yours just comes out the fitting that went in the air box? I have two on my 76 but I also have a 1981 parts motor and have the single from that.

 

[5twins]

How often you need to clean them and re-oil will depend how dusty it is in your area. It's not very dusty where I live so I only clean them maybe every 4 or 5 years. For the breather hoses, we use a $5 power brake check valve from the auto parts store. It fits perfectly in the end of the stock hose and being a 90° configuration, directs any oil or oil mist that may come out back through the pod filter .....

That later single output breather is a nice little upgrade. I would swap it out for your current double output one. Then you'll only need one check valve .....

If you swap the breather, swap the whole thing, guts and all (internal baffle plate). You would probably have to anyway as I don't think the baffle plates are the same.