Driveability/Performance issues

[16VGTIDave]

I'm frustrated! I've been trying to sort out a couple of driveability issues with my '82 Maxim 400:

1) Abrupt throttle tip in. Especially noticeable when cruising along in 5th or trying to ride slowly in lower gears. I'm either accelerating or decelerating. Nothing in between. I THINK that this is related to pilot screw adjustment...

2) stumble / misfire / bucking between ~4000 - 7000 RPM with heavy or full throttle. I can hear the carbs roaring while the exhaust gets loud and spluttery (that is a technical term isn't it?). The bike runs strong above 7000 RPM to 10,000 RPM or smoothly at any RPM with light throttle. I can't reproduce the misfire in neutral, I must have a load on the engine for it to occur. For example, climbing a grade of any sort at 100 kph in 5th gear (~5200 RPM) results in loosing speed with the throttle held steady, as expected. If I open the throttle, very gently, I might be able to maintain speed. But if I try to accelerate, it starts to miss and gets worse the more I open the throttle. If I rev the engine out in lower gears it will pull through the misfire and I can maintain the higher RPM's with the engine running smoothly.

What I've done:
- Cleaned carbs (to get bike initially running)
- Replaced mufflers with universal fit ones without crossover pipe (factory were shot)
- run strong fuel system cleaner (no change)
- running Shell Gold gasoline (91 octane but with no alcohol)
- Ultrasonically cleaned carbs (no change, but I'm not positive I got it spotless)
- Adjusted pilot screws and synced carbs
- Tested needles in every clip position, returned to middle (3 of 5) slot
- Rebuilt sparkplug wires and cleaned all connections I encountered
- Exchanged new NGK DR8ES-L sparkplugs for used Champion RA6HC sparkplugs (THIS made a noticeable difference reducing the misfire / stumble and narrowing the RPM band affected)
- Cursed and muttered

I don't know if the stumble is ignition related, carburation, or a combination of both. I probably should check and adjust valve clearances as well. And I'm obviously frustrated by my inability to resolve this misfire.

Any thoughts or words of wisdom would be appreciated!

Thanks,
Dave

 

[Lou Ranger]

1) This engine does NOT like to run at less than 2,500, maybe 3,000. Just road vibration sometimes makes it hard to hold the throttle steady. I began pushing down on the "cruise" lever with my thumb to take these variations out of the equation. It is also more pronounced when the chain is too loose. Try the first, check the second.
Once I got used to the higher revs, I began trying to keep the revs over 3k even in town (but not over 4k - don't want to be one of "those" - do we?). That helps quite a bit too.

2) This has happened to me on occasion, but it has been barely noticeable when it has.
When it happens, it is usually after a period of time riding around town or cruising leisurely around - e.g. into town, through a couple of lights, pull into Tim's, 15 minutes later, put-put out into traffic, 2 lights, up the hill out of town at 50 kph. Then at the top of the hill, kick it down into 3rd and wind it right up to nearly 10k. At around 7k (I think) sometimes it will be uneven. I say uneven, because I can't tell if it's a slight miss or a slight surge, it's just uneven. It will do it again (but less) in 4th. If I were to slow right down and then do the run again right away, it will be do it noticeably less.
Sometimes it won't do it. We should keep a log of what gas is in the tank, what kind of riding immediately precedes it, and what the air temperature and humidity are.

I have concluded that it's overall state of tuning (for the 7k+ rpm range under WOT load) must be close to "the edge", because I swear that sometimes the only variable is the weather. Or, it's right after it has been run at low rpm for a long time. Reminds of the old days when we would take the old Mustang or Chevy out for a run to "blow out the carbon". Maybe that's all it is.

Yesterday it did it a bit while I had regular gas (and therefore ethanol) in the tank. I filled up with premium last night, and today it didn't do it. But today was also warmer and hazier than yesterday and I rode hard over to Tim's on the back roads - instead of the leisurely cruise on the straight road.

I have read a number of articles and bike reviews which talk about bikes in the early '80s being set up too lean from the factory (air pollution or mileage concerns (?), I forget). That would mean that it came jetted at the lean end of the ideal range. If that's the case, then the slightest additional restriction in a carb orifice would push it past the boundary between just okay and too lean, would it not?

 

[bentwrench]

I agree with Lou's #1 point - for your 1st point, that is.

What you are describing in #2 is the same issue I had as soon as I upgraded my exhaust to the 2-2 MAC that I'm running (you're probably running the same, as I don't think anyone else is manufacturing a replacement exhaust for our bikes.)

I had to go up several steps (like, 6) for my main jet in my carbs (but then, I also removed the intake.) You may want to consider shopping at Mike's XS for jets (they are cheap.) If I didn't have a newborn and a toddler, I would invite you over to Oakville for some tuning experimentation with my very large collection of DOHC main jet sizes...

To tune your carbs, you need to get that WOT between 4K and higher running strong all the way to redline before adjusting your needle. Mike's has a large selection of jets for about $7 a pair...

But then, you could always sweet talk me with the offer of beer and I'll let you experiment with my collection (I have every size larger than stock up to 137.5s... so yeah, a lot of jets.)

 

[Lou Ranger]

... I had to go up several steps (like, 6) for my main jet in my carbs (but then, I also removed the intake.) ... you could always sweet talk me with the offer of beer ...

When you say you had to go up, am I correct that the up-sizes are larger - i.e., they let more fuel into the carb at a given throttle setting?
If so, then the new MAC exhaust has a bit less back pressure resulting in the carbs sucking a bit more air at a given throttle setting?
Now my big question, if I have it right so far, then take my stock exhausts. As the years go by they would wear slightly - at least the inside baffles would. As they wore, wouldn't they gradually reduce back-pressure thus letting the carbs gradually take in more air?
Assuming so, then eventually (29 years?) the carbs would be making a mixture that is too lean, n'est-ce pas?
If so, do you think slightly bigger jets would bring my bike back into the middle of the good mixture range and eliminate the surging that occurs some days but not all days?
I'd be in for a two-four.

 

[bentwrench]

There are two ways to tell if your jetting is off (well, more than 2, but 2 easy ones.)

You can pull your spark plugs and take a look. If they're tan coloured, then you're fine. White for too lean, black for too rich, broken for much bigger problems in your engine...

Or, you can just give'er. At WOT, you should be able to pull right to redline in 5th (do it on the highway - you won't get above 140 km.h with stock gearing.) If you can get there, then you're fine. If your engine starts to cough and give out, then you could use some jetting.

I tell ya, Lou, when your bike is finally down South again, we'll go for a ride. I'll bring my tools, we'll pull your plugs, and find out immediately.

 

[16VGTIDave]

A quick update:

Yesterday the bike was running better with virtually no misfire as previously described. Today I put ~12 Litres of Shell Gold gasoline in the tank and 160+ km on the odometer. The bike was running well even in the cool and rainy weather. The exhaust still has a bit of a bark to it though.

I'm quickly coming to the conclusion that the new NGK plugs I had installed are garbage and that the carbs are jetted lean.

I'm quite interested in taking you up on your offer Bentwrench. Getting together to talk bikes and test jets over beer sounds very appealing! Looks like Lou is also interested.

How much sweeter do we need to talk?

Dave

 

[Lou Ranger]

How much sweeter do we need to talk?

Dave

... =

 

[bentwrench]

Well, I'm in Oakville, Lou is in Guelph, and the only real obstacle is that I have a 6 week old who doesn't really let me sleep... but we can get past that, somehow.

Where in Southwestern Ontario are you, Dave?

 

[16VGTIDave]

I'm in Drumbo (just east of Woodstock), which makes us a triangle. I also passed through Guelph twice on my bike yesterday, I wonder if Lou heard me?

Congrats with the fresh kid! I know how you feel.

My schedule is flexible as I'm self-employed. But I've only got my M1, so I have that stupid curfew and highway restriction to consider. And that could make the ride to you rather scenic.

Dave

 

[Lou Ranger]

... My schedule is flexible as I'm self-employed. But I've only got my M1, so I have that stupid curfew and highway restriction to consider. And that could make the ride to you rather scenic. Dave

I'm likely trapped on the scenic roads of the Bruce Peninsula until Thanksgiving, but thereafter, have a flexible (weather based) schedule due to being old (retired ).
Even though I've been "grandfathered" a full M, if there isn't a scenic route to somewhere I don't go. I think you should be glad that you can't be tempted onto 401 or 403.
Getting there is half the fun - the other half is finding out that the ride will take longer

 

[16VGTIDave]

Getting there is half the fun - the other half is finding out that the ride will take longer

While I can't completely disagree, being late for an appointment isn't good for my clients either. Being located minutes from the 401 and 403 highways was one of the reasons I moved to Drumbo. Not being able to use them is frustrating! There often isn't a good (timely) alternative route...

 

[frwinks]

scenic roads is where it's @ I often find myself a "lil" late for work due to all the goodness in the Wellington/Peel area
Welcome aboard