Any ideas why my bike looses power at freeway speeds (cont)?

[Dave_A]

I have an 82 450TX. When I got it, it was dead, I did a quick carb cleanup, and got it running. However, unless I ride with the choke on, it can only sustain 45-55+ mph for a few minutes, then it sputters out and dies. With the choke on, I can reach (but not exceed) somewhere around 75-80mph.

I have no problems with idling or operating below 45mph...

I am using the air filter that was in there when I got the bike, and I have no idea how old this is. Also, there is slightly less exhaust pressure on the left-hand pipe (as determined by placing my hands at the opening of each pipe and feeling which blows harder) leading me to believe that I'm loosing compression on the left cylinder...

[earlfor]

Re: Any ideas why my bike looses power at freeway speeds (co

Sounds to me like an induction problem. I would replace the seals between the carb intake rubbers and the cylinder head.

Earl

[argonsagas]

A compression check is easy to do, and will show you what you are working with. Remember to do it with the throttle fully OPEN.

You may have to have another look at the carbs.

On many bikes, when they are below about 40/45 MPH, they are running mainly on the carb pilot jets, and above that the needles come into play.

It may be that something is sticking, or a diaphragm is faulty.

[Anonymous]

any idea why my bike looses power

How do the plugs look after a choke-off run and after a choke-on run? Does it idle without the choke on and if so, how does it run up to the 45 mph mark? My CB550 Honda will not go past 50 mph without an airfilter installed. Maybe 60mph with the choke pulled out all the way. Put an air filter into it and it goes like a scalded dog (runs properly). Proper airflow and vacuum on the induction side of the carbs is very important to some motors.

What Argonsagas said sounds right. Pull of the airbox, rev the motor and see if both carb slides go up equally.

[argonsagas]

Re: any idea why my bike looses power

How do the plugs look after a choke-off run and after a choke-on run?

Normally, they should be a light to medium brown but if they are running rich, as with choke on, they will be black, and might even have a fluffy appearance due to soot buildup from improper burning.

[earlfor]

Re: Any ideas why my bike looses power at freeway speeds (co

A compression check is quick and easy to do. Low compression could cause your problem. However, an induction leak will not be detected by a compression test. Less exhaust/pressure/heat on the left pipe indicates the mixture on the left cylinder is weaker. Since it idles fine, we can assume the idle jets are clear. At idle, the idle circuit operates in a high vacum, low volume condition. An induction leak, unless major, would not stop the engine from idling relatively normally because you have plenty of vacum to draw fuel with. As rpm increases, flow/volume increases but vacum decreases. At 75 mph or so, the engine is operating at a very low vacum. A loss of a few pounds of vacum at that point would result in having almost no vacum at all. Without vacum, fuel does not flow. Pulling out the choke is replacing your loss of fuel drawing ability at speed, hence the engine runs richer and you are able to reach 75 mph.
At that point, the engine fuel requirement exceeds the flow capability of the choke. Retarded timing could cause the problem, but it is unlikely it would only cause one cylinder to run weak. Low compression usually results in a poor idle. Incorrect fuel pilot and airscrew settings cause a poor idle, but have minimal effect on upper rpm running as their volume capabilities do not constitute enough of the overall requirements to have much effect.

Check the compression first. Check the timing to make sure it is not retarded. (my hunch is it will be neither of these problems) We know the carb jets are clear and it idles fine, so I would pass on touching the carbs.
All that leaves is the induction boots on the engine, and a leaking induction boot will result in your problem.

Earl

I have an 82 450TX. When I got it, it was dead, I did a quick carb cleanup, and got it running. However, unless I ride with the choke on, it can only sustain 45-55+ mph for a few minutes, then it sputters out and dies. With the choke on, I can reach (but not exceed) somewhere around 75-80mph.

I have no problems with idling or operating below 45mph...

I am using the air filter that was in there when I got the bike, and I have no idea how old this is. Also, there is slightly less exhaust pressure on the left-hand pipe (as determined by placing my hands at the opening of each pipe and feeling which blows harder) leading me to believe that I'm loosing compression on the left cylinder...

[argonsagas]

Impeccable logic, Earl. (even if I must resort to re-spelling vacuum for ewe)

[earlfor]

Hmmm Ron, how distressingly embarrassing. :-) :-) :-)

Earl


[quote="argonsagas"]Impeccable logic, Earl. (even if I must resort to re-spelling vacuum for ewe)

[Dave_A]

Ok...

I know it's not the carb boots, as I tested them (soaked them in starting fluid, with the engine on, with no increase in RPM or other change in operation).

The plugs (left side especially) do get black. It used to leak fluid from the left side plug hole, but new plugs fixed that.

Someone did bore a 1/8in hole in the airbox between the filter and the carbs, but I plugged it with duct tape to prevent a leak...

I'm more and more inclined to think it's air-filter related (since dying from having the choke off would indicate running rich, right???)...

[Dave_A]

=delete=

[Dave_A]

Ok...

I think it's not the carb boots, as I tested them (soaked them in starting fluid, with the engine on, with no increase in RPM or other change in operation). Also, IIRC my bike has electronic ignition (no points), hence non-adjustable timing... It runs fine below 45 (although it looses power the closer it gets to 45-50mph...). I have rarely seen it above 5.5k RPMS in gear.

The plugs (left side especially) do get black. It used to leak fluid from the left side plug hole, but new plugs fixed that.

Someone did bore a 1/8in hole in the airbox between the filter and the carbs, but I plugged it with duct tape to prevent a leak...

Since you guys say it's running lean, maybe a petcock issue (although even with the petcock in prime, it still needs choke, just a little less than with it in 'on')??

[earlfor]

Dave, since the bike is adjusted to idle normally and it is probable you soaked the boots with starting fluid as the bike idled sitting still, its unlikely that would tell you much if the leak is small. As I said, idle is high vacuum, low volume, so a little missing vacuum would not be particularly noticable.
A inexpensive way to be sure it is not the boots is to pull the carbs and induction boots off and apply a coating of hi temp silicone sealer to the mating surfaces. Take a rag moistened with alcohol and wipe any excess out of the inside of the boots. Reinstall the carbs and take it for a test ride. For two carbs and two cylinders, that should be about a 30-45 minute job and all it will cost you is a tube of silicone sealer. Then you'll know for sure. I think you will be surprised. :-) :-)

Earl


[quote="Dave_A"]Ok...

I think it's not the carb boots, as I tested them (soaked them in starting fluid, with the engine on, with no increase in RPM or other change in operation). Also, IIRC my bike has electronic ignition (no points), hence non-adjustable timing...

[Anonymous]

any ideas why my bike looses power

Dave,

What you are describing sounds very very similar to the way my CB550 Honda was behaving. The need to use the choke to get more speed means that the engine is running lean at that speed without the choke. With my Honda I tried everything to richen the mixture (multiple cleaning and raising the slide needle, new O-rings between the intake manifold and the head, checking the petcock for proper flow, running it with gas cap loose to ensure no vacuum in the tank, cleaning and dressing intake boots, etc.). But, nothing worked until I put a stock air filter into it. When I got the bike it was not running and had no airfilter. I wanted to get the thing running right before spending $35 for the stock filter. I spent almost a year trying to figure out what was wrong with it but to no avail. Finally a friend of mine said " Put an air filter into it. How do you expect to get the mixture right without an airfilter?" I still didn't want to do it. My friend had to order the filter for me and pick it up for me and deliver it to my door. I didn't want to take it out of the package because I didn't believe it would do any good. When I finally installed it and test drove the bike, I almost crapped my pants. What a difference!!! The restriction of the air filter is designed to richen the mixture to the correct operating mixture.

Now I cringe evertime somebody says they want to take off the stock airbox and put on something else because it looks cool. This should only be done if you have the science to back it and the dyno to test it. I do like buying bikes from guys who have tried to make these kind of modifications because the bike usually runs like a piece of crap and the owner can't figure out why the bike that he has put all the work into and put all the expensive stuff on doesn't run right. They give up and will practically give the bike away. Sorry, the tiraid wasn't directed at you. It's probably just my compulsive behaviour disorder!

Earl,

I'm sorry but I totally disagree with your reasoning on the vacuum leak. If you wish I will explain my reasoning.

Junkman

P.S. Did you try checking for air leaks in the carb boots on the airbox side of the carb?

[earlfor]

Re: any ideas why my bike looses power

A differing viewpoint doesnt bother me a bit. I would be PLEASED to listen.
:-) :-) I'm all ears :-) :-) :-)

Earl



[quote="Junkman Frankenbiker"]Dave,
Earl,
I'm sorry but I totally disagree with your reasoning on the vacuum leak. If you wish I will explain my reasoning.

[Anonymous]

any ideas why my bike

Hi Earl,

Sorry I took so long to get back. My wife thinks posting to this forum is another obsessive compulsive disorder. I tell her it's just a manifestation of the same one.

Air leaks are bad. No dispute. Check for them. Eliminate them and show no mercy. However, I think that a vacuum leak between the carb and the head is much more significant at idle than at high speed. Here's why.

Air behaves as a fluid (a thin fluid) and is governed by fluid properties (pressure, flow rate, etc).

You have a situation of a vacuum leak between the carb and the head. Let's say it's a hole 1 sq mm. The engine is ticking over at 1000 rpm drawing air into the cylinder at a rate of 225cc X 500/min = 112.5L/min. The throttle slide is only open a slit (lets say 5 sq mm). Vacuum is high. The vacuum (pressure gradient) at the leak is the same as the vacuum (pressure gradient) at the throttle slide. But, the leak is allowing 1/5 = 20% more air to get into the cylinder. Rpm goes up and the mixture leans out.

At higher speed (let's say 4000 rpm) there is 225cc X 2000 = 450L of air being sucked into the cylinder every minute but, the throttle is open wider (let's say 3/4 throttle at 4.9 sq cm = 490 sq mm) and the motor is under load so it doesn't have the opportunity to run freely and keep the vacuum up, so the vacuum drops. The vacuum (pressure gradient) at the leak is still the same as the vacuum (pressure gradient) at the throttle slide. But now the ratio of the leak area (1 sq mm) versus the throttle opening area (490 sq mm) has dropped to a much less significant figure (1/490 = 0.2%). If the vacuum at both sites (leak and throttle opening) is the same, the rate of flow is dependent on the relative size of the openings.

So, whether it is a high vacuum situation or a low vacuum situation is really a moot point. It is the relative rate through the openings, determined by the relative sizes of the openings, that is important.

It's like having a leak in a barrel that has a tap on it. If the leak is only a drip and tap is only open a crack, over time you will lose a significant amount of the contents of the barrel due to the leak. If, however, you have the tap wide open the barrel will empty in no time and the lose due to the leak is less significant.

I hope that explains my point.

POST ON!!

Junkman