Odd carb issue... Slides flutter Video included


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754
Posts: 26
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Motorcycle: 1992 GL1500

1978 CB750 Supersport

Odd carb issue... Slides flutter Video included

Post by 754 » Wed Nov 01, 2017 2:51 pm



Good afternoon all I decided to start a new thread to highlight a new problem I have with my bike. I am dumbfounded by this, I have not seen this issue before, ever. I have limited experience with CV style carbs. They have been cleaned twice all passages. The pilots have also been taken out cleaned and new o-rings put in. In fact a rubber bits have been replaced with factory Honda parts. The video is needed because trying to use words just won't work. Anyone have any ideas? I have never seen vacuum slides flutter like this. I know the video isn't great but my phone is all I had at that moment.

As Always thanks for any ideas offered.

Travis





1992 Goldwing Gl1500

1978 CB750 Supersport

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Mh434
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Re: Odd carb issue... Slides flutter Video included

Post by Mh434 » Wed Nov 01, 2017 5:04 pm

To be honest, I've never seen CV-type carbs where the slides didn't flutter!! They can make a fair bit of noise doing it, too, although this is usually damped out by the air cleaner circuit etc.

The carb slides in my Concours fluttered a lot more than that, and it ran beautifully. As the carb slides react to manifold vacuum, and as there are a lot of pulses in the vacuum, it's not surprising that they do this.

Personally, as long as it's running well otherwise, I wouldn't worry about it - it seems like normal action to me.

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754
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1978 CB750 Supersport

Re: Odd carb issue... Slides flutter Video included

Post by 754 » Wed Nov 01, 2017 5:16 pm

I wish it ran well. This is my issue. When they flutter like that the engine just goes flat no power is being made. Actually if you hold the throttle at about 3000RPM or so, the engine will die. The RPM's just being to fall until the engine stalls, but it will start back with a simple bump of the switch.

Travis
1992 Goldwing Gl1500

1978 CB750 Supersport

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ct1500
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Re: Odd carb issue... Slides flutter Video included

Post by ct1500 » Wed Nov 01, 2017 5:48 pm

From the too short vid I gotta ask why is the engine idling at 1500RPM? Slides do oscillate when revving. As I described earlier there are three fuel circuits in the carb with slides only coming into play under heavy acceleration. Leave the airbox off and go for a ride on level ground accelerating through the gears like an eighty year old man driving not going over 3k RPM or a little less. The slides should only have the most minimal action. How does it run? If still not right then the problem is narrowed down to vacuum loss below the carbs or the pilot and especially main fuel system of the carbs is still questionable. While stationary and twisting the throttle slowly open from idle it should rev smoothly up to any RPM with no slide action whatsoever and run perfect, if it can't then the problem lies elsewhere as described above.
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ct1500
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Re: Odd carb issue... Slides flutter Video included

Post by ct1500 » Wed Nov 01, 2017 6:00 pm

754 wrote:
Wed Nov 01, 2017 5:16 pm
Actually if you hold the throttle at about 3000RPM or so, the engine will die. The RPM's just being to fall until the engine stalls, but it will start back with a simple bump of the switch.
Travis
When it does this at 3k put your thumb over the noisy rubber hose that attaches to the bottom right front of airbox. Assuming the shot air system hoses are intact with no defects this will rule that system out as the problem if no change in running. To get at 3k RPM stationary the throttle needs to be turned ever so slightly, how much do you need to turn the throttle?
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754
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1978 CB750 Supersport

Re: Odd carb issue... Slides flutter Video included

Post by 754 » Wed Nov 01, 2017 6:35 pm

The bike actually idles at the normal 800 RPM's with some surging. Sorry the video was short, the bike was not up to operating temp at the time I made the video the "choke" was still slightly activated. I have tried to ride "like an old man" and there is just no bottom end torque, it's difficult to keep alive and get moving. While riding after the throttle get past 3000RPM it begins to pull like I know it should. With this issue the bike just falls flat until after 3000RPM. I am leaning to a possible fuel issue. I have not checked the pump output yet, that is on the list. Any other ideas? I have a shop manual and have been working through that as well.

Travis
1992 Goldwing Gl1500

1978 CB750 Supersport

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Re: Odd carb issue... Slides flutter Video included

Post by ct1500 » Wed Nov 01, 2017 7:18 pm

754 wrote:
Wed Nov 01, 2017 6:35 pm
The bike actually idles at the normal 800 RPM's with some surging. Sorry the video was short, the bike was not up to operating temp at the time I made the video the "choke" was still slightly activated. I have tried to ride "like an old man" and there is just no bottom end torque, it's difficult to keep alive and get moving. While riding after the throttle get past 3000RPM it begins to pull like I know it should. With this issue the bike just falls flat until after 3000RPM. I am leaning to a possible fuel issue. I have not checked the pump output yet. Any other ideas?
Travis
With choke on there is a small bleed off of slide vacuum signal which limits their opening and has nothing to do with the current problem. Forget for now your fast running problems where multiple carb circuits are involved along with possible vacuum leaks, shot air system and air jet controller problems thrown into the mix. Surging at idle is a baseline symptom of a lean running condition with minimal systems involved which can be confirmed by the carb clean sprayed into the carbs which I linked to earlier in the other thread. It will tell you if one or the other or both sides of engine are screwed up.
Yeah, the thumb over the hose?
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754
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Re: Odd carb issue... Slides flutter Video included

Post by 754 » Wed Nov 01, 2017 8:22 pm

I'm at work for the 48 hours. I'm hoping to get back to the bike on Friday.
1992 Goldwing Gl1500

1978 CB750 Supersport

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754
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Re: Odd carb issue... Slides flutter Video included

Post by 754 » Tue Nov 07, 2017 10:15 am

I had a chance to work with my bike on Sunday. This was after a second cleaning and removing the pilot needles and cleaning that circuit as well, plus replacing the o-rings in that circuit as well. Before starting I also decided to check the fuel pump output which flowed much more than needed (3.5oz in 5 seconds. When multiplied by 12 as per the shop manual turns out to be 48oz per minute. The shop manual says a minimum of 21.6oz). It started right up, and after warming up to operating temp there was still a small surge. However after reading all the responses here I decided to try a few things, most importantly I operated the throttle much slower than I have been. So long as I easily accelerated the engine it reved right through the range with no issues at all, including no slide involvement. I was able to recreate this multiple times. Now if I operated the throttle as I had been, the slides would react and the engine would run poorly. I did also notice that at some point the surge resolved on its own, while it was not the sewing machine smooth idle it should be it was not surging. And with the large hose to air cleaner housing plugged, no real changes in the engine were noted. And no the carbs have not been balanced yet (my last manometer was destroyed by someone, so I need to pick up a new one). After letting it sit for an hour or so I started it again, and the surge had returned and I sprayed either under the rubber mat getting all of the vacuum lines with no engine speed change (thinking the carb imbalance may be the issue) and after a few minutes the surge resolved again.

So I say all that to say this, the bike is probably only a small part of the issue. I believe most of the issue is me. I have been trying ride my Wing much like my old CB750, 2 very different machines. I'm so used to being heavy handed with the throttle (2 very different carb set ups), I hadn't noticed the negative effect on the Wing I will be working on being easier and smoother on the throttle. I'll try to get a test ride in tomorrow and see how everything is running at speed.

Travis
1992 Goldwing Gl1500

1978 CB750 Supersport

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Mh434
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Re: Odd carb issue... Slides flutter Video included

Post by Mh434 » Tue Nov 07, 2017 3:34 pm

Operating the engine without the air filter system attached will affect the running. The carbs are designed to work with a certain amount of resistance to air flow, so without them the carbs may not run like they should. The resistance helps to damp out intake pulses, as well. This is similar to running a stock engine with wide open exhaust - in stock tune, the engine will run like crap until the exhaust is connected again, or the engine is tuned for zero back pressure.

Just a thought...

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Re: Odd carb issue... Slides flutter Video included

Post by MikeB » Tue Nov 07, 2017 5:37 pm

You have not said and no one else has mentioned it but is the fuel petcock doing its job? If it is not opening like it should with vacuum or the diaphragm is torn, the fuel supply will not be sufficient for the bike to run properly. Try bypassing the fuel petcock and then do your tests again to take it out of the equation.
MikeB
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754
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1978 CB750 Supersport

Re: Odd carb issue... Slides flutter Video included

Post by 754 » Tue Nov 07, 2017 8:22 pm

I have been wondering about that too, it's one of the few things I have not checked. Might do that tomorrow if I get a chance. Simple enough to do.

True about having the air cleaner off, but even with the air cleaner on I still had the same issue.

Travis
1992 Goldwing Gl1500

1978 CB750 Supersport

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Re: Odd carb issue... Slides flutter Video included

Post by RangerDanger » Tue Nov 07, 2017 9:46 pm

Did you check the slide diaphragms? It sure acts like they are cracked. Also it's a good time to replace the accelerator pump diaphragm and very thoroughly clean both check valves while your in there.

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754
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Re: Odd carb issue... Slides flutter Video included

Post by 754 » Fri Nov 10, 2017 1:09 pm

I have checked the diaphragms and while holding them up to a bight light I found no holes or cracks in them.


1992 Goldwing Gl1500

1978 CB750 Supersport

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