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Vibration/?miss at full throttle no matter what the RPM
glad u seem to have finally solved the problem
1987 Aspencade 129K
1986 SEI 93K
2014 Tri-Glide HD 17K

#16 05-28-2012, 11:25 AM,
Well good to know. My wifes 1200 I trike is doing that exact same thing. It ran on two cyl when we got it. I pulled the carbs and cleared out the jets and bottom end of the carbs but after a bit of hesitation didnt pull the carb diaphram tops to check the diaphrams or for a sticking slider. She was anxious to ride it and I was a bit afraid of a diaphram tearing from old age when I pulled them so I left them alone. It runs smooth up to 75 or so as long as you dont full throttle it and start that vibration which she never does and 70 is about as fast as shed be going on the interstate anyway.

Figure to let her ride it this summer then when it gets cold Ill take em back off and do the whole thing. unless of course it clears up on its own which with my luck I doubt LOL
Ghost.President: G.M.M.C.C.
Member:Patriot Guard Riders
#17 05-28-2012, 09:59 PM,
Recall these carburetors operate by a difference of air pressure, the pressure of the air trapped at the top of the carburetor and the air rushing through the carburetor as this air makes its way to the combustion chamber.
There are several reasons why the needle slide will not slide, or will not slide smoothly: torn or improperly installed diaphragm; chrome carburetor caps (poor mating surfaces causing a poor seal); slider piston/carburetor cylinder (where piston slides) has some debris causing sticking or hesitation; needle is bent.

Easy fixes:
Make sure the diaphragm is seated properly; also make sure the return spring is seated properly in both the slider and the carburetor cap.
Replace the chrome caps with factory caps – I have heard many horror stories about poor performance only to be resolved AFTER the factory caps were reinstalled.
Clean the walls of the slider and clean the walls of the cylinder the slider slides in – I used gas (take the regular precautions) because that is the chemical the carburetor was designed to handle. Note too, there is a small hole on the slider piston running top to bottom. That too must be unobstructed.
If the needle is bent, replace it.

I worked one carburetor at a time so as not to mix ‘n’ match. After reassembling the carburetors, I pushed each slider up noting its resistance (travel should be smooth and continuous) and then they should close with a ‘snap’! Travel up and down should be smooth and without hesitation because remember, it is a difference of air pressure which causes the sliders to glide up!

Another symptom of sticky sliders is the bike running up through the rpm then having a flat spot before taking off like a raped ape. That flat spot may be one of more sliders seeing resistance as they move up. The engine is in need of more air to match the fuel, but the sticky slider will not allow it, until the difference in air pressure overcomes the resistance. And then the raped ape appears.

After the bike is reassembled, let it reach normal operating temperature. Slowly, and I mean s l o w l y open the throttle and listen to the engine as it increases its rpm. The transition from low rpm to higher rpm should be smooth and stumble free.
Keep in mind the age of your bike and its mileage as not all bikes will be perfectly smooth and stumble free, but should also not have flat spots or max out at 4000 rpm.

Be thorough in your cleaning….

-Ride On
enjoying the view from the saddle....... due mainly to the people and information found within this site
#18 06-11-2012, 03:47 PM,
AHHHHHHHHHHHHHHHHHHH.................... I am getting frustrated. I thought it was fixed but today I was riding and it seems to be back, and my gas mileage really sucks. I struggle to get 30 mpg when I used to get over 40 routinely. Also it is very snappy getting up to speed but I was on the expressway today and anything over 70-75 mph, 3600 rpm and up it seems to have a bad vibration feeling like it has a bad miss. It has never run this good getting up through the low gears. I have balanced these dang things so many times I lost count. I replaced the plugs and wires and caps. Once it cools off I will put the factory carb covers back on, but I rode with the chrome ones on for over 10 years without problem so I'm not thinking it will make any difference.
Could one of the coils be breaking down at high speed? It seems to me if that was the case I would notice it hen I get in the high rpm's in the low gears too, but I don't.
Any ideas will be greatly appreciated
1985 Aspencade Biege/Brown
Ride Safe, but ride often, none of us know how long we are here for.
#19 06-13-2012, 01:50 PM,
First: take notes as to when the vibration occurs: indicated ground speed
indicated engine speed
ambient temperature
the engine should reach normal operating temperature
flat/level ground

When the bike vibrates are you increasing ground speed, maintaining ground speed, or decreasing ground speed? Or all three?
70-75 mph and 3600 rpm; is this in OD? - try 3rd gear @3600 rpm
try 4th gear @3600 rpm
note any vibration
If no vibration @3600 rpm in 3rd or 4th, increase rpm in those gears (not past redline though)
note any vibration

I have found my bike will handle OD at no less than 2500 rpm.
You will need a long stretch of road for this – get into OD at or about 2500 rpm and slowly increase ground speed without bogging down the engine. Slowly increase ground and engine speed until you get the vibration you have felt at 70-75 mph @3600

If you don’t get vibration at or above 3600 rpm in 3rd or 4th, I would rule out the coils breaking or the pulse generators breaking down
If you have access to another, known good, set of coils, beg, borrow or steal (kidding) them and rule out the coils

Do the same kind of testing at night (be safe) with the shelter removed. In this way, any stray arcing can be easily detected

If all of the above proves no vibrating I would suspect the ignition advance unit. Make certain the vacuum hose from #4 intake is in good working order and its routing is not pinched or kinked

If you do detect vibration @3600 and above in 3rd and 4th, suspect the coils, pulse generators.

When you performed the carburetor rebuild, were the air/fuel screws adjusted?
Are the intake o-rings seated properly?
Are the fasteners tight? Intake bolts, intake bands

Do you still have the old diaphragms? Try replacing the new with the old (if useable) and check performance again.

A ton of experimenting is involved but that is what it can take when chasing down gremlins.

Good Luck and keep us posted.

-Ride On
enjoying the view from the saddle....... due mainly to the people and information found within this site
#20 06-14-2012, 01:41 PM,
thanks, I will take note of all of these. I already know, as I have been tracking this down since April, that in the lower gears I can feel a slight vibration but it is barely noticeable. As I increase in gears it gets worse. Yesterday it seemed to be much worse than it had been since I found the stuck throttle valve so I put in some Seafoam and it did seem to improve but it was certainly still there and it took everything it had to get up to 80 mph. Also this vibration is only on acceleration, if I just back off the throttle the tiniest bit it goes right away.
Thanks again for the info and I will keep you posted.
1985 Aspencade Biege/Brown
Ride Safe, but ride often, none of us know how long we are here for.
#21 06-16-2012, 06:51 AM,
I took your advise and put the stock carb covers on. When I took #4 cover off I noticed that the diaphragm had not been seated right under the cover the last time I put it on. I fixed this and took it for a ride. I no longer feel the vibration in 1st, 2nd, 3rd or 4th. And when I get on the highway I don't feel it anything like before but it is hard to tell because above 85 the buffeting from my windshield shakes the bike some. It does have much more pickup accelerating from 75 on up to 90 mph. I hate to say it but this problem may be fixed. Now if the gas mileage improves I would be all set. If the float levels are off a bit would that have a big effect on the gas mileage?
Thanks again for the suggestions
1985 Aspencade Biege/Brown
Ride Safe, but ride often, none of us know how long we are here for.
#22 06-19-2012, 05:27 PM,
Good to hear you may have found it. Getting those diaphragms and return springs seated properly are key to a smooth transition across the power band.
You may consider checking the synch of the carbs. The mis-seated diaphragm on number 4 may or may not have changed the synch.
Also note, number 4 is the carb. whose vacuum feeds the ignition advance box for fifth gear. So if that carb was acting up, its vacuum would be acting up which might confuse the ignition advance algorithms.

I don't think float levels being off a minute amount would be detrimental to m/g. It has more to do with starving that particular carb.
But setting every adjustment to factory has its merits and is the best starting point when faced with a problem.

Let us know and watch your ground speed!

-Ride On
enjoying the view from the saddle....... due mainly to the people and information found within this site
#23 06-21-2012, 09:30 AM,

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