Low RPM growl and vibration

[Dave-85-LTD]

Hi guys, it's been a couple of years since I was very active on this forum, or had much chance to ride my LTD. Moved to a different house and have been doing an extensive remodel on it.

Anyway back to my question.

My LTD has done this since I got it about 5 years ago.

If I am in any gear and let the RPM go below 2350 and then roll on throttle gently or really crank it, the motor growls and the bike has a definite motor vibration in it. It will run smooth below that RPM if I don't add more throttle. When it growls, it sounds and feels like it is running on 2 cylinders and pushing the others along.
I did some tests yesterday, in 3 rd or 4 th gear, I let the bike slow down until I'm down to 2000, then add a small amount of throttle and it starts growling, as the bike picks up speed, the noise and vibration stops instantly when the tach reads 2400. Also the power output doubles or more when the motor hits that 2400 RPM "switch".
I know these motors are not a low RPM power house, and are happier at 2500-3000 and above. It's been many years since my wife and I borrowed my dad's 84 interstate, but we putted around at the lowest possible speed to balance, all around the Victoria BC, Canada, waterfront. His bike put-up with us riding at about 1500 or so,I think it was actually at total idle some of the time, yet it would increase speed smoothly, and of course pull stronger as the RPM's came up, but not the sudden "switch" effect I get.

I don't know how I could check the spark out-put while I am riding, any ideas?

[GoneFishin]

My 84 is more or less the same way. They don't like to roll on the power unless they are in the right gear. Heck the day I bought mine 4 yrs. ago I thought something was wrong with it on the way home and then realized they do not like to be lugged! Man will they go when they get to 2500-3000, reminds me of the Suzuki GT750 I had years ago. It had the "flip the switch" at 2000. Ah but ain't it great? Gotta love these old wings!

[bs175dths]

Welcome back to this forum.
I hope all went well with the remodel. As my wife said when we finished ours and I had other things ‘I should have done’ during the remodel, “there is always next time!”
Yea, a, no!
Anyway, recall these carburetors are constant velocity carburetors meaning, the primary jet needle operates off of a difference of pressure and are not directly connected to the throttle cable.
The way they operate is thus, there is trapped air (with a breather passage) in the top of the carb., and as the throttle is opened, an increase of gas causes the engine to run faster. The faster it runs, the more air it sucks in through the carb. body. As this air increases, a difference in pressure is created between the air passing through the carb. and the air trapped in the top of the carb. thus causing the needle piston to float up, thus constantly increasing the size of the main jet, thus increasing the amount of fuel passing through the carb. and into the combustion chamber, causing the engine to run faster, and on and on, and on until redline!
This needle is attached to a piston which is supposed to smoothly float up and down which would then allow the engine to smoothly transition across its rpm range.
Imagine, then if one of these needle pistons ran into some resistance and ‘hung up’ until that resistance was overcome. The engine would not transition smoothly.
The carb. walls are made of aluminum and the piston is made of a plastic material and they have to have a very close tolerance. The smallest amount of dirt or dried gas can cause resistance between the piston and the wall, and this can reduce the smooth floating.
Similarly, if the diaphragm which seals the top of the piston from the bottom, is torn or not seated properly, the piston will not smoothly float.
Also, the return spring in the top of the carb. must be seated properly.
When I cleaned my carbs., I also cleaned (using just gas and a well, ventilated area) the plastic piston and the piston wall. I made absolutely sure all mating surfaces were clean, shiny and the piston went up with ease and returned home with a ‘snap’.
One way to test for this hesitation you are having is after reaching normal operating temperature, bike on center stand for stability, slowly, and I mean slowly open the throttle (bike does not need to be in gear). You are listening to the engine, watching the tachometer and sensing the travel of the throttle grip.
There should be a smooth transition from idle to 3000/3500 rpm. You may have to do this several times before getting the hang of it.
I am sure at some point you are going to hear/see/feel where your ‘switch’ is and the first place I would look are the needle pistons.

Unfortunately it means carb. removal.

You may not ‘see’ the resistance, use the back side of you index finger (it is more sensitive to irregularities than the tip of your fingers) to feel inside the carb. cylinder and needle piston.
Again, I used gas as a cleaner, then a cloth to ‘polish’ the surfaces of the piston and cylinder walls. Do one carb. at a time and see if you can tell if there is a difference between a clean carb. and a not yet cleaned carb.

Good luck.

That is just my $.02 (more like a buck fifty)

-Ride On ~O)

[unionjack]

You are loading the engine with far too low an RPM, that is why it is grumbling about it. You should be up over 2500 before putting any serious load on it.

The maximum torque is not reached until the engine reaches 5000 rpm or more, and will not redline until you get up to 7500 rpm.

Don;t be afraid to give her a little throttle, she will appreciate being turned loose once in a while, and will show up quite a number of crotch rockets if you use the lower gears and turn up the wick instead.

[Burnout]

Welcome back to the forum Dave!

I'm gonna go out on a limb here and say that the carbs on your LTD are just fine.... :d

When I had my LTD, I was able to go down as far as 30mph in 5th gear and it would pull itself out of the hole. It lugged, but it still pulled smoothly with no hesitation, vibration, etc. You say it's like a light switch, with instant-on power after a certain rpm. Could be one of several things and have a look at the following... Is the battery fresh and fully charged? LTD's and SEI's need a GOOD battery to run right. Also "ALL" of the power distributions and grounds must be clean and tight including the fuses and the starter relay. Is the engine in a good state of tune? Fresh plugs, filters, oil, etc? Have you pulled the plugs yet to see what's going on inside of the engine? How do the plug wires and cap ends look? ALL of the vacuum hoses ok? Dry rotted, cracked?(especially the PB sensors). Have a look at the basic's first, before you start digging too deep into things. After you confirm that everything is in good shape, we can move onto other things.

Good luck and keep us posted....

[ghostrider52005]

Id say the LTD carbs are fine if you can actually find carbs on an LTD :-\ So far I havent found mine but I think the pesky fuel injectors are in the way :lol:
My LTD is the same way. Below a certain point it grumbles. On the way up it hits is power curve and away we go with nary a gripe from it.
Wifes 84 that does have carbs is pretty much the same thing. It will lug but it doesnt like it. Actually it grumbles worse than the ltd and it just had a carb job and sync. Nature of the beast I think

[Dave-85-LTD]

Yes I would say my "carbs" are good also.

I know these engines are happier at 3000 and up, beyond that point on my bike you better damn well hang on :d cause things are happening fast.
I have had some size of Honda for about 45 years, I know what excessive lugging is and what it sounds like. As in my post, I said I was in 3 rd or 4 th gear at about 2000 RPM, then I added a SMALL amount of throttle, not enough to cause lugging, and that causes the sound and vibs. I am not twisting the grip as far as it will go, Used to do that on my CB650-4. :ymblushing: Plus the switch effect always happens at the same RPM, if it was simple lugging, the "switch" would move a little dependent on how much too much throttle you apply

Burnout, I think you have some good things I need to check.

New Deka battery, kept on a maintenance charger all winter, lots of power, all good clean tight connections, (sort of a hard ass on my wiring, it has to be right, I hold a federal electronics license)

[Dave-85-LTD]

Yes I would say my "carbs" are good also.

I know these engines are happier at 3000 and up, beyond that point on my bike you better damn well hang on :d cause things are happening fast.
I have had some size of Honda for about 45 years, I know what excessive lugging is and what it sounds like. As in my post, I said I was in 3 rd or 4 th gear at about 2000 RPM, then I added a SMALL amount of throttle, not enough to cause lugging, and that causes the sound and vibs. I am not twisting the grip as far as it will go, Used to do that on my CB650-4. :ymblushing: Plus the switch effect always happens at the same RPM, if it was simple lugging, the "switch" would move a little dependent on how much too much throttle you apply

Burnout, I think you have some good things I need to check.

New Deka battery, kept on a maintenance charger all winter, lots of power, all good clean tight connections, (sort of a hard ass on my wiring, it has to be right, I hold a federal electronics license)
All connections of the charging system are soldered, stator checked and in good shape.
About 500 miles on oil & filter & timing belts

I will start checking some of the other things you mention,

It just puzzles me as to why it happens at the same RPM every time.
I'll have to poke and test as time will allow.

Thanks bs175dths. These electronic fuel injected are not real common, I know you are trying to help. I was cleaning and fixing motorcycle carbs 40 years ago, I even stuffed a 125 Honda 2cyl into a minibike, damn near killed me, but man it was fun.

[ghostrider52005]

Dave you might have an injector that is hanging up a little. Feeding to much gas for the demand till a certain rpm. Havent had it happen on LTD but have in cars. If its not much it probably wont smell like flooding and might not even color the plug since after a point the mix is right and plug clears up. Injectors can do some weird things that are pretty subtle. You might try running a little sea foam or lucas thru it just to see what it does.