Engine rough at partial throttle

[Blaze]

If there is a post about this I can't find it. My engine is running roughly (a little spitting and sputtering) at partial throttle. Once I get above about 1/8th throttle its good. The RPM can be from just above idle to around 2500 RPM when this happens. Its sounds to me like the timing is to retarted at low throttle. Or maybe I need to sync the carbs. I know the timing advance is controlle by ECT and the timing wheel but how does it suppose to change from idle through the acceleration curve? Is that all through the wheel?

[PAVALKER]

Carb related?   Maybe a can of seafoam to clean em out will cure it.  Worth trying and couldn't hurt.

[Patrick]

I agree.. Its sounds like the low circuits are starting to get gummed up.. Dump a can of Sea-foam in the tank, fill it up and ride at low engine speeds [<2500].. It may take a few tanks..  See what happens..

[Blaze]

I've ran one can of sea foam through it so far. Got some advice from some other members and I'm gonna change out slow jets. Thanks.

[salty1]

I would agree that the slow speed jets are probably getting fouled, but if it runs reasonably well, keep trying a cleaner, maybe Berryman's Chemtool.

[PAVALKER]

A good Honda mechanic did my state inspection right after I got my (out of state) Interstate, and notice the low speed jets stumbling.  He recommended a can of seafoam and not riding it as "normal" or at low speed, but cranking it up now and then and especially when going down hill, crank on it and then back off and let the engine pull the fuel and gunk thru.  Seemed to work.... 

[Ricky-D]

I suggest to index the throttle.

Simple, efficient and more accurate,

when dealing with these kind of problems.

***

[Valkpilot]

While you're at it, check for vacuum leaks:

• Vacuum lines and plugs on the back of the intake tubes
• Clamps on the intake tubes.  There are three clamps per carb/intake tube (18 total)
• O-rings between the intake tubes and the engine block

[Patrick]

Good point !!  I sometimes tend to think folks have already looked at the normal problems like vacuum lines/caps, etc.

[gordonv]

How did you use the carb cleaner?

Recommendation is to use it at a stronger mixture. Like 1/2 can in a 1/2 tank.

Others have found good results trying different brands, with each doing a little more. Sea Foam, Techron, Berrymans B-12, etc. I gas up as much as I can at Chevron now, figure I might as well get my cleaner with my gas.

[Blaze]

I suggest to index the throttle.

Simple, efficient and more accurate,

when dealing with these kind of problems.

***

How do you "Index the throttle"?

[Blaze]

How did you use the carb cleaner?

Recommendation is to use it at a stronger mixture. Like 1/2 can in a 1/2 tank.

Others have found good results trying different brands, with each doing a little more. Sea Foam, Techron, Berrymans B-12, etc. I gas up as much as I can at Chevron now, figure I might as well get my cleaner with my gas.

I am using 1 can per tank. Stronger than suggested. Sea Foam, STP and Techron so far. Don't have time or the parts to change the jets right now so I will continue to run more through it in the mean time.

[Valkpilot]

How did you use the carb cleaner?

Recommendation is to use it at a stronger mixture. Like 1/2 can in a 1/2 tank.

Others have found good results trying different brands, with each doing a little more. Sea Foam, Techron, Berrymans B-12, etc. I gas up as much as I can at Chevron now, figure I might as well get my cleaner with my gas.

I am using 1 can per tank. Stronger than suggested. Sea Foam, STP and Techron so far. Don't have time or the parts to change the jets right now so I will continue to run more through it in the mean time.

I spent a long time chasing what I thought were gummy slow jets once, only to discover a vacuum leak was the culprit.  Easy and cheap to start there.

[Blaze]

How did you use the carb cleaner?

Recommendation is to use it at a stronger mixture. Like 1/2 can in a 1/2 tank.

Others have found good results trying different brands, with each doing a little more. Sea Foam, Techron, Berrymans B-12, etc. I gas up as much as I can at Chevron now, figure I might as well get my cleaner with my gas.

I am using 1 can per tank. Stronger than suggested. Sea Foam, STP and Techron so far. Don't have time or the parts to change the jets right now so I will continue to run more through it in the mean time.

I spent a long time chasing what I thought were gummy slow jets once, only to discover a vacuum leak was the culprit.  Easy and cheap to start there.

I'll definately start there. Thanks.

[YoungPUP]

When mine did this this past spring it seemed to clear up fairly quickly after adding a full can of seafoam and taking her to the basment in 5th (about 20-25mph) and then bringing her back out of it. Did this several times through the tank of fuel. (5-6 times per ride before refuel).

[Ricky-D]

You can't solve a carburetor jetting problem until you know which fuel circuit is involved. Using a throttle index is an easy way to determine which circuit to investigate. From the December 2005 issue of Motorcycle Cruiser magazine. By Mark Zimmerman
From the April, 2010 issue of Motorcycle Cruiser


"All right guys, listen up. If I've said it once, I've said it a thousand times: Trying to diagnose a jetting or problem based on engine rpm is a waste of time.

Here's why: When you tell me or any other tech that the bike farts and burps at 3000 rpm, we've no way of knowing what circuit the carburetor is using at the time. As you can appreciate, 3000 rpm in fifth gear is a very different throttle opening from 3000 rpm in first gear. Even if you fill in the blank spots, the best anyone can do is guess which fuel circuit you're using when the problem occurs. And, if the problem is with carburetion, knowing which fuel circuit you are primarily using when it occurs is essential to correct the problem.

To make an accurate assessment of any carburetion problem, you need to know where the throttle is positioned, indicating which fuel delivery circuit is in use when things go haywire.

Trying to figure out where your throttle is while dodging traffic can get a little hectic, so try using this simple tool next time you've got jetting issues. Wrap a piece of masking tape around the throttle housing. On the tape, mark the closed position, wide open throttle, and the points in between, where the throttle is 1/4 open, 1/2 and so on. With the throttle closed (that is, released and at rest), stick a colored pushpin in the grip, adjacent to the Closed mark. Now, using your indexed markings, note the point on the throttle where the problem occurs. This will let you (or at least the tech) determine which carburetor circuit is primary when the problem occurs.

Once you know which fuel circuit is affected, solving your problem should prove much easier."

Read more: http://www.motorcyclecruiser.com/tech/find_carburetion_problems/index.html#ixzz1xUDfTy6i

***

[Led]

It could be something really simple!!
I am in the process of installing a Pingel petcock.  So, while things are all apart, I checked all the clamps on my carbs.  A lot of them were quite loose!
My bike seemed to "lope" at idle,  and have a slight stumble at just above idle.  Now it seems to have improved dramatically after I cinched them all down!!
  Even the smallest of vacuum leaks on these engines, can make a world of difference!!

[Blaze]

Thanks for all the great advice. As per some of the advice I ordered some low speed jets, .38's to replace the original .35's. But I've not had time to install them yet. So in the mean time I've ran through 5 tanks of gas. 2 can's of seafoam, one can of STP fuel system cleaner, and 2 cans of B12. In that order. In the first half of the 5 tank cycle I did notice a gradual improvement. So much so that I had to readjust my idle down twice to get it back to 900. I adjusted it up to 900 from 600 when I first got the bike. So I truly believe it worked on the low speed jets, but not completely. The last half of the 5 tank cycle there was no change. But then I tightened all 6 of the intake boot clamps. And they were all very loose, so I thought I must have it now! Although it was the greatest quick improvement it still is not completely gone. So is there more clamps that I can only get to with the tank off? Originally the idle was rough and it stumbled from 0 throttle to just under 1/4 throttle. Now it has a much smother idle and is only rough right at 1/8 throttle. I am getting closer.

[Michvalk]

Thanks for all the great advice. As per some of the advice I ordered some low speed jets, .38's to replace the original .35's. But I've not had time to install them yet. So in the mean time I've ran through 5 tanks of gas. 2 can's of seafoam, one can of STP fuel system cleaner, and 2 cans of B12. In that order. In the first half of the 5 tank cycle I did notice a gradual improvement. So much so that I had to readjust my idle down twice to get it back to 900. I adjusted it up to 900 from 600 when I first got the bike. So I truly believe it worked on the low speed jets, but not completely. The last half of the 5 tank cycle there was no change. But then I tightened all 6 of the intake boot clamps. And they were all very loose, so I thought I must have it now! Although it was the greatest quick improvement it still is not completely gone. So is there more clamps that I can only get to with the tank off? Originally the idle was rough and it stumbled from 0 throttle to just under 1/4 throttle. Now it has a much smother idle and is only rough right at 1/8 throttle. I am getting closer.

There are three clamps for each carb

[Led]

If you remove the little chrome covers (three screws each side) over the "enrichener" system, there are clamps hiding underneath those.........

Six of those you need an 8mm wrench.......unless you own some really wierd screwdrivers!  LOL!!  They are that hard to get to........!!!!!

[Blaze]

Just to follow up. I replaced the slow jets, desmoged and added a Dan Marc and fuel filter. Not it runs perfect.  The only thing now is (and I may be getting to picky) after my oil change I thought the valves sounded a little louder. I shouldn't need to adjust at 16700 miles should I? I know you can hear the valves normally but it just seems louder to me know. I used 20W 50 Synthetic with a K&N filter.

[pocobubba]

Congrats on the good job . Yeah maybe picky ,

                                               Pocobubba

[salty1]

Have you ever checked the valve clearance before? Honda recommends that they be checked at 12000 mile intervals. Too loose is better than too tight! If you preform a valve adjustment, you probably should follow that up with synchronizing you carbs. FWIW.