Flat spot/lag when opening throttle

[GoneForTheBorder]

I recently bought an 86 Interstate. When opening the throttle, the engine lags badly until the throttle is opened more than 1/4 turn. It feels as though it's missing. It's not as bad when the engine is warm, but only slightly less then when cold. When releasing the throttle, RPMs drop slowly to idle.

I have already synched the carbs (they were way out of synch) and corrected the throttle cable adjustment (PO had taken all slack out so that the throttle wouldn't fully close). That eliminated some of the lag but it is still very evident.

Any suggestions as to what to look at next would be very appreciated!

[admin]

I need to know what happens if you run the bike with the choke on? Does it run better or worse?

[GoneForTheBorder]

It's raining pretty hard in my area today, I probably won't get the chance to try it until the weekend. I will post as soon as I can test that. I appreciate the help!

As an additional data point in the meantime, with the top shelter door open, I could hear backfiring on 1 and possibly two cylinders when the throttle was cracked open.

Glen

[admin]

If you pull the choke on as you rev it does the backfiring diminish?

[rcmatt007]

shouldn't be back firing.... I know on the 1000-1100's the first place to look would be a stuck cv slide, but the 1200 carbs are different. one inexpensive thing to try is to drain your carbs and then refill them with a 50:50 carb cleaner gas max.... let it sit for a few hours, then drain (just dumo this into the gas tank) if you just have a gummed jet this will sometimes work
don't try to run it on the 50:50 mix as the carb cleaner does not ignite well, and it will really sputter until fresh gas is in the carbs
if this doesn't work you are only out some time and a few $'s

[GoneForTheBorder]

Mangaged to test it today (between rain showers), results as follows:

The missing/hesitation is slightly reduced when riding with the choke partially on but is still very evident.

When revving the motor in the garage, I can't hear the backfiring through the carb(s) due to the increased exhaust note. The missing/hesitation remains pretty much the same whether the choke is on or not when the motor is not under load.

A stuck slide sounds like a possibility, as the throttle is opened, there is a point when power increases suddenly and the motor seems to smooth out. Maybe one of the slides isn't closing all the way?

Glen

[admin]

What is the condition of the spark plugs? Are they new? Properly gapped? Have you checked the resistance of the spark plug wire circuit? Weak spark can come across like carb problems. Let me know if you need specs and procedures.

[GoneForTheBorder]

I'll check the plugs next. The PO said they'd just been replaced, but considering the condition of the throttle cable and the carb synch, I'll do well to check them for myself.

I've never checked plug wires for resistance, but do have the Honda service manual. If the specs and procedure are in there, I'm good to go on that. I'll post the results and consult further as to what to do.

Good thing it's going to be a rainy weekend... perfect for spending it in the garage, I guess!

Glen

[admin]

It's good to know that the ignition is sound before you start playing with the carbs as it can save you a lot of aggravation if you find one of the resistors in the plug caps is bad or a coil is shorting to ground. Check it over carefully.

[Guest]

New to the forum. Twenty seven years line mechanic for import dealership. I have more than a fair working knowledge of mechanical, fuel and electrical systems. We have a saying in the trade...Keep it simple.
You stated that you bought the bike recently. Has it always run like this or did these symptoms suddenly occur? Since using the choke doesn't make a significant improvement, your problem probably isn't fuel related. (Assuming the choke actually works. Check your linkage as you apply the choke and verify it actually moves.) Since the carbs were "way" out of sync, someone has probably been tinkering to try to improve the "miss".
Try to isolate the cylinder (cylinders, there may be more than one) that's causing the miss. Hold the rpm @2000, based on your explanation, and remove one plug wire at a time. If there is NO noticeable change in rpm, you've located the guilty culprit. Do this on all cylinders. If there is only one affected, you can concentrate on things like a bad spark plug, or a bad plug wire. If there are two cylinders affected, then you need to start looking for things they have in common. Such as a timing belt off a tooth, or a bad coil, blown headgasket, etc.

[GoneForTheBorder]

glhonda... good suggestion! Removing the #1 plug wire doesn't change the RPM - removing any other one does. Sounds like the likelihood is a bad plug wire. I'll pull and check the #1 wire and post when that's done. Thanks!

Glen

[Guest]

Whoa, big fella! All we've done is isolate the problem. Remember, keep it simple.
Remove #1 plug wire from the spark plug and insert a small screw driver in the resistor cap. Hold it @ 1/16 gap from the valve cover and see if the spark will jump the gap. If you get good spark, the wire and resistor are probably okay. If the spark is good, swap #1 spark plug with another hole and see if the condition follows the component. Could just be a bad plug.
If you have good spark AND the problem is still on #1 after swapping the plugs, your miss just got expensive. Mechanical. (Burned valve, flatened cam lobe, broken spring) I'm keeping my fingers crossed. My eyes too! :roll:

[GoneForTheBorder]

In answer to your earlier question, the bike exhibited the condition when I bought it - but the price was right.

Now it's a little strange - I swapped the #1 plug, (which was light tan, appeared wet, gap was OK) with the #4 plug to reduce the possibility of a jumped timing belt affecting the test. Now, removing either plug causes the RPMs to drop an equal amount. The hesitation is still there, but much less than before.

Could it have been just a bad connection between spark plug cap and the plug?

Glen

[GoneForTheBorder]

And now even stranger - I repeated the test of pulling 1 plug at a time to see if idle speed was affected. When I pulled the #2 plug, a component which I hadn't changed and which tested good before, RPMs didn't change at all. I'm beginning to think there's a gremlin somewhere in my ignition system, laughing at me!

Glen

[Guest]

ANYTHING is possible. But I suspect since the PO supposedly changed the plugs, that's not the case. You said you have an equal drop between #1 and #4. Do you have an equal drop between ALL cylinders? When you removed #1 plug, did you notice any water in the plug well? Something was causing #1 to missfire. Putting it in #4 may be allowing the plug to clear. Run it for awhile, see if the condition reoccurs. If it doesn't, re-sync your carbs. Remember, you did your last sync on a bike that was only bangin' on three cylinders.