"Flat Spot"?

[pick]

Hey all, after totaling my 01, replaced it with an 07 Midnight that was bought "New" off the showroom in 10. I know the owner, he did no mods other than aftermarket mufflers "Bubs" I bought it from him with 7k on it. Having trouble with what I can only call a "flat spot"? Bike will start fine, idle ok, but from idle and a hard acceleration, if stalls for a bit before really catching my 01 never did this? Hard acceleration at 40 and no issues, it is only from a low speed. I run seafoam regularly. Carb sync is spot on, and other than that, the bike runs great. Any ideas? Can the add on mufflers effect the low end acceleration? I would gladly go back to stock mufflers if that is the case?

Thanks!!!!!!

[M61A1MECH]

Are you sure he did not change the air filters to K&Ns or open up the air boxes?

I had a similar issue on my 07 when I added K&Ns and opened up the air boxes to much.

I fixed the problem by adding one shim under the diaphragms, still had just a little hint of it on occasion. Later I put stock air boxes back on , but kept the K&Ns and no more flat spot.

[Ace]

I had the same "flat spot" you describe on my 06 RSV after opening up the air boxes with K&N's. I had to go back to stock air boxes to get rid of the flat spot but I did leave the K&N's in.

[darthandy]

And to answer your original question, yes - the mufflers could cause the "flat spot". The effect, as with freer breathing air filters, is to cause the bike to run lean. When you crack open the throttle with freer flowing filters, the intake system pulls in too much air for the amount of gas the jets can provide and there is a momentary hesitation till the amount of gas provided catches up to the amount of air being pulled in. With freer flowing mufflers, part of the intake charge can get pulled out of the cylinder as the exhaust gases leave more quickly than with stock mufflers, again causing a lean condition. It does, however, take very free flowing mufflers to cause this unless the bike was already running lean from the factory or the jets have become partly plugged for some reason. I would start by running a full tank of gas through the system, with at least a half a can of Sea Foam added to the fuel. If the jets are only partially plugged, this could clean them up enough to get the bike running properly again. If doing this helps alleviate the problem, then run another tankful with Sea Foam through the system to see if you can get more improvement. If this has no effect, check to see if your mufflers can accept some "packing" or baffles to reduce the exhaust flow through them. If this helps, then you'll have to look at either keeping tha "blockage" in place or re-jetting your carbs to compensate for the greater amount of air flowing into the system.

 

Andy

Edited February 26, 2014 by darthandy
Clarified my explanation (I hope...)

[MikeWa]

You might want to check the carb sync. It is surprising how often this helps stumbling problems.

 

Mike

[bongobobny]

Mike, he stated his carb sync is "spot on." As mentioned in the other responses, yes indeed the free flow mufflers can indeed cause unexpected strange things to happen. OK to make an engine "Breathe" more efficiently it is a combination of allowing the air in less restricted, allowing the exhaust gasses to evacuate faster, and to properly jet the carbs. Unfortunately many well intention-ed mods can go haywire if the modifier does not really know what they are doing. The real issue is improper metering in the carbs, or even though they are drawing the same vacuum at idle, the richness of even one carb not being exactly spot on will effect the overall response of the unison at all throttle positions. To properly set the richness you need to monitor each exhaust with a CO analyzer separately.

 

So, the answer to your issue is not necessarily an easy one...

[ACE50]

Off idle flat spots can be rid of by putting one size larger slow jets. I've done this to numerous bikes.

It gets worse by adding K&N's and opening the air box some.

Bigger slow jets fixes this and you'll love your bike even more!

 

It will even help with getting rid of popping on decal.

[Eck]

sent you an email...........check your choke..

[pick]

Thanks all, geez' I have absolutely no idea if I have K&N filters???????? I know I had them off to clean the filters, I will have to recheck. I consistently run seafoam, bike seems to be getting normal mileage, 35-42 depending on load and "habits". Lol. I really don't like the sound of the Bubs, so when it warms back up down here in the sunny south,l I think I will swap out the mufflers first. Do another carb sync, check carb heaters. My fear is that the bike sat on a showroom floor for several years without being run and there is crap in the jets? This is not something I can just live with, so this might go on for a bit!!!! Lol, thanks for all the ideas!!!!!!!!!!!!!

This forum is still the best!...........oh, and not to get off topic, but hey Eck, maybe an upcoming road trip???? Redbeard and I could renew our vows?????????????? Whatcha think Sleeperhawk??????? Roflmao!

[bongobobny]

OK stock is white, K&N are purplish pink...

[pick]

You mean the actual color of the paper inside the pastic boxes?

[bongobobny]

Yup!!

[billmac]

if it's only happening when you take off or very slow speeds check the idle screw. very easy to be a little low. some people have good ears for this, thought I did but one of the guys I ride with said the idle was low barely turned the screw and the flat spot went away.

[V7Goose]

What vacuum are you pulling at idle? It is possible the pilot jets are mostly plugged and you have the idle stop cranked up to run off the mains - this would cause the problem while the carb "waits" for the slide to get sucked open where you normally get the transition from pilot to main. Normal vacuum at idle is around 10" - anything lower than 9" is a real problem.

Goose

[pick]

Goose, forgive the ignorance, how/where do I check that vacumn?

[csdexter]

With a vacuum gauge, which looks just like a pressure gauge only works in reverse (i.e. measures negative PSIs or inHg from the atmospheric pressure downwards). You can plug that into the same nipple you'd use to sync your carbs.