Bonneville America Forums Forums Technical Forums General Tech Throttle response

Before my question, let me put a disclaimer in. Most of the bikes I've had in the past had fuel injection and shaft drive. Ok. That said, my '06 Speedmaster has what I can only describe as a lag and a surge. It's had this from day one. I notice this mostly in the lower gears. It's quite annoying. I recently changed my pipes an re-jetted the carbs, but no change. I adjusted the chain - still no change. Is this a normal by-product of carburation versus fuel injection or is something else going on? This happens when I roll off the throttle and roll it back on. Any suggestions?

Only a small part of it is FI vs. Carb. On ethiong you might want to do is let some more air in by removing the air filter snorkel (if you haven't already). By changing the pipes and rejetting, you have only done two-third's of the job. It should help.

Soren

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my '06 Speedmaster has what I can only describe as a lag and a surge

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At what throttle position(s)?

Don't forget, these carbs have vacuum controlled slides. You cannot snap the throttle on takeoff, you have to roll on the throttle. Also, the power band is typically above 4K rpm.

Is it sort of like gear lash? Even with a well adjusted chain, there is still going to be a bit of transfer from engine braking to acceleration as the chain slack reverses.
As opposed to a shaft that is which is always directly connected via hard gears.

Really whenever you are having a percieved problem it is best to do it one thing at a time. Makes it a more scientific approach to the problem. If you go changing 4 things, who is to say which did the trick? Or maybe it gets worse, or stays the same, but maybe one thing fixed it, and another put it back to where it was. It takes longer, but one thing at a time with a test in between is best. What did you jet to? Check out Pat's jetting calculator for some good starting points.

Benny...This has done this when the bike was new with no mods at all. I am starting to think this may be the nature of the bike. But I will take it one step at a time when experimenting.

JOE

Just because the bike is, er was, new don't think that for a minute it was un-crated correctly or assembled at the factory correctly. Loads of members have found their air mixture screws bottomed out (under the old epa caps too). Dealers don't care enough to un-crate or do anything else but hand you a bill and take your money. Try balancing your carbs first. The early days of mapping had lots of fi bikes running below par. Throttle response shortcomings have no easy direct answer. Some have drilled the slides 'hole'. (standard for 07 models now btw) Tuning the carbs after rejetting and more open pipes can cause your issues too. Spacers under the needles,different needles etc.

Could be a definite case of M M A R N P, or Muhmarnip for short --

Monday Morning After a Rough Night at the Pub syndrome

I took it for a ride again this morning. I think Benny you may have hit the nail on the head. It does seem to be drive train play. How noticeable should this be?? Mostly this happens in 1 or 2nd gear when I let off the throttle and reapply it going up or down a hill. You just have to touch the throttle to feel it. I did remove the snorkel no real improvement on this lag thing but seemed to give me a bit more power

Joe

Whatever you do, balance (synchronize) the carbs. This makes all the difference in the world. Everything else can be great, but if the carbs are not synchronized, there will always be a delay and a much slower power response. My bike just "jumps" when I crack the throttle, and it always has. I always check the synch after doing any related fuel or electrical work. Be sure to synch both the differetial screw between the carbs at a higher than idle rpm (eg. 2000 RPM) and the idle air (pilot) screws afterwards at idle. Then recheck the higher speed setting.

It could also be play in the cush drive. Some of our members have replaced the cush drive rubber with one from a Thunderbird Sport and say it fits tighter.

Some great info. Much appreciated.

JOE

That's another thing I forgot, after each step, when playing with carbs, balance them.

As far as the driveline thing, it's a chain. Chains can only pull, not push, so when you let off the throttle, the tension has to switch from the "top" of the chain to the "bottom". Proper adjustment of a chain DOES involve some slack. Has to, or else it would snap. So there will always be a little bit of a snatch like that.

Hi Joe, Your problem is caused by unsynchronised carbs, make sure your fuel levels are identical and mixture screws are set identically before you synchronise the carbs at approx 1200 rpm, also make sure you have absolutely no vacuum leaks, cheers, Tadpole..
ps...I have a brand new 07 and let it be tillit had 1600 kms up and then did the job, now purfect.

the screws do not actually have to be even. there have been a few cases of the carbs actually needing a bit different tuning than one another, there seems to be a bit of variance between many of the stock cv's

Frank

Well.. When I took out the snokel it made it worse. Funny thing. I tried pulling out the choke a bit because I think the bike was running lean ( pipes started to blue) and sure enough the drive train lash disappeared. I have the air mixture screws turned out 3 turns. Should I go 2.5 turns or would this not help the situation and I would need to rejet??

JOE

I think you really need to distinguish between drivetrain slack, and engine/carbs. Adjusting the chain won't fix an out of tune carb, and neither will the opposite. Rolling on and off the throttle will produce SOME drivetrain (chain) slack feelings, especially in the lower gears...Drive your car in 1st gear at around 10mph, then let off the gas, I guarantee you'll feel pulled forward out of your seat. The bike will feel the same way.
What you need to do is ride it at a steady speed (throttle position) and note any stumbling, missing, spitting, whatever, then go on to examine setups, jetting, air screws, etc...I'm afraid you're mixing apples with oranges, and confusing yourself...

P.S. Turning the screws OUT richens the mixture, so if you're lean, ie needing some choke, try turning them out a 1/4 turn at a time, riding it in between to see the difference.

Moe has a very good point. Don't assume it was set up right new. Also doing 1 thing at a time with a test is the best way to pinpoint the problem. My bike has always jumped as soon as I hit the throttle and does take a fuel injected Hardley twice its size. It is a hardley so not saying a whole lot.I have never noticed any lag but I have never had anything newer than 1972 except a 2 stroke triple Suzuki and thats a whole different animal.