Unusual Sensation at 5k rpm on highway ~75mph.

tedkraus · April 24, 2011, 09:11:09 PM

Hello everyone! It has finally been nicer in MN! I noticed something odd on the interstate today wondering if anyone has some input. Around 5k rpm cruising in 5th gear if I "hit the gas" so to speak the bike momentarily seems to pull hard, bog and pull hard again... If that did not make sense let me explain one more way, when I go WOT at 5k rpm in 5th the bike seems to rapidly accelerate, hesitate for a second and than accelerate again. This all happens in just a second or two, and I am not trying to refer to the typical dead spot at 5-6k rpm. I cannot recreate this problem below 4800rpm or above 5200pm, if I go WOT at those RPM it just pulls like it should with no unusual sensations right through 5k with no problems. I can also recreate this problem in other gears like 4th and I also think 3rd. ( I get pulled back initially, pushed forwards, and then pulled back again all in a very brief time frame in my opinion under 2 seconds)

Extra info : I have been running 91 octane, with a small dab of seafoam the past few tanks, the bike idles fine after 10 seconds or so (50-65F outside), and there is no "surging" when not under stress like in neutral. The bike has K&N filter, cobra pipes, and has a cobra jet kit. Also for those who helped me previously the coolant system continues to be fine after the winter problems you guys helped me with earlier no loss of fluid smoke etc.

Could this be the jetting? Carb synching? Chain (seems good tension to me)? Gunk in the carbs from winter still? Clutch briefly slipping and catching again? Also I am at about 14.5K miles?

Happy Riding!

Magniac · April 24, 2011, 09:56:35 PM

Only thing I can suggest is GET RID OF THE 91 OCTANE. All Magnas are designed to run on 87 octane. The bike will run smoother and actually make more power with 87 octane. You should only run 91 in an engine that requires it, otherwise you're just lightening your wallet unnecessarily.

tedkraus · April 24, 2011, 10:00:06 PM

I had been running 87 ever since I bought the bike (2 years ago) but recently switched to 91 because the 87 is stuffed with so much crap around here it is un-believable...

hootmon · April 24, 2011, 10:13:44 PM

My bike pings on 87, so I've always used 91 without issue.
As to your 5K issue.. One think I would check is make sure all of your boots are tight between the carbs and the cylinders (two clamps per hose).
Sound like it's a reaction to the sudden change in vacuum pressure from pulling the throttle open.. I don't know if this is it, but it's something to check..

tedkraus · April 24, 2011, 10:25:17 PM

the "hose clamps" on the top and bottom of the boots?

roboto65 · April 25, 2011, 01:09:24 AM

The hesitation you are feeling is common among the 3rd gens what you need to do is shim the needles that will take care of the flat spot in the range you are talking about..

LIMagna · April 25, 2011, 07:11:37 AM

He's already got a Cobra Jet kit, which I'm pretty sure is simply a rebadged DynoJet kit. If that's the case, there should be adjustable needles on the bike and you may need to move the clip one position down. Also, since you've got both exhaust and intake mods, it's possible that even larger jets may be required but it's hard to make recommendations without knowing for certain what's in there (both the size and whether they're DynoJet jets or Honda jets).

tedkraus · April 25, 2011, 10:26:19 AM

I have the kit found here http://www.cruisercustomizing.com/detail.cfm?Category_ID=88&manufacturer_ID=4&product_ID=8755&sblid_name=Cobra_Jet_Kit___Magna_for_Honda_750_Magna&model_id=15

I am using the DJ096 jets with the needle clip in the third position as recommended.

Why would I be able to accelerate through the 5k flat spot with out noticeable problems (ie starting at lower RPM)? But if I start at 5k rpm have noticeable hesitation? Its a nice day... I am going to do some riding let me know what you guys think!

roboto65 · April 25, 2011, 01:10:54 PM

[quoteWhy would I be able to accelerate through the 5k flat spot with out noticeable problems (ie starting at lower RPM)? But if I start at 5k rpm have noticeable hesitation?][/quote]

Well the reason is that the needle has does not come into the picture till about mid throttle heck the 3rd gens make most of there HP till around 7000 to 8000 and then you are on the main jet.

On the Cobra jets the needle has a thinner taper but the jets are different to you can add a shim and it should put you midway between notches..

Greg Cothern · April 25, 2011, 05:54:36 PM

The 96 jets might be a bit small, maybe try the 100's for the dyna-jets.

Remember at 5k your on the mains mostly.

Carb sync is something I would think of if the carbs have been mucked with a lot. I have found its typically of not much help..

LIMagna · April 25, 2011, 06:20:27 PM

Here's a jet size cross reference I just found:

http://www.hondaatvforums.net/forums/repair-maintenance/10476-carb-jet-size-cross-reference-keihin-dynojet-mikuni.html

it might be helpful in translating what you've got vs. where you want to be. I'm not sure if the indicated Keihin sizes are the same as the stock Honda jets ... I know the stock Honda main is 102 and you probably want to get to at least a 105 and quite possibly a 108 since you've got both exhaust and air filter upgrades. You also might want to go to the next size pilot jets as well (42 vs the stock 40's).

hootmon · April 25, 2011, 08:06:45 PM

Quote from: LIMagna on April 25, 2011, 06:20:27 PM
Here's a jet size cross reference I just found:

http://www.hondaatvforums.net/forums/repair-maintenance/10476-carb-jet-size-cross-reference-keihin-dynojet-mikuni.html

it might be helpful in translating what you've got vs. where you want to be. I'm not sure if the indicated Keihin sizes are the same as the stock Honda jets ... I know the stock Honda main is 102 and you probably want to get to at least a 105 and quite possibly a 108 since you've got both exhaust and air filter upgrades. You also might want to go to the next size pilot jets as well (42 vs the stock 40's).

What Charlie said.. I have a K&N and drilled stock pipes and 105's helped.. SO, if your are not at least there you are too lean!!

Greg Cothern · April 25, 2011, 08:07:16 PM

The Dyna-jet kits have different jets vs stock as well as needles. So you have to use their style/type jet.

Pilot jets are not in the mix at 5k. Although 42's are probably of benefit.

Chad in Michigan · April 26, 2011, 10:41:28 AM

Quote from: Greg Cothern on April 25, 2011, 08:07:16 PM
The Dyna-jet kits have different jets vs stock as well as needles. So you have to use their style/type jet.

Pilot jets are not in the mix at 5k. Although 42's are probably of benefit.

pilot jets are always on, never off. they are in play throughout the entire range of the throttle with these carbs. since he has the k&n filter, it would only help if he installed the 42 pilot jets. Going up on the main jet wouldn't hurt either.

Greg Cothern · April 26, 2011, 06:50:33 PM

Chad I disagree, the pilot jets are for the idle circuit and low rpms. They along with the taper needle make up the mid-range, then the mains take care of the upper range of engine rpm fuel metering.

Sure changing from #40's to #42's would richen low end and mid-range but at 5K and up it would not change the air-fuel mixture.

I had the Dyna-jet kit in my Project 96 Magna with 100 main jets and 42 pilots, with K&N, and Supertrapps, worked GREAT!