A bit sluggish out of first...

Here's one that's been bugging me for a while now. It seems like when I give a lot of throttle while in first gear from a stop the bike bogs and then after about 2-3 seconds it hooks up and away I go. I've been around the forum for a while now and haven't come across this scenario. Any ideas? I'm convinced it is a carb issue but am open to other ideas.

My thoughts were that it was getting too much fuel at idle - so I synched and colourtuned and repeated but it only improved a bit. Now I am thinking it could be a float level issue having seen the float level posts recently. I don't understand how the float level can have such a big impact though? Maybe someone could explain this...? I have checked the float level and it looks in line with the specs in the haynes manual.

My brother in law has a '72 Honda CB400 and beats me off the line everytime! I think I should be able to destroy the Honda!!!!!!

Rob

 

Fraps:

Not sure why it would only do it in first, but you might want to try enrichening your idle circuit by backing out your mixture screws a little bit.

Last fall I cleaned my carbs, and after putting them back on, had an incredible bog right off idle. I backed the mixture screws out to 2 1/2 - 3 turns and that took care of it.

If your idle mixture is too lean, the bike will starve for gas when you crack the throttle, until the main circuits respond.

Good luck!
Herb

 

It might be your petcock. I was having the same problem, and found that if I set the petcock to PRI instead of ON, it had all kinds of power off the line.

Worth a try.

 

I'll give it a try. When I think of it, I have an inline fuel filter that may be causing a restriction as well.

Thanks for the info.

 

I'll go the other way. I reckon too rich on idle. Too much fuel going in to ignite correctly on take off. Check air filter for clogging as at idle the bike does not run much vacuum so until the revs increase then it needs all the air it can get.

Also check pilot jet size to make sure they haven't been replaced by bigger ones and also that they are clean.


Also have a read up on Ricks "clunk" test. If those sliders are slow to rise so will the revs.

 

Thanks HG. What would you recommend is the best way to check filter flow problems? It is a new filter so it shouldn't be dirt. Just run the bike with no filter to see if that is the issue?

I'll check my pilot jets and "clunkers"

 

Yep, run it without for a few miles. It's not uncommon to have a bike bog down taking off, but fixing it can be frustrating.

You've done the sync and colortune so they shouldn't be the issue. The fix is somewhere in the airflow system not creating enough vacuum for proper combustion at low revs. Either too much fuel or not enough air.

If it were petcock or filter restrictions then I would expect starvation issues to be more at higher revs.

Another thing not mentioned but worth checking is valve clearances. My 550 has the same bogging issue but I also know my clearances are way out. If the valves are not closing properly then again, vaccum is compromised.

Keep missing out on shims on ebay to fix it. :cry: .

Hope you get it sorted, can't have piddly Hondas beating you.

 

The goal is to DESTROY the Honda! The bike pulls hard once it hooks up so I can easily catch up and pass - but I shoudn't have to catch up!

I have never checked the shims - I have all the tools to swap them out if need be so I'll bump that one to the top of my list.

 

Look at this website for low, mid range and full throttle problems and how to set.
http://www.factorypro.com/tech_tuning_p ... gines.html
Hope it helps

 

Don't mind me chiming in, but that is not true, when an engine is idling or decelerating is when it's making the most vacuum.

I see a lot of people seriously seeking carb tuning help & advice and lots of dis-information coming across which isn't going to help them.

Not that I'm trying to claim I'm the Big Kahuna of tuning, and I am new in finding this XJ forum as they are bikes I love and I'm certain I can gain information from some here. But in tuning, 25 years dragracing and building motorcycles I have accumulated a lot of information that I see folks here are in need of so here's a fantastic write up on tuning CV carbs that I think everyone on this forum can benefit from.

This will be the biggest benefit in learning CV Carb tuning. I also have information if someone wants it on the generation ,function and development into motorcycle carburators.

CV Carb Tuning 1
CV Carb Tuning 2
CV Carb Tuning 3

Good Luck Tuners, this should help

 

Hello Fraps, does the problem also occurs when you drive away from 0-Mph in first gear upwards to about 50-Mph in fifth gear with a minimum of throttle and how does the engine runs? I put in some documents about Motorcycle Carburetor Theory 101 and Carb Repair Hints. Carbs101.pdf, Carb Repair Hints.pdf

 

I think there was a large and valuable discussion thread on this somewhere before.

Basically on the basics of vacuum vs airflow but for simplicity sake for those who have a bit of trouble comprehending the dark depths of CV carbs, I tend to use the generic term of vacuum.

This discussion was interesting as the difference of opinion on what vacuum really is was quite interesting.

I still believe vacuum is higher above idle as airflow is higher. The basic proof of this is in the sliders of a CV carb. At idle they are closed. at WOT they are open. What lifts the sliders?? vacuum. Of course this vacuum is created by the increased airflow (and increased vacuum) as the butterfly's are opened.

Of course the question covered before is does the vacuum increase or decrease when the butterflies are opened. Initially vacuum decreases as restrictions are removed but as engine revs increase then more air must be taken in to maintain the correct air/fuel ratios for perfect combustion.

Measured at the filter end of the carb this is a higher vacuum than at idle.

Now, to put this into Fraps problem, we need to analyse which particular part of the problem is causing the "bogging" in lower gears.

When he "loads" up the engine taking off in low gear then what is happening. It has been idling at low revs and opens the throttle then the vacuum initially decreases but the fuel is already being sucked through the carbs therefore causing overfueling as the airflow has been changed but the fuel feed hasn't.

Now, disregard jetting as we are presumably working with a bike that has the correct jets for the relevant modifications. This should basically be fine unless the pilots have been changed anyway.

Now, where do the valves come into it.

All the jetting changes in the world will not compensate for an engine that does not combust perfectly due to "leaking" pressures at the valve heads. Too much fuel in not enough air will not combust correctly and cause a lack of power. Essentially the "bogging" affect at lower revs.

To correct this, the valves need to be adjusted to create a more "static" vacuum by sealing the combustion chamber so the vacuum created on the downstroke of the piston is higher as the restriction(butterfly) is opened rather than dragging exhaust gases back into the chamber.

The other side effect badly adjusted intake valves leaking is the pressure created by the piston on the compression stroke. If the intake valve is leaking then combustion pressure is then forced back into the the intake system negating the vacuum tat the carbs. This can be noticeable, often at starting by a little backfire through the carbs as the compression stroke fires and combusts back into the carb intakes.

So, after all this does anybody have a better grasp on the workings of the internal combustion engine or just confused as hell now. :lol:


Open for comment/rebuttal asthis stuff gets me excited. 8O

 

HG - that seems like a fine description and I'm always for the more detailed description of how things work. Thank you.

 

Hired_goon,

Your interpretation and detailed explanation is correct.
In the document Carbs101.pdf you will find interesting supplementary information for trouble shooting the problem.

 

been there done that
it's the starlight sparkle switch

 

You are essentially right, just the reception of the use of the term "vacuum" is blurred I guess. If you've ever watched a rack of 4cv carbs flow on a flow bench and introduce a smoke, you can really study their operation, and using vacuum gauges around different area's on the box and the carbs, and a good airflow gauge.
CV carbs operate much like a quadrajet 4barrell carb, they are basically a self calibrating airflow gauge, that automatically adjust to the desired air/fuel mixture, that's why on street bikes they are so good for self adjusting for altitude and changing barameteric pressure.

But I guess it's the understanding of vacuum, yes it's an increasing vacuum feeling at the intake throat if you put your hand over it, but that is the intake of the carb, not by which most mechanics mean when you talk about vacuum, so I felt it miss leading to someone doing tuning, as the term "vacuum" as used in service manuals, and most mechanics and performance people are meaning in the term "vacuum" is the ported vacuum by which is used to connect a vacuum gauge to the inside (between carb and intake chamber of the intake valve) that is used for tuning.
Ported Vacuum is highest at idle, which exists to serve as the Boost when the throttle is cracked open to assist the pull up of the slider from the rubber seal cap under the top of the carb. From there, the intake charge velocity increases to push up the slider or give it lift.
The underside of a slider has a very slight angle, like the wing of a plane, that the airflow is able to also push up the slider, as ported vacuum has now dropped off.
This is the purpose of the air pilot jet, to allow an intake of air to the ported vacuum chamber which allows free movement of the rubber slider back down as the pilot jet now allows more air then the vacuum. Once the throttle is closed, the vacuum draw can now overtake the pilot jet and again restore a resevior of vacuum. This whole curcuit is to assist the slider to move up before the intake charge can get it's velocity going and keep from having a terrible bog to accelerate.
The airflow velocity is also a pulsing intake charge which is the pulsing you'll feel with your fingers over the carb, as the charge forward is constantly being halted each time the intake valve closes, the vacuum curcuit also assists in smoothing the slider some.

This is the understanding I've gotten from messing around with carbs for years, a homemade test bench and working with some dragracers far more knowledgable then I. CV's are an automatic adjusting air flow valve, quite interesting, but can be hard to understand.
I've seen carb boring done, to turn a set of 35mm carbs into 39mm's and I don't remember the increase in flow numbers he got, but it was impressive. The slider's groove was reduced alot, but was enough to keep the slider retained ok.

 

Air flow "Creates" the various "Vacuums" needed for these carbs to function properly.

Rapidly moving air-flows across tuned ports reduces Barometric pressures with cause and effect. As a rapid flow of air passes over a tiny metering port Atmospheric PRESSURE is reduced. The reduced pressure allows fluids to rise, internal chamber air pressures to fall, and liquids to flow to areas of reduced pressure above the orifices ... it's Venturi, Bernoulli and Gravity all doing a beautiful thing ... and, we get to tweak it!!!

Its not really "Vacuum"
But we call it that; anyway.

 

No offense here Fraps but it might be your technique, slip the clutch a little bit
Or you be the one to say go, reaction time gets’m every time
For his birthday buy him the biggest countershaft sprocket they make

 

Polock - I definitely can't rule out operator slothness. This is my first bike and I am not the most experienced rider. I was thinking just the other day that I have way more hours wrenching on my XJ than actually riding it! I'll have my Honda buddy ride my Seca and see what he thinks.

Well here is an update:

1. Checked the alternator brushes - damn near new.
2. Check the valve shims - surprisingly ALL within tolerances in manual
3. Fuel Filter was not causing restrictions - I forgot I pulled it off last year.
4. Installed new fuel filter with faster flow rate.
5. Checked carb boots for vacuum leaks - none found (these are new boots and gaskets 2 years ago)
6. Full teardown and cleaning of carbs including "Clunk Test", main jet, pilot jet, air jets, needle and seat cleaning. Everything was fairly clean - I did clear the holes in the emulsion tube but it was minor.
7. Synch
8. Colourtune
9. Synch
10. Colourtune
11. Colourtune (cus it's fun!)

I took the bike out today - still same problem - just not responsive out of first. Almost like a stutter.

Still to try:

1. Running without air cleaner to see if it is restricting air flow
2. Check the float levels.

What are everyone's thoughts on this being the pilot jet? I have a 4-1 exhaust but I didn't put it on the bike so I have no idea if the carbs have been jetted for it.

Here are the jet sizes:
Main = 112
Pilot = 43
Air 1 = 50
Air 2 = 205
Needed = Y-10

Any other thoughts?

 

This sounds all too familiar.
We have to take a peek at the two AIR JETS in the Upper Body, on the right side, under the rubber Vacuum Piston Diaphragm.

As we look into the top of the Carb ... there are THREE (3) threaded ports.
Two are for AIR JETS.
One is for the threaded-end of the special cover retaining fastener -- if your carbs have the cover.
The PORT ~> Closest to the Enrichment Valve (Choke) ... is for the BIG Air Jet.
The PORT ~> In the "Center of the three" ... is for the SMALL Air Jet.

The remaining PORT is EMPTY -- a dead port -- where the cover fastener screw goes if your carbs are so equipped.

WHY is it necessary to check these air jets after a complete overhaul?

Because the Haynes Manual ... used by nearly everyone doing a Carb Overhaul ... ~~> IS WRONG ... SHOWS THESE TWO JETS REVERSED!

Worth a peek if you think you might have them in backward!

 

RickCoMatic,

I have headed your warning in the very detailed writing you posted on cleaning the carbs. I noted which threaded hole was for the cover and placed an 'x' beside it. I should note that even if I wanted to, I couldn't screw a jet into it - there was epoxy or something inside - I left it. The pilot air jet (50) was installed in the center and the main air jet (205) was installed to the side of the pilot air jet.

I am certain these are in correctly - thanks to you Rick for that.

 

Let's experiment and see if we can get that hesitation out.

Run High-test for this little tuning festival.
Operation: "Too Blue"

The ColorTune has you rescued from being too lean, the way the bike is set-up at the Factory.
But, with who-knows-what the volatility of the fuel you had in there for initial tuning ... it's set too rich for good gas.

Idle bike.
Tweak -- micro-adjust -- Mixture screw IN until the Idle starts to stumble.
Tweak OUT until Idle is steady.
Do 1 - 4
Take another "Almost didn't even move-it" teenie-tiny little Tweak IN -- On all four ... listening as you do.
As long as you have a good, strong, steady idle ... you can continue takin' another 10,000ths IN ... across the board.

As you do ... either run fans and work-out the hesitation in the shop ... or just take the bike out for a run and see what its REALLY doing after all the TWEAKING!

 

Hey Rick - maybe you stumbled on something. The bike (IMO) doesn't really have a "real strong steady idle". It's almost like it's missing. WHen colourtuning - It goes dark for a second then steady ignition, then dark etc. Following beardkings posts - I'm leaning towards pilot jets. It has 43's installed and I think they should be 45's.

I run high test already (with a fan when colourtuning) but I'll give your suggestions a shot.

Thanks.

 

Alright - i just checked. Stock HSC32 Hitachi Carbs have 110 mains and 40 pilots for jetting. Obviously, mine have been modified.

Just a thought - what is the easiest way to tell the difference between Mikuni and Hitachi carbs? My carbs are off another bike and I'm not convinced they are Hitachi.

 

Look for the Hitachi Logo on the side of the carb body casting.

 

Thanks Nick.

Good news all! I fixed this problem. Turns out it was the float levels. They were horribly wrong. Between 2mm and 4mm out of spec. Bike is way more responsive out of first now and it's peppy in the other gears too. Thanks for all your help.

 

Just thought I would update everyone. My RJ is a rocket off the line now and I'm destroying the old 72 cb450 Honda now.

In retaliation - my bro-in law went out and bough at 2006 CBR600F4.

It was good while it lasted..