terrible flat spot at 2500-3000

[Anonymous]

OK here's the rub

1982 Katana 750
1983 Katana 1100 Engine - just rebuilt the top end w stage 1 Yosh cams
stock 750 carbs w/ 130 dynajet main jets
K&Ns
4into1 exhaust w/ Vance & Hines can

won't hold it's idle (gradually dies) if idle is set below 1600rpm
terrible flat spot (bog?) from a stop at about 2500 rpm
idle increases drastically as engine warms
a little snap, crackle, pop at 4k
seems a little hot

I've read every related post on GS resources and tried everything:

replaced the aftermarket needles with the stockers and moved the clip to the bottom mark (raising the needle as high as it'll go)

adjusted the mixture screw out to 3.5 turns (no joy) moved 'em back to stock (2.5 turns) no joy

removed the vent tubes from the carbs

synched, synched and resynched the carbs

replaced the plug caps

checked, replaced and rechecked all wiring connections

the pipes all heat up evenly, the engine seems a little hot, it runs smoothly at 4k and beyond and the throttle response seems crisp at that speed.

anybody have any ideas?

thx so much,

Marcus

 

[tfb]

G'day Marcus,

Well it seems like you've done a fair bit of work there. But have you cleaned the carbies? You may have blocked pilot jet(s) which would affect engine performance in the rev-range you describe.

Mike.

 

[Dink]

Yup blocked pilot's is my guess too, make certain your emulsion tubes are clear as well.
Dink

 

[Detman101]

Emulsion tubes?

Aren't those the ones that connect the carbs together?

 

[mark m]

Emulsion tubes?

Aren't those the ones that connect the carbs together?

Nope. I do believe that they are the tubes that feed the pilot jet fuel into the carb throat and vaporise the pilot circuit fuel. They have many rows of tiny holes and if those holes get blocked up, the fuel will not get properly distributed into the intake air.


Mark

 

[Anonymous]

thanks so much for the quick responses, you folks are awfully swell

oh yes, sorry, should have mentioned, I cleaned the carbs several times (to double/triple check that I hadn't missed anything), pulled the emulsion tubes, all the little holes are clear, and the pilot jets, again all the little holes are clear, I'm gonna take 'em apart again today and make sure the floats are properly adjusted,
also I read on the dynojet pdf - http://www.dynojet.com/pdf/3133.pdf - that you're supposed to enlarge the slide lift holes when you rejet cv carbs, so I'll do that today as well. I'm gonna put the dynojet needles back in as well and put the e-clip at the 3rd position. I'll let you folks know what comes of it.

some other facts I neglected to mention:

late model Katana coils (you can change the wires on these suckers!)
valve clearance performed with the rebuild

 

[Detman101]

Emulsion tubes?

Aren't those the ones that connect the carbs together?

Nope. I do believe that they are the tubes that feed the pilot jet fuel into the carb throat and vaporise the pilot circuit fuel. They have many rows of tiny holes and if those holes get blocked up, the fuel will not get properly distributed into the intake air.


Mark

Ah ha!!! okay, now I know what you are talking about!!

Thank you!

 

[tfb]

I always thought "emulsion tubes" was an alternative term for "needle jets". :?:

Mike.

 

[KEITH KRAUSE]

I always thought "emulsion tubes" was an alternative term for "needle jets". :?:

Mike.

It is.

 

[KEITH KRAUSE]

I think the 750 carbs being installed on an 1100 motor is the problem.
Do both models use the same MM carbs? Even if so, there must be some differences that could easily cause your problem. Different air jets, needle jets, jet needles, etc? Have you cross-referenced the two models?
Drilling out the vacuum port to the diaphragm chamber will help throttle response. Be careful to drill them correctly.
Be sure to put the stock plugs back in. There should be no need to change them.

 

[gsBert]

i had a similar problem, only much worse.

in my case, i was bogging down because the mix was way too rich at all ranges except WOT.

the needles i had were too small a diameter for my needle jets (emulsion tubes).

needles and needle jets should ALWAYS be changed together, they are a matched set. this is very important, and knowing it a year ago could have had my bike working, well, a year ago...

try 140s as main jets for the 1100 to start out with. put the original plugs back in (dont mess with heat ranges, stick with stock unless you run a turbo) and READ YOUR PLUGS at all throttle openings.

i think different carb parts affect different throttle openings:
closed throttle to 1/8th pilot circuit
1/8 to 3/4 slides and needles
3/4 to WOT: main jet
so get a pllug reading for all these (run at that opening a while, hit the clutch and killswitch and yank the plugs).

i consider this good advice, but remember it's coming from a guy whose bike is still not jetted right...

 

[Anonymous]

I think the 750 carbs being installed on an 1100 motor is the problem.

well I ran these same carbs on the 1100 engine previous to the rebuild and they worked nicely, no change (in throttle response) from the 750 engine. I know that doesn't mean a heck of alot but these problems I'm having are post rebuild.

There were quite a few changes with the rebuild - I previously had ported heads but went back to stock and I believe my previous cams were stage 2s (that's what I was told but I can't find the marks on 'em) and my new ones are stage 1's

to my knowledge 750 Katanas and 1100 Katanas both used 34mm carbs I don't have an 1100 manual to cross reference 'em though - anyone know any different?

OK, here's what I've done since my last post:

I drilled the slides (1/8")
put the dynojet needles back in (matched to the mains which are 130 not 125 as I originally posted - originals are 112.5) and put the clip on the needle 3rd slot (of 6) from the top
adjusted the floats to 22.4mm - they were all low by like 5mm except 1 of them which was dead on (probably the source of alot of my problems)
made very sure that the carbs were indeed clean as per carb cleanup article
removed the stock clamps from the manifolds and replaced them with hose clamps, they seemed a little loose
turned the mixture jets to 2.5 turns out from the bottom as suggested by dynojet
synched the carbs

it runs alot better now though still not perfectly, it bogs a bit from a stop still, I have to slip the clutch a little though not nearly as bad as before, also it still won't hold an idle below 1500, it gradually peters out until it stops.

thx so much for the all the help any other advice would be most welcome.

take 'er easy

Marcus

oh, here's a pic of the bike:

'82 Katana 750 frame and bodywork - monoshocked, with GSXR rearsets
'83 Katana 1100 engine
'92 GSXR 1100 USD forks
'96 GSXR 600 triples and clipons
'86 GSXR 1100 swingarm
'92 GSXR 110 wheels
buncha other stuff I can't remember

http://motorama.net/images/katana.jpg

 

[KEITH KRAUSE]

The DJ needles being at position 3 seems lean. Just my gut feeling.
Take some plug reads after a 1/3 throttle chop test.
The bog off idle could be either the needle or the pilot circuit. But the bike stalling during idling would be the pilot circuit, if carburetion related.
Does one or both problems get worse as the motor warms up? (too rich)
Try turning the mixture screws in to 2 turns and test. (may help the idle problem and possibly the bog)
I'm thinking the needle is too lean (bogging) and the idle problem is rich related. Before it starts to stall, is the bike idling at the correct rpm...about 1,100?
In your first post, you said you removed the overflow lines. You meant the two float bowl vent lines, correct? Are the standard plugs back in and correctly gapped? Do your manifolds have o-rings? Did you replace them? Are the manifolds in good condition?
Let us know what the 1/3 throttle test reads are.

 

[Anonymous]

Does one or both problems get worse as the motor warms up?

yes the problem worsens as the motor warms

Before it starts to stall, is the bike idling at the correct rpm...about 1,100?

when I drop the revs below say 1600 the revs drop on their own and then it stalls

In your first post, you said you removed the overflow lines. You meant the two float bowl vent lines, correct?

yes the float bowl vent lines

Are the standard plugs back in and correctly gapped?

not yet, I'll do that tommorrow

Do your manifolds have o-rings? Did you replace them? Are the manifolds in good condition?

no o-rings in these manifolds they clamp on both ends, they're in good condtion, I sprayed 'em with WD-40 with the engine running to test for leaks, I've had that issue with another bike in the past, one of the first things I checked

Let us know what the 1/3 throttle test reads are.

I'll certainly do that

Thanks so much for your help Keith

take 'er easy

Marcus

 

[KEITH KRAUSE]

Let me add to my pevious post now that you mention it won't hold an idle under 1,600 rpm's.
This is usually a rich condition, either to much fuel or too little air.
Going by what you say you've done to try to fix it, did you remove the rubber plugs that go over the pilot jets? Some members have said they removed them and the bike idled badly. (too easy to draw fuel)
My thought on too little air would be the throttle plate(s) closed too much. I know you said you synched them, but any chance you did this wrong? It only takes one carb to cause a bad idle.
Also, is the ignition timing correct? Is it advancing correctly too?
Are the valve clearances correct?

 

[Anonymous]

did you remove the rubber plugs that go over the pilot jets? Some members have said they removed them and the bike idled badly. (too easy to draw fuel)

nope, left 'em in, made sure they were nice and soft too

My thought on too little air would be the throttle plate(s) closed too much. I know you said you synched them, but any chance you did this wrong? It only takes one carb to cause a bad idle.

I've been synching 'em everytime I take the carbs apart, I'm gonna do it again though, just to make absolutely sure.

Also, is the ignition timing correct? Is it advancing correctly too?

well it's an electronic ignition, I imagine it's alright

Are the valve clearances correct?

yes, performed the valve clearance when I put the top end back together

I'll put in the old plugs, change the mixture adjustment to 2 turns out and resynch, I'll let you know how it goes, thx again for all the help!

take 'er easy

Marcus

 

[KEITH KRAUSE]

OK. Good luck and we'll see what the 1/3 throttle test says about the jet needle position.
PS: I always put a gun on the ignition to be sure the timing is correct and advancing. Electronic or otherwise.

 

[Anonymous]

a gun, huh, I threw mine away when I got rid of the bike that had points, I knew I shoulda kept it, dayum!

terrible weather, haven't had any time to get to the bike, I'll let you know as soon as I do.

thx again Keith, it's people like you who make this the great resource that it is.

take care

Marcus

 

[Anonymous]

OK many things have gone on since my last post, ages ago I know but I've been working hard on this sucker.

replaced my manifolds, they were actually in bad shape.

went back to the proper plugs: D8EA or whatever the heck.

got myself a carb rebuild kit and changed the seats and float valves.

synched, synched and resynched.

results:

bike is running better thought it still bogs at about 2500-3000 rpm and still wont hold an idle below 1500 also there's a little cracking noise/backfire when I hit about 4500. It still seems like it's running hot as well.

here's what I'm gonna do:

I bought some new mikuni jets 137.5 - gonna install those suckers

I'm gonna check my plugs before I put 'em in though, I think I'm running lean but I haven't checked 'em yet.

I'm getting a new exhaust can, mine's all dented so I can't pull the innards to put some packing in, it is empty presently.

I've also ordered a set of K&Ns they should arrive pretty soon.

I'll let you know what the plugs look like and whatnot.

 

[Anonymous]

OK, I checked my plugs and they're pretty sooty

I changed my pipe to a Vance & Hines sounds much better than it did (the exhaust tone) seems a little smoother

I replaced my mixture screws, backed 'em out to 3 turns

I haven't put in the 137.5 mikuni jets yet, I'm not sure if it's a good idea because the plugs are showing a rich condition. ideas?

one other problem I've neglected to mention is that my idle speed does increase drastically as my engine warms, I've read in Keith Krause's other posts that this is indicitive of a air leak, I've already changed my intake manifolds, they're the kind with a clamp on either side, no o-rings, is there some other place that air could be leaking in? Maybe through the choke mechanism or something?

any help advice is most welcome

thx.

M.