Topic: Bogging, surging, starving not sure what the word is? Need advice

[cakey]

Bike is a CB750k1. On a previous thread I spoke about on one of the bikes first rides after rebuild I went 9 miles down the road then stopped the bike for a while to get it inspected at the rego department. On the way home the bike started playing up, almost like it was starving for fuel at about 50 m/hr. Twist the wrist a bit and it sprang to life and then held it constant ant it spluttered and slowed until gave it another twist. Stopped at the lights a few mile down the road and then it didn't do it again. I couldn't find filters that would fit on the fuel lines but tank has been coated inside, new petcock and new Carty kits throughout. Doesn't play up at any other time.


Went to the show and shine today. Didn't play up at all on the way there other than the new neutral switch failing which is in another thread. Sat the bike for a few hours. Won the trophy for best Jap bike  and then on the way home exactly the same Symons again, feels almost like starving for fuel until you give the wrist a twist. Only does it around that 50m/hr range.

Do the fuel tanks get air locked or like vacuum problems? I have new tank cap and latch fitted.

Can it be air screws or is that only for idle?

Really not sure what problem could be. I have done the clear tube method with the bowls. I have no leaks, all new gaskets, o rings and brass wear.

Anything I should be looking for hear, bike has done a total of 41 mile after rebuild and is running great except for this one issue, the neutral light and front squealing brake.

[bjbuchanan]

My first guess is clogged pilots. Only the tiniest of crud blocks it up. Unusual given your setup but still worth checking. Blow out and ride

[cakey]

Any idea if I can get to pilots whilst on the bike. Do you mean blow out with a bit of compressed air?

[HondaMan]

You mentioned the carbs are rebuilt with new brass? Does this mean the needles were changed in the slides? I have seen no proper needles in any aftermarket kits, yet. The needles start working right about 50m/hr, too.

For example: the ones that come in Keyster kits are so lean at the low throttle settings (2500-5500 RPM range) that the engines fall flat on their face there. Keyster tries to "fix" this by recommending the mainjet be made bigger, but that does not fix the too-thick taper. The proper needles are the only fix that this one.

I have about a dozen slightly-used Keyster needles that caused this problem, have been replacing them with the right size mainjet and a Keihin needle. Works perfectly.

[cakey]

Not sure of what brand the kit is but here is a pic of the packet.

I also have the 95 jets and the needles that came out of the carbies but I don't know if they are honda or not.

The kits came with 95, 110 and 125 jets and I just put the 95 ones out the kit in.

Any suggestions?

[cakey]

I forgot I had one question though Hondaman, if the problem was the jet or the needle then wouldn't it happen all the time?

Both times now I have travelled about 20 mile. 10 mile there and no problems. 10 miles home and then problems. That's what got me puzzled.

[PeWe]

Similar here...
My Mikuni VM29's are too lean @ 4000-5000rpm /5:th gear. ~100-120km/h. Worse after 20km. Thats a little bit strange, it should need richer mix when cold...
Larger needle jets maked it better, still in need of even larger. 2 larger sets during shipment from sunny Florida.

[cakey]

Bt if everything is stock except it has been taken .50 oversize and its got the new Yamiya no number pipes then 95 should be good to go I would have thought

[PeWe]

your needles are not stock? too thick as Hondaman mentioned, causing too lean.
(As my carbs that has too small needle jets, almost same thing as too thick needles)

[cakey]

your needles are not stock? too thick as Hondaman mentioned, causing too lean.
(As my carbs that has too small needle jets, almost same thing as too thick needles)

Sorry PeWe do you mean that the ones that I had in the photo with the jets that came out of the carbies originally are not Honda and too thick or as Hondaman suggested that the needles I now have in the carbs could be too thick.

I don't know what original ones look like so cannot tell the difference anyway.

I know very little about carbs so I was wondering if the fuel is struggling to get through because the needle is too thick should I then change the position of the circuit on the needle or is this not what it is for?

[Kevin D]

I also have the 95 jets and the needles that came out of the carbies but I don't know if they are honda or not.

The kits came with 95, 110 and 125 jets and I just put the 95 ones out the kit in.

Any suggestions?

120's are stock mains for k1. I put 115s in after years of plug fouling. Where are your 120's?

Congrats on the trophy!

[cakey]

I will have too double check tomorrow but I went back to my old emails to the kit supplier and said "95 were removed from the carb but the kit comes with 95, 110 and 125, I don't think 95 is correct, which should I install?" They said to put 95 back in.

When I went through the parts I took out the carbs I found the no 95's. There is no 120's.

This could definitely be a problem. Are they the main jets in the photo or have I stuffed up and it's a different jet?

Pic of the bike with its trophy

[Kevin D]

Yep, those look like main jets to me, screws into the emulsion tube, which screws into the carb body and receives the needle.

Cakey, I'm over my head in deep water posting in the tech forum. What I know is whats stock and what works for me and my K1, maybe a little more, probably not. I don't think i've ever read about 95's in a K1.

[ekpent]

The smallest main in any stock 750 was a 105 and that was during the brutal gas and EPA crisis. As mentioned earlier stock K1 was a #120.

[cakey]

Thanks for all that guys. Crack of dawn I have checked out the jets that are in the carbs. I have put the 120 jets from the kits. The jets that were in there were 95.

The pics above are the needles that were in the bike. They are stamped with the Keihin logo and then 27201- these might be original needles.

So best bet is back to Hondamans original and take out the kit needles and put these Keihin needles back in if they are suitable and leave the kit jets in there.

Should I leave the circlip in the middle position?

[cakey]

Maybe I am using the wrong search words because I can't seem to find info on if there is a way to remove the carb tops from the Keihin without removing from the bike?

I only found one on mikuni which I know nothing of......like a lot of things!

[harisuluv]

No it is not possible, you will need to take them off the bike and off the rack to do this. 

Your kits look like those sold by "cruzinimage" on ebay.  They are pretty much bottom of the barrel as far as quality goes, and the price reflects that quality.  You get exactly what you pay for.  They are absolutely not OEM in any shape or form in regards to quality and tolerances.  However on those kits I have seen some pretty sad fake keihin logos on parts.  Even the stamp looks poor quality.

Yes your old needles look like originals.  I would stay with the 3rd position.

[cakey]

Thanks Harisuluv. I was hoping you would chime in.

If I am taking off the bike then I might as well put all the old brass back in. Anyone have any idea where to get Keihin 120 main jets. I havnt checked on eBay yet but am a bit dubious if they will be copies. I can check by phone some motorcycle shops in OZ tomorrow.

Don't know why the originals had 95 jets in them.

The ones in the kit don't have any logos at all but from everyone's experience this must be a prime candidate for my probs. thanks.

[cakey]

I got Yamiya 4/4 no number pipes Nic

[RevDoc]

Thanks Harisuluv. I was hoping you would chime in.

If I am taking off the bike then I might as well put all the old brass back in. Anyone have any idea where to get Keihin 120 main jets. I havnt checked on eBay yet but am a bit dubious if they will be copies. I can check by phone some motorcycle shops in OZ tomorrow.

Don't know why the originals had 95 jets in them.

The ones in the kit don't have any logos at all but from everyone's experience this must be a prime candidate for my probs. thanks.

Cakey, I'd be surpised if you couldn't find what you need here:  http://www.jetsrus.com/index.html

[HondaMan]

Not sure of what brand the kit is but here is a pic of the packet.

I also have the 95 jets and the needles that came out of the carbies but I don't know if they are honda or not.

The kits came with 95, 110 and 125 jets and I just put the 95 ones out the kit in.

Any suggestions?

Well, that's too lean: use at least the #110 if your needles are too thick (likely are). The 125 jet would help a little bit, but this combination will make the curve incorrect, probably causing surging around 4000 RPM.

I'd suggest the Keihin needles above all: yours should be numbered "K27201", then use the #110 or #115 mainjet. (Note: fixed the number...)

[cakey]

Not sure of what brand the kit is but here is a pic of the packet.

I also have the 95 jets and the needles that came out of the carbies but I don't know if they are honda or not.

The kits came with 95, 110 and 125 jets and I just put the 95 ones out the kit in.

Any suggestions?

Well, that's too lean: use at least the #110 if your needles are too thick (likely are). The 125 jet would help a little bit, but this combination will make the curve incorrect, probably causing surging around 4000 RPM.

I'd suggest the Keihin needles above all: yours should be numbered "K27102", then use the #110 or #115 mainjet.

Thanks Hondaman. If my needles are K27201 then have I got the right carbs for the bike? Is there a way to identify the carbs?

Also a couple of my slides are a bit sticky. Is it ok to cut and polish these to get them running smoother?

Under the magnify glass the kit needles are heaps more pointy than the original ones and heaps different shape.

[cakey]

Must have been a typo Hondaman. Just searching through your book on the carbs and it says that the K1 needle is K27201. That's what I have in their now.

[harisuluv]

Thanks Harisuluv. I was hoping you would chime in.

If I am taking off the bike then I might as well put all the old brass back in. Anyone have any idea where to get Keihin 120 main jets. I havnt checked on eBay yet but am a bit dubious if they will be copies. I can check by phone some motorcycle shops in OZ tomorrow.

Don't know why the originals had 95 jets in them.

The ones in the kit don't have any logos at all but from everyone's experience this must be a prime candidate for my probs. thanks.

Who knows, #120 is a main jet we can still get over here in the US, don't know about OZ.  If  you get in a pinch and can't find them I can send you some NOS honda ones.

[harisuluv]

Not sure of what brand the kit is but here is a pic of the packet.

I also have the 95 jets and the needles that came out of the carbies but I don't know if they are honda or not.

The kits came with 95, 110 and 125 jets and I just put the 95 ones out the kit in.

Any suggestions?

Well, that's too lean: use at least the #110 if your needles are too thick (likely are). The 125 jet would help a little bit, but this combination will make the curve incorrect, probably causing surging around 4000 RPM.

I'd suggest the Keihin needles above all: yours should be numbered "K27102", then use the #110 or #115 mainjet.

Thanks Hondaman. If my needles are K27201 then have I got the right carbs for the bike? Is there a way to identify the carbs?

Also a couple of my slides are a bit sticky. Is it ok to cut and polish these to get them running smoother?

Under the magnify glass the kit needles are heaps more pointy than the original ones and heaps different shape.

Pretty sure he meant 27201.  It depends on what the reason your slides are sticky.  If the slides are damaged IE from someone prying them up if they were stuck, polishing isn't going to fix that.  Also, what do you mean by "cut" in cut and polish.  The slides are chrome so if you mean cutting like a black compound I would say stay away from that.  If your chrome slides are so bad they need to be cut then you have bigger problems!