4-1, blue pipes, white plugs, poor acceleration...

[gorbys]

Hello folks! 
I rescued a 1980 GSX750 last month from a garage where it had been sitting for the last 5-6 years... I cleaned the carbs and got it running, and now the fun begins...
Basically it starts and runs fine. But acceleration is sluggish. I rode it in anger on the backroads here for about 30-40 minutes and checked the plugs, 1-4 where white, 2-3 had a little bit more color but not much really. 

It pulls great from the get go but then as you roll on through a quarter to half throttle and beyond there's a noticeable change in behaviour as you get on the needle and main jet.

Twist the throttle from off idle and acceleration dips at 4k rpm. Every time. And from there on it accelerates but it feels lacking. Especially in higher gears.

So white plugs, blue pipes, lean condition right? I believe someone has swapped to  a 4-1 exhaust but never rejetted. Because it should be rejetted right even if you still have the stock airbox and filter?

OR? Is shimming the needle an alternative? 
Or in case of rejetting; I have a dynojet kit that I used on my 1100. I used the needles and the stage 3 jets, which means I still have three jet sizes left over in that box. Can I use those dynojets with the stock needles or is that a no go, I need proper mikuni main jets as long as I use the mikuni needle?

Edited September 14, 2023 by gorbys

[slayer61]

 

White plugs are lean, yes. But, unless you did a proper "plug chop" you might be looking at the wrong circuit.

 

The main jets and the needle height both affect different throttle positions. The needle is out of the picture by the time the mains come in.

Edited September 14, 2023 by slayer61

[TonyGee]

they don't look to bad TBH !!!! a tad weak maybe, what are the carb settings right now ? 

[Gixer1460]

DJ Mains won't fit in Mikuni Emulsion tubes. Most steel aftermarket pipes with chrome tubes will blue at first bend from head. And the Mains are flowing fuel up the emulsion tubes that is metered by the needle which never completely leaves the tube, so to say they are separate is incorrect.

[gorbys]

49 minutes ago, TonyGee said:

they don't look to bad TBH !!!! a tad weak maybe, what are the carb settings right now ? 

Weak? As in a weak spark? 

Carbs are stock as far as I know, I'm gonna double check the main jet as I forgot to check size when I had it apart. But I'm pretty sure they're stock. 

1 hour ago, slayer61 said:

White plugs are lean, yes. But, unless you did a proper "plug chop" you might be looking at the wrong circuit.

 

The main jets and the needle height both affect different throttle positions. The needle is out of the picture by the time the mains come in.

No i don't have new sparkplugs laying around so no not a proper plug chop. Just 30-40mins of high speed driving and wide open throttle pulls. 

But in general I feel like it needs more fuel under wot. I realise in hindsight I should have tried some pulls while giving it some choke to see if it helped but one can't think of everything when you're freezing your balls off on a late night 

[gorbys]

6 minutes ago, Gixer1460 said:

DJ Mains won't fit in Mikuni Emulsion tubes. Most steel aftermarket pipes with chrome tubes will blue at first bend from head. And the Mains are flowing fuel up the emulsion tubes that is metered by the needle which never completely leaves the tube, so to say they are separate is incorrect.

What do you mean? I have dj mains in my 1100 with the stock tubes. 

[Wee Man]

This might help. 

 

http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,cv,high_rpm_engines.html

 

[Gixer1460]

1 hour ago, gorbys said:

What do you mean? I have dj mains in my 1100 with the stock tubes. 

Mikuni's being Japanese have metric threads, Dynojet being American will have Imperial / peculiar threads - draw your own conclusions! Tried it once and jet tightened up before fully done up so stuck with stock in stock and DJ with DJ!

[gorbys]

1 hour ago, Gixer1460 said:

Mikuni's being Japanese have metric threads, Dynojet being American will have Imperial / peculiar threads - draw your own conclusions! Tried it once and jet tightened up before fully done up so stuck with stock in stock and DJ with DJ!

Sure you didn't get the wrong jets? No issues seating them in the 1100 carbs and I just compared the spare ones I have to a genuine mikuni jet and threads are the same. Screwed them both into the bs32 carbs I have on the shelf no problems. 

[Gixer1460]

It was 30+ years ago - maybe they put their sensible head on and changed the threads?

[gorbys]

17 minutes ago, Gixer1460 said:

It was 30+ years ago - maybe they put their sensible head on and changed the threads?

One would assume so, I haven't heard anything about this when browsing the Web. 

 

Anyways, browsing the dynojet pages I have found a kit for the 750 and I have checked the installation manual but unfortunately the jets recommended doesn't match the extra jets I have. So I have to buy new ones. 

According to this guide I need to go up two sizes. https://www.caferacerwebshop.com/en/blogs/blog/how-to-rejet-your-cafe-racer-carburetor/

If my stock ones are 112.5 that would mean a 115 jet right? But should I buy a set of 117.5 just incase? I need to order from the UK so shipping takes a while. 

[BigT]

The first thing is that the Mikuni and Dynojet numbers are not the same

Second is the Dynojet kit should have adjustable needles

[ral]

My guess is the carbs are still not clean enough, I had similar issues to your description and it took three carb strips to get the internals squeaky clean, have you had the emulsion tubes out and cleaned all the holes out for example.

Once spotless get it on a dyno to set up optimum delivery and you won’t regret it, he’ll of a difference when done right.

[gorbys]

1 hour ago, BigT said:

The first thing is that the Mikuni and Dynojet numbers are not the same

Second is the Dynojet kit should have adjustable needles

I know this. The numbers I mentioned are mikuni numbers. 

And yes dj kits have adjustable needles with different tapers. 

But none of that matters since I can't use the dj jets anyway. Cheaper as well to just buy four new mikuni jets instead of one more dj kit 

56 minutes ago, ral said:

My guess is the carbs are still not clean enough, I had similar issues to your description and it took three carb strips to get the internals squeaky clean, have you had the emulsion tubes out and cleaned all the holes out for example.

Once spotless get it on a dyno to set up optimum delivery and you won’t regret it, he’ll of a difference when done right.

Yes, every thing that could be taken apart was taken apart and cleaned, poked and blasted. 

Dyno would be great but this is Norway. I know of one, and that's a cross country trip for me... 

Edited September 14, 2023 by gorbys

[gorbys]

So. I checked the ignition system. And found out that all the plug caps are wack except for one. 

Primary resistance is good in both coils. Secondary was way higher than specified. One was at 70k ohm and the other one at over 800k ohm(this is the one that supplies plug 1-4) 

The one plug cap that was good, was number two. Coincidentally the same plug that shows a little bit richer mixture than the three other. So weak spark=lean burn? 

Secondary resistance with caps removed checked out at 11k ohms on both coils so they're good.

I'm gonna try with the caps off of my 1100 since they're pretty new and see if it goes any better but is there any thing detrimental to using 5kOhm caps instead of the oem 11kOhm? I haven't noticed anything on the 1100...

[gorbys]

Drove pretty much the same route again with the new plug caps. Not sure I felt that much of an improvement. I mean the plugs are indicating a richer condition but it still doesn't feel like it pulls as hard as it should. And number two carb is leaking fuel through the fuel needle so maybe that's why it's not going as good, since it looks like it's drowning. 

 

Edit: I did a couple of pulls 1-2 gear to redline and when I hit 8 or 9.5k rpm it kind of stopped, then came back again. Like a really lazy rev limiter. That's not right either I guess. 

Edited September 15, 2023 by gorbys

[prutser]

How is the float height of the carbs?

I ve had similar issues with my 1981 gsx750, having a weak spot around 3-4k. Found out one carb was having a low fuel level.

Running a 4-1 exhaust with stock airbox and stock jetting by the way. Once warmed up it pulls smoothly through all the rev range.

[johnr]

did you remove the emulsion tubes when you cleaned the carbs?

[gorbys]

17 hours ago, prutser said:

How is the float height of the carbs?

I ve had similar issues with my 1981 gsx750, having a weak spot around 3-4k. Found out one carb was having a low fuel level.

Running a 4-1 exhaust with stock airbox and stock jetting by the way. Once warmed up it pulls smoothly through all the rev range.

I've found one carb leaks, and the petcock leaks in the on position. So I'll bet the fuel level in that one is to the max! 

Also noticed a bit of smoke today when I started it up. 

So we have at least to high fuel level on one and possible some oil burning. 

Weird thing is I watched a video yesterday I took from last week where I did a pull from standstill and it was freaking flawless. It's like it worked alright right after fixing the carbs and then it's gotten worse the more I drove it... 

3 hours ago, johnr said:

did you remove the emulsion tubes when you cleaned the carbs?

Yes. The where clogged as well. Had to poke out every single hole on every single tube... 

 

I'm starting to think it needs another clean out. Like something has loosened and clogged up something.

[johnr]

Did you clean out the fuel tank whilst you were at it? Don't assume a fuel filter will help, the fine dust from a rusty tank will go right through most petrol filters.

[et1170]

Have you done a compression test on the engine?

[Safra]

Have you changed your plugs between all this carb fiddling sounds like your plugs are toast 

[gorbys]

On 9/19/2023 at 1:41 AM, johnr said:

Did you clean out the fuel tank whilst you were at it? Don't assume a fuel filter will help, the fine dust from a rusty tank will go right through most petrol filters.

No I didn't, thanks for the info. 

On 9/19/2023 at 8:27 AM, et1170 said:

Have you done a compression test on the engine?

I was gonna but I didn't have the correct size adapter for the spark plug hole. 

On 9/19/2023 at 9:27 AM, Safra said:

Have you changed your plugs between all this carb fiddling sounds like your plugs are toast 

No. You're right, they might be bad. I remember I experienced spark blow out on my Datsun due to bad plugs now that you mentioned it. 

 

Anyways, I don't think I'll be fiddling anymore with it now. I don't have the time and space for it at the moment so I've put it up for sale. If it goes it goes, if it doesn't then maybe I'll come back to it next year when it's hot again and we get our three days of summer.