terrible flat spot at 2500-3000

[KEITH KRAUSE]

Jeez. :? Sooty plugs, rpm's increase with engine heat, won't hold an idle under 1,500 rpm's...could be so many things.
The rpm's increasing with engine heat is the classic intake leak. I've seen some people do strange carb synchs and throttle cable routing that can cause high rpm's/hanging, but if your bike will start cold at 1,500 rpm's (which is already too high), and then the rpm's rise to say 2,000+ "all by themselves" with the only change being engine temp, then you've got an intake leak somewhere. This will have to be fixed before trying anything else. You didn't mention this before. This would be a reason why your bike won't idle under 1,500 rpm's too.
Sooty plugs could be simple needle/main jet problems, as long as you know the timing/spark is good and filters are good. But I'm still not convinced about these 750 carbs working well on this 1100 motor. You said before these carbs ran good on the better flowing 1100 motor. Now you have them on this motor, which should not flow like before. That means the carbs would have to be jetted leaner. I also still wonder about any jetting differences between the 750 and 1100, such as primary air jet, pilot air jet, needle jet, float levels, etc, that you haven't addressed. I feel there's more than just the typical main jet/jet needle position, pilot jet changes needed here.
Soooo, after the intake leak is fixed...
I do know I would first look up the carb spec's for the 750 and 1100 carbs. I'd note any differences, how these differences would effect this motor, and also if changes can be made. If there's really no significant differences, then I'd do a complete cleaning/o-ring check, float level check, bench synch, check timing, valves, then set the mixture screws and vacuum synch.
Then I'd test at 1/3 throttle position and see what the performance/plug reads say to do. After the needle's good, I'd test at full throttle for the main jet and get that right. Then I'd test at mimimal throttle position/cruising and get the pilot circuit right.
As I've said in other posts, get the basic maintenance things done first. Then you can focus on the jetting. In your case though, there's the 750/1100 carb questions that need to be checked out first.

 

[Anonymous]

Thanks for the reply Keith \m/

OK, I'll take a look at the specs for both model's carbs and see if there're any differences and get back to you.

What do you think about upping my main jet size? The stock jets were 112.5 I've got K&Ns and a Vance and Hines 4-1, should I put in the Mikuni 137.5 mains? I'm presently running dynajet 130s.

rockitgetiton!

Marcus

 

[KEITH KRAUSE]

You need to find the intake leak before anything else. Performance and plug reads will be effected.
As for main ideas, you said the plugs were sooty at last check, but you don't say at what throttle position, which matters greatly. Only after a full throttle run and chop test of the plugs and performance noted, could I make a suggestion. Your plugs could be sooty from the jet needle. You still have to fix the intake leak too.
By the way, DJ and Mikuni size their jets differently. Roughly, a 130 DJ main is about the same as a 122.5 Mikuni main. So going up to a 137.5 main is an increase of 3 full sizes (15).
If stock is 112.5, then I'd guess about 5 full sizes up (137.5) would get you close, maybe right. If you have sooty plugs, I doubt it's because of 122.5 mains. That's only an increase of 2 full sizes, not enough for K&N's and a VH pipe. Your sooty plugs are most likely the jet needle position. Test the jet needle at 1/3 throttle position. As with the main, do what performance/plugs say to do.

 

[Anonymous]

thx Keith,

I recieved my K&Ns today, put 'em on and rode around a bit, I didn't do the various throttle position checks quite yet, I'm trying to suss out a good place to do it, it's in the works, but I pulled my plugs and they're all the same color for the first time, a little black around the edges but tan in the center and all consistent from cylinder to cylinder so I think the emgos had quite a bit to do with my problems.

I pulled my carbs off, changed the mains and put 'em back on, making sure they're nicely seated in the manifolds and the clamps are nice and tight, the manifolds are new and there's no cracking inside or out, I don't know where else there could be any air leaks, the mixture screws are new with new springs, o-rings and washers, I'm starting to think It's a synch problem not an air leak problem.

I use the motion pro mercury synchronizer but I always get air bubbles in the mercury, is there a way to avoid this? Would this lead to an improper synch? It seems to me that it would.

If the plugs are all the same color it seems to me that it rules out an air leak but could still mean the carbs are out of synch, would this be a correct assumption?

I'm certainly going to check my plugs from the various throttle positions real soon, it seems to me that it's one of those things you almost have to set a day aside for but I certainly agree with you that it's crucial in determining the necessary adjustments for the various circuits, I'll let you know the verdicts once I manage to get it done.

thx again Keith

 

[KEITH KRAUSE]

If you take awhile to synch with mercury, you'll get condensation in the mercury. With the bike running, all you can do is quickly pull off the hose at the manifold/adapter tube and quickly re-install it. This usually helps and might take a few tries. If you can't get it all out and there are gaps, sometimes you can see where the levels would be without the gaps. Best thing is to get good at synching and not let the bike get too hot in the first place. As soon as you turn it off to cool, on re-start the condensation mixes in.
The way you describe the rpm gain, it does sound like an intake leak, but bad carb synchs can do similar things to rpm's. If you can't find an intake leak, best way to know is to synch the carbs and be sure they're right. Check the throttle plates for smooth operation. If the rpm's still rise as before, you know you have an intake leak.

 

[Anonymous]

First, I have no knowledge of the specific carburetor in question but have rejetted the gsxr 750 -90 carbs fitted on my slightly tuned gs1100. Changing from the stock 34 mm carburetors to the 38 mm (European) gsxr carburetors resulted in a large power increase. Upon using gs1150 rubbers and KN dual pod filters (2 carbs fitted to 1 filter) these carbs is an easy upgrade that I recommend.

A carburetor related problem on the GS engines in the range 1000-4000 rpm region is most likely not related to improper main jet selection. More likely your problem is related to improper pilot jet size, wrong needle position or improper needle-needle jet combination. From the description of symptoms, especially upon regarding the idle problem, I would recommend try using a smaller pilot jet.

Best regards Kristian