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Jetting for rebuilt top end

  • Thread starter Thread starter JimmyR
  • Start date Start date
J

JimmyR

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I recently rebuilt the top end of my GS1100G's engine - new rings, honed bore, reseated valves with new seals. I haven't gone up a size in rings as the bore still measured in spec - basically just did the cylinder because it was available to do while I was getting the valves done.

So yesterday was the first time I have started the bike since all the work. It sounds different from how I remember, doesn't smoke on startup and even smells different now. But it seems to want bigger pilot jets. Is this normal?

I have Mikuni RS36 carbs with K+N pods and had #25 pilots in there. It was hesitant and poppy so I put 27.5s in there and it's a lot better. The 25s were good before the work but now I think I may have to go to 30s.

So far high throttle performance seems good. I have 127.5 mains in there but may go to 130s if I think they are needed. Why would I need to richen things up? I would love to understand why the jetting needs to be changed. Any clues guys?
 
Hi babes. If it isnt smoking and sounds good, why do you think bigger pilot jets are.needed?
maybe try running it in for a while and see what occurs in a few days... Cant you adjust the mixture screw to see if that helps some.

Good luck.
Xxx
 
So far high throttle performance seems good. I have 127.5 mains in there but may go to 130s if I think they are needed. Why would I need to richen things up? I would love to understand why the jetting needs to be changed. Any clues guys?[/QUOTE]

I'm running 132 mains on my 1100 using Mikuni 36's off an 1150. The main jet is changed to take advantage of high rpm power especially if you are running pods & a pipe & a Dynojet kit. The factory jets are meant to be a balance between emissions & power. I noticed a good top end boost when I installed the larger carbs.
 
Hey Seana - I think it needs richening because it's running like crap right now! Actually today I think I discovered that it might be leaking from the exhaust as it comes out of the engine, which would explain part of why it's running so crap.

Thanks Nastyjones - It's good to hear others experiences. I've found that for general riding the pilot jets seem to make much more difference than mains. I'm not riding at WOT very much!
 
Ok then! I don't need much encouragement. :-) I'm trying to keep the revs changing as much as possible. The engine is really good at higher revs, it's only crappy at lower revs right now. I have been changing down a fair bit because it runs so much better over 5000rpm.
 
You say you lapped the valves and put in new stem seals correct? Did you check the valve clearances after doing this work. I am not sure if lapping the valves would change the clearances at all or how much, just a thought I had.

Also did you synch the carbs after the rebuild? Mine ran bad after top end work until I vacuum synched the carbs. Makes a huge difference.

I cannot imagine it would have anything to do with jetting it all the intake and exhaust parts are the same before and after the rebuild.
 
If it was running well before then it won't need a jetting change just because you have rebuilt the top end.
 
Thanks guys. I found out why it was running so badly - leaking exhaust at the headers. So it gave symptoms of running lean (carb farting) but when I checked the plugs they were very black. Turns out all 4 headers had leaks.

So now I am back to my (pre-valve work) pilots at #25. Runs really well once I get past slightly open throttle but just open throttle is still crap - better but crap. Checked the plugs and now the tip of the electrode thing is going greyish. So I don't know whether to go up or down now or what! I'll work it out.
 
Did you adjust valves and re-synch the carbs after the rebuild?

Even if you left the carbs on the rack while doing the top end, the new rings, valve adjustments, etc. will throw off your vacuum synch.

Also, whether you pulled the valves or not, once you have disrupted the cams, you need to check and adjust the valve clearances after reassembly.
 
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