920cc GS750 build - head gasket, deck height, valve sizes, cams, ?'s

[Steve]

Now looking for a machine shop in town well versed in motorcycle engines to bore the cylinders out...

If you have not yet found your machine shop, I would suggest that you "go west, young man."

Give Pearson Racing a call at 937-839-9723. He is about an hour and a half west of you, in West Alexandria, oHIo, and is very highly regarded in the racing community. He even has his personal GS sitting in the shop.

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[Guest]

I checked. We have a stack of about 25 copper head gaskets. Can't tell if they are 750 or 1000. Can't remember if that even matters. They are 75 mm.

 

[TeamDar]

I checked. We have a stack of about 25 copper head gaskets. Can't tell if they are 750 or 1000. Can't remember if that even matters. They are 75 mm.

They will fit GS750/850/1000/1100 8 valve models. Only difference would be the cam chain area o ring access.

 

[49er]

Yes stock 850 needle jets (0-2), but running 17.5 pilots.

I tried 108 mains, but the engine richened to 12.3-1 at 9000 rpm. This also dropped 2 hp off the peak.

Some say I run too lean, but "there's a fine line between pleasure and pain".

I'm tuning the 29 Smoothbores to the 894, and then I'll fatten them up slightly. They will then be close enough to fire up the 1023 and get it bedded in as quickly as possible. I'm a strong believer of the Motorman running in method.

The 29's aren't as sweet in the midrange as the 26's, so I'm fitting a set of 5DL36 needles, 0-4 jets and leaning up the mains for a trial. Hoping to do tests over the next couple of weeks. Plug checks and the butt dyno indicated a slight richness throughout the rev range when using the stock 5DL31 needles and 0-6 jets.

Well..........the experiment using 0-4 needle jets on the 29's wasn't very successful. The engine wouldn't pull once the needle started taking over. Besides being only .002" less clearance around the needle, it made a huge difference. I refitted the 0-6 needle jets, set the needles at 5DL36-2 and ran 112.5 mains.

The result was a great torque improvement in the mid and upper rev ranges.

I'm not happy with the pilot circuit...........it's either too rich or too lean for some reason. After reading many tuning attempts on these carbs from the archives, it appears they don't perform well in this area anyway, having been designed strictly for high performance applications. I've rechecked the condition of my float needles, the floats and their heights. The heights were set at 23 mm (off the gasket). I will try 25 mm. The needles have light wear rings showing (cant feel a ridge when rubbing a finger nail across them), so that combination may be causing a flooding condition.

After returning from a ride, I left the petcock on prime for 15 minutes while taking a phone call, and found a nice pool of fuel on the garage floor. No oil contamination though, so not all bad. I guess that tells me that the needles really do need replacing.:-\\\

After fitted the 26's back onto the 894, I wonder why I'm bothering with 29's. Their airflow velocity is obviously superior throughout the whole rev range, giving a strong even torque progression.

Thinking of boring my spare set of 26's out to 28's as an experiment for the 1023 engine.

 

[Chuck78]

Well, I am paying very careful attention to your tuning trials & errors so that I can get the best tuning possible on my 920cc gs750 when the time comes to swap piston & bore the 850 jugs & rebuild the carbs to match. Thanks a lot for sharing your findings!

My bike, even with dirty carbs & old o-rings, continually impresses me on theway it performs with the wiseco 844 kit. Above 70mph, the acceleration is just "adequate," but up to that speed, the bike's acceleration feels absolutely monstrous! 0-60 is insanely fast, I cant imagine how much torque I'll have going to a 10:1 920cc engine with really dialed in carbs, k&n filter, & HEI ignition!
Looking for wider alloy rims 3.50/3.75/4.25 rear for a 140/70/18 rear tire & 2.50x18 from for a 110/90/18. Got ninja twinpot dual calipers & cbr900rr rotors, ninja adjustable lever & master cylinder, going for sonic or progressive front springs & probably Hagon rear schocks, & gs1100e box aluminum swingarm. Really trying to make this bike a real performer all around.
Having trouble finding a 36 spoke 3.50"+ rear rim in an 18, but I find a lot of 3.50x17 supermoto rims, & some gs1100's apparently had 3.5x17 rims

 

[tirebiter]

49er,

Any chance you have a chart that shows the needle profile for the 5DL36-2 ? I can't find one anywhere. In fact i can't find a profile chart for any of the older Mikuni needles.

Considering a swap from the stock 5D50 in my '80 GS850LT. Hoping to remedy a slight hessitation just off of idle and have made numerous jet changes already to great effect but still looking for better.

Searching for a needle profile that richens slightly more but just at the top of the needle than the 5D50. Any other suggestion are very welcome.

 

[Chuck78]

It's on a german language pdf file which is something like a 50 or 100 or 200 page document...big file. I cut and pasted together a jpg of the pertinent ones for my application and loaded them onto one of my threads somewhere on GSR... i don't think I have it on my computer anymore. I will dig for a minute but you can search under my username for jet needle and 5DL36. FYI - the "-2" at the end is not a part of the part number, but rather the slot position that clip goes in to set the height up or down in the slide.

 

[Chuck78]

http://www.thegsresources.com/_forum/showthread.php?210977-VM26-needle-swap-modified-GS750

 

[Chuck78]

The needle style that fits the VM26 may be completely different from the 1980+ BS32's, particularly in overall height, so you may need to dig up that entire pdf chart mikuni needle reference listing and search for different needles. They are expensive, typically like $15 each, so you are almost better off just buying a dynojet kit for your particular model bike so you get a performance needle that fits your carb and is designed for your bike's approximate displacement.

I have been searching for the german link on here in all of my old posts with no luck so far.

 

[tirebiter]

Chuck78,

The link is in post #10 here:

http://www.thegsresources.com/_forum/showthread.php?210977-VM26-needle-swap-modified-GS750/page2

and it still works after registering. Downloaded the file, just now. Thank you MUCHLY !

I hear ya about getting the dynojet kit. If only they published their needle profiles but I imagine that info is probably a well-guarded secret.

 

[Chuck78]

I posted that earlier but it was of those times where I couldn't get any forum page to load, only getting an error page saying that the server is too busy. Looked like it posted to me, strange, not here now.

I also mentioned in the missing post that the needles for the VM26 carbs probably won't work on your BS32's because the 32mm tall CV carb venturi's are taller than the 26mm vm's. You'll need to look up height on your stock spec needle, or total length rather, and find one of that spec with notches in the same spot, and a skinnier taper.

I also had mentioned that the Mikuni needles run like $14-17 each, so a Dynojet kit for your specific carb and bike will give you some agressive tapered needles and extra jets to play with for a slight bit more cost but definitely will be a good taper needle with no guesswork, very minimal chance of getting their needles and saying "whoops, way too rich now!" as compared to picking a Mikuni needle and then the harder part, trying to find one or find out what bikes they came on and source that. Some of them are darn hard to track down. Cheaper route would be Mikuni's with a lot of research and knowledge of exactly what kind of taper your bike setup is going to run best with, but there are a lot of variables there.

 

[tirebiter]

Yes, it's not easy to make the correct choice but I have done considerable work with similar carbs on automobiles so I already have a good idea of what's ahead.

One thing that worries me already is an apparent discrepancy. I measured my 5D50's that are in the bike as stock. The chart is nowhere near what I measured.

Apparently I need to study more on how to look at the chart ! Definitely off-topic here. I'll start a new thread once I figure out what the chart is telling me ... or not.

 

[Chuck78]

keep in mind that unless you want to change your needle jets also, keep the upper part of the needle you are shopping for very near the exact diameter. any skinnier at the thickest points at the beginnings of the taper, and you will run very rich at low throttle/slide opening positions, and it will not be adjustable in any way other than changing needle or needle jets. a skinnier needle or a larger needle jet do the same thing, more fuel. A wideband o2 AFR gauge setup for $160 really saves a TON of tuning time (doing plug reads and plug chops), hassle, and guesswork. the initial diameter is the most critical thing to get correct, as you can move the taper around by changing the needle height, but you can't change the base diameter to make it run more lean. The incremental differences in diameter are so tiny that sanding one with fine grit emory cloth would be far too inaccurate to try and richen a lean idle and low throttle openings condition.

 

[tirebiter]

Chuck78,

I started a new thread regarding the Mikuni jet needle chart. Feel free to chime in with any info you want to share.

http://www.thegsresources.com/_foru...Mikuni-jet-needle-chart&p=2169319#post2169319

 

[tirebiter]

A wideband o2 AFR gauge setup for $160 really saves a TON of tuning time

Oh YES !!! But where do you find one for $160.00 ? Have they come down THAT much ? Last I looked it was in the $350.00 range.

 

[Chuck78]

I posted in the product reviews. There are a few AEM and Innovate brand options under $200. Look for a post from last year in there by me on wideband's. Only difference I would look for now is maybe one that just had a small LED readout panel that I could incorporate permanently into an LED light indicator lamp panel so I could leave it on full time. There are a few companies that make those. Having one transferable to multiple bikes is very nice, however.

 

[tirebiter]

Found it. Thanks again Chuck78