another first-ride: help me get my bike running well.

[MrSelfdestruct]

Hey,

I picked up a 1978 GS750 a few months ago. Since then I've:
-cleaned/rebuilt carbs
-new o-rings and gaskets in carbs
-new intake o-rings (boots are in really good shape)
-adjusted valve clearance
-new points
-new plugs
-adjusted timing
-adjusted float height
-new hoses all the way around (fuel/vacuum)
-rebuilt petcock
-a few other non-engine related things...

I also changed the rusty and old air box out for some emgo(?) pods (they flow better than stock airbox, but not as well as K&N's). To compensate for these I bumped the main jets up to #112's from stock #102's, and I raised the needle up one notch. Mind you, I live in Denver which is a little over 5000 ft elevation. Well I took it out for the first time today to see how the re-jetting worked and I'm a little confused. It feels and sounds lean at low throttle (1/8 - 1/2)and low rpm. I can hear lots of intake noise and power is relatively low. Once the revs come up to 4-5000 It feels peppier and sounds pretty awesome, but i'm sure there's more power to be had. I couldn't really wind it out because I'm in the middle of the city on mostly residential streets, so I don't really know what full throttle/high speed feels like yet. I pulled the plugs expecting to see white insulators typical of a lean mixture, but they're black except for the tip by the electrode which is a medium brown...almost coffee brown. So if it feels and sounds lean, but has mostly black plugs, what do I do next. What does an overly rich mixture feel like, I don't have much experience with that. I didn't really notice and strong exhaust smells while I was out. Any advice would be appreciated, but first, the stock air box is not going back on. It's in a bad way and It would be more trouble to make it work than getting these pods to work right.

 

[BassCliff]

Greetings and Salutations!!

Greetings and Salutations!!

Hi Mr. Selfdestruct,

It looks like you're doing a great job. The thing about pod filters is that it's not an exact science. Plug chops may be necessary to get the jetting right. Do you have a stock exhaust? I'm not sure if a Dynojet Stage 1 or Stage 3 would be better for you. I know that many folks here do not like the EMGO pods and like the K&N better. Many also prefer a new petcock rather than a rebuild kit. Your mileage may vary. It is quite possible that you can be lean on one carb circuit and rich on another. Perform plug chops to get a read on your plugs at various throttle openings. You'll find links to this information on my website. Let me welcome you to the forum.

If there's anything you'd like to know about the Suzuki GS model bikes, and most others actually, you've come to the right place. There's a lot of knowledge and experience here in the community. Come on in and let me say "HOoooowwwDY!"....

Here is your very own magical, mystical, mythical, mind-expanding "mega-welcome". Please take notice of the "Top 10 Common Issues", the Carb Rebuild Series, and the Stator Papers. Now let me roll out the welcome mat for you...

Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike!

Thanks for joining us. Keep us informed.

Thank you for your indulgence,

BassCliff

 

[MrSelfdestruct]

thanks for the mega-welcome. I've actually been lurking here for a few weeks and I hijacked someone else's mega-welcome so I read up on all sorts of things GS-related. Nice collection of info you've got there. As for my bike, it still has stock exhaust. It's gonna stay that way for a while. As for the emgo's... I can't afford anything nicer for the time being. The petcock actually works really well just as a rebuild. It was just sitting for a while before I got to it, so the rubber internals were dried out. The plugs are similar across the board. I tuned the bike up using a colortune spark plug, so according to those tests, all should be well: Nice bunsen blue flame at idle, changing to yellow upon acceleration then returning to bunsen blue when rpm stabilizes when you hold the revs at 3000. I'll try to do some plug chops, I just need to ride farther out of the city where I can go more than two blocks without a stop sign and I'm worried about riding too far as is If it is in fact lean. I also don't know much about jetting carbs (mechanical fuel injection is my strong suit) so I don't know when to mess with the pilot circuit vs. needle vs. main jet. I know the needle controls mid-throttle settings, but if I hold a consistent 1/2 throttle from a stop it runs like crap until I get to 4 or 5k rpm then it runs better. I don't know what that means.

 

[MrSelfdestruct]

I'll try to get some pics up soon btw.

 

[BassCliff]

Hi,

This info from Factory Pro may be of some help to you:

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


Thank you for your indulgence,

BassCliff

 

[Nessism]

Where are the fuel and air screws set? Common starting point for a stock bike is 3/4 to 7/8 turn on the fuel screw and 1.5 on the air screws. Also, did you check the voltage at the coils yet? If it's less than 10.5 volts you may need to do the coil/relay mod.

Good luck and thanks for doing your homework before posting here.

BTW, those Emgo pods shouldn't be oiled and can not be cleaned. You might want to consider APE pods once the Emgo's are worn out.

 

[MrSelfdestruct]

the air screws are still at factory settings (where the little marks are). I don't know exactly where the fuel screws are because I used a colortune plug to set those. I turned them out until the flame was yellow (rich) and then leaned them out just enough for the yellow to go away. Ignition is really strong on this bike, I guess I got lucky in that regard. I'm on the way to the parts store right now to pick up some fork seals and a hand full of main jets and I'll start there. That link that bass cliff posted looks like it might help - I'm assuming the principals are the same between cv and vm carbs.

thanks for all the help guys

 

[waterman]

If I am reading this correctly, you have replaced stock airbox with pods, and then increased Main jet with a 4 size larger jet and lifted the needle a position in effect richening mid throttle to wide open. You also live in Denver, with 5000 ft elevation.

I thought as one goes up in elevation, you need to lean out the mixture not enrichen it to compensate for the lower air pressure/lower oxygen concentration. I think this is why your plugs are showing that you are rich.

Likewise, the VM carbs that I assume you have for your 78 are more prone to bogging from low rpms, hence the reason that it does not run well from idle at half throttle. From my understanding of their operation, it is better to roll on the throttle as opposed to simply slamming them open.

I am sorry I don't have specifics for your situation, but it just seems that your bike may be running richer than you think. Likewise, it may not run as well as you may think due to your altitude.

 

[MrSelfdestruct]

Is that 4 sizes larger? I always heard to increase jetting in 10% increments. Maybe i'll throw the 102s back in and see what happens

 

[Skunky]

Not that this helps but I put EMGO pods on my 79 750L... didn't change the jets and it ran fine. But I'm also at .0005 ft above sea level (Maybe less with all the rain we've been getting) I didn't like the louder hollow sound it made so I put the stock box back on.

 

[BassCliff]

That link that bass cliff posted looks like it might help - I'm assuming the principals are the same between cv and vm carbs.

Hi,

Yes, the principles are the same. However, when you are tuning the VM carbs, do them in the reverse order of the CV carbs. In other words, start with "idle and low rpm" and work your way up to "full throttle".

This may help to: http://www.mikuni.com/pdf/vmmanual.pdf


Thank you for your indulgence,

BassCliff

 

[Big T]

Your best bet may be to find an airbox and return the jetting to stock

The 78 VM jetting was already on the lean side, so you should run good at 5,000' with the stock set up