Jetting '78 750 for pods, headers only

[verde]

Hi everyone. My '78 bobber will soon be roadworthy and I'm getting antsy about it. Trying to finish before school semester starts up next week. Eventually plan on a more modest exhaust note, but will be running just stock headers for now. Also, I've read a few posts saying these bike aren't fans of pods, but too late for that now. Running crappy chrome ebay pods with end caps(don't flow as good as full foam uni's, I know). Maybe a custom 2 piece airbox will come with the exhaust mods later.

I understand these carbs are a little finicky, and due to the pipe situation I can't cheat like I did on my 2 other bikes and just weld a bung on for a wide band 02 sensor with accompanying air fuel meter Hence the too many questions I'm asking now to help me through a quick and dirty seat of the pants tune

Mains: Only found one thread in another forum that makes me think 125ish for starters on the mains. I have a large range of generic jets available by 5's(105, 110, 115 etc). Climate: sea level, san francisco

Pilots: whatever was in the rebuild kits, didn't order any others.

Screws: The dual adjustment screws are new to me... When I rebuilt the carbs I went with whatever base setting was in the old shop manual I got with the bike... I think 1.5 turns and 2 turns out for the bottom screw and side screw respectively. Any tips there appreciated.

Finally tips on the needles. Start in the middle, all the way at the top notch? Leave it at wherever the PO had it w/box and silencers? I know this is the last thing to dial in, but if they all generally end up in a specific range it'd be nice to guess at that and find I don't have to pull the carbs again.

Thanks for any help!

 

[Bennys78]

I've got exactly what you have above, 78 GS750 with pods and pipe. I've also started with similar adjustments except the 'bottom screw' is just under one turn out - varies depending on which carb. It runs, and it's ok - where it's going next is my local, where the owner did his apprenticeship on GS750s and GS1000s 30 years ago. After looking through all the threads on advice, I just wanna give it to someone else. If you can find someone similar and you have the cash - I'd recommend it and save your sanity.

 

[tejasmud]

Your carbs should be mechanical slide VM carbs. They accept pod filters without too much fuss.


But, Emgo pod filters, and from reading your post the exhaust is cut off.

Good luck.

I would start 4 sizes larger on the mains, needles 2nd notch from the bottom/tapered end pointing down, 1 up on the pilots, and adjust the screws using the highest idle method.

Assumung the valves are in spec, carbs are dipped and were bench synced.

From there spark plug color will be your best friend. Jet accordingly.

 

[BassCliff]

Greetings and Salutations!!

Greetings and Salutations!!

Hi Mr. verde,

After intake and exhaust modifications, carb jetting is a mystical voodoo black magic art.

Cheap pod filters make it more difficult. You'll find carb tuning tips and links on my little website. I agree with Mr. tejasmud and his recommendations, they will get you in the ballpark. Normally your carbs will use 7/8 turn out from lightly seated on the fuel screws and 1 3/4 turn out on the air screws. Adjust from there using the 'highest idle method'. Plug chops may be in order to really dial them in. CLICK HERE for the factory VM tuning manual. Now let me get to the business at hand, sharing some GS lovin'!

Yes, you'll find lots of GS lovin' on my little website. What you don't find there you can certainly find here.

I just stopped by to welcome you to the forum in my own, special way.

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

 

[Scudder]

I own a 78 gs750. I've been all over the place when it comes to pods, jets and carb adjustments. Carbs have been off my bike too many times to count. Started with emgo pods and eventually got uni pods and oiled them. Made a big difference. I have a 4 to 1 exhaust. I don't run standard muffler. I just use an exhaust tip that I bought at an auto parts store. It has some holes in it to kind of act as a muffler, I suppose, but it allows quite a bit more air flow than the muffler. It is pretty loud compared to what it was, but it runs way better.

If you really want emgo pods (they do look good I think), then a mechanic friend of my said that he has run a bead of hot glue in the creases of the filter to reduce some of the air flow. I've done something like this on a bobber that I have and it worked fine.

Just resently I found what works for my bike.

Needles: Stock
Fuel mixture screw: 1 turn out
air mixture screw: 1 1/2 turn out
Main Jets: 110 (Stock is 100)

Scudder

 

[BassCliff]

One other thing...

One other thing...

Hi,

I forgot to mention that with quality pod filters and 4-n-1, the DynoJet Stage 3 kit is pretty much "plug-n-play", but it's not cheap.



Thank you for your indulgence,

BassCliff

 

[rustybronco]

A lot of factors to consider as each pipe/engine mod/pod combo will be different. quite a few posts on this board with the info you need. the search function is your friend here.

with that said and 'only' down and dirty numbers... 125's, forth groove down on the needles, one turn from lightly seated on the fuel and two turns on the air screws should get you in the ball park.

chop, read the plugs and rejet/tune from there.

 

[verde]

Thanks for all the replies guys!

After looking through all the threads on advice, I just wanna give it to someone else. If you can find someone similar and you have the cash - I'd recommend it and save your sanity.

Sanity is long gone so I'll try my hand at it for now.

Your carbs should be mechanical slide VM carbs. They accept pod filters without too much fuss.


But, Emgo pod filters, and from reading your post the exhaust is cut off.

Good luck.

I would start 4 sizes larger on the mains, needles 2nd notch from the bottom/tapered end pointing down, 1 up on the pilots, and adjust the screws using the highest idle method.

Assumung the valves are in spec, carbs are dipped and were bench synced.

I was thinking they should take the pods fine since they're mechanical, but I found a lot of google hits of people returning to air boxes. Maybe they just didn't take the time to dial 'em in? I know the airflow around the engine affects the two twins I've done w/ pods, so I was also thinking this same effect could be worse on a 4 cyl and maybe that's what people were having problems with.

Pipes aren't cut. The '78 has separate headers that clamp into the y collectors on either side. I could see where you might think that though as I also have a '79 I picked up at the same time. That has the pipe all one piece from the finned clamp to the very back(and a funky cross-over underneath between the 2/3 pipes).

Valves were way out of spec... got to do my first real engine work on this project(just valve lapping/decarb). ...speaking of which, need to see about tips on running it around the block and then retorquing the head down if I can't find it in the shop manual.

After intake and exhaust modifications, carb jetting is a mystical voodoo black magic art.

This will be my third, so my brain can grasp some of the voodoo. I seriously love my wide band a/f meter, tells me so much more than seat of the pants or a dyno run can. Unfortunately don't have time or patience to build the exhaust on this bike right now, so I don't have a place to install it.

Great site you've got. Thanks for taking the time to put that together for the rest of us:clap:

with that said and 'only' down and dirty numbers... 125's, forth groove down on the needles, one turn from lightly seated on the fuel and two turns on the air screws should get you in the ball park.

Thanks for the down and dirty verification on top of all the other suggestions I got. That's a more refined version of about where I was gonna start

I'll let you guys know how it goes. Still wrapping up details. Might finish the wiring tonight. Still got to figure out tank mounts. Paint some pieces. Etc. Hopefully it's not raining this weekend.

 

[rustybronco]

Thanks for the down and dirty verification on top of all the other suggestions I got. That's a more refined version of about where I was gonna start

I just re-read your first post. stock headers? you mean the full factory exhaust is on the bike with pods added? your post said pods and headers...if that's the case 125's are way to big!!!! search for posts on re-jetting your '78 with just pods added and see what's recommended.

 

[verde]

Stock headers, sans stock silencers.

clear?


Edit: what I replied to tejasmud above:

Pipes aren't cut. The '78 has separate headers that clamp into the y collectors on either side. I could see where you might think that though as I also have a '79 I picked up at the same time. That has the pipe all one piece from the finned clamp to the very back(and a funky cross-over underneath between the 2/3 pipes).

Dunno if that clears it up any more. Both bikes exhaust systems certainly don't look aftermarket to me. Not sure when/what models suzuki put out with the different systems.

 

[tejasmud]

Yes, not aftermarket.

I see now.

From your first post it sounded like you were thinking of only running the down tubes ("YES" in green) without the mufflers ("NO" in green).

My words "cut off" may have not been the best choice of terms.

To be honest, if you have both sides of the mufflers it would be easier to jet the carbs for pod filters with the mufflers on the bike. Much less hassle. At least until you find a 4 into 1 header if that is the plan for the future.

Now, the carbs can be jetted to run just the down tubes with pod filters, but the lowend torque will most likely be lost somewhat. Might be some goofyness with the idle that can drive you insane trying to eliminate it as well.

The full stock exhaust and pod filters it will ride better in opinion.

E

 

[verde]

Yeah, but check the hard tail. Crotch of the collectors hit the new sections of tube. I know of some voodoo claims of an eye bolt in the tail of a straight piped exhaust can help with the back pressure for the lower end torque. Might try that. I saw one guy who had a strip of metal across his slash cut pipes which I assume was an attempt at the same principle. May just pipe clamp something of the same idea in place, that way I don't cut, weld, or drill into the pipe at all and it can still accept the clamps latter on.

 

[tejasmud]

So, you are saying the mufflers of the stock system has a routing issue due to the tubing of the hard tail conversion.

The carbs can be tuned to run with just the down tubes, but like I say there may and probably will be some additional headache doing so.

I have read about the "eye bolt" trick. Seems like it was basically about adding an obstruction to get the pressure pulse to change directions to aid in scavenging. aka "back pressure.

Sounds plausable in theory.

Similar to what GSXRs of today use with the valve in the exhaust system to incrase the low end torque.

Eye bolt is suppose to give it a little adjustability. I think.

The method if I remember correctly was an eye bolt with a washer welded to it, or possibly a bolt and washer, which would serve the same function.

At this point it is all a matter of experimentation with an assortment of jets, and some spark plugs. A pneumatic spark plug cleaner would be a good investment.

Without an O2 sensor it is all about reading plugs, one change to the carbs at a time, and patients.



E

 

[verde]

Took it on it's first test ride today! Starts up like a champ. Wants to fall flat on it's face if I try to move before the engine is good and warmed up. Think I'm a little lean just off idle, even once it's hot. How much can I do with the screws here or should I order up some pilots? Rode pretty good other than that. Will get some fresh plugs soon...

Had a weird not very consistent engine noise, timing chain maybe? Will post a video if I can figure out my new camera. EDIT: Engine noise video posted, here's the thread: http://www.thegsresources.com/_forum/showthread.php?p=1344527#post1344527

Here's my attempt at a non-invasive version of the eye bolt idea. Not sure if it helps or hurts at all.

 

[cowboyup3371]

What are the caps, for lack of a better word, on the ends of your pipe supposed to do? I don't quite understand the idea.

 

[verde]

What are the caps, for lack of a better word, on the ends of your pipe supposed to do? I don't quite understand the idea.

It's an attempt at a non-invasive version of this: http://www.nightrider.com/biketech/exhaust.htm Dunno how effective it is in my case. Might try to make the strips a little flatter to impede the exhaust flow more. Could also do the lollipops but on tabs that are pipe clamped in place at the end of the pipes. It's only for a little while anyway until I have time to rework the exhaust system(spring break probably).

 

[verde]

Getting Close...

Getting Close...

I would start 4 sizes larger on the mains, needles 2nd notch from the bottom/tapered end pointing down, 1 up on the pilots, and adjust the screws using the highest idle method.

Wasn't getting very far with the screws. Finally got some 20's in the mail and she's a lot happier Getting a rare cough back into the carbs at "just puttin' about" speed/rpms. I think #3 most frequently, so maybe a screw setting. Petcock vac hook-up is capped.

First attempt at a plug chop wasn't all that successful and was before the new pilots. Didn't get a long enough run before having to cut it before catching up to too much traffic. Plugs looked right out of the box. As I understand it modern fuel burns a lot cleaner so you can only really tell if it's pretty far one way or the other. My next move might be to bump up on the mains and see when I can start to feel it bog, back off from that and see how my cough is doing. If I'm still getting that putting speed cough then I'll try raising the needles. Sound like a decent plan?

 

[GabrielGoes]

What are the caps, for lack of a better word, on the ends of your pipe supposed to do? I don't quite understand the idea.

hes trying to create back pressure

 

[tejasmud]

Wasn't getting very far with the screws. Finally got some 20's in the mail and she's a lot happier Getting a rare cough back into the carbs at "just puttin' about" speed/rpms. I think #3 most frequently, so maybe a screw setting. Petcock vac hook-up is capped.

First attempt at a plug chop wasn't all that successful and was before the new pilots. Didn't get a long enough run before having to cut it before catching up to too much traffic. Plugs looked right out of the box. As I understand it modern fuel burns a lot cleaner so you can only really tell if it's pretty far one way or the other. My next move might be to bump up on the mains and see when I can start to feel it bog, back off from that and see how my cough is doing. If I'm still getting that putting speed cough then I'll try raising the needles. Sound like a decent plan?

When the screws don't seem like they are adjusting anything changing the pilot jet size is what you need to do. The low speed cough could be the valves need adjusting. Didn't see anything posted if the valves were in spec or not.

As to the "fuel burning cleaner" these days, I read that in a
thread where Dynojet replied to someone installing a jet-kit. Don't know the validity of that statement. Could be true in some locations on this big blue planet where fuels are a different blend. Here, I run regular 87 octane unleaded that has 10-15% ethanol, and with an AFR of 12.5:1 the plugs do color. Light brown in color on the base of the insulator like always has been my observation.

Anyways, sounds like you are on the right path.

E

 

[verde]

Engine was out of spec when I got it. I had the head off, lapped all the valves. First time doing valve work. Exhaust valves took about 3 times the elbow grease that the intake valves did, but they all looked nice and concentric even after their neglect. Mixing and matching 2 engines and an extra set of shims I got all but one exactly where I wanted them. #1 intake is the only one that was borderline tight, and that's not the side I'm getting my cough on most frequently.

If there's anywhere on this big blue planet that will have different fuel blends, I'm right in the middle of it... paying $3.40 a gallon. Might try some plug chops again later, but will continue with seat of the pants erroring on the side of rich for now. Thanks for the help so far.