79 gs850 4-1

[Jwhit89]

Greetings all,

I’ve recently acquired a Vance and Hines 4-1 for my bike. While removing the factory exhaust a bolt snapped off into the head. During my process of attempting to extract (still unsuccessful) I decided to toss the 4-1 on and start the bike to get some heat into the head. After about a minute the pipes got really hot and instantly turned blue. I haven’t had a chance to pull plugs yet, does anyone have any suggestions for base tunes on the vm26 carbs? I’m assuming it’s running stupid lean and i immediately shut it down. (Valve clearances good, new boots to head for carbs, had it tuned pretty good for stock pipes). I’ve got the two next size jets coming for the carbs to address the mains but was looking for pilot air and fuel base settings. Appreciate any feedback!

[steve murdoch]

Just installing a 4-1 exhaust system shouldn't require any rejetting.
Still got the stock airbox and filters or have you switched to pods?

[Jwhit89]

Just installing a 4-1 exhaust system shouldn't require any rejetting.
Still got the stock airbox and filters or have you switched to pods?

im stock air box, I’ve got no interest in going pod filters. The only piece missing from my air box is the little snorkel off the back.

[zed1015]

As already suggested , just fitting a 4-1 shouldn't require any jetting change.
Even fitting pods or open carbs , which has not been done , only requires going up on the main a couple of sizes ( alongside correction of the main air jet to cure off idle hesitation )..
The blueing or yellowing chrome is common on single skin pipes such as the V&H and is not really any indication of lean jetting .
The only indicator you get from exhaust discolouration is if one pipe is markedly different to the others which suggest that cylinder was running hotter or cooler and adjustment to the a/f ratio or valve clearances may be needed.

[rphillips]

Back in the early "80"'s V&H sent recommendations for jetting with their 4 into 1 pipes, some of them anyway. It's got to be on this computer somewhere, but I can't find it...Oh wait I just found it, looks like from a member here. Just punch in "This is the V&H literature included with their exhaust" on your computer & you'll see it. I knew it should be here somewhere. Good luck.

[Jwhit89]

Back in the early "80"'s V&H sent recommendations for jetting with their 4 into 1 pipes, some of them anyway. It's got to be on this computer somewhere, but I can't find it...Oh wait I just found it, looks like from a member here. Just punch in "This is the V&H literature included with their exhaust" on your computer & you'll see it. I knew it should be here somewhere. Good luck.

I searched and found the document you were talking about, it does not mention the 79 gs850 in the table, it references 80-82 gs850 and 2v 750s. This is where my confusion lies I’m not sure which one I should go with, the settings for 2v 750 or the 78-79 gs1000 (I believe they use the vm26ss) appreciate everyone’s input

[rphillips]

Just my opinion, go with the GS750 2V recommendations, carbs were the same but the 750 & 850 share the same air intake systems. The GS1000 has a completely different air intake system, with a removable lid that is recommended to be removed & that will affect the air intake vacuum pressure & jet sizing. They recommend for a 2V 750 stock mains were 100's, they advise go up to a 115, so that's up 6 sizes, I'd see what stock was on your 850 & go up 6 sizes from that. Also on 750 they advised for pilot, go up 1 size, find stock size pilot on 850 & go up 1 size there also.

[mattd1205]

The brass for the 1979 GS850 was identical for the GS750s built after January 1, 1978 to comply with the new emissions rules that went into effect for 1978 so I would think the pilot tuning would be pretty close to the post 1/1/78 GS750. The post 1/1/78 gs750s pilots were flow tested from the factory to keep the maximum allowable emissions at idle and just off idle in spec and as such didn’t have a “set” amount of turns for the fuel screw like the pre-1/1/78 carbs did. Suzuki did publish a TSB that indicated the pre-1/1/78 carbs had a set amount of turns for the pilot screws but that doesn’t apply here. Most of the post 1/1/78 GS750 carbs I’ve dealt with were never monkeyed with and still had the tamper evident paint intact. I’ve noticed these carbs seem to have “indexing marks” on the piece of aluminum that surrounds the pilot fuel screw (visible in attached picture). These indexing marks have lined up with the slots on the pilot fuel mixture screws on all of the un tampered carbs that I’ve run into over the years. These later style carbs generally had their pilot fuel screws turned out ~1/2 a turn out ±1/8. There’s also indexing marks on the side air screws but those were usually set at about 1 turn out. I don’t have any literature to back this up so these are just my observations working with these stricter emissions carbs. It just seems like more than a coincidence to me but I’m not an expert. I’d probably find out if you’re actually running lean first before making any changes and if you make changes, thoroughly document how the carbs are set-up with pictures so you can easily go back to where you were. A colortune would easily verify your pilot settings provided you follow the instructions to a “T”. As mentioned earlier, it could just be that the headers are single-walled and discolor easily.

Matt

[Jwhit89]

Little update, ended up ordering 115 mains and a 17.5 pilot, tossed em in and it's going pretty good! Still trying to fine tune the pilot jet as I'm getting a slight hesitation at 1/8th throttle like when cruising at 60 in 5th gear. Anything outside of the pilot circuit and she's an absolute animal!