Main jet swap

[Guest]

So I have a 82 gs550m that has V&H 4-1 exhaust with a K&N air filter in the original airbox. The main jet size in there now is a 97.5. The factory size is 92.5. Does anyone have a similar setup on their 550 (4-1 exhaust with K&N in original air box) and if so, what size jet are you using? If I want to make it run a little richer will the main jets from my 78 gs750 work? So basically swapping from vm carbs to cv. The main jet from my 78 gs750 is 100 and they are not being used. I would rather it be rich than too lean. I haven't done a plug test yet, but from pulling them today after a normal ride, they look white, no color on the tip of the porcelain at all.

I have looked at the tutorial on how to do the plug chop, but I read on a link that you actually look at the rim of the threaded part of the spark plug as well as deep inside the porcelain part to see if you are getting any color. I had never hear of this before, I always thought you looked at the porcelain for the right color. It seemed that from the article I read, the tip of the spark plug wasn't an accurate way of testing a lean or rich mixture.

[Steve]

I don't have a bike with the configuration you are asking about, but can give you what I understand as some general guidelines.

Depending on how restrictive the exhaust is, add 2 to 4 sizes to the main jet.

For a stock airbox with a K&N insert, add 1, maybe 2 sizes.

For pod filters (individuals), add 3 to 5 sizes.

So, for your situation, I would guess you need 4 or 5 sizes up from stock. Since you should have had 92.5s in there, I would guess 102.5 or 105.
I don't think the VM carbs use the same jets as the BS carbs, but I have been wrong before, and your 750 carbs should have 100s in them, anyway.

.

[hampshirehog]

Those mains might be alright - you can't tell diddly from your normal ride today. Chances are you rode on the pilot circuit and you need to wind out your mixture screws. You need to do proper plug chops.

[B-rent]

I have tried looking for different options for the main jet and all I can find is a stage 3 dynojet kit. This might be a little overkill, and I'd rather not spend the 100+ dollars on trying to get it to run a little richer. I couldn't find any jets on z1enterprises web site that were specific for my 550. Does raising the needle help solve a lean mixture at 6k rpm and higher?

My 78 gs750 has pods filters, and the PO put harley davidson mufflers on it, So I put bigger jets in it. The 100's are what I pulled out of there.

[B-rent]

I did test the plugs. I took it up to around 8k rpm for about 5 seconds. Is this enough time to get a reading, or should I have held it there for a bit longer? The lower rpm reading looked the same as the high rpm. Both showed no carbon buildup at all. The porcelain is still just about as white as when I bought them a few days ago. The top of the metal rim where the threads are shows some carbon, but not any black buildup at all.

[Montanaman]

8 seconds isn't long enough.

When I was jetting for power, I'd put in new plugs, stick a plug wrench in my pocket, hit the abandoned blacktop, hold it wide open for about a mile, shut it down, and then pull the plug.

That's how I discovered that the '83 was very willing to hit redline in top gear -

That's for WOT, as far as the mid-range jetting goes, you just have to see how cleanly it carburetes, no burbles, or lean stutters.

[B-rent]

8 seconds isn't long enough.

When I was jetting for power, I'd put in new plugs, stick a plug wrench in my pocket, hit the abandoned blacktop, hold it wide open for about a mile, shut it down, and then pull the plug.

That's how I discovered that the '83 was very willing to hit redline in top gear -

That's for WOT, as far as the mid-range jetting goes, you just have to see how cleanly it carburetes, no burbles, or lean stutters.

I was afraid that wasn't long enough. I'm not sure I can find enough pavement around here without a stop sign or something else to slow me down. I hate the traffic here in Provo. Looks like I will have to get out of town far enough to do a proper test. The bike didn't have any complaints about going to 9k rpm, almost like it had been made to do just that.

[hampshirehog]

For WOT testing you can always pull off a plug lead and let the thing run on 3 cylinders - that way you can avoid going too fast and attracting the unwanted attentions of the rozzers.