Running lean on 650GD...new main jets needed?

[BigD_83]

I let my housemate take the 650 for a quick run tonight. He says he never got the revs over 5000rpm. I've been a bit suspicious that I'm running lean because the new BUB 4:1 header is bluing a little, but I haven't had the opportunity to do proper plug chops.

I pulled the plugs and they were definitely showing signs of being lean. Brown, but with a tad of chalky white to them.

Here's the skinny:

Stock carbs in every way (main jets 110)
Stock airbox and filter (oiled)
Vacuum synch and highest idle done. 2 1/8 turns out on the idle mix, IIRC
valves adjusted

The bike starts up at first go with full choke and I'm off it in a minute or two.

There is no popping on decel, and I have not noticed any flat spots or surging in the throttle response.

One odd bit that I don't see affecting midrange but I'll mention it for completeness...once warm, it'll idle happily with no choke at 1100 rpm. Applying 1/2 - 3/4 choke the engine runs up to 4000rpm from idle (1100 rpm +/-). At full choke it'll struggle to keep going, and eventually die


Does this scenario sound like I should be bumping the mains up to 115 or beyond?

[tkent02]

Does this scenario sound like I should be bumping the mains up to 115 or beyond?

Hopefully before the engine burns up.

[koolaid_kid]

Yes, you need to try larger mains and do some plug chops.

[BigD_83]

Hopefully before the engine burns up.

Tom,

that IS the response I was expecting, and the bike is parked until I get this sorted.

I've found a great thread started by Duaneage when he was going through some rejetting due to the JAMA/Marshall 4 into 1 he installed. Opinions varied on whether a 115 is enough of a jump.


Is there anything else I can do before I can get new jets in to make the situation tenable?

Based on experience, are 115s going to be enough, or should I go to 117.5?

[tkent02]

I haven't jetted a 650 for a pipe yet, but usually the air box makes a bigger difference than a pipe. 110 is stock, 115 might be enough with the pipe and stock air box, maybe it will take more. Careful testing and doing plug chops will tell you. Just a guess, I might buy 120s, 117.5s and 115s to start with, and try the biggest ones first.

Please post your results, I need to do this with my 650 soon as I get the pipe on there.

Z1 sells them the cheapest.

[BigD_83]

I haven't jetted a 650 for a pipe yet, but usually the air box makes a bigger difference than a pipe. 110 is stock, 115 might be enough with the pipe and stock air box, maybe it will take more. Careful testing and doing plug chops will tell you. Just a guess, I might buy 120s, 117.5s and 115s to start with, and try the biggest ones first.

Please post your results, I need to do this with my 650 soon as I get the pipe on there.

Z1 sells them the cheapest.

Thanks for the input, guys.

I've posted up a wanted ad for jets. I was just on the Z1 website and didn't find the Mikuni jets. I'll have to give them a call next week.

I'll be sure to post up the results.

[7981GS]

Do you receive E-mails or PM's?

Eric

[BigD_83]

Do you receive E-mails or PM's?

Eric

I get both emails and PMs. I've responded via email.

[tkent02]

Z1 has them, just call. I think they are just under $2 each.

[Nessism]

I'd raise the needle in addition to changing the mains. Experiment with thinning the plastic spacer on top of the needle, either by sanding down the spacer or using a stack of washers with a thinner total height. Raising the needle will richen up the mid range.

[surviverguy]

I'd check "ATV Superstore" they had jets for my GS450 at $1.95 Ea.

[surviverguy]

enrichening the midrange

I've read and reread that carb tutorial and have two important questions- 1)Shouldn't the needle be raised to make the midrange richer? 2)To raise the needles shouldn't thicker spacers/washers be used?- The tutorial seems to say the opposite....

[Nessism]

I've read and reread that carb tutorial and have two important questions- 1)Shouldn't the needle be raised to make the midrange richer? 2)To raise the needles shouldn't thicker spacers/washers be used?- The tutorial seems to say the opposite....

Yes, raise the needle. To do this you need to reduce the thickness of the spacer on top of the needle clip. The needle has a spring under the clip that pushed upward, so to change the needle height you reduce the thickness of the spacer so the needle gets pushed up further, thus raising it.

[surviverguy]

Ok got it. When I had my carbs apart I didn't have a C clip tool skinny enough to remove the tiny clip down low in the recess. I wasn't sure how to remove the pointy plastic cover over the needle. Can you explain how to remove that plastic needle cover? Does the dammit clip , I mean C clip, have to come off first? I didn't want to damage anything.

[tkent02]

I would wait to see. With CV carbs, a bigger main jet will enrich the midrange as well, so there might not be any needle raising required.
Also a 4-1 pipe doesn't really flow more than the stock system until the engine is wide open and wound out, again it might not need any needle raising. It's easy enough to do later if needed, you don't have to remove the carbs to raise the needles or to replace main jets.