Carburetor and Fuel System
Title: Linkert
adjustments
It wouldn't run without choke on initially, and was really rich on the high speed. I began with the suggested baseline starting point (1.5 and 2.5? Can't remember right now, I'm not in the shop) and worked from there until it wasn't smoking or stumbling when on the gas. I went for a ride last night, and all seemed good but I think I'm not quite there with my settings. This is not my forte and I was hoping for a little advice to help me dial things in rather than just clicking away blindly.
My two observations were;
1. When maintaining a cruising speed, barely on the throttle (40ish) there was an almost imperseptable stumbling. None at all when I accelerate hard, and no smoke during acceleration. Seems to cruise along good otherwise.
2. Back in my shop afterward, when I rev it up, there is no stumbling, but a slight popping on deceleration.
I think I'm only a couple clicks from being in my happy place, but would like some input if you could. Like I said, this is not my specialty and I would rather get closer than further from my sweet spot. I suspect the popping on deceleration might be a rich setting on the low, and maybe the high is starving slighly when cruising at around 40?
I'm gonna go for a short ride with new plugs and check them, sorry didn't do that last night. And I made sure my timing was bang on before anything else.
Post by 108 on Aug 15, 2006, 2:12am
Panfreak, I'm no expert but I know from mine and others I've adjusted that... the idle will sound better and throttle response off-idle will be better if the idle mixture is a little on the rich side. That slight stumble sounds like a lean missfire because at 40 in high gear , your running on the idle circuit. Here's a test- at a normal stable idle , if you can twist the throttle wide open real quick and it revs right up with out poping back or cutting out , the idle mix is aboput right. About the only way to get the high speed right is to start out rich , run through second or third gear wide open to see how it feels, turn it down two or three clicks , and and do it again until you get the strongest and highest acceleration.
Post by ozwick on Aug 15, 2006, 6:28am
I have found that popping on the deceleration was a lean condition on the low speed screw. Open the low speed screw until it goes away.
Post by panfreak on Aug 16, 2006, 5:02pm
I experimented on a good ride last night to get this figured out. When I gunned it around 40ish it was sluggish to respond and just felt like it was getting too much fuel as it would smooth out a bit when I let off. I tried two clicks down with the high speed needle, it was BETTER but not quite there. Two more, much better. One more and it now accelerates hard with no popping or smoke. Five clicks? Man I was rich.
Post by Cotten on Aug 17, 2006, 4:27am
All should please remember that the M74B operates its highpeed circuit off of a fixed metering orifice (mainjet) screwed into the bowlstem, and the needle only trims a minimal excess, about the difference that a mile in altitude makes. Thus, the HS needle has often been dubbed "The Denver Jet".
Normally you will find a #18 or #19 installed, but only trial and error will determine if either places your HS needle within a sensitive range. The things are so forgiveable that the jet can be plugged and still be supplied by the needle. Yet the idle circuit must supply both fuel and air as well to balance out.
It is often normal for the HS needle to have very little effect, so Panfreak's is dialed in most sweetly!
....Cotten
Post by jellero on Aug 20, 2006, 2:53am
cotten or anyone, do you know what the #18-#19 jets convert to in drill size numbers and which is the leaner? my plugs are black and i'm fouling plugs. my hs needle is shut tight. live at 7000'. i have a #19 which seems to be a #43 and took it down to #46 today, helpful but still black plugs. idle and starting are great with new plugs.
and how would a manifold leak cause richness? j
Post by Cotten on Aug 20, 2006, 2:24pm
I haven't the # drills handy, but Palmer lists the #18 jet as .0795" and the #19 as .082".
Manifold leaks vary in their own leakage as RPM, temperature, etc., vary.
Therefor the mixture must often be set over-rich to accommodate it at its leanest condition, sooting the plugs under other conditions.
....Cotten
Post by jellero on Aug 20, 2006, 6:42pm
hmmm... well .08" is pretty close to 5/64" which is real close to a #46 bit which i used yesterday and as i said, still way too rich. the float isn't close to hitting anything and didn't have any gas in it i could hear (brass float). float valve seemed to work fine so i'm stumped. think i will put some miles on it today since it stopped raining, then check again. j
Post by Cotten on Aug 21, 2006, 12:16am
Brass float?
How much does it weigh, and how deep did you set it?
.....Cotten
Post by jellero on Aug 22, 2006, 7:34pm
i just pulled the float bowl again and measured 7/16" from lip of bowl to float, manual says it should be 1/4". if i bend it up to 1/4", that will let even more gas in. not good. j
Post by jellero on Sept 1, 2006, 4:45am
well i moved float to 1/2" and re-jetted to a #51, then a #54. 51 was still rich, 54 requires two turns on high speed needle so i am set for the mountains and finally have some control. am i the only one at altitude?? bike is running better than ever now, my friend again. j
