Question Posted By: MeatandCheese on 5-13-2007
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||My bike is pretty much stock, except for cam, tappets, pushrods, and exhausst
|What's your question?
||When not under a load, my bike runs uneven and pops
||Two yrs ago I put a new Crane Fireball 316 2B in my bike, and it runs great at rpms > ~2400, but not when decelerating or when letting off gas (in the corners and when rpms drop). Then it misses and pops.. It idles great at 980, but letting the motor drop from ~2500 to below 2000, it pops and misses. Is this the problem of too big a cam, or could this problem be fixed with a different carb than the stock Keihin? It's always bothered me, but since I put in a working enricher assembly, its really annoying. My bike is fast, and runs great on the highway, but around town this popping/missing/running uneven I'd like to fix. Any help's much appreciated.
- First, check the manifold at the carburetor and the intake spigots for vacuum leaks. If you don't find any vacuum leaks - then what's happening is when you shut the carburetor throttle down, the higher RPM during deceleration runs the carburetor lean. Try this:
Remove the cap over the air/fuel mixture screw and set the screw at 2 turns out as a base setting. You should be able to get the best low speed performance and a smooth idle within half a turn from there.
Install a #45 or #46 idle jet. For most Evo's a #45 works fine but some do better with a #46 when cammed. Start with a #45 and work your way up. While you're in there check the float level height and adjust if needed. Usually the stock main jet works fine so leave it alone for now.
Next, use one .020" Yamaha needle shim, part #90201-03331, to shim up the slide needle. This puts it's taper in a better working range. Use a modified 3/8 drill, reground like an endmill, to remove a little material from the inside of the needle retainer so the needle will float a bit and not stick or cock in the emulsion tube. I'm not sure this is absolutely needed but do it as an added bit of assurance. After all is done and you've test ridden the bike to get it up to proper operating temp, check the idle and mid range operation with an EGA machine that tells if the air/fuel mixture is burning too lean or rich. Also, use a fresh set of properly gapped plugs to ensure the best readings.
If you ride WFO a lot, you may need to use more than one shim and tinker with the main jets (#180/185) to get the results you want. These mods should give you good performance all across the power band and decent gas mileage.
If you're still running the stock air filter...all the stuff you've added is being choked off by that air filter. Before you do any mods...install a K&N air filter system and don't look back! By: 47Knuckle-Dragger on: 5-14-2007
- Thanks, 47K-D...forgot to say I have a K&N air filter setup, with the breather routed down to a little filter forward of the trans.
I had this guy, who's a lot better than me with carbs(wow), etc., tell me that the drilling and the shimming is a lot of trouble. He said that if I had a mikuni HSR42 on it, it would be easy to dial in. Then another mechanic tells me the mikuni would be much harder to setup, but that an S&S 'E' would be easier.
From the look of the plugs, the bike's been running lean/hot, but after the working enricher assembly installation, I've fouled the rear plug a few times. I haven't done that since the bike was new--before air filter, and crank case breather; Then I ran it hard and hammered the cam, replaced that (plus tappets and one block) and have had this running problem since. Not enough to keep me from riding, but still irritating.
To me it's just intimidating. I really don't know if I'm the guy to be drilling on the carb, since it's the only carb I've got. I've also heard that by changing the plug gap, you can make a small adjustment to the mixture? Something about the time difference-distance the spark has to travel before firing? I don't want to get another carb unless it would really make a difference, and could fix this problem.
Thanks. By: MeatandCheese on: 5-15-2007
- i have a device over here in uk called a colourtune. you screw it into the sparkplug hole in cyl head ,then while bike is idlinh you can turn mix screw while watching the dial change colour to a rich blue and then your mix is spot on. dont know if they do them in the states but its so easy to use. By: biggles on: 5-15-2007
- "I had this guy, who's a lot better than me with carbs(wow), etc., tell me that the drilling and the shimming is a lot of trouble."
There is no trouble to it...it's a straight-up modification. "Trouble" is a word used by people that are either lazy, or don't know what the fuck they're doin'.
"He said that if I had a mikuni HSR42 on it, it would be easy to dial in."
For whom, you or him?
"Then another mechanic tells me the mikuni would be much harder to setup, but that an S&S 'E' would be easier."
Again, another opinion. I offer advice on another tech forum where guys constantly post questions about their fucked up S&S carburetors. The problem is...it isn't the carburetor per se...it's the guys that fuck with them before they get ahold of them. You go to the S&S website and you can download the instructions for installing an S&S "E". Can't say I've ever found anyone that followed those directions religiously. They usually get frustrated about three-quarters of the way through and end up with problems. I put 40MM Bendix carbs with the added high-speed external adjustment on all the bikes I build. They have a choke; an accelerator pump; an adjustable idle; an adjustable low-speed; and, an adjustable high-speed. No problems...EVER!
"I've also heard that by changing the plug gap, you can make a small adjustment to the mixture? Something about the time difference-distance the spark has to travel before firing?"
Someone's blowin' smoke up your ass. Changing the spark plug gap in a stock/near-stock engine does nothing but fuck with it. Any fine tuning that involves spark plug gap verses air/fuel mixture should be left to guys with thousands of dollars worth of air/fuel mixture test equipment that do that at the races every Sunday!
By: 47Knuckle-Dragger on: 5-15-2007
- I put 40MM Bendix carbs with the added high-speed external adjustment on all the bikes I build. They have a choke; an accelerator pump; an adjustable idle; an adjustable low-speed; and, an adjustable high-speed. No problems...EVER!
- I guess the last part of my post(#5 above) wasn't sent. Anyway, the Bendix you describe sounds like what I want. What's the price on the one you use on your bikes? And where do you get one?
Thanks for the answers. By: MeatandCheese on: 5-16-2007
- J&P Cycles sells the Zenith/Bendix caburetor. There's also a couple for sale on eBay, new. If you find one used, J&P Cycles also sells the high-speed adjustment kit, and rebuild kits. By: 47Knuckle-Dragger on: 5-16-2007
- trying to correct italics... By: 47Knuckle-Dragger on: 5-16-2007
- Thanks for the scoop on the carbs, Knuck. I printed out your directions and think I'll try the modification first, but not until I buy a Z/B carb. first as reinforcement. That's a good price for them in the J&P catalog, and I've been real happy dealing with J&P.
They readily took two primary covers and gaskets back, and even apologised for not being able to get me the correct one. Real good service with their techs and everything. It's nice dealing with people who give a damn..especially a big co. like that:) By: MeatandCheese on: 5-23-2007