I'm a budding DIY mechanic and I'm trying to sort out this issue before I just throw bigger mains in. I'm in Denver, so I wouldn't think it'd run lean stock at this altitude but maybe each bike is different/I'm missing something totally obvious. If you guys have any ideas on what to check, I'm all ears and hands 🙌

I'm running lean (I think?) at higher rpm, i.e. sputtering and gasping. It's almost as if the rev limiter is being hit at ~10.5k 😧 I'm not having any of these issues at lower rpm, and in neutral she revs all the way up just fine.

​​​As far as I know the bike is stock except for a K&N air filter and a Yoshi exhaust.

I've cleaned the jets in Berryman's dip and sprayed everything out. Bench-syncd the carbs, checked for diaphragm leaks, everything appears good to the naked eye. Put in new plugs, too. They appear to be in good condition at a light gray.

I'm going to try a vacuum test next, but I've never done one before (I also tried to do a compression test but I couldn't get the nozzle to screw in more than two turns? It says hand-tighten only but I'm not sure if that would cause a compression leak).

Also, do I need to somehow connect to all four cylinders in order to check for vacuum or should I do one cylinder at a time? Would reading only one cylinder have a difference in vacuum to all at the same time? How the hell am I supposed to get an air-tight connection to the number 1 cylinder's screw port? 😡

And last question: would it be unreasonable for my bike to just need bigger mains? I'm not sure if it's the whole "every bike has their own personality" situation or if running lean at altitude means there is definitely a problem.

Thanks guys! I've attached a pic of the ports in question, with the red asterix denoting the port that I found uncovered about a week ago.

 

Nobody? Okay, TL;DR:

What are some reasons for sputtering at high RPM besides jetting?

 

Hi the 4 vacuum nipples you are showing should all be blocked, the one on the right evidently isn't and can be upsetting the running of you bike, when you synchronise the carbs they should be all connected to your tester, 1 carb will be the master and you sync the rest to that one, but you will need something like a Morgans Carbtune in order to do that, take a look in your workshop manual.

 

I know this thread is a a few months old but I found it while searching for some info. Just to clear something up for anyone looking, jets should be SMALLER for high altitude. There's less air at higher altitude so you likewise need less fuel to maintain the proper fuel-air ratio.

 

You're right - (proper combustion is a ratio [about 14.5:1 air to fuel], so less air means less fuel for a proper burn), but the OP acknowledges this.

Messing with jets is a last resort and would only be necessary if some dunderhead has been in there before you with too many spare parts and not enough brains. Vacuum leak or blocked carb passage is 100x more likely.

Pertinent question: Had the bike for a while? Been running fine in the past at altitude and now it's suddenly got issues (perhaps after sitting for a while)? Or is this something you just acquired and are now trying to sort out?

First things first: if you have a vacuum leak, you ought to be able to spray carb cleaner around the carbs and the rubber intakes and the intake side of the head. If there's a leak and you manage to get some carb cleaner near it, you'll hear the engine respond.

If you can't find signs of a vacuum leak, more than likely you are looking at removing and tearing down the carbs for a serious cleaning. Not terribly difficult, but getting the carbs off the head can be a PITA (hint: two-person job, one on each side of the bike).

Do some readings here: Dan's Motorcycle Carburator theory and Tuning Great website!


Annnnd...bloody hell. Just noticed the OP was in June. Well, I guess an update would be cool, but I doubt he's been staring at the bike all summer waiting for a response.