Rpm skyrocketing

[WelderDave]

so I got this bike running a week ago, idled fine for a little while then died, then used the choke to keep it running for a bit. Just wanted to see if it would run.

This weekend I started it up and the rpms just skyrocketed with no intent of coming down so I keep having to kill it before it gets out of hand. I have the emgo mufflers and the uni pod filters, so I jumped up a pilot jet and played with the mixture screw at 1.5 out, 2, 3, and 4, and it always does the same thing, so I gave it a rough bench sync with feeler gauges, even cleaned the carbs again checked float height, everything. I’m at a loss of ideas here.

The valves are within tolerance, only thing I didn’t check was the points but I don’t see how that would make such a change happen over a week of sitting, not to mention cause this problem where it just accelerated rpms on its own on first kick over.

Carb boots to the engine are brand new, everything is tight, throttle disconnected. I’d appreciate all the help! It’s a 78 xs400E

 

[WelderDave]

Alright, so I I did find that the carb boots were not all the way tight, don’t know how they loosened up, might need some blue lock tire on those. So it didn’t immediately rev all the way up but now it’s backfiring, I’m assuming the points to be the culprit for this. Never did points before, so not quite sure what I’m doing. Maybe there’s a topic on here for this?

 

[WelderDave]

So I was reading if the fuel mixture screw can cause the backfiring depending where you have it, but after messing with it, I’m still seeing the rpms slowly rise and backfire when I kill it.

Any input is much appreciated! I’m getting frustrated.

 

[spectra]

Have you sprayed starter fluid around the boots to make sure they are sealed 100% I had a brand new set on a 1100 and they wouldnt seal for crap. Had to double gasket them and then got them to seal up.

So main jets are the same? I know on my latest build I jumped one size up on both and it seems to run ok so far. Carbs have been sinced with a guage I have and did play with the idle mixture screw a little I think 2 turns out.

These little bikes can be temperamental at times.......

 

[xschris]

Sync with a manometer.

 

[WelderDave]

To my surprise I actually figured it out, was definitely a learning experience for me, never worked with carbs before, just fuel injection.

The stock pilot on the left was plugged so I went with the new 1 size up, since a lot of people had good luck with it, and it’s to big. I’m 1 full turn out and everything smoothed out and no more backfiring, spark plug said it was running really rich. I also am going to have to put locktite on the carb boot bolts because they came lose causing an intake leak. All in all, I learned a lot.

Based on what I was reading since I’m only 1 turn out on the mixture screws, I need one size smaller pilot which is the stock size, so I’ll be ordering that.

Appreciate the responses everyone! Hopefully someone else who may have this issue will find this helpful, I know I was getting frustrated.

 

[NewHavenMike]

The bike should stall if you screw an idle mix screw in all the way. If it doesnt stall, the pilots are too big. Im not sure if theres any sort of " over lapping" with pilot jets though.. If I set my mix screws at 1.5-2 turn out, I will get a backfiring upon deceleration. If I screw in all the way, it stalls.

You need to figure out what main jet you need to use first, worry about idling and mid range later.

 

[WelderDave]

I guess my thought process is, if I can get it to idle properly, I can ride it and figure out the rest. How are you supposed to drive a bike safely that doesn’t stay at a steady idle because the mixture is way off?

As long as I work on one system of the carb at a time, I should be alright. I did a lot of looking up stuff, everything I was reading was saying to work on one system at a time but didn’t give a specific order.

Up to 1/8 throttle is idle jet, 1/4 to I think it was 1/2 is needle setting, then from that it’s main jet. I may be wrong on the actual positions but I have it written down somewhere. You get what I’m trying to say, each has a spot on throttle so you know what system you’re working on.

Edit* I should also mention at 1.5 turns out it slowly accelerated on it own really high, but at 1 turn out it idles smooth with no backfire, idk why it is but I’m assuming because the butterfly valves were just bench synced for the moment it’s letting in extra air, I will be using the manometer I made first before I switch jets just to make sure, don’t want to switch out pilot jets if I have the one I need already in it just to use the manometer and find out I’ve the right pilots and go through the unnecessary hassle.

 

[NewHavenMike]

Well just keep in mind that if the bike doesnt stall when you screw a mix in all the way, theyre still too big.

Ideally, you want a nice open stretch of road where you can open up the throttle. It took me all Summer to get my bike tuned well enough to ride when i went through all this.

If the idle rises, youre still too lean. Back the mix screws out to 2.5 turns. Make sure the cable adjustment at the handlebar is not tightened. You should almost be doing this with the throttle cable disconnected actually.. i had the same problem with my bike and part of the problem was that the old cable was not returning to idle position.

Im not sure if anyone linked you the Factory Pro tuning guide?? This was supe helpful for me.

 

[xschris]

Poorly syced carbs will cause those issues also. These tunning issues are the reasons why I tell people to stick with a stock intake and exhaust. I took me less than 10 min to tune a bike and never had to touch it again. No flat spots, bucking, weird idling, and hits red line with ease. Yamaha knew what they where doing.

 

[WelderDave]

Well it’s my project bike. I’m just getting eager to ride. Throttle isn’t connected, had to order one as the original one was junk. Should be here by the weekend to put on, and I’ll be using the manometer, just needed to make sure the butterfly valves were close enough to idle somewhat properly to use it, so there is somewhat of s gap letting extra air in on idle.

I will double check and see if the bike does with mix screw all the way in first before I switch them out again. There’s gas I. The bowl now so the less draining I have to keep doing the better

 

[NewHavenMike]

So you have a 78. That has points right? Another problem for me was with the points. The mechanical advancer was sticking on full advance and that would make it idle very high. This was partially why I picked up an electronic ignition.. I ended up switching back to points early this year.

The swing weights on the mechanical advancer can get rusty/dry and start sticking. Youll have to remove the points plate and strip down the advancer and get everything clean and lightly oiled. I use this stuff called EEZOX, its a gun oil and comes in a little tin. It has amazing anti corrosion properties far beyond anything Ive ever heard of. You will soak your parts and let them sit overnight, then wipe everything clean and dry and youre good to go.

that's if you have points.

 

[WelderDave]

I might do that eventually, but everything in there looked pretty clean and the points plates look brand new, but useful information! Thanks.

 

[WelderDave]

So I screwed the mixture screw all the way in and it still ran, choke made it go way up, sprayed the carb boot area to check for air leaks, no leaks. However, since I’m taking the carbs out I’ll see where the needle is placed. If the needle is place all the way up (lowest notch) could that effect this? Gave it some idle, it backfires, and hangs for a while before coming back down, but it doesn’t dip low and come back to idle. Wouldn’t that indicate to lean though?

I thought if it was rich it would dip down and come back up and settle at idle?

 

[WelderDave]

I know I posted a lot on other stuff before here again, so here’s an update. Had a broken mix screw, previous owner did it, I was very careful cuz I knew they snapped, so I drilled it out.

Spend 2 hours trying to get it out with other known ways, none of which worked so I said f it, 1/16” drill bit and drilled it out. I’m gonna need a new mixture screw for that side, yeah hole is a bit bigger but nothing I could do. Far to much time and money into this bike, I can’t buy new carbs, I’ll just have to deal with it.

Engine still backfires and starts racing without throttle. Could be the broken mixture screw I’m guessing, didn’t have another to replace it so I put it back in.

One guy said he had to drill his out and his bike ran ok, not great, guess that’s what I’ll have to deal with for the rest of the season. Spent $100 in jets, so I’m not trashing these carbs however it did the racing thing before too and backfiring, I don’t get it.

Broken mixture screw all the way in, the left cylinder doesn’t get warm so I’m assuming it’s not firing which means the mix screw closes the port, I don’t know how far up from the slope the inlet is, but it must be sealed when screwed in all the way.

No air leaks, carbs have been cleaned 3 times, contemplates a carb swap but not many do this with success, contemplates selling the bike because I’m getting fed up and now I’m out of fun money. What can I do?!

 

[spectra]

Sounds like you need to work on the fun money situation all kidding aside I would look for another set of carbs if I was you. They are out there you just need to look. I see them pop up every once in a while.

I have a feeling that since you made the hole bigger thats just going to open up a whole different can of worms.......

 

[WelderDave]

Probably, lol but it’s all I could do, it was fudged anyway. Either case, I’m still 4 turns out now with 1 size up from stock and running lean, pilot is still to small choke only made a blip in the rpms coming down so I need to riches it up quite a bit.

idles at 4500 rpms lean, granted I have uni pods and the mufflers I have are half the length of stock and no baffles, so yeah I guess that’s to be expected. I wanna smell it running rich first then I can back it off. Hopefully I can kinda tune the carb I drilled out a bit differently so it runs somewhat close to the other

 

[WelderDave]

So I decided I’m gonna try the ones from mikesxs, the “performance” ones. Purchased and on their way. Some people say they’re junk, some are able to work them. So I’m gonna have a go, will let you all know how it turns out. Mixed reviews, but I’m fairly confident I’ll get them running.

Mixed reviews, but Saw the one video on YouTube, they fired the bike right up, may not be the same for me, but I’ll play around with them. Guy said they ran the bike lean but hadn’t changed jets. Never saw another video, so either they worked and he didn’t make another or they didn’t and he didn’t post another, lol. Guess that’s the thing with mixed reviews.

I’m determined to get them running.

 

[SoulMan]

If you want to get rid of you original carbs very cheaply let me know. I might be interested in trying some type of backyard fix on them lol.

 

[NewHavenMike]

Theres a great set of carbs for sale in the classifieds for an even better price.