Light Throttle/Cruise Carb Issues

setexascustoms · Apr 3, 2014

My '85 has some issues at light throttle/cruise especially in higher gears/heavier load. The bike is stock other than the 4 into 4 Cobra slip-ons. I pulled the carbs and cleaned them, replaced a coast enricher diaphragm that was bad, jetted it down to 147.5 base on Sean's advice due to the exhaust and set the floats based on the dry method using the casting circle in the jet block as a guide. It also has new jet block gaskets and emulsion tube o-rings and was synced when the carbs were installed. I have checked for vacuum leaks and can't find any. Once you get over third gear any where between 2,500 and 5,000 it's pretty choppy at cruise, and when you open the throttle it has a short surge of power then goes flat for a moment until you get the throttle open further. At WOT it pulls strong and runs good on the top end. Idle quality is also good. Any ideas? I'm at a loss. Just because there laying around tomorrow I'm going to try the stock jets to see of that makes a difference. Thanks for any help!

one2dmax · Apr 3, 2014

You can also try adding a washer (to richen) between the clip and retainer. Not sure if this will help or hurt it more.

dannymax · Apr 4, 2014

one2dmax said:
You can also try adding a washer (to richen) between the clip and retainer. Not sure if this will help or hurt it more.

+1 Was thinking the same thing.

tugla · Apr 4, 2014

checking wet method for float height is a good idea....

1st Shirt Maxer · Apr 4, 2014

So my experience with this is that the cobra slash cuts suck. They were on my vmax when I bought it. Since I have no way to dyno tune my bike, everything I do is based on feel and the tremendous amount of experience on this forum. My bike was a rescue and needed some work to get her running. After a deployment, I had cleaned and rebuilt my carbs, forks, brakes and clutch. Once I got the bike running I synced the carbs and started down a 2 month aggravation trying to find the cause of a low speed ill running bike. It spit and backfired, not smooth at low speeds and felt like it layer down at WOT runs. After reading a lot of posts about how the cobra exhaust robs power due to lack of back pressure I purchased a set of the supertrapp slipons. I installed them with 6 disks and with no other changes felt and heard an immediate difference. The low speed surging was eliminated, the bike pulled hard all the way to redline and I felt the power band was much steadier. Again, I did not get into messing with the carbs to try to fix it due to the time it takes to get anything on Guam so I just paid for new slip ons. Just my two cents and hope you get your bike running like you want it.

mabdcmb@yahoo.com · Apr 4, 2014

I would try raising the needles 1/2 to a full clip.

Did you use OEM emulsion tubes and jet block gaskets and emulsion tubes or generic parts out of a rebuild kit. There have been some issues with generic rebuild parts. I believe Keyster was one of the off brands with issues.

I would think your main jet is in the right area with cobra slip ons.

Traumahawk · Apr 4, 2014

[email protected] said:
I would try raising the needles 1/2 to a full clip.

Did you use OEM emulsion tubes and jet block gaskets and emulsion tubes or generic parts out of a rebuild kit. There have been some issues with generic rebuild parts. I believe Keyster was one of the off brands with issues.

I would think your main jet is in the right area with cobra slip ons.

Unfortunately I think that he has stock needles. These might be kinda high, but Ive used these on my bike, and they are very accurate.

http://www.exhaustgas.com/ProductDe...entID=0&CategoryID=0&MenuID=&BasketID=&RepID=

I bought 2 sets of shim kits to do all of my carbs

dannymax · Apr 4, 2014

Don't know if they still sell them but Radio Shack sold a little bag of washers for something like $1.95....a little poking thru and I always found what I wanted.....018", .015", .020", whatever.

They were stamped, not ground so a little work with a mill file or emory cloth was required.

setexascustoms · Apr 5, 2014

[email protected] said:
I would try raising the needles 1/2 to a full clip.

Did you use OEM emulsion tubes and jet block gaskets and emulsion tubes or generic parts out of a rebuild kit. There have been some issues with generic rebuild parts. I believe Keyster was one of the off brands with issues.

I would think your main jet is in the right area with cobra slip ons.

The parts are OEM, I got them from Sean. I put the stock jets back in and it's better, but still not right. I think the 147.5's were probably right with the exhaust and the richer jets are masking the problem. I'll check the floats wet, I probably should have before now. I went over it again for vacuum leaks, none I can find. I shimmed the needles with some washers I had laying around, just to see if that would help. It may have, but is was not a significant enough change to be sure. The problem is definitely affected by load. In first and second gear all is well, third you can just feel a slight hesitation, and forth and fifth are pretty bad. I pulled the plugs and checked the gap. They're NGK and look new, and the ground electrode is a medium to light tan. Any suggestions or problem ares to check are greatly appreciated!

setexascustoms · Apr 5, 2014

1st Shirt Maxer said:
So my experience with this is that the cobra slash cuts suck. They were on my vmax when I bought it. Since I have no way to dyno tune my bike, everything I do is based on feel and the tremendous amount of experience on this forum. My bike was a rescue and needed some work to get her running. After a deployment, I had cleaned and rebuilt my carbs, forks, brakes and clutch. Once I got the bike running I synced the carbs and started down a 2 month aggravation trying to find the cause of a low speed ill running bike. It spit and backfired, not smooth at low speeds and felt like it layer down at WOT runs. After reading a lot of posts about how the cobra exhaust robs power due to lack of back pressure I purchased a set of the supertrapp slipons. I installed them with 6 disks and with no other changes felt and heard an immediate difference. The low speed surging was eliminated, the bike pulled hard all the way to redline and I felt the power band was much steadier. Again, I did not get into messing with the carbs to try to fix it due to the time it takes to get anything on Guam so I just paid for new slip ons. Just my two cents and hope you get your bike running like you want it.

Thanks. I do plan on changing the exhaust, but I'm on a tight budget (baby on the way). So if there is a problem in the carbs that can be remedied for less money that's the preferred method for now. Can anyone else chime in on the exhaust? Does everyone who runs Cobras have issues?

mabdcmb@yahoo.com · Apr 7, 2014

setexascustoms said:
Thanks. I do plan on changing the exhaust, but I'm on a tight budget (baby on the way). So if there is a problem in the carbs that can be remedied for less money that's the preferred method for now. Can anyone else chime in on the exhaust? Does everyone who runs Cobras have issues?

I wouldn't say problems. They are a popular exhaust for the vmax and the bike can be made to run ok with them. However, they are the worst performing exhaust available. You lose power compared to stock.

setexascustoms · Apr 7, 2014

[email protected] said:
I wouldn't say problems. They are a popular exhaust for the vmax and the bike can be made to run ok with them. However, they are the worst performing exhaust available. You lose power compared to stock.

Yeah, I knew that. I'm more concerned with ridability right know. I'll change the exhuast when money permits. I just want to make sure I'm not wasting my time chasing down this problem if the exhaust is to blame.

lawman504 · Apr 7, 2014

setexascustoms said:
Thanks. I do plan on changing the exhaust, but I'm on a tight budget (baby on the way). So if there is a problem in the carbs that can be remedied for less money that's the preferred method for now. Can anyone else chime in on the exhaust? Does everyone who runs Cobras have issues?

They were on mine when i bought it. The look and sound was frikin awesome and i loved it. I thought the bike ran great and had lots of power. Then took them off after reading stuff on here and put the stock pipes on. Dont sound near as good but holy cow i realized the bike ran like shit with the cobra's on. I never touched anything in the carbs and there was an amazing amount of difference in power when i took them off. Cant afford any other pipes right now either but im much happier with stock.

Sent from my SPH-D710 using Tapatalk

tugla · Apr 7, 2014

I had Cobras on my bike....never had a issue...carbs were alll stock with stock head pipes and no hessitation or hick ups anywhere....yes a little down on power but for what I wanted no big deal. so no reason you cant get the bike to drive nice with those mufflers.

did you end up checking float level......when you look at the carbs from the top do 'll the slides return to closed if you move them by hand at the same rate....not hanging up?
have you run a little bit of seafoam for a few tanks. and I forget if you said. d but you got stock needles and stock air box right.

setexascustoms · Apr 7, 2014

tugla said:
I had Cobras on my bike....never had a issue...carbs were alll stock with stock head pipes and no hessitation or hick ups anywhere....yes a little down on power but for what I wanted no big deal. so no reason you cant get the bike to drive nice with those mufflers.

did you end up checking float level......when you look at the carbs from the top do 'll the slides return to closed if you move them by hand at the same rate....not hanging up?
have you run a little bit of seafoam for a few tanks. and I forget if you said. d but you got stock needles and stock air box right.

Stock needles and air box, I have ran a few tanks of seafoam through it. The slides all move freely and return to closed. I've had the carbs off and they were each Chemdipped over night, so they should be clean. One thing I did not do is pull all the jets, so I'll do that and make sure it has the right jets in the right location. Is there a guide or chart on here for that? I'll also check the float levels wet, I set them dry when I had the carbs apart, but never double checked my work.

Thanks for all the input guys, I do appreciate it!

alorio1 · Apr 7, 2014

The PO may have drilled out the slides for stage 7. You may want to search stage 7 installation and verify the size hole required for this mod. If you find the hole in the slide is too large you can fill it in with JB Weld and redrill to stock size hole.

setexascustoms · Apr 7, 2014

alorio1 said:
The PO may have drilled out the slides for stage 7. You may want to search stage 7 installation and verify the size hole required for this mod. If you find the hole in the slide is too large you can fill it in with JB Weld and redrill to stock size hole.

I did look at the slides and it looks like a factory hole, but I'll try to verify the size. Thanks!

tugla · Apr 7, 2014

id have to dig out all the stock jet sizing. besides main 152.5hrs
one on top of carb 90 and other inside I think 37.5 but cant remeber....

Dannymax should have them tattooed on the inside of his eye lids by now....
hey dan....close your eyes....what do you see for stock carb guts....

setexascustoms · Apr 10, 2014

Ok, so yesterday I went to leave work and one of the bowls overflowed and started pouring fuel out of the vent tube. I emailed Sean to let him know and to get prices on the needle valve and float, thinking that might have been the problem the whole time, and that it just now got bad enough to overflow. He told me to tap that carb with a rubber mallet, that the float was probably just stuck. He was right. I went ahead and checked the float levels wet, and they're all right in the 16-17mm window. I'm still going to check the diameter of the whole in the slides to make sure they haven't been drilled. Any other ideas? Thanks guys!

setexascustoms · Apr 18, 2014

Ok, so here's the latest update. I replaced the slides and diaphragms today, and the made a big difference, but it still has that issue.

Symptoms: at cruise above 3,000 RPM, in fourth and fifth gear it sputters, misses and bucks. If I roll on the throttle it goes away. If I jab the throttle it has a quick burst of power, then goes flat for a moment, then comes on strong. If I have run it hard, get off the throttle, then back on the dead spot is most noticeable. I noticed that the hole under the slide diaphragm, where the o-ring is, is wet on some carbs and dry on others. Is that normal? Or should it be either wet or dry?

About the bike: '85 Vmax, stock other than Cobra 4 into 4 slip ons. Plugs look new, NGK and are gaped at .035". Wires look new, also NGK. Charging system is very good, 13.3v at 1,000 RPM idle, 14.7 from 1,200 RPM up. The carbs were removed, chem dipped over night with new jet block gaskets and new emulsion tube o-rings. Floats read between 16mm and 17mm wet. No vacuum leaks I can find.

This bike is starting to frustrate me an any advice is greatly appreciated! Thanks guys!

Light Throttle/Cruise Carb Issues

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