Carb Problems

[mikemax04]

I was wondering Shawn if we could discuss the carbs on the Max and possibly come up with some answer to the problems that many (even new) Maxes experience. I've tried to get some answers from the Yahoo community on the subject but none were willing/able to diagnose the problem. I know several guys (me included) that have newer stock bikes and have had idle problems and worse, running on three cylinders. It's worth a discussion by all in the know about these particular carbs.

 

[Dreamster66]

My local yamaha dealer/shop told me they do 3 1/2 turns out on the fuel air mixture screws and it is always almost right on.....I trust them and just wanted additional input.
Anymore than that and I think I'd be getting ALOT of rich burning smell from my exhaust.
On a stock, no mods other than a drop in k&n....does this sound about right?
Randy

 

[shawn kloker]

Its alot harder to talk about than to just do.If I feel mine are wrong I take a ride and have a little screw driver in my pocket.
I mostly adjust them for off idle.No hesitation with a small twist of the throttle.A big twist will cover up small problems if that makea any sense to you.It takes alot of feel.
One thing that effects this adjustment is valve clearances.It is also good to have mercury sticks hooked up and watch the reaction of each carb.
I am looking into boring my carbs to 37mm or replacing them with something bigger right now.I personally do not like the sound of flatslides though.The first time I heard them I thought the guy had a rod going out or something.
The most important thing to do first is make sure there are no air leaks in any of the boots.Then make sure everything is clean inside.These carbs tend to plug up when they are not opened up fully every once in a while to clear the circuts.
I had a torn vboost boot that drove me crazy last summer as I was convinced it was a jetting problem till I found where it was sucking air.
Basically we will have to address each problem as it comes since it can be one of many causing varied problems.

 

[Dreamster66]

Its alot harder to talk about than to just do.If I feel mine are wrong I take a ride and have a little screw driver in my pocket.
I mostly adjust them for off idle.No hesitation with a small twist of the throttle.A big twist will cover up small problems if that makea any sense to you.It takes alot of feel.
One thing that effects this adjustment is valve clearances.It is also good to have mercury sticks hooked up and watch the reaction of each carb.
I am looking into boring my carbs to 37mm or replacing them with something bigger right now.I personally do not like the sound of flatslides though.The first time I heard them I thought the guy had a rod going out or something.
The most important thing to do first is make sure there are no air leaks in any of the boots.Then make sure everything is clean inside.These carbs tend to plug up when they are not opened up fully every once in a while to clear the circuts.
I had a torn vboost boot that drove me crazy last summer as I was convinced it was a jetting problem till I found where it was sucking air.
Basically we will have to address each problem as it comes since it can be one of many causing varied problems.

I bought ALMOST new lower carb boots....need to put them on tho..... maybe I should check and possibly replace my vboost boot too.
The lower carb boots that are on it now has some tiny cracks....just like dry rot on a tire.
Thats why I bought these.... 30.00 for 4 on ebay.
Look just like new!
Tomorrow I'll get to them, hopefully before my daughters concert!
I do have the carbtune II....LOVE IT!
Randy
aka.... D66

 

[shawn kloker]

The biggest problem that I have found is where the boots attach to the airbox.When they start to get hard they do not seal and actually pop down off the airbox by themselves.Stage 7 eliminates this problem!

 

[Buster Hymen]

Great thread Mike and some great answers. I find the amount of work I have to do on the carbs frustrating. I'm still thinking about springing for the FI unit... but it is a lot of cash!

 

[mikemax04]

Mostly I had in mind not so much the tuning part, but just out of the blue the carbs go wacko. I agree that a quick response from idle is most desirable. I know several guys with almost new Maxes where for no reason they run crappy. Myself, I experienced this at about 7k and found by process of elimination, that one carb was not getting gas for some reason. After a good dose of carb cleaner down the throat a few times while idling, it cleared up. It's just something I never ran into and have rebuilt many various carbs. Always fresh gas, no water, no dirt, clean filter (air and gas). This problem seems to be inherent in the Max only from what I've been able to find out. Wish I had a used carb so I could take it apart and study it's functions. The diagram in the manual isn't sufficient for that purpose or it could be my brain is out to lunch.

Finally sold my CB750 that I bought in '69 and after 35 years, I only overhauled the carbs once with no problems like the Max is showing me. With so many Max owners in the world and so many years of the same problems, one would think there would be plenty of answers out there. Not looking for THE answer, but a discussion along these lines might be beneficial to all and perhaps we might just stumble upon something.

I too am switching over to FI with the Motodama engine attached to it. Come on Yamaha!!!!!!!!!

 

[maleko89]

I have found most of the problems is with the gas. The bikes I have worked on have sit unused for a lengthy period of time. When draining the bowls, the fuel is brown. For those that do ride often....I have two theories. Could it be that the fuel filter doesn't do the job good enough? Could it be from the crankcase vent? "Dirty" air from the vent goes through the carbs too. Another possibility is that the rubber carb parts rot because of the fuel. If you take the float bowl cover off, you see bits of the cover o-ring in the mating surface.

 

[mikemax04]

Good observations Mark. I know in several cases like mine, there was always fresh gas with no chance of getting old. The bikes are too new for any rubber to deteriorate. The crank case ventilation just might have some influence. I was also thinking that the four in-line carbs on the Honda were vertical and moved the slides via a mechanical cable = positive control of the slides. From the little bit I know about these carbs is that the slides seem to be operated by engine vacuum. The more the butterfly is opened, the stronger the pull on the diaphragm of the slides. Since these slides are in a horizontal position to start with, their own weight is always at rest and contributes nothing to their movement. Is it possible that a slide can hang up and not permit gas to enter through the jet? Is it possible that there is a closure of the orifice to any degree which would prevent enough suction from the engine to work the diaphragm? Sorry about all the questions but perhaps they are food for thought. Another head scratcher for me is the enrichment circuit, it's function and how it affects the rest of the carb, when not in use, if any.

 

[Dreamster66]

Good observations Mark. I know in several cases like mine, there was always fresh gas with no chance of getting old. The bikes are too new for any rubber to deteriorate. The crank case ventilation just might have some influence. I was also thinking that the four in-line carbs on the Honda were vertical and moved the slides via a mechanical cable = positive control of the slides. From the little bit I know about these carbs is that the slides seem to be operated by engine vacuum. The more the butterfly is opened, the stronger the pull on the diaphragm of the slides. Since these slides are in a horizontal position to start with, their own weight is always at rest and contributes nothing to their movement. Is it possible that a slide can hang up and not permit gas to enter through the jet? Is it possible that there is a closure of the orifice to any degree which would prevent enough suction from the engine to work the diaphragm? Sorry about all the questions but perhaps they are food for thought. Another head scratcher for me is the enrichment circuit, it's function and how it affects the rest of the carb, when not in use, if any.

Its funny, and I have probably said it here before, but I've been digging into my carbs soooo much this past summer my neighbor finally asked me if I got a bad motor....lol!

 

[maleko89]

Mike, the slide could stick. I regularly take them out and clean 'em good with wd-40 and also clean the slide bore too. Anyway, you can take the slides completely out and it will idle just fine. You could just stick a long screwdriver down through the airbox to push the slides back and forth. That'll give you a good idea if they're binding at all.

As for the coasting enrichener, think of it as a smaller version of the piston slide/diaphragm. Instead of making the main carb bore variable, it opens/closes off the pilot air jet #2. At idle or when you close the throttle during engine braking, the piston closes off this jet thus making the pilot mixture richer. Starting at off idle, a small vent for this piston creates negative pressure and opens up PAJ2 thus leaning out the idle mixture. The idle mixture comes into play up to 5000 rpms. Air from the pilot jet #2 combines with air from pilot air jet #1 to mix with fuel from the pilot fuel jet. When you open or close PAJ2, you're effectively leaning and richening the pilot mixture.

Next time you look at the top of your carbs, notice the little vent. It's the oval hole. When the throttle is opened, air is sucked into the vent. Some air is used to open the main slide. A smaller portion is used to open the coasting enrichener plunger and another small portion flows through PAJ2.

If the enrichener is stuck closed, then you'll probably run rich in that cylinder. If it's stuck open, the bike will pop at deacceleration or even at idle.

 

[mikemax04]

Thanks Mark, now there's some food for thought. Let me digest that and let others ask or contribute.

 

[Dreamster66]

Mike, the slide could stick. I regularly take them out and clean 'em good with wd-40 and also clean the slide bore too. Anyway, you can take the slides completely out and it will idle just fine. You could just stick a long screwdriver down through the airbox to push the slides back and forth. That'll give you a good idea if they're binding at all.

As for the coasting enrichener, think of it as a smaller version of the piston slide/diaphragm. Instead of making the main carb bore variable, it opens/closes off the pilot air jet #2. At idle or when you close the throttle during engine braking, the piston closes off this jet thus making the pilot mixture richer. Starting at off idle, a small vent for this piston creates negative pressure and opens up PAJ2 thus leaning out the idle mixture. The idle mixture comes into play up to 5000 rpms. Air from the pilot jet #2 combines with air from pilot air jet #1 to mix with fuel from the pilot fuel jet. When you open or close PAJ2, you're effectively leaning and richening the pilot mixture.

Next time you look at the top of your carbs, notice the little vent. It's the oval hole. When the throttle is opened, air is sucked into the vent. Some air is used to open the main slide. A smaller portion is used to open the coasting enrichener plunger and another small portion flows through PAJ2.

If the enrichener is stuck closed, then you'll probably run rich in that cylinder. If it's stuck open, the bike will pop at deacceleration or even at idle.

sooo....if its stuck closed.....could that give you a nasty exhaust smell (gas smell)? plus....sometimes when I turn the key on....it "pops". (carbs...sounds like a small backfire from pressure maybe?)
I just took my carbs back off to put the lower boots on.
Does anyone remember who to get ahold of for ordering max parts?
I'm going to replace all 4 vboost boots while I'm at it.
Please post his info in the vendor section.
Thanks again.....Randy
ps..... Shawn Kloker....I might make a road trip this summer down to see you..... need to get away from NY for a bit and might go to NC to see my brother and stop in to see you on the way down, if its ok.
Randy

 

[shawn kloker]

Randy,Did you check your enricheners after I told you that I thought one was giving you the popping.
I'd love to see you come down with your max.The true master of these carbs lives in pittsburgh,About 30 miles south of me.

Mike Barth at University Motors.

 

[Dreamster66]

I'm gonna check them tomorrow since the carbs are off....I did check 1 on the bike....the screws were tough to get to and get off.....I'll let you know tomorrow.
Any luck on the guy who sells parts at a discount to vmaxers on the board or to VMOA members?
I ordered from him before but cant remember who it was.
ANYONE....just post his info in the vendor section.
Randy

 

[shawn kloker]

Mike Barth @ University Motors


(701)235-6479

 

[gleno]

Having read this thread (which makes lots of sense) I'd like to ask some advice from Sean Kloker, Maleko89 and anyone else with a few clues.

While on a recent tour (bike running fine on highway) I ended up doing a couple of days inner city stop start, low revving, low speed, crawling in heavy traffic.

During this duty the bike started to miss and bog down when rolling the throttle on from idle. This was typical when starting off in first and slipping the clutch or when moving in first or second and rolling on the throttle while slipping the clutch.

This symptom is intermittent but if you ride like this for a half hour it will start to do it again. Opening up the throttle and reving the motor 'clears its throat' for a while and it runs fine.

This has now become more frequent and has started to happen when exiting a low speed corner while rolling on throttle, so last night I sprayed the carby throats with carb cleaner and spayed cleaner down PAJ1. Took it for a run this morning and all was fine at anything above 2500 rpm but below this the same intermittent symptoms persist.

Upon returning to my home garage and shutting of the bike I could smell raw fuel.

So I appear to have an overfuelling problem at low RPM that is obvious below 2500 rpm and shows up as a choking motor when rolling on throttle from idle or low rpm.

What are the likely causes and what should I check first.

 

[maleko89]

Gleno, pull the slide covers and see if fuel runs off the piston slide diaphragms. If so, one of the purge jets is clogged and that'll cause a rich condition. Other things to check are sync, float level, dirty air filter.

Another test is to remove the air box and slides and start the bike up. Do you see fuel pooling on top of the throttle valve? That usually means that your idle is set too high or out of sync carbs. The bypass holes are uncovered in this situation causing fuel to collect on the throttle valve. The last thing I'd check is for a stuck coasting enrichener. If it's stuck closed, PAJ2 is closed so idle mixture would go way rich at off-idle up to 3500 rpms or so.

Mark
#1098

 

[shawn kloker]

Seafoam or another quaility fuel treatment time.Put some in the fuel tank and go get some on a highway.These bikes are known to clog up when putted around for a while.Need to open it up once in a while and clear all the carb circuts.

I put seafoam in my tank every 3 or 4 tankfulls.It will only get worse over the summer here in the states as summertime means ethenol gas season time.It clogs the carbs bigtime.