Maxim-X carb conundrum

Mikuni BS33 carbs for the Maxim-X: 8K on the engine.

I removed the carbs and removed the jets and emulsion tubes, cleaned them thoroughly.

I made the passageway at the bottom of the float bowl & the top of the bowl (where the brass tube slips into) open as they were clogged.

I made sure there were no holes in the diaphragms.

I put new O rings on air idle needles.

I cleaned the slides and made sure they go clunk.

I adjusted all four float bowl fluid levels to the proper level.

All float valves are working properly.

New plugs and I cleaned all contacts in the cap to give a clear free voltage path. Plugs are gapped properly.

But...

When I put the carbtune on and started it, it ran terribly rough and the metal tubes did not move. The bike would not idle properly and the engine would suddenly race. At high RPM the tubes moved upward as expected. When I'd blip the throttle it would slow down and work its way to a stall.

I removed carbtune hose from the intake manifold when it was idling at around 1000 rpm and instead of suction, I was feeling puffs coming from the vacuum nipple. Finally after fudging with the sync settings when it was idling at about 2,500 rpm I got it to idle for several minutes at 1K rpm but out of nowhere the engine would race to 5K, blip the throttle and it went right back down but I couldn't get it to idle reliably no matter what I did.

When it was idling steady around 1500 I sprayed starter fluid all around the carbs & manifold to see if the engine spiked due to an air leak but it made no difference. I also tried it when the engine was idling around 3K and the starting ether made no difference in the engine so I think there's no suction leaks and the throttle shaft seals are intact.

I put a heavy dose of Seafoam in the Aux tank I use when syncing and a good amount of smoke came after doing that so I ran it like that for 10 minutes, shut it off and am leaving it sit for a day with the seafoam in the carb.

My first thought when I felt puffs coming up from the manifold nipples was all the intake valves were out of adjustment but I started seeing the bars rise at lower & lower RPMs as I fiddled with the carbs but at 1K they still don't rise, around 1500 RPM they do rise as expected.

I did a pretty thorough job of cleaning the jets and soaked them in carb cleaner for at least an hour and did find three of the idle jets in the float bowl were plugged. I used a ultra thin mandolin wire to break the blockage free and then re-soaked them just to be sure. I used the same wire to free the blockage in the float bowls.
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So the issue is why I can't get it to idle without suddenly and unpredictably racing to 5K or eventually stumbling and stopping from rapidly decreasing RPMs.

I don't understand why I would get positive pressure at the manifold nipples at lower RPM but excellent suction at 2K rpm.

Anyone have a suggestion what I should do at this point?

I'm stumped...

 

Are you running an airbox? It will not run right without an airbox and (if stock) a paper filter.

 

Oh yes, 100% stock.

 

Conduct a Compression Test.

You should always do a Compression Test on an Engine as part of the Tuning Process.

The Compression Test is easier and quicker than Measuring the Valve Clearances.
Measuring the Valve Clearances does NOT give you information about RINGS.

GOOD Compression Values let you commence Tuning.
OUT of SPEC Compression Values indicate the need for a Valve Adjustment or other work necessary to obtain peak performance.

 

ooooooooooooooooooooh......that's the SECOND person to tell you that

DaveF

 

And here's the third person.

Do a compression test.

Check your valve clearances.

 

I understand a compression test will tell if there's low/no compression but how would knowing valves or rings are, or are not seating properly tell me why the engine suddenly races?

I hope my metric feelers are narrow enough to test these shims.

Earlier I went to run some seafoam through the suction nipples and when I removed the cap and went to connect a 3/32 vinyl tube to it, seconds later the engine raced. It was as if the lowered suction resistance induced the engine to race. I stopped the engine and started it again with the port plugged and it idled nice around 1K removing the plug again and it did the same thing after a few seconds.

I repeated this with all 4 suction ports, and ran 1/4 can of seafoam into each to get to the valves & hopefully break up any sticking valve or deposits. I'm letting it rest for a bit, am going to get some dextron ATF and when I get back, will do as I was suggested to do & this will lube the top end & run the engine a bit and then do the same with the Dextron, remove the plugs & pour some into the cylinders, rotate the engine by hand a few times to get this detergent oil to the top ring and let it sit a couple of days to lube & break up deposits.

I just placed an order with Chacal for rebuild parts for the carbs including shaft seals (just sent yesterday) and will go over them again while I'm waiting for the dextron to do what it may. I'll have the carbs done in a couple of days & then I'll check to see how it runs.

Although I do feel those puffs from each of the ports, the seafoam sucked in with great vigor. There's definitely great suction in each cylinder but it's the low rpm puffs from each nipple on an X with only 8K that seems unlikely. One cylinder with an intake valve problem, maybe, but all four? Vegas wouldn't take those odds. Well maybe Vegas...

 

If you have a valve that's not sealing properly at low RPM and "blowing back" through the intake instead, it MAY be revealed by a compression test; or by a compression test and valve clearance check. A tight valve can greatly affect compression.

The same feelers that work on your "airhead" should work on the Max; the aspirin-sized shims are under the buckets which is why they're so small. The bucket itself and cam lobe aren't significantly smaller.

If I were you I'd at least check the valve clearances before you drive yourself crazy trying to do a vac sync. A couple of too-tight valves and you won't be getting a decent sync.

 

You check your valve clearances, especially on an 8k motor, because valves wear faster when the engine is new. I just did the valve adjustment on my 6k mile turbo seca, 7 were tight. If they haven't been done on your bike, then one could be burned (that'll be found in the compression test). Odds are actually high that they're tight, and the house will take all your money if you bet against them for too long.

You don't pull nipple caps on the intake while running. That just introduces a BIG air leak, and if the engine is running rich at all, that air leak will cause the engine to race. Pulling those caps off doesn't test anything. Also, "puffing" might be happening between intake strokes. It only pulls a vacuum 1/4 of the time. Those nipples are for syncing and nothings else, not sea foam, not dextron, not testing the vacuum. In fact, I've never even seen a manifold vacuum spec for these bikes. Don't worry about cleaning the cylinders (I think you only do that if you pull the head) and just compression test the engine.

 

I'll do a valve check while I'm working on the carbs & the engine is cold. Good to know the feelers will work, I took your advice Fitz & bought a nice metric only set.

According to the periodic maintenance schedule for the Maxim-X, the shims are to be checked every 42,000 km / 26,000 miles (page 7-3 of the owners manual). I wouldn't think they'd be tight at 8K but anything's possible. If the valves are stuck then hopefully doing the upper cylinder lube/cleaning with dextron will free them. Seems like checking the shims would be best to do after this as I would think the gap readings would be different between stuck and not stuck?

I didn't pull the caps off to do a test, I removed them to insert the tubing with seafoam at the other end while it was idling so the seafoam would get sucked into the valves to hopefully break-up any deposits. Indeed the seafoam did get through as seafoam was dripping out from where the exhausts bolt to the headers. That's just what I wanted, so both the exhaust and intake valve stems got a good dose of seafoam. Hope it worked, seafoam is pricey...

 

Ah, I knew the X was different, but didn't know the valves went that long between checks.

The valves wouldn't "stick." Rings stick, and usually that's what you are trying to break free by soaking the cylinders in some kind of solvent. I'm not sure what good it does running it through an idling engine, it'll just combust or flood it. I think you'd usually just putseafoam/mmo/oil something else and let it sit to break the rings free if, in fact, they are stuck. But I'm pretty sure if your valves are in spec, and it blows good compression, you don't need to get any more SeaFoam :wink:

I was trying to explain the behavior when you pulled the caps to put seafoam in. Seafoam is combustible, so opening the cap introduces air, then dumping in sea foam introduces fuel, so you get more fuel mixture since you are bypassing the carbs, and the rpm increases. Not unusual, and it doesn't really help your diagnosis.

 

Any combustible liquid introduced into the Manifold while the Engine is running will induce the Plant to increase rpm's.

It's just using the Seafoam as more Fuel.

 

the rpm increase happened before I got to dip the tube into the seafoam. It seemed to occur seconds after the cap was removed from the vacuum nipple. With the seafoam rushing in, the plugs were pretty well overwhelmed and the engine bogged but did not choke out.

Unlike me at my first dorm party...

 

hahaha :lol:

 

I would have enjoyed hearing the thoughts of what about the carbs would have made the engine race randomly as it did. The suggestions to check valve clearance is good and would have been a possibility as to why there was positive pressure instead of suction from the suction nipples.

I just finished tearing the carbs apart as far as they go and made it a point to check every orifice and be sure it wasn't plugged. Things like the little copper tube that goes into the bottom of the float bowl; it has two holes on the side that must be open and there is a destination for that tube which also must be clean. I wrapped the tube with tape and wrapped the end of the tube from a carb cleaner to make the flow continuous and held it tight at the junction & squeezed till the fluid came out elsewhere. I then went back and made sure the copper tube had the two side holes open. I did that again with each carb....

Things like that; I went over those carbs with a 10x magnifier looking for anything that might lead to something plugged and I used three cans of carb cleaner and dipped the bodies & bowls & jets in carb cleaner for 12 hrs each. I did everything by the book and then some to be sure these carbs are as new. Took me two days... & Thanks Chacal for getting me the parts I ordered so quickly, that helped.

I put the carbs on the X today and it fired up with a high RPM, I adjusted the idle adjustment and played with the carb tune and the rods were acting exactly as they should, elevation at low idle & no more unexpected engine racing. It's now steady as a rock. Without a doubt, the problem was confined to the carbs but all I did was clean and replace the carb parts, I still have no idea exactly what I did that made the difference. I did replace the throttle shaft seals and I replaced the entire float mechanism in each carb, new everything that can wear.

I like to know what makes things go wrong so I have a better idea of what is going on. This one will have to remain a mystery.

 

Sooo... you're just going to ignore the valve clearances and hope for the best? Every 26,000 after the initial check at ???

The "airhead" interval is 5K, but the INITIAL check was to have been at only 3K. What was the "initial" for the "X" supposed to have been done at? And was yours?

 

Ah you assume I'm not going to check the clearances... MMmmuuuwwwwwaaahhhhaaaa Come closer over here, I have a bridge for sale.

However... in the manual it says carte blanche, 26K, no earlier mention of an abbreviated initial schedule



I guess the 5 valve approach was maybe more stable. Considering the much greater effort to adjust the valves, that's a good thing.

 

The reason for the longer intervals is the valve springs do not have near as much tension as the air cooled models. Valves are much smaller and lighter so it does not need as much pressure to close the valve. Plus Its a majorly updated design over the air cooled models not even sure if the valves are made of the same material it was designed in about 84 vs the 70s for the 2 valve models. The engine racing when you pull the caps off is normal your letting in a lot more air kinda like cracking the throttle with the slides all the way down. When the rpm rises your sucking more fuel thru the throttle blade as long as the motor can get enough fuel thru it it will run. I would definitely check the valves there is a lot more overlap on the cams compared to the 2 valve models its designed off the fzr platform 10k-11k so it will spit some air back to the carbs.
This is just me i would not let the motor suck any liquid thru the vac port all it takes is just a bit 2 much and your engine is done. Liquid does not compress so be careful

 

Thanks for the reply. I don't think my explanation gave most people the picture. When I removed the caps from the vaccum plug, the RPM would change from 1,000 to 6,000. If I replaced the cap, the racing persisted. Sometimes the engine would idle at 1.5K and then without my doing a thing, the engine would race to 5-6K and stay there. Blipping the throttle did nothing but shutting off the bike & restarting would have the engine at idle again.

It was like someone else had the controls, something from Stephen King or the Sci-Fi channel. Yes, allowing the cylinder to get more air will make the engine race higher, maybe 1-2K but not more. Replacing the cap will make properly running engines return to normal. What happened to this engine was not normal.

Since it's running perfectly now that I anally rebuilt every possible weak point in the carbs, I won't know exactly what was the issue or what will make that happen... Knowing that kind of information is key to really understanding how engines work. I like to know everything that I can & don't care for mysteries except on PBS...

It is purring like a kitten now though!

 

Did you add the ATF to the cylinders as you mentioned?

If so, all the while you were re-cleaning and rebuilding the carbs, everything in the top end was soaking nicely in a film of ATF and seafoam.
You got the carbs all nice and rebuilt, and by the time you got it back together, the top end was much happier too? (maybe)

I would bet on the throttle shaft seals though....The X seems to be very picky about throttle shafts and after seeing the instant improvement on 4 or 5 of them, that is one of the first things I consider with an unhappy X
The YICS really seems to reduce the impact of a leaky throttle shaft since the leak is basically balanced across all four cylinders.

I wish I had time to test an "aftermarket YICS" and connect all the vacuum ports together with a couple tees just for kicks. But sadly mine still sits in the garage with a dead battery and a layer of winter dust still on the tank waiting for my other projects to get finished.

 

No, I haven't used the ATF yet but I like the idea of the detergent and lubricating action on the valve stems, I'll get around to it. I tend to keep old stuff and I looked around in the garage and found a can of original Dextron and one of Dextron II on my shelf, covered in dust. I'll use those, I doubt there's much collector value yet for a 60's-70's can of transmission fluid.

Before I stripped the carbs I ran seafoam into the heads via the suction nipples, let it set overnight & then I used a heavy dose of seafoam in the lawnmower tank I use when syncing. When I started it the next day it was smoking like crazy and I ran it for maybe 10 minutes like that. I put the bike's tank back on and went for a ride hoping that the issue would now be able to resolve under operating conditions. Funny but the smoking lasted much longer than I expected, for 8-10 miles perhaps and when I got back the problem might have been a tad better but all the problems still remained so I did the total restore on the carbs.

I just had time to put them on yesterday and the vacuum/sync behaved like my XJ650 & XJ1100 do, just perfect. It idles like a sewing machine. I didn't have time to take it for a ride, I had to help someone out with a problem and won't be able to test anything further till Sunday evening at the soonest.

Even when it was running badly I could tell there was a lot of power under the hood. I couldn't WOT and get acceleration, I had to back it down and there was a point it would take off so it seems like the engine was in a rich state. Doesn't matter now, that problem is resolved.

Now I need to get more familiar with how to care & feed this bike. And... I need to ready the 650 Maxim for sale. I'll hate to see that bike go, it's really a perfect bike and it's one I can always rely on to run without any issues. Might be late in the year to sell it and sale-wise I might be better off readying it now & pack it away for sale early next year when the riding season is longer.

 

All's well, ... that ends well!

 

If an engine is racing and you suspect it is the carburetors, keep in mind that the racing could very well be related to a lean/very lean condition. The lean condition was likely caused by your removal of the carbs and air leaking into the combustion chamber; if the rubber carb holders are cracked, the air leak can focus on the piston crown and will eventually cause the piston to be holed in the spot where air is focused. Rubber carb holders should be replaced on a routine basis as the rubber dries and they don't seal as they should or crack. Other potential air leak places are the points where vacuum is checked for sync or is connected to the petcock, etc. If rubber caps or hoses are connected, one or more can be cracked or compromised and the leak will cause the engine to race. If your pilot jets are grossly out of adjustment (mixture screws set wrong) or are partially plugged, the engine can race. Throttle cables don't cause engine racing but if the cables don't have the slack called for in the owner's manual, the carbs can race. If the carb assembly uses choke plungers rather than choke butterflies, and the rubber is worn where seat occurs, the engine can race and you can check this by jiggling the choke actuating mechanism. The racing will stop and then begin again randomly when the bike idles. Last thing off the top of my head... if you have removed your exhaust head pipes, the gaskets use are crush gaskets and can only be used once effectively; air can leak into the combustion chamber through the leaky gaskets and the engine can race.

 

That's it? That's all you can come up with? :roll:
:twisted: :twisted: :twisted:




Good solid reply.

 

Hi George & welcome to xjbikes!

Some really good ideas you mentioned. I do wonder though if in some way the enrichment circuit was the culprit? I can't ID anything that was reproducible in resolving the issue but still, I think that's the area of concern.

Thanks for the thoughtful answers.

 

The replacement of Manifolds, Brake Lines and Exhaust Port Flange Donut Gaskets are universally the items most neglected by owners of Asian Bikes.

Very often these parts are not serviced until they fail.

Because of the low-quality of fasteners and hardware used in the build; carb manifold and exhaust pipe leaks are commonplace.
The owners would rather have to work-around the leak than wrestle with the consequences of contending with a seized fastener.