Carb Adjustment

New rubber slide diaphrams can be gotten from www.jbmindustries.com. These do NOT use the plastic diaphram holders on the slides, instead they are a perfect fit and work much better. Check out the website...

Damn, I like those alot ^.

xschris said:

Some of you older gents might remember one of these. It was given to me by a friend that had it in his basement since the 70's works awesome. carb stix mercury gauges for syncing carbs.

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Yep, I have one and it works just fine. Never tried one with just air pressure guages.

Yamaha Newbie said:

Thanks XS. Per HoughMade's advice I pulled the carbs last night and have them sitting on the workbench waiting for dismantling and cleaning, probably early next week. Since they are off I looked inside the rubber connecting mounts that sit between the carbs and engines. The outsides are cracked pretty much all the way around, but there are NO cracks or holes on the insides, so I'm hoping I can get by without replacing them until I do a full tear down this winter.

XSchris, are those rubber pieces in your pictures available from Mikesxs? Part number? Also do the boots from the 650 work on the 400. Same carbs and diameters from what I can tell. I found a pair of 400 mounts on ebay for like 30 bucks, but I always like to have a back up plan just in case.

Thanks again for the assist guys.

Edit: I think I just found the parts you were talking about XSchris on Mikes Part #48-1408.

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i found a set of NEW carb intake boots for my 81' 400 from a guy in Canada www.siriusconinc.com

with the two gaskets for 44.99 plus shipping (58.74)

Hi All, I have had my bike for 3 years now and it has never run right.
I am currently the closest I have ever been, but still have some problems.

If I try and use more than about 3/4 throttle the engine just bogs right down and won't pull any more, however below 3/4 throttle it will rev all the way to the red line.

I am running with standard air filters, but with short, mostly empty silencers. I tried rejetting from 155 to 160 in the mains and that seemed to make little difference.

Anybody encountered a similar problem?

cheers

If you can run to red line that is full throttle. These are vacuum operated carbs so going WOT under load the bike will bog. 150-160 jets are way to big. I would start with stock and see what that does. I run a 2-1 exhaust with a free flow muffler and uni air filters. All I need is a 137.5 main jet. How do your plugs look?

Hey guys anyone know a good starting point for pilot jet would be I cleaned my carb and didn't pay attention to how many turns it needs amateur mistakes

i just emailed mikesxs asking about jets and such for my 80 (or 81?) xs400 with the B34 Mikunis.

they emailed me back and said NONE of their parts will work for the 400!!!!!! i emailed back and said i thought they were the same carbs, but here was their response:

The carb bodies are the same, the air,fuel jets, emulsions tube, CV slide cut-away and spring,etc are all different.

anyone have any other recommendations on where to get the jets and whatever else parts i need to rejet?

Have some questions on carb adjustments on a 1978 XS400. Had the carbs off to clean as bike has set for 2-3 years with almost no running. It has some kind of aftermarket mufflers but everything else is, to the best of my knowledge, original, such as stock air filters.

From what I have read the low speed jet screws are to be approx. 3 turns out from lightly seated and that's were I started. Started bike and let it idle for a while till I could push in choke and it would keep running. Then adjusted idle speed up some till it would run on one cylinder with other coil unhooked. It would idle on on cylinder pretty good at about 1,500 rpm(maybe this is too high?) Then tried adjusting the low speed screws there was almost no difference if they were 4 turns out or lightly seated. Tried both carbs with the same results.

I've adjusted dozens of carbs on small engines like Briggs and Tecumseh, old tractors and a few old Chevys and Fords among many others. On just about every one of those moving the low speed screws 1/4 to 1/2 turn in either direction will make a major difference to the point of killing the motor.

Just looking for ideas on which way I should head. Like pull the plug out of the side not being adjusted to let engine possibly idle slower. As I said to the best of my knowledge all the jets in the carbs are stock, I believe I did check them against the ones listed for this model. I did buy a carb kit from Japan(probably China) but only thing I used was the bowel gasket and the float valve. Did note that the Pilot jet was different in regards to holes through the side, mine had I believe two cross drillings where the ones in the kit had none but I reused the originals.

I'll stop there so someone can ask questions if I left something out.

kshansen said:

Have some questions on carb adjustments on a 1978 XS400.

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Well I just finished trying the carb adjustments again.

Idle set high enough so it would run with one coil unhooked.

This time I tried it with engine running at about 1,000 rpm per the tach. Now it still did not respond anywhere as much as I would like but when turning in screws in it would stumble and stall at some point. Turning out from that point would, once to the point it was not trying to stall, it picked up a little speed, maybe a bit over 100 rpms. Going past that point for two or so turns did not seem to change anything. So I put it back to the point it raised up and then added abut 1/4 turn.

After doing this to both cylinders I checked the adjustments between the two. Right side was right on 2 turns and left was at 2 1/4. Set idle speed to 1,500 rpm

Revving up the engine and letting it go back to idle and it drops fast to about 2,000 hangs there for 1 or maybe 2 seconds.

For now I think I'm going to say "Close enough for this outfit". I'll finish reinstalling the sidecar and keep fingers crossed we get a bit of rain. Rain? Yea! want to flush the salt off the roads before I do any test runs.

Have you sync them with a manometer? That is the best way to balence carbs.

xschris said:

Have you sync them with a manometer? That is the best way to balence carbs.

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Well I used the dual gauge set up I bought from a guy a few years back. Used the screw down between them and adjusted till both were showing the same vacuum reading.

Got the sidecar remounted and just need to slip the fuel tank and seat back on then when roads are less salty I can start doing some short test rides.

If things seem pretty good after that I'll change the oil another time and install the adapter for the spin-on oil filter

We're there anymore post about this thread on how his bike turned out after rejetting for pods and open pipes

Jetting for pods/exhaust is usually around a 145 main and 45 pilot. That's a good starting point anyway as every bike will be different.

mcrowell75 said:

Oh yeah,

I also set my floats at 32mm. I know that seems ridiculously high. But that comes straight from the haynes and yamaha manual.

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that is high. how did u manage it