Washer in crankcase

[crazy steve]

For compression numbers, I've been personally seeing right around 170 lbs up to 200 depending on the year (the early motors seem to run a little higher). This is with the carbs off the bike, and all plugs removed when spinning the motor with a 'booster battery'. Carbs on, using the bike battery, etc can all effect the numbers. But anything over 140 is considered 'good' and lower doesn't mean the bike won't run well, it may just be down on power. The main thing you want here is even numbers (all the same or close). Do be aware that valve adjustment can drastically effect compression if any valves are real tight; the XJ with 49K miles I'm working on went from 170/90/60/170 to 170/170/160/170 after doing the valves...

As to fork seals/brake rebuild kits, you can still get those from at least some Yamaha dealers, as well as aftermarket vendors.

[BA80]

Even if the compression is a little low squirt a little oil in the cylinder and see if that brings it up. If it has been sitting the rings will be dry and they need a little oil to seal propery.

If that brings it up just running it wll get those rings working right. Same with valves that have been sitting. They will get surface rust and dust on the faces just from sitting and may need to be run to get them seated again.

[fredintoon]

- - - does anyone have an idea of what the average compression for these engines at about 30K on them? - - -

Hi KJ,
what Steve sez. But if the motor has low compression numbers but runs OK give it a few hundred miles for the rings etc. to come unstuck after their long rest then check again.

[pcmcse1]

Carb tuning

Okay great! Thanks for all the input guys!

Now does anyone have any advice in adjusting/tuning Mikuni RS series carbs on this thing?

When this starts, it sounds like the vacuum advance is going to beat it's self apart. Not sure but think there is much more vacuum generated with these carbs and too much vacuum on this line. It dies when I push the choke back in, and seems to be running rich.

These were supposed to be factory set to OEM factory specs on the bike. Mikuni says these should need minimal adjustment to perform correctly on this bike.

Mikuni RS 36MM racing carbs. They are flat slides with Teflon coated intake bore and slide assembly.

The bike sounds real cocky when running. I have a new pair of Screaming Eagle HD mufflers on it and they sounds great.

Any ideas welcome.

[James England]

Hello all!

Good news, the crankcase pan was removed and I found both washers.

Now... I bet you feel much better having found them, don't you? You'll not have that little nagging voice that keeps telling you they might knock your camchain off or something. It's got to be worth it, just for that, eh?

[fredintoon]

- - - When this starts, it sounds like the vacuum advance is going to beat it's self apart. Not sure but think there is much more vacuum generated with these carbs and too much vacuum on this line. It dies when I push the choke back in, and seems to be running rich. - - -

Hi KJ,
what dies; the vacuum advance rattle or the engine?
The vacuum advance should be hooked up to a tapping on the #2 stock carb body, NOT one of the 4 carb manifold vacuum taps.
Check where on the new carb rack yours is hooked up to.

[pcmcse1]

carb tuning

The vacuum line is connected to the #2 intake boot, with the petcock lines connected to #1 & 4, 3 is plugged.

I was told by a certified Yamaha mechanic...that there was no way I would get these carbs tuned right on this bike.

Not sure which school he went to but a gasoline engine with proper air fuel mixture and timing and the is no doubt this thing should have great response to the throttle.

Anyone had any experience with changing carbs on the these inline fours with success?

Thanks in advance!

[fredintoon]

The vacuum line is connected to the #2 intake boot, with the petcock lines connected to #1 & 4, 3 is plugged. - - -

Hi KJ,
wrong place!
If you plug the vacuum advance line onto any of the 4 intake boots the advancer will rattle around like crazy and won't work right.
The stock #2 carb has a vacuum connector built into it's body to work the vacuum advancer.
If your orphan carbs don't have that feature, I think you are screwed.

[crazy steve]

The vacuum line is connected to the #2 intake boot, with the petcock lines connected to #1 & 4, 3 is plugged...

That's why your vacuum advance is hammering. DO NOT connect it to the manifold, if the carbs don't have the correct type of port for this leave it disconnected.

Look in the 'modifications' forum here (for ZRX carbs I think) for a post on this.

[motoman]

Has to be ported vacuum, not manifold vacuum, and the mechanic is likely right in this case. IIRC, those carbs dont have a ported vacuum location. The original carbs had a ported vacuum hook-up on #2 carb. Some have tried to drill and make a ported vacuum on those carbs, but IMHO, if off a couple thousands, the carb is trashed. You'd be better off getting a good rebuildable set of BS34Mikunis.

[skids]

Yoy could find a way to put a vacuum nipple on one of the carbs. If you do, the hole that penetrates through the carb (outlet side of the butterfly plate and cloose to it) will need to be tiny Alternatively, you could install a restrictor inline in the vacuum hose. This is to dampen the impulses from the vacuum.

[motoman]

[QUOTE=skids;377462 Alternatively, you could install a restrictor inline in the vacuum hose. This is to dampen the impulses from the vacuum.[/QUOTE]

Won't work correctly Sid, again, has to be ported vacuum. He'd be better off with the vacuum can dis-connected than using manifold vacuum. Manifold vacuum, it would be fully advaced(vacuum anyways) at idle and no vacuum advance under throttle load.