Poor Power <2500RPM Adjusting Carburetors and Valves?

If you go right to a yamaha dealer you can get them for about the same cost as mikes. Get actual mikuni jets. The ones mikes sell are Chinese knock offs and don't work as well. I f you compare the two side by side the the orifice in the back is way bigger on the cheap ones and the fuel is not metered as is should be. Do what you want but that is just my 2 cents:laugh:
 

Attachments

  • DSC02632.jpg
    DSC02632.jpg
    151.2 KB · Views: 348
Drilling jets can be an exercise in frustration. A tiny burr can reduce flow through the jet and make it flow less fuel despite being larger. For the price, you'd be better off buying an assortment of different sized jets.

I wouldn't even consider jetting until the carbs are clean and everything else is in good order.
 
robbiecampbellsoup said:
And does anyone know what the bolt on the right is? It was jammed between the boot thing and the block.
Click to expand...
It almost looks like the little bolt on the head which you loosen in order to check oil pressure and flow after an oil change. But you couldn't drive it, unless a P.O. lost it and then bought a replacement.
Sounds like it just got dropped from above and lost (could have been years ago) by a P.O.
 
Lou Ranger said:
It almost looks like the little bolt on the head which you loosen in order to check oil pressure and flow after an oil change. But you couldn't drive it, unless a P.O. lost it and then bought a replacement.
Sounds like it just got dropped from above and lost (could have been years ago) by a P.O.
Click to expand...

The bolt that is in there does look aftermarket - it has a 10mm hex drive as well as a philips head.

I'll also check the tach drive cover - thanks for the input guys.
 
These things look pretty damned clean. I'm going to clean them anyways...

Was going to boil them in vinegar. I've removed everything so far but the butterfly valve assembly - should I do this before boiling?

Here are the Jet sizes; I have stock airbox (with shitty filer that smells of fuel...) and Mac 2-1 exhaust.

Side of Bike: RH LH
Pilot Air Jet: 170 170
Pilot Jet: 45 45
Main Jet: 117.5 127.5

I've figured that the idle is lean - I'll attach a video of the idle and throttle response on an edit.

Youtube video of idle
 
Last edited:
Another update;

These are my plugs (after idling only)
Right


Left


1. They were "wet" when I took them out... not sure if that's oil, or fuel, or whatever.
2. The right plug 'looks' like it's a bit lean
3. The left plug 'looks' rich

Looking at the jets that were put into the carbs...

Due to the airbox, I'm assuming that the LEFT cylinder gets more air flow initially, due to a less restrictive air path. My brain tells me that this would mean that the LEFT carb should have a higher flowing main jet, the jet with the larger number.

Ahhhh there's so much to do :(
 
Judging by the extreme blueing of your exhaust headers the bike was running really lean at some point. Give the darker plug a sniff when you take it out and if it smells like oil.
 
When life (PO) gives you lemons (beled screw broken off and sheared into caliper) you say fuck you and drill and tap the hole to M8 and then spend way too much time making a custom stainless steel bleed nipple.

 
Yes you should take out the butterfly assembly. I made that mistake and my butterfly seals came out in pieces. You can order them from the yamaha dealership as well as replacement screws that are VERY easy to strip, even with JIS screwdrivers. Standard screwdrivers will almost definitely strip them and then you will have a screw in there that will be a real SOB to get out.
 
robbiecampbellsoup said:
These things look pretty damned clean. I'm going to clean them anyways...

Was going to boil them in vinegar. I've removed everything so far but the butterfly valve assembly - should I do this before boiling?

Here are the Jet sizes; I have stock airbox (with shitty filer that smells of fuel...) and Mac 2-1 exhaust.

Side of Bike: RH LH
Pilot Air Jet: 170 170
Pilot Jet: 45 45
Main Jet: 117.5 127.5

I've figured that the idle is lean - I'll attach a video of the idle and throttle response on an edit.

Youtube video of idle
Click to expand...


Nice vacuum leak test in the video!


robbiecampbellsoup said:
When life (PO) gives you lemons (beled screw broken off and sheared into caliper) you say fuck you and drill and tap the hole to M8 and then spend way too much time making a custom stainless steel bleed nipple.

Click to expand...

That looks good.
 
16VGTIDave said:
That bolt looks like the one that retains the tach drive in the cam cover.
Click to expand...

Luckily for me, the PO JB welded a hole on the top of the crank drive cover. I'm going to hope that it is not that hole... but really, any hole.. why do people JB weld bolt holes :(.

Woe is me.

On a side note - brakes - How do you bleed them effectively?

I'm not getting any leaks, but the lever feels very spongey.

I followed this process:

All fluid was drained (I had to make a new bleed nipple) - hoses re-attached, 1200 grit sanded the copper washers.

Open reservoir - add DOT3 fluid.

Squeeze lever - open bleed nipple - close bleed nipple - release lever

repeat.

Do you guys typically take the caliper off the fork to do this? Jam some 1/4" stock in the caliper so the piston doesn't eject?

I wanna make this bike purr.

I think.. that I will put on the UNI filters as well as rejet to larger pilots/mains while I have the carbs out - the exhaust was already modified - the pilot jets have been upped up to 45 each, but the main jets are stock - I can't find any info on the pilot air jet stock value; mine is 170...

I suppose there will be more tuning to do; but does anyone have any feedback on the idling that I displayed in the video?

Thanks for the continued support and tips everyone!
 
Okay - so update on this whole thing.

I've removed and cleaned the carbs, and re-installed them with the jetting that had existed from the previous owner. I also readjusted the float height according to specification with new float needles. The main jet needle and boot are new, and look to be in great shape.

45 pilots and stock main jets LH 127.5 RH 117.5.

I synced the carbs using the home-made style manometer (U-Loop of tubing). Those things are all over the place as you rev it! but I synced them at idle.

I did a compression test:

~130 PSI right, ~125 PSI left. The right cylinder could run the engine at idle on it's own while I compression tested, the left could not, it had to be fed about 1/4 throttle to stay alive.

The throttle response is much better if I simply take off the vacuum tube on the left carb, allowing more air into the stream, after the carb. This means the ratio is more lean, and considering the left was burning rich at idle before anyways, this makes sense. However the right cylinder seems to be burning 'okay'.

I also measured my valve clearance;

LH Intake: 0.006"
LH Exhaust: 0.009" (over spec)
RH Intake: 0.006"
RH Exhaust: 0.009" (over spec)

The sparks from each plug looked similar.. but I'm not sure if they are good or not? Is there a way to measure the spark 'power' etc? I checked the gap and they were within tolerance.


So - I need some help. Can anyone point me in the right direction?? I want to get this thing back on the road. It's been sitting idle for two weeks while I tinker (mostly) blindly.

I will add a youtube video showing some simple testing, and so you can hear the throttle response etc.

I picked up 130 mains and 132.5 as well as some Uni pod filters, and the materials to do an H-pipe with breather a la Dave - I think I'll save that for winter though.

I also picked up 42.5 Pilot Jets

I may also do a fuel injection retrofit assuming a get the bike running and have fun for the rest of the summer.(I have a co-worker who's converting a 1940s lawnmower engine to a fuel injected generator... he's offered to help with a more advanced conversion).
 
For syncing carbs you do it at idle. Your manometer needs to be much longer. To test compression you do it with the motor off and cold. With one piston at a time and with the throttle wide open turn it over. If you are getting 125 with a warm engine I would say your compression is too low and a lot or you problem.
 
I think I would be looking at both cam and ignition timing at this point, maybe even a leak down test just to be sure!
 
This is what you want.
 

Attachments

  • user3153_pic120_1294889478.jpg
    user3153_pic120_1294889478.jpg
    86.9 KB · Views: 352

Similar threads

Myron
Idle RPM climbs at stoplight.
Replies
2
Views
612
Myron
Myron
G
DOHC Seca no power after 4000 rpm
Replies
2
Views
503
CaptChrome
CaptChrome
pulmenti
No Start - Shims / Valve Adjustment
Replies
11
Views
704
tstidham
tstidham
A
1980 XS400 Special, jet issues.
Replies
2
Views
510
Archilles
A
AmbientMoto
What am I missing?
Replies
18
Views
2K
CaptChrome
CaptChrome
Back
Top