Low compression

[MNS494]

81 mns 1100..21000 miles.....just had the valves adjusted.......they were very tight........compression test was 115 (+/- .005) across the board....went to 150 with oil in cylinders............book says 128 min.....guess that means it got very hot at some time and needs new rings.....so, would the low compression make it run rich?.......also.....the engine revs are slow to come back down to idle..........i don't know what tag to use.......

[Incubus]

You didn't say if this bike was a runner or if you'd ever heard it run. If the engine sat a long time the rings could have become stuck in the piston grooves. If so, and if the engine turns freely, it is possible that the rings will loosen and the compression would go up afer the engine has run for a while. If this were my bike I would start it and run it for a while before I decided to tear it down.

Patrick

[MNS494]

Sorry............yes it was running, but very rich..........also at idle, it would blow the vaccumn caps off the carb boots........i was told that meant the valves were tight............

[Ivan]

Did you have the throttle held open when you did the compression test?

The oil in the cylinders bringing up the ratio a bit does kinda indicate rings though. If the bike has sat for any length of time without running, its possible the rings have been gummed up with coke (carbon from old oil and such, weird name I know.) and are sticking. In this case, a dose of automatic transmission fluid while running will clean up the cylinders nicely. I usually use a vacuum hose on whatever engine I am cleaning and put it in a quart of ATF, with some form of valve to close the hose. Then start the engine, and rev it while allowing the ATF to pull into the engine. Gradually allowing more and more until the engine finally dies. Then let it sit for half an hour, and keep starting it until it runs again.

A word of warning. This makes smoke. A LOT OF SMOKE. If you live in a congested metropolis, it may be smart to call the local fire department to give them a heads up, or people will call and report a fire. If there isn't much wind, or you are in a shop, you will choke, cough and hack and gag and get diarrhea. Best to do it on a breezy day outside.

[MNS494]

History of the bike........tony rode the bike from 82-98 puting on19,000 miles........saw a bad wreck...bike sat from 98-06 ...tony tried to clean the carbs and broke off a float post.......sold it to a friend (mike).........mike took it to the local cycle salvage shop where they put on tires , fork seals, battery, different carb.......history is a little murky here.........$1800 worth of work..........didn't run right and mike just gave it back to tony..............it sat til nov of 08 when tony took it to the local old yamaha fix-up shop where they went thru the carbs again......$400..........i bought it in spring 09......test ride, it ran good.........got home and it blew off one vacumn cap ..........i started taking extra caps with me and rode it about 1,00 miles on short (60miles or less)rides.........i took off the carbs at least 6 times cause it would always run rich...........it seemed to run good when it was cold and had new plugs.........so i was never sure if it was the plugs fouling that made it run bad.........

Things i've done.....
On the 1st cleaning... I replaced the o-rings under the float seats and weighed the floats (all the same)....set the floats down as per instructions in the front of my clymer book.... The guy that was helping me said that didn't look right so we only went half way down.........still ran rich...

2nd cleaning.....took the floats all the way down..........one plug actually looked pretty good

3rd cleaning was after i found the instructions in the back of the clymer book...........now i had to just guess and then put them on and check..........they were a little low...........still ran rich

4th cleaning i took the carbs to a friend who knew about 2-stroke carbs and we went thru looking for numbers.....everything seemed to be stock when we could find a number..........still ran rich

5th cleaning raised the floats up a tad....number 3 is a bit high......this is when i took it to a friend who adjusted the valves for me.....i'll call him today to see if he had the throttle open when he did the compression check ..i'm assuming you're suppossed to......wife's b-day so nothing's going to get done today..........would like to know if the 115 lb compression is something that makes running good impossible?

[BossMaverick]

As soon as the blizzard clears up I'll be putting seafoam in each cylinder via the spark plug hole to see if it helps my compression. I'd try the same if I was you.

With your running rich problem, I'm assuming the air filter is clean and the air intake hose doesn't have a mouse nest or something, right? Also, the exhaust is open, right?

[skids]

Blowing the vacuum caps off can mean that it is either running too lean or you have leakage through an intake valve. I can tell you that a worn out baffle in the exhaust can also do that. Bad baffles or untight exhaust systems can cause weird scavenging (or the lack thereof) of the exhaust gases and the vacuum can disappear. Exhaust afterfire can occur when both valves are open during the cycle. This condition can also cause the automatic operation of the petcock (standard) or octopus (special) to quit flowing fuel at certain rpm ranges. If this is the case, you might also try running on prime to help diagnose the problem. I dont pretend to know everything about this condition, but I have been there-done-that with one of mine. PS, you might find a strange condition with one or more of your spark plugs if this is happening, like a brown color on the porcelin that appears glazed like ceramic. You might also see fouling in the associated carburator.

[trbig]

Sounds like you either need to get a color-tune and adjust the mixture screws or let someone who knows about these things adjust the mixture screws by ear. Also.. if it's blowing off the vacuum caps, it's because the carbs aren't synch'd. A burnt intake valve could do that too, but the compression is too high for you to have that. And yes.. an extremely tight clearance on the intake valve could also do it.. and if they were all equally tight, they could be holding the intake valve open too long and getting some of the compression stroke back into the carb and blowing the caps off. Are you the one that actually checked them, or are you trusting someone else's work?

Were these compression numbers done with the bike good and warm and the throttle held wide open with a good and charged battery and the other spark plugs out? Being that they are the same across the board and there is only 29k miles on the bike, I really don't think you have a problem besides carbs. Tight valves aren't going to make you run rich.

Tod

[MNS494]

Ok....
Boss...i'll recheck the compression again.....it's not the filter cause i took the bottom of the air box off, cleaned the plugs and rode out to the desert and back....about 10 miles.....checked the plugs and they all were sotty black..........i do have to ride slow when i come back into the subdivision.....

Skids......i like the idea of the muffler being screwed up cause there is at least one bolt missing and the bike puts sooty deposits on the floor when it's idleing......the po drilled 5 -1/2 inch holes in the back of the pipes.......... .i'll check that...the ocopus is gone but it still has the original petcocks on it.......two hoses into a plastic "y" into an inline filter into the carbs........the plugs are always sotty black........

Tod...........i am trusting my friend (dave)'s work but will relay the messages and try rechecking the compression...........yesterday was the wife's b-day so i didn't get over to his shop.....but it got over 40 degrees so maybe he took it for a ride to see what it was doing after the valve adjustment..........i borrowed a sync adjuster and dave said the carbs weren't that far off.........it seems weird to me that all four carbs are running rich....

I'll probably have to work to day since now they're saying partly cloudy........i'll let you know what happens tues.
Thanks d

[MNS494]

Ok........the compression test was done correctly............so i have low compression.......waiting to hear how boss maverick did with his seafoam test......got the valves adjusted......dave said it must have gotten very hot cause they appeared to be "cupped"...anyway, thin shims got it fixed.........dave took it out this morning and took it up horseshoe bend hill...a raise of about 900 ft in elevation.........it didn't run very good til he put some chevron gas in........then it ran great up the hill.........wheel shimmy at 95 he said...we checked the plugs and they were way better...not perfect but tolerable........pulled the carbs and the pilot jets had 8 side holes in them . We sized them and they were all about 55 +......one was the vm style and marked 42.5......the other three had no markings we could find.......took them down to yamaha and talked to dennis burgess who worked on them back in the day.........he looked it up and said my bike was supposed to have jets with no holes and caps on top........i've seen where you guys have talked about the caps but i didn't think my bike used them......now i don't know if i have the right carbs on it........anybody able to tell me how to identify what i 've got.??.......doesn't seem to be any markings or numbers on the carbs .......i saw a pic of a bs 34 ii and thats not them ..and they do have the 4 round holes..........bs 34 iii???...and that one has the caps?

[DiverRay]

Early carbs have the mixture adjustment screw that you can see. the 1980 and later carbs, the one that SHOULD be on the bike, have the mixture screw hidden in a recess in the front top of the carb. You will need to know what year carbs you have BEFORE you start adjusting. The early carbs need a larger main jet because it flowed fuel to the pilot as well. the later carbs have a smaller main, and the pilot jet has it's own fuel pick up inside the carb.
Not totally clear description, but it should help some.

[MNS494]

OK.............MY ADJUSTING SCREWS ARE RECESSED BUT THE CAPS, IF THERE WERE ANY ARE GONE.......I WAS TOLD THERE WAS SUPPOSED TO BE A TUNNEL GOING FROM THE SLIDE TO THE PILOT JET AND THAT THE PILOT JET HAD NO HOLES ON THE SIDE... LIKE THESE?

Mikuni N151.067 Pilot Jet
14mm in length with a 4.8mm Head (outside of threads)
(Description Note - No Bleed holes around the body)



Partial Applications List:
Late model OEM (BS) CV Carburetors.
Kawasaki KZ1000J/K/M/P/R, KZ1100
Suzuki GS1000, GS1100
Yamaha XS1100
JET SIZES AVAILABLE
30 32.5 35 37.5 40 42.5 45 47.5 50 52.5 55 57.5 60

Mikuni N151.067 Pilot Jet
Price: $7.95ea
Qty Size
HMMMM.....THERE WAS SUPPOSED TO BE A PIC.......IS THERE ANYTHING ELSE ON THE OUTSIDE OF THE CARB TO TELL ME IF THEY'RE STOCK? IS THE RECESSED ADJ. SCREWS ENOUGH?

[rpgoerlich]

My 80's carbs had rubber plugs in the pilot with the tunnel as you described from the main to the pilot.
My 81's carbs did not.
I dont remember if the pilot jets had holes in either.

[MNS494]

How do i find that tunnel?...can you see it by lifting the slides or do i need to take them out?

[DGXSER]

Only way I know to see it is to pull the float bowl off. Then if your pilot jet is not plugged off from the float bowl, look down the tube it is in and see if you see the elongated hole in the side toward the main jet/needle jet. That would be the crossover hole between the two.

The early 80s carbs seem to be the likely beast to have these bastardized carbs. From the outside they look just like any other 80 set of carbs, but for some reason they kept the crossover. If your has that crossover, then you will need to plug the bottom of the pilot jet and I believe you need the larger 137.5 (for stock) main jet.