ive got a YAMAHA NIGHTMARE on my hands!!!!

[oonighttrain]

as you may know from my previous posts.. i have a 1998 yamaha 50 horse 4 stroke..

it had low compression on #1 and #4 of the 4 cylinders.. so, i took the head off, took it to the machine shop, they did a vacuum test on all the ports and told me it was fine..

i got some info froma few guys on here that said to do a leak down test..

so, i did, had air coming out of the dipstick tube when i put air in the #1 cylinder..

so, my bud and i took the powerhead out, took it all apart, honed the cylinders and put new rings in....

after hours and hours of work.. the motor is back together.. it runs....

BUT, i still have the same issue.. runs a bit rough and still have the same compression problems..

btw, i miked the cylinders and they all were within specs...

2 and 3 had over 120 p.s.i. at first.. and still do...


the weird thing is, im getting erratic compression readings on #1.. it will show almost no compression, then it will read 30, then 60... ive tried 3 different guages.. pretty much all the same...

MY BRAIN IS FRIED FROM TRYING TO FIGURE THIS OUT!!!!!!

 

[rodbolt]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

somewhere ya did something wrong. the mechanical side is easy but requires close attention to detail.

 

[oonighttrain]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

any chance it could be a bent valve or bad valve guide?

how about timing???

 

[rodbolt]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

think mon think
tell us how timing can affect compression/leakdown?
no you have a mechanical issue that MUST be resolved before fighting anything else.
ya did something wrong at reassembly or the head is bad.
like I told ya before. aint NO vacum test for valves.
double check the machine shop.

 

[4.0l sahara]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

think mon think
tell us how timing can affect compression/leakdown?
no you have a mechanical issue that MUST be resolved before fighting anything else.
ya did something wrong at reassembly or the head is bad.
like I told ya before. aint NO vacum test for valves.
double check the machine shop.

If valve timing is off and the valve is open at the wrong time comression can leak out. Also could be the valve not seating right.

 

[oonighttrain]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

If valve timing is off and the valve is open at the wrong time comression can leak out. Also could be the valve not seating right.

thats what i thought too...

but, i still have a leakdown problem.. i put air into #1 cylinder and i get it coming out the dipstick hole..

im wondering if the cylinders are outta round??


also, wondering about the timing.. i took a light to it and found that it appears ok according to the flywheel marks but, is it possible to check the timing at the cam pulley also???

 

[BruceAML]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

Timing determines when the ignition is supplied to a spark plug. It appears that you have a bad cylinder head, head gasket or valve. Did you use a new genuine Yamaha head gasket? If so, what was the part number? What exactly was done to the head and valves?

 

[JustJason]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

if your hearing ALOT air out of the dipstick then your rings are not sealing.. period. Now that may or may not be your only problem. You said you mic'd the cylinders. But did you take the time to fit the pistons? You mic'd the cylinder but did you do it on both axis at 3 different points per axis?

1. what is the specified piston to cylinder wall clearence, and what is yours?
2. what is the specified ring end gap, and what is yours?
3. what is the specified taper, and what is yours?
4. what is the specified TOR (total out of round), and what is yours?
5. are the pistons directional, and are yours in the correct direction?

I'm pretty sure your answer is going to be in 1-5 somewhere.
When you say Mic i'm assuming your using an outside micrometer to measure your piston, and a dial bore gauge calibrated with the same micrometer to measure the cylinder wall.
A set of dial/digital calipers wont do, especially to measure the bore.

 

[rodbolt]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

if cam (valve) timing was off it will affectr all cylinders equally as it will affect all cam lobes the same.
minor cam timing issues usually show up as low engine compression or low intake manifold vacum.
not the issues going on.
as this engine was dissasembled and reassembled we now have to find which assymbly point is creating this issue.
this is why a compression test on this issue is worthless.
a leakdown test with a leak down tester is imperative.
as the pistons have ring gaps you will always have a certain % of leakage.
what we need to know is where the leak is and which component is leaking.
barring a casting flaw in the head your down to piston ring leakage,cyl head gasket leakage,ex valve and intake valve leakage.

 

[oonighttrain]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

well,
i spoke to a guy from a machine shop today that gave me some info..

i told him the whole story, including my problem kindof intermittent compression readings. and when i put a timing light on the #1 wire, checked
the timing on the flywheel and also at the cam pulley. the cam pulley appeared to be off the mark (#1). also, i told him i pulled off the rocker arm rail and turned the motor by hand, whn i did that i had much more compression out of #1.. he said, he would have to assume that i had a timing issue... he said maybe the timing belt is off a tooth one way or the other..

is there a certain rpm that i should check the timing?

 

[MikDee]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

well,
i spoke to a guy from a machine shop today that gave me some info..

i told him the whole story, including my problem kindof intermittent compression readings. and when i put a timing light on the #1 wire, checked
the timing on the flywheel and also at the cam pulley. the cam pulley appeared to be off the mark (#1). also, i told him i pulled off the rocker arm rail and turned the motor by hand, whn i did that i had much more compression out of #1.. he said, he would have to assume that i had a timing issue... he said maybe the timing belt is off a tooth one way or the other..

is there a certain rpm that i should check the timing?

From what you're saying, If you're compression is good on both cyliners with the rocker arms off, then it has to be the in the valve, or rocker arm system, did you check the valve springs on the 2 bad cylinders to see if there's play, or slack, in the rocker, or a broken valve spring.

 

[oonighttrain]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

From what you're saying, If you're compression is good on both cyliners with the rocker arms off, then it has to be the in the valve, or rocker arm system, did you check the valve springs on the 2 bad cylinders to see if there's play, or slack, in the rocker, or a broken valve spring.

well, i wouldnt say its good.. it was however, a whole lot better than running it with the head on it.. i turned it with a wrench on the flywheel and got 60 out of it..

i did check the valve springs, valves and all to see if i could see anything.. all looks ok..

im still wondering about the timing, as far as maybe the belt is off by a tooth or something..

 

[MikDee]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

well, i wouldnt say its good.. it was however, a whole lot better than running it with the head on it.. i turned it with a wrench on the flywheel and got 60 out of it..

i did check the valve springs, valves and all to see if i could see anything.. all looks ok..

im still wondering about the timing, as far as maybe the belt is off by a tooth or something..

You keep narrowing it down yourself, if the compression isn't good with the rocker arms off, then maybe the valves themselves aren't seating? as another poster said if your valve timing was off, all your cylinders would read different compression, besides don't you have some way of checking the valve timing now, with it partially apart?

 

[oonighttrain]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

You keep narrowing it down yourself, if the compression isn't good with the rocker arms off, then maybe the valves themselves aren't seating? as another poster said if your valve timing was off, all your cylinders would read different compression, besides don't you have some way of checking the valve timing now, with it partially apart?

i guess you could roll the cam around when the head is off and make sure the valves are closed when it hits each 1,2,3,4 mark????

anyone know if there is a certain rpm to check the timing with a light?

 

[JustJason]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

there are 2 rpms you need to check... your idle... and your wot timing.

I do suspect you need a leakdown test first my friend. anything else and your chasing chickens.

do a leakdown. post your readings. then answer my 5 questions.

 

[MikDee]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

i guess you could roll the cam around when the head is off and make sure the valves are closed when it hits each 1,2,3,4 mark????

anyone know if there is a certain rpm to check the timing with a light?

All those valves should be sprung tightly to the head, or block, no up, & down play, or slack, and you shouldn't be able to turn them by hand, nor should they be bent, or off their seat for any reason, if your rocker arms are off.

 

[oonighttrain]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

i just got done re-installing all the valves back in the head..

they all look fine, all the springs look good, seats look good..

the only thing that i question is why only #1 and #4 exhaust valve where pretty carboned up (on the face, between the stem and the seat).. .. #2 and #3 were fine, no deposits at all..

 

[oonighttrain]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

ok.. i put the head back on, adjusted the valves according to specs.. re-installed the timing belt as close to perfect as humanly possible.. it started fine, sounded pretty good when it was running, smooth.. ..(better than it has)..


my buddy thought it was a good idea to check the compression.. RUINED MY NIGHT AGAIN!!! SAME CHIT!!! 15-20 on #1, over 120 on 2 and 3.. couldnt get the guage down to #4 and was just to pizzed off to care... had we not checked the compression, i would have thought everything was good...

its a good thing im really not a violent person..

 

[JustJason]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

leak down time.

 

[rodbolt]

Re: ive got a YAMAHA NIGHTMARE on my hands!!!!

unless you can turn the flywheel with a wrench at 300 RPM minimum your wasting your time.
you MUST use a fully charged battery and the starter with the throttles at or near the WOT position or the numbers will be all over the place.
preferably on a warm motor but that part is not so imperative,
a leakdown test can be done with the engine on the bench.
there again warm engine would be prefered but not absolutly nessasary.
start rotating the cam or the crank with the cam timing off or the belt off and you will find out the difference between a clearence engine and a non clearence engine.
the F50 is in the NON catagory.
you really need to slow down, use the proper test equipment with the proper service specs.
all your trying to test is the ability of the cylinder to seal.
it can leak past the valves,head gasket or piston/rings.
but ya cant do a compression test using a wrench on the flywheel,aint gonna happen.
ign timing on the F50 is fixed, nothing you can do to change it.
cam timing is a simple mater of lining up the marks on the flywheel and the mark on the head and cam gear as per service manual.