Carb spitting gas and bogging | GTAMotorcycle.com

Carb spitting gas and bogging

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andrewsnyder

Member
*1989 Yamaha FZR 400*
So I've had this bike apart so many times now and have done a valve job twice now. The first time was to get them in spec but once i had it running it never ran right, it always bogged from idle, extremely hard bogging. I've cleaned the carbs an infinite amount of times now. I recently took it into a shop where the mechanic checked my carbs which were fine, sync'd them. but then when he did a compression test all of the sudden Cyl#1 had next to none compression and when a leak-down test was performed it had about 95%-100%. He thought it would be my valve clearances(too small and opening too early) but when i got the bike back i took it all apart and all my clearances except for a too big clearance on 1 exhaust valve. I changed all the shims to be on the high end of specs as some were low. I also lapped or re-seated my valves with a valve grinding compound because I thought if the clearances are fine it must not be sitting right. The valves looked fine to me, no rounding, seals good, and not burnt(cracking or wear). I finally put it back together yesterday, timing is bang on. BUT, when it started up after a few seconds of trying to get it going, ITS ALL THE SAME. Gas just spits out of the intake and you can feel lots of air coming our of that Cyl, it also still has a large bog but it does seem like it has gotten a bit better, but very little, it'll still die if i give it for long enough. I am going to do another compression test tonight and if its gone I may bring it to another shop or something as I've had this for over a year now and its taken all my money, sanity and happiness away because this damn issue causing me not to be able to finish it.

Whenever I take the carbs off, i see a lot of white "mist" or "smoke?" in the intake boots, mostly for Cyl2,3,4 less in #1, which is odd because it used to be complete opposite.
Bike has Pods(weren't on when spitting gas), Muzzy Can, 110 Main Jet(way higher than stock 87.5), new intake boots.

Another thing was gas suddenly was leaking out the bottom of my carb which I have never had before, also when I take the carbs off there is a lot of gas sitting on the boots and I have no idea where it came from. It sits on the top of the boot on the outer ring. Any ideas are EXTREMELY APPRECIATED. Thank you.Thanks,
 
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andrewsnyder said:
*1989 Yamaha FZR 400*
So I've had this bike apart so many times now and have done a valve job twice now. The first time was to get them in spec but once i had it running it never ran right, it always bogged from idle, extremely hard bogging. I've cleaned the carbs an infinite amount of times now. I recently took it into a shop where the mechanic checked my carbs which were fine, sync'd them. but then when he did a compression test all of the sudden Cyl#1 had next to none compression and when a leak-down test was performed it had about 95%-100%. (This is important) He thought it would be my valve clearances(too small and opening too early) but when i got the bike back i took it all apart and all my clearances except for a too big clearance on 1 exhaust valve. I changed all the shims to be on the high end of specs as some were low. I also lapped or re-seated my valves with a valve grinding compound because I thought if the clearances are fine it must not be sitting right. (Point 2) The valves looked fine to me, no rounding, seals good, and not burnt(cracking or wear). I finally put it back together yesterday, timing is bang on. BUT, when it started up after a few seconds of trying to get it going, ITS ALL THE SAME. Gas just spits out of the intake and you can feel lots of air coming our of that Cyl, it also still has a large bog but it does seem like it has gotten a bit better, but very little, it'll still die if i give it for long enough. I am going to do another compression test tonight and if its gone I may bring it to another shop or something as I've had this for over a year now and its taken all my money, sanity and happiness away because this damn issue causing me not to be able to finish it.

[...] Removed for relevancy
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1. The first thing you have to address is the mechanical sealing issue as found by the leak down. Based on those readings, cylinder #1 is not sealing at all. You need to find out why before you go any further. Did the tech say which valve(s) were leaking? Usually with a leak-down you can pinpoint whether the leak is from the intake or exhaust side fairly easily. If he suggested it was a clearance issue, and your clearances were in spec, then it is not a clearance issue. There is something else wrong. Did you tell him that you had just been doing a valve service? If so, that would lead me to believe that they need to be confirmed before moving on to something more serious. Which is what he suggested (I'm inferring this based on your relation of the discussion and your actions before and after).

2. You said your clearances were fine, and you adjusted them anyway? I am assuming that you did these in the reverse order as you wrote them.. Lapped, then reassembled, then reset clearances.

At this point you should be looking for the leaky valve, assuming that is where the leak is coming from (which can be heard with a leak down test). Once you know where the leak is, you can begin solving the problem which may require another head dis-assembly.
 
People way more experienced than me are going to chime in, but needless to say, you need to re-check compression on cylinder #1 before trying anything else. Let us know how that goes.

Edit - ^^ this guy beat me to it.
 
LPakkala said:
1. The first thing you have to address is the mechanical sealing issue as found by the leak down. Based on those readings, cylinder #1 is not sealing at all. You need to find out why before you go any further. Did the tech say which valve(s) were leaking? Usually with a leak-down you can pinpoint whether the leak is from the intake or exhaust side fairly easily. If he suggested it was a clearance issue, and your clearances were in spec, then it is not a clearance issue. There is something else wrong. Did you tell him that you had just been doing a valve service? If so, that would lead me to believe that they need to be confirmed before moving on to something more serious. Which is what he suggested (I'm inferring this based on your relation of the discussion and your actions before and after).

2. You said your clearances were fine, and you adjusted them anyway? I am assuming that you did these in the reverse order as you wrote them.. Lapped, then reassembled, then reset clearances.

At this point you should be looking for the leaky valve, assuming that is where the leak is coming from (which can be heard with a leak down test). Once you know where the leak is, you can begin solving the problem which may require another head dis-assembly.
Click to expand...

The leak was found to be in the Intake more than the exhaust as during the test you could hear the exhaust close but the intake was open the whole time through the carb. I told him that I had done a valve job before and he was surprised as he thought it would be clearances.
As for lapping I went and checked clearances, lapped then adjusted clearances to better spec. After lapping the valves I tried putting soapy water on the valve heads and blowing air through the bottom to check for leaks which, there weren't any which now is screwing my head.

Sent from my A0001 using Tapatalk
 
Matt Rain said:
People way more experienced than me are going to chime in, but needless to say, you need to re-check compression on cylinder #1 before trying anything else. Let us know how that goes.

Edit - ^^ this guy beat me to it.
Click to expand...
That's what I plan to do in a few hours, will reply with an update.

I said I'd do this in my original post.

Sent from my A0001 using Tapatalk
 
The leak might be very difficult to detect with the head still installed. If there is still a leak it could be:

Valve seats - may need to be re-cut
Valve guide to stem clearance - if worn the seating might be off intermittently.
Cracked head
Valve itself

Easy way is to remove the head and the cam(s) to locate the leak. Fill the back side of the valve up with something thin (I have used brake cleaner in the past) and watch for any moisture leaking through to the combustion chamber. I would probably do another leak-down before dismantling, in case you corrected the leak and you are chasing two (or more) problems.
 
So after doing a test, no compression, again (fml). Im not sure if somewhere theres a pin hole leak as I dont have a leak down test I had brought it to someone before which I may do again.
A BIG THING THOUGH is that I USED to have compression in this cylinder but then I went back and did my valve seals and thats when it went downhill but the seals didnt look bad this time around.

When inspecting valves before, valves seats LOOKED okay but doesnt mean they were that good. Valve guide im not sure how I would inspect that but it seemed okay before. The head was not cracked anywhere I could see. could my piston rings cause the spitting? I know if they were bad(they werent before) they can cause low compression. The valve itself didnt look bad or bent at all, I COULD order new ones except they are so goddamn expensive. I will probably start pulling the head and stuff again soon but I want to talk to some local guys first.

with the spark plugs disconnected it still shot out gas which I thought to be normal, I also check lead wire and it didnt look like it had a good connection which I fixed but it did nothing.
 
LPakkala said:
Yup. Can be checked by measuring free length and comparing to spec. Also spring alignment or valve stem could be stretched too.
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But the spring seemed totally fine, same height as others, same amount of work to compress it when removing valves.

Sent from my A0001 using Tapatalk
 
lots of engines have carbs that will have a stand off fog of gas... and you can't make adjustments to the tune up with the air box off.. they require that fog in place when the throttle opens.. the fog is drawn in and keeps the mixture correct..

that said... you MUST find out why there is no compression in that one cylinder..

can you watch the valve stems / springs move while spinning the engine over? ... spark plugs out..
spin it over and watch the valve stems move,

put your thumb over the spark plug hole... spin the engine over.. and it should blow your thumb off the hole if there is good compression.. redneck compression test

I have seem compression testers that mess with you as the needle is loose, and when you put it on left cyl.. good reading,, but installed on the right side.. the gauge is leaning the other way,, and the needle falls to zero.
 
meester_jamie said:
lots of engines have carbs that will have a stand off fog of gas... and you can't make adjustments to the tune up with the air box off.. they require that fog in place when the throttle opens.. the fog is drawn in and keeps the mixture correct..

that said... you MUST find out why there is no compression in that one cylinder..

can you watch the valve stems / springs move while spinning the engine over? ... spark plugs out..
spin it over and watch the valve stems move,

put your thumb over the spark plug hole... spin the engine over.. and it should blow your thumb off the hole if there is good compression.. redneck compression test

I have seem compression testers that mess with you as the needle is loose, and when you put it on left cyl.. good reading,, but installed on the right side.. the gauge is leaning the other way,, and the needle falls to zero.
Click to expand...
No I can't do any of that on this engine

Sent from my A0001 using Tapatalk
 
UPDATE: So checking clearances tonight, one of my intakes is totally out of spec. Will fix it shortly and respond. When I fired the bike up today it seemed to run MUCH better than before which one of the valves is now in spec which could mean that it's this last valve causing it's screw up. AS for the carb shooting gas, that's definitely because the leak of air coming from the cylinder through the carb is spraying it up words <br /><br />Sent from my A0001 using Tapatalk

Sent from my A0001 using Tapatalk
 
FINAL UPDATE:

So went back to check clearances, turns out one of the intakes had a WAY bigger shim (170) vs the shim in the intake valve beside it (135) so once switched, put bike back together and that cylinder works again(YESSSSS). It did bog but didnt spit any gas. Fixed bogging with Pilot Adjustment screws so now it runs mint. Time to fix fairing, check cooling, and RIDE IT FOR THE FIRST TIME!


THANK YOU ALL SO MUCH WHO HELPED ME FIX THIS ISSUE, IT MADE THIS PROJECT SO MUCH BETTER.
 
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