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Isolating Air Leak

Discussion in 'Technical and Troubleshooting Torque' started by Zealous, Sep 13, 2012.

  1. Hi,

    I'm trying to isolate an air leak around the area of the number 2 carburetor and intake manifold. The bike has been in storage for near 4 years and only recently started. I did the following before attempting to start it:


    Spark Plugs
    Cleaned tank (Only little rust was present, used ball bearings to remove it)
    Inspected electrical system
    Dismantled, cleaned and balanced carburetors
    Replaced fuel line and filter

    I started it and after balancing the carburetors it was running quite well for about 5 minutes (Idling) when all of a sudden the revs increased without me touching it, I then moved the throttle a tad and it near red-lined indicating an airleak; to ensure it was indeed an airleak I managed to get it to idle again and sprayed a tad of WD40 around the boots and the revs increased.

    I've since removed the carburetors and intake manifolds to inspect them and even though the outside of the manifolds have cracks the inside of each are in excellent condition.

    Before reinstalling the manifolds I placed 0.4mm of gasket material under each and replaced the clamp screws and manifold bolts with SS Allen heads as the OEM stripped easily. I started it not long ago and its idling much much better but the number 2 manifold has a leak somewhere however I cannot isolate it to a particular area. I'm spraying in the area shown below so if anyone has any ideas I would greatly appreciate your input.


    Once the engine warms up its difficult to get your hands in there. I was thinking of placing some silver tape over parts of the manifold to try and limit the rev increase that way I would have a good idea of where the leak is occurring.

    Tomorrow I will inspect the spark plugs and post a picture if that helps.

    Bike information:

    1992 Yamaha FZX250 ZeaL

    EDIT: I'm not directly spraying WD40 into the bore, when I spray the carb boots the carburetors and airbox are installed

  2. I would think the most likely cause would be one of these cracks being worse than you think - or opening up under vacuum.

    I had a similar problem with my old grey import, which I fixed by coating the cracks in the rubber manifold with gasket goo. Try taping it first and see if it improves.
  3. Will do. How long did that fix hold up for you (gasket goo)?
    Checked with my local Yamaha dealer today, turns out the manifolds are $108.00ea which combined equals a greenslip so I will try all repair ideas.
  4. A great way of checking for a leak is as follows.

    Have the motor running, idle is good as it should be pulling a good vacuum.
    Accurately pour a decent amount of water over the suspect areas one at a time.

    What you are looking for is the motor to splutter or stall, this is due to water being sucked in under vacuum.

    Continue doing so until it does stall, pull apart that area and inspect for where the leak maybe.

    If do need to use a sealant, I've always found RTV gold to be the best, will even resists exhaust temps.

    Oh another way is to listen for a leak. I know this will seem stupid until you try it.
    Run the motor on idle and use a long blade screw driver.

    Place the tip of the driver against the suspect area and stick your ear on the end of the handle. It works like a stethoscope....trust me it does work, great for chasing that noise in a motor.
  5. Was still holding fine 30,000kms later.
  6. Thanks fellas, will try your suggestions out in the morning.

    - Zealous
  7. Do the boots have a vacuum port for carb balancing? If so you're in luck. Find something that will bung up the carb end of the boot, set the engine at top dead centre so the inlet valve is shut, wet the boot with some soapy water and blow down the vac port. If there's a leak, the soap will bubble and pinpoint it. If there's no vac port, you'll have to find some way of putting a bit of tube through whatever you used to block up the boot.
  8. Stainless steel bolts into alloy ?
    Really they should be mild steel?
    But not likely relevant
  9. Technically stainless into ally is bad. Practically I've always found it to be better than mild steel in corrosive environments (wet salt etc) provided that decent anti-seize is used. Again, copper is theoretically bad but has never caused me any problems. Nickel is better but harder to find from "high-street" type outlets.
  10. In the past I have also used silastic and tape to seal broken manifolds. Works until it stops working then do it again..
  11. Silastic is not compatable with a fuel area
  12. Its a rare thing that silicone is not the go
    Old fashioned product is the go but
    Name escapes me atm.
    Anyway a gasket compound cud do it.
    Or maybe think out side and be sure its
    Just not running too lean .
    Spray vapor cud of got in through top
    Of carbs ?
  13. Excuse this idea if ur a mechanic.

    The alloy surface quite often is less than
    Pristine on manifolds etc.

    You have to be pristine as you do with
    Head gaskets etc.

    Nice n clean n shiny.

    Just trying to throw ideas on the table
    For ya bloke
  14. Thanks again for all the suggestions. I used a non-hardening sealant to seal the cracks and used 0.4mm gasket under each of the manifolds. After starting it only moments ago there's a leak but I've nailed it down to one of the clamps. Initially this clamp was on number 2 (The problematic intake), so to eliminate the clamp being the issue I changed its position to the number 3 intake and now 3 is leaking around the clamp area. I will replace all 4 clamps with standard hose clamps tomorrow and report back.

    Thanks again,

    - Zealous
  15. Hi again,

    Unfortunately I spoke too soon, the problem still remains. It was worse with the hose clamps I purchased from the hardware so I returned to the OEM ones, covered all the cracks with gasket goo and taped the complete manifold but a leak persists around the number 2 manifold. I've been playing around with the tightness of the clamp but cannot work out if its having any effect. I only get like 4 shots per day because the engine must cool otherwise it starts running near normal.

    Additional information:

    - Idles different when upright, if on side stand it's hard to get it to idle in the one spot (e.g. 1600) - revs will increase without anything touched.
    - Sucking fuel, actually lots of fuel but not spitting fuel.
    - Headlight is really hot, not sure if normal because different headlight has been added (Twin instead of round). Will need to check that out.
    - Removed the added gasket under the manifolds, it was causing a difference in carburetor position between 1 and 4 thus carburetors were not sitting right in the boots.

    Still haven't checked spark plugs but will do when it cools.

    Can an electrical problem cause idle issues?

    Thanks in advanced.

    - Zealous
  16. Are you not able to buy replacement hose and make new manifold tubes for it? Places like Supercheap and Autobarn sell silicon hose in verious sizes and you could cut some short lengths of the right diameter to replace the cracked ones you have. This hose is used on intercooler and turbo inlet manifold connections.
  17. Can't offer any suggestions re your other queries, but any headlamp not subject to a cooling air flow will get bloody hot. It's perfectly normal and one of the reasons I don't like hard wired headlamps.
  18. Hey guys,

    I'm still needing help here, this is way out of my depth. I purchased new manifolds (all 4) costing near $300, but same problem persists. I don't believe it to be an air leak as I have sprayed near 5 cans of carburetor cleaner around the area of the manifolds, head/engine gaskets and bolts and the reason revs were increasing, causing me to believe it was an airleak, is because the carb cleaner was evaporating toward the intake and it was sucked in (I lol'd too).

    Anyway this problem I've narrowed down to the carburetors. Watch the video below:


    Notice when I only slightly touch the throttle adjuster (Where the cables are placed), the revs increase as normal but do not return to idle.

    Turn the engine off, restart and idles perfect but touch throttle thingy and it shoots up and stays there.

    Things I've done:

    Thoroughly cleaned carburetors
    - stripped and placed in dip for 48 hours
    - Cleaned off with water, carburetor cleaner in can and compressor
    - Reassembled making certain diaphragms in working condition and jets, passage ways all clean and clear ready to accept fuel.
    - Adjusted floats, sitting at 9mm when seated and 17mm when angled open
    - Triple checked vacuum hoses to make certain nothing loose or cracked
    - Checked manifolds again (even though brand new) for cracks and to see if flush with engine and carburetors)

    Checked spark plugs, see images below:

    Spark plug 1
    Spark plug 2
    Spark plug 3
    Spark plug 4


    I'm at a loss right now, cannot afford service so must learn with help from you.

    Thanks in advanced.

    _ Aaron
  19. Choke.

    You say it's using fuel?

    At least two of those plugs could be bordering on wet.

    Check the routing of the choke cable, see if it's pinched, or misrouted, causing the inner to be pulled tighter than it should be. If the engine is getting more fuel than it should, it will cause the revs to hang like that. The hanging revs are also a diagnostic tool when you are rejetting carbs, have the carbs been rejetted?

    Best way to check that choke cable would be to disconnect it from the carb side, careful not to lose a spring that it may or may not have on that end, and check that the inner moves freely in and out of the sleeve. Then check wherever the lever end of the choke connects, move it in and out, make sure it's not jammed or twisted. Then, if it is one of the chokes that is connected to the handlebars/switchblock, check they are rotated to the right position so they aren't fouling on the frame/chassis/handlebars anywhere.

    It may not be it, but hopefully this helps.
  20. Additionally, watching your video again, maybe check that the spring is working properly in your throttle tube, the grip can slide down and foul against the switch block, stopping it from returning properly. After that, check the throttle cable tension on the top and bottom cables, to make sure that the grip is returning the carbs to the right position when you are off the throttle. There should be a wheel with a cable over the top and one around the bottom, they push and pull against each other.

    After that, make sure that your idle screw, it usually has a spring around it, make sure that it is in contact with the throttle stopping plate when the throttle is closed. If it is not, then the bike is free to choose it's own throttle position for idle, and this can be different every time if things are a little old/sticky.
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