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hawk carb

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  • hawk carb


    My 63Hawk is hard to get started from cold.
    After using magic spray it gets going but gives out a tinny sound on acceleration as if there is a vacuum leakor a leak of some kind.After a run of 5-6 miles there seems to be a slight surge and on one occasion the motor has cut out but could be re started immediately afterwards.Am in the process of checking the inlet manifold in case the exhaust may be leaking into the inlet.I have installed teflon bushes on the front butterfly spindle on the carb as it was very loose but the noise and hard starting persists.Fuel pressure is 4Lbs
    and vacuum at idle is about 14.The PCV valve into the carb is clear.

    Apart from the problem with the front suspension and the starting,2 years of restoration should be finished!Any help would be appreciated

    bill j

  • #2
    If you're measuring vacuum in inches, that's low. It should be up around 17-18 inches. Retarded timing could account for the low reading. When setting the timing remember to disconnect the vacuum advance. On my Hawk I set the timing by the vacuum gauge to get a reading of 18 inches at a slightly fast idle. According to the timing marks on the harmonic balancer, this was slightly advanced but gear wear could account for that. That's why I like to use a vacuum gauge instead of relying solely on the marks. It's also very much easier to see the marks if you rub yellow shop chalk, livestock marking crayon or even kid's chalk on the timing marks before you start. The power steering pump makes it quite hard to get the timing light down in there and with chalk, you can see the marks easily even in broad daylight. It's hard to describe in words, but with retarded timing, the engine sounds labored, ok, laboured for you Aussies, and sluggish. When advancing the timing, it begins sounding more perky and "happy". This is one case where a rusted out muffler comes in handy. In fact, I've set the timing on engines strictly by sound alone. With a bit of practice, you can check later with a timing light and be right on.

    A good way to look for vacuum leaks is with the engine idleing, take a small handheld propane (butane) torch, open it up but don't light it. Move the tip very closely along any joint, shaft, gasket or seal where vacuum can be found inside. If the leak is of any size at all, the engine will speed up a bit when it sucks the propane through the leak. Don't worry about anything blowing up because the fan is going to dissipate the gas, sometimes too well on small leaks. If so, remove the fan blade. Be smart and do this outside (yeah, one of those safety disclaimers).

    You say the engine sounds tinny...I don't know exactly what that is. A large vacuum lead will hiss audibly, loudest at idle when the vacuum is the highest and become quieter under low vacuum conditions such as acceleration or any time the throttle opening is large. A sound reminicient of shaking a tin can with marbles is "pinging" and is a result of detonation. This occurs when you step down on the throttle under load. It can be caused by a fuel air mixture too lean, ignition too far advanced, compression too high for the octane rating of the fuel or carbon deposits in the combustion chamber. Normally the fuel air mixture burns evenly (in milliseconds), it starts at the spark plug and advances to the extremes of the combustion chamber. Detonation occurs when the mixture explodes instead of burning progressively. This will burn holes in pistons in a hurry of you're not careful. Here's why. Under normal conditions, a layer of gas bubbles seperate the burning fuel mix from the metal of the piston. That's why an aluminium piston that melts at 800 degrees can live in an environment of 1,500 degrees or more. When the fuel mix detonates, the shock wave (the ping you hear) knocks the layer of gas bubbles away and then you have 1,500 degree heat on a 800 degree piston and it melts.

    You mentioned surging and that sounds like a too lean fuel mixture. This could be caused by a vacuum leak or other carburation problems. Check the spark plugs and compare them. If you have an intake manifold leak, sometimes the plug closest to the leak will run hotter than the others. (Lean mixtures run hotter than richer ones.) A white surface means it has ran hot and if its brothers are brown or tan, check in that area, especially with your propane torch. A blistered surface means it's been plenty hot and if the electrode is eroded away or partially melted, you have a serious case of detonation.