Testing for Vac leaks on intake system

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Tony Z
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Testing for Vac leaks on intake system

Post by Tony Z » Tue Oct 21, 2008 5:55 pm

I have posted this a few times in the past, so decided to do a "how-to" so it can be saved for future reference......

To test for a Vacuum leak, go thru the following and you should come right.

Buy some quick start. This comes in a spray can
Start the engine, in Neutral and hand-brake on.
Open the engine lid.
with engine running spray a tiny amount into the air filter. You will hear the engine change revs, sound, whatever, it will do something. Take a note of how it changed, you will be looking for something similar soon.
Now at each joint, rubber and gasket spray a bit of quick start around it. You can be a bit more liberal here than at the air filter. If you have a vac leak, the quick start will get sucked in and you will hear the engine sound change.
Check for more leaks.
Once you have found and noted any leaks, go back to them. Spray quick start again to check if you got it right.
Spray again, this time a lot less - this makes 100% sure you are not feeding the carbs indirectly with quick start.
Be 100% sure you are feeding the leak and not the air filter with quick start.
Repair
Test by doing the same.

Good luck


A lean engine often has decent flat spots and a very warm exhaust. Also the temp of the exhaust gas will he higher than usual.
On a near stock engine, a rule of thumb is to hold you hand about 20cm from the exit of the exhaust pipe. The gas should be warm. If it feels hot, then you have an issue (timing, lean mixture, dead or unthreading plugs....).
Another rule of thumb is that if you suspect the carb being the culprit, then you probably suspect wrong. Make sure everything else is right first cause the carb is often the last item to give issues.



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Re: Testing for Vac leaks on intake system

Post by ZeroAxe » Tue Oct 21, 2008 7:51 pm

Tony Z wrote:cause the carb is often the last item to give issues.
Unless the brass bushing on the bottom shaft is worn out oval.
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Re: Testing for Vac leaks on intake system

Post by Rusty » Wed Nov 05, 2008 9:56 pm

Thanks TZ - that's very informative. Now how about telling me (us) how to tell when the carb is running lean or rich & which way to turn those screws on a 34 pict 3 to change the situation.

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Re: Testing for Vac leaks on intake system

Post by Tony Z » Wed Nov 05, 2008 10:29 pm

thats another topic altogether.... post a question in the advice section and you will get most of your answers

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Re: Testing for Vac leaks on intake system

Post by bonolo » Fri Mar 20, 2009 10:03 am

Can a carburator cleaner be used as a substitute if I dont have a quick start spray ?

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Re: Testing for Vac leaks on intake system

Post by Tony Z » Fri Mar 20, 2009 10:16 am

probably, you could prob use deodorant if you wanted, but it wont be as easy to tell the difference and I have no idea how the engine will like it.
Quick start is a fuel (Ether I think), anything else isnt, they just happen to burn too.

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Re: Testing for Vac leaks on intake system

Post by subie » Fri Nov 27, 2009 7:03 am

I really like this piece of advice.
It always bemuse me when I see the oke's from the "rev right motors brigade" furiously
attack the carb first when there is any problem.
As an AC rookie I would think if you run with a working air and fuel filter and know your carb and intake system is airtight then there should be nothing that can un-adjust itself realistically.
Bliksem die engels is swaar!! :lol:

subie

Another rule of thumb is that if you suspect the carb being the culprit, then you probably suspect wrong. Make sure everything else is right first cause the carb is often the last item to give issues[/b





[quote="Tony Z"]I have posted this a few times in the past, so decided to do a "how-to" so it can be saved for future reference......

To test for a Vacuum leak, go thru the following and you should come right.

Buy some quick start. This comes in a spray can
Start the engine, in Neutral and hand-brake on.
Open the engine lid.
with engine running spray a tiny amount into the air filter. You will hear the engine change revs, sound, whatever, it will do something. Take a note of how it changed, you will be looking for something similar soon.
Now at each joint, rubber and gasket spray a bit of quick start around it. You can be a bit more liberal here than at the air filter. If you have a vac leak, the quick start will get sucked in and you will hear the engine sound change.
Check for more leaks.
Once you have found and noted any leaks, go back to them. Spray quick start again to check if you got it right.
Spray again, this time a lot less - this makes 100% sure you are not feeding the carbs indirectly with quick start.
Be 100% sure you are feeding the leak and not the air filter with quick start.
Repair
Test by doing the same.

Good luck


A lean engine often has decent flat spots and a very warm exhaust. Also the temp of the exhaust gas will he higher than usual.
On a near stock engine, a rule of thumb is to hold you hand about 20cm from the exit of the exhaust pipe. The gas should be warm. If it feels hot, then you have an issue (timing, lean mixture, dead or unthreading plugs....).
Another rule of thumb is that if you suspect the carb being the culprit, then you probably suspect wrong. Make sure everything else is right first cause the carb is often the last item to give issues.[/quote]

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Re: Testing for Vac leaks on intake system

Post by TonyE » Fri Nov 27, 2009 2:46 pm

Another way to test for air leaks is to turn the air screw on the carb close , if it still idles it getting air from somewhere else.( A leak)

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Re: Testing for Vac leaks on intake system

Post by bugspray » Sun Apr 18, 2010 2:55 pm

bump. will try this out as im told that this could be a problem to my car not startin... oh and free bump for the peeps that missed this.
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Re: Testing for Vac leaks on intake system

Post by sacem » Wed Mar 30, 2011 4:57 am

Excelent advice specially if you are running non stock multiple carburetores
SACEM

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Re: Testing for Vac leaks on intake system

Post by John Alves » Wed Mar 30, 2011 8:48 pm

I have fuel leaking from the intake rubbers, but the motor runs fine.
Could this be the start of a vac leak?
No sense being pessimistic. It wouldn't work anyway.

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Re: Testing for Vac leaks on intake system

Post by John Alves » Wed Mar 30, 2011 9:18 pm

Tony Z wrote: you could prob use deodorant if you wanted, .
Doom also works, but that kills bugs dead
No sense being pessimistic. It wouldn't work anyway.

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Re: Testing for Vac leaks on intake system

Post by Tony Z » Thu Mar 31, 2011 8:07 am

John Alves wrote:I have fuel leaking from the intake rubbers, but the motor runs fine.
Could this be the start of a vac leak?
My guess is you already know the answer, which is why you are asking....
My guess is that when the engine is warm, the rubbers work fine, but if they are cold, there is a slight vac leak. Check for leaks at idle just after starting the engine on a cold morning.

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Re: Testing for Vac leaks on intake system

Post by John Alves » Thu Mar 31, 2011 12:22 pm

Thanks, I will try that
No sense being pessimistic. It wouldn't work anyway.

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Re: Testing for Vac leaks on intake system

Post by fig » Thu Mar 31, 2011 12:34 pm

If fuel is leaking out then air is definitely leaking in. I think you may be surprised at the performance boost once you've fixed the leaks and retuned your carb.
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