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Engine racing - Quadrajet carburetor

jackman

Contributing Member
I recently dismantled, cleaned and re-assembled a Rochester Quadrajet carburetor which is on a Mercruiser 228 engine. No trouble starting engine and after it had been running steadily at 900rpm for about 10 minutes whilst waiting for the temperature guage to rise to normal level, the engine revs suddenly rose to almost peak revs and stayed there. I had not been revving the engine it just suddenly happened. I switched off and tried it a couple of more times but the same happened after a moment or so.. I checked the various linkages, etc but couldn't see any problem. Any advice gratefully received.
 
With the flame arrestor (air cleaner for easy understanding) off and motor running you need to watch the carb and see what happens when this issue occurs.
Looking at it after the fact wont help.

My first thought would be the secondary (UPPER) butterfly's are not staying shut and are opening. They are vacuum operated.

Other possibilities are a fuel leak or sever vacuum leak. The motor should idle easily down to 400-500 rpms and if it is not then you have fuel or vacuum issues also normal idle setting should be 650-750 rpms when in gear at idle speeds.

Also make sure that the throttle cable has a small amount of positive pressure forcing the throttle linkage at the carb to stay shut all the way.

so you must visually watch what happens and report back with your observations.
 
Sorry about delay in replying. I wasn't able to re-start the engine so I took carb' off and left it with a repair shop for overhaul and check-over. When I re-fit it I'm hoping to find problem solved. I'll let you know outcome. Could you confirm if gasket sealant(non-setting) should be used on the gasket between base of carb' and inlet manifold? I've been advised Yes and No.
 
Fuel enrichment only will not increase RPM as to what you have described.
In order to increase RPM as to what you have described, the throttle plates would need to be opening.

As Jack noted, look at the secondary throttle plates and air door.
The throttle plates would require being off closed position, and air door would be opening up some.


Also make sure that the throttle cable has a small amount of positive pressure forcing the throttle linkage at the carb to stay shut all the way.
Ditto.


.
 
Thank you all for your advice. I didn't have the time so I left carb' into a repair shop where it was cleaned out and rebuilt and it's fine now although they told me that they didn't find any particular problem. So I'm still as wise.
 
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