Need help with Quadrajet

[Capt.Erich]

I have a pair of Crusader 454's with Quadrajets. Both carbs have had rebuild kits installed in the last year. One works great and the other will allow the engine to run under load up to about 3200 rmps and then it will bog down to around 1200 and then go back up to 3200. It repeats the process and will not maintain rpms much over 2700. Both engines rev easily and maintain rpms while NOT under load. New fuel lines and filters were installed this year.

The mechanic says the jets are clogged. He pulled the carb and "soaked" it for a few days. It seemed to help but did not fix the problem. I have suggested swapping the carbs to see if the issue follows the carb.

Should I spring for a new carb? If so, can you reccomend a supplier?

Thanks in advance.

 

[Boatsb]

You are on the right track. Switch the carbs and see if the trouble follows the carb or stays on the engine.

Fuel volume and pressure may be an issue. Check the float height, see if the jets are cloged, Fuel pumps weak? air leaks?

 

[yachtsmanbill]

Check the accelerator pump check ball at the bottom of the plunger cylinder. The other problems arise from flooding due to leaking plugs that are pressed in between the body and the base plate. Fill them with epoxy.
My gas boat sat for the last three years and was starving for fuel... the inlet screen/filter was full of crud from between the pump and the carb. More than likely varnish debris from sitting.
When the secondary flaps open the main metering jets should lift also. They are on the yolk shaped linkage between the secondary plates. They should lift by finger with the engines off. ws

 

[yachtsmanbill]

One other thing... The vibration line that runs to the fuel filter base was a braided unit and mine was cracked so that it sucked air. I changed that out to a piece of USCG approved hose and problem solved... that is until the hose failed from ethanol. Went with some gray silicone stuff and its good to go... ws

 

[oldboat]

here are two links to a couple of quadrajet papers. they are from a corvette site but it is the same carb.

Q-jet problems
http://www.lbfun.com/warehouse/tech_info/fuel & intake/Q-Jet Problems.pdf

Q-jet tuning
http://www.lbfun.com/warehouse/tech_info/fuel & intake/Q-Jet Tuning Paper.pdf

 

[Maynard Rupp]

Those things that look like choke plates on the back, (larger), 2 barrels have a spring pre-load. There is an allen screw on the outside that frees the spring to move. There is a tension setting for those springs. They make a little indicator which you press on the closed plate and slowly increase preasure until the plate just starts to move. Now read the gage and adjust the spring tension until you get the recomended value. If the plate spring is too stiff, the engines won't accelerate when the throttle is advanced as the back barrels kick in. If the spring is too loose, the engine will draw too much air and bog badly when the throttle is advanced enough for those rear barrels to open. The throttle plates are controlled by the linkage and your throttle position, those choke plates bleed the air in slowly and that rate is controlled by the tension on those springs.

As far as a good replacement, Vic Edelbrock has been making his quadrajunk replacements for years. They are as good as that type of carburator will ever get.

 

[REBrueckner]

That's a classic sign of an engine starved for fuel...unlikely its an acceleration pump or float valve issue with those symptoms...be sure the fuel flow to the carb is free and clear....thesame for both engines....I'll bet switching carbs will almost certainly switch the problem to the other engine....

 

[Finalee]

Put in a new fuel filter first. Definitely sounds like fuel starvation. Check fuel pump pressure. Sounds like the carb is carburating correctly when there is fuel.

 

[fixin-to-cruise]

I'd check the fuel delivery system to the carb. I've seen that sams symptom due to clogs in the fuel system, once on a boat and 2 times on cars. On the boat it was a clogged check valve at th fuel tank fitting elbow. On cars...one had a clogged sock type filter in the tank, another had an in-line in the fuel.