Quadrajet carb question.

[ccustomrides]

4.3 Mercruiser with a quadra jet and mechanical fuel pump. Problem I have is when under full throttle once in a while I will loose few rpm. All I have to do is back the throttle off just a bit and rpm and speed comes back. I might loose 100-200 Rpm and 2 mph when this happens. This usually happens right at the end of the throttle travel only. I have no other loss of power of any kind. Like I said all i have to do is tap the lever back a bit and it picks back up. I called few places and they said it might be my carb. I guess there are some shaft seals inside the carb that wear out and can cause the same problem.
My fuel pump is original but I think it works good. I did want to replace it but then said I will wait till this season. My ignition system is new and all fuel filters are repalced and new.
Boat idles well and runs to full rpm and speed. Throttle plates are fully open at the end of the throttle lever travel. So if anyone has any tips or pointers I am all ears.

 

[boaterinsd]

Re: Quadrajet carb question.

So what year of boat is this? Has it ever had a kit put in it? Are you sure the cable is pushing it out to W.O.T. and if so is there a throttle stop in the linkage? Are you certian this is a carb problem? Check fuel pressure, i am thinking about 4- 7 psi.

 

[ccustomrides]

Re: Quadrajet carb question.

1990. No I never touched the carb. I did try to run it on 89 octane and did put some seafoam thru it which I think helped a little. I did run 87 and I did see a diffrence when I started using 89.
Yes throttle plates open all the way and carb is open when lever is pushed all the way forward. I dont have any other problems and it pulls hard and gets rpm and mph in any other position.
Like I said once in a while it will drop 100rpm when I jam the lever forward. All I have to do is tap on it in reverse direction and I gain my rpm. I mean like small amount of movement. Engine never drops a beat if I hold it at wot or after I loose few rpm and tap it back. Yes there is a linkage stop.

 

[kmarine]

Re: Quadrajet carb question.

Sounds like a carb problemb to me, although fuel filter would be mt first step. Did you see if you have one in the carb where the fuel ilne comes in. You may also have a fuel float or needle stuck. If the carb was never rebuilt since 1990 I suggest looking into having it done. You may justify it with possible fuel savings. good luck

 

[ccustomrides]

Re: Quadrajet carb question.

Yes all filters clean or replaced. Should I go get it rebuilt or buy another one on Ebay or something new/rebuilt. Problem I have is then tuning the dang thing. LOL not looking forward to that. Like I said just really weird but no backfires or loss of power besides that small drop of rpm. In the end it goes back to normal when I tap it back a notch which is the weird part but it works and it never skips. I did notice towards the end of the year if I wanted to run Wot then I had to position the throttle lever just a click or 2 away from the full forward position. If i push it further then it will bog for a second and drop rpm. When I click it back it goes back up to normal and can run there all day long. Gets on plane quick and with no problems and no other problems at all. Like I said some guy I talked to told me to get it rebuilt. He said they have some seals or bushings that wear and can created such a problem.

 

[ac0j]

Re: Quadrajet carb question.

Have you checked the timeing, or distributor advance setting?

 

[ccustomrides]

Re: Quadrajet carb question.

Yes I check it every spring. This spring it was 6 btdc. So it moved 2 deg from last spring. This is normal dude to vibration and chain stretch. I did try to set it around 10 or 9 this year so if it does move this season it will still be around 8. I did check to see if the advance works. Whne throttle is added it moves in a + direction. I dont have a $$$ light that shows digital readout of advance but I do see advance on the scale as it moved from 9 to 16 and so on. I dont know if there is a setting? I just check timing and advance each year. Is there a setting or reset I can do or something.

 

[sstocker31]

Re: Quadrajet carb question.

Is there a chance that pushing the throttle all the way open is moving the throttle blades over centre slightly, and thus disturbing the airflow.

 

[dan t.]

Re: Quadrajet carb question.

Possibly the spring tension on the secondary air valves, if it is a little loose it could cause a lean condition at wot, also check the vacuume pot on the side of the carb, it also helps controle air valve opening, it must hold vacuume and open slowly when it released,