AQ 271C Not reaching WOT

[benet]

I have a 1990 Glastron with the AQ271C 350 engine 290 DP running B4's
cannot get above 3200 RPM,s full load on the water when I try to go full throttle it backfires
at the dock no load it can be reved up to over 3,000
have rebuilt the carb, new plugs wires, dist cap and rotor new anti-siphon valve, coil OK
timing set to 8Deg btdc changed fuel filter several times, new gas
any suggestions
Benet

 

[Maclin]

Re: AQ 271C Not reaching WOT

I will ask for some history regarding you and this boat. Have you had it a while and it just started acting like this? Or did you just get it and this is how it behaved? Has it had a compression check lately?

Usually a backfire thru the carb like that is a super lean condition, could be from an incorrect accellerator pump shot, incorrect timing, not-so-fresh fuel, or even leaking intake valves can cause that.

There are other causes, if this engine is new to you I would do a compression check, then if those numbers are not good you can do a leakdown test to see if it is valves or gaskets. If low compresson it just may be due for a valve job.

 

[Maclin]

Re: AQ 271C Not reaching WOT

Say, have you posted questions about this same engine recently?

 

[benet]

Re: AQ 271C Not reaching WOT

I will ask for some history regarding you and this boat. Have you had it a while and it just started acting like this? Or did you just get it and this is how it behaved? Has it had a compression check lately?

Usually a backfire thru the carb like that is a super lean condition, could be from an incorrect accellerator pump shot, incorrect timing, not-so-fresh fuel, or even leaking intake valves can cause that.

There are other causes, if this engine is new to you I would do a compression check, then if those numbers are not good you can do a leakdown test to see if it is valves or gaskets. If low compresson it just may be due for a valve job.

I have had the boat for 23 years started doing this early may fairly new engine 5 years less than 300 hours
just had the boat timed last week starts up at the dock will rev up

 

[benet]

Re: AQ 271C Not reaching WOT

first post today

 

[Walt T]

Re: AQ 271C Not reaching WOT

I assume this is a Holley? When you rebuilt the carburetor did you replace the power valve? Set the float properly? Did you verify all the itty bitty teensy weensy holes were clear? If the float is set wrong it will cause this as it starts running out of fuel at upper rpms under load. If the timing is too far advanced it can do this. If it is a very regular sounding backfire or popping it can be an intake valve problem. If it is a prestolite distributor the module inside may be bad. So, free and easy like Cindy was back in the day, Check the float level first, may as well verify the choke operation too. Verify ignition timing. Most carburetor rebuilds are not done correctly. If it is a Rochester well then that's a little harder.

 

[benet]

Re: AQ 271C Not reaching WOT

I assume this is a Holley? When you rebuilt the carburetor did you replace the power valve? Set the float properly? Did you verify all the itty bitty teensy weensy holes were clear? If the float is set wrong it will cause this as it starts running out of fuel at upper rpms under load. If the timing is too far advanced it can do this. If it is a very regular sounding backfire or popping it can be an intake valve problem. If it is a prestolite distributor the module inside may be bad. So, free and easy like Cindy was back in the day, Check the float level first, may as well verify the choke operation too. Verify ignition timing. Most carburetor rebuilds are not done correctly. If it is a Rochester well then that's a little harder.

The carb is a Rochester Quadrajet I had someone rebuild it that says they are expert with t5his type of carb
If I were to guess this is over 15 years old changed out the electronic module
Thanks Benet

 

[Walt T]

Re: AQ 271C Not reaching WOT

You can check the float pretty easy on those but might freak you out if you've never done it. Checking fuel pressure will tell you if the fuel pump is working.
Anyway, this is a quadrajet and it is a mechanical secondary with restrictor flaps over the secondaries. Its possible they are not opening correctly and causing the backfire. With mechanical secondaries you must have a way to allow the rush of sudden incoming air to get fuel into it when the secondaries open. Holley uses a second accelerator pump, Rochester uses that restrictor flap to slow down the incoming air until the fuel rods have time to catch up. The restrictor flap may not be adjusted correctly, it is quite easy to do but here again if you don't know how it can be confusing. Especially since the adjustments are where you cant really see the screws and if you have a boat with the engine buried in the bowels of the bilge where you need to go to the circus and hire a contortionist. Hey how do those contortionists bend their spines like I've seen them do? Good grief I mean "head up where the sun don't shine" is entirely possible here. Anyway if its not correct it can open too soon causing the lean condition spoken of earlier and voila! backfiring at upper rpms. Your symptoms fit. Too bad my jeans don't.

 

[benet]

Re: AQ 271C Not reaching WOT

You can check the float pretty easy on those but might freak you out if you've never done it. Checking fuel pressure will tell you if the fuel pump is working.
Anyway, this is a quadrajet and it is a mechanical secondary with restrictor flaps over the secondaries. Its possible they are not opening correctly and causing the backfire. With mechanical secondaries you must have a way to allow the rush of sudden incoming air to get fuel into it when the secondaries open. Holley uses a second accelerator pump, Rochester uses that restrictor flap to slow down the incoming air until the fuel rods have time to catch up. The restrictor flap may not be adjusted correctly, it is quite easy to do but here again if you don't know how it can be confusing. Especially since the adjustments are where you cant really see the screws and if you have a boat with the engine buried in the bowels of the bilge where you need to go to the circus and hire a contortionist. Hey how do those contortionists bend their spines like I've seen them do? Good grief I mean "head up where the sun don't shine" is entirely possible here. Anyway if its not correct it can open too soon causing the lean condition spoken of earlier and voila! backfiring at upper rpms. Your symptoms fit. Too bad my jeans don't.

Thanks for the advice I am going to have a me4chanic check this Thursday I will let you know what I find out
Benet

 

[Walt T]

Re: AQ 271C Not reaching WOT

If he says he adjusted it, ask him the procedure...

QuadraJet 4 This site goes through the critical adjustments including the secondary flaps. The procedure is outlined in the specification sheet that comes with the carb kit used in rebuilds. The best way to adjust it is on the water.