Weber Idle Mixture

[Loren Warner]

Installed a new carb this weekend. Initially set Idle mix screws 1 1/4 turns. Started engine and set idle using idle screw at 650 rpm in forward gear. Idle was steady. When fine tuning the idle mix screws, the idle speed is not decreasing as it should no matter how many turns either direction. In fact, I've completely closed the idle mix screws and the rpms still remain steady. I'm also using a vacuum gage and although the reading is fluctauting slightly (within 1 hg), it's holding at 15 hg regardless of the mix screw adjustment. A friend believes this may be caused if the float is set too rich (slightly flooding). Any other ideas what may cause the idle mix screws to be ineffective. I'm returning the carb for another anyway but curious as to what would be the cause.

thank you,
loren

 

[Don S]

Re: Weber Idle Mixture

Welcome to iboats Loren, still have that 28 with the 7.4? If so, do you have Thunderbolt V ignition on your engine? If you do, you need to put it in base timing mode to adjust the carb. Otherwise the timing will change and try to keep the engine running.

Also, with 15" of vac, that is within limits, but you could have some air leakage in the carb gaskets.
Usually a carb rebuild, along with soaking in carb cleaner and blowing out all the passages and setting everything, usually cures the no adjustment problem.

 

[Loren Warner]

Re: Weber Idle Mixture

Hi Don, glad to see you're still around. Yes, still have the same boat w/ 7.4L and Thunderbolt V. I did place the ignition in base timing prior to starting the engine. I've done this before with the old carb and always noticed a little drop in rpm when adjusting the mix screws so I was surprised when nothing was happening with the new carb. I believe it stayed in base timing throughout my adjustments because just prior to stopping the engine, I disconnected the ground wire and the rpm would increase. Unlikely that I had any leakage in the gasket as this carb uses rubber quad gaskets and I torqued to 120 inch lbs. This carb is brand new, not a rebuild, so I would like to think it came with clean ports and properly adjusted floats. Does it make sense that a misadjusted float could cause this problem?

A week earlier I had run the engine with the old carb after winter lay-up doing the same adjustments in base timing mode. Everything worked as it was supposed to and the engine ran very smooth. Still does. The reason I changed carb is that the idle was no consistant after coming down in speed because of slop in the cabs body where the throttle linkage connects to the primary throttle.

Don, I'm very happy to see that your around sharing your technical knowledge. I'm in the phone book when you get up this way. I'll let you know whatever I find but won't be able to get back to it until next weekend.

Thanks,
Loren

 

[Loren Warner]

Re: Weber Idle Mixture

The dealer I purchased the carb from said that the problem could also be caused by fuel boiling if the fuel line is routed to close to a heat source. I find this to be an unlikely cause since the line going to and from the carb is a pre-bent stainless steel tubing and have never had this problem before. But will verify when I get back to Whittier. Anyway, he's also sending me another new carb in case the floats or something is in fact the problem. Is there anything that I should do or check on a new carb prior to or after installation that's not in the Mercruiser Manual? Otherwise, I intend to just install a new gasket and preset the Accelerator Pump 7/16", Mixture settings 1 1/4 turns, and Choke Pull-off 15/64". Also, should the choke pull-off for the electric choke be set the same as a diaphragm type? With regard to what to look for, he says when idling I should not see any fuel coming out of the accelerator pump discharge or venturi's as the fuel should only be coming out below the throttle plates. Does this sound right?

 

[bjcsc]

Re: Weber Idle Mixture

With regard to what to look for, he says when idling I should not see any fuel coming out of the accelerator pump discharge or venturi's as the fuel should only be coming out below the throttle plates. Does this sound right?

Yes. The idle circuit is the only thing controlled by the mixture screws, i.e. their adjustment does not affect your carb running rich or lean during normal operation. The mixture screws if all the way in (easy-not forced) should shut the engine down. If it has no effect, fuel is entering the carb through some other circuit...