Engine "choking" between 800 - 1200 rpm

[SimeC]

Good day,
I am a new guy on this forum and need advice
I have a 1997 MerCruiser 7,4 Thunderbolt ignition.
The engine start normally, let the engine heat up for a couple of minutes at the idle rpm's (around 700).
Manoeuvre around to clear the berth at the idle speed, but when I increase the throttle above 700 rpm's the engine start to "choke".
The "choking" range is between 800 till 1200 rpm's, above 1200 rpm's the engine runs normally.
It started few days ago.
New fuel filters installed and tank cleaned, the engine has run only 50 hours after filter installation.
I assume it is carburettor or bad fuel.

Thanks in advance

Sime

 

[alldodge]

Howdy Sime

Sounds like the accelerator pump in the carb and it needs overhauled. The pump pushes in a little more fuel as the throttle is advanced. With the motor OFF, look down the throat of the carb and have some move the throttle and see if you see two streams of gas being injected in.

http://www.mercruiserparts.com/bam/subassembly/31916/2345/8

 

[harleyman1975]

I'm confused. does it start to miss and blow black smoke or does it fall on its face?

 

[SimeC]

Howdy Sime

Sounds like the accelerator pump in the carb and it needs overhauled. The pump pushes in a little more fuel as the throttle is advanced. With the motor OFF, look down the throat of the carb and have some move the throttle and see if you see two streams of gas being injected in.

http://www.mercruiserparts.com/bam/subassembly/31916/2345/8

Hi AllDodge,
Thanks a lot mate. Will do that.

 

[SimeC]

Hi AllDodge,
Thanks a lot mate. Will do that.

Hello AllDodge,

First of all appreciate a lot your advice.
I did what you advice. With the motor off, I look down the throat of the carb, moved a throttle and I saw two streams of gas

 

[alldodge]

Hello AllDodge,

First of all appreciate a lot your advice.
I did what you advice. With the motor off, I look down the throat of the carb, moved a throttle and I saw two streams of gas

OK, Should have said this before you went to check. Wonder if your getting the streams as soon as the throttle is moved, or does it move a little and then start streaming? May want to try this with the motor running and have someone try to throttle up and watch (with safety glasses) to see what you see.

The accelerator pump has a mushroom type seal. So as the throttle is opened, the gas streams in. Now if the check valve is weak and the bore does not get a full load of fuel, or if the mushroom seal is worn, or if the bore that the seal moves on is worn of ruff, the motor will not get a full stream.



This is not your carb but note the check ball for inlet and for discharge



In the end I see you needing some carb cleaning

 

[Bondo]

In the end I see you needing some carb cleaning

Ayuh,.... Welcome Aboard,.... Ya missed a Golden diagnostic opportunity if ya didn't inspect the Contents of the ole fuel filter before ya tossed it,....

Bein' a big block, I'm guessin' ya got the Merc/ Weber/ Carter AFB carb,....

If so, Pull it, 'n rebuild it,....
It's a carbon copy of the Edlebrock BBC carb(1410, maybe), parts, 'n pieces all interchange with the Merc carbs,...

It's a Great easy to work on carb, easy to tune as well,...

 

[wellcraft-classic210]

1 comment on possible cause

When you come off of idle and the throttle plates open the fuel stops getting sucked through the idle passages that driven by the high vacuum are under the throttle plate and starts coming out of the venturi's as the air flow increases around them / the Venturi's are shaped to create a vacuum as the air rushes past them in the same way an airplane wing create lift. The accelerator pump gives a squirt of fuel to help with this transition.

So any debris that might be in the passages between the float bowl fuel and the venturi's can cause issues as you come out of idle such as bogging, stalling or general lack of power.

Checking for debris in your float bowls and clean and ( and all ) passages with Gumout jet spray ( or similar ) and compressed air usually resolves those problems in short order.

( Part of the rebuild process -- as suggested above by the experts )

 

[SimeC]

Hello AllDodge, Bondo and wellcraft-classic210,

Guys THANKS VERY MUCH.
Sorry on the late reply, was quite busy.
So far I changed and clean ignition coil, all spark plugs cleaned, (no oil trace) only a lot of sot. Means it is getting more fuel then require when I speeding.
Believe it or not, the chocking problem I solved by retighten flying wheel for the water pump belt, cause fuel pump is connected to the water pump.
Anyway, fuel mixture is reach and I have to adjust/repair accelerator pump, and all other thingies Alldodge said before regarding the carb. (Cleaning first)
Guys once again thanks a lot.
I have to do it by my self cause I live in a pretty shi..y city, where only one guy have some clue about Weber carbs, and he is pretty busy.
If I send my carb to his workshop I will get it back maybe 1st of Linburg .), means forget this season and maybe another.
Guys all the best, and appreciate your help A LOT!!!
Sime