Engine no power under load

[achris]

In the meantime ive been fiddling with the carb, the larger back(transom side) throttle plates connected to the choke dont seem to open, they are not frozen, i can open them with fingers, but seem to stay closed at WT.

These ones?

They are NOT choke plates, they are air valves, and they will not open until there is (a lot of) air flow. ie, the engine needs to be running, and under load with a high throttle setting to see them open...

Chris........

 

[bones774]

These ones?
View attachment 363985
They are NOT choke plates, they are air valves, and they will not open until there is (a lot of) air flow. ie, the engine needs to be running, and under load with a high throttle setting to see them open...

Chris........

yes those ones. How do i test if they are opening? maybe under load they are not opening and thats the problem?

 

[achris]

yes those ones. How do i test if they are opening? maybe under load they are not opening and thats the problem?

There's a spec for the load required to move them (effectively the spring tension). As long as they are free to move, they'll move...

 

[bones774]

There's a spec for the load required to move them (effectively the spring tension). As long as they are free to move, they'll move...

yes they are free to move

 

[jimmbo]

On the earliest Q-jets there was a Tab, that prevented the Air Valve from opening, unless the Coke was fully open. The Secondary Throttle Shaft could be fully open, but the blocked Air Valve did not allow much air through.
Later Q-jets got rid of the Interlock on the Air Valve, and modified how the Throttle Linkage on the Carb opened the Secondaries. The Secondary Throttle Shaft was no longer directly linked to the linkage from the Primary Shaft. It was now Spring Loaded Shaft, that had a Lockout, linked to the Choke Linkage, that prevented the Secondary Throttle shaft from rotating whenever the Choke is utilized.

 

[bones774]

I went out this AM and ran with spark arrestor off to view carb operation. The boat starts nice and choke seems to be working. at around 2500 RPM engine starts to bog under load, the rear throttle did not open as throttle was applied, i manually opened them at full throttle and engine started to die. I removed vent to atmosphere line on top of tank and that didnt do anything, same problem. Removed gas cap too, no improvement. any thoughts. thanks

 

[achris]

Take a standard 20L outboard fuel tank out with you and see if it runs properly on that. If it does you have a fuel supply problem. If not, check and verify the ignition timing, and that the timing advances properly. If that's all ok, pull the carb and do a complete clean and reset everything...

 

[bones774]

On the earliest Q-jets there was a Tab, that prevented the Air Valve from opening, unless the Coke was fully open. The Secondary Throttle Shaft could be fully open, but the blocked Air Valve did not allow much air through.
Later Q-jets got rid of the Interlock on the Air Valve, and modified how the Throttle Linkage on the Carb opened the Secondaries. The Secondary Throttle Shaft was no longer directly linked to the linkage from the Primary Shaft. It was now Spring Loaded Shaft, that had a Lockout, linked to the Choke Linkage, that prevented the Secondary Throttle shaft from rotating whenever the Choke is utilized.

My secondaries are spring loaded and connected to choke thru a rube goldberg type linkage. The choke can lockout secondaries.

Any chance it could be ignition? The wires are Sierra(blue), about 10 years old. What type wires should i replace with? Thinking to do wires, cap, rotor and plugs.

 

[bones774]

Take a standard 20L outboard fuel tank out with you and see if it runs properly on that. If it does you have a fuel supply problem. If not, check and verify the ignition timing, and that the timing advances properly. If that's all ok, pull the carb and do a complete clean and reset everything...

how to hookup that tank to my fuel supply?

 

[alldodge]

Nothing has been said that the motor is missing. You can throw parts at it but that isn't going to find the real issue.

Try the portable fuel tank, and you can use a 2 to 5 gal gas can and longer piece of fuel line. If gas can doesn't work then I would look at the carb

 

[bones774]

OP here, sorry for the delay, dont see my boat that often.
To close this thread, I replaced the rotor and cap, spark plugs and wires. The boat runs fine now, I can't say which it was because i replaced all at the same time. I think it may have been wires, they felt very soft and came off plugs and cap very easy, not tight. Felt good about it all, regular maintenance parts anyway. Thanks all

 

[alldodge]

What ever helps you get through the night