Too fast!! 1982 485, 3.7litre 2bbl rochester

[billywhizz]

Hi, I have the above carb and engine, all cables are fine, lubed and not sticking, but the problem i am faced with is that there seems to be no range on the carb,ie: 1/8" movement on the control and she jumps from 650 rpm to 2000rpm as you can imagine this makes things pretty difficult when trying to manoeuvre at low speed, not to mention the danger aspect of catapulting my crew overboard!, the engine has low hours and is in good service order, so i have to put this problem down to carb, i also have disconnected the linkage and moved the lever on the carb by hand , same again, tiny movement and it revs too much, should i be thinking of replacing the carb to achieve a more moderate acceleration,particularly as i use the boat on inland waters not the ocean and do not require all this power, or can i get the desired effect with a rebuild kit? or would it be possible for me to change jets in order to reduce the power output?also i have looked down into the carb and i assume that both inlet flaps/valves are supposed to open at the same time and not one before the other? could you confirm this? thank you for your time,
Billy

 

[Bondo]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

or can i get the desired effect with a rebuild kit?

Ayuh,.........

It sounds like you're dealing with either an internal Carb issue,....
Or,...
A Linkage issue......

You might get to the issue with a Carb Kit,......
It's Way Cheaper than a New Carb,+ would be My 1st choice.....

or would it be possible for me to change jets in order to reduce the power output?

Nope, It Don't work that way.....

i assume that both inlet flaps/valves are supposed to open at the same time and not one before the other? could you confirm this?

Ayuh,...... That is Correct....

 

[steelespike]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

I don't know anything about that carb but I wonder if something is loose inside.A throttle plate perhaps.Something loose that allows the throttle to pop farther open suddenly.

 

[Don S]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

I would sure do a vacuum gauge test, check for excess fuel being pumped into the carb (float-bad fuel pump etc.)
It doesn't sound like you are very familiar with carbs or troubleshooting, so you might want to download an OEM service manual for your engine. Sure couldn't hurt.
Manual #4 is the one that covers your engine.

http://www-alt.mercurymarine.com/mnetdata/sisdata/srvcovr.pdf

It's 301 MB in size, so don't expect it to download in an instant.

 

[billywhizz]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

Ayuh,.........

It sounds like you're dealing with either an internal Carb issue,....
Or,...
A Linkage issue......

You might get to the issue with a Carb Kit,......
It's Way Cheaper than a New Carb,+ would be My 1st choice.....


Nope, It Don't work that way.....


Ayuh,...... That is Correct....

I dissconnected the linkage and it was just the same, as soon as the valve flaps start to open it revs right up?

 

[billywhizz]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

I don't know anything about that carb but I wonder if something is loose inside.A throttle plate perhaps.Something loose that allows the throttle to pop farther open suddenly.

Nope, checked em my freind, thanks for the reply

 

[billywhizz]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

I would sure do a vacuum gauge test, check for excess fuel being pumped into the carb (float-bad fuel pump etc.)
It doesn't like you are very familiar with carbs or troubleshooting, so you might want to download an OEM service manual for your engine. Sure couldn't hurt.
Manual #4 is the one that covers your engine.

http://www-alt.mercurymarine.com/mnetdata/sisdata/srvcovr.pdf

It's 301 MB in size, so don't expect it to download in an instant.

thanks for that, downloading as we speak, yes fairly competent mechanicly, never had a marine engine before, enjoying the challenge, :

 

[oops!]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

time for a rebuild.

good luck
oops

 

[a70eliminator]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

Look at the plastic cam that actuates the accelerator pump, someone may have changed that cam to a steeper more aggressive one, that would be my guess if the rate of throttle up is the only issue, these cams also have two or three different positions also, the mounting holes are usually designated as #1,2 and 3 position, you need to get the spec on your carb and varify that cam position.

 

[billywhizz]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

Thanks my freind, but i ran the engine with the accelerator pump completely disconnected from the carb, no difference tho,

 

[bjcsc]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

I'm confused about something: You asked if "both flaps" should be opening at the same time and Bond-O said they should. Assuming you're both talking about the choke plates, why would the secondary always be opening?

 

[Don S]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

There are no secondaries on a 2bbl carb.

 

[billywhizz]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

Don, I downloaded manual 4 as you suggested, unfortunately it covers only the sterndrive unit, it looks like manual 3 is the one i need, however as soon as i start to download it stops after a few seconds, do you no if it is prohibited or limited access? thanks Bill

 

[Don S]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

Not sure what I was thinking when I said #4, sorry about that. Yes, #3 is the one you want, it was working, but that download is 1.36 GB, (Not MB) in size. I just downloaded 20 MB of the #3 manual, and it seems to work. Might have been your ISP was very busy yesterday.

 

[ziggy]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

the engine has low hours and is in good service order

so does that mean ya've confirmed compression, done a tune up, points, plugs, cap, rotor, fuel filter on the carb, fuel filter on the fuel pump. set dwell, then timing, then idle speed...... when did ya do this to know it's in good service order?

have ya tested fuel pump pressure?

got any air leaks? have ya hooked up a vacuum gauge to see what it reads?

got any wear in the bottom of the venturis? where the throttle plates close. maybe it aint never gettin down to the idle circut if there's a gap between the plates and the venturis maybe the throttle body assy. is wore. though i reckon it'd be hard to achieve 650rpms if this was the case...

how did this problem occur. over time, just happened. have ya been rebuilding on the carb or someone else perhaps?

have ya checked the advace curve? maybe a spring is broke allowing for instant advance.....

short of everything else. it sounds internal to the carb. to me.....a rebuild being sure that all specs are met......them new service manuals ya just downloaded are gonna be a whole lotta help with that......

i aint the pro, but just thought i'd throw a few thoughts out.......sounds like a hard problem ya got.....good luck.....

 

[John_S]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

Some how you are getting gas and air without the throttle opening and without the accelerator pump???

With engine off and looking at the butterflies, and you move the throttle, you should see the butterflies open in a very linear way, especially in the area of just off idle.

Start you motor and let it warm-up and the choke to fully open. Being careful, remove flame arrestor. Observe down the barrels. If you are seeing dribbling of gas and puddling, or gas coming from fuel pump vent, stop. You have a bad fuel pump if the latter, and probably sticking needle valve if former, which calls for carb rebuild. (both of these items were mentioned by Don)

If everything looks good idling, advance the throttle to the problem area. Do you see anything abnormal, like the butterflies swinging open, even though you didn't advance them any more? If that is the case, you are probably hooked to wrong linkage or return spring broke, or combination of both. They should only open like they did in the prior engine not running condition.


"have ya checked the advace curve? maybe a spring is broke allowing for instant advance....."

I'd pull the distributer cap and take a look. And verify your base timing is correct.

 

[bjcsc]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

There are no secondaries on a 2bbl carb.

Whoops...I missed that it was a 2bbl...

 

[billywhizz]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

Thank you for your thoughts, not sure what you mean by
"have ya checked the advace curve? maybe a spring is broke allowing for instant advance....."
where do i look for the spring?

 

[John_S]

Re: Too fast!! 1982 485, 3.7litre 2bbl rochester

If you have a mechanical advance distributer, it is controled by two weights and two springs. When the distributers turns fast enough, centrifical force causes the weights to move out against the force of the springs. As the weights move out, the timing advances, until the weights hit a stop. They are usually located under the rotor. If a spring is broke or both are very weak, the advance would come all the way in at very low rpms.