Stumbling anywhere below 2500 RPM. Driving me nuts!!

This problem has been plaguing since half-way through the summer. '74 Mercruiser 140, Pertronix Ignitor ignition, Pertronix 3.0 ohm flamethrower coil, 1-year old plug wires, carb rebuilt last year. Good squirt from accelerator pump, solid idle. Idle adjust screws and idle speed set correctly. Clean flame arrestor. Compression 130/128/125/132. Good premium gas.

Toward the first half of the summer everything was great. Good hole shot, got to within 200 RPM of WOT and all was smooth. Now, I get it out on the lake and hit the throttle and it stumbles and runs poorly until it hits fairly high RPM, at which point everything suddenly smooths out. If I keep it above 2500 RPM, it runs very smoothly with lots of power. If I let it drop below 2500, it stumbles and continues to bog.

I've tried it getting on the accelerator slow, getting on it fast, and just about any other way I could think of. About the only way that really made any difference was if I revved it up above 2500 in neutral, dropped the RPM down to idle, then got on the throttle immediately.

I'm trying to teach my kid to water ski, and with no hole shot he's going to be miserable.

Tomorrow I'm installing my backup carb which soaked in cleaner for three days and has been meticulously rebuilt. I've been swapping back and forth between the two and either they both have the same thing wrong with them, or it's something other than the carb.

Any suggestions?

Re: Stumbling anywhere below 2500 RPM. Driving me nuts!!

did ya do a tune up on it this year? with new parts? time it? check out pp 10-17 of yer service manual. in the trouble shooting guide part....

there's always dons' list... bet the issue would be found somewhere inside it....

http://forums.iboats.com/showthread.php?t=208547

post #2 in this thread....

Re: Stumbling anywhere below 2500 RPM. Driving me nuts!!

ziggy said:

did ya do a tune up on it this year? with new parts? time it? check out pp 10-17 of yer service manual. in the trouble shooting guide part....

there's always dons' list... bet the issue would be found somewhere inside it....

http://forums.iboats.com/showthread.php?t=208547

post #2 in this thread....

Click to expand...

What's to tune? With the Pertronix, the gap and dwell are set. I haven't checked the timing or the advance--I'll do that tomorrow, but I find it hard to believe that they'd just go out like that unless somehow the advance weights seized in the advanced position.

Any other ideas?

Re: Stumbling anywhere below 2500 RPM. Driving me nuts!!

What's to tune?

Click to expand...

well, ya named a couple of them. timing and advance. just because ya got hei don't mean your plugs won't get dirty, or your distributor cap could have a crack in it, or the rotor either. or filthy contacts on the cap and rotor. or that your fuel filter is clean. go check out the service manual. it'll give you ideas of where to look. these are some of the things to go look at. just because yesterday it was ok, don't mean today it is. one of them is probably wrong even or ya wouldn't be having an issue.

the sevice manual also suggest a sticking choke. bad plug wires, insuficiat fuel supply, faulty ign wireing or loose connections.... go look at it... there's others too.... or use dons' format which mirrors the service manual pretty closely.....

Re: Stumbling anywhere below 2500 RPM. Driving me nuts!!

ziggy said:

well, ya named a couple of them. timing and advance. just because ya got hei don't mean your plugs won't get dirty, or your distributor cap could have a crack in it, or the rotor either. or filthy contacts on the cap and rotor. or that your fuel filter is clean. go check out the service manual. it'll give you ideas of where to look. these are some of the things to go look at. just because yesterday it was ok, don't mean today it is. one of them is probably wrong even or ya wouldn't be having an issue.

the sevice manual also suggest a sticking choke. bad plug wires, insuficiat fuel supply, faulty ign wireing or loose connections.... go look at it... there's others too.... or use dons' format which mirrors the service manual pretty closely.....

Click to expand...

I guess I could have been clearer. Plugs are clean and properly gapped. Plug wires, rotor and cap were replaced less than fifteen hours of runtime ago, and boat is stored indoors on a trailer. Fuel filter was replaced three weeks ago. I will check these things, but I'm asking because I have a set of symptoms which could be caused by a lot of different things, and I'm looking for more suggestions as to what it could be.

Basically, what I'm saying is that I've fixed everything that the manual tells me normally causes these symptoms, so I'm looking for abnormal things that people on the forum might know about.

Re: Stumbling anywhere below 2500 RPM. Driving me nuts!!

it does sound like ya've coverd your bases on the tune up part. your compression is good. that leaves fuel. bubba could be onto something with the carb base gasket. another idea. right outa the book. have ya run it on a remote fuel tank? eliminates everything from the fuel tank to the fuel pump as possible problems......

Re: Stumbling anywhere below 2500 RPM. Driving me nuts!!

Have you looked at the timing advance plate, under the Petronix unit, or where the points used to be?

If I remember right, there are springs that control the advance plate. If one of those springs is broken, it will bog on acceleration until the engine can catch up with the timing advance.

Easy to see with a timing light. The timing advance will shoot up too rapidly. It should be a smooth advance.

Re: Stumbling anywhere below 2500 RPM. Driving me nuts!!

If I remember right, there are springs that control the advance plate.

Click to expand...

2 of them even, if i remember right, . a quick test short of looking at them is to just turn the rotor with yer fingers. ya can feel the spring action of the advance weights against the springs....

Re: Stumbling anywhere below 2500 RPM. Driving me nuts!!

You say you rebuilt the carb. The accelerater pump is good?