Won't rev past 4500

[tcbelise]

Wondering if anyone had any advice on a problem I've been encountering on my 67:

Just had the carbs tuned and the engine had been running great but recently when I'm driving on the freeway typically at 4000-4500, the engine will start to stumble and I can't get the revs past 4000. Even if I give more throttle the engine feels like it's lugging and the revs won't rise. If I downshift to 4th the revs still won't go past 4000-4500. If I drop the revs and try to accelerate the acceleration is very slow. In the past this problem would be intermittent and just go away after a few minutes of constant throttle and things would go back to normal. Yesterday this seemed to have gotten worse to the point where I had to pull off the freeway. As I cruised off the offramp to stop the engine died and once I restarted the engine was back to normal with full rev capacity. I got back on the freeway and after a few minutes the problem started again. Each time it happened restarting the engine would seem to clear the issue for a few minutes but then things would start again.

I'm thinking it might be either a fuel delivery issue or an ignition issue. Any thoughts on where to start?

Thanks!

[ejboyd5]

Perhaps not a strictly typical presentation, but I would look at the ignition coil.

[edmayo]

As ejboyd said it could be an ignition coil that gets hot, fails, then works again after a cool down. Could also be the in tank filter, when was that checked last? Start with the easy stuff, replace the coil. If problem continues then remove in tank filter and clean. Could also be fuel pump getting hot and not pumping enough. Was pump running when this quit? If you can catch it at a quit, check for fuel and spark,,,see which is missing.

[Freitasporsche]

I would also look for something in the ignition system. Does your rotor have the built in Rpm limiter? maybe that a good place to have a first look. just a though...

[Frank Beck]

I would start by taking it to a qualified shop.
They'll probably be able to pinpoint it quickly.

SO MANY things it could be you're pissing in the wind asking strangers who can't even drive it.

[2.5MFI]

I think asking "strangers" for ideas is a good thing. I agree on the fuel tank screen/filter is #1 and electrical checking next.

[fizeus]

many of the given suggestion are really good starting points.

If I were you I'll try to isolate the problem: if it's screen filter related problema you should hear pops through the carbs and your plugs should be whiteish on inspection.

You can try your fuel delivery (aka screen filter hogged), when the problem appears, downshift to 4th from 5th, push clutch pedal for some seconds and let the RPM drop to the minimum for 4-5 seconds (this should allow more fuel to accumulate into lines and carb bowls). After 4-5 seconds release the pedal and hard puce accelerator to see if you can override the 4.5 limit.
(If your screen filter is hogged you will be able to rev at full range and after some seconds you will newly be at 4.5K RPM as soon the fuel in carbs bowl and tubes finish).

If it's an electrical problem, when you hit the limit, simply put in neutral and press accelerator pedal to see if you can override the 4.5 RPMs: if in neutral you can't rev over 4.5 this sounds to an electrical problem. (check the rotor, if yours has a REV limiter the return spring can be worn and can disengage electrical continuity too soon)

Then I'll try, as already suggested, a new coil ... and then CDI ... did you already checked connections ???


Let us know how the story goes!

[uai]

Even if this sound like a mantra of mine....
The answer is WBO2. If you get a Wideband Lambda this well tell you a lot.
Costs perhaps 200-300 USD and welding a bung to the collectors - but I would not want to miss mine.
Apart from what was set before I'd check ignition advance too.
But the WBO2 is the answer...

[tcbelise]

Thanks for all the advice! I'll post an update how things work out.