Why can't I rev past 4,500

[cvair4life]

So when I'm cruising in 1st and 2nd (and a little bit in 3rd)... I never really make it in 4th. The car kind of struggles with 4,500 rpm and greater... like it's rev limited or mis-timed or starved or something.

This is on my 110 engine with primary stock carbs from a 140. Everything is balanced. I think timing is advanced to +20° I'll have to check but I remember dialing it in there.

Any ideas why? I'll post a video soon of this happening.

[UNSAFE]

Did this just start happening - or has it never run right for you yet ?

As long as the internals of the motor are good you are likely looking at a timing or fuel delivery problem.

First thing I'd do is confirm that both carbs are fully open when the pedal is floored. Block the pedal or have a helper hold it to the floor while you look at the carbs.

Then I'd check the mechanical timing advance . The advance should move the timing as the rpms increase . If it is stuck or not working properly you will be way down on power. Just check it with a timing light to confirm that it's working as you increase the rpms .

Next it could be a restriction inside the carb(s) but I'd double check the first 2 things first.

Also if the valves are adjusted too tight and/or you have weak springs the valves may be floating at 4500 rpms.

[miniman82]

Blocked jet?

[cvair4life]

The timing advance definitely works. I've been through that several times.

I've never opened up these carbs up so a blocked jet could be possible... I'd expect to feel that stutter more in even lower rpms though?

What kind of fuel delivery problem are you referring to Kevin?

[miniman82]

I've never opened up these carbs up so a blocked jet could be possible... I'd expect to feel that stutter more in even lower rpms though?

If a main is blocked it will run fine in the lower RPM's, but fall on it's face when wide open. It would mean the issue is throttle position dependent, and should happen in any gear besides first. I don't think first puts enough load on the engine to pull on the mains, but I could be wrong. I'm not a carb guy anymore, my engines have fuel injectors now. @ Kevin

[UNSAFE]

What kind of fuel delivery problem are you referring to Kevin?

Basically the jets or the internal passageways. Some of those tiny orifices let air in instead of gas. Also a little water in the carbs can cause lotsa problems.

If you have eliminated ignition and timing and internal problems like low compression or bad cam -- it's likely a carb problem. If the carbs sat with some water in them at some time in the past the internal passageways may be plugged with a white chalky corrosion.

I'd try cleaning them first or even better if you could borrow or steal a known good pair of carbs to try.

Closing the chokes most of the way will richen the mixture . You might try wiring the chokes almost closed and then drive it to see what kind of difference it makes..

You could also try shorting out one plug wire at a time and then driving around the block noting how much difference it makes. This may pinpoint something that didn't show up in the idling cylinder balance test.

[flat6_musik]

I would also be thinking about a lean mixture also. Maybe the power valve needle/weight is lodged and not being able to lift off its seat to add more fuel when it's needed. You didn't mention if the lack of power/revvability includes any jerking, backfiring or lurching or is it just a "going away" of power?

I think it's a good idea to take apart your carbs now and then just to eyeball and check everything and keep them really clean.

[cvair4life]

how about this..... I seem to be able to rev past 4500 no hesitation when the heads are warm but I haven't been running very long... (e.g. idled for 5 minutes before I pulled out and drove normal for a block before hammering it through 1st and 2nd)

When I've been driving for a while and everything is good and hot it seems to show a more noticeable stumbling around 4500. Also it seems like I can get through it if I gradually accelerate up to and through 4500 rpm... while if I'm full throttle all the way up it stumbles...??