Why do engines top out in RPM

[Mekanix]

Hi, what would cause an engine to pull strong but top out before red-line without any kind of rev limiting device?

I understand the need for strengthening the valve train to prevent floating at high rpm's but I cant even get there.

What would limit how high and engine can rev mechanically ?

thanks

[rapidray]

If the jetting is too rich or too lean it will limit rpms. Ray.

[GregT]

Several things can act as a limiter.
Inlet - undersized carb(s) - or not opening fully.
Ignition - either not reaching full advance or a curve with a built in retard,
either deliberate or maybe components failing.
Cam timing - not hard to do by accident.
Exhaust system - maybe partially blocked, eg collapsed muffler internals
or just plain wrong lengths and/or diameters for that motor.

Are you looking for an answer to a specific problem ?

Some engines are designed to run out of breath well before the mechanical limits are reached - Harleys come to mind here.

[Mekanix]

Thanks Ray
I'm working on jetting now and low to mid-range was rich but tan is all I'm getting at WOT. After lowering the needle I get tan through the mid-range (4000-7000rpm) but after 7500 there is no increase in pull and seems to drop off very quickly


Thanks GregT

Its for the engine in my sig, An 81 gs400l. Everything is stock except a set of slip-on pipes.


"Inlet - undersized carb(s) - or not opening fully" stock VM34SS carbs and internals, although I'm not completely certain they are opening up all the way but I don't really know of any way to tell if they are.


"Ignition - either not reaching full advance or a curve with a built in retard" with a timing light I can see the advance happening at 3-4k and the timing marks align with the rotor and stator plate.


"either deliberate or maybe components failing" I just finished rebuilding the engine but I do suspect the pistons and rings for wear as they are hard to find, therefore I reused the original's which still give me 146/148psi dry

"Cam timing - not hard to do by accident" I checked it quite a few times and the first 3 times i did have the exhaust cam 1 tooth off until I set it 1 tooth advanced and then it aligned properly when the cam caps were tightened. Although I didn't degree the cam's yet. I'm getting a second set of cam gears slotted for that.


"Exhaust system - maybe partially blocked, eg: collapsed muffler internals
or just plain wrong lengths and/or diameters for that motor" Stock down tubes and cut equally on both sides, Slip-on pipes set to the same length on both sides. But as for proper I don't think I have a way to tell but I am going to try moving them in and out and see what effect it has on the power band.



I'm trying to get the bike up to stock performance and then modify it slightly.

Including:

Degree both cams

Boring and using 10.5/1 Wiseco pistons

Using pod filters and jetting accordingly


There is one thing that I do remember during the build: The original head gasket was shaped the same as the combustion chamber and the replacement was round. That would make the combustion chamber bigger but only slightly. Although I'm not sure how much impact that would have.

This is my second gs400l and I remember the first one could rev past red-line easily and I had to back off. But this one just doesn't have it.

Thanks again

[spchips]

7500 is about right. This is where you make peak torque (pull). You make peak horsepower slightly higher, maybe 8500 rpm, because while you are making slightly less torque you are spinning faster (HP = torque * RPM /5252), and after that it's all downhill because your torque starts fading fast as you approach redline.

This is because (on any bike):

The carburetor and/or exhaust are limiting the peak airflow. (see next)

The valve openings are becoming restrictive and not allowing more air in as the speed increases.

The valve timing and ignition timing are no longer at the ideal point for the speed that everything is happening in the engine.

The valves may be getting close to floating (not closing all the way each time around because the spring can't pull them up fast enough), although this is usually a few thousand RPM beyond redline.



In general, these are all good things, because on a stock bike a sensible person will shift as the power fades away even if they are not looking at the tach. "Redline" is considered the how fast you can spin the engine without significant reduction in how long it will last.

[tkent02]

I've never seen a GS quit making power at 7,500 RPM, unless there's something wrong with it. Especially the little ones, they are eager to spin, and pull hard to red line and more.

Not to skip the easy stuff, is the ignition timing correct and advancing as it should?

Clogged exhaust is very likely. Mouse's nest up the pipe? Or the mouse itself? A potato?
These are all things I have seen. I bought a GS 1000 that had a huge ball of steel wool jammed up the 4-1 pipe, sitting in the collector. It wouldn't rev at all.

I had a car that had something shift inside the muffler, blocking the airflow. It stopped making power completely.

[GregT]

Try nudging the ignition advance up a little at a time. Even though it seems ok with a light there's no guarantee the backplate is original - or correct.
Is it a 2v/cylinder or 4v/cylinder 400 ?
If it's a 2v 400 does the motor still have points ignition ?
Poor coil saturation due to incorrect dwell can knock revs off the top end too.
When it wants to stop rising in the revs, does it miss ?

[Mekanix]

Thanks for the reply's

Spchips: All great points. It doesn't feel like it has the ability to rev even up to the red-line of 9500 Its like having a soft cushioned rev limiter.


Tkent02: It has a good pull from 4200 to 5000, a little soft from 5 to 6and another from 6 to 7 but it really likes 7 to 7500 and then fades above 8500.
I'll keep an eye out for potato's It has only been a month since a complete rebuild of the entire bike including new pipes however the slip on pipes might just be too small over all.

I've watch the mechanical advance do its thing with a timing light and it does shift up at the correct RPM in the service manual and the timing marks for advance line-up.



GregT: I'll try 2 degrees advance and see what happens, I only have I think 4 degrees either way. It is definitely the stock plate and pickups not points.
Its the 4 valves per cylinder model.
Doesn't miss at high end, just feels like its backing off on the throttle even though I'm holding it open.
I have modified the wiring harness for more power handling. The coils and ignitor get near full 14.5v just off idle.

Maybe that's too much since it spent most of its life only getting 11-12v max. Coils could be saturating or the ignitor could be breaking down.

[GregT]

Hmm - Will it redline in the lower gears ? Does it pull better cold than hot ?

There are 2 possibilities in the carbs - fuel starvation, maybe partially blocked filter in the tank, restriction in the fuel line, vacuum petcock playing up ?
OR - maybe a PO has drilled the vacuum holes in the bottom of the carb slides. They're usually a single hole around 2.8 mm diameter. If they are drilled excessively the slides can drop at WOT.

Ignition - is the ignitor box the same for all the 8v 400 twins ? Does the L have a rev limiter the others don't ?

[Mekanix]

It acts the same cold or warm. But when hot its a little less responsive.

3rd gear is where I do most of the testing and it will hit 1100 and completely run out or steam after that.
I doubt it would even rev that high in neutral and I think it was mostly momentum carrying it that high.

The specs for the ignitor are a mystery to me.

Slides are stock and weigh the same but you might be onto something with the holes.

I wonder if the problem is: not having enough restriction in the air-box to create the vacuum needed to raise the slides ?

I think i need to get a camera in there

[GregT]

Suzuki ignitors are identified by the number moulded into the case. I'm not internet savvy enough to track which belongs to which model but someone here will be able to.

Can you get access to a rolling road type dyno ? You don't want or need a dyno run as such, but WOT with someone looking on & maybe a camera in the airbox would answer some questions.

[wst33]

The engine is nothing more than an airpump, in stock form there is only so much air that can be pumped thru it. To get more air thru it carbs, cams, headers, air filters, or modifying the cylinder head will need to be modified.
Nitros will also work.

[Roger P.]

Plenty of bikes won't pull redline in top, it doesn't mean that there is anything wrong with them. A lot of bikes are geared for lower highway rpm at normal cruising speeds, with a resultant loss in top end speed. I know some of the 1980's Honda V-fours and v-twins had very tall overdrive 6th gears, and some of these bikes had a higher top speed in 5th gear rather than top gear.