engine will not rev passed 5000 rpm why

[advinnie]

Hi all I have a 1699cc engine ( stock pistons with a 74mm crank) in my beetle and it will not rev passed 5000 rpm anyone got any ideas why?
Here's my engine spec
Dual relief case
CW 74mm crank
Lightened flywheel
gene berg 307 cam with 1.4 rockers. 477 lift at valves and 278 degrees duration
dual single chock kadron 40s

I have tried a 4 into 1 header with a hideaway exhaust and a stock exhaust system and the engine will still not break the 5000 rpm mark.
I am using stock heads.
The carbs are not running out of fuel the engine will rev nicely up to 5000 RPM without any problems and will sit there all day it just will not rev any faster. Any help would be great.
Cheers

[helowrench]

What ignition system do you have?

[FJCamper]

Hi Advinnie,

Check first to make sure you are getting full ignition advance.

Then check to see if you're getting full thottle on both carbs. That means with the acellerator on the floor, you can look down into the Kads and see the throttle plates standing straight up.

If not, adjust the carb linkage or accel cable so you do get full throttle.

But, if you are getting full throttle and you're still stuck at 5000 RPM, it's more complicated, such as your heads are flowing as much as they can. Maybe your valves are not opening to full lift. Take off a valve cover and rotate the engine around, watching one of your intake valves as the rocker pushes it into full depression. Compared to any one of the still-closed valves on that head, your intake should have compressed down so it is at least 12mm lower as measured from a line across the tops of your valve stem tips.

It's hard to be accurate with this, but if you're seeing less, your rocker geometry is off, or maybe the valve springs arebinding on full depression.

Check these possible problems out and let us know.

FJC

[advinnie]

I have a svda distributor set to 32 degrees BTDC at 3000 rpm with the vacuum line disconnected and 44 degrees BTDC with it on. Carbs are synchronised and both fully open at full throttle. Valve springs are not binding and all valves are opening fully ive all ready done that one.

[helowrench]

Points or electronics in the dizzy?
I have seen points float and cause this.

(also have seen someone forget they had a rev limiting rotor, but figure you have checked that)

[FJCamper]

Hi Advinnie,

OK, If the basics are out of the way ... I am presuming you have 28mm venturis in your Kads, (which would take you past 5K). I have seen Kad venturis loose and actually dropped down about half an inch in the carb throat. If this has happened on one or both sides, you will have an RPM limit.

A leaking (meaning sucking) crossover tube between carbs can badly limit RPM.

With the air cleaners removed, do you smell gas, an overrich condition, at any time the engine is running, to include with the engine just turned off? That can indicate a high RPM flooding.

A techical point is if, somehow, one or both of your carbs were failing to engage the main power circuits. Again, watching down the throat as you manually move the throttle and pass 3000 RPM, you should see gas spraying out of what looks like a smaller venturi just about the main venuiri.

Check 'm out and let us know.

FJC

[RHough]

Is it a hard limit or a soft limit? Does the engine shut off at 5000 RPM like a switch? > Check Ignition 1st
Does the engine pull the same RPM in every gear? > Check Ignition 1st

Fuel problems tend to be "soft". The load (and hence manifold vacuum) at 5000 RPM will be different in each gear. Carbs have no idea what the RPM is, they only react to the pressure differential between atmosphere and manifold. It would take a very unique set of circumstances for carburetors to cause the RPM limit problem since airflow at 5000 RPM is not always the same.

It might something as simple as an electrical connection going intermittent due to vibration at 5000 RPM.

Good luck!

Randy

[advinnie]

Hi all the engine does not shut off at 5000 rpm it just stops accelerating. It will pull hard from idle all the way to 5000 rpm and sits there every now and again it will go passed it but this morning I was racing a boy in his car he thought he could over take me in town, anyway for the first time ever the rpm went to 5500 in 1st 5400 ish in 2nd and every time I change it a gear the rpm limit dropped. I couldn't believe it so i turned around and tried it again same weather same road same conditions but this time NO up to 5000 rpm in any gear and that was it.

[helowrench]

SP or DP heads, and what size valves, and level of porting?

you may just be reaching the airflow limits of your heads.

[advinnie]

Hi they are dual port heads Stock valves and no portion g or polishing.

[Ol'fogasaurus]

advinnie: for a start, I would pull the plugs and look at them and see what condition they are in; e.g., rich, lean or OK. This alone might tell you something... or not. At least it would tell you if you might have a fuel problem, a weak cylinder, etc.. I would also do a leak down test on all of the cylinders.

Also: look at the plug wires and the rest of the spark delivering components, from the coil south, if you could buzz it up there once then why not all the time. Along with the other things suggested carburation wise: I would also check the condition of the area the butterfly cross-shaft goes through (bushings, especially if these are used carbs) to see if they are leaking. Also check for gasket leaks on the carbs to manifold and manifold to head (lean out). Also, are there any air passage blockages inside of the carbs (especially if they are used but there can be problems even with new ones. What about air cleaner elements? Also check to see about fuel flow and the fuel filter or are you getting a vacuum condition in the tank because the fuel cap isn't venting properly.

Since I am not familiar with Gene Berg cams it is hard to say if the cam is a limitation or not. The cam I have (intentionally) limits me to around 5K usable RPMa before the engine falls on it's face but then you were able to get the engine up to 5500 once in first gear. Not being able to do so in the other gears after the one good RPM pull might be telling you something too. Did you degree the cam in when installing it? Are the valves too tight, weak spring, etc.?

Some very basic and inexpensive things to look into. This sounds like one of those things you have to keep after, one step at a time until you get an Oooooh Yeah! Probably something very simple.

Lee

[advinnie]

Hi looks like something is wrong on the right hand side of the engine cylinder 1&2. There seems to be very little compression/ the compression is leaking out somewhere. When you turn the engine over by hand you can feel the compression building up when cylinders 3&4 are coming up to TDC and the engine has a slight kick when you go passed that point on them two cylinders. But on cylinders 1&2 you can start to feel the compression coming on but it never builds up you can here all the compression hissing away and you can easily turn the engine up to and passed TDC on them two cylinders. Does this make sense?

[Ol'fogasaurus]

By doing the rotation test you did now gets you started on a real diagnosis to the problem... good job! If you come up with what you think is a soft cylinder or two then a compression test is in order; don't forget to do both a wet and dry test.

If you find a problem but can't place the problem then leak down test may be in order. It it allows you to be more precise in finding where the problem is; here is how to do one: http://www.bing.com/videos/search?q=how ... 55590090EA A bit more complicated but it may be the next step to check/verify the condition of the suspect (or all) cylinders.

Lee

[helowrench]

adjust the rockers out a turn or two

then check the rotation again.

if you have a valve not closing this will tell you.

[helowrench]

before you do what I posted above, check your valve adjustment and see if it has changed any.

btw how old is this engine since build?