3.9 won't revv over 4500 rpm

Vulture · Post by **Vulture** » Sat Aug 11, 2007 2:08 pm

Hi everyone.

At long last, the 3.9 engine in my SD1 is running again. It starts well and has lots of power. However, it won't revv over 4500 rpm. The specs are as follows:

RV8 3.9, CR 9.35:1
RPi Engineering Stage III heads
Piper 285 cam
Fully balanced
Lightened and balanced flywheel
Edelbrock 500 cfm carb
Offenhauser intake manifold
Rimmer Brothers Sports exhaust system and manifolds

I'm using the jets and rods RPi sent me to go with this setup.

When driving full throttle, it pulls away very smoothly from 500 rpm, but the closer I get to 4500, the less power there is, until the engine keeps the revs steady at 4500 with full throttle. The engine itself should be healthy, and I suspect that the carb isn't set up correctly so the engine doesn't get enough fuel at higher revs. Could anybody confirm my suspicion or am I completely wrong?

PS: as always, my English is slightly awkward, but I hope you understand what I mean.

ian.stewart · Post by **ian.stewart** » Sat Aug 11, 2007 4:41 pm

have you checked with the engine off you are getting full throttle at the butterflies so they are wide open

Eliot · Post by **Eliot** » Sat Aug 11, 2007 5:34 pm

Maybe your fuel pump isn't keeping up.
Do a plug-chop (turn ignition off at full throttle) and inspect the plugs.

Vulture · Post by **Vulture** » Sat Aug 11, 2007 5:37 pm

@ian.stewart: I've checked right now. The smaller set of butterflies opens all the way, the larger one doesn't open at all when operating the gas pedal without the engine running.

@ Eliot: how are the plugs supposed/not supposed to look?

ian.stewart · Post by **ian.stewart** » Sat Aug 11, 2007 6:57 pm

Next thing to have a look at is the plugs, If they are wet with petrol it is unlikely they will clear, and need to be dried, what plug gaps are you running??

Post by **katanaman** » Sat Aug 11, 2007 10:11 pm

You can flick the metal vac operated flap out the way and you will see if the secondary butterflies are opening fully or not. Is this the first this engine has ran or has it ran ok before but been laid up for a while?

Eliot · Post by **Eliot** » Sun Aug 12, 2007 9:51 am

Plug colours
http://www.dansmc.com/Spark_Plugs/Spark ... talog.html

Vulture · Post by **Vulture** » Sun Aug 12, 2007 11:34 am

katanaman wrote:You can flick the metal vac operated flap out the way and you will see if the secondary butterflies are opening fully or not. Is this the first this engine has ran or has it ran ok before but been laid up for a while?

I had the engine running for about 1600 miles last February/March, but the RPi heads were faulty, a valve spring broke and I had them all replaced. The engine hasn't run since then. The problem already existed back then, but I thought it was because of the faulty springs (6 of them were about 5 mm shorter than the others). I saw something move below the flap, but I don't think it opened all the way.

The plugs I'm using are standard Beru plugs as I had in my Vitesse engine; I think I even reused them because they hadn't run much. I can't check on them currently because I'm at work until midnight or so, and I don't have the necessary tools here. Thanks for the link, Eliot, pretty useful info there!

Eliot · Post by **Eliot** » Sun Aug 12, 2007 12:57 pm

Vulture wrote:
katanaman wrote: I saw something move below the flap, but I don't think it opened all the way.!
The top flap is to keep the air velocity high when at wide open throttle but low rpm.
push it open, then get someone to push the accelerator (or hold it with a brick) - the two lower butterflies should be wide open.
They are on a progressive linkage, the smaller ones open first, then the secondaries start to open.

Vulture · Post by **Vulture** » Sun Aug 12, 2007 5:58 pm

I've checked again. The larger butterflies open only a tiny little bit whereas the small ones open all the way. Is there a way to adjust the secondary butterflies?

ian.stewart · Post by **ian.stewart** » Sun Aug 12, 2007 6:10 pm

Right, do you know if the carb has mechaninal secondaries or vacuum secondaries

sidecar · Post by **sidecar** » Sun Aug 12, 2007 6:52 pm

Edelbrock 500 has mechanical secondaries, if they are only opening a fraction then there is a mechanical problem with the carb. It should not be too difficult to sort out as the mechanism is on the outside of the carb.

By the way don't try junking the vac operated "flap" valve, I tried and found out that I could kill the motor stone dead even at 3.5k just by flooring the throttle!

Nice easy fix, I hope.

Cheers,

Pete

mgbv8 · Post by **mgbv8** » Sun Aug 12, 2007 7:57 pm

My 500 flips the secondaries fully open at about 60% throttle. You must have a problem with the secondary linkage. Mine is fixed and doesnt appear to be adjustable.
Are you sure you are getting full travel on the accelerator??

Eliot · Post by **Eliot** » Sun Aug 12, 2007 7:57 pm

Vulture wrote:I've checked again. The larger butterflies open only a tiny little bit whereas the small ones open all the way. Is there a way to adjust the secondary butterflies?

That's your problem then. There's no adjustment as such, theres a fixed linkage between them. I cant even quite imagine how its failing to open the secondaries to full.

Ive got a 600 i can take photo's or video of the linkage of, if it helps

mgbv8 · Post by **mgbv8** » Sun Aug 12, 2007 8:26 pm

It must be lack of travel on the throttle cable, or the secondary linkage is jammed or has something bent. Its a fixed rod between the primary and secondaries. I've just had a look at mine and the only real way you wont open the secondaries fully (assuming nothing is wrong with the carb) is if you run out of cable travel or something is getting in the way of the secondary linkage.