RPM goes up when shifting

[ratsar_nx]

I've had this issue for a while, but it doesn't happen all the time. I figured I'd ask here in case it is actually something that could cause serious problems...

 

When I put the clutch in to shift, sometimes the RPMs will go up instead of down. I've noticed that this usually happens when I shift about around 2000-2500RPM or above. When the RPMs climb when shifting, it doesn't sound the same as when they climb from applying more throttle. I did notice once when I let them climb that it went to about 3000 RPM, there was a pop from the exhaust and the tach dropped. It didn't sound like a backfire.

 

It's an E16 engine, stock Hitachi carb, distributor is equipped with vacuum advance. I'm running a 1.75" exhaust with no muffler, just a resonator at the back (and a few rust holes along the way). I noticed that it started happening after I removed the muffler and changed to the current setup, although I never noticed it happening when my muffler inlet was rusted out and I was running pretty much straight piped (inlet pipe fell off the muffler).

 

Is it that I'm not getting enough back pressure and the carb is expecting more EGR? Could it be a vacuum leak? And most importantly.... is it something that could damage my engine?

[datsunaholic]

I'd be more suspect that one of the throttle plates is sticking open.

[DaTsUn72WaG]

Yeah the plate could be sticking or the linkages but if it is quieter than a b-f it could be missing badly or timing on distributor off. Is the car still relatively cold when it does this?

[ggzilla]

Sounds like carb icing. If it is under 40F when this happens, the throttle plate may be iced-up. Make sure your hot air tube is connected and more importantly, that the vacuum diaphragm on the air clean actually works to raise the hot-air flap.

[ratsar_nx]

The engine isn't missing. I know what it sounds like when it's running on 3 cylinders from doing plug tests. It does intermittently stutter at idle, but I'm hoping that goes away with a new distributor cap. My timing may be a bit off, but I know it's close to what it should be. I notice it more when the engine is cold, but it will still happen when the engine is hot.

 

I never thought about the throttle plate sticking. I guess it would likely be the secondary if it's happening above around 2000RPM. It's well below 40F, and has been for a while (probably won't get warmer than that for at least another month). I can't remember what the temperature outside was when I first noticed it. Last I checked, my hot air flap was raised properly - it lowers as the air gets warmer.

 

I have a Holley 5200 that I'm in the process of rebuilding, which I hope to put on the car once the weather warms up in the spring. I'll have to so something with the air cleaner so I can keep the hot air intake.

[jefe de jefes]

Sounds like a similar problem I had. Turns out the linkages on my carb were getting stuck. I haven't had a problem since cleaning and lubing them up.

[datzenmike]

This might happen if the choke were on and the fast idle cam were set. Usually it runs in the 1800 to 2300 rpm range. Check that the choke works.. by works I mean that it opens completely when warmed up.

[ratsar_nx]

I checked the choke - it's definitely open all the way when the engine is at operating temp. I have noticed that my fast idle doesn't always seem to work though. Often it'll idle at under 1000RPM just after starting when the engine is cold. I figure that the choke must still be operating though, since the air temperature is always well below freezing and the engine always idles by itself.

 

With all the helpful advice I've gotten here, I'm starting to think that maybe my secondary is sticking partially open, especially since it's vacuum operated and doesn't depend on the primary. It only does this when the RPM is above 2000, usually when I'm giving it more than half throttle, and it it doesn't climb as strongly as if I were to maintain constant throttle when putting the clutch in. I'd probably notice a lot of throttle play if it were the primary sticking.

 

Speaking of throttle play, how much play should there be in the gas pedal? I think I might need to adjust my throttle cable tension.

 

I think I'm going to leave it alone, since it doesn't do it all the time and I have other work to do on the engine that's a bit more important (Oil change and valve clearance).

Edited January 30, 2009 by ratsar_nx
spelling

[ggzilla]

No play is required in the throttle pedal. You can if you want adjust it right up to zero play.

[datzenmike]

With engine off depress the throttle and let up slowly.... does it stick or hang up at all? Try when warm or cold. It might just not quickly return to idle when you let off to shift.