engine and drivetrain High Revving Between Gears

[AdamKaz]

I've got a bit of mystery on my hands and hoping the community here can help. Here's the situation:

 

After the car is warmed up and choke is off, I'm getting a high rev event whenever I change gears. This only happens if the revs are above 3k, and when I engage the clutch to change the tach heads up to about 4K (if I'm already at 4K, it will run up to about 4.7K, if I'm at 5K it seems to not increase much but I'm not at 5K very often). If I shift around 2-2.5K, the car behaves brilliantly.

 

Also, sometimes on idle the engine will just sit at 3K until a gear is enganged and the clutch is let out. Blipping the throttle when this happens does not fix the problem. BUT GENERALLY the car idles just fine (+/- 1K).

 

I've adjusted the dual Weber carbs, and because 90% of the time the idle is right in the correct range I have a hard time thinking that it is an idle problem.

 

Anyone have any ideas what could cause this?

 

Thanks for your time.

[M3M3M3]

your throttle linkage is sticking somewhere between your foot and the carb butterflys.

 

have you had the carbs a long time or are they new to you?  i ask because another issue may be how the throttle cable is attached between car and engine.

[Rocan]

Marshall said it exactly. fix this issue asap before driving on it; you don't want to over-rev you motor or cause an accident because of a stuck throttle. Carefully look over your linkage. I have a high idle issue that is caused by a worn throttle linkage that causes it to get hung up just off idle, but what you mentioned sounds like it is only getting hung up halfway through the travel. With the engine and ignition off, have someone activate the throttle slowly (or do it yourself from the engine bay) and see if you can find an exact spot where it hangs up.

[AdamKaz]

I've installed a new throttle rod (the piece directly connected to the plastic pedal), so I've gotten a good look at the linkage below the car and I'm confident that that is all moving smoothly. I've also inspected the linkages towards the top of the engine bay and on the carbs - all okay. In addition, I've opperated the throttle rob connecting the carbs together by hand while the engine is running and it springs back to rest immediately, no problem. I don't see any mechancial peice that is sticking.

 

Did I understand your suggestions correctly or am I missing the plot? Because I can't SEE anything wrong/sticking, I'm thinking something in the carbs (butterfly valves sticking?). But maybe I just want it to be something that is invisible so I don't feel stupid. That's a distinct possibility...

[Rocan]

Could be a bent butterfly shaft, though unlikely. check that the carb closes completely. Next place to look would be vacuum leaks and your idle adjustment. 

[tim385]

Are your engine mounts both OK , reason I ask is during acceleration as the motor rocks it can cause the linkages to bind a bit (if they are not free enough)which will not be obvious at lower revs (idle) or during sedate driving, but may show up on quicked changes /higher revs.Just a thought!

[M3M3M3]

pictures of your linkage please.

[kbmb02]

Here's one more thing to check: Check the clearance between the throttle linkage 'arm' which exits the pedal box as it relates to the clutch fork / linkage. I've seen a combination of misalignment which causes the throttle rod to hang-up on the clutch linkage when engaged, resulting in a condition something like you describe. -KB

[TobyB]

all good suggestions above.

 

2 more- the butterflies on one carb are free to shift side- to- side just a bit, and they're binding in the bore.

 

or

 

- the weights in your mechanical advance distributor are sometimes sticking.

 

I've had both things give an intermittent high idle.

 

hth

 

t

[AdamKaz]

Thank you all, many things to check.

Travelling for a few weeks, then updates and pictures to come!

Thanks!

[AdamKaz]

Pictures coming soon, forum is giving me grief for the JPG format. What the?

 

Quick thought - how could it be linkages/mechanical when (if I'm driving at street speeds) the revs jump 1000rpm as soon as I depress the clutch (while removing pressure from the accelerator of course)? In my (feeble and niave) mind, this points to carb trouble. Lastly, I recently changed the in-line fuel filter. Could this now be delivering too much pressure? Wouldn't the float valves in the carbs solve that?

[M3M3M3]

because when clutch engaged, engine is under load.  push clutch, no load.  revs go up because linkage sticking at the "loaded" position...

[Rocan]

It could also be a sticky throttle pedal. Take it off and check that the nylon bushing is intact and make sure everything is CLEAN.

[mike472]

There is an internal springs in each carb. Check under the plate on the face of the carb flange. One of mine was missing and the carb will not return to idle well without it. You need to take the top cover off the carb to see the top of its bore. Better yet, get a diagram online and look at it.

[AdamKaz]

Sorry for the long delay, been travelling a lot with work. Here's the promised pics of my linkages. You've all given me plenty of stuff to look at in the next few days - I'll report back here if I find anything.