High rpm upon startup

[trentonm]

I mentioned this issue in a previous post of mine but I figured I'd make another post to get more eyes on it, and include photos
A few days ago my granpda and I were working on our U20 and it seemed like we were finally getting close to getting the timing and carburetors adjusted just right. idle was right around 800rpm, engine was beginning to smooth out, it was looking good. then seemingly out of nowhere rpm shot way up. by the time i made it to the key to shut it down, we were probably around 5k rpm with no signs of slowing down. now every time i go to start the car, rpm shoots way up the same way. im not sure if we have a vacuum leak issue, or if somehow we have a timing issue? i really have no idea. hopefully the photos help out. otherwise i can try and answer questions best i can. hopefully its a small issue i'm overlooking, im still learning so its very possible. thank you
https://imgur.com/a/9sezCjT
https://imgur.com/a/PjkETjq
https://imgur.com/a/lkQap59
https://imgur.com/a/QTK2G1p

[Habitat.pat]

I’m not a 2L guy, but are the throttle return springs in place?

Back out whatever you did before the speed change.

On the 1.6L there is a idle adjusting screw on the linkage to hold the butterflies open that needs to be backed off fully when the engine is in use. Does the 2L have the same that might be holding the throttle open?

Verify that the carb dashpot oil level is good.

When you start it do both carb pistons rise the same or does one rise faster/sooner than the other?

Peace,
Pat

[trentonm]

I’m not a 2L guy, but are the throttle return springs in place?

Back out whatever you did before the speed change.

On the 1.6L there is a idle adjusting screw on the linkage to hold the butterflies open that needs to be backed off fully when the engine is in use. Does the 2L have the same that might be holding the throttle open?

Verify that the carb dashpot oil level is good.

When you start it do both carb pistons rise the same or does one rise faster/sooner than the other?

Peace,
Pat

Thank you for the response.

The return springs are in place on both sides.
The 2L has a similar setup with the butterflies. I don't think the adjustment screws are backed off ALL the way but they're pretty close to it.
We've checked on our oil level, that's all good. We did notice while running previously that the piston in the carb closest to the firewall was raised higher than the front carb's piston. we could physically fit a finger under it while we couldn't on the other one.

Thanks again,
Trenton

[Bwk2000]

I’m assuming you’ve already checked and don’t have a stuck hand throttle, accelerator or choke cable? Then, since it happened “out of nowhere” when it was otherwise running pretty good, there are really only two possibilities: Something is now jamming your throttle plates wide open or there is a major vacuum leak (likely from a hose that was knocked off).

Let us know.

[Gregs672000]

" We did notice while running previously that the piston in the carb closest to the firewall was raised higher than the front carb's piston. we could physically fit a finger under it while we couldn't on the other one. "
[/quote]

What? Wait... are you saying that when the engine was idling that you could fit a finger between the SU piston and the bottom of the carb throat? That condition at idle (the piston pushed upwards) turns off that carb... you actually use a push pin to achieve that so as to test the idle mixture in the other carb.

Somethings amiss... are you sure it was running on both carbs? Did you check airflow??? First time I put SUs on my Datsun PU as a teenager it ran on one carb before I took it to a grumpy old Datsun guy who I befriended... he found it was running on one carb and taught me how to balance them. Same issue here? Maybe you thought you had it adjusted but it wasn't, and then something changed that increased the airflow in what was previously (one of the carbs) not working, and now it is (at least more than it was) and it's reving up.

CHECK AIRFLOW.

[Gregs672000]

BTW, I know 5000 rpms sounds a bit scary, it's not going to hurt the engine. 7000+ starts to become concerning. You can control the engine by cutting off airflow through the carbs by putting some restrictive object (like strong card board) over the throats and kill it if you have to... no air, no go. I might not use my hand... it sucks pretty hard!

[MattC]

I know you said return springs were attached but in this pic it looks like this spring is not anchored to the heat shield. Could that be a possibility?

C7F93480-317E-48AC-A7BD-E37E24A6E208.png

[MattC]

Oh never mind, I think it is but it’s hard to see the tab.

[keith0alan]

Just a guess but it sounds like it was running on one carb then the second carb kicked in. The second carb has the throttle open too far so away it went. Back to basics. Valves, timing, carb static setup then airflow balance. That is incorrect float bowl to nozzle fuel line and it will give you a world of problems. Also you are missing hardware on the manifold throttle pivot point.

[rwmann]

Throttle cable?

[unklpat]

I'm not letting go of the "lean mixture" theory, especially after seeing those fuel lines. Get the proper tubing, not too long, and try it again. Those lines can kink inside and look fine on the outside. Pat

[RBMann]

You can see clean threads where this idle screw has been turned in. This position gives a higher idle. This might have been done when it was running on mainly one carb and when the second one kicked in, off you go to 5,000. Good luck.

UfVwZBYl.jpg

[nismou20]

Yeah, I would remove the domes and dampers to be sure the butterfly’s are not jammed open by something. Just a small vacuum leak wouldn’t get it up to 5k rpm.

[trentonm]

" We did notice while running previously that the piston in the carb closest to the firewall was raised higher than the front carb's piston. we could physically fit a finger under it while we couldn't on the other one. "

What? Wait... are you saying that when the engine was idling that you could fit a finger between the SU piston and the bottom of the carb throat? That condition at idle (the piston pushed upwards) turns off that carb... you actually use a push pin to achieve that so as to test the idle mixture in the other carb.

Somethings amiss... are you sure it was running on both carbs? Did you check airflow??? First time I put SUs on my Datsun PU as a teenager it ran on one carb before I took it to a grumpy old Datsun guy who I befriended... he found it was running on one carb and taught me how to balance them. Same issue here? Maybe you thought you had it adjusted but it wasn't, and then something changed that increased the airflow in what was previously (one of the carbs) not working, and now it is (at least more than it was) and it's reving up.

CHECK AIRFLOW.
[/quote]

interesting. i hadn’t known this. what would cause us to just be running off of one carb?
thank you for the response. joining this forum has been great for learning from responses like yours

[trentonm]

You can see clean threads where this idle screw has been turned in. This position gives a higher idle. This might have been done when it was running on mainly one carb and when the second one kicked in, off you go to 5,000. Good luck.

UfVwZBYl.jpg

so the idle screw, i’ve been confused by it and where it needs to be. i’ve read that it needs to be backed all the way out, but when i do that and increase throttle, it catches on, i don’t know what it’s called, the sort of “T” junction right above the screw. then the throttle is stuck open. so just back it out as far as i can so it doesn’t catch?