Spitfire Spitfire HS4 help

[tomkatb]

Yesterday my helpful son put a “rebuilt” set of HS4’s on his mothers 77 Spitfire. Prior to that event the rebuilt motor ran perfectly on the old Stromberg.

We cannot get the idle right.

The back carb seems not to completely cut off air flow to the engine when the idle screw is all the way out. I.E. the car will not idle below 1100 rpm. The front carb will close off all air flow to that side.

Are there any obvious things to look at?

Is there a right side up for throttle discs?

Should there be grease on the throttle shafts when reassembling them?

Larry

 

[DrEntropy]

Throttle plate alignment is critical to 'shutting off' flow, also check the choke actuator to be sure it's not interfering with the full closure of the plate.

This bit you may already know, but I thought it worth mentioning: To set 'em up you have to loosen the cross-shaft linkage on one or the other and do the initial idle settings with them running independent of one-another.

 

[tomkatb]

Thanks for the reply Doc.

We were trying to set the carbs up disconnected from each other.

I am thinking perhaps my son installed the disc backwards in the carb. That might do it.

Do you have any idea what the Poppet valves do?

The new discs did not have them.

I will check the choke for interferance later.

Larry

 

[tdskip]

I'd double check this info before doing anything irreversible, but the Poppet valves are designed to help reduce emissions and don't seem to be well liked since as they wear they seems to impact idle quality.

The Poppet valve should be on the top with the spring facing the engine.

Have you checked for vacuum leaks?

Is the throttle shaft aren't proper/tight/sealed? That could also be a cause of the high idle.

 

[tomkatb]

tdskip

My kid put new throttle shafts in yesterday. Seemed easy.

We did the vacuum leak check and had none.

Larry

 

[piman]

Hello Larry,

the throttle discs are designed with overlarge holes where the screws pass through. The correct way to fit them is to loosely assemble the disc and then, while keeping the throttle shaft closed move the disc to get the minimum air showing as you look up the choke. If you just slip them in and tighten the screw the chances are you won't get it centered properly.

The other thing that needs centering are the jets, if one is binding the needle it may not allow the piston to fully drop at idle.

Alec

 

[tomkatb]

thanks Alec. That is what I needed.

This afternoon I will take the carb off again.

Larry

 

[DrEntropy]

You'll get it. It can't be anything you won't see. Good that you have solid plates, too. I'd next have been asking if you'd solder'd the poxy poppet valve thingies shut and recommend to if you hadn't.

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[tomkatb]

The throttle plate was not centered well enough. We reset it and cleaned some debris from the carb body. Runs ok now. Needs tweeking and maybe needles.

We have a 1500 with a recent rebuild. Quality 3 axis valve job, new stock cam, balanced better than the factory, High compreession pistons, Aluminum flywheel. and slight decking of the head.

The European carbs are hs4 and have the ABM meedles. Do these sound about right???

Larry

 

[SilentUnicorn]

ABT needles i think were stock needles. I started with those, and have been fiddling with them a bit. Here is an online needle chart where you can compare different needles https://www.mintylamb.co.uk/suneedle/ Hs-4 should have an .090 jet. I found the abt's not to my liking....1500, +.020, cam , decked head. i would tell you what i am running now , but my feeble mind doesn't remember at the moment and i am 300 miles from my car and the home computer where all my info is.



mark

 

[tomkatb]

The car is running well. I would guess a 7-10% hp improvement. Very different.

To boot, the sound is much better.

Worth the $300 it cost.

I would say that with the single Stromberg would be advised it is worth the effort!!

Larry

 

[tdskip]

Good going Larry!