Trouble with carb tuning, idling

[Sportsdoc]

Can get the car to start with choke pulled, but then runs at very high rpm. When choke is released, it dies unless the throttle is pumped keeping the rpm at over 3000. I did the initial carb stuff such as tightening the jet adjustment screw and then backing off two turns. The idle speed screws are not engaging the housing so I don't think that is a problem.

I'm kind of stumped at this point, and can't find anything in the books to help. I would appreciate any ideas.

 

[65sprite]

Air leaks, or worn throttle shaft bushings. Carefully spray WD40 at the throttle shafts while running. If RPMS pickup you may need to rebush the carb body. Also apply wd 40 at the junction of the carb to manifold spacers, and manifold to head. Check that the pistons are free and the dash pots are filled. Try and set the Idle with the linkage separated between the carbs and with the throttle cable disconnected. Make sure the butterflies are completely sealing when closed.

Others more knowledgeable will add to the list. Good luck
ron.

 

[Jim_Gruber]

Again check intake manifold bolts. I had this happen all of a sudden and discovered intake manifold bolts had come loose from engine block. Engine kept idling faster and faster. Discovered I could make major movements of carbs, tigtening bolts solved the problem.

 

[BWillis]

Richard,

Is this a 1098? How do you have the PCV plumbed?

If it runs with the choke open then its obviuosly getting to much air. Did you start with both jets 12 flats (2 turns) from level with the bridge?

 

[dklawson]

As above, look for big vacuum leaks (manifold gaskets, open PCV port...)

 

[Sportsdoc]

Yes, this is a 1098. The PCV is plumbed to the manifold port like it was when I got the car. I have not done anything with the PCV, though, except remove and replace it. I did the twelve turns from the bridge, and made sure the jets were centered.

It runs with the choke open but at an excessive RPM and when I close the choke it promptly dies. I can sometimes keep it running by pumping the throttle.

 

[BWillis]

Are your dampers moving up when the engine is running? Do they have oil in them?

 

[dklawson]

Along that line... is there a chance you did not put the springs back in when you re-installed the vacuum pistons?

You also said:
"The idle speed screws are not engaging the housing so I don't think that is a problem."

What do you mean by that? The idle screws should be in contact and tightened in at least one full turn to re-start the engine prior to tuning and proper setting.

 

[Jim_Gruber]

Got the Throttle and Choke cables connected to the right spots on the carb? Don't ask me how I know?

 

[jlaird]

Sounds like the old air leak problem.

 

[Sportsdoc]

I did put the springs back, but the cable right spots could be a problem. What are the right spots??

What I meant by the idle speed screws was that they are not tightened down to increase idle speed so I don't think they are the problem.

 

[spritenut]

Where are the choke fast idle screws set? Keep them off the cam for now and set the throttle idle screws.
I bet the choke fast idle screws are too far in.

 

[BWillis]

Richard,

The throttle goes to the center of the carbs on the shaft. The choke goes to the rear carb and connects to the front linkage.

When the choke is operated you should see the jets drop down lower than when it is off. This lowers the level of the tip and causes more fuel to exit enriching the mixture.

Let us know how yours is set up. Also a few pics may show us something that you arent explaining.

 

[SilentUnicorn]

this sounds like sticking jets due to mal-adjusted linkage


mark

 

[Sportsdoc]

I'm quite sure I have the throttle and choke attached properly. When I pull the choke, the jets drop and return when I push the choke back.

I will try to post a pic soon. I'm getting a new fuel pump, low pressure, so hopefully I can get it running without the bowl flooding fire hazard and adjust the carbs/vacuum and get her to idle properly.

Thanks for the help everyone.

 

[dklawson]

At the risk of supplying a link you don't need...
https://www.1978mgmidget.com/Tuning_SU_Carbs.pdf