I have a pair of Stromberg 97's on a Thickstun PM-7 I spent the afternoon setting up the carbs with a balancer that indicated the intake vacuum. Bit of an odd thing was happening, I could get a steady reading on the rear carb but the front one diminished whilst the gauge was on there! Happened quite quickly, but happened as regular as clockwork. The motor has been running quite lumpy recently, hence the need to setup again. What could the reason be for this strange reading? I have one slightly tappy lifter (Adjustables) but the only thing I could think of was a intake gasket leak. Would this give the indication I have outlined? Or could it be a problem in the carb itself? Any help greatly appreciated before I start to take it apart.
Did you have both carbs connected with the linkage when you were using the sync unit ? I think they should separated ( no linkage ) when you are doing this...........I could be dead wrong
I've just installed new leads, plugs and a rebuilt properly set-up Ford crab style dissy! I can check the timing again though! I did have the linkage still attached to both carbs. I thought that the balancing was done via this linkage!
Linkage should be loose. Do one carb at a time until they're both close, then tighten linkage when you get the same reading. Otherwise, they're fighting each-other.
Hi Max Am I missing something here? I thought that the 'balancing' was done by sychronising the two linkages. My procedure was to set the motor idling at operating temp. and then slacken the linkage between carbs. The rear carb was still on the slow running stop on the linkage so in order to balance the two I then adjusted the linkage between the two so that the vacuum reading was the same on both carbs! I then checked the adjustments on the slow running jets. Is that right or am I missing a trick here?
The advise to sync separate makes more sense. From idle, would you not use the base idle screw, then adjust linkage to couple up without moving the throttles.
this worked for me, turn both idle screws out, set your linkage up, set 1 idle screw for both carbs, I also turn in the air screws on 1 carb, start the engine and sync, I run 3 carbs and this works for me
So do the air screws make a difference to the vacuum reading at the carb mouth? It sounds as though the method I used was as close as can be. I guess its stripdown later!
Well I feel a bit of a jerk! You know how you should always look for the simple things first? I have had the front carb apart, all checked out fine, fresh seals and gaskets. Couldn't see that the intake was leaking anywhere. What did I find that made a difference? The front slow running stop screw had gone missing! The rear stop was still in place, but I guess that the linkage was settling once the stop at the back was reached! The carbs both balance up now and I have an equal reading on both! Now to double check the timing (again!) but its back to chirping the tyres again! Thanks for all the input everyone.
On your linkage issues...as Max said, linkage inop for this because you want both carbs at their true idle...you probably need to go around the 4 fuel screws and two idle speed screws several times to reach optimum as all affect one another. Once idle is slow and smooth and high on yor gauge, hook up the linkage without moving either carb off its screw stop so both start moving from your good idle stting and not held partially open by the connection to other carb
Thanks for that Bruce! I went through them half a dozen times last night, the motor runs as smooth as it ever did (for a Flathead!) went out in the pouring rain to spin the wheels! What fun in the dark and wet on country roads! Thanks again to everyone! Cheers
The Jalopy Journal
