backfiring through rebuilt 97 Strombergs

[burly]

Guys, I've got the Dual Carb set-up idling and accelerating well other than a fairly routine slight hesitation and backfire through carburetors when I shift from 1st into 2nd gear? Warm or cold. Didn't do that with single intake and carb. Fresh stock build 24 stud, bubba's rebuilt crab dizzy, new 6v coil, wires and plugs. Appreciate any suggestions you'd be willing to share. Thanks!

[Brian]

Too lean?

[Ronnieroadster]

Being to lean will cause the problem try increasing the jet size. If theres two positions to set the accelerator pump rod try the second location the idea is to add some additional gas during the shift when you return onto the gas pedal that might help.
Ronnieroadster

[rotorwrench]

Might be a fuel blockage or a large vacuum leak.

[Talkwrench]

Originally Posted by rotorwrench

Might be a fuel blockage or a large vacuum leak.

Yes I'd say that, and lean . But how about telling us what type of manifold, jet sizes, p/v even fuel delivery etc and it might help a little more..

[B-O-B]

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Before doing a bunch of stuff try pulling the choke out a wee bit & see what happens.

[burly]

Originally Posted by Talkwrench

Yes I'd say that, and lean . But how about telling us what type of manifold, jet sizes, p/v even fuel delivery etc and it might help a little more..

It's a early thickstun intake, 43 jets and 69 p/vs. Factory fuel pump with regulator set at 2 lbs. Thanks to all who've replied.

[big job]

Old faithfull Vacuuam gauge

[glennpm]

I agree with B-O-B, pull out the choke making it run richer and see what happens. You can also run an unlit propane torch around the carbs and intake to see if the RPM increases and if it does you've identified a vacuum leak.

Glenn

[jimTN]

Tight intake valves

[Bill OH]

I had the same problem with 43 main and 65 pvs. I went to 46 mains and 71 pvs. 43s are just toooo lean.

[solidaxle]

When you say rebuilt carbs, most are half rebuilt. Worn and loose throttle shafts will give you a lean condition. If they were not professionally rebuilt, I would be suspicious on them being done right.

[Talkwrench]

OK just two cents here.. I have a slingshot setup with 97's One thing I did notice when setting up is that the throttle linkage it critical because at idle the throttle rod attachment is forward and low of 12 o'clock [ I-2 o'clock] and as the linkage pulls back it rises up to the 12 o'clock and then falls away to around the 9 0'clock position at full throttle. Depending on your throttle pull and angle I believe its very 'hard' on the shafts and may cause old worn ones to leak bad..
I don't think you have any heat risers in your manifold? I only have it at the base on the slingshot and it runs crap until the engine is close to temp, cold its a pig, fine balance of choke , slipping the clutch revs up etc ..aghh!

[john in illinois]

I agree with Bill. Stock .045 or .046 jets. Read JWL book section on 2 strombergs on a stock flathead. I fought jetting for a long time and finally looked to see if he had anything on it. Should have looked first.

John

[scicala]

Just a note of interest to avoid confusion. I'm sure most people here realize that on Stromberg 97 power valves, the orifice gets smaller (leaner) as the power valve number gets bigger.
In other words a 65 power valve is richer than a 70 power valve. The opposite of what you would normally expect.


Sal